PB90-159807
HANDBOOK OF SUGGESTED PRACTICES FOR THE DESIGN
AND INSTALLATION OF GROUND WATER MONITORING WELLS
National Water Well Association
Dublin, OH
Feb 90
U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
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PB90-159807
EPA/600/4-89/034
February 1990
HANDBOOK OF
SUGGESTED PRACTICES FOR THE DESIGN
AND INSTALLATION OF
GROUND-WATER MONITORING WELLS
by
Linda Aller, Truman W. Bennett and Glen Hackett
Bennett & Williams, Inc.
Columbus, Ohio 43231
and
Rebecca J. Petty
Ohio Department of Natural Resources
Division of Ground Water
Columbus, Ohio 43215
and
Jay H. Lehr and Helen Sedoris
National Water Well Association
Dublin, Ohio 43017
and
David M. Nielsen
Bias land, Bouck and Lee
Westerville, Ohio 43081
and
Jane E. Denne
Advanced Monitoring Systems Division
Environmental Monitoring Systems Laboratory
Las Vegas, Nevada 89193-3478
Cooperative Agreement CR-812350-01
Project Officer
Jane E. Denne
Advanced Monitoring Systems Division
Environmental Monitoring Systems Laboratory
Las Vegas, Nevada 89193-3478
This study was conducted in cooperation
with National Water Well Association
Dublin, Ohio 43017
ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89193-3478
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA/600/4-89/034
3. RECIPIENT'S
VaP
/AS
I. TITLE AND SUBTITLE
HANDBOOK OF SUGGESTED PRACTICES FOR THE DESIGN AND
INSTALLATION OF GROUND WATER MONITORING WELLS
5. REPORT DATE
February 1990
6. PERFORMING ORGANIZATION CODE
AUTHORS L> AUer^ 1
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NOTICE
The information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under Cooperative
Agreement Number CR-812350-01 to the National Water Well Association. It
has been subjected to the Agency's peer and administrative review, and it
has been approved for publication as an EPA document. Mention of trade
names or commercial products does not constitute endorsement or
recommendation for use.
This document has been prepared in cooperation with EMSL-LV, Office of
Research and Development. It is intended to be used as a general reference
and will not supersede program-specific guidance (e.g., the RCRA
Ground-Water Monitoring Technical Enforcement Guidance Document).
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ABSTRACT
The Handbook of Suggested Practices for the Design and
Installation of Ground-Water Monitoring Wells is intended to assist
personnel involved with the design, construction and installation of
ground-water monitoring wells. This document does not focus on specific
regulatory requirements, but instead presents state-of-the-art technology
that may be applied in diverse hydrogeologic situations. The "Handbook"
addresses field-oriented practices to solve monitoring well construction
problems rather than conceptual or idealized practices. The information in
this Handbook' is presented in both matrix and text form. The matrices
use a relative numerical rating scheme to guide the user toward appropriate
drilling technologies for particular monitoring situations. The text
provides the narrative overview of the criteria that influence ground-water
monitoring well design and construction in various hydrogeologic settings.
The "Handbook" addresses topics ranging from initial planning for a
monitoring well to abandonment. Factors influencing monitoring well design
and installation include: purpose, location, site hydrogeology,
contaminant characteristics, anthropogenic activities, and testing
equipment that the well must accommodate. Decontamination procedures
should be planned and executed with care. 'Detailed recordkeeping from the
time of well installation through sampling to abandonment is very
important. Numerous drilling and formation sampling techniques are
available, and many factors must be considered in selecting an appropriate
method. Materials for well casing, screen, filter pack, and annular
sealants also should be selected and installed carefully. Well completion
and development procedures should allow collection of representative
ground-water samples and levels. Maintenance of monitoring wells is an
important network management consideration. Well abandonment procedures
should include consideration of the monitoring well construction,
hydrogeology, and contamination at the site. The "Handbook" serves as a
general reference for the numerous factors involved in monitoring well
design, construction, and installation.
This report was submitted in fulfillment of Cooperative Agreement
Number CR-812350-01 by the National Water Well Association under
sponsorship of the Environmental Monitoring Systems Laboratory, Las Vegas,
Nevada. This report covers a period from June 1985 to May 1989, and work
was completed as of June 1989.
Ill
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CONTENTS
Notice ii
Abstract iii
Figures viii
Tables xiii
Acknowledgements xv
Section
1. Introduction 1
Objectives and Scope 1
Purpose and importance of proper ground-water
monitoring well installation 7
Organization of the document 8
References 10
2. Factors influencing ground-water monitoring well
design and installation 11
Geologic and hydrogeologic conditions 11
Hydrogeologic regions of the United States 11
Site-specific geologic and hydrogeologic conditions 33
Facility characteristics ....'. 35
Type of facility 35
Waste characteristics 39
Other anthropogenic influences 45
Equipment that the well must accommodate 46
Borehole geophysical tools and downhole cameras 46
Water-level measuring devices 51
Ground-water sampling devices 51
Aquifer testing procedures 53
References 54
3. Monitoring well planning considerations 59
Recordkeeping 59
Decontamination 62
Decontamination area 64
Types of equipment 66
Frequency of equipment decontamination 66
Cleaning solutions and/or wash water 66
Containment of residual contaminants and cleaning
solutions and/or wash water 68
Effectiveness of decontamination procedures 68
Personnel decontamination 68
References 70
4. Description and selection of drilling methods 71
Introduction 71
Drilling methods for monitoring well installation 71
Hand augers 71
iv
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Driven wells 73
Jet percussion 75
Solid-flight augers 77
Hollow-stem augers 80
Direct mud rotary 35
Air rotary drilling gg
Air rotary with casing driver 93
Dual-wall reverse-circulation 95
Cable tool drilling ... .91
Other drilling methods 102
Drilling fluids ... 102
Influence of drilling fluids on monitoring well
construction 102
Drilling fluid characteristics 104
Mud-based applications 108
Air-based applications 109
Soil sampling and rock coring methods 110
Split-spoon samplers HI
Thin-wall samplers 113
Specialized soil samplers 114
Core barrels 120
Selection of drilling methods for monitoring well
installation 123
Matrix purpose 123
Matrix description and development 123
How to use the matrices 127
How to interpret a matrix number 127
Criteria for evaluating drilling'methods 128
Versatility of the drilling equipment and technology
with respect to the hydrogeologic
conditions at the site 128
Reliability of formation (soil/rock/water) samples
collected during drilling 128
Relative drilling costs 135
Availability of equipment 136
Relative time required for well installation
and development 136
Ability of drilling technology to preserve
natural conditions 137
Ability of the specified drilling technology to permit
the installation of the proposed casing diameter
at the design depth 138
Ease of well completion and development 139
Drilling Specifications and contracts 139
References "... . 143
5. Design components of monitoring wells 145
Introduction 145
Well casing 145
Purpose of the casing 145
General casing material characteristics 145
Types of casing materials 149
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Coupling procedures for joining casing 167
Well casing diameter 170
Casing cleaning requirements 170
Casing cost 171
Monitoring well intakes 173
Naturally-developed wells 174
Artificially filter-packed wells 174
Well intake design 185
Annular Seals 192
Purpose of the annular seal 192
Materials used for annular seals 194
Methods for evaluating annular seal integrity 200
Surface completion and protective measures 200
Surface seals 200
Above-ground completions 201
Flush-to-ground surface completions 201
References 203
6. Completion of monitoring wells ... 208
Introduction 208
Well completion techniques . . 208
Well intake installation . . 208
Filter pack installation . . 210
Annular seal installation. . . 211
Types of well completions 218
Single riser/limited-interval wells 218
Single riser/flow-through wells. 219
Nested wells 220
Multiple-level monitoring wells. 222
General suggestions for well completions 224
References 227
7. Monitoring well development. ... : . . . 228
Introduction/philosophy. ... 228
Factors affecting monitoring well development 229
Type of geologic material. . 229
Design and completion of the well 230
Type of drilling technology. 231
Well development 232
Methods of well development. . 235
Bailing 237
Surge block 238
Pumping/overpumping/backwashing 242
References 244
8. Monitoring well network management considerations 246
Well documentation 246
Well maintenance and rehabilitation 249
Documenting monitoring well performance 249
Factors contributing to well maintenance needs 250
Downhole maintenance 253
Exterior well maintenance. . 254
Comparative costs of maintenance 255
Well abandonment 255
Introduction 255
VI
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Well abandonment considerations 256
Well abandonment procedures 257
Grouting procedures for plugging 260
Clean-up, documentation and notification 261
References 262
Master References 264
Appendices
A. Drilling and constructing monitoring wells with hollow-stem
augers 278
B. Matrices for selecting appropriate drilling equipment 324
C. Abandonment of test holes, partially completed wells and
completed wells (American Water Works Association, 1984) . . . 367
Glossary 371
VII
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FIGURES
Number Page
1 Ground-water regions of the United States 12
2a Location of the Western Mountain Ranges region 14
2b Topographic and geologic features in the southern
Rocky Mountains part of the Western Mountain
Ranges region 14
3a Location of the Alluvial Basins region 15
3b Common ground-water flow systems in the Alluvial
Basins region 15
4a Location of the Columbia Lava Plateau region 17
4b Topographic and geologic features of the Columbia Lava
Plateau region • 17
5a Location of the Colorado Plateau and Wyoming Basin region 18
5b Topographic and geologic features of the Colorado Plateau
and Wyoming Basin region 13
6a Location of the High Plains region 20
6b Topographic and geologic features of the High Plains region .... 20
7a Location of the Nonglaciated Central region 22
7b Topographic and geologic features of the Nonglaciated
Central region 22
7c Topographic and geologic features along the western
boundary of the Nonglaciated Central region 22
8a Location of the Glaciated Central region 23
8b Topographic and geologic features of the Glaciated
Central region 23
VIM
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9a Location of the Piedmont and Blue Ridge region 25
9b Topographic and geologic features of the Piedmont and
Blue Ridge region 25
100 Location of the Northeast and Superior Uplands region 26
lOb Topographic and geologic features of the Northeast and
Superior Uplands region 26
lla Location of the Atlantic and Gulf Coastal Plain region 28
lib Topographic and geologic features of the Gulf Coastal Plain .... 28
12a Location of the Southeast Coastal Plain region 29
12b Topographic and geologic features of the Southeast Coastal
Plain 29
13a Location of the Alluvial Valleys ground-water region 31
13b Topographic and geologic features of a section of the
alluvial valley of the Mississippi River 31
14 Topographic and geologic features of an Hawaiian island 32
15 Topographic and geologic features of parts of Alaska 32
16 Migration of a high density, miscible contaminant
in the subsurface 41
17 Migration of a low density, soluble contaminant
in the subsurface 41
18 Migration of a low density, immiscible contaminant
in the subsurface 43
19 Migration of a dense, non-aqueous phase liquid (DNAPL)
in the subsurface 43
20 Sample boring log format 61
21 Format for an "as-built" monitoring well diagram 63
22 Typical layout showing decontamination areas at a
hazardous materials site 65
23 Diagram of a hand auger 72
24 Diagram of a wellpoint 74
25 Diagram of jet percussion drilling 76
IX
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26 Diagram of a solid-flight auger 73
27 Typical components of a hollow-stem auger 81
28 Diagram of a screened auger 34
29 Diagram of a direct rotary circulation system 36
30 Diagram of a roller cone bit 57
31 Diagram of a down-the-hole hammer 90
32 Range of applicability for various rotary drilling methods 91
33 Diagram of a drill-through casing driver 94
34 Diagram of dual-wall reverse-circulation rotary method 95
35 Diagram of a cable tool drilling system 98
36 Diagrams of two types of bailers 99
37 Practical drilling fluid densities 105
38 Viscosity-building characteristics of drilling clays 107
39 Schematic of the behavior of clay particles when
mixed into water .• 10g
40 Diagram of a split-spoon sampler 112
41 Diagram of a thin-wall sampler 115
42 Two types of special soil samplers 116
43 Internal sleeve wireline piston sampler 117
44 Modified wireline piston sampler 118
45 Clam-shell fitted auger head 119
46 Types of sample retainers 119
47 Diagram of a continuous sampling tube system 121
48 Diagram of two types of core barrels 122
49 Format for a matrix on drilling method selection 126
50 Forces exerted on a monitoring well casing and screen
during installation 147
51 Static compression results of Teflon® screen 153
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52 Types of joints typically used between casing lengths 168
53 Effect of casing wall thickness on casing inside and outside
diameter 171
54 Envelope of coarse-grained material created around a
naturally-developed well 175
55 Plot of grain size versus cumulative percentage of sample
retained on sieve 176
56 Determining effective size of formation materials 177
57 Determining uniformity coefficient of formation materials 178
58 Envelope of coarse-grained material emplaced around an
artificially filter-packed well 179
59 Artificial filter pack design criteria 183
60 Selecting well intake slot size based on filter pack grain
size 184
61 Types of well intakes 189
62 Cross-sections of continuous-wrap wire-wound screen 191
63 Potential pathways for fluid movement in the casing-borehole
annulus 193
64 Segregation of artificial filter pack materials caused by
gravity emplacement 211
65 Tremie-pipe emplacement of artificial filter pack materials . . . .212
66 Reverse-circulation emplacement of artificial filter pack
materials 213
67 Emplacement of artificial filter pack material by backwashing . . .214
68 Tremie-pipe emplacement of annular seal material (either
bentonite or neat cement slurry) . 216
69 Diagram of a single-riser/flow through well 219
70 Typical nested well designs 221
71 Field-fabricated PVC multilevel sampler 222
72 Multilevel capsule sampling device installation 223
73 Multiple zone inflatable packer sampling installation 225
xi
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74 Diagrams of typical bailers used in monitoring well development . .239
75 Diagram of a typical surge block 240
76 Diagram of a specialized monitoring well surge block 241
XII
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TABLES
Number
Page
1 Summary of federal programs and activities related to the
protection of ground-water quality 2
2 Federal ground-water monitoring provisions and objectives 3
3 Use and limitations of borehole geophysical tools 48
4 Descriptive information to be recorded for each
monitoring well 60
5 List of selected cleaning solutions used for equipment
decontamination 67
6 Applications and limitations of hand augers 73
7 Applications and limitations of driven wells 75
8 Applications and limitations of jet percussion drilling 77
9 Applications and limitations of solid-flight augers 79
10 Applications and limitations of hollow-stem augers 83
11 Applications and limitations of direct mud rotary drilling 88
12 Applications and limitations of air rotary drilling 92
13 Applications and limitations of air rotary with casing
driver drilling 95
14 Applications and limitations of dual-wall reverse-circulation
rotary drilling 97
15 Applications and limitations of cable tool drilling 101
16 Principal properties of water-based drilling fluids 104
17 Approximate Marsh Funnel viscosities required for drilling
in typical types of unconsolidated materials 106
18 Drilling fluid options when drilling with air 109
19 Characteristics of common formation-sampling methods Ill
xiii
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20 Standard penetration test correlation chart 113
21 Index to matrices 1 through 40 125
22 Suggested areas to be addressed in monitoring well bidding
specifications 140
23 Suggested items for unit cost in contractor pricing schedule. . . .142
24 Trade names, manufacturers and countries of origin for various
fluoropolymer materials 151
25 Typical physical properties of various fluoropolymer materials. . .151
26 Hydraulic collapse and burst pressure and unit
weight of stainless steel well casing 155
27 Typical physical properties of thermoplastic well casing
materials at 73.4°F 159
28 Hydraulic collapse pressure and unit weight of PVC well casing. . .160
29 Hydraulic collapse pressure and unit weight of ABS well casing. . .161
30 Representative classes of additives in rigid PVC materials
used for pipe or well casing 164
31 Chemical parameters covered by NSF Standard 14 164
32 Volume of water in casing or borehole 172
33 Correlation chart of screen openings and sieve sizes 187
34 Typical slotted casing slot widths 190
35 Intake area (square inches per lineal foot of screen) for
continuous wire-wound well intake 192
36 Summary of development methods for monitoring wells 233
37 Comprehensive monitoring well documentation 247
38 Additional monitoring well documentation 247
39 As-built construction diagram information 248
40 Field boring log information 248
41 Regional well maintenance problems 251
42 Chemicals used for well maintenance 254
43 Well abandonment data 261
XJV
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ACKNOWLEDGEMENTS
This document presents a discussion of the design and installation of
ground-water monitoring wells without specific regulatory recommendations.
The information contained within the document is the product of many
experiences, both published and unpublished to date. Assisting in the
direction of the project and in the review of various stages of the
document was an able and knowledgeable advisory committee. Although each
of the individuals contributed positively, this document is a product of
the authors and may not be entirely endorsed by each of the committee
members. To the following named persons, grateful acknowledgement of their
contribution is made:
Roger Anzzolin, U.S. EPA, Office of Drinking Water
George Dixon, formerly with U.S. EPA, Office of Solid Waste
Tyler Gass, Bias land & Bouck Engineers, P.C.
James Gibb, Geraghty & Miller, Inc.
Todd Giddings, Todd Giddings & Associates
Kenneth Jennings, U.S. EPA, Office of Waste Programs Enforcement
Thomas Johnson, Levine-Fricke, Inc.
Ken McGill, formerly with U.S. EPA, Region 3
John Mann, formerly with U.S. EPA, Region 4
Roy Murphy, U.S. Pollution Control, Inc.
Marion R. Scalf, U.S. EPA, Robert S. Kerr Environmental Research
Laboratory
Dick Young, U.S. EPA, Region 7
John Zannos, U.S. EPA, Region 1
The basic conceptual foundation for this report was supported in its
infant stages by Leslie G. McMillion, U.S. EPA, retired. A special note of
acknowledgement and gratitude is extended to him for his inspiration and
support in starting this document.
In addition to the committee members, many individuals assisted in
adding practical and regional perspectives to the document by participating
in regional group interviews on many aspects of monitoring well design and
installation. Thanks is also extended to the following individuals for the
donation of their time and invaluable input:
John Baker, Anderson Geotechnical Consultants, Inc., California
Dala Bowlin, Jim Winneck, Inc., Oklahoma
John Braden, Braden Pump and Well Service, Mississippi
Harry Brown, Brown Drilling Company, Inc., Michigan
Kathryn Davies, U.S. EPA, Region 3
Hank Davis, Mobile Drilling, California
XV
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Tim De La Grange, De La Grange and Sons, California
Lauren Evans, Arizonia Department of Health Services, Arizona
Jerry Frick, Walkerville Well Drilling and Supply Co., Michigan
Tyler Gass, Blasland & Bouck Engineers, P.C., New York
Jim Hendry, National Water Well Association, Ohio
Tom Holdrege, Anderson Geotechnical Consultants, Inc., California
Joseph Keely, Oregon Graduate Center, Oregon
Larry Krall, Layne Environmental Services, Arizona
Bruce Kroecker, Layne Environmental Services, Kansas
David Lang, U.S. EPA, Region 1
Bill Long, Jim Winneck, Inc., Oklahoma
Carl Mason, C.M. Consulting, Pennsylvania
Bill McKinnel, West Corp., Wyoming
Bruce Niermeyer, Interstate Soil Sampling, Inc., California
Harry Ridgell, Jr. Coast Water Well Service, Inc., Mississippi
Charles 0. Riggs, Central Mine Equipment Co., Missouri
Scott Sharp, Layne Environmental Services, Arizona
Bill Snyder, William Stothoff Company, Inc., New Jersey
Steve Story, Layne Environmental Services, California
Fred Strauss, Layne Environmental Services, Calif iornia
Dave Sullivan, D.P. Sullivan & Daughters Drilling Firm, Inc.,
Massachusetts
Bud Thorton, Associated Well Drillers. Inc., Idaho
Taylor Virdell, Virdell Drilling, Texas
Dick Willey, U.S. EPA, Region 1
Douglas Yeskis, U.S. EPA, Region 5
John Zannos, U.S. EPA, Region 1
In addition, gratitude is expressed to those individuals who
participated in reviewing the document:
Joe Abe, U.S. EPA, Office of Solid Waste
Doug Bedinger, Environmental Research Center, UNLV
Regina Bochicchio, Desert Research Institute, UNLV
Jane Denne, U.S. EPA, EMSL - Las Ve«as
Joe DLugosz, U.S. EPA, EMSL - Las Vegas
Phil Durgin, U.S. EPA, EMSL - Las '.'*«»
Larry Eccles, U.S. EPA, EMSL - Las V»gas
Steven Gardner, U.S. EPA, EMSL - Las <>gas
Jack Keeley, U.S. EPA, retired
Eric Koglin, U.S. EPA, EMSL - Las Vegas
Lowell Leach, U.S. EPA, Robert S. Kerr Environmental Research
Laboratory, Ada
Wayne Pettyjohn, Oklahoma State University
Mario Salazar, U.S. EPA, Office of Drinking Water
Kenneth Scarbrough, U.S. EPA, EMSL - Las Vegas
Peter Siebach, U.S. EPA, Office of Solid Waste
Kendrick Taylor, Desert Research Institute, University of Nevada
System, Reno
John Worlund, U.S. EPA, EMSL - Las
Kendrick Taylor also provided information contained in the borehole
geophysical tool section of the document.
XVI
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SECTION 1
INTRODUCTION
OBJECTIVES AND SCOPE
The Handbook of Suggested Practices for the Design and Installation of
Ground-Water Monitoring Veils has been prepared as an aid to owners and
operators of facilities as well as others concerned with proper
installation of ground-water monitoring wells. This document is also
designed to assist state and federal authorities in evaluating all aspects
of monitoring well design and installation in varying hydrogeologic
settings. Information contained within this publication does not address
specific regulatory requirements, which must be followed, but rather
presents state-of-the-art technology that can be used in differing
situations.
This document is intended to be both informative and descriptive in
nature. The objectives are to provide a concise description of the
components of monitoring well design and installation and to detail t*,u
applicability of various drilling techniques in diverse hydrogeologic
regimes. The information is presented in both text and matrix form.
Through a relative numerical rating scheme, the matrix guides the user
toward appropriate drilling technology for particular monitoring
situations.
Impetus for the development of the Handbook of Suggested Prf 'tices for
the Design and Installation of Ground-Water Monitoring Wells was provided
by the passage of a series of federal laws which addressed the need to
protect ground-water quality. Table 1 lists the laws enacted by Congress
and summarizes the applicable ground-water activities associated with each
law. Of the sixteen statutes listed in Table 1, ten statutes have
reguiatory programs which establish ground-water monitoring requirements
for specific sources of contamination. Table 2 summarizes the objectives
and monitoring provisions of the federal acts. While the principal
objectives of the laws are to obtain background water-quality data and to
evaluate whether or not ground water is being contaminated, the monitoring
provisions contained within the laws vary significantly. Acts may mandate
that ground-water monitoring regulations be adopted, or they may address
the need for the establishment of guidelines to protect ground water.
Further, some statutes specify the adoption of rules that must be
implemented uniformly throughout the United States, while others authorize
adoption of minimum standards that may be made more stringent by state or
local regulations.
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TABLE 1. SUMMARY OF FEDERAL PROGRAMS AND ACTIVITIES RELATED TO THE PROTECTION OF GROUND-WATER QUALITY
(AFTER OFFICE OF TECHNOLOGY ASSESSMENT, 1984)
Statutes
Atomic Energy Act
Clean Watef Act
Coastal Zone Management Act
Comprehensive Environmental
Response, Compensation and
Liability Act
Federal Insecticide. Fungicide
and Rodenliade Act .
Federal Land Policy and
Management Act (and
associated mining laws)
Hazardous Liquid Pipeline
Safety Act ...
Hazardous Materials
Transportation Act
National Environmental
Policy Act
Reclamation Act
Resource Conservation and
Recovery Act
Sale Drinking Water Act
Surface Mining Control and
Reclamation Act
Toxic Substances Control Aci
Uranium Mill Tailings
Radiation Control Act
Water Research and
Development Acl
Investigations/detection
Ground-water
Ambient monitoring Water
Inventories ground-water related supply
of sources8 monitoring to sources* monitoring
X
XXX
X X
X
X
X
X
X X
x xx
X
X
X
Correction Prevention
Federally
funded Regulatory Regulate Standards lor
remedial requirements chemical new/existing
actions for sources' production sources3
XX X
X X
X
X X
X
X
X
X
X X
X
X X
X X
XX X
Aquiler
protection Standards Other6
X
X X
X
X
X
X X
X
•Programs dm achwiies under lha heading relate directly 10 specific sources ol ground-wafer conlanwwtwn
bThis category includes dciuilm inch as research and development ana gidtiis to ate Stales lo develop ground walei related programs
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TABLE 2. FEDERAL GROUND-WATER MONITORING PROVISIONS AND OBJECTIVES (AFTER OFFICE OF TECHNOLOGY
ASSESSMENT. 1984)
Statutory authority
Atomic Energy Act
Clean Waiei Act
-Sections 201 and 405
-Section 208
Coastal Zone Management Act
Comprehensive Environmental
Response. Compensation
and Liability Act
f-uder.il Insecticide Fungicide
and Hodenleide Acl-
Seciion3
Federal Land Policy and
Management Act (and
associated mining laws)
Hazardous Liquid Pipeline
Salely Aci
Hazardous Materials
Transportation Act
National Environmental
Policy Act
Reclamation Act
Resource Conservation and
Recovery Act
Monitoring provisions'
Ground water monitoring is specified in Federal regulations lor low level
radioactive waste disposal sites The facility license must specify the monitoring
requirements lor the source The monitoring program must include
—Pie-operalional monitoring program conducted over a 12-month penod
Parameters not specified
--Monitoring during construction and operation to provide early warning ol
releases ol radionuchdes from the sile Parameters and sampling frequencies
not specified
- Post-operational monitoring program to provide early warning ol releases ill
radKNiuclides from the site Parameters and sampling frequencies not
specified System design is based on operating history closure and
stabilization ol the site
Ground-water monitoring related to ihe development ol geologic repositories will
be conducted Measurements mil include Ihe rate and location ol water inflow
into subsurface areas and changes in ground-water conditions
Ground-walei monitoring may be conducted by DOE as necessary as pan ui
remedial action programs at storage and disposal facilities lor radioactive
substances
Ground water monitoring requirement! are established on a case-by-ran II.IMS
lor ihe land application ol waslewaler and sludge horn sewage treatment
plants
No enpncil rcquiiements are established, however giound-waier monitor my
studies are being conducted by SCS under Ihe Rural Clean Water Program lo
evaluate the impacts ol agricultural practices and lo design and determine ihe
ellectiveness ol Best Management Practices
The statute does not authorize development ol regulations lor sources Tims jny
ground-water monitoring conducted would be Ihu result ol requirements
established by a Slate plan (e g. monitoring with respect lo salt -water
intrusion) aulhoriied and funded by CZMA
Ground-water monitormg may be conducted by EPA (o> a Slate) as neceis.iry tu
respond lo releases ol any haiardous substance contaminant or pollutant ias
defined by CERCLA)
No monitoring requirements established lor pesticide users Howevet muiiiiiiiniij
may be conducted by EPA n instances where certain pesticides aic
contaminating ground water "
Ground-water monitoring is specified in Federal regulations lor geotheini.il
recovery operations on Federal lands lor a period ol al least one year pnui lu
production Parameters and monitoring frequency are not specified
Explicit ground-water monitoring requirements lor mineral operations on Federal
lands are not established in Federal regulations Monitoring may be required
(as a permil condition) by BLM
Although the statute authorizes development ol regulations lor certain pipelines
tor public safety purposes Ihe regulatory requr.emenls locus on design and
operation and do not provide lor ground-water monitoring
Although Ihe statute authorizes development ol regulations lor transportation lur
public safely purposes Ihe regulatory requirements locus on design and
operation and do not provide loi ground-water monitoring
The slatuie does noi authorize development ol regulations lor sources
No implicit requirements established however monitoring may be conducted ,is
necessary as part ol water supply development projects
Ground-water monitoring is specified in Federal regulations lor all hazardous
waste land disposal facilities (e g landfills, surface impoundments waste piles
and land treatment units!
Monitoring objectives
To obtain background water quality data jiul lu
evaluate whether ground wdlei is being
contaminated
To confirm geoiechnical and design pcirdmeicrs und lu
ensure that the design ol Ihe geologic repository
accommodates actual field conditions
To characterize a contamination problem diirj to select
and evaluate Ihe effectiveness ol corrective
measures
To evaluate whelhui ground water is being
contaminated
To characterize a contamination problem unU 10 select
and evaluate Ihe effectiveness ol corrective
measures
To characterize a contamination problem (e g lo
assess the impacts ol the situation lo identify or
verily Ihe source(s) and lo select and evaluate Ihe
eilecitveness ol collective measures)
To characterize a contamination problem
Tu obtain background walet-quality delta
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TABLE 2. FEDERAL GROUND-WATER MONITORING PROVISIONS AND OBJECTIVES (AFTER OFFICE OF TECHNOLOGY
ASSESSMENT. 19M)-CONTINUED
Statutory authority
Monitoring provisions*
Monitoring objectives
Resource Conservation and
Recovery Act (confO)
-Subtitle C
Facilities in emslenceon the effective dale ol statutory or regulatory dmundmenls
under ine eel thai mould make the facility subiecl lo the requirements to haw a
RCRA permit musi meet Interim Sutut monitoring requirements unit a linal pui
mil is issued These requirement specify Ihe msiallation ol at least one upgia-
dienl well end three downgradieni wells Samples must be taken quarterly duung
Ihe first year end analysed for the National Drinking Waler Regulations water
quality parameters ichlotirJa iron manganese, phenols, sodium and suilatel and
indicator parameters (pH. specific conductance. TOC and TOX) In subsequent
years each well is sampled end analyzed annually for the si« background water
quality parameters and sem-annually tor Ihe four indicator parameters
II contaminant leakage has been delected during detection monitoring inu owner or
operator ol an interim status facdily must undertake assessment monitoring rue
owner or operator must determine Ihe vertical and horizontal concentration i»o-
hles ol all Ihe hazardous waste constituents in Ihe plume(s) escaping liom waste
management units
Ground-water monitoring requirements can be waived by an ownei/operalur it d
written determination indicating thai there is low poiential lor waste migidhon via
Ihe uppermost aquifer lo water supply wells or surface weter is made and certified
by a qualified geologist or engineer Ground-water monitoring requirements lor a
surface impoundment may be waived il (1) it a used lo neutralize wastes which
are hazardous solely because they exhibit Ihe corroswily characteristic under
Section Ml 22 or are toted m Subpart O ol Part 261 and (2) contains no other
hazardous wasle The owner or operator must demonstrate that there is no poten-
tial lor migration ol the hazardous wastes Irom Ihe impoundment The demonslra
lion must be in writing and must be certified by a qualified piofessional
I lie niiiiiiionng lequnemenis lor a lulir permitted facility are comprised ol .1 thici-
To obtain background water-quality data and evaluate
whether ground water is being contaminated
To obtain background water quality data or evaluate
whether ground water is being contaminated
(detection monitoring) to determine whether
groundwaier quality standards are being mel
(compliance monitoring), and lo evaluate Ine
effectiveness ol corrective action measures
11 hi r«*M> AAMiilfi'ii; lniptomaiilvO *rifii « pviinil ib i»&ui •! .iinl Unit, ib
no indication ul leakage from a facility Parameters aie speciln.-d in Ihe
permit Samples must be taken and analyzed at leasl semi-annu
-------
TABLE 2. FEDERAL GROUND-WATER MONITORING PROVISIONS AND OBJECTIVES (AFTER OFFICE OF TECHNOLOGY
ASSESSMENT. 1984)—CONTINUED
Statutory authority
Resource Conservation and
Recovery Act (confd)
- Subtitle C(conld)
Monitoring provisions*
containing tree liquids, is designed and operated to eiclude liquids
precipitation an." -.iher nm-on and run oil. tias both inner and outer
containment layui... has a leak detection system built into each
containment layer, owner of operator mil provide continuing operatiun
and maintenance ol leak detection systems, and to a reasonable degree ol
certainly will not allow hazardous constituents to migrate beyond the
outer containment layer prior 10 end ol post-closure care period
Monitoring objectives
-Subtitle O
The 1964 Hazardous and Solid Waste Amendments requite EPA to revise CIHUIM
tor sobd waste management facilities thai may receive household haiardous
waste or small quantity generator hazardous waste At a minimum ine
revisions must require grounoVwater mentoring establish location cnlena ami
provide lur ra>roclwe action
On August 30.1988 EPA published proposed rules requiring ground-water
moratonng at all new and eirstmg municipal soM waste landfills
cn
-Subtitle I
Sale Dunking Waier Act
—Pan C—Underground
Injection Control Program
Surface Mining Control and
Reclamation Act
Giound water monitoring a one ol the r
owners and operators ol petro4eun ur
dergn
s available Ion
id storage tanks Itisarsuan
Tone Substance Control Act
-Section 6
option al eirswig haiardous substance underground storage tanks until
December 22. 1998 Al the end ol this period owners and operators must upgrade
or replace this release detection method with secondary containment and inieisn-
lial monitoring unless a variance is obtained
Ground water monitoring requirements may be specified in a facility permit lur
injection wells used lor in-silu or solution mining ol minerals (Class III wulls)
where inieclton is into a lormalion containing less than 10 ODD mg/l TOS
Parameter and monitoring frequency not specified except in areas subject to
subsidence or collapse where moniiorrng a required on a quarterly basis
Ground-water monitoring may also be specihed n e permit lor wells which inject
beneath the deepesl underground source ol drinking water (Class I wells)
- iiamelers and monitoring frequency not speeded in Federal regulations
uiound-walei monitoring is specihed in Federal regulations lor surface and
underground coal mtrung operations to delermm" the impacts on the
hydrologic balance of the mining and adjacent at . A ground-water
monitoring plan must be developed lor each mutiny .liberation (including
reclamation) Al a minimum, parameters must include total dissolved solids or
specific conductance pH, total iron, and total manganese Samples must be
taken and analyzed on a quarterly basis
Monitoring of a particular wafer-ocarina stratum mar be waived by the leyulaiuiy
authority il il can be demonstrated that il is not a stratum which serves as an
aquifer that significantly ensures Itie hydrotogic balance of the cumulative
impact area
Ground-water monitoring specified in Federal regulations requires momioiing
prior lo commencement ol disposal operations lor PCBs Only three wells arc
required il underlyng earth materials are homogenous, impermeable and
uniformly sloping in one daemon Parameters include (al a minimum) PCBs
pH specrtac conductance and chlomaied oiganici Monitoring frequency nui
specified
No requirements are established for active life or allei closure
To evaluate whelhei ground waler is being
contaminated
lo ubldin background watei-u.ii idly ddld anil evdluale
whether ground watei is being contaminated
Tu obtain backgiunnd water-quality data
-------
TABLE 2. FEDERAL GROUND-WATER MONITORING PROVISIONS AND OBJECTIVES (AFTER OFFICE OF TECHNOLOGY
ASSESSMENT. 1984>-CONTINUED
^* Statutory authority Monitoring provisions* Monitoring ob|eclives
Uranium Mill Tailings Federal regulatory requirements lor active mill tailings sites are lor the most part To obtain background water-quality dala evaluate
Radiation Control Act the same as those established under Subtitle C ol RCRA whether ground water is being contaminated.
determine whether ground-water quality standards
are being met. and evaluate the ellectiveness ol
corrective action measures
Ground-water monitorng lor inactive sites may be conducted il necessary to To obtain background water-quality data and to
determine the nature ol the problem and lor the selection of an appropriate characterize a contamination problem
remedial action
Water Research and The statute does not aulhonie the development ol regulations lor soun.es
Development Act Ground-water monitoring miy be conducted as pan ol protects lunded by
the act
MM moMixng prOMiam pmanlad m DM nun we oann mow lOKitea by raauUMm tot ••alng ind now tourca a lor around •«« moniianng inn may M conducno n pan ol an invnnguoiy iludy or inn«i.al
•Chan program
°Pnncdt lunulKluwt"»» >» rrjquiiM t>, EPA lu tulxn.1 ground MM momlonng dau a pan al in* icgiutmon loquircmenli lur < pi^i^.ue [i.ouuci to euli.au ita aofcnui loi • peucKk lu conun.«utL y,o..rvl -MI
-------
With such diverse statutes mandating ground-water monitoring
requirements, it is not surprising that the regulations promulgated under
the authority of the statutes also vary in scope and specificity. In
general, most regulations further define the objectives of the statute and
clarify the performance standards to achieve the stated objectives.
More specific ground-water monitoring recommendations can be found in
the numerous guidance documents and directives issued by agencies
responsible for implementation of the regulations. Examples of guidance
documents include the Office of Waste Programs Enforcement Technical
Enforcement Guidance Document (TEGD)(United States Environmental Protection
Agency, 1986), the Office of Solid Waste Documents SW-846 (Wehran
Engineering Corporation, 1977) and SW-6L1 (United States Environmental
Protection Agency, 1987j. The purpose of this "Handbook" is to be a
general (non program-specific) reference to provide the user with a
practical decision-making guide for designing and installing monitoring
wells, and it will not supercede program-specific guidance.
PURPOSE AND IMPORTANCE OF PROPER GROUND-WATER MONITORING WELL INSTALLATION
The primary objective of a monitoring well is to provide an access
point for measuring ground-water levels and to permit the procurement of
ground-water samples that accurately represent in-situ ground-water
conditions at the specific point of sampling. To achieve this objective,
it is necessary to fulfill the following criteria:
1) construct the well with minimum disturbance to the formation;
2) construct the well of materials that are compatible with
the anticipated geochemical and chemical environment;
3) properly complete the well in the desired zone;
4) adequately seal the well with materials that will not
interfere with the collection of representative
water-quality samples; and
5) sufficiently develop the well to remove any additives
associated with drilling and provide unobstructed flow
through the well.
In addition to appropriate construction details, the monitoring well
must be designed in concert with the overall goals of the monitoring
program. Key factors that must be considered include:
1) intended purpose of the well;
2) placement of the well to achieve accurate water levels
and/or representative water-quality samples;
3) adequate well diameter to accommodate appropriate tools
for well development, aquifer testing equipment and
water-quality sampling devices; and
4) surface protection to assure no alteration of the
structure or impairment of the data collected from the well.
If proper monitoring well design and construction techniques are not
employed during monitoring well installation, the data collected from the
well may not be reliable. For example, Sosebee et al. (1983) determined
that the solvent used to weld lengths of polyvinyl chloride (PVC) casing
together can leach significant amounts of tetrahydrofuran, methylethyl
7
-------
ketone, methylbutyl ketone and other synthetic organic chemicals into water
that comes in contact with the solvent-welded casing joint. This could
result in false determinations of the presence of certain chemical
constituents in water samples taken from PVC wells in which the joints were
solvent welded.
Monitoring well installation procedures can also have a significant
impact on the integrity of ground-water samples. For example, Brobst and
Buszka (1986) found that organic drilling fluids and bentonite drilling
muds used in mud rotary drilling can have an effect on the chemical oxygen
demand of ground water adjacent to the wellbore in a rotary-drilled well.
This, in turn, can affect the quality of a water sample taken from such a
well, resulting in the acquisition of non-representative ground-water
samples.
Vertical seepage of leachate along well casing can also produce
non-representative samples. Monitoring wells ace frequently sealed with
neat cement grout, bentonite or a cement-bentonite mixture. The correct
choice of a grout and the proper emplacement method to ensure a seal are
critical to assure ground-water sample integrity and prevent cross
contamination of aquifers. Wehrmann (1983) noted that while a neat cement
grout is often recommended, shrinkage and cracking of the cement upon
curing can create an improper seal. Kurt and Johnson (1982) have presented
the case that the smooth surface of thermoplastic casing provides a
potential path for vertical leakage between the casing and the grout
material. The implications of the impact of adhesion, including chemical
bonding, versus swell pressure have not been documented in the literature.
However, it is known that vertical leakage between the casing and the grout
material may occur because of swelling and •shrinkage during the curing of
the grout.
This brief synopsis of potential problems associated with improper
monitoring well design and installation illustrates that there are a number
of design elements that must be addressed in proper monitoring well
construction. This manual attempts to discuss the basic elements that lead
to the construction of a viable monitoring well. Where appropriate,
potential problems or pitfalls are discussed.
ORGANIZATION OF THE DOCUMENT
This document contains 8 major sections and 3 supporting appendices.
A complete list of references can be found immediately following Section 8.
Section 1, "Introduction," provides an explanation of the impetus for this
"Handbook" and includes a brief discussion of the regulatory framework for
ground-water monitoring regulations. Section 2, "Factors Influencing
Ground-Water Monitoring Well Design and Installation," discusses the
importance of sizing a monitoring well in accordance with the intended
purpose of the well. Section 2 also describes the importance of monitoring
well location and the influence of hydrogeology, contaminant
characteristics and anthropogenic influences on monitoring well design.
Section 3, "Monitoring Well Planning Considerations," explains the
importance of keeping detailed records during the entire existence of the
monitoring well from installation through sampling to abandonment. A
discussion of the necessity of decontamination procedures for drilling
8
-------
equipment used during monitoring well installation is also included in this
section. Section 4, "Description and Selection of Drilling Methods,"
includes a brief discussion of drilling and sampling methods used during
monitoring well construction and the advantages and disadvantages of each
technique. The focus of this section is a set of matrices (included in
Appendix B) that indicate favorable drilling techniques for monitoring
wells with certain specifications drilled in selected hydrogeologic
settings. Section 5, "Design Components of Monitoring Wells," describes
the materials and installation techniques for casing, well intakes and
filter packs. A discussion of grout mixtures and emplacement techniques is
also presented. Section 6, "Completion of Monitoring Wells," provides a
description of well completion techniques and types of well completions
designed to maximize collection of representative ground-water samples.
Section 7, "Monitoring Well Development," discusses the importance of
proper development and describes techniques used in monitoring wells.
Section 8, "Monitoring Well Network Management Considerations," discusses
the importance of maintenance and proper well abandonment coupled with the
necessity for recordkeeping.
Also included within the document are a glossary and three supporting
Appendices. The glossary contains pertinent ground-water monitoring terms.
Appendix A contains a detailed discussion of installing monitoring wells
with a hollow-stem auger. Appendix B includes a set of matrices designed
to assist in the selection of drilling technologies. Appendix C is a
reproduction of a standard for well abandonment.
-------
REFERENCES
Brobst, R.D. and P.M. Buszka, 1986. The effect of three drilling fluids on
ground-water sample chemistry; Ground Water Monitoring Review, vol. 6, no.
1, pp. 62-70.
Kurt, Carl E. and R.C. Johnson, Jr., 1982. Permeability of grout seals
surrounding thermoplastic well casing; Ground Water, vol. 20, no. 4, pp.
415-419.
Office of Technology Assessment, 1984. Protecting the nation's ground
water from contamination, vols. I and II; United States Congress,
Washington, D.C., 503 pp.
Sosebee, J.B., P.C. Geiszler, D.L. Winegardner and C.R. Fisher, 1983.
Contamination of ground-water samples with PVC adhesives and PVC primer
from monitor wells; Proceedings of the ASTM Second Symposium on Hazardous
and Industrial Solid Waste Testing, ASTM STP 805, R.A. Conway and W.P.
Gulledge, eds., American Society for Testing and Materials, Philadelphia,
Pennsylvania, pp. 38-50.
United States Environmental Protection Agency, 1986. RCRA ground-water
monitoring technical enforcement guidance document; Office of Waste
Programs Enforcement, Office of Solid Waste and Emergency Response,
OSWER-9950.1, United States Environmental Protection Agency, 317 pp.
United States Environmental Protection Agency, 1987. Test methods for
evaluating solid waste, physical/chemical methods (SW-846); Office of Solid
Waste and Emergency Response, Government Printing Office, Washington, D.C.,
519 pp.
•'Wehran Engineering Corporation, 1977. Procedures manual for ground water
monitoring at solid waste disposal facilities (SW-611); National Technical
Information Service, Springfield, Virginia, 269 pp.
Wehrmann, H. Allen, 1983. Monitoring well design and construction; Ground
Water Age, vol. 17, no. 8, pp. 35-38.
10
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SECTION 2
FACTORS INFLUENCING GROUND-WATER MONITORING WELL
DESIGN AND INSTALLATION
GEOLOGIC AND HYDROGEOLOGIC CONDITIONS
The geologic and hydrogeologic conditions at a site affect the
occurrence and movement of ground water and contaminant transport in the
subsurface. Concomitantly, these two factors significantly influence the
design and construction techniques used to install a monitoring well. The
following discussion of the geologic and hydrogeologic conditions pertinent
to the design and construction of monitoring wells is divided into two
parts. The first part addresses regional geologic and hydrogeologic
conditions that impact ground-water occurrence, and hence the types of
water-bearing materials that are likely to be monitored. Non-exploitable
aquifers in some cases must also be monitored. The second part of this
discussion focuses more on site-specific geologic and hydrogeologic
conditions that can affect the design of a monitoring well and selection of
an appropriate method for drilling and constructing the well.
Hydrogeologic Regions of the United States
Heath (1984) has developed a classification system that divides the
United States into ground-water regions based on ground-water occurrence
and availability. Because the presence of ground water in the subsurface
is closely related to geologic conditions, areas with similar rock
composition and structure tend to fora similar ground-water regions. The
classification system developed by Heath (1984) uses the type and
interrelationship of the aquifers in an area as the major division for
regional designation. Additional factors including: 1) primary versus
secondary porosity, Z) mineral composition of the aquifer, 3) hydraulic
characteristics of the aquifer, and 4) the effects of recharge and/or
discharge areas were used to further define each region. Figure 1
illustrates the division of the United States into 15 ground-water regions.
For the purposes of this discussion, however, Puerto Rico and the Virgin
Islands will be excluded. Because the primary focus of this discussion is
limited to the hydrogeologic conditions pertinent to monitoring well
construction, the reader is referred to Heath (1984) for additional
information on each ground-water region.
Western Mountain Ranges--
The Western Mountain Ranges are comprised of tall, massive mountains
separated by narrow, steep-sided valleys. In many areas, the mountains
have been subjected to alpine glaciation. Major lowland areas occur
11
-------
2. Alluvial Basin
7. Glaciated
Central
region
. Non9l.ci.ted
Central region
6. Nonglaciated
Central
region
14. ALASKA
o
5 \
C y*=&v
7/ -
'. >^ x
• «.. -/^
w~y
13.
HAWAII
'' "i.
D
«, — ^2
500 MILES
15. PUERTO RICO
AND
VIRGIN ISLANDS
800 KILOMETERS
Figure 1. Ground-water regions of the United States (Heath, 1984).
12
-------
between the mountain ranges in the southern part of this region. With
geologic origins related to major orogenic and tectonic events, most of the
mountain ranges are comprised of metamorphic and igneous rocks flanked by
consolidated sedimentary rocks of Paleozoic to Cenozoic age. Other
mountain ranges such as the Cascades and the San Juan mountains are
composed primarily of basaltic lava.
Bare bedrock exposures or a thin layer of weathered material cover the
slopes and summits of the mountains. The weathered layer tends to thicken
toward the base of the mountains and in the alluvial valleys. Figures 2a
and 2b illustrate the location and main geologic and hydrogeologic features
of this region. Despite high precipitation rates in the region, ground-
water resources are primarily limited to the storage capacity of the
fractures in the crystalline rocks that serve as an aquifer for this area.
The lowlands between the mountain ranges contain thick deposits of fine to
coarse-grained alluvium eroded from the adjacent mountains. These deposits
serve as aquifers that are capable of supplying moderate to large yields to
wells. The alluvial aquifers are often in direct hydraulic connection with
the underlying bedrock.
Alluvial Basins--
The Alluvial Basins region is comprised of thick alluvial deposits in
structural lows alternating with igneous and metamorphic mountain ranges.
This region covers two distinctive areas: 1) the Basin and Range area of
the southwest and 2) the Puget Sound/Willamette Valley Area of the Pacific
Northwest (Figure 3a).
The Basin and Range area consists of basins filled with thick deposits
of unconsolidated alluvial material eroded from the adjacent mountains and
deposited as coalescing alluvial fans. The alluvial materials in the fans
are typically coarsest near the mountains and become progressively finer
toward the center of the basin. These basins typically form closed-basin
systems where no surface or subsurface flow leaves the region. However,
water may move through the permeable deposits and actually move between
basins in a complex hydrogeologic relationship as illustrated in Figure 3b.
Most ground water in this region is obtained from the permeable sand -...
gravel deposits that are interbedded with finer-grained layers of saturated
silts and clays.
The alluvial deposits of the Puget Sound were deposited by
sediment-laden meltwater from successive glaciations. Thick layers of
permeable sands and gravels that are interbedded with discontinuous clay
layers provide the majority of trie water resources for this area. The
Willamette Valley consists of interbedded sands, silts and clays deposited
by the Williamette River and related streams. High precipitation rates in
the region provide the major source of recharge to these aquifers.
The mountains bordering these alluvial basins consist of igneous and
metamorphic rocks ranging from Precambrian to Tertiary in age. The limited
water resources in the mountains are derived from water stored in fractures
in the bedrock.
13
-------
(a)
(bi
Figure 2a. Location of the Western Mountain Ranges region (Heath, 1984).
Figure 2b. Topographic and geologic features in the southern Rocky Mountains part of the Western
Mountain Ranges region (Heath, 1984).
14
-------
(a)
X. / -.', Oryplavi x '
(b)
Figure 3a. Location of the Alluvial Basins region (Heath, 1984).
Figure 3b. Common ground-water flow systems in the Alluvial Basins region (Heath, 1984).
15
-------
Columbia Lava Plateau--
The Columbia Lava Plateau consists of a sequence of lava flows ranging
in total thickness from less than 150 feet adjacent to mountain ranges to
over 3,000 feet in south-central Washington and northern Idaho (Figure 4a).
The lava is composed of basalt that erupted from extensive fissures and
produced large sheet-like flows. The lava beds comprise the principal
water-bearing unit in the region.
Ground water in basalt flows through the permeable zones that occur at
the contacts between the lava flow layers (Figure 4b). The permeable zones
result from the cooling of the crust on the molten lava as it continues to
flow thus producing a zone of fragments and gas bubbles near the top of the
lava sheet. Cooling of the lava sheet itself also produces vertical
fracturing within the basalt. These interflow zones, created by the
cooling crust, form a complex series of relatively horizontal aquifers
separated by denser layers of basalt that are often hydraulically
interconnected by the intersecting fractures and faults within the lava
sheets.
The region can be divided into two separate hydrogeologic flow
regimes. The Columbia River Group, in the western part of this region,
consists of relatively thick basalt flows that have been offset by normal
faults. Primary water movement is through shallow interflow zones. The
aquifers are typically poorly hydraulically interconnected because the flow
is controlled by the faults which form barrier-controlled reservoirs.
The remainder of the region, occupied by the Snake River Plain,
consists of a series of thin lava flows with we11-developed interflow zones
and extensive fracturing. These interflow zones exhibit high hydraulic
conductivities and are hydraulically interconnected by cooling fractures.
The large differences in hydraulic conductivity between the interflow zones
and the denser basalt often result in significant differences in hydraulic
head between aquifers. Consequently, there is the potential for the
movement of water between aquifers through uncased or improperly cased
wells.
Recharge to the aquifer is from precipitation and infiltration from
streams that flow onto the plateau from adjacent mountains. Irrigation of
crops in this region provides additional recharge to the aquifer through
the interflow zones when the source of water is not from the aquifer.
Colorado Plateau and Wyoming Basin--
The Colorado Plateau and Wyoming Basin region is characterized by a
broad structural plateau underlain by horizontal to gently dipping beds of
consolidated sedimentary rock. In some areas, the structure of the plateau
has been modified by faulting and folding that resulted in basin and dome
features. The region contains small, isolated mountain ranges as well as
extinct volcanoes and lava fields (Figures 5a and Sb).
The sedimentary rocks in this region consist of Paleozoic to Cenozoic-
age sandstones, limestones and shales. Evaporitic rocks such as gypsum and
16
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(a)
Older moumemi
River canyon
EXPLANATION
Present coil rone
interflow rone
Sid ind ciiy
Cooling fractures
(b)
Figure 4a. Location of the Columbia Lava Plateau region (Heath, 1984).
Figure 4b. Topographic and geologic features of the Columbia Lava Plateau region (Heath, 19*4).
17
-------
(a)
Canyon
Extinct volcanoes
Ridge*
Dome
Fault scirp
Fault
EXPLANATION
Freshwater •/.>! Sandetone
J Salty water ~±= Umeitone
t-I-r-1 Shale
Metamorphic
rocki
(b)
Figure 5a. Location of the Colorado Plateau and Wyoming Basin region (Heath, 1984).
Rgure 5b. Topographic and geologic features of the Colorado Plateau and Wyoming Basin region
(Heath, 1984).
18
-------
halite also occur in some areas. The sandstones serve as the principal
source of ground water. Water within the sandstone is contained within
pore spaces and in fractures and bedding planes. Minor deposits of
unconsolidated alluvium occur in major river valleys and contribute small
to moderate yields of ground water.
Recharge to the aquifers is from precipitation and from infiltration
from streams that cross the outcrop areas. The gentle dip of the beds
causes unconfined conditions in outcrop areas and confined conditions
downdip. Aquifers in the region frequently contain mineralized water at
depth. Aquifers typically discharge to springs and seeps along canyon
walls.
High Plains—
The High Plains region represents a remnant of an alluvial plain
deposited by streams and rivers that flowed eastward from the Rocky
Mountains during the Tertiary period. Extensive erosion has subsequently
removed a large portion of the plain, including most areas adjacent to the
mountains.
The High Plains region is underlain primarily by the Ogallala
formation, a thick deposit of serai-consolidated alluvial materials
consisting of poorly-sorted sands, gravels, silts and clays (Figures 6a and
6b). The Ogallala formation is the major aquifer and is overlain locally
by younger alluvial material that is often saturated and forms a part of
the aquifer. In places where the Ogallala is absent, these younger
alluvial deposits, that are comprised of unconsolidated sand, gravel, silt
and clay, are used as the major aquifer. Extensive areas of surficial sand
dunes are also present. In some areas, older underlying consolidated
deposits that include the fine-grained sandstones of the Arikaree Group and
Brule formation are hydraulically connected to the Ogallala. Where these
deposits are absent, the Ogallala is underlain by other sedimentary rocks
that often contain unusable, highly mineralized water.
Recharge to the aquifer from precipitation varies across the area.
The presence of caliche, a low permeabilitv • alcium carbonate layer at or
near the land surface, limits the amount of precipitation that infiltrates
to the aquifer, thereby increasing the amount of water lost to evapotrans-
piration. In the sand dunes area, however, the permeability of the surface
materials allows increased recharge to the aquifer.
Extensive development of the aquifer for agricultural irrigation has
led to long-term declines in water levels Where ground-water withdrawal
rates have exceeded available recharge to the aquifer, ground-water mining
has occurred. The depletion of water from storage in the High Plains
region has resulted in a decrease in the saturated thickness of the aquifer
in areas of intensive irrigation.
Nonglaciated Central Region--
The Nonglaciated Central region covers a geologically complex area
extending from the Appalachian Mountains to the Rocky Mountains. Most of
19
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(a)
Pltttt Rivtr
EXPLANATION
t>^j Cl.v
L«*« j Grlv«l "••'•'•'"•I Sinfliton*
Figure 6a. Location of the High Plains region (Heath, 1984).
Figure 6b. Topographic and geologic features of the High Plains region (Heath, 1984).
20
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the region is underlain by consolidated sedimentary rocks, including
sandstones, shales, carbonates and conglomerates that range from Paleozoic
to Tertiary in age (Figures 7a, 7b and 7c). These rocks are typically
horizontal to gently dipping with the exception of a few areas, notably the
Valley and Ridge section; the Wichita and Arbuckle mountains in Oklahoma;
the Ouachita Mountains in Oklahoma and Arkansas; and the Triassic basins in
Virginia and North Carolina. The Triassic basins contain interbedded
shales, sandstones and conglomerates that have been faulted and invaded by
igneous rocks.
Chemical and mechanical weathering of the bedrock has formed a layer
of regolith that varies in thickness and composition depending on the
composition and structure of the underlying parent rock and the effects of
climate and topography. The sandstones and limestones constitute the major
aquifers in the area. Water occurs primarily in bedding planes and
fractures in the bedrock. Many of the limestones contain solution channels
that increase the permeability. Limestones in this region often form
extensive cave systems that directly affect patterns of ground-water flow.
Recharge in the region occurs primarily from precipitation in outcrop
areas and varies widely. Small to moderate well yields are common; higher
yields may be available in karstic areas. Well yields often depend on the
size and number of fractures intersected by the well, the recharge to the
area and the storage capacity and permeability of the bedrock and/or
regolith. In many parts of this region, mineralized water occurs at depths
greater than 300 feet.
Glaciated Central Region--
The geology of the Glaciated Central region is characterized by
relatively horizontal sedimentary rocks of Paleozoic to Tertiary age
consisting of sandstones, shales and carbonates. The bedrock is overlain
by varying thicknesses of poorly-sorted glacial till that is interbedded
with: 1) well-sorted sands and gravels deposited from meltwater streams,
2) clays and silts from glacial lake beds and 3) wind-blown silt or loess
deposits (Figures 8a and 8b).
In the eastern part of the region, the glacial deposits are typically
thin on the uplands and thicken locally in valleys. Toward the central and
western parts of the region, glacial deposits are thicker and often mask
the location of preglacial river valleys. These thick deposits in the
preglacial river valleys often contain permeable sands and gravels that
form major aquifers with significant well yields. Overlying till deposits
often act as confining layers for the underlying sand and gravel aquifers.
The underlying bedrock in this region also commonly serves as an
aquifer. Water occurs primarily along bedding planes and in fractures.
Frequently the glacial deposits and the bedrock are hydraulically
interconnected. The glacial deposits often provide recharge to the bedrock
aquifers and serve as a source of water for shallow wells. Movement of
poor-quality water from the bedrock into the glacial deposits may cause
local ground-water quality problems. Recharge to the glacial deposits is
provided by precipitation and by infiltration from streams. Recharge rates
21
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XPLANAT1ON
i Frwhw«»f ...... Sandtton.
I I S*ltv*«w L>>d Shalt
(c)
Figure 7a. Location of the Nonglaciated Central region (Heath, 1984).
Figure 7b. Topographic and geologic features of the Nonglaciated Central region (Heath, 1984).
Figure 7c. Topographic and geologic features along the western boundary of the Nonglaciated Central
region (Heath, 1984).
22
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\ ; f A —
v I r~>
(a)
1 Freth water
I _] StUvwwtr
(b)
Figure 8a. Location of the Glaciated Central region (Heath, 1984).
Figure 8b. Topographic and geologic features of the Glaciated Central region (Heath, 1984).
23
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primarily vary with precipitation rates, evapotranspiration rates,
permeability of the glacial materials and topography.
Ground-water supplies are abundant in this area; well yields are
moderate to high. Smaller yields are expected in areas where the glacial
deposits are fine-grained or where the underlying bedrock has an
insufficient amount of fractures or solutioning. Because of the widespread
occurrence of carbonate rocks, ground water in these areas frequently
exhibits high hardness.
Piedmont and Blue Ridge--
The Piedmont lies between the coastal plain and the Appalachian
Mountains. The region is characterized by a series of low, rounded hills
that gradually increase in height toward the west and culminate in the
parallel ranges of the Appalachian Mountains in the north and the Blue
Ridge Mountains in the south. The bedrock of the region consists of
Precambrian to Mesozoic-age igneous, metamorphosed-igneous and sedimentary
rocks (Figures 9a and 9b).
Active chemical and physical weathering of the bedrock has formed a
clay-rich, unconsolidated deposit that overlies bedrock. This deposit,
called saprolite or regolith is typically thinner on ridges and thickens on
slopes and in valleys. Larger streams in many valleys have deposited
significant thicknesses of well-sorted alluvial materials that often
overlie the saprolite.
The regolith serves two purposes in the ground-water system: 1) the
regolith yields small to moderate quantities ;t water to shallow wells and
2J the regolith serves as a storage reservoir *.o slowly recharge the
bedrock aquifer. The storage capacity in the bedrock is limited because
the ground water occurs along fractures and in joints. Water-supply wells
are often completed in both the regolith and :n :he bedrock.
Well yields in this region are extremely variable; bedrock wells that
intersect fractures and/or have sufficient recharge from the overlying
regolith are the most productive. A higher iensity of fractures typically
occurs along valleys and in draws bordering ridges.
Northeast and Superior Uplands--
The Northeast and Superior Uplands cover two geographic areas: 1) the
Northeast includes the Adirondack Mountains and most of New England, and 2)
the Superior Uplands include most of northern Minnesota and Wisconsin. Both
areas are underlain by Precambrian to Paleozoic-age igneous and metamorphic
rocks that have been intruded by younger igneous rocks and have been
extensively folded and faulted (Figures lOa and lOb).
The bedrock is overlain by unconsolidated glacial deposits that vary
in thickness. These glacial deposits include poorly-sorted glacial tills,
glacial lake clays, and well-sorted sands and gravels laid down by
meltwater streams. The glacial sands and gravels serve as important
aquifers and are capable of producing moderate to large yields. Ground
24
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(a)
Bedrock outcrop.
(b)
Rgure 9a. Location of the Piedmont and Blue Ridge region (Heath, 1984).
Figure 9b. Topographic and geologic features of the Piedmont and Blue Ridge region (Heath, 1984).
25
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(a)
(b)
Figure I0a. Location of the Northeast and Superior Uplands region (Heath, 1984).
Figure 10b. Topographic and geologic features of the Northeast and Superior Uplands region (Heath,
1984).
26
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water in the bedrock is typically found in fractures or joints and the rock
has a low storage capacity. The glacial deposits provide recharge by slow
seepage to the underlying bedrock. Wells are often completed in both
bedrock and the glacial deposits to provide maximum yields. Recharge to
the glacial deposits occurs primarily from precipitation.
Atlantic and Gulf Coastal Plain--
The Atlantic and Gulf Coastal Plain region extends southward from Cape
Cod to the Rio Grande River in Texas. The region is underlain by Jurassic
to Recent-age semi-consolidated to unconsolidated deposits of sand, silt
and clay laid down by streams draining the adjacent upland areas. These
deposits are very thin toward the inner edge of the region and thicken
southward and eastward. The thickest deposits occur in a down-warped zone
termed the Mississippi Embayment. All deposits either dip toward the coast
or toward the axis of the embayment; therefore, the older formations
outcrop along the inner part of the region and the youngest outcrop along
the gulf coastal area. Coarser-grained material is more abundant updip,
and clay and silt layers tend to thicken downdip (Figures lla and lib).
Limestone and shell beds also occur in some areas and serve as productive
and important aquifers.
Recharge to the aquifers occurs in outcrop areas from precipitation
and from infiltration along streams and rivers. In some areas an increase
downdip in the percentage of clay in the deposits limits recharge and
affects ground-water flow paths. Ground-water withdrawals in these areas
sometimes exceed recharge to the aquifer and result in declining water
levels and land subsidence.
Southeast Coastal Plain—
The Southeast Coastal Plain includes all of Florida and the southern
parts of Alabama and Georgia. The surficial deposits in this area are
comprised of unconsolidated Pleistocene-age sand, gravel, silt and shell
beds. The semi-consolidated limestone beds of the Biscayne aquifer outcrop
in southern Florida. Throughout much of the region, surficial deposits are
underlain by the Hawthorn formation, a Miocene-age clay and silt layer.
The Hawthorn formation often serves as a confining layer. The Hawthorn
formation overlies a thick sequence of semi-consolidated to consolidated
limestones and dolomites known as the Floridan aquifer (Figures 12a and
12b).
The Floridan aquifer is one of the most productive aquifers in the
United States and is the principal ground-water resource for the entire
region. In the northern part of the region, the Floridan is unconfined.
Most recharge to the aquifer occurs from direct infiltration of
precipitation in this area. In central and southern Florida, the aquifer
is semi-confined by the Hawthorn formation and recharge from the surface is
limited. Natural discharge from the Floridan occurs from springs and
streams and from seepage through confining beds. Many springs with high
discharge rates can be found where the Floridan outcrops.
27
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(a)
Figure 11a. Location of the Atlantic and Gulf Coastal Plain region (Heath, 1984).
Figure lib. Topographic and geologic features of the Gulf Coastal Plain (Heath, 1984).
28
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- 1
i v~-J---i .
>-J. ] L V-^
/ -T-J f x
4 / lr--L, \ _r
\;-' r~^. vt^-
f I-^--T-^^--
^t
v-^J
— J
-4^^
h
\ ^
(a)
Recharge •'»»-
i***^ ^-T5««a
(b)
Figure 12a. Location of the Southeast Coastal Plain rvgkM (Heath, 1984).
Rgure 12b. Topographic and geologic features of the Southeast Coastal Plain (Heath, 1984).
29
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In southern Florida, water in the Floridan is typically saline. In
this area, water supplies are developed in the shallower Biscayne aquifer.
The Biscayne is unconfined and is recharged directly by precipitation and
by infiltration from streams and impoundments.
The surficial sands and gravels also serve as aquifers in many parts
of the region, particularly where the Floridan is saline. These aquifers
supply small to moderate yields to wells and are recharged by infiltration
of precipitation.
Alluvial Valleys--
The Alluvial Valleys region encompasses the thick sand and gravel
deposits laid down by streams and rivers. Figure 13a illustrates the
extent and location of these major alluvial valleys. Alluvial valleys
typically contain extensive deposits of sands and gravels that are often
interbedded with overbank deposits of silts and clays. The origin of many
of the alluvial aquifers is related to Pleistocene continental and alpine
glaciation. Sediment-laden meltwater from the glaciers deposited extensive
sands and gravels in many stream valleys. These permeable sands and
gravels are capable of yielding moderate to large water supplies to wells.
These aquifers are typically confined to the boundaries of the flood plain
and to adjacent terraces (Figure 13b).
In many of the alluvial valleys, ground-water systems and surface-
water systems are hydraulically interconnected. Recharge to the aquifer
occurs from streams and from precipitation. Withdrawals of ground water
near a stream may cause a reversal of hydraulic gradients; ground water
previously flowing from the aquifer and discharging to the stream may now
receive recharge from the stream by induced infiltration.
Hawaiian Islands--
The Hawaiian Islands were formed by volcanic eruptions of lava. These
shield volcanoes rise from the ocean floor and form the eight major
Hawaiian islands. Erosion of the volcanoes has carved distinctive valleys
and has created an adjacent narrow coastal plain.
The islands are formed from hundreds of separate lava flows composed
primarily of basalt. The lavas that were extruded beneath the sea are
relatively impermeable. Lavas that were extruded above sea level contain
permeable interflow zones, lava tubes and cracks and joints formed while
the lava cooled. Lava flows in the valleys are often covered by a thin
layer of alluvium eroded from the basalt.
The mode of deposition of the basalt largely controls the occurrence
and flow of ground water on the islands. The ground-water system consists
of three major parts: 1) dike-impounded water, 2) basal ground water, and
3) perched (fresh) water (Figure 14). Dike-impounded water is found in the
joints developed along the vertical fissures through which the lava
erupted. Basal ground water is found in the permeable zones of the
horizontal lava flows extending from the eruption centers and is partially
hydraulically interconnected to the dike-impounded water. The perched
30
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r^hV
MISSISSIPPI
RIVfP
ȣ.
" J' _-J Silt «nd el«y
Limenon*
Figure I3a. Location of the Alluvial Valleys ground-water region (Heath, 1984).
Figure 13b. Topographic and geologic features of a section of the alluvial valley of the Mississippi River
(Heath, 1984).
31
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I I f'**t\
J S
-------
(fresh) water system is found in permeable lava or alluvial deposits above
thick impermeable lava flows or basal ground water.
Recharge to these aquifers occurs through the infiltration of
precipitation. Because the volcanic soils are highly permeable,
approximately thirty percent of the precipitation infiltrates and recharges
the aquifer.
The basal ground-water system is the principal source of water to the
islands. The basal system occurs as a fresh-water lens floating on the
denser sea water. Basal and dike-impounded ground water is often withdrawn
from horizontal tunnels and vertical and inclined wells constructed into
the lava flows.
Alaska--
Alaska can be divided into four physiographic divisions from south to
north: 1) the Pacific Mountain System, 2) the Intermontane Plateaus,
3) the Rocky Mountain System and 4) the Arctic Coastal Plain. The mountain
ranges are comprised of Precambrian to Mesozoic-age igneous and metamorphic
rocks. These are overlain by younger sedimentary and volcanic rocks. Much
of the region is overlain by unconsolidated deposits of gravel, sand, silt,
clay and glacial till (Figure 15).
Climate directly affects the hydrology of Alaska. Much of the water
at the surface and in the subsurface is frozen throughout much of the year,
forming a zone of permafrost or perennially frozen ground. Permafrost
occurs throughout the state except along the southern and southeastern
coasts. The depth of permafrost varies, but is typically deeper in the
northern areas and becomes shallower toward the south.
In zones of continuous permafrost, ground water occurs beneath the
permafrost and in isolated zones beneath deeper lakes and alluvial
channels. In zones of discontinuous permafrost, ground water occurs below
the permafrost and in sand and gravel deposits in major alluvial valleys.
In the areas where permafrost is absent, ground water occurs both in the
bedrock and in the overlying unconsolidated deposits.
Recharge to the aquifers is limited due to permafrost. Even in
non-permafrost areas, shallow ground water is usually frozen when spring
runoff occurs. Most recharge to the aquifers occurs from stream
infiltration as the streams flow across the alluvial deposits when
permafrost is absent.
Site-Specific Geologic and Hvdrogeologic Conditions
The geologic and hydrogeologic conditions at a specific site influence
the selection of an appropriate well design and drilling method. Prior to
the installation of monitoring wells, exploratory borings and related
subsurface tests must usually be made to define the geology beneath the
site and to assess ground-water flow paths and velocity. Formation samples
and other data collected from this work are needed to define the hydraulic
characteristics of the underlying materials. The logs of these borings are
33
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used to correlate stratigraphic units across the site. An understanding of
the stratigraphy, including the horizontal continuity and vertical
thickness of formations beneath the site, is necessary to identify zones of
highly permeable materials or features such as bedding planes, fractures or
solution channels. These zones will affect the direction of ground-water
flow and/or contaminant transport beneath the site. Because the occurrence
and movement of ground water in the subsurface are closely related to the
geology, the geologic conditions at the site influence the location, design
and methods used to install monitoring wells.
The required depth of a monitoring well is determined by the depth to
one or more water-bearing formations that need to be monitored. Where two
or more saturated zones occur beneath a site and the intent of the
monitoring program is to monitor water quality in the lower zone, the
monitoring well may require surface casing to "seal-off" the upper
water-bearing formation prior to drilling deeper.
The formations at the site, whether consolidated or unconsolidated,
also influence the type of well completion. In unconsolidated deposits,
screened intakes are typically designed. The well may have either a
naturally-developed or artificially-emplaced filter pack, depending on the
grain-size distribution of the water-bearing materials. Artificial filter
packs and screened intakes are also often required in poorly-consolidated
formations to minimize potential caving of the borehole and/or to reduce
turbidity in water samples collected from the completed well. In some
consolidated formations, the well may be completed as a cased borehole with
no screen intake or filter pack. Where conduit-born fines are a problem in
consolidated formations, an artificial filter pack and a screen intake may
be required.
Drilling methods must be chosen based at least in part on geologic
considerations. Hard, consolidated formations restrict or eliminate
certain drilling methods. For example, in karstlc formations, cavernous
openings create significant problems in maintaining circulation and in
protecting drilling equipment. Unconsolidated deposits can also present
severe limitations for various drilling methods Some drilling techniques
cannot be used where large boulders are present. Conversely, cohesive
geologic deposits and the resultant stability of the borehole may expand
drilling options. Variations in equipment, drilling techniques and
installation procedures may be necessary to overcome specific limitations
when using particular drilling methods.
Consideration of the hydrogeology at the site is also important when
selecting a drilling method. The depth to which the well must be drilled
to monitor a selected water-bearing zone may exceed the practical depths of
a particular drilling technique. In addition, certain saturated geologic
materials, under high hydrostatic pressures, may either: 1) impose
increased frictional resistance (i.e. expanding clays) which limits the
practical depths reached by some drilling methods or 2) create unstable
borehole conditions (i.e. heaving sands) that may preclude the use of some
drilling methods for installation of the monitoring well.
For a complete discussion of well drilling methods and a matrix for
selecting a drilling method based on the general hydrogeologic conditions
34
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and well design requirements, the reader is referred to Section 4,
"Description and Selection of Drilling Methods."
FACILITY CHARACTERISTICS
Frequently the purpose of a monitoring program is to evaluate whether
or not ground water is being contaminated from a waste disposal practice or
a commercial operation associated with the handling and storage of
hazardous materials. In these instances, the design and construction of
the monitoring wells must take into account the type of facility being
monitored and the fate and transport in the subsurface of the waste
materials or commercial products.
Recognition of the type of facility being monitored is necessary to
determine whether the facility is regulated under existing federal and/or
state statutes and administrative rules (see Section 1). Some regulated
facilities must comply with specific ground-water monitoring requirements,
and program-specific guidance documents may describe the design and
construction of the monitoring wells. The type of facility or operation
may also determine the types of materials and potential contaminants which
have been handled onsite, past or present, and whether or not those
contaminants were stored or disposed of on or below the ground surface. The
design of the facility may also include a system for waste or product
containment that impacts potential release of contaminants, both onsite and
offsite, and may require separate monitoring.
The physical and chemical characteristics of the contaminants,
including volatility, solubility in water and specific density, influence
the movement of the contaminant in the subsurface. Additional factors that
affect contaminant fate and transport include: oxidation, sorption and
biodegradation. Monitoring wells must be located and designed with these
environmental factors and contaminant characteristics in mind.
Construction materials for the well should be selected based on their
ability to withstand attack by contaminants that are anticipated at the
site.
The following two-part discussion focuses on facility characteristics
that impact the design and construction of monitoring wells. The first
part presents the more prominent types of waste disposal facilities or
commercial operations for which ground-water monitoring wells are designed.
The second part focuses on those physical and chemical characteristics of
contaminants that significantly influence the transport of the contaminant
in the subsurface.
Type of Facility
Landfills--
A landfill is a facility or waste unit where solid waste is typically
disposed of by spreading, compacting and covering the waste. The landfill
design, construction and operation details vary depending on the physical
conditions at the site and the type and amount of solid waste to be
35
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disposed. Wastes are usually emplaced and covered in one of three
settings: 1) on and above the natural ground surface where surface
topography is flat or gently rolling, 2) in valleys, ravines or other land
depressions, or 3) in trenches excavated into the subsurface. The design
of the landfill determines the boundaries of the fill area and the lowest
elevation at which the solid waste is disposed. The physical dimensions of
the landfill are important criteria for locating and designing the depth of
monitoring wells used to monitor the quality of ground water in the first
water-bearing zone beneath the bottom of the landfill.
The wastes that are disposed of in landfills are generally classified
as either hazardous or non-hazardous. Wastes that are characterized as
hazardous are regulated in Title 40 of the United States Code of Federal
Regulations (CFR) Part 261. The distinction between a landfill receiving
hazardous waste versus non-hazardous waste is important from a regulatory
standpoint when developing a ground-water monitoring program. Landfills
receiving wastes classified as hazardous are subject to minimum federal
regulations for the design and operation of the landfill (40 CFR, Parts 264
and 265, Subpart N and Part 268) and for ground-water protection and
monitoring (40 CFR, Parts 264 and 265, Subpart F). These regulations are
mandated under the Resource Conservation and Recovery Act (RCRA) and
subsequent amendments to RCRA. Individual states may be authorized by the
United States Environmental Protection Agency to enforce the minimum
federal regulations and may adopt separate state regulations more stringent
than the federal standards.
Landfills receiving non-hazardous wastes are also regulated under
RCRA; however, these facilities are addressed under different federal
guidelines or recommendations for the design and operation of sanitary
landfills and for ground-water protection measures (40 CFR, Part 241,
Subpart B). Properly designed landfills should include a bottom liner of
compacted, low permeability soil and/or synthetic liner to minimize the
percolation of leachate from the landfill into the subsurface. A leachate
collection system should also be installed beneath the landfill to control
leachate migration and permit the collection of leachate for final
treatment and disposal. Hazardous waste landfills are subject to minimum,
federal technological guidelines for "composite double liner systems"
(including compacted low permeability soils and two flexible synthetic
membranes) that incorporate both primary and secondary leachate collection
systems. Many older or abandoned landfills containing both hazardous
and/or non-hazardous wastes are unlined and have been unregulated
throughout the operational life of the facility.
Ground-water monitoring programs at hazardous waste land disposal
facilities are also subject to federal requirements, including performance
criteria. The regulations require that a sufficient number of wells be
constructed at appropriate locations and depths to provide ground-water
samples from the uppermost aquifer. The purpose of ground water monitoring
is to determine the impact of the hazardous waste facility on ground water
in the uppermost aquifer. This is done by comparing representative samples
of background water quality to samples taken from the downgradient margin
of the waste management area. The ground-water monitoring wells must be
properly cased, completed with an artificial filter pack, where necessary,
36
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and grouted so that representative ground-water samples can be collected
(40 CFR, Sections 264.97 and 265.91). Guidance for the design and
construction of these monitoring wells is provided in the RCRA Ground Water
Monitoring Technical Enforcement Guidance Document (TEGD). Owners and
operators should be prepared to provide evidence that ground-water
monitoring measures taken at concerned facilities are adequate.
A potential monitoring problem at all landfills, particularly older
facilities, is the accurate location of the boundaries of the landfill. If
the boundaries of the fill area are unknown, monitoring wells may not be
accurately placed to properly define subsurface conditions with respect to
the actual location of the disposal site. Accidental drilling into the
landfill causes safety and health concerns. All personnel involved in the
drilling of monitoring wells at hazardous waste treatment, storage and
disposal facilities, or in the direct supervision of such drilling, should
have received initial training in working in hazardous environments in
accordance with the regulations of the Occupational Safety and Health
Administration (29 CFR, Section 1910.120).
Surface Impoundments--
Surface impoundments are used for the storage, treatment and/or
disposal of both hazardous and non-hazardous liquid wastes. Impoundments
or lagoons can be constructed either in natural depressions or excavations
or created by surface diking. The impoundments typically are used to
settle suspended solids. Liquid wastes within the impoundment are usually
treated chemically to cause precipitation or coagulation of wastes.
Surface impoundments may be either "discharging" or "non- discharging."
Discharging impoundments are designed to intentionally permit the
supernatant fluid to overflow into receiving streams for final treatment
and disposal. Non-discharging impoundments can either intentionally or
unintentionally lose liquids through seepage into the subsurface or through
evaporation.
The size of a surface impoundment can range from a fraction of an acre
to thousands of acres in surface area. The depths of these impoundments
reportedly range from 2 feet to more than 30 feet below the ground surface
(Office of Technology Assessment, 1984). The specific design and operation
requirements for surface impoundments that contain hazardous materials are
regulated under RCRA (40 CFR, Parts 264 and 265, Subpart K). To prevent
waste infiltration, hazardous waste impoundments are subject to minimum
federal technological guidelines for a "compacted soil double liner system"
(including compacted, low permeability soil and a single flexible synthetic
liner). A leachate collection system is also required to contain any
leachate that does infiltrate into the subsurface.
Hazardous waste impoundments are subject to the same minimum federal
ground-water protection and monitoring regulations discussed above for
hazardous waste landfills. Water levels in monitoring wells located too
close to impoundments often reflect the effects of mounding on the water
table and lead to inaccurate interpretation of the water-level data (Beck,
1983). The design depth of the monitoring wells also depends on the depth
of the bottom of the surface impoundment below ground level and the depth
of the first water-bearing zone underlying the bottom of the impoundment.
37
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Waste and Material Piles--
Large quantities of both wastes and materials may be stockpiled for
storage. Stockpiled material may include potentially hazardous material
such as highway deicing salts, copper, iron, uranium and titanium ore,
coal, gypsum and phosphate rock. Hazardous waste piles can also be
generated by other industrial operations and vary in composition. Waste
piles typically include two types of mining wastes: 1) spoil piles and 2)
tailings. Spoil piles are the overburden or waste rock removed during
either surface or underground mining operations. Tailings are the solid
wastes generated from the cleaning and extraction of ores. Both types of
mining waste include waste rock that can contain potential contaminants
such as uranium, copper, iron, sulfur and phosphate. Waste piles
containing hazardous wastes are regulated under RCRA and are subject to
minimum federal design and operational requirements (40 CFR, Parts 264 and
265, Subpart L) and ground-water protection requirements (40 CFR, Part 264,
Subpart F), particularly where the waste piles are unprotected from
precipitation and surface drainage. In many instances, waste and material
piles remain uncovered and exposed to the atmosphere. Precipitation
percolating through the material can dissolve and leach potentially
hazardous constituents into the subsurface. For example, ground-water
quality problems have occurred due to the dissolution of unprotected stock
piles of highway deicing salt. Cyanide leaching to extract gold from mine
tailings is potentially dangerous and a widespread problem in some areas.
Surface runoff from stockpiles can also be a source of potential
ground-water contamination. Ground-water monitoring efforts in waste and
material pile areas need to be designed to detect or assess ground-water
contamination occurring onsite and to determine that surface runoff has not
contaminated adjacent areas.
Land Treatment—
Land treatment involves the application of waste liquids and sludges
onto the ground surface for biological or chemical degradation of the waste
or for the beneficial use of nutrients contained in the waste. Land
treatment operations commonly involve spray irrigation or land spreading of
sludges on agricultural, forested or reclaimed land. Municipal wastewater
or sludge application to agricultural land is the most common form of land
treatment. Industrial waste sludge includes effluent treatment waste,
stack scrubber residue, fly ash, bottom ash and slag (Office of Technology
Assessment, 1984). Control measures must be instituted to prevent surface
runoff, wind erosion and excessive percolation into the ground water during
site operation. The rate and duration of sludge application depends on the
waste, soil type and the level of anticipated degradation.
Wastes applied to the ground surface at a land treatment facility may
be hazardous or non-hazardous. Hazardous waste land treatment facilities
are regulated under RCRA and are subject to minimum federal design and
operational requirements (40 CFR, Parts 264 and 265, Subpart M) and
applicable ground-water protection and monitoring requirements (40 CFR,
Parts 264 and 265, Subpart F).
38
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Underground Storage Tanks--
Underground storage tanks are used to store hazardous and non-
hazardous waste, industrial products and raw materials. The primary
industrial use for tanks is the storage of fuel oils. It is estimated that
half of all steel tanks in use store petroleum products. Both steel and
fiberglass tanks are also used to store other products including solvents,
acids and technical grade chemicals.
Recent amendments to RCRA now specify design, maintenance and
operation requirements for tanks containing hazardous waste and commercial
petroleum products (40 CFR, Parts 264 and 265, Subpart J). These
regulations include requirements for a double liner system and/or cathodic
protection of steel tanks, leak detection and inventory control.
Radioactive Waste Disposal Sites--
Radioactive wastes are produced during the development and generation
of nuclear fuel and other radioactive materials. Waste products include:
1) spent fuel from nuclear power plant operations, 2) high-level
radioactive waste from initial processing of reactor fuels, 3) transuranic
waste from fuel processing, 4) low-level wastes from power plants, weapons
production, research and commercial activities and 5) medical waste (Office
of Technology Assessment, 1984).
The radioactive waste disposal method depends on the radiation levels
and the waste characteristics. Low-level radioactive wastes are usually
disposed of in shallow burial sites. High-level radioactive wastes are
stored in specially constructed facilities' and may be reprocessed. Spent
reactor fuels may be stored on site or transferred to disposal facilities.
All radioactive waste disposal facilities are regulated by the Nuclear
Regulatory Commission. Ground-water monitoring requirements for specific
facilities coupled with the design configuration of the facility directly
affect the location and installation of monitoring wells.
Waste Characteristics
The physical and chemical characteristics of the waste(s) present at a
site should be carefully evaluated and considered together with site
hydrogeology when designing a monitoring program. The mechanisms that
govern the fate and transport of contaminants in the subsurface affect the
occurrence and configuration of a contaminant plume. By considering these
effects a monitoring program can be designed to monitor or detect
subsurface contamination. The monitoring well locations, the depth of the
screened intervals, the method of well installation and the appropriate
construction materials must all be compatible with the specific waste and
hydrogeological characteristics of the site.
Two physical properties that affect transport and fate of a compound
in the subsurface are the relative solubility and density of the
contaminant. Based on these properties, contaminants can be classified
into categories that subsequently influence monitoring well design: 1)
39
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compounds that are primarily miscible/soluble in ground water and 2)
compounds that are relatively immiscible/insoluble in ground water. These
categories can be further subdivided based on the relative density of the
compound.
Primarily Hiscible/Soluble Contaminants—
This category of contaminants exhibits a relatively high solubility in
water and typically is mobile in the subsurface. Soluble contaminants can
exhibit densities greater than, less than or equal to water. In general,
where the density of the contaminant closely approximates that of water,
the contaminant moves in the same direction and with the same velocity as
ground water.
The primary processes that affect dissolved contaminant transport in
porous media include advection and dispersion (Freeze and Cherry, 1979;
Anderson, 1984; Mackay et al., 1985). Advection is the process by which
solutes are transported by the motion of ground water flowing in response
to hydraulic gradient, where the gradient reflects the magnitude of the
driving force. Dispersion refers to the dispersal of contaminants as they
move with the ground water. Dispersion occurs by mechanical mixing and
molecular diffusion. Seasonal changes in gradient may affect lateral
movement of a contaminant more than dispersion. Interactions that occur
between the contaminant and the porous media include retardation, sorption
(Freeze and Cherry, 1979; Cherry et al., 1984; Mabey and Hill, 1984; Mackay
et al., 1985) and biodegradation (HcCarty et al., 1981; McCarty et al.,
1984; Wilson et al., 1985). These mechanisms can affect the rate of
movement of a contaminant plume or alter the chemistry within the plume.
The effects of contaminant density oust also be considered in waste
characterization (Bear, 1972). Figure 16 illustrates the migration of a
high density, miscible contaminant in the subsurface. As shown, the
contaminant sinks vertically through the aquifer and accumulates on top of
the lower permeability boundary. The contaminant then moves in response to
gravity and follows the topography of the lower permeability boundary,
possibly in opposition to the direction of regional ground-water flow.
Because the contaminant is also soluble, the contaminant will concomitantly
move in response to the processes of advection and dispersion. Therefore,
two or more zones of different concentration may be present within the
plume: 1) a dense pool of contaminant at the bottom of the aquifer and 2)
a dissolved fraction that moves with the ground water. Because the dense,
pooled portion of the plume is also soluble, the contaminants will continue
to dissolve and migrate in response to ground-water flow conditions.
Ground-water monitoring wells installed in the aquifer may more easily
detect the dissolved portion of the plume unless a specific monitoring
program is devised for the dense phase of the plume. A knowledge of
subsurface topography, determined from a top-of-bedrock map or overburden
thickness maps and confirmed by surface geophysics and/or borings assist in
accurately locating and monitoring the denser portion of the plume.
Figure 17 illustrates the migration of a low density, soluble
contaminant. The contaminant initially accumulates at the top of the water
40
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Figure 16. Migration of a high density, miscible contaminant in the subsurface.
fs//s///
7 7
Figure 17. Migration of a low density, soluble contaminant in the subsurface.
41
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table. Dissolution and dispersion of the contaminant occurs as the
accumulated contaminant migrates with the ground water. Continued
dissolution of the contaminant causes eventual dissipation of the plume
Monitoring for contaminants with these characteristics is frequently most
effective in the shallow portion of the aquifer.
Contaminants with a density similar to water migrate in response to
advection and dispersion. Contaminants in this category include inorganic
constituents such as trace metals and nonmetals. Because of the similarity
of contaminant movement to the ground-water movement, certain nonmetals
such as chloride, are commonly used as tracers to estimate the boundaries
of contaminant plumes. The dissolved portion of certain organic
contaminant plumes can also have a density similar to water and migrate
with the ground water. Monitoring and detection schemes for plumes of
these contaminants must be based on the calculated effects of advection
dispersion, chemical attenuation and subsurface hydrogeology. '
Relatively Immiscible/Insoluble Contain inants--
In both the saturated and unsaturated zones, immiscible compounds
exist as either free liquids or as dissolved constituents depending on the
relative solubility of the contaminant. The migration of dissolved
constituents in the aqueous phase is primarily governed by the processes of
advection-dispersion and biological/chemical attenuation (Schwarzenbach and
Giger, 1985). The distribution of free liquids is complexly interrelated
to capillary pressure, density (gravitational forces) and viscosity (shear
forces) (Kovski, 1984; Villaume, 1985). The relative density of the
contaminant affects the occurrence and moveMnt of the contaminant in the
subsurface and must be considered when locating monitoring wells and when
determining the interval(s) to be screened In the aquifer.
Figure 18 illustrates the migration of a low density, immiscible
contaminant. The contaminant moves downward through the vadose zone and
accumulates at the top of the water table and/or within the capillary
fringe. A residual amount of fluid is retained In the vadose zone in
response to surficial and interstitial forces (Kovski, 1984; Yaniga and
Warburton, 1984). The contaminant plume accumulates on the water table and
typically elongates parallel to the direction of ground-water flow (Gillham
et al., 1983). The movement and accumulation of immiscible hydrocarbons in
the subsurface has been discussed by Blake and Hall (1984), Kovski (1984)
Yaniga and Warburton (1984), and Hinchee and Reisinger (1985). Depending'
on the physical properties of the contaminant, a volatile gas phase may
accumulate in the unsaturated zone.
Monitoring wells designed to detect or assess low density immiscible
contaminants should be screened in the upper part of the aquifer. In many
instances the screen should span the vadose zone and the upper portion of
the aquifer to allow the floating contaminant to enter the well. Many
immiscible contaminants depress the water table in the well and create an
apparent free liquid thickness that is greater than the thickness of the
floating contaminant within the aquifer. Where volatiles accumulate in the
vadose zone, an explosion hazard may exist. Various mapping and detection
42
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Figure 18. Migration of a low density, immiscible contaminant in the subsurface.
Figure 19. Migration of a dense, non-aqueous phase liquid (DNAPL) in the subsurface.
43
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techniques including soil gas sampling and geophysical techniques can be
utilized in planning the monitoring well locations to intercept the plume
and reduce the risk of an explosion (Noel et al., 1983; Andres and Canace,
1984; Marrin and Thompson, 1984; Saunders and Genneroth, 1985; Lithland et
al., 1985).
High density immiscible fluids are called dense non-aqueous phase
liquids (DNAPLs). DNAPLs include most halogenated hydrocarbons and other
aliphatic compounds because the density of most organic compounds is
significantly greater than water. A density difference of one percent or
greater has been shown to cause migration of contaminants in the subsurface
(Mackay et al., 1985).
Figure 19 illustrates the movement of DNAPLs in the subsurface.
Movement of DNAPLs in the unsaturated zone is primarily governed by
capillary forces and density (Villaume, 1985). The contaminant sinks
through the aquifer and pools at the bottom of the aquifer on top of the
lower permeability boundary (Schwille, 1981). The pool of contaminant
migrates in response to the topography of the lower permeability boundary
independent of regional ground-water flow. Residual material is retained
in the pore space of the unsaturated and saturated zones. This residual
typically occurs as discrete fingers of globules. The formation and
movement of the globules in the subsurface depends on the extant pore-size
distribution and capillary forces (Schwille, 1981; Villaume, 1985). As
much as five percent by volume of a compound may be retained in the aquifer
after plume migration.
Both residual contaminant and the contaminant plume may continue to
contribute dissolved constituents to the ground water for an extended
period of time. Thus, small spills of persistent compounds have the
ability to extensively contaminate ground water. A vapor plume from the
contaminant source may also form and migrate in the vadose zone. These
plumes can often be detected through soil gas sampling techniques.
Field investigations at hazardous waste sites have supported the
phenomena of sinking DNAPLs as demonstrated by Schwille (1981) in physical
model experiments (Guswa, 1984; Reinhard et al., 1984; Villaume, 1985).
Monitoring for these DNAPLs poses special problems. The actual contaminant
plume may migrate independently of regional ground-water flow and may be
very difficult to locate. Analysis of maps of aquifer thickness and
bedrock topography will aid in determining potential migration pathways.
The dissolved constituents will migrate according to the ground-water flow
regime. Vapor plumes can be detected by using soil gas sampling
techniques.
Villaume (1985) indicates that monitoring well installation through
DNAPL-contaminated zones should proceed with caution to avoid cross
contamination. Where the borehole is open during drilling or where the
annulus is not properly sealed, DNAPLs may migrate down the hole or annulus
and cause cross contamination.
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OTHER ANTHROPOGENIC INFLUENCES
The hydrogeology of a site and the characteristics of the facility are
primary factors that should be assessed when choosing specifications for a
monitoring well program. However, a variety of factors that relate to the
activities of man also should be assessed to determine any potential
impacts to the monitoring program. These factors can affect ground-water
gradients and flow direction and might have had past impacts on
ground-water quality that will affect a current monitoring program.
To minimize the possibility of unknown anthropogenic influences, any
initial investigation should include a detailed review of the site history.
This review should encompass a study of any land use prior to the current
or proposed activity at the site. Additionally, a design and operational
history for any existing operation also should be compiled that includes
the location of all site activities and the type(s) of waste accepted
during the operation of the disposal facility. For example, information
about tank age, volume of product delivered and sold, location of the tank
and similar information is needed to assess a gasoline-dispensing
operation. Another example is where a presently regulated disposal
facility is located on the site of a previously unregulated landfill or a
turn-of-the-century industrial facility. Prior waste disposal practices
may already have caused ground-water contamination. Knowledge of the past
site practices might lead the investigator to the conclusion that
contaminants are held in the vadose zone and could be periodically released
to the ground-water during recharge events (Pettyjohn, 1976 and 1982).
Cyclic fluctuations in ground-water quality are sometimes difficult to
evaluate because naturally-occurring const'ituents in the vadose zone can
also cause similar fluctuations. Additional sources of data to assess site
history include: 1) historical photographs, 2) air photos, 3) zoning
plats, 4) interviews with local citizens and 5) local newspapers.
A complete site assessment must frequently include an investigation
outside the legal boundary of the property. An evaluation of past and
present land use practices in the area to be monitored can alert the
investigator to potential contamination problems not related to the
activity to be monitored. For example, non-point sources such as
agricultural practices may affect natural background water quality.
Adjacent industrial or commercial facilities may also influence background
water quality or may serve as a source of contamination.
Pumping or injection wells near an area to be monitored can affect
ground-water flow direction and velocity and/or can influence ground-water
quality. The presence of a well or collection of wells with resultant
cones of depression or impression might reverse anticipated ground-water
flow directions or alter the rate of migration of contaminant plumes. The
influence of a pumping well(s) should be determined before completing final
design of the monitoring program. Collection of water-level measurements
and evaluation of pump test data and velocity plots can be used to
determine the possible hydraulic effects of the other wells in the
monitoring program (Keely and Tsang, 1983). A more detailed discussion of
monitoring strategies that are useful near well fields can be found in
45
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Keely (1986). Potential water-quality effects from injection wells near
the site must also be evaluated.
Other activities that can alter ground-water velocity and/or direction
include infiltration galleries and ground-water recharge facilities.
Mounding of the water table beneath these areas will locally affect
ground-water gradients. Where the quality of the recharge water differs
from background water quality, the ground-water quality in the area may
also be affected.
Storm sewers, surface runoff catchments, sanitary sewers, buried
underground cables, underground pipelines or other subsurface disturbances
may affect ground-water flow paths and ground-water quality. Preferential
flow paths can be created when subsurface trenches or excavations are
refilled with unconsolidated backfill and bedding materials. These more
permeable materials provide conduits that can influence or control the flow
of contaminants in the subsurface and can also serve as a vapor migration
pathway. Storm and sanitary sewer lines and other buried pipelines may be
a source of contamination if leakage occurs. The precise location of
buried pipelines and cables should be determined to avoid inadvertently
drilling into or through the lines. For example, drilling into natural gas
pipelines poses an immediate health and safety risk to anyone near the
drilling site. Drilling into pipelines for sanitary or storm sewers poses
less of a safety risk, but may exacerbate the contamination problem. In
summary, a review of all site activities and subsurface structures serves
to contribute valuable information to the monitoring program.
EQUIPMENT THAT THE WELL MUST ACCOMMODATE '
The purpose of a monitoring well is to provide access to a specific
zone from which water-level measurements and/or ground-water quality
samples, representative of the extant water quality in the monitored zone,
can be obtained. These conditions and the size of equipment necessary to
obtain the desired measurements or collect the desired samples will
determine the diameter of the well that must be drilled. For example, if
the transmissivity of the monitored zone is to be evaluated, then the well
diameter must accommodate a pump or other device capable of providing the
necessary water demand to make the transmissivity determination.
Similarly, if representative ground-water quality samples are to be
collected from the well, then an appropriate well diameter must be selected
that accommodates the needed sampling equipment. Equipment and procedures
that influence the choice of a well diameter include: 1) borehole
geophysical tools and downhole cameras, 2) water-level measuring devices,
3) ground-water sampling devices and 4) aquifer testing procedures.
Borehole Geophysical Tools and Downhole Cameras
Use and Limitations of Borehole Geophysical Tools - -
Borehole geophysical methods are often used in monitoring wells to
obtain hydrogeologic information. Under appropriate conditions, porosity,
hydraulic conductivity, pore fluid electrical conductivity and general
46
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stratigraphic logs can be obtained. Unfortunately, borehole geophysical
methods are frequently limited by the materials and the drilling and
completion methods used to construct the well. If it is anticipated that
borehole geophysical methods will be conducted in a well, it is necessary
to consider the limitations that are imposed by the various methods and
materials that are used to construct the well.
Virtually all borehole methods that are likely to be used in shallow
ground-water investigations can be conducted in a 2-inch diameter well.
Four things that commonly restrict the use of borehole methods are well
fluid, casing type, perforation type and gravel pack. Each one of these
imposes limitations on the geophysical methods that can be conducted in the
well. A summary of the limitations is presented in Table 3, and the
limitations are discussed below.
Some geophysical methods require that a fluid be present in the well.
Sonic tools will not operate in an air-filled borehole because the acoustic
source and receivers are not coupled to the formation. Television systems
can operate in air or fluid, but only if the fluid is not murky.
Radiometric methods, such as natural gamma, gamma density or neutron
moisture logs can operate in air or fluid-filled wells. However, the
calibration of these tools is different between air and fluid-filled wells.
Standard resistivity tools that measure the electrical conductivity of
the formation will not operate in air-filled boreholes because of the lack
of an electrical connection between the electrodes and the formation. Some
individuals have modified resistivity tools to operate in air-filled
boreholes by altering the electrode design to insure that the electrode is
always in contact with the formation. If the well fluid electrical
conductivity is two orders of magnitude or store greater than the formation
electrical conductivity (electrical conductivity is the reciprocal of
electrical resistivity), then the lateral and normal electrical resistivity
tools cannot be used because the well fluid distorts the electric field to
such a degree that it cannot be corrected. This situation can occur in low
porosity formations. The induction log, which measures formation
electrical conductivity by electromagnetic coupling, does not require fluid
in the well to operate and is usually not affaced by the well fluid.
The casing material also influences which methods can be used. No
measurement of the electrical properties of the formation can be made if
the well is cased with metal. Quantitative resistivity measurements can
only be made in open boreholes; limited qualitative measurements can be
made in perforated PVC or perforated teflon wells. The formation
electrical conductivity can be measured qualitatively with induction logs
in wells cased with PVC or teflon. Sonic methods have not been
demonstrated to be useful in cased wells, although this is an area that is
currently being researched. The calibration of radiometric logs is
affected by the thickness and material used in the casing. This is
particularly true when neutron moisture methods are used in PVC casing
because the method is unable to distinguish hydrogen in the PVC from
hydrogen in the pore fluid.
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TABLE 3. USE AND LIMITATIONS OF BOREHOLE GEOPHYSICAL TOOLS (K. TAYLOR, DESERT RESEARCH
INSTITUTE, RENO, NEVADA, PERSONAL COMMUNICATION. 1988)
Borehole Method
Sonic
Resistivity
Induction
Natural Gamma
Gamma Density
Neutron
Caliper
TV
Borehole Fluid
Fluid Resistivity
Vertical Flow
Horizontal Flow
Air
4
4
1
2
2
2
1
1
4
4
4
Fluid Casing Material Perforations
Water Open Metal Plastic Screen No Screen
11444 4
11433 4
1141
2222
2222
2222
1
2
1
1 1 4
1 3 4
Radius of
Investigations
(cm)
5-50
5-400
100-400
5-30
5-15
5-15
0
0
0
0
2-6 cm
Comments
Big elfect with PVC
Clear fluid only
Strongly influenced
by screen
'Works, this well property does not adversely aftecl the log
'Works, but calibration affected
JWorks qualitatively
4 Doesn't work
-------
The type of perforations influence which methods can be used.
Qualitative resistivity measurements can be made in non-metallic wells that
are uniformly perforated, but not in wells that are not perforated because
there is no path for the current between the electrodes and the formation.
Vertical flow in the well is controlled by the location of perforated
intervals. Hence, the location of perforations will dictate what intervals
can be investigated. Horizontal flow through the well is controlled by the
radial distribution of perforations. Attempts to measure the horizontal
flow must have perforations that are continuous around the well.
In cased holes, the material in and the size of the annulus between
the casing and the undisturbed formation will influence geophysical
measurements. This occurs because all borehole geophysical measurements
are a weighted average of the property being investigated over a cylinder
portion of the formation adjacent to the borehole. The radius of this
cylinder is referred to as the radius of investigation. The radius of
investigation is a function of the geophysical method, tool design, and, to
a lesser degree, the formation and annular material. Table 3 lists typical
radii of investigation for common borehole geophysical methods. Because it
is generally the formation, not the material in the disturbed zone, that is
of interest, it is important to ensure that the radius of investigation is
larger than the disturbed zone.
The radius of investigation for the sonic tool is on the order of a
few wavelengths of the sonic pulse. Hence, it is less for high frequency
tools (greater than 30 kHz) than for low frequency tools (less than 20
kHz). The radius of investigation of resistivity tools is controlled by
the type of array that is used. Resistivity tools with multiple radii of
investigation can commonly be used to correct for the effects of a
disturbed annulus. The radiometric logs have a very limited radius of
investigation and usually require a driven casing or open borehole to be
accurate. The spacing between the source and the detector influences the
radius of investigation. Some tools use two spacings to correct for
disturbed zones less than approximately 4 inches in radius. Horizontal
flow through the borehole is strongly affected by the hydraulic
conductivity of the material in the disturbed zone. Hydraulic testing of
discrete intervals with straddle packers is adversely affected if the
annular material adjacent to the packers has a hydraulic conductivity
significantly greater than the formation.
When using tools that have a radioactive source (gamma density or
neutron moisture), state regulations vary. Host states severely restrict
the use of these tools in water wells. At a minimum, it is usually
required that the measurements be made in cased wells. This complicates
the use of these tools because the casing influences the calibration and
creates a disturbed zone. Another common restriction is that the well not
be perforated in an aquifer with potable water. This further limits the
use of these methods to areas that are already contaminated.
General Applications - -
Natural gamma and self potential (SP) logs are commonly used to detect
lithologic boundaries and to identify formations containing clays and
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shales (Keys, 1968; Keys and MacCary, 1971; Voytek, 1982; Mickam et al.,
1984; Taylor et al., 1985). Both natural gamma and SP logging tools can be
accommodated by 2-inch diameter or larger wells and are frequently
available in combination with other logging tools as a portable unit that
may be easily transported to sites with restricted access.
Formation porosity and density may be determined through the use of
neutron, sonic and gamma-gamma logs (Keys, 1968; Keys and MacCary, 1971;
Senger, 1985). The use of the neutron tool is generally accepted as an
indicator of moisture content (Keys, 1968). Wilson (1980) and Everett et
al. (1984) have pointed out limitations in using the neutron tool inside
plastic casing, in the presence of certain contaminants and in certain
geologic settings. Tool detector sizes are limited to 2-inch diameter wells
or greater and are available as portable units for remote field access.
Various types of caliper logs are used to maintain a continuous record
of well or borehole diameter that can be used to detect broken casings, the
location of fractures, solution development, washed-out horizons and
hydrated clays (Keys and MacCary, 1971; Mickam et al., 1984; DeLuca and
Buckley, 1985). Diameters are "sensed" through the use of multiple feeler
arms or bow springs. Calipers are available for borehole or well diameters
ranging from 1.65 inches to 30 inches.
Other borehole logging tools may be used to derive information about
the character of water in the borehole and the formation. Induction tools
are used to measure pore fluid conductivity (Taylor et al., 1985).
Selected resistivity tools with different formation penetration depths are
used to detect variations in pore fluids CKeys, 1968; Keys and MacCary,
1971; Kwader, 1985; Lindsay, 1985). Temperature logs have recently been
applied to the detection of anomalous fluid flow (Urban and Diment, 1985).
Induction, resistivity and temperature logging tools have been designed to
fit 2-inch diameter or larger monitoring wells.
Flowmeters are used to monitor fluid rates in cased or uncased holes.
This tool provides direct ground-water flow measurement profiling.
Flowmeters can also be used to detect thief zones, lost circulation zones
and the location of holes in casing. Flowmeters measure flow using low
inertia impellers or through changes in thermal conductance as liquids pass
through the tool (Kerfoot, 1982). Many professionals remain unconvinced,
however, as to the effectiveness of flowmeters. Impeller flowmeters are
available as small as 1.65 inches in diameter; conductance flowmeters are
typically 1.75 inches in diameter.
Some uncertainty exists in the application of almost all borehole
equipment including geophysical logs. The correct interpretation of all
such data often depends on precise knowledge of geologic and hydrogeologic
conditions that are frequently not available. Therefore the interpretation
of these data are invariably subjective.
Downhole television cameras can be used to gather in-situ information
on boreholes and monitoring wells (Huber, 1982; Morahan and Doorier, 1984).
Television logging may be used to check monitoring well integrity (i.e.,
50
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casing and screen damage), to inspect installation and construction
procedures and to accurately characterize subsurface fractures and geologic
strata. Borehole television cameras have recently become available for
wells as small as 2 inches in diameter. Cameras are available that provide
multi-angle viewing, black/white or color images and recorded depth data
during imaging.
Many of the logging tools discussed in this section are available as
either combination probes or single probes. These tools have been designed
so that they can be run from truck mounted winches and loggers or from
portable units that can be transported by backpack to sites where vehicular
access is restricted. In addition, a variety of portable data loggers are
available to record logging data gathered onsite.
Water-Level Measuring Devices
The basic water-level measuring device is a steel tape typically
coated with ordinary carpenter's chalk. This is the simplest water-level
measuring device and is considered by many to be the most accurate device
at moderate depths. In addition to a standard steel tape, the five main
types of water-level measuring devices are: 1) float-type, 2) pressure
transducers, 3) acoustic probes, 4) electric sensors and 5) air lines.
Float-type devices rest on the water surface and may provide a continuous
record of water levels on drum pen recorders or data loggers. Float sizes
range from 1.6 inches to 6.0 inches in diameter, but are only recommended
for wells greater than 4 inches in diameter due to loss of sensitivity in
smaller diameter boreholes. Pressure transducers are suspended in the well
on a cable and measure height of water abdve the transducer center.
Transducers are available in diameters as small as 0.75 inches. Acoustic
well probes use the reflective properties of sound waves to calculate the
distance from the probe at the wellhead to the water surface. Acoustic
probes are designed for well diameters as small as 4 inches and are limited
to water depths greater than 25 feet (Ritchey, 1986). Electric sensors are
suspended on the end of a marked cable. When the sensor encounters
conductive fluid, the circuit is completed and an audible or visual signal
is displayed at the surface. Air lines are installed at a known depth
beneath the water; and by measuring the pressure of air necessary to
discharge water from the tube, the height of the water column above the
discharge point can be determined.
Steel tapes coated with a substance that changes color when wetted are
also used as water-level measuring devices (Garber and Koopman, 1968).
Tapes are available as small as 0.75 inches in width. Specially coated
tape with physical and chemical resistance has recently been developed that
is 0.375 inches in width and contains electrical conductance probes at the
end of the tape to sense water levels (Sanders, 1984).
Ground-Water Sampling Devices
A wide variety of ground-water sampling devices are available to meet
the requirements of a ground-water monitoring program. A discussion of the
advantages and disadvantages of sampling devices is provided by Barcelona
51
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et al. (1983) and (1985a), Nielsen and Yeates (1985) and Bryden et al.
(1986).
Bailers are the simplest of the sampling devices commonly used for
ground-water sampling. They can be constructed from a variety of materials
including polytetrafluorethylene (PTFE), polyvinyl chloride (PVC) and
stainless steel. Diameters of 0.5 inches or larger are common. Because
bailers are lowered by hand or winch, the maximum sampling depth is limited
by the strength of the winch and the time required for bailing.
Grab samplers such as Kemmerer samplers can be used to collect samples
from discrete sampling depths. These samplers can be constructed from a
variety of materials and can be manufactured to fit in wells with 0.5-inch
diameter or larger.
Syringe samplers allow for depth discrete sampling at unlimited depths
while reducing effects on sample integrity (Nielsen and Yeates, 1985).
Syringe samplers have been constructed from stainless steel, PTFE and
polyethylene/glass with various modifications (Gillham, 1982). These
samplers may be utilized in wells with a casing diameter 1.5 inches or
larger.
Suction lift or vacuum pumps include both centrifugal and peristaltic
pumps. These types of pumps are limited to sampling depths of less than
25 feet. However, they can be utilized in wells of 0.5-inch diameter or
larger.
Gas drive samplers can be used in wells with a casing diameter of 0.75
inches or larger. These samplers operate on the principal of applied gas
pressure to open/close check valves and deliver samples to the surface
(Robin et al., 1982; Norman, 1986). Sampling depth is limited by the
internal working strength of the tubing used in sampler construction.
Positive displacement bladder pumps can b« constructed of various
inert materials for wells with a diameter of 1 5 inches or larger. The use
of pressurized bladders ensures that the saaple does not contact the
driving gas. Most bladder pumps are capable of lifting samples from 300 to
400 feet, although models capable of 1000 feat of lift have been recently
advertised.
Both gear-drive and helical rotor submersible pumps have been
developed for wells with a casing diameter of at least 2 inches. These
pumps are capable of lifts of up to at least ISO feet. Submersible
gas-driven piston pumps have been developed that operate on compressed air
or bottled gas without contact of the sample with the air. These pumps are
available for 1.5 and 2-inch diameter monitoring wells and have pumping
lifts from 0 to 1000 feet. All of these types of pumps can be constructed
from various inert materials and may provide continuous, but variable flow
rates to minimize degassing of the sample.
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Aquifer Testing Procedures
The diameter, location, depth, and screened interval of a monitoring
well should be chosen based on the need for and the type of aquifer testing
procedures that will be performed on the well. Observation wells generally
do not have to be designed with the same diameter criteria in mind. The
type of aquifer testing procedure should be based on the hydraulic
characteristics of the aquifer such as transmissivity, storage coefficient,
homogeneity and areal extent.
Pumping tests are typically performed in wells with a high
transmissivity and in wells with a diameter large enough to accommodate the
pumping equipment. Conversely, slug injection or recovery tests, that add
or remove smaller amounts of water, are typically performed in formations
with low transmissivity and in smaller diameter wells. Packer tests can be
conducted in wells as small as 2 inches in diameter, but the optimum well
diameter for packer testing is 4 inches. Bailer tests to evaluate aquifer
characteristics can be performed in wells of all diameters. Tracer tests
are also used to evaluate aquifer characteristics and can be performed
regardless of well diameter.
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REFERENCES
Anderson, M.P., 1984. Movement of contaminants in ground water: ground
water transport-advection and dispersion; Ground-Water Contamination,
Studies in Geophysics; National Academy Press, Washington, D.C., 179 pp.
Andres, K.G. and R. Canace, 1984. Use of the electrical resistivity
technique to delineate a hydrocarbon spill in the coastal plain deposits of
New Jersey; Proceedings of the NWWA/API Conference on Petroleum
Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection
and Restoration; National Water Well Association, Dublin, Ohio, pp.
188-197.
Barcelona, M.J., J.P. Gibb, J.A. Helfrich and E.E. Garske, 1985a.
Practical guide for ground-water sampling; Illinois State Water Survey, SWS
Contract Report 374, Champaign, Illinois, 93 pp.
Barcelona, M.J., J.P. Gibb and R. Miller, 1983. A guide to the selection
of materials for monitoring well construction and ground-water sampling;
Illinois State Water Survey, SWS Contract Report 327, Champaign, Illinois,
78 pp.
Bear, J., 1972. Dynamics of fluids in porous media; Elsevier, New York,
764 pp.
Beck, B.F., 1983. A common pitfall in the design of RCRA ground-water
monitoring programs; Ground Water, vol. 21, no. 4, pp. 488-489.
Blake, S.B. and R.A. Hall, 1984. Monitoring petroleum spills with wells:
some problems and solutions; Proceedings of the Fourth National Symposium
on Aquifer Restoration and Ground-Water Monitoring; National Water Well
Association, Dublin, Ohio, pp. 305-310.
Bryden, G.W., W.R. Mabey and K.M. Robine, 1986. Sampling for toxic
contaminants in ground water; Ground-Water Monitoring Review, vol. 6, no.
2, pp. 67-72.
Cherry, J.A., R.W. Gillhara and J.F. Barker, 1984. Contaminants in ground
water: chemical processes; Ground-Water Contamination, Studies in
Geophysics; National Academy Press, Washington, D.C., 179 pp.
Deluca, R.J. and B.K. Buckley, 1985. Borehole logging to delineate
fractures in a contaminated bedrock aquifer; Proceedings of the NWWA
Conference on Surface and Borehole Geophysical Methods in Ground-Water
Investigations; National Water Well Association, Dublin, Ohio, pp. 387-397.
54
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Everett, L.G., L.G. Wilson and E.W. Hoylman, 1984. Vadose zone monitoring
for hazardous waste sites; Noyes Data Corporation, Park Ridge, New Jersey,
360 pp.
Freeze, R. A. and J.A. Cherry, 1979. Ground Water; Prentice-Hall, Inc.,
Englewood Cliffs, New Jersey, 604 pp.
Garber, M.S. and F.C. Koopman, 1968. Methods of measuring water levels in
deep wells; Techniques of Water-Resources Investigations of the United
States Geological Survey, Book 8, Instrumentation; United States Government
Printing Office, Washington, D.C., 23 pp.
Gillham, R.W., 1982. Syringe devices for ground-water sampling; Ground-
Water Monitoring Review, vol. 2, no. 2, pp. 36-39.
Gillham, R.W., M.J.L. Robin, J.F. Barker and J.A. Cherry, 1983.
Ground-water monitoring and sample bias; API Publication 4367,
Environmental Affairs Department, American Petroleum Institute, Washington
D.C., 206 pp. '
Guswa, J.H., 1984. Application of multi-phase flow theory at a chemical
waste landfill, Niagra Falls, New York; Proceedings of the Second
International Conference on Ground-Water Quality Research; Oklahoma State
University Printing Services, Stillwater, Oklahoma, pp. 108-111.
Heath, R.C., 1984. Ground-water regions of the United States; United
States Geological Survey Water Supply Paper 2242; Superintendent of
Documents, United States Government Printing Office, Washington, D.C.,
78 pp.
Hinchee, R.E. and H.J. Reisinger, 1985. Multi-phase transport of petroleum
hydrocarbons in the subsurface environment: theory and practical
application; Proceedings of the NWA/API Conference on Petroleum
Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection
and Restoration; National Water Well Association, Dublin, Ohio, pp. 58-76.
Huber, W.F., 1982. The use of downhole television in monitoring
applications; Proceedings of the Second National Symposium on Aquifer
Restoration and Ground-Water Monitoring; National Water Well Association,
Dublin, Ohio, pp. 285-286.
Keely, J.F., 1986. Ground-water contamination assessments; Ph.D.
dissertation, Oklahoma State University, Stillwater, Oklahoma, 408 pp.
Keely, J.F. and C.F. Tsang, 1983. Velocity plots and capture zones of
pumping centers for ground-water investigations; Ground Water, vol. 21, no.
6, pp. 701-714.
Kerfoot, W.B., 1982. Comparison of 2-D and 3-D ground-water flowmeter
probes in fully penetrating monitoring wells; Proceedings of the Second
National Symposium on Aquifer Restoration and Ground-Water Monitoring;
National Water Well Association, Dublin, Ohio, pp. 264-268.
55
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Keys, W.S., 1968. Well logging in ground-water hydrology; Ground Water
vol. 6, no. 1, pp. 10-18.
Keys, W.S. and L.M. MacCary, 1971. Application of borehole geophysics to
water-resources investigations, Book 2; United States Department of the
Interior, Washington, D.C., 126 pp.
Kovski, J.R., 1984. Physical transport process for hydrocarbons in the
subsurface; Proceedings of the Second International Conference on Ground
Water Quality Research; Oklahoma State University Printing Services,
Stillwater, Oklahoma, pp. 127-128.
Kwader, T., 1985. Resistivity-porosity cross plots for determining in situ
formation water-quality case examples; Proceedings of the NWWA Conference
on Surface and Borehole Geophysical Methods in Ground-Water Investigations;
National Water Well Association, Dublin, Ohio, pp. 415-424.
Lindsey, G.P., 1985. Dry hole resistivity logging; Proceedings of the NWWA
Conference on Surface and Borehole Geophysical Methods in Ground-Water
Investigations; National Water Well Association, Dublin, Ohio, pp. 371-376.
Lithland, S.T., T.W. Hoskins and R.L. Boggess, 1985. A new ground-water
survey tool: the combined cone penetrometer/vadose zone vapor probe;
Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons and
Organic Chemicals in Ground Water: Prevention, Detection and Restoration;
National Water Well Association, Dublin, Ohio, pp. 322-330.
Mabey, W.R. and T. Mill, 1984. Chemical transformation in ground water;
Proceedings of the Second International Conference on Ground-Water Quality
Research; Oklahoma State University Printing Services, Stillwater,
Oklahoma, pp. 61-64.
Mackay, D.M., P.V. Roberts and J.A. Cherry, 1985. Transport of organic
contaminants in ground water; Environmental Science & Technology, vol. 19,
no. 5, pp. 384-392.
Marrin, D.L. and G.M. Thompson, 1984. Remote detection of volatile organic
contaminants in ground water via shallow soil gas sampling; Proceedings of
the NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in
Ground Water: Prevention, Detection and Restoration; National Water Well
Association, Dublin, Ohio, pp. 172-187.
McCarty, P.L., M. Reinhard and B.E. Rittmann, 1981. Trace organics in
ground water; Environmental Science & Technology, vol. 15, no. 1, pp.
40-51.
McCarty, P.L., B.E. Rittman and E.J. Bouwer, 1984. Microbiological
processes affecting chemical transformation in ground water; Ground-Water
Pollution Microbiology, G. Bitton and C.D. Gerba, editors, Wiley and Sons,
New York, pp. 90-115.
56
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Mickam, J.T., B.S. Levy and G.W. Lee, 1984. Surface and borehole
geophysical methods in ground-water investigations; Ground-Water Monitoring
Review, vol. 4, no. 4, pp. 167-171.
Morahan, T. and R.C. Doorier, 1984. The application of television borehole
logging to ground-water monitoring programs; Ground-Water Monitoring
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Nielsen, D.M. and G.L. Yeates, 1985. A comparison of sampling mechanisms
available for small-diameter ground-water monitoring wells; Proceedings of
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Ground-Water Monitoring; National Water Well Association, Dublin, Ohio, pp
237-270.
Noel, M.R., R.C. Benson and P.M. Beam, 1983. Advances in mapping organic
contamination: alternative solutions to a complex problem; National
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D.C.; Hazardous Materials Control Research Institute, Silver Spring,
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sampling device; Ground-Water Monitoring Review, vol. 6, no. 2, pp. 56-60.
Office of Technology Assessment, 1984. Protecting the nation's ground
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Pettyjohn, W.A., 1982. Cause and effect of cyclic changes in ground-water
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distribution of selected trace organics in the leachate plume of a
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108-113.
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devices; Ground-Water Monitoring Review, vol. 2, no. 1, pp. 63-65.
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57
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Saunders, W.R. and R.M. Germeroth, 1985. Electromagnetic measurements for
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Geophysical Methods in Ground-Water Investigations; National Water Well
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ground-water monitoring; Proceedings of the Second National Symposium on
Aquifer Restoration and Ground-Water Monitoring; National Water Well
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contaminants in the subsurface; Ground-Water Quality, C.H. Ward, W. Giger
and P.L. McCarty, editors; John Wiley and Sons, New York, 547 pp.
Wilson, L.G., 1980. Monitoring in the vadose zone: a review of technical
elements and methods; U.S. Environmental Protection Agency Publication No.
600/7-80-134, 168 pp.
Yaniga, P.M. and J.G. Warburton, 1964. Discrimination between real and
apparent accumulation of Immiscible hydrocarbons on the water table: a
theoretical and empirical analysis; Proceedings of the Fourth National
Symposium on Aquifer Restoration and Ground-Water Monitoring; National
Water Well Association, Dublin, Ohio, pp. 311-315.
58
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SECTION 3
MONITORING WELL PLANNING CONSIDERATIONS
RECORDKEEPING
The development of an accurate recordkeeping process to document the
construction, installation, sampling and maintenance phases of a monitoring
well network plays an integral part in determining the overall success of
the program. An accurate account of all phases is necessary to ensure that
the goals of the monitoring program (i.e. accurate characterization of the
subsurface hydrogeology and representative water-quality samples, etc.) are
met. It is from these records that information will be used to resolve any
future monitoring problems that will be encountered.
Recordkeeping begins with the drilling of the monitoring well.
Complete documentation of the drilling and/or sampling process should be
accurately recorded in a field notebook and transferred to a boring log.
Notations about weather, drilling equipment, personnel on the site,
sampling techniques, subsurface geology and hydrogeology should be
recorded. Lithologic descriptions should be based on visual examination of
the cuttings and samples and confirmed with laboratory analyses where
appropriate. The Unified Soil Classification System is one universally
accepted method of soil description. In the Unified Soil Classification
System, soils are designated by particle size and moisture content.
A description of the system can be found in a publication by the United
States Department of Interior (1974). Identification and classification of
rock should include typical rock name, notations on pertinent lithology,
structural features and physical alterations. Although there is no
universally accepted system for describing rock, one system is described by
Williamson (1984). A list of information that should be recorded in the
field notebook is contained in Table 4. Information in the field notebook
is transferred to the boring log for clarity of presentation. Figure 20
illustrates the format for a sample boring log. Both the boring log and
the field notes become part of the permanent file for the well.
In addition to the boring log, an "as-built" construction diagram
should be drawn for each well. This differs from a "typical monitoring
well" diagram contained within the design specifications because the "as-
built" diagram contains specific construction information about the
materials and depths of the well components. An "as-built" diagram
eliminates confusion if the monitoring well was not built exactly as
conceived in the design specifications. In addition, the drawing provides
an "at-a-glance" picture of how the well is constructed (similar to the
59
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TABLE 4. DESCRIPTIVE INFORMATION TO BE RECORDED FOR EACH MONITORING WELL
General Information
Bonng number
Date/time to start and finish well
Location of well (include sketch of location)
Elevation of ground surface
Weather conditions during drilling
Name of driller, geologist and other personnel on site
DiNIng Information
Type o1 drilling equipment
Type and design of dnll bit
Any uniting fluid used
Diameter ol drill bit
Diameter ol hole
Penetration ratedunng dnllmg (reel/minute, minutes/foot, feel/hour, etc}
Depth to water encountered dunng dnihng
Depth to standing water
Soil/rock classification and dexnptmn
Total well depth
Remarks on miscellaneous dnihng conditions, including
a) loss or gain of fluid
b) occurrence of boulders
c) cavities or voids
d) borenow conditions
e) changes in color of formation samples or fluid
f) odors while dnlhng
Sampling Information
Types of samplers) used
Diameter and length of Hmpier(s)
Number of each sample
Stan and finish depth of each sample
Split spoon sampling
a) size and weight of dnve hammer
b) number of ttows required tor penetration of 6 inches
c) free tall distance used to drive sampler
Thin-walled sampling-
a) relative ease or difficulty of pushing sample OR
b) pounds per square men (psi) necessary to push sample
Rock cores
a) core barrel dnll bit design
b| penetration rate (feet/minute, minutes/tool, feet/hour, etc)
Percent of sample recovered
Well Completion Infotmfltlon
Elevation ol lop ot casing (± 01 fool)
Casing
a) matenal
b) diameter
c) total length of casing
d) depth below ground surf ace
e| how sections joined
<) end cap (yes or no)
Screen
a) matenal
t» diameter
c) slot size and length
d) depth to top and bottom ol screen
Filter pack
a) type/sue
b) volume empiaced (calculated and actual)
c) depth to top ot filter pack
d) source and roundness
e) method of emplacement
Grout and/or Sealant
a) composition
b) method of emplacement
c] volume empiaced (where applicable) (calculated and actual)
d) depth ot grouted interval (top and bottom)
Backfill matenal.
a) depth of backfilled interval (top and bottom)
b) type ot matenal
Surface seal detail
a) type of seal
b) depth of seal (must be below frost depth)
Well protector
a) type
b) locking device
c) vents (yes or no)
Woll davatopfnant
a) mothod
b) data/time, start/stop
c) volume and source water (if used)
-------
BORING NO
2A
SH 1 OF __L
PROJECT AML Manufacturing
BORING LOG
LOCATION Sussex County
CASING I.D. 4?5"
CONTRACTOR Sprowls & Sons
DATE START Aug. 30. 1987 FINISH Aug. 31 1987
GROUND ELEV 33709' TOTAL DEPTH (FT) 23.50'
. CORE SIZE _ NX
TYPE Air Rotary w/Casing Hammer
LOGGED BY
S. Smith
SCALE ' LITHOLOGIC
IN SYMBOL
FKT
SAMPLE
MPTH
AND NO. I ORREC i "*"«
l^T 1 —
ROD
RATE
OF PEN
MIN/F7.
SOIL AND ROCK DESCRIPTION/COMMENTS
' Unified SO" ciass system fiocn aescnonon Deotn to *aier
table Loss 01 dmi tiuia. etc i
10'
0.5
SS-1 45-29 5.0'
-36 6.5'
70
SS-J
4345
-5>
10.0'-
11.5'
0.7
Cora
Breaks
Gravelly SILT little sand trace clay
About 30°'o oeooies ano granules
Moderately moist Mooerate yellowisn crown
HOVR5/4 mottlefl 5Y5/2) arab Till
[GM|
Gravelley SILT httie sana. trace clay
About 30% pebbles ana granules Dry to sligntly
moist Mfyiprate yellowish brown HO YR5/4),
drab Till
JGM|
15'
20'
NX-1
NX-2
Medium oarK gray lo flam gray SILTSTONE. sanay
SILTSTONE. with minor shale seams Fresh ana
hard except at breaks along slightly to moderately
weathered snaie seams Jointed and broken approx-
imately as depicted Coauma seam us V-1S2') very
calcareous Generally only calcareous in sandy
SILTSTONE layers Wet @>17 5'
Medium aark gray lo aarx gray SILTSTONE. sanoy
SILTSTONE ano minor snaie seams same as above
DfBo
25' -
Piezometer 2A installed with screened interval of
180'to 230'
Overburden
Rock
13.0'
10.5'
| Water Level
i Date
Time
Total Depth 2350'
Comments Surface casing driven 8" Into rock.
16.2'
8/30/87
1 00 p.m
1635'
8/30/87
3 00 p.m.
Elevation Measuring Point i Top of Casing Top of Casing
Figure 20. Sample boring log format (After Electric Power Research Institute, 1985).
61
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function of a boring log). The "as-built" diagram should contain
information about the elevation, depth and materials used in well
construction. Figure 21 illustrates the format for an "as-built" diagram
of a monitoring well.
Finally, records should be kept for each well illustrating not only
the construction details for the well, but also a complete history of
actions related to the well. These include: 1) dates and notations of
physical observations about the well, 2) notations about suspected problems
with the well, 3) water-level measurements, 4) dates of sample collection
(including type of sampler, notations about sample collection and results
of laboratory analyses), 5) dates and procedures of well maintenance and 6)
date, method and materials used for abandonment. This record becomes part
of a permanent file that is maintained for each well.
DECONTAMINATION
Decontamination of drilling and format ion-sampling equipment is a
quality-control measure that is often required during drilling and
installation of ground-water monitoring wells. Decontamination is the
process of neutralizing, washing and rinsing equipment that comes in
contact with formation material or ground water that is known or is
suspected of being contaminated. Contaminated material that adheres to the
surface of drilling and formation sampling equipment may be transferred via
the equipment: 1) from one borehole to another and/or 2) vertically within
an individual borehole from a contaminated to an uncontaminated zone. The
purpose for cleaning equipment is to prevent this "cross-contamination"
between boreholes or between vertical zones within a borehole. Although
decontamination is typically used where contamination exists,
decontamination measures are also employed in uncontaminated areas as a
quality control measure.
Planning a decontamination program for drilling and formation sampling
equipment requires consideration of:
1) the location where the decontamination procedures will
be conducted, if different from the actual drilling site;
2) the types of equipment that will require decontamination;
3) the frequency that specific equipment will require
decontaminat ion;
4) the cleaning technique and type of cleaning solutions and/or
wash water needed for decontamination;
5) the method for containing the residual contaminants and
cleaning solutions and/or wash water from the decontamination
process, where necessary; and
6) the use of a quality control measure, such as equipment
blanks or wipe testing, to determine the effectiveness
of the decontamination procedure, if appropriate.
The degree to which each of these items are considered when developing a
decontamination program varies with the level of contamination anticipated
at the site. Where the site is "clean," decontamination efforts may simply
consist of rinsing drilling and formation sampling equipment with water
between samples and/or boreholes. As the level of anticipated or actual
62
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Well Number 7H
start 8/13/87 8:00a.m -1 00pm
Finish 8/14/87 10a.m. -I2:00pm
Vented cap Drilling Method Hollow Stem Auger
Steel (Schedule 40) Protective Casing with Mmgeo Cao
Master LOCK »632
m Concrete Pad (mm 4" thick on undisturbed or compacted soil)
Elevation 856 03 feet
2" PVC Casing iScneauie 40 tiusn |omt. threaded)
260'
280'
• 0
1.'
1 . •
.• *
~ o
• •',
•/,
It::
• » . '
'•*.
.. .
Silica Fine-grained Sand (Mortar Sand)
2" PVC Well Screen witn ooio men slot open
Filter Pack (Clean Medium to Coarse Silica Sand)
2" PVC Casing
102S"
Figure 21. Format for an 'as-built" monitoring well diagram.
63
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contamination increases, so should the decontamination effort. A document
by the United States Environmental Protection Agency (1987) discusses
decontamination at CERCLA sites.
One important factor when designing a decontamination program is the
type of contaminant(s). The greater the toxicity or the more life-
threatening the contaminant, the more extensive and thorough the
decontamination program must be. The following discussion focuses on
measures to be employed at sites where contamination is known or suspected
or decontamination is desired as a quality control measure. Less formally
defined decontamination efforts may be employed at any site.
Decontamination Area
An appropriate decontamination area at a site is selected based on the
ability to: 1) control access to the decontamination area, 2) control or
contain residual material removed from the surfaces of the drilling and
formation sampling equipment and 3) store clean equipment to prevent
recontamination before use. In addition, the decontamination area should
be located in close proximity to the drilling area to minimize further site
contamination. The importance of these considerations during the selection
process for a decontamination area will be influenced by the type of
contaminants involved and the extent of contamination at the site. For
example, the decontamination area for drilling and formation sampling
equipment may be located near the drilling rig when: 1) the ground surface
is regarded as noncontaminated, 2) the known or suspected subsurface
contaminants are non-hazardous and 3) the drilling method permits good
control over the containment of cuttings from the borehole. However, the
decontamination area should be located an 'adequate distance away from the
rig to avoid contamination of clean equipment by airborne lubricating oil
or hydraulic fluids from the drilling rig. Once drilling and sampling
equipment is cleaned, the equipment should not be placed directly on the
ground surface even though the area is generally regarded as
noncontaminated. Clean equipment should be placed, at a minimum, on top of
plastic ground sheeting, and the sheeting should be discarded after each
borehole is drilled. Clean equipment may also be stored off the ground on
storage or equipment racks until used for drilling or formation sampling.
Heavy equipment, such as the drilling rig, water truck or any other support
vehicle, should be cleaned in the decontamination area prior to
demobilizing from the site.
The presence of hazardous materials at a drilling site dictates that a
more controlled access area be established for equipment decontamination to
prevent cross-contamination and to provide worker safety. Figure 22 shows
a general layout of a contaminant reduction zone where systematic
decontamination procedures are employed as personnel and equipment move
from the hazardous material exclusion zone to a clean, non-hazardous
support zone.
64
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Heavy Eauipment
Decontamination
Area
EXCLUSION
ZONE
—x *~T~ *&& *Q- x—i— X—
r<
; a.
o
SUPPORT ZONE
CONTAMINATION
REDUCTION ZONE
>— *—
N
ME
i
) 0-
i
E»
Pat
ImilMllUM
1 =
u
I
-ofi
t
n
'
i 1
|3
J-O
r* —
LEGEND
HOTLINE
O-O- CONTAMINATION
CONTROL LINE
ACCESS CONTROL
POINT • ENTRANCE
ACCESS CONTROL
POINT* EXIT
-O O-
i Dreuout |
i Afea !
U— ____—J
^
T
Entry
Patrt
Redress
I Area I
' J
Figure 22. Typical layout showing decontamination areas at a hazardous materials site (United States
Environmental Protection Agency, 1984).
65
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Types of Equipment
Decontamination of drilling and formation sampling equipment involves
cleaning tools used in the borehole. This equipment includes drill bits,
auger sections, drill-string tools, drill rods, split-barrel or thin-wall
tube samplers, bailers, tremie pipes, clamps, hand tools and steel cable.
Equipment with a porous surface, such as natural rope, cloth hoses and
wooden blocks or handles, cannot be thoroughly decontaminated and should be
disposed of properly after completion of the borehole. The specific
drilling and formation sampling equipment that needs to be cleaned should
be listed in the equipment decontamination program.
A decontamination program for equipment should also include cleaning
heavy equipment, including the drill rig and support trucks. Advanced
planning is necessary to ensure that the decontamination area is adequately
sized to accommodate large vehicles, and that any contaminants removed from
the vehicles are properly controlled and contained within the
decontamination area. This should include the "tracking zone" created by
vehicles as they move into and out of the area.
Frequency of Equipment Decontamination
A decontamination program for equipment should detail the frequency
that drilling and formation sampling equipment is to be cleaned. For
example, drilling equipment should be decontaminated between boreholes.
This frequency of cleaning is designed to prevent cross-contamination from
one borehole to the next. However, drilling equipment may require more
frequent cleaning to prevent cross-contamination between vertical zones
within a single borehole. Where drilling'equipment is used to drill
through a shallow contaminated zone and to install surface casing to
seal-off the contaminated zone, the drilling tools should be decontaminated
prior to drilling deeper. Where possible, field work should be initiated
by drilling in that portion of the site where the least contamination is
suspected.
Formation sampling equipment should be decontaminated between each
sampling event. If a sampling device is not adequately cleaned between
successive sampling depths, or between boreholes, contaminants may be
introduced into the successive sample(s) via the formation sampling device.
Cleaning Solutions and/or Wash Water
Decontamination of equipment can be accomplished using a variety of
techniques and fluids. The most common and generally preferred methods of
equipment decontamination involve either a clean potable water wash, steam
cleaning or water/wash steam cleaning combination. Water washing may be
accomplished using either low or high pressure. If a low pressure wash is
used, it may be necessary to dislodge residual material from the equipment
with a brush to ensure complete decontamination. Steam cleaning is
accomplished using portable, high-pressure steam cleaners equipped with
pressure hose and fittings.
66
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Sometimes solutions other than water or steam are used for equipment
decontamination. Table 5 lists some of the chemicals and solution
strengths that have been used in equipment decontamination programs. One
commonly used cleaning solution is a non-phosphate detergent. Detergents
are preferred over other cleaning solutions because the detergent alone
does not pose a handling or disposal problem. In general, when a cleaning
solution for equipment decontamination is necessary, a non-phosphate
detergent should be used unless it is demonstrated that the environmental
contaminant in question cannot be removed from the surface of the equipment
by detergents.
TABLE 5. LIST OF SELECTED CLEANING SOLUTIONS USED FOR EQUIPMENT
DECONTAMINATION (MOBERLY, 1985)
Chemical Solution Uses/Remarks
Clean Potable Water None used unoer mgn pressure or steam to remove heavy mud etc . or to
rinse other solutions
Low-Sudsing Detergents Follow Manufacturer s General an-ourpose cleaner
(Aiconoxi Directions
Sodium Carbonate «/io Gal Water Effective tor "euiranzmg organic acids heavy metals metal
(Washing Soflai orocess-nq .astes
Sodium Bicaroonate 4»/io Gal Water used to -euirane enner base or neutral acio comammants
I Baking Soaai
Tnsodium Phosphate 2D/10 Gal Water S
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Containment of Residual Contaminants and Cleaning Solutions and/or Wash
Water
Contaminated material removed from the surfaces of equipment and
cleaning solutions and/or wash water used during decontamination usually
require containment and proper disposal. If non-hazardous contaminants are
involved, the decontamination program for equipment may not require
provisions for the disposal of wash water and residual material removed
from the equipment. Conversely, a decontamination program for equipment
exposed to hazardous materials requires provision for catchment and
disposal of the contaminated material, cleaning solution and/or wash water.
Where contaminated material and cleaning fluids must be contained from
heavy equipment such as drill rigs and support vehicles, the
decontamination area must be properly floored. Preferred flooring for the
decontamination area is typically a reinforced, curbed, concrete pad which
is sloped toward one corner where a sump pit is installed (Moberly, 1985).
Where a concrete pad is impractical, planking can be used to construct a
solid flooring that is then covered by a nonporous surface and sloped
toward a collection facility. Catchment of contaminants and cleaning
fluids from the decontamination of lighter-weight drilling equipment and
hand tools can be accomplished by using small trenches lined with plastic
sheeting or in wash tubs or stock cans. The contaminated cleaning fluids
can be stored temporarily in metal or plastic cans or drums until removed
from the site for proper disposal.
Effectiveness of Decontamination Procedures
A decontamination program for drilling and formation sampling
equipment may need to include quality-control procedures for measuring the
effectiveness of the cleaning methods. Quality-control measures typically
include either equipment blank collection or wipe testing. Equipment
blanks are samples of the final rinse water that are collected after
cleaning the equipment. Equipment blanks should be collected in
appropriate sampling containers, properly preserved, stored and transported
to a laboratory for analyses of contaminants known or suspected at the
site. Wipe testing is performed by wiping a cloth or paper patch over the
surface of the equipment after cleaning. The test patch is placed in a
sealed container and sent to a laboratory for analysis. Laboratory results
from either equipment blanks or wipe tests provide "after-the-fact"
information that may be used to evaluate whether or not the cleaning
methods were effective in removing the contaminants of concern at the site.
Personnel Decontamination
A decontamination program for drilling and sampling equipment is
typically developed in conjunction with health and safety plans for field
personnel working at the site. Although a discussion of site safety plans
and personnel protective measures are beyond the scope of this manual, the
health and safety plan for field personnel should be of foremost concern
when drilling in known or suspected contaminated areas. Specific health
and safety procedures necessary at the site depend on the toxicity and
68
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physical and chemical properties of known or suspected contaminants.
Where hazardous materials are involved or suspected, a site safety program
should be developed by a qualified professional in accordance with the
Occupational Safety and Health Administration requirements in 29 CFR
1910.120. Field personnel at hazardous sites should receive medical
screening and basic health and safety training, as well as specific on-site
training.
69
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REFERENCES
Electric Power Research Institute, 1985. Ground water manual for the
electric utility industry: groundwater investigations and mitigation
techniques, volume 3; Research Reports Center, Palo Alto, California,
360 pp.
Moberly, Richard L., 1985. Equipment decontamination; Ground Water Age,
vol. 19, no. 8, pp. 36-39.
United States Department of Interior, 1974. Earth manual, a water
resources technical publication; Bureau of Reclamation, United States
Government Printing Office, Washington, D.C., 810 pp.
United States Environmental Protection Agency, 1984. Standard operating
safety guides; United States Environmental Protection Agency Office of
Emergency Response, United States Government Printing Office, Washington,
D.C., 166 pp.
United States Environmental Protection Agency, 1987. A compendium of
Superfund field operations methods; United States Environmental Protection
Agency Publication No. 540/P-87/001, 644 pp.
Williamson, D.A., 1984. Unified classification system; Bulletin of
Engineering Geologists, vol. 21, no. 3, The Association of Engineering
Geologists, Lawrence, Kansas, pp. 345-354.
70
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SECTION 4
DESCRIPTION AND SELECTION OF DRILLING METHODS
INTRODUCTION
Monitoring wells can be, and have been, installed by nearly every
conceivable type of drilling and completion technique. However, every
drilling technology has a special range of conditions where the technique
is most effective in dealing with the inherent hydrogeologic conditions and
in fulfilling the purpose of the monitoring well. For example,
constructing wells by driving wellpoints or by jetting provides low-cost
water-level information but severely limits the ability to collect detailed
stratigraphic information.
The following section contains a description of common methods of
monitoring well construction and includes a discussion of the applications
and limitations of each technique. A matrix that helps the user determine
the most appropriate technology for monitoring well installation in a
variety of hydrogeologic settings with specific design objectives is also
included in this section.
DRILLING METHODS FOR MONITORING WELL INSTALLATION
Hand Augers
Hand augers may be used to install shallow monitoring wells (0 to IS
feet in depth) with casing diameters of 2 inches or less. A typical hand
auger, as shown in Figure 23, cuts a hole that ranges from 3 to 9 inches in
diameter. The auger is advanced by turning into the soil until the auger
is filled. The auger is then removed and the sample is dumped from the
auger. Motorized units for one- or two-operators are available.
Generally, the borehole cannot be advanced below the water table
because the borehole collapses. It is often possible to stabilize the
borehole below the water table by adding water, with or without drilling
mud additives. The auger may then be advanced a few feet into a shallow
aquifer and a well intake and casing installed. Another option to overcome
borehole collapse below the water table is to drive a wellpoint into the
augered hole and thereby advance the wellpoint below the water table. The
wellpoint can then be used to measure water levels and to provide access
for water-quality samples.
Better formation samples may sometimes be obtained by reducing the
hole size one or more times while augering to the desired depth. Because
71
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Figure 23. Diagram of a hand auger.
72
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the head of the auger is removable, the borehole diameter can be reduced by
using smaller diameter auger heads. Shaft extensions are usually added in
3- or 4-foot increments. As the borehole size decreases, the amount of
energy required to turn the auger is also reduced. Where necessary, short
sections of lightweight casing can be installed to prevent upper material
from caving into the borehole.
A more complete list of the applications and limitations of hand
augers is found in Table 6.
TABLE 6. APPLICATIONS AND LIMITATIONS OF HAND AUGERS
Application
Shallow soils investigations
Soil samples
Water-bearing zone identification
Piezometer, lysimeter and small-diameter monitoring well
installation
Labor intensive, therefore applicable when labor is inexpensive
No casing material restrictions
Limitations
• Limited to very shallow aeptts
• Unable to penetrate extremely dense or rocky
soil
• Borehole stability difficult to maintain
• Labor intensive
Driven Wells
Driven wells consist of a wellpoint (screen) that is attached to the
bottom of a casing (Figure 24). Wellpoints and casing are usually 1.25 to
2 inches in diameter and are made of steel to withstand the driving
process. The connection between the wellpoint and the casing is made
either by welding or using drive couplings.' Drive couplings are specially
designed to withstand the force of the blows used to drive the casing;
however, if the casing is overdriven it will usually fail at a coupling.
When constructing a well, a drive cap is placed on top of the uppermost
section of casing, and the screen and casing are driven into the ground.
New sections of drive casing are usually attached in 4 or 5-foot sections
as the well is driven deeper. Crude stratigraphic information can be
obtained by recording the number of blows per foot of penetration as the
wellpoint is driven.
Wellpoints can either be driven by hand or with heavy drive heads
mounted on a tripod, stiff-leg derrick or similar hoisting device. When
driven by hand, a weighted drive sleeve such as is used to install fence
posts is typically used. Depths up to 30 feet can be achieved by hand in
sands or sand and gravel with thin clay seams; greater depths of SO feet or
more are possible with hammers up to 1000 pounds in weight. Driving
through dense silts and clays and/or bouldery silts and clays is often
extremely difficult or impossible. In the coarser materials, penetration
is frequently terminated by boulders. Additionally, if the wellpoint is
not structurally strong it may be destroyed by driving in dense soils or by
encountering boulders. When driving the wellpoint through silts and/or
clays the screen openings in the wellpoint may become plugged. The screen
may be very difficult to clean or to reopen during development,
particularly if the screen is placed in a low permeability zone.
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Drive cap
- Coupling
• Casing
. Coupling
• Screen
• Wellpomt
Figure 24. Diagram of a wdlpoinL
74
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To lessen penetration difficulties and screen clogging problems,
driven wells may be installed using a technique similar to that used in
cable tool drilling. A 4-inch casing (with only a drive shoe and no
wellpoint) may be driven to the targeted monitoring depth. As the casing
is driven, the inside of the casing is cleaned using a bailing technique.
With the casing still in the borehole, a wellpoint attached to an inner
string of casing is lowered into the borehole and the outer casing is
removed. As the casing is removed, the well must be properly sealed and
grouted. A second option can also be used to complete the well. With the
casing still in the borehole, a wellpoint with a packer at the top can be
lowered to the bottom of the casing. The casing is then pulled back to
expose the screen. The original casing remains in the borehole to complete
the well. Either of these completion techniques permit the installation of
thermoplastic or fluoropolymer in addition to steel as the screen material.
A more complete listing of the applications and limitations of driven
wells is found in Table 7.
TABLE 7. APPLICATIONS AND LIMITATIONS OF DRIVEN WELLS
Applications
Water-level monitoring in shallow tarnations
Water samples can oe collected
Oewatenng
Water supply
Low cost encourages multiple sampling points
• Depth limited to approximately SO leet (except in sanay
material)
Small diantetar casing
No sal samples
Steel casing interferes with some chemical analysis
Lack of stratigraphic detail creates uncertainty regarding
screened zones and/or cross eontumnMion
Cannot penetrate dense and/or some dry materials
No annular SSWGB tor completion procedures
Jet Percussion
In the jet-percussion drilling method, a wedge-shaped drill bit is
attached to the lower end of the drill pipe (Figure 25). Water is pumped
down the drill pipe under pressure and discharges through ports on each
side of the drill bit. The bit is alternately raised and dropped to loosen
unconsolidated materials or to break up rock at the bottom of the borehole.
Concomitantly, the drill pipe is rotated by hand, at the surface, to cut a
round and straight hole. The drilling fluid flows over the bit and up the
annular space between the drill pipe and the borehole wall. The drilling
fluid lubricates the bit, carries cuttings to the surface and deposits the
cuttings in a settling pit. The fluid is then recirculated down the drill
pipe.
In unconsolidated material the casing is advanced by a drive-block as
the borehole is deepened. If the casing is positioned near the bottom of
the borehole, good samples can be obtained as the cuttings are circulated
to the surface and stratigraphic variations can be identified. Where the
borehole is stable, the well can be drilled without simultaneously driving
the casing.
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Jetting pipe
Cuttings washed up annular space
Drilling fluid discharged through port in bit
Bit
Figure 25. Diagram of jet percussion drilling (After Speedster Division of Koehring Company, 1983).
76
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After the casing has been advanced to the desired monitoring depth, a
well intake can be installed by lowering through the casing. The casing is
then pulled-back to expose the well intake. Casing diameters of 4 inches
or less can be installed by jet percussion. Depths of wells are typically
less than 150 feet, although much greater depths have been attained. This
method is most effective in drilling unconsolidated sands.
A more complete listing of applications and limitations of jet-
percussion drilling is found in Table 8.
TABLE & APPLICATIONS AND LIMITATIONS OF JET PERCUSSION DRILLING
Applications
• Sample collection m form ot cuttings to surface
• Primary use in unconsoMated formations, but may be used m
some softer consolidated rock
• Best application is 4-mch borehole with Zinch casing and
screen insulted, scaled and grouted
UmttMom
• Drilling mud may be needed to return cuttings to surface
• Diameter limited to 4 inches
• installation slow m dense, boutdery clay/till or similar
formations
• Disturbance 01 the formation possible if Borehole not
caaeo immediately
Solid-Flight Augers
Solid-flight augers (i.e. solid-stem, solid-core or continuous flight
augers) are typically used in multiple sections to provide continuous
flighting. The first, or lowermost, flight is provided with a cutter head
that is approximately 2 inches larger in diameter than the flighting of the
augers (Figure 26). As the cutting head is advanced into the earth, the
cuttings are rotated upward to the surface by moving along the continuous
flighting.
The augers are rotated by a rotary drive head at the surface and
forced downward by a hydraulic pulldown or feed device. The individual
flights are typically 5 feet in length and ire connected by a variety of
pin, box and keylock combinations and devices. Where used for monitoring
well installation, available auger diameters typically range from 6 to Ik
inches in outside diameter. Many of the drilling rigs used for monitoring
well installation in stable unconsolidated material can reach depths of
approximately 70 feet with 14-inch augers and approximately 150 feet with
6-inch augers.
In stable soils, cuttings can sometimes be collected at the surface as
the material is rotated up the auger flights The sample being rotated to
the surface is often bypassed, however, by being pushed into the borehole
wall of the shallower formations. The sample often falls back into the
borehole along the annular opening and may not reach the surface until
thoroughly mixed with other materials. There is commonly no return of
samples to the surface after the first saturated zone has been encountered.
Samples may also be collected by carefully rotating the augers to the
desired depth, stopping auger rotation and removing the augers from the
77
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Auger connection
Flighting •
Cutler head
Rgure 26. Diagram of a solid-flight auger (after Central Mine Equipment Company, 1987).
78
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borehole. In a relatively stable formation, samples will be retained on
the auger flights as the augers are removed from the borehole. The inner
material is typically more representative of the formation at the drilled
depths and may be exposed by scraping the outer material away from the
sample on the augers. Because the borehole often caves after the saturated
zone is reached, samples collected below the water table are less reliable.
The borehole must be redrilled every time the augers are removed, and the
formation not yet drilled may be disturbed as the borehole above collapses.
This is particularly true in heaving formations.
Because the core of augers is solid steel, the only way to collect
"undisturbed" split-spoon or thin-wall samples is to remove the entire
string of augers from the borehole, insert the sampler on the end of the
drill rod, and put the entire string back into the borehole. This sampling
process becomes very tedious and expensive as the borehole gets deeper
because the complete string of augers must be removed and re- inserted each
time a sample is taken. Sampling subsequent to auger removal is only
possible if the walls of the borehole are sufficiently stable to prevent
collapse during sampling. Boreholes are generally not stable after even a
moderately thin saturated zone has been penetrated. This means that it is
usually not possible to obtain either split-spoon or thin-wall samples
after the shallowest water table is encountered.
The casing and well intake are also difficult to install after a
saturated zone has been penetrated. In this situation, it is sometimes
possible to auger to the top of a saturated zone, remove the solid augers
and then install a monitoring well by either driving, jetting or bailing a
well intake into position.
A more complete listing of the applications and limitations of
solid-flight augers is found in Table 9.
TABLE 9. APPLICATIONS AND LIMITATIONS OP SOLID-FLIGHT AUGERS
A m M|| , —.1 , _ -
Appncanoni
• Shallow soils investigations
• Soil samples
• Vadose zone monitoring wells (lysnneterst
• Monitoring wells in saturated, stable soils
e identification of depm to bedrock
e Fast and mobile
1 ImHmitnn m
UfiUIBUOfls
• Unacceptable son samples unless split-spoon or thin-wall
samples are taken
• Soil sample data limited to areas and depths where stable
soils are predominant
• Unafile to install monitoring wells in most unconsolidated
aquifers because ol borehole caving upon auger removal
• Oeptt capability decreases as diameter ot auger increases
• Monitoring well diameter limited by auger diameter
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Hollow-Stem Augers
Similar to solid-flight augers, hollow-stem auger drilling is
accomplished using a series of interconnected auger flights with a cutting
head at the lowermost end. As the augers are rotated and pressed downward,
the cuttings are rotated up the continuous flighting.
Unlike the solid-flight augers the center core of the auger is open in
the hollow-stem flights (Figure 27). Thus, as the augers are rotated and
pressed into the ground, the augers act as casing and stabilize the
borehole. Small-diameter drill rods and samplers can then be passed
through the hollow center of the augers for sampling. The casing and well
intake also can be installed without borehole collapse.
To collect the samples through hollow-stem augers, the augers are
first rotated and pressed to the desired sampling depth. The inside of the
hollow-stem is cleaned out, if necessary. The material inside the auger
can be removed by a spoon sampler with a retainer basket, jetting and/or
drilling with a bit attached to smaller-diameter drill rods. If the
jetting action is carried to the bottom of the augers, the material
immediately below the augers will be disturbed. Next, either a split-spoon
(ASTM 1586) or thin-wall (ASTM 1587) sampler is placed on the lower end of
the drill rods and lowered to the bottom of the borehole. The split-spoon
sampler can then be driven to collect a disturbed sample or the thin-wall
sampler can be pressed to collect an "undisturbed" sample from the strata
immediately below the cutting head of the auger. Samples can either be
taken continuously or at selected intervals. If sampling is continuous,
the augers are rotated down to the bottom of the previously-sampled strata
and cleaned out if necessary. The sampler is then re-inserted through the
auger and advanced into the undisturbed sediments ahead of the auger.
With the augers acting as casing and with access to the bottom of the
borehole through the hollow stem, it is possible to drill below the top of
the saturated zone. When the saturated zone is penetrated, finely-ground
material and water may mix to form a mud that coats the borehole wall.
This "mud plaster" may seal water-bearing zones and minimize inter-zonal
cross connection. This sealing is uncontrolled and unpredictable because
it depends on: 1) the quality of the silt/clay seal, 2) the differential
hydrostatic pressure between the zones and 3) the transmissivity of the
zones. Therefore, where possible cross contamination is a concern, the
seal developed during augering cannot be relied upon to prevent cross
contamination. One other potential source of cross contamination is
through leakage into or out of the augers at the flighting joints. This
leakage can be minimized by installing o-ring seals at the joints
connecting the flights.
While drilling with hollow-stem augers with the center of the stem
open, formation material can rise into the hollow stem as the auger is
advanced. This material must be cleaned out of the auger before formation
samples are collected. To prevent intrusion of material while drilling,
hollow-stem auger boreholes can be drilled with a center plug that is
installed on the bottom of the drill rods and inserted during drilling. A
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Drive Cap
Pilot Assembly
Components
Center Plug
Pilot Bit
Rod 10 Cap
Adapter
Auger Connector
Hollow Stem
Auger Section
Center Roo
Auger Connector
Auger Head
\X** Replaceable
^0 Carbide insert
Auger Tooth
Figure 27. Typical components of a hollow-stem auger (After Central Mine Equipment
Company, 1987).
81
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snail drag bit may also be added to prevent intrusion into the hollow stem.
An additional discussion on drilling with hollow-stem augers can be found
in Appendix A entitled, "Drilling and Constructing Monitoring Wells with
Hollow-Stem Augers." Samples are collected by removing the drill rods and
the attached center plug and inserting the sampler through the hollow stem.
Samples can then be taken ahead of the augers.
When drilling into an aquifer that is under even low to moderate
confining pressure, the sand and gravel of the aquifer frequently "heave"
upward into the hollow stem. This heaving occurs because the pressure in
the aquifer is greater than the atmospheric pressure in the borehole. If
a center plug is used during drilling, heave frequently occurs as the rods
are pulled back and the bottom of the borehole is opened. This problem is
exacerbated by the surging action created as the center plug and drill rods
are removed.
When heaving occurs, the bottom portion of the hollow stem fills with
sediment, and the auger must be cleaned out before formation samples can be
collected. However, the act of cleaning out the auger can result in
further heaving, thus compounding the problem. Furthermore, as the sand
and gravel heave upward into the hollow stem, the materials immediately
below the auger are no longer naturally compacted or stratified. The
sediments moving into the hollow stem are segregated by the upward-flowing
water. It is obvious that once heaving has occurred, it is not possible to
obtain a sample at that depth that is either representative or undisturbed.
Four common strategies that are used to alleviate heaving problems
include:
1) adding water into the hollow stem in an attempt to maintain
sufficient positive head inside the augers to offset the
hydrostatic pressure of the formation;
2) adding drilling mud additives (weight and viscosity control) to
the water inside the hollow stem to laprove the ability of the
fluid to counteract the hydrostatic pressure of the formation;
3) either screening the lower auger section or screening the
lowermost portion of the drill rods both above and below the
center plug, in such a manner that water is allowed to enter the
auger. This arrangement equalizes the hydraulic pressure, but
prevents the formation materials from entering the augers; and
4) drilling with a pilot bit, knock-out plug or winged clam to
physically prevent the formation from entering the hollow stem.
The most common field procedure is to add water to the hollow stem.
However, this method is frequently unsuccessful because it is difficult to
maintain enough water in the auger to equalize the formation pressure as
the drill roda are raised during the sampling process. Adding drilling mud
may lessen the heaving problem, but volume replacement of mud displacement
by removal of drilling rods must be fast enough to maintain a positive head
on the formation. Additionally, drilling mud additives may not be
desirable where questions about water-quality sampling from the monitoring
well will arise. A third option, screening the lowermost auger flight,
serves two purposes: 1) the formation pressure can equalize with minimal
formation disturbance and 2) water-quality samples and small-scale pumping
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tests can be performed on individual zones within the aquifer or on
separate aquifers as the formations are encountered. Wire-wound screened
augers were developed particularly for this purpose and are commercially
available (Figure 28). By using a pilot bit, knock-out plug or winged
clam, heaving is physically prevented until these devices are removed for
sampling. In essence, the hollow stem functions as a solid stem auger.
However, once these devices are dislodged during sampling, problems with
heaving may still need to be overcome by using an alternative strategy.
Hollow-stem augers are typically limited to drilling in unconsolidated
materials. However, if the cutting head of the auger is equipped with
carbide-tipped cutting teeth, it is often possible to drill into the top of
weathered bedrock a short distance. The augers can then be used as
temporary surface casing to shut off water flow that commonly occurs at the
soil/rock interface. The seal by the augers may not be complete;
therefore, this practice is not recommended where cross contamination is a
concern. The rock beneath the casing can then be drilled with a
small-diameter roller bit or can be cored.
The most widely-available hollow-stem augers are 6.25-inch outside
diameter auger flights with 3.25-inch inside diameter hollow stems. The
equipment most frequently available to power the augers can reach depths of
150 to 175 feet in clayey/silty/sandy soils. Much greater depths have been
attained, but greater depths cannot be predictably reached in most
settings. A 12-inch outside diameter auger with a 6-inch inside diameter
hollow stem is becoming increasingly available, but the depth limit for
this size auger is usually 50 to 75 feet. Because of the availability and
relative ease of formation sample collection, hollow-stem augering
techniques are used for the installation of the overwhelming majority of
monitoring wells in the United States.
A more complete listing of the advantages and disadvantages of
hollow-stem augers is found in Table 10. A more comprehensive evaluation
of this technology is presented in Appendix A.
TABLE 10. APPUCATIONS AND LIMITATIONS OF HOLLOW-STEM AUGERS
A M«Uj.«lftA.M
Appncraofn
Limitations
• All types o< soil investigation
• Permits good Mil sampling with split
-------
- Continuous slot screen
Auger flighting
• Auger head
Figure 28. Diagram of a screened auger.
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Direct Mud Rotary
In direct mud rotary drilling, the drilling fluid is pumped down the
drill rods and through a bit that is attached at the lower end of the drill
rods. The fluid circulates back to the surface by moving up the annular
space between the drill rods and the wall of the borehole. At the surface,
the fluid discharges through a pipe or ditch and enters into a segregated
or baffled sedimentation tank, pond or pit. The settling pit overflows
into a suction pit where a pump recirculates the fluid back through the
drill rods (Figure 29).
During drilling, the drill stem is rotated at the surface by either
top head or rotary table drives. Down pressure is attained either by
pull-down devices or drill collars. Pull-down devices transfer rig weight
to the bit; drill collars add weight directly to the drill stem. When
drill collars are used, the rig holds back the excess weight to control the
weight on the bit. Most rigs that are used to install monitoring wells use
the pull-down technique because the wells are relatively shallow.
Properly mixed drilling fluid serves several functions in mud rotary
drilling. The mud: 1) cools and lubricates the bit, 2) stabilizes the
borehole wall, 3) prevents the inflow of formation fluids and 4) minimizes
cross contamination between aquifers. To perform these functions, the
drilling fluid tends to infiltrate permeable zones and tends to interact
chemically with the formation fluids. This is why the mud must be
removed during the development process. This chemical interaction can
interfere with the specific function of a monitoring well and prevent
collection of a sample that is representative of the in-situ ground-water
quality.
Samples can be obtained directly from the stream of circulated fluid
by placing a sample-collecting device such as a shale shaker in the
discharge flow before the settling pit. However, the quality of the
samples obtained from the circulated fluid is generally not satisfactory to
characterize the formations for the design of monitoring wells.
Split-spoon, thin-wall or wireline samples can and should be collected when
drilling with the direct rotary method.
Both split-spoon and thin-wall samples can be obtained in unconsol-
idated material by using a bit with an opening through which sampling tools
can be inserted. Drilling fluid circulation must be broken to collect
samples. The rotary drill stem acts as casing as the sample tools are
inserted through the drillstem and bit and a sample is collected.
Direct rotary drilling is also an effective means of drilling and/or
coring consolidated rock. Where overburden is present, an oversized
borehole is drilled into rock and surface casing is installed and grouted
in place. After the grout sets, drilling proceeds using a roller cone bit
(Figure 30). Samples can be taken either from the circulated fluid or by a
core barrel that is inserted into the borehole.
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>— Cuttings circulated to
' surface through
Borehole wall
Figure 29. Diagram of a direct rotary circulation system (National Water Well Association of Australia,
1984).
86
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Figure 30. Diagram of • roller con« bit
87
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For the rig sizes that are most commonly used for monitoring well
installation, the maximum diameter borehole is typically 12 inches.
Unconsolidated deposits are sometimes drilled with drag or fishtail-type
bits, and consolidated formations such as sandstone and shale are drilled
with tricone bits. Where surface casing is installed, nominal 8-inch
casing is typically used, and a 7 5/8 or 7 7/8-inch borehole is continued
below the casing. In unconsolidated formations, these diameters permit a
maximum 4-inch diameter monitoring well to be installed, filter-packed and
sealed in the open borehole. In consolidated formations, a 4 5/8-inch
outside diameter casing can be used in a 7 5/8-inch borehole because there
are relatively few borehole wall stability problems in consolidated rock.
This smaller annular space is usually sufficient to permit tremie placement
of filter pack, bentonite seal and grout.
A more complete listing of applications and limitations of direct mud
rotary drilling is found in Table 11.
TABLE 11. APPLICATIONS AND LIMITATIONS OF DIRECT MUD ROTARY DRILLING
Application* | Umttationt
• Rapid drilling of clay, sill and reasonably compacted Sana and
gravel
• Allows spirt-spoon and turn-wall sampling in unconsolidated
materials
• Allows core sampling in consolidated rock
• Drilling rigs widely available
• Abundant and Hex** range ot tool sizes and depth capabilities
• Very sophisticated dnlling and mud programs available
• Geophysical borehole logs
• Difficult to remove dnlling mud and wall cake Irom outer
penmeier ol filter pack during development
• Sentomte or other dnlling fluid additives may influence quality
ot ground-water samples
• Circulated (ditch) samples poor tor momtonng well screen
selection
• Split-spoon and thin-wall samplers are expensive and ot
questionable cost effectiveness at depths greater than 150 feet
_• Wireline conng techniques for sampling both uneonsolidated
and consolidated formations often not available locally
• Difficult to identity aquifers
• Drilling fluid invasion at permeable tones may compromise
validity ol subsequent momtonng well samples
Air Rotary Drilling
Air rotary drilling is very similar to direct mud rotary with the
exception that the circulation medium is air instead of water or drilling
mud. Air is compressed and circulated down through the drill rods and up
the open hole. The rotary drill bit is attached to the lower end of the
drill pipe, and the drill bit is advanced as in direct mud rotary drilling.
As the bit cuts into the formation, cuttings are immediately removed from
the bottom of the borehole and transported to the surface by the air that
is circulating down through the drill pipe and up the annular space. The
circulating air also cools the bit. When there is no water entering the
borehole from the formation, penetration and sampling may be enhanced by
adding small quantities of water and/or foaming surfactant. Foam very
effectively removes the cuttings and lubricates and cools the bit.
However, the drilling foam is not chemically inert and may react with the
formation water. Even if the foam is removed during the development
process, the representativeness of the ground-water quality sample may be
questioned.
-------
As the air discharges cuttings at the surface, formation samples can
be collected. When the penetrated formation is dry, samples are typically
very fine-grained. This "dust" is representative of the formation
penetrated, but is difficult to evaluate in terms of the physical
properties and characteristics of the formation. However, when small
quantities of water are encountered during drilling or when water and
surfactant are added to the borehole to assist in the drilling process, the
size of the fragments that are discharged at the surface is much larger.
These larger fragments provide excellent quality samples that are easier to
interpret. Because the borehole is cleaned continuously and all of the
cuttings are discharged, there is minimal opportunity for recirculation and
there is minimal contamination of the cuttings by previously-drilled zones.
Air discharged from a compressor commonly contains hydrocarbon-related
contaminants. For this reason, it is necessary to install filters on the
discharge of the compressor.
When drilling through relatively dry formations, thick water-bearing
zones can easily be observed as drilling proceeds. However, thin
water-bearing zones often are not identifiable because either the pressure
of the air in the borehole exceeds the hydraulic pressure of the
water-bearing zone or the combination and quantity of dust and air
discharged is sufficient to remove the small amount of moisture indicative
of the thin water-bearing zone. Where thin zones are anticipated, the
samples must be carefully evaluated and drilling sometimes must be slowed
to reduce absorption of the water by the dust. It may be desirable to
frequently stop drilling to allow ground water to enter the open borehole.
This technique applies only to the first water-bearing zones encountered,
because shallower zones nay contribute water to the open borehole. To
prevent shallow zones from producing water or to prevent cross
contamination, the shallower zones must be cased off. Identification of
both thin and thick water-bearing zones is extremely important because this
information assists greatly in the placement of well intakes and/or in the
selection of isolated zones for packer tests.
In hard, abrasive, consolidated rock, a down-the-hole hammer can be
substituted for a roller cone bit to achieve better penetration (Figure
31). With the down-the-hole drill, the compressed air that is used to cool
the bit is also used to actuate and operate the down-the-hole hammer.
Typical compressed air requirements range from 100 pounds per square inch
to as much as 350 pounds per square inch for the latest generation of
down-the-hole hammers. When a down-the-hole hammer is used, oil is
required in the aiz stream to lubricate the hammer-actuating device. For
this reason, down-the-hole hammers must be used with caution when
constructing monitoring wells. Figure 32 shows the range of materials in
which roller cone bits and down-the-hole pneumatic hammers operate most
efficiently.
Air rotary drilling is typically limited to drilling in consolidated
rock because of borehole instability problems. In air rotary drilling, no
casing or drilling fluid is added to support the borehole walls, and the
-borehole is held open by stability of the rock and/or the air pressure used
during drilling. In unconsolidated materials, there is the tendency for
89
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Cuttings discharge
through pipe
Air to
actuate
hammer
and remove
cuttings
\
t
t
Hammer
• Button bit
Figure 31. Diagram of a down-the-hole hammer (After Layne-Western Company, Inc., 1983).
90
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WELL DRILLING SELECTION GUIDE
Type of Formation
Geologic Ongm »
Examples*
Hardness »
Orttng Methods
[Diameter
|t)«pth
igneous ana Metamof phic
Granite Ouartzite
Basalt Gneiss Schst
Very hard to hard
1-
1
\'
I
n
ae
1
Downhole
hammer
Carbide
nsertbit
a^^^^™
•—Carbide tooth
Smal (4 - 8 in)
Shatow(50-200rt)
Sedimentary
Limestone Sandstone Shale
Hard to soil
«
a
NP
R
te
Rotary dri
Air or foam rotary
MS— ••
Clay Sand Gravel
UnconsoMated
k.
">
Smal to meoxvn (6 - 12 m)
ShaJow 10 oeeo (SO • 1 .000 It)
Figure 32. Range of applicability for various rotary drilling methods (Ingersoll-Rand, 1976).
91
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the borehole to collapse during drilling. Therefore, air rotary drilling
in unconsolidated formations is unreliable and poses a risk for equipnent.
Where sufficient thicknesses of unconsolidated deposits overlie a
consolidated formation that will be drilled by air rotary techniques,
surface casing through the unconsolidated material is installed by an
alternative technique. Drilling can then be accomplished using air with
either a roller-cone bit or down-the-hole hammer.
Monitoring wells drilled by air rotary methods are typically installed
as open-hole completions. Because the borehole is uncased, the potential
exists for cross connection between water-bearing zones within the
borehole. Further, the recirculated air effectively cleans cuttings from
the borehole walls so that the borehole is usually not coated with a wall
cake such as occurs with mud rotary drilling or with augering techniques.
This cleaner borehole wall increases the potential for cross connection,
but increases the effectiveness of well completion and development.
Additionally, the air introduced during drilling may strip volatile
organics from the samples taken during drilling and from the ground water
in the vicinity of the borehole. With time, the effects of airstripping
will diminish and disappear, but the time necessary for this recovery will
vary with the hydrogeologic conditions. The importance of these factors
needs to be evaluated before choosing the air rotary drilling technique.
The diameter of the roller-cone or tri-cone bit used in air rotary
drilling is limited to approximately 12 inches, although larger bits are
available. For the down-the-hole hammer, the practical limitation is
8-inch nominal diameter. There is no significant depth limitation for
monitoring well construction with the air rotary technique, with the
possible exception of compressor capacity Units in deep holes with high
water tables and back pressure.
A more complete list of applications and limitations of air rotary
drilling is found in Table 12.
TABLE 12. APPLICATIONS AND LIMITATIONS OF AIR ROTARY DRILLING
Rapid ending of semKonsoiidated and consolidated rock • S^rm cwng frequently required to protect top of hole
Good quality/reliable formation samples (particularly it small t Dnang KstncMd to semi-consoiuJateO and consolidated
quantities of water and surfactant are used) , •omenr*
Equipment generally available • Sarxxw Viable but occur as small particles thai are
Allows easy and quick identification of hthologic changes **e»i« to interpret
Allows rtentitieunn ot moat water-bearing zones * 0^ •«»•« ot air may mask lower yield watw
9>oouang zones
Allows estimation of yields in strong water-producing zones witn
short "down time' ' * *" •"••""•Quires contaminant filtration
; • ».i may modify chemical or biological conditions Recovery
t.«ne it uncertain
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Air Rotary With Casing Driver
This method is an adaptation of air rotary drilling that uses a
casing-driving technique in concert with air (or mud) rotary drilling. The
addition of the casing driver makes it possible to use air rotary drilling
techniques in unconsolidated formations. The casing driver is installed in
the mast of a top head drive air rotary drilling rig. The casing can then
be driven as the drill bit is advanced (Figure 33).
The normal drilling procedure is to extend the drill bit 6 to 12
inches ahead of the casing. The distance that the drill bit can be
extended beyond the casing is primarily a function of the stability of the
borehole wall. It is also possible to drive the casing ahead of the bit.
This procedure can be performed in unconsolidated formations where caving
and an oversize borehole are of concern. Once the casing has been driven
approximately one foot into the formation, the drill bit is used to clean
the material from inside the casing. This technique also minimizes air or
mud contact with the strata.
Where drilling through unconsolidated material and into consolidated
bedrock, the unconsolidated formation is drilled with a drill bit as the
casing is simultaneously advanced. When the casing has been driven into
the top of the bedrock, drilling can proceed by the standard air rotary
technique. The air rotary with casing driver combination is particularly
efficient where drilling through the sand-gravel-silt-boulder-type
materials that commonly occur in glaciated regions. The sandy and/or
gravelly, unstable zones are supported by the casing while the boulder and
till zones are rapidly penetrated by the rotary bit. Because the upper
zones within the formation are cased-off As the borehole is advanced, the
potential for inter-aquifer cross-contamination is minimized. The
protective casing also permits the collection of reliable formation samples
because the entire formation is cased except for the interval that is
presently being cut. An additional advantage of the drill-through casing
driver is that the same equipment can be used to drive the casing upward to
expose the well intake after the casing and well intake have been installed
in the borehole.
Water-bearing zones can be readily identified and water yields can be
estimated as drilling progresses. However, as with the direct air rotary
method, zones that have low hydrostatic pressure may be inhibited from
entering the borehole by the air pressure exerted by the drilling process.
Additionally, the dust created as the formation is pulverized can serve to
seal off these zones and then these water-bearing zones may be overlooked.
For these reasons, it is necessary to drill slowly and carefully and even
occasionally to stop drilling where water-bearing zones are indicated or
anticipated.
A more complete list of applications and limitations of the air rotary
with casing driver method is found in Table 13.
93
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II L
JH
\
Air supply
• Top-head
drive
Mast
• Casing
driver
Discharge for
cuttings
Casing
Drill pipe
- Drive shoe
Drill bit
Rgure 33. Diagram of a drill-through casing driver (Aardvark Corporation, 1977).
94
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TABLE 13. APPLICATIONS AND LIMITATIONS OF AIR ROTARY
WITH CASING DRIVER DRILLING
Applications
Limitation*
• Rapid dniimg of unconsolidated sanos. silts and clays
• Drilling in alluvial material (including boulder formations)
• Casing supports borehole thereby maintaining borenole integnty
ana minimizing inter-aguifer cross contamination
• Eliminates circulation problems common with direct mud rotary
method
• Good formation samples
• Minimal formation damage as casing pulled back i smearing of
clays and silts can be anticipated)
• Thin, low pressure waterbearing zones easily overlooked if
drilling not stopped at appropriate places to observe whether
or not water levels are recovering
• Samples pulvenzed as in all rotary drilling
• Air may modify chemical or biological conditions Recovery
time is uncertain
Dual-Wall Reverse Circulation
In dual-wall reverse-circulation rotary drilling, the circulating
fluid is pumped down between the outer casing and the inner drill pipe, out
through the drill bit and up the inside of the drill pipe (Figure 34).
Continuous sample discharge
Top-head -
drive
Figure 34. Diagram of dual-wall reverse-circulation rotary-method (Driscoll, 1986).
95
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The circulation fluid used in the dual-wall reverse circulation method can
be either water or air. Air is the suggested medium for the installation
of monitoring wells, and, as such, it is used in the development of the
ratings in Appendix B. The inner pipe or drill pipe rotates the bit and
the outer pipe acts as casing. Similar to the air rotary with casini
driver method, the outer pipe: 1) stabilizes the borehole, 2) minimizes
cross contamination of cuttings and 3) minimizes interaquifer cross
contamination within the borehole.
The dual-wall reverse-circulation rotary method is one of the better
techniques available for obtaining representative and continuous formation
samples while drilling. If the drill bit is of the roller-cone type the
formation that is being cut is located only a few inches ahead of the
double-wall pipe. The formation cuttings observed at the surface represent
no more than one foot of the formation at any point in time. The samples
circulated to the surface are thus representative of a very short section
of the formation. When drilling with air, a very representative sample of
a thin zone can be obtained from the formation material and/or the
formation water. Water samples can only be obtained where the formation
has sufficient hydrostatic pressure to overcome the air pressure and dust
dehydration/sealing effects. (Refer to the section on air rotary with
casing driver for a more complete discussion.)
Unconsolidated formations can be penetrated quite readily with the
dual-wall reverse-circulation method. Formations that contain boulders or
coarse gravelly materials that are otherwise very difficult to drill can be
relatively easily penetrated with this technique. This increased
efficiency is due to the ability of the method to maximize the energy at
the bottom of the borehole while the dual^wall system eliminates problems
with lost circulation and/or borehole stability.
When drilling in hard rock, a down-the-hole hammer can be used to
replace the tri-cone bit. When the down-the-hole hammer is employed, air
actuates the hammer by: 1) moving down through the hammer, 2) moving back
up the outside of the hammer and 3) re-entering the center drill pipe in a
cross-over channel just above the hammer. When drilling with the hammer,
the full length of the hammer is exposed below the protective outer casing
(approximately 4 to 5 feet). Thus the uncased portion of the borehole is
somewhat longer than when drilling with a tri-cone bit. This longer
uncased interval results in formation samples that are potentially
representative of a thicker section of the formation. Otherwise, the
sampling and representative quality of the cuttings are very similar to
that of a formation drilled with a tri-cone bit. This method was developed
for and has been used extensively by minerals exploration companies and has
only recently been used for the installation of monitoring wells. Depths
in excess of 1000 feet can be achieved in many formations.
When drilling with air, oil or other impurities in the air can be
introduced into the formation. Therefore, when drilling with air and a
roller-cone bit, an in-line filter must be used to remove oil or other
impurities from the airstream. However, when using a down-the-hole hammer
oil is required in the airstream to lubricate the hammer. If oil or other
96
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air-introduced contaminants are of concern, the use of a down-the-hole
hammer may not be advised.
When the borehole has been advanced to the desired monitoring depth,
the monitoring well can be installed by either: 1) inserting a small-
diameter casing and well intake through an open-mouth bit (Driscoll, 1986)
or 2) removing the outer casing prior to the installation of the monitoring
well and installing the monitoring well in the open borehole. When
installing a casing through the bit, the maximum diameter casing that can
be installed is approximately 4 inches. This is controlled by the 10-inch
maximum borehole size that is readily available with existing drill pipe
and the maximum diameter opening in the bit. When installing a casing in
the open borehole, the borehole must be very stable to permit the open-hole
completion.
A more complete list of applications and limitations of the dual-wall
reverse-circulation technique is found in Table 1A.
TABLE 14. APPLICATIONS AND LIMITATIONS OF DUAL-WALL REVERSE-CIRCULATION
ROTARY DRILLING
rt • iiln»Hn«iM
Appucanons
• very rapid drilling througn both unconsoiidated and • L.m,wo oorenole size that limits diameter of momtonng veils
consolidated formations • ,n un*uwe formations, well diameters are limited to
• Allows continuous sampling in all types of formations • »og*o»ifnateiy 4 inches
• very good njpresentatiw samples can be obtained with minimal • Eowomeni availability more common in the southwest
nsk of contamination of sample and/or water-bearing zone ; . *„ ^ ^a^ chemical or biological conditions, recovery
• in stable formations, wells with diameters as large as 6 inches
can be installed in open hole completions
. un^m to install fitter pack unless completed ope
Cable Tool Drilling
Cable tool drilling is the oldest of all the available modern drilling
technologies. Prior to the development of direct mud rotary, it was the
standard technology used for almost all forms of drilling.
In cable tool drilling, the drill bit is attached to the lower portion
of the weighted drill stem that, in turn, is attached by means of a rope
socket to the rope or cable (Figure 35). The cable and drill stem are
suspended from the mast of the drill rig through a pulley. The cable runs
through another pulley that is attached to an eccentric "walking or
spudding beam." The walking beam is actuated by the engine of the drilling
rig. As the walking beam moves up and down, the bit is alternately raised
and dropped. This "spudding action" can successfully penetrate all types
of geological formations.
When drilling in hard rock formations, the bit pounds a hole into the
rock by grinding cuttings from the formation. The cuttings are
periodically excavated from the borehole by removing the drill bit and
inserting a bailer (Figure 36). The bailer is n bucket made from sections
of thin-wall pipe with a valve on bottom that is actuated by the weight of
97
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Crown —
sneave
Shock —
aosoroer
— Casing ana sand
line sheaves
iv*
k
rp 3nH'
H (ii
_Ufci
Tool t»x —
Bull reel shaft
Catnead-
JJ
.^
Sana reel snattJ
Casing reel-1 -Crankshaft
Figure 35. Diagram of a cable tool drilling system (Buckeye Drill Company/Bucyrus-Ene Company, 1982).
98
-------
r\
Flapper
valve
(a)
Rgure 36. Diagrams of two types of bailers:
a) dart valve and b) flat bottom.
99
-------
the bailer. The bailer is run into the borehole on a separate line. The
bailer will not function unless there is sufficient water in the borehole
to slurry the mixture of cuttings in water. If enough water is present the
bailer picks up the cuttings through the valve on the bottom of the bailer
and is hoisted to the surface. The cuttings are discharged from either the
top or bottom of the bailer, and a sample of the cuttings can be collected.
If the cuttings are not removed from the borehole, the bit is constantly
rcdrilling the same material, and the drilling effort becomes very
inefficient.
When drilling unconsolidated deposits comprised primarily of silt and
clay, the drilling action is very similar to that described in the previous
paragraph. Water must be added to the borehole if the formations
encountered during drilling do not produce a sufficient quantity of water
to slurry the mud and silt. If the borehole is not stable, casing must be
driven as the bit advances to maintain the wall of the borehole.
When drilling unconsolidated deposits comprised primarily of water-
bearing sands and gravels, an alternate and more effective drilling
technique is available for cable tool operations. In the "drive and bail"
technique, casing is driven into the sand and gravel approximately 3 to 5
feet and the bailer is used to bail the cuttings from within the casing.
These cuttings provide excellent formation samples because the casing
serves, in effect, as a large thin-wall sampler. Although the sample is
"disturbed," the sample is representative because the bailer has the
capability of picking up all sizes of particles within the formation.
When drilling by the drive and bail technique, "heaving" of material
from the bottom of the casing upward may present a problem. When heaving
occurs, samples are not representative of the material penetrated by the
casing. Instead, samples represent a mixture of materials from the zone
immediately beneath the drill pipe. Heaving occurs when the hydrostatic
pressure on the outside of the casing exceeds the pressure on the inside of
the casing. The heaving is exacerbated by the action of the drill stem
that is suspended in the borehole as the pipe is driven and by the action
of the bailer that is used to take the samples. If the bailer is lifted or
"spudded" rapidly, suction is developed that can pull the material from
beneath the casing up into the casing. This problem is particularly
prevalent when the drill advances from a dense material into relatively
unconsolidated sand and gravel under greater hydrostatic pressure.
Several techniques have been developed to offset the problem of
heaving. These techniques include:
1) maintaining the casing full of water as it is driven and as the
well is bailed. The column of water in the casing creates a higher
hydrostatic head within the casing than is present in the
formation;
2) maintaining a "plug" inside the casing as the samples are taken
with the bailer. This plug is created by collecting samples with
the bailer between 1 and 3 feet above the bottom of the casing.
The plug maintained in the bottom of the "borehole" offsets
heaving when the pressure differential is low;
100
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3) overdriving the casing through the zone that has the tendency to
heave; and
4) adding drilling mud to the borehole until the weight of the mud and
slurried material in the casing exceed the hydrostatic pressure of
the heaving zone. This fourth option is the least desirable
because it adds drilling mud to the borehole.
If it is necessary to maintain a slurry in the casing in order to control
heaving problems, it is still possible to collect both disturbed and
undisturbed samples from beneath the casing by inserting smaller-diameter
drill rods and samplers inside the casing at selected intervals.
Cable tool drilling has become less prevalent in the last 25 years
because the rate of formation penetration is slower than with either rotary
techniques in hard consolidated rock or augering techniques in
unconsolidated formations. Because cable tool drilling is much slower, it
is generally more expensive. Cable tool drilling is still important in
monitoring well applications because of the versatility of the method.
Cable tool rigs can be used to drill both the hardest and the softest
formations. Cable tool rigs can drill boreholes with a diameter suitable
to fulfill the needs of a monitoring well or monitoring well network.
There is no significant depth limitation for the installation of monitoring
wells.
When comparing cable tool to other drilling technologies, cable tool
drilling may be the desired method. In a carefully drilled cable tool
borehole, thin individual zones and changes in formations are often more
easily identified than with alternative technologies. For example,
smearing along sidewalls in unconsolidated formations is generally less
severe and is thinner than with hollow-stem augering. Therefore, the
prospect of a successful completion in a thin water-bearing zone is
generally enhanced.
A more complete listing of advantages and disadvantages of cable tool
drilling is found in Table IS.
TABLE 15. APPLICATIONS AND LIMITATIONS OF CABLE TOOL DRILLING
AfifJLja^Mfifia
• Drilling in Ail typBS of gaoteQic foiniaiions
• Ease of monitoring wen installation
• Ease and practicality of w«U development
Uniltatlons
• Drilling relairaery stow
• Heaving of unconsoixttua materials must Be controlled
• Equipment availability more common m central, norm central
and northeast sections of the United Stales
101
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Other Drilling Methods
There are two other drilling techniques that are commonly available to
install monitoring wells: 1) bucket auger and 2) reverse circulation
rotary. Bucket augers are primarily used for large-diameter borings
associated with foundations and building structures. Reverse-circulation
rotary is used primarily for the installation of large-diameter deep water
WeX XS•
While either of these technologies can be used for the installation of
monitoring wells, the diameters of the boreholes and the size of the
required equipment normally preclude them from practical monitoring well
application. Unless an extraordinarily large diameter monitoring well is
being installed, the size of the zone disturbed by the large diameter hole
excavated by either of these techniques severely compromises the data
acquisition process that is related to the sampling of the monitoring
wells. While either of these techniques have possible application to
monitoring well installation, they are not considered to be valid for
regular application.
DRILLING FLUIDS
Prior to the development of rotary drilling, water and natural clay
were added to the borehole during cable tool drilling to: 1) cool and
lubricate the bit, 2) slurry the cuttings for bailing and 3) generally
assist in the drilling process. With the development of rotary drilling
the use of drilling fluid became increasingly important. In rotary
drilling, the drilling fluid: 1) cools and lubricates the bit, 2) removes
the cuttings and 3) simultaneously stabilizes the hole. Drilling fluid
thus makes it possible to drill to much greater depths much more rapidly.
As fluid rotary drilling programs became increasingly sophisticated, it
became possible either to temporarily suspend cuttings in the mud column
when the mud pump was not operating, or. under appropriate circumstances
to cause the cuttings to drop out in the mud pit when the cuttings reached
the surface. These improvements served not only to enhance the efficiency
of the drilling operation, but also to improve the reliability of the
geologic information provided by the cuttings
Today, the fluid system used in mud rotary drilling is no longer
restricted to the use of water and locally-occurring natural clays.
Systems are now available that employ a wide variety of chemical/oil/water-
base and water-base fluids with a wide range of physical characteristics
created by additives. The predominant additives include sodium bentonite
and barium sulfates, but a variety of other chemicals are also used. This
drilling fluid technology was initially developed to fulfill the deep-
drilling requirements of the petroleum industry and is not generally
applied to monitoring well installations.
Influence of Drilling Fluids on Monitoring Well Construction
Monitoring well construction is typically limited to the use of simple
water-based drilling fluids. This limitation is imposed by the necessity
102
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not to influence the ground-water quality in the area of the well. Even
when water-based fluids are used, many problems are still created or
exacerbated by the use of drilling fluids. These problems include:
1) fluid infiltration/flushing of the intended monitoring zone, 2) well
development difficulties (particularly where an artificial filter pack has
been installed) and 3) chemical, biological and physical reactivity of the
drilling fluid with the indigenous fluids in the ground.
As drilling fluid is circulated in the borehole during drilling
operations, a certain amount of the drilling fluid escapes into the
formations being penetrated. The escape, or infiltration into the
formation, is particularly pronounced in more permeable zones. Because
these more permeable zones are typically of primary interest in the
monitoring effort, the most "damage" is inflicted on the zones of greatest
concern. If the chemistry of the water in the formation is such that it
reacts with the infiltrate, then subsequent samples taken from this zone
will not accurately reflect the conditions that are intended to be
monitored. Attempts to remove drilling fluids from the formation are made
during the well development process. Water is typically removed in
sufficient quantities to try to recover all the infiltrate that may have
penetrated into the formation. When a sufficient quantity of water has
been removed during development, the effects of flushing are arbitrarily
considered to be minimized.
Most monitoring wells are typically 2 to 4 inches in diameter. They
are frequently surrounded by a filter pack to stabilize the formation and
to permit the procurement of good ground-water samples. Because of the
small well diameter, it is very difficult, and often not possible, to fully
develop the drilling mud from the interface between the outside of the
filter pack and the inside of the natural formation. Failure to fully
remove this mudcake can interfere with the quality of the samples being
obtained for a substantial period of time.
In practice, when ground-water sampling is undertaken, samples are
usually collected and analyzed in the field for certain key parameters,
including specific conductance, temperature and pH. Water is discharged
from the well and repeated measurements are taken until the quality of the
water being sampled has stabilized. When this "equilibrium" has been
achieved and/or a certain number of casing volumes of water have been
removed, the samples collected are commonly considered to be representative
of the indigenous quality of the ground water. It is assumed that the
drilling fluid filtrate no longer impacts the results of the sample
quality. This is not necessarily the case. If, for example, the chemical
reactions that took place between the drilling fluid and formation water(s)
resulted in the precipitation of some constituents, then the indigenous
water moving toward the well can redissolve some of the
previously-precipitated constituents and give a false result to the sample.
Theoretically, at some point in time this dissolution will be completed and
the samples will become valid. However, there is currently no reliable
method in practice that postulates the time frame required before reliable
quality is attainable.
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Biologic activity induced by the introduction of the drilling fluid
may have a similar reaction. In particular, the use of organic drilling
fluids, such as polymeric additives, has the potential for enhancing
biologic activity. Polymeric additives include the natural organic
colloids developed from the guar plant that are used for viscosity control
during drilling. Biologic activity related to the decomposition of these
compounds can cause a long-term variation in the quality of the water
sampled from the well.
The use of sodium montmorillonite (bentonite) can also have a
deleterious long-term impact on water quality. If the sodium-rich
montmorillonite is not fully removed from the well during development,
constituents contained in the ground water being monitored will come in
contact with the montmorillonite. When this happens, the tendency is for
both organic molecules with polar characteristics and inorganic cations to
be attracted to positions within the sodium montmorillonite structure.
This substitution results in the release of excess sodium ions and the
retention of both selected organic molecules and cations. Organic
molecules and cations that might otherwise be indicative of contamination
can be removed from the sample and possibly be re-dissolved at an undefined
rate into subsequent samples.
Drilling Fluid Characteristics
The principal properties of water-based drilling fluids are shown in
Table 16. Selected properties are discussed in this section. Monitoring
well construction typically starts by using only the simplest drilling
fluid*-water; however, water should only be used when necessary. Any water
added as a drilling fluid to a monitoring well should be the best quality
of water that is available. The chemical and bacteriological quality of
this water must be determined by laboratory analyses in order to identify
potential interference with substances being monitored. As this "clean"
water is circulated in the borehole, the water picks up clay and silt that
form a natural drilling mud. During this process, both the weight and
viscosity of the drilling fluid increase. The degree of change in these
properties depends on the nature of the geologic formations being
penetrated. It is possible to attain a maximum weight of approximately 11
pounds per gallon when drilling in natural clays. The same maximum weight
can also be achieved by artificially adding natural clays or bentonite to
make a heavier drilling mud where the formation does not naturally have
these minerals.
TABLE 16. PRINCIPAL PROPERTIES OF WATER-BASED DRILLING FLUIDS (DRISCOLL, 1986)
Density (weight) Gel strength
Viscosity Fluid-loss-control effectiveness
Yield point Lubricity (lubrication capacity)
104
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Where additional weight is needed to maintain stability of the
borehole, heavier additives are required. The most common material used
for drilling mud weight control is barite (barium sulfate). Barite has an
average specific gravity of approximately 4.25; the specific gravity of
typical clay additives approximates 2.65. Figure 37 shows the range of
drilling fluid densities that can be obtained by using a variety of
different drilling additives.
0
Weignt ol drilling liuio ID/gal
5 10 15 2o
Air
Mist
| Stiff foam
| Wet foam
Water
Maximum practical density
using polymers
Polymeric drilling fluid saturated witn NaCl
Maximum practical density using Oentonne
Polymeric drilling fluid saturated with CaCI
Weighted bentome dnumg flud using name
0 600 1.200 1800 2.400
Weight of dnflmg flud. kg/m>
Figure 37. Practical drilling fluid densities (Driscoll. 1986).
When the weight of the drilling fluid substantially exceeds the
natural hydrostatic pressure exerted by the formation being drilled, there
is an excessive amount of water loss from the drilling fluid into the
formation penetrated. This maximizes the filtrate invasion and
consequently maximizes the adverse impact of filtrate invasion on the
reliability of water-quality samples collected from the monitoring well.
Another important property of a drilling fluid is viscosity.
Viscosity is the resistance offered by the drilling fluid to flow. In
combination with the velocity of the circulated fluid, viscosity controls
the ability of the fluid to remove cuttings from the borehole. In
monitoring wells where water is the primary drilling fluid, the viscosity
is the result of the interaction of water with the particulate matter that
is drilled. Viscosity is also affected by the interaction of water with
the clays that are sometimes added during the drilling process. Sodium
montmorillonite (sodium bentonite) is the constituent most often added to
increase viscosity.
Viscosity has no relationship to density. In the field, viscosity is
measured by the time required for a known quantity of fluid to flow through
an orifice of special dimensions. The instrument used for this measurement
is called a Marsh Funnel. The relative viscosity of the drilling mud is
described as the Marsh Funnel viscosity, in seconds. Table 17 presents the
105
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approximate Marsh Funnel viscosities required for drilling in typical
unconsolidated materials. These typical values are based on the assumption
that the circulating mud pump provides an adequate uphole velocity to clean
the cuttings from the borehole at these viscosities. For comparison, the
Harsh Funnel viscosity of clear water at 70°F is 26 seconds.
TABLE 17. APPROXIMATE MARSH FUNNEL VISCOSITIES REQUIRED FOR DRILLING IN TYPICAL
TYPES OF UNCONSOLIDATED MATERIALS (DRISCOLL, 1986)
Appropriate Marsh Funnel
Material Drilled Viscosity (seconds)
Fine sand 35 - 45
Medium sand 45-55
Coarse sand 55-65
Gravel 65-75
Coarse gravel 75-85
Clays are frequently a mixture of illite, chlorite, kaolinite and
mixed-layer clays. These minerals all have a relatively low capability to
expand when saturated. The reason that sodium montmorillonite is so
effective in increasing viscosity is because of its crystalline layered
structure; its bonding characteristics; and the ease of hydration of the
sodium cation. Figure 38 demonstrates the variation in the viscosity-
building characteristics of a variety of clays. Wyoming bentonite (a
natural sodium-rich montmorillonite) is shown at the extreme left.
The impact of the mix water on sodium bentonite is indicated by Figure
39. This figure shows the viscosity variation that results from using soft
water versus hard water in drilling mud preparation. Sodium
montmorillonite is most commonly used as the viscosity-building clay.
However, in hard water the calcium and magnesium ions replace the sodium
cation in the montmorillonite structure. As a consequence, a much lower
viscosity is obtained for a given quantity of solids added. As previously
discussed, this sodium cation replacement is similar to the activity that
occurs in the subsurface when bentonitic materials are left in the
proximity of the well. These materials have the capacity to prevent ions
from reaching the borehole and to release them slowly back into the ground
water at an indeterminate rate. This process can have a profound influence
on the quality of the ground-water samples collected from the monitoring
well.
The loss of fluid from the borehole into permeable zones during
drilling occurs because the hydrostatic pressure in the borehole exceeds
that of the formation being penetrated. As fluid moves from the borehole
into the lower pressure zones, fine particulate matter that has been
incorporated during the drilling operation, plus any clay additives that
have been added to the drilling fluid, are deposited in the pore space of
106
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the zone being infiltrated. When this happens, a "filter cake" is formed
on the borehole wall. Where a good quality bentonitic drilling mud
additive is being used, this filter cake can be highly impermeable and
quite tough. These characteristics minimize filtrate invasion into the
formation, but make it difficult to develop these clays out of the zone
penetrated.
(
60
50
a
£ 40
I
o 30
it
9 ?o
>
10
0
Weight. iD/tt1
33.7 675 71.2 75.0 78.7 82.5 86.2 90
Weight. ID/gal
8.5 9.0 9.5 10.0 10.5 11.0 11.5 12
1 i ; 1
i ' 1 /; / /
Wyorningf~Premiurnr f - Common i / Low-yield /!
bentonite 1 dnllinq dav / drilling clav / drilling clav /
/ / /
I i
I
I
1 i 1
\ 1 f
/ / /
1 / /' / /
\i / x Z. i
s ^ .-^ — — • — . r
5 10 15 20 25 30 JS 40 45 50
Percent sows by «*qnt
20010075 50 40 30 25 20 18 16 '4 -2 10 9 8
Yield (1 5-centipoise drilling fiud) CMTTM per ton
0
0
Figure 38. Viscosity-building characteristics of drUMng days (After Petroleum Extension Service,
1980).
Yield point and gel strength are two additional properties that are
considered in evaluating the characteristics of drilling mud. Yield point
is a measure of the amount of pressure, after a shut down, that must be
exerted by the pump upon restarting, in order to cause the drilling fluid
to start to flow. Gel strength is a measure of capability of the drilling
fluid to maintain suspension of particulate matter in the mud column when
the pump is shut down. There is a close relationship between viscosity,
yield point and gel strength. In monitoring well installation these
properties are rarely controlled because the control of these properties
requires the addition of additives that can impact the quality of the water
produced by the completed well. They are important, however, in evaluating
the reliability of samples taken from the mud stream. Where drilling fluid
quality is uncontrolled, ditch samples are generally unreliable.
107
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Bentonite
II
Complete
mixing
a
Higher viscosity per It) of clay solids
a a.
Incomplete
mixing
u
Lower viscosity per ib of clay solids
a u
Deftoccuiated
Low gel strength
Lowest rate of
filtration
Firm filter cake
Flocculated
Progressive gel strength
High rate of filtration
Deftoccuiated
• Low gel strength
• Low rate Of filtration
% Firm fHter cake
Flocculated
Sudden, non-progressive
gel strengtn
Highest rate of filtration
Soft filter cake
Figure 39. Schematic of the behavior of day particles when mixed Into water (Driscoll. 1986).
Mud-Based Applications
It is desirable to install monitoring wells with the cleanest,
clearest drilling water that is available. In monitoring well
applications, the properties related to mud weight and the properties that
relate to flow characteristics are only controlled under exceptional
conditions. This control is usually exercised only on relatively deep
boreholes or boreholes with moderately large diameters.
When drilling using either cable tool or hollow-stem augering
techniques, it is sometimes necessary to add water to the borehole in order
to effectively continue drilling. The addition of water may be required
to: 1) stabilize the borehole, 2) improve the cutting action of the bit or
3) enable the driller to remove the cuttings from the borehole. With
drive-and-bail and hollow-stem auger techniques, it may be necessary to add
water to the borehole to minimize heaving of the formation upward into the
casing or hollow stem. When the zone immediately below the augers or the
casing heaves, the samples collected from this zone are considered
disturbed and are not representative of the natural undisturbed formation.
108
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When drilling fluid is added during either cable tool drilling or
hollow-stem augering, the effectiveness of the water is enhanced by the
addition of bentonite to the drilling fluid. The bentonite is added to the
borehole for formation stabilization. When either clean water or clean
water plus additives are added to the borehole, the problems of flushing,
potential contamination and water-quality modification are the same as when
using fluid rotary drilling. For these reasons, it is suggested that
addition of drilling fluid additives and/or even clean water be avoided
when using cable tool or hollow-stem augers if at all possible. If it is
anticipated that the addition of fluids will be necessary to drill with
either cable tool or hollow-stem augers, it is suggested that alternative
drilling techniques be considered.
Air-Based Applications
In addition to water-based drilling fluids, air-based drilling fluids
are also used. There are a variety of air-based systems as indicated in
Table 18. When using air-based drilling fluids, the same restrictions
apply as when using water-based drilling fluids. When a monitoring well is
drilled using additives other than dry air, flushing, potential
contamination and water-quality modification are all of concern. Even with
the use of dry air, there is the possibility that modification of the
chemical environment surrounding the borehole may occur due to changes in
the oxidation/reduction potential induced by aeration. This may cause
stripping of volatile organics from formation samples and ground water in
the vicinity of the borehole. With time, this effect will diminish and
disappear, but the time necessary for this to occur varies with the
hydrogeologic conditions.
TABLE 18 DRILLING FLUID OPTIONS WHEN DRILLING WITH AIR (AFTER DRISCOLL, 1986)
• Air Atone
• Air Mot
• Air plus a small amount of water/perhaps a small amount of surfactant
• Air Foam
• Stable foam —air plus surfactant
• Suit town — air. surfactant plus polymer or bentonite
• Aerated mud/water base — drilling fluid plus air
Where dry air is being used, a filter must be placed in the discharge
line to remove lubricating oil. Because a down-the-hole hammer cannot be
used without the presence of oil in the air stream, this particular variety
of dry-air drilling cannot be used without the danger of contaminating the
formation with lubricating oil.
Monitoring wells can be installed in hard rock formations using air as
the circulation medium and employing roller-cone bits. Air can also be
used successfully in unconsolidated formations when applied in conjunction
with a casing hammer or a dual-wall casing technique. For effective
109
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drilling, the air supply must be sufficient to lift the cuttings from the
bottom of the borehole, up through the annular space and to the discharge
point at the surface. An uphole velocity of SOOO to 7000 feet per minute
is desirable for deep boreholes drilled at high penetration rates.
SOIL SAMPLING AND ROCK CORING METHODS
It is axiomatic that: "any sample is better than no sample; and no
sample is ever good enough." Thus, if there are no samples except those
collected from the discharge of a direct rotary fluid drilled hole or those
scraped from the cutting head or lead auger of a solid core auger, then
these samples will be collected and analyzed to the best of the ability of
the person supervising the operation. In general, however, it can be
stated that in a monitoring well installation program these types of
samples are not sufficient.
When evaluating the efficiency of a sampling program, the objectives
must be kept in mind. Where formation boundaries must be identified in
order to establish screened intervals, continuous samples are important.
If identification of isolated zones with thin interfingers of sand and
gravel in a clay matrix is important for the monitoring program, then the
samples must allow identification of discrete zones within the interval
being penetrated. If laboratory tests will be performed on the samples,
then the samples must be of sufficient quality and quantity for laboratory
testing. Specific laboratory tests require that samples be undisturbed;
other tests permit the use of disturbed samples. The sample program must
take these requirements into account.
Table 19 demonstrates the characteristics of the sampling methods
available for the drilling techniques that are most frequently employed in
the installation of monitoring wells. The table is arranged such that the
general overall reliability of the samples increases downward in the table
for both unconsolidated and consolidated materials. The least favorable
type of sampling is the scraping of samples from the outside of the flights
of solid-flight augers. This sampling method: 1) permits only
discontinuous sampling, 2) does not allow identification of discrete zones,
3) provides no sample suitable for laboratory testing and 4) generally
provides unreliable sample quality. It can also be seen from Table 19 that
split-spoon and thin-wall sampling techniques are the minimum techniques
required to obtain: 1) good sample quality, 2) continuous sampling, 3)
samples suitable for laboratory testing and 4) samples that allow the
identification of discrete zones.
Split-spoon sampling has become the standard for obtaining samples in
unconsolidated materials by which other techniques are compared. Split-
spoon samples are "driven" to collect disturbed samples; thin-wall samples
are "pressed" to collect undisturbed samples. Undisturbed samples cannot
be taken using driving, rotational or vibratory techniques in
unconsolidated materials. Split-spoon and thin-wall sampling techniques
are the primary techniques that are used to obtain data for monitoring well
installation.
110
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TABLE 19. CHARACTERISTICS OF COMMON FORMATION-SAMPLING METHODS
Type of
Formation
Unconsolidatea
Consolidated
Sample Collection
Method
Solid core auger
Ditch (direct rotary)
Air rotary with casing driver
Dual-wall reverse circulation
rotary
Piston samplers
Split spoon and thin-wall
samplers
Special samplers (Damson.
Vicksburg)
Cores
Ditch (direct rotary)
Surface (dry air)
Surface (water/foam)
Cores (wireline or conventional)
Sample
Quality
Poor
Poor
Fair
Good
Good
Good
Good
Good
Poor
Poor
Fair
Good
Potential for
Continuous
Sample
Collection
No
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Samples
Suitable
for Lab
Tests
No
NO
NO
NO
Yes
Yes
Yes
Yes
No
No
No
Yes
Discrete
Zones
Identifiable
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Increasing
Reliability
l
P
i
Sample description is as important as sample collection. It is often
difficult to collect good formation samples of non-cohesive materials
because the fine, non-cohesive particles are frequently lost during the
sampling process. The person using and describing such sampling data must
make an on-site, sample-by-sample determination of sample reliability if
the data are to be used in a meaningful manner. Another sampling bias is
that particulate material with an effective diameter greater than one-third
of the inside diameter of the sampler frequently cannot be collected. It
is not unusual for a single large gravel or small cobble to be caught at
the bottom of the sampler and no sample at all recovered from a sampler
run. It is also possible in a sequence of alternating saturated clay/silt
and sand to "plug" the sampler with the clay/silt materials and to drive
through the sands without any indication of sand. It is also common for
the sample to be compacted so that if a 2-foot sampler is driven completely
into the sediments, only 1.5 feet or less may actually be recovered.
It must be stressed that regardless of the sampling equipment used,
.the final results frequently depend on the subjective judgment of the
person describing the samples. Therefore, in order to properly screen and
develop a well in a potentially contaminated zone, it is often necessary to
employ auxiliary techniques and substantial experience.
Split-Spoon Sa|"plers
Split-spoon sampling techniques were developed to meet the
requirements of foundation engineering. The common practice in foundation
evaluation is to collect 18-inch samples at 5-foot intervals as the
borehole is advanced. The split-spoon sampler is attached to the end of
the drill rods and lowered to the bottom of the borehole where it rests on
top of fresh undisturbed formation. In order to obtain valid samples, the
111
-------
Head assembly •
Split barrel
Spacer
Shoe
Figure 40. Diagram of a split-spoon sampler (MobHt Drilling Company, 1982).
112
-------
bottom of the borehole must be clean and the formation to be sampled must
be fresh and undisturbed. It is, therefore, easy to see why: 1) the
difficulties of a heaving formation must be overcome prior to sampling and
2) a good sampling program can only be conducted in a stabilized borehole.
A split-spoon sampler, as shown in Figure 40, is of standard
dimensions and is driven by a 140-pound weight dropped through a 30-inch
interval. The procedure for collecting split-spoon samples and the
standard dimensions for samplers are described in ASTM D1586 (American
Society for Testing and Materials, 1984). The number of blows required to
drive the split-spoon sampler provides an indication of the
compaction/density of the soils being sampled. Because only 18-inch
intervals are sampled out of every 5 feet penetrated, drilling character-
istics (i.e. rate of penetration, vibrations, stability, etc.) of the
formation being penetrated are also used to infer characteristics of
unsampled material. "Continuous" samples can also be taken with the split-
spoon method by augering or drilling to the bottom of the previously-
sampled interval and continuously repeating the operation. In order to
obtain more accurate "N" values, a better approach is to attempt to collect
two samples every five feet. This minimizes collection of samples in the
disturbed zone in front of the bit. Continuous sampling is more time
consuming, but is often the best way to obtain good stratigraphic data in
unconsolidated sediments.
Table 20 shows the penetration characteristics of a variety of
unconsolidated materials. The samples collected by split-spoon sampler are
considered to be disturbed samples. They are, therefore, unsuitable for
running certain laboratory tests, such as permeability.
TABLE 20. STANDARD PENETRATION TEST CORRELATION CHART (AFTER ACKER. 1974)
SoH Type Designation
BlOWvFOOi
{
0-10
*-S
Veiy Dense >50
Clay
VeiySott
son
Medium
Stiff
Hard
»
•;2
3-5
16-25
>25
•Assumes: a) 2-inch outside diameter by IH-inch inside diameter sampler
b) 140-pound hammer falling through 30 inches
Thin-Wall Samplers
Work performed by Hvorslev (1949) and others have shown that if
relatively undisturbed samples are to be obtained, it is imperative that
113
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the thickness of the wall of the sampling tube be less than 2.5 percent of
the total outside diameter of the sampling tube. In addition, the ratio of
the total area of the sampler outside diameter to the wall thickness area
(area ratio) should be as small as possible. An area ratio of
approximately 10 percent is the maximum acceptable ratio for thin-wall
samplers; hence, the designation "thin-wall" samplers. Because the
split-spoon sampler must be driven to collect samples, the wall thickness
of the sampler must be structurally sufficient to withstand the driving
forces. Therefore, the wall thickness of a split spoon sampler is too
great for the collection of undisturbed samples.
The standard practice for collecting thin-wall samples, commonly
referred to as Shelby tube samples, requires placing the thin-wall sampling
tube at the end of the sampling drill rods. The sampler and rods are
lowered to the bottom of the borehole just as is done with the split-spoon
sampler. Instead of driving the sampler into the ground, the weight of the
drill rig is placed on the sampler and it is pressed into place. This
sampling procedure is described in detail in ASTM D1S87 (American Society
for Testing and Materials, 1983). A typical thin-wall sampler is shown in
Figure 41.
The requirement that the area ratio be as small as possible presents a
serious limitation on obtaining undisturbed samples in compact sediments.
A thin-wall sampler may not have sufficient structural strength to
penetrate these materials. A standard 2-inch inside diameter thin-wall
sampler will frequently collapse without satisfactorily collecting a sample
in soils with "N" values of 30 or greater. "N" values are a standard
method of comparing relative density as derived from blow counts and are
explained in ASTM D1S86 (American Society-for Testing and Materials, 1984).
Specialized Soil Samplers
Many special-function samplers have been developed to deal with
special conditions. These include: 1) structurally strong thin-wall
samplers that collect "undisturbed" samples, 2) large-diameter samplers
that collect coarse sand and gravel for gradation analyses and 3) piston
samplers that collect samples in heaving sands. Two good examples of the
reinforced-type design are the Vicksburg sampler and the Dennison sampler,
as shown in Figures 42a and 42b. Both samplers were developed by the
United States Army Corps of Engineers and are so named for the districts in
which they were first developed and used. The Vicksburg sampler is a
5.OS-inch inside diameter by 5.25-inch outside diameter sampler that
qualifies as a thin-wall sampler but is structurally much stronger than a
Shelby tube. The Dennison sampler is a double-tube core design with a thin
inner tube that qualifies as a thin-wall sampler. The outer tube permits
penetration in extremely stiff deposits or highly cemented unconsolidated
materials while the inner tube collects a thin-wall sample.
Examples of piston samplers are the internal sleeve piston sampler
developed by Zapico et al. (1987) and the wireline piston sampler described
by Leach et al. (1988) (Figures 43 and 44). Both samplers have been
designed to be used with a hinged "clam-shell" device on the cutting head
of a hollow-stem auger (Figure 45). The clam shell has been used in an
114
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Head assembly
> Cap screw
Tube
Figure 41. Diagram of a thin-wall sampler (Acker Drill Company, Inc., 1985).
115
-------
Drill rod
Air hose
N" rod coupling
Plug
Sampler head
Clamp
Wrench holes
Adapter
Allen set screws
Rubber gasket
Sampling tube
• Outer tube
• Bearing
Inner head
• Inner tube
• Liner
Sawtooth bit
Basket retainer
Inner tube extension
Bit
(a)
(b)
Figure 42. Two types of special soil samplers: a) Vicksburg sampler (Krynine and Judd. 1957) and
b) Denison sampler (Acker Drill Company, Inc., 1985).
116
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Upper drive
head with left
threaded pin
Piston cable
Hardened drive
shoe
Schematic
inner core barrel
'dedicated)
Outer core barrel
0 «oo *iin rubber
& brass
Figure 43. Internal sleeve wireline piston sampler (Zapico
et si., 1987).
117
-------
Brass bushings
Teflon wiper disc
Swivel
Neoprene seals
Figure 44. Modified wireline piston sampler (Leach et aL, 1988).
118
-------
Auger-head
Bit
Clam-shell
Figure 45. Clam-shell fitted auger head (Leach et al., 1988).
(a) Basket
(b) Spring
(c) Adapter ring
(d) Flap valve
Figure 46. Types of sample retainers (Mobile Drilling Company, 1982).
119
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attempt to: 1) improve upon a non-retrievable knock-out plug technique,
2) simplify sample retrieval and 3) increase the reliability of the
sampling procedure in heaving sand situations. The Zapico et al. (1987)
device requires the use of water or drilling mud for hydrostatic control
while the Leach et al. (1988) device permits the collection of the sample
without the introduction of any external fluid. The limitation of using
this technique is that only one sample per borehole can be collected
because the clan shell device will not close after the sampler is inserted
through the opening. This means that although sample reliability is good,
the cost per sample is high.
In both split-spoon and thin-wall sampling, it is common for a portion
of the sample to be lost during the sampling process. One of the items to
be noted in the sample description is the percent recovery, or the number
of inches that are actually recovered of the total length that was driven
or pressed. To help retain fine sand and gravel and to prevent the sample
from being lost back into the borehole as the sample is removed, a "basket"
or a "retainer" is placed inside the split-spoon sampler. Figure 46 shows
the configuration of four commercially-available types of sample retainers.
A check valve is also usually installed above the sampler to relieve
hydrostatic pressing during sample collection and to prevent backflow and
consequent washing during withdrawal of the sampler.
Except for loss of sample during collection, it is possible to collect
continuous samples with conventional split-spoon or thin-wall techniques.
These involve: 1) collecting a sample, 2) removing the sampler from the
borehole, 3) drilling the sampled interval, 4) reinserting the sampler and
5) repeating the process. This effort is time consuming and relatively
expensive, and it becomes increasingly expensive in lost time to remove and
reinsert the sampler and rods as the depths exceed 100 feet.
To overcome this repeated effort, continuous samplers have been
developed. One such system is shown in Figure 47. A continuous sample is
taken by attaching a 5-foot long thin-wall tube in advance of the cutting
head of the hollow-stem auger. The tube is held in place by a specially
designed latching mechanism that permits the sample to be retracted by wire
line when full and replaced with a new tube. A ball-bearing fitting in the
latching mechanism permits the auger flights to be rotated without rotation
of the sampling tube. Therefore, the sampling tube is forced downward into
the ground as the augers are rotated.
Core Barrels
When installing monitoring wells in consolidated formations the
reliability and overall sample quality of the drilled samples from either
direct fluid rotary or air, water and foam systems is very similar to that
of the samples obtained in unconsolidated formations. Where reliable
samples are needed to fully characterize the monitored zone, it is
suggested that cores be taken. Coring can be conducted by either wireline
or conventional methods. Both single and double-tube core barrels are
available as illustrated in Figures 48a and 48b.
120
-------
Auger drill
rig
Auger column
Barrel sampler
Non-rotating
sampling rod
Auger head
Figure 47. Diagram of a continuous sampling tube system (After Central Mine Equipment Company,
1987).
121
-------
.Tube
Hanger
bearing
assembly
Reaming
shell
Reaming
shell
'PI
Figure 48. Diagram of two types of core barrels:
a) single-tube and b) double-tube (Mob*. Drilling Company, 1982).
122
-------
In coring, the carbide or diamond-tipped bit is attached to the lower
end of the core barrel. As the bit cuts deeper, the formation sample moves
up the inside of the core tube. In the single-wall tube, drilling fluid
circulates downward around the core that has been cut, flows between the
core and the core barrel and exits through the bit. The drilling fluid
then circulates up the annular space and is discharged at the land surface
Because the drilling fluid is directly in contact with the core, poorly-
cemented or soft material is frequently eroded and the core may be
partially or totally destroyed. This problem exists where formations are
friable, erodable, soluble or highly fractured. In these formations very
little or no core may be recovered.
In these circumstances a double-wall core barrel may be necessary. In
a double-wall core barrel, the drilling fluid is circulated between the two
walls of the core barrel and does not directly contact the core that has
been cut. As drilling fluid circulates between the two walls of the core
barrel, the core moves up into the inner tube, where it is protected. As a
result, better cores of poorly-consolidated formations can be recovered.
Good recovery can be obtained even in unconsolidated clays and silts using
a double-wall coring technique.
SELECTION OF DRILLING METHODS FOR MONITORING WELL INSTALLATION
Matrix Purpose
The most appropriate drilling technology for use at a specific site
can only be determined by evaluating both the hydrogeologic setting and the
objectives of the monitoring program. To assist the user in choosing an
appropriate drilling technology, a set of 'matrices has been developed that
lists the most commonly used drilling techniques for monitoring well
installation and delineates the principal criteria for evaluating those
drilling methods. A matrix has been developed for a unique set of
hydrogeologic conditions and well design requirements that limit the
applicability of the drilling techniques. Each applicable drilling method
that can be used in the described hydrogeologic setting and with the stated
specific design requirements has been evaluated on a scale of 1 to 10 with
respect to the criteria listed in the matrix. A total number for each
drilling method was computed by adding the scores for the various criteria.
The totals represent a relative indication of the desirability of the
drilling methods for the specified conditions.
Matrix Description and Development
A set of 40 matrices has been developed to depict the most prevalent
general hydrogeologic conditions and well design requirements for
monitoring wells. The complete set of matrices are included as Appendix B.
The matrices were developed from a combination of five factors including:
1) unconsolidated or consolidated geologic formations
encountered during drilling,
2) saturated or unsaturated conditions encountered during
drilling,
3) whether or not invasion of the monitored zone by drilling
fluid is permitted,
123
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4) depth range of the monitoring well: 0 to IS feet,
15 to 150 feet or greater than 150 feet and
5) casing diameter of the monitoring well: less than 2 inches,
2 to 4 inches or 4 to 8 inches.
Table 21 indicates the number of the matrix that corresponds to the
combination of factors used to develop the numbers on each matrix.
Each matrix provides a relative evaluation of the applicability of
selected drilling methods commonly used to construct monitoring wells. The
drilling methods evaluated in the matrix include:
1) hand auger,
2) driving,
3) jet percussion,
4) solid flight auger,
5) hollow stem auger,
6) mud rotary,
7) air rotary,
8) air rotary with casing driver,
9) dual-wall rotary and
10) cable tool.
A complete description of these drilling techniques and their applicability
to monitoring well installations can be found in the beginning of this
chapter under the heading entitled "Drilling Methods for Monitoring Well
Installation."
The drilling techniques have been evaluated with respect to a set of
criteria that also influences the choice of a drilling method. These
additional criteria include:
1) versatility of the drilling method)
2) sample reliability,
3) relative drilling cost,
4) availability of drilling equipment,
5) relative time required for well installation and development,
6) ability of drilling technology to preserve natural conditions,
7) ability to install design diameter of well and
8) relative ease of well completion and development.
A complete discussion of the importance of these factors can be found in
this section under the heading entitled "Criteria For Evaluating Drilling
Methods."
Each matrix has three main parts (Figure 49). The top section of the
page contains a brief description that delineates which unique combination
of general hydrogeologic conditions and well design requirements apply to
evaluations made in that matrix. The middle of the page contains a chart
that lists the ten drilling methods on the vertical axis and the eight
criteria fox evaluating the drilling methods on the horizontal axis. This
chart includes relative judgments, in the form of numbers, about the
applicability of each drilling method. The bottom of the page contains
explanatory notes that further qualify the general hydrogeologic conditions
and well design requirements that have influence on the development of the
numerical scheme in the chart.
124
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TABLE 21. INDEX TO MATRICES 1 THROUGH 40
\ «*ss*
\
Matrix \
Number \
i
2
3
4
5
s
1
B
9
10
11
12
13
14
15
16
17
18
19
20
21
2?
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
|
i
X
X
X
X
1
X
X
X
x
x
X
X
X
X
X
X
X
X
X
X
X
„
X
3
X
x
X
X
X
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
£
X
X
X
X
X
X
X
X
X
X
X
Depth 0-15 Feel
x
X
X
X
X
X
X
X
X
X
X
Depth 15- ISO Feel
x
,
X
I
x
x
x
x
X
x
X
X
Depth > 150 Feet
X
*
X
X
X
x
x
X
X
X
X
x
< 2-Inch Diameter Casing
x
X
X
X
X
X
X
x
x
Jf
X
2-4 Inch Dtameler Casing
X
x
x
x
—
x
x
x
x
]l
x
x
x
4*8 Inch Diameter Casing
x
X
x
x
X
x
x
x
x
x
x
X
125
-------
MATRIX NUMBER 1
General Hydrogeotogic Conditions & Well Design Requirements
Unconsolidated; saturated: invasion of formation by drilling fluid permitted; casing diameter 2 inches or less; total
well depth 0 to 15 feet.
\ H
\ li
\ "* 5
\ si
\ H
\ t°
\ o
DRILLING \
METHODS \
Hand Auger
Driving
•o
o
£
0)
I
Q
"5
00
e
1
i
^
a
IS
i
91
Q.
i
5
1
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
2
3
10
8
NA
7
7
9
1
4
10
10
NA
5
a
10
a
I
i
2
1
(C
9
10
8
7
9
a
NA
6
6
5
1
c
a
5
UJ
ai
c
i
"5
2-
2
1
10
10
10
9
9
10
NA
4
1
7
I
si
' o
II
ll
l§
II
ll
5
5
0
ll
c ^
H
f 3
Q|
B §
11
9
5
1
5
10
10
•7
NA
6
6
4
1
4
8
4
NA
9
9
10
5
U
Q
c
I
1
£
o
IT!
a *
< 0
6
1
1
5
10
10
NA
10
10
10
§
is
Q.
3
1-
• "3 I
U) 0
-I
1 =
C a
4
4
TOTAL
49
37
1
2
9
5
NA
10
9
10
29
44
75
62
NA
57
56
65
EXPLANATORY NOTES.
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3 The anticipated use ot the monitoring well permits the use of drilling fluid and additives in construction
4. The shallow depth of up to 15 feet and small completed well diameter of 2 inches or less allows maximum flexibility in equipment
5 Samples collected in solid flight auger, hollow-stem auger, mud rotary and cable-tool holes are taken by standard split-spoon
(ASTM 01586) or thin-wall sampling (ASTM 01587) techniques, at 5-loot intervals.
Figure 49. Format for a matrix on drilling method selection.
126
-------
The numbers in the charts are generated by looking at each of the
criteria for evaluating drilling methods and evaluating each drilling
method on that one criteria with respect to the conditions dictated by the
prescribed five general hydrogeologic conditions and well design
requirements. The most applicable drilling method is assigned a value of
10 and the other drilling methods are then evaluated accordingly. The
process always includes assigning the number 10 to a drilling method. Once
each of the criteria is evaluated, the numbers for each drilling method are
summed and placed in the total column on the right. Where a drilling
method is not applicable, the symbol, "NA," for not applicable, is placed
in the row for that drilling method.
How To Use the Matrices
The matrices are provided as an aid to the user when selecting the
appropriate drilling technique under selected conditions. The user should
begin by referring to Table 21 and choosing the number of the matrix that
most closely parallels the hydrogeologic conditions at the site and that
has the same anticipated well depth and casing diameter requirements. The
user should then refer to that matrix in Appendix B, read the explanatory
notes and refer to the relative values in the "total" column of the matrix.
Explanatory text for both the drilling methods and the criteria for
evaluating drilling methods should be reviewed to understand the
assumptions and technical considerations included in the relative numbers.
How To Interpret a Matrix Number
The numbers contained in the "total" column of the chart represent a
relative indication of the desirability of" each drilling method for the
prescribed conditions of the matrix. Higher total numbers indicate more
appropriate drilling methods for the specified assumptions. When numbers
are relatively close in value, drilling methods may be almost equally as
favorable. Where numbers range more widely in value, the matrix serves as
a relative guide for delineating a favorable drilling method. The numbers
cannot be compared between matrices; numerical results are meaningful only
when compared on the same chart. The purpose of the numerical rating is to
provide the user with a relative measure of the applicability of drilling
methods in specific situations.
Once the user consults the matrix for a preliminary evaluation, it is
necessary to re-evaluate the numbers in terms of the factors that locally
impact the ultimate choice of a drilling method: equipment availability
and relative drilling cost. A drilling method might be indicated as the
most favorable technique according to the matrix totals, but the equipment
may not be available or the cost factor may be prohibitive. In these
situations, an alternative drilling method will need to be chosen or the
design criteria modified. The drilling costs have been evaluated in the
matrix based on relative national costs. Recognizing that relative costs
may vary, the user of the matrix should look carefully at the relative cost
column to determine if the relative costs are applicable for the specific
geographic location of interest. Adjustments should be made if costs
differ significantly.
127
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CRITERIA FOR EVALUATING DRILLING METHODS
In determining the most appropriate drilling technology to use at a
specific site the following criteria must be considered. These criteria
must encompass both hydrogeologic settings and the objectives of the
monitoring/drilling program.
Versatility of the Drilling Equipment and Technology with Respect to the
Hydrogeologic Conditions at the Site
The drilling equipment must effectively deal with the full range of
conditions at each site and also allow the satisfactory installation of
well components as designed. The choice of proper drilling techniques
requires specific knowledge of: 1) the objectives of the monitoring well,
including desired well depth and casing diameter, 2) the type(s) of
geologic formations to be penetrated and 3) the potential borehole
instability and/or completion difficulties. Additional factors that
influence the choice of a drilling method include: 1) saturation or
unsaturation of the zone(s) to be drilled, 2) necessity to install a filter
pack in the monitoring well and 3) potential adverse effects on the final
monitoring program by drilling fluid invasion into the monitored zone.
The interaction between the geologic formations, hydrologic conditions
and the equipment to be used is best illustrated by example. After
reviewing the discussion of drilling methods in the beginning of this
section, it should be obvious that hollow-stem augers can be used
effectively in unconsolidated materials, but are not applicable to the
installation of monitoring wells in solid rock such as granite. It may be
less obvious that drilling through the saturated, unstable overburden
overlying solid rock, such as granite, may be very difficult with the air
rotary technique; however, the air rotary technique would be very effective
in drilling the granite. The overburden, conversely, can be very
effectively dealt with by hollow-stem augers.
If the monitoring objectives in this illustration include pumping at
relatively high rates, then a 4-inch or larger casing may be required. The
installation of the casing mandates the use of a large inside diameter
hollow-stem auger unless the overburden is sufficiently stable to permit
open-hole casing installation. If either the casing diameter is too large
or the depth is too great, then hollow-stem augers are not appropriate and
an alternative drilling technique (e.g. mud rotary, cable tool, drill
through casing hammer, etc.) must be evaluated. Thus, Judgment has to be
made for each site whether or not the preferred drilling technology can
deal with the extant hydrogeologic conditions and the objectives of the
monitoring program.
Reliability of Formation (Soil/Rock/Water) Samples Collected During
Drilling
The purpose of a monitoring well is to provide access to a specific
zone for which water level (pressure head) measurements are made, and from
which water samples can be obtained. These water samples must accurately
128
-------
represent the quality of the water in the ground in the monitored zone. To
this end, it is essential to acquire accurate, representative information
about the formations penetrated during drilling and specifically about the
intended monitored zone. Sample reliability depends partially on the type
of samples that can be taken when using various drilling techniques. The
type of samples attainable and the relative reliability of the samples are
summarized in Table 19 and discussed below in terms of drilling methods.
An additional discussion of sampling techniques is found in the section
entitled "Soil Sampling and Rock Coring Methods."
Hand Auger--
Soil samples that are taken by hand auger are disturbed by the
augering process and are usually collected directly from the cutting edge
of the auger. Deeper samples may be non-representative if sloughing of
shallow materials occurs. Drilling by hand auger is usually terminated
when the saturated zone is encountered. It is possible to continue
drilling below the saturated zone in some situations by adding water and/or
drilling mud. However, when water and/or drilling mud are added, reliable
samples cannot usually be obtained. An additional discussion of hand
augering can be found in the section entitled "Drilling Methods for
Monitoring Well Installation."
Driven Wells--
No samples can be taken during the construction of a driven well,
although some interpretation of stratigraphlc variation can be made from
the driving record. Water-quality samples can be obtained in any horizon
by pumping from that depth of penetration. An additional discussion of
driven wells can be found in the section ant It led "Drilling Methods for
Monitoring Well Installation."
Jet Percussion--
Neither valid soil samples nor valid water samples can be obtained
during the construction of wells by this mathod. Only gross lithology can
be observed in the material that is washed to the surface during the
jetting procedure. An additional discussion of jet percussion drilling can
be found in the section entitled "Drilling Methods for Monitoring Well
Installation."
Solid Flight Augers—
Soil samples collected from solid, continuous flight augers are
rotated up the auger flights to the surface during drilling or scraped from
the auger flights upon extraction. The disturbed samples from either of
these sources provide samples of moderate quality down to the first
occurrence of water, and generally unreliable samples below that level.
More valid samples can be obtained where the borehole is stable enough
to remain open. In this situation, the auger flights can be removed from
the borehole and samples can then be taken by either split-spoon (ASTM
D1586) or thin-wall (ASTM D1587) sampling techniques. It is generally not
129
-------
possible to use these techniques in saturated formations with the augers
removed because the borehole frequently collapses or the bottom of the
borehole "heaves" sand or silt upward into the open borehole. The heaving
occurs as a consequence of differential hydrostatic pressure and is
exacerbated by the removal of the augers. When caving or heaving occurs,
it is very difficult to obtain reliable samples. An additional discussion
on solid-flight augers can be found in the section entitled "Drilling
Methods for Monitoring Well Installation."
Hollow-Stem Augers--
Where samples are collected from depths of less than 150 feet, the
hollow-stem auger technique is the method most frequently used to
obtain samples from unconsolidated formations. Samples may be taken
through the hollow-stem center of the augers by split-spoon (ASTM D1586),
thin-wall (ASTM D1587) or wireline piston sampling methods (refer to
Figures 40 through 44). The maximum outside diameter of the sampler is
limited by the inside diameter of the hollow stem. If 3.25-inch inside
diameter augers are being used, then a maximum 3-inch outside diameter
sampler can be used and must still retain the requisite structural strength
and meet the requirement to optimize (minimize) the area ratio. An
additional discussion on soil sampling can be found in the section
entitled "Soil Sampling and Rock Coring Methods."
The rotation of the augers causes the cuttings to move upward and
debris to be ground and "smeared" along the borehole in the thin annular
zone between the borehole wall and the auger flights. This smearing has
both positive and negative connotations. Because the movement of debris is
upward, the cuttings from the deeper zones may seal off shallower zones.
This minimizes cross-connection of fluids from shallow to deep zones, but
increases the possibility of deep to shallow contamination. Shallow zones
that may have been penetrated in the upper portion of the borehole are also
difficult to develop once smearing occurs. With the shallow zones sealed
off by cutting debris and with the auger flights serving as temporary
casing, it is often possible to obtain valid formation samples of discrete
saturated zones as they are initially penetrated.
Water samples are difficult to obtain in the saturated zone during
drilling due to formation instability. A special type of lead auger flight
has been designed to overcome the problem of collecting water samples
concurrent with drilling and to make it possible to sample and/or pump test
individual zonea as the augers are advanced. This specially reinforced
screened auger serves as the lead, or lowermost, auger and is placed just
above the cutting head (Figure 28). This screened section can be used to
temporarily stabilize the borehole while a small-diameter pump or other
sampling device is installed within the hollow stem. Appropriate testing
can then be performed. The advantage of this technique is low-cost
immediate data and water sample acquisition during drilling. The major
disadvantages are: 1) doubt about cross-connection of zones and ultimate
data validity and 2) the risk of losing both the equipment and the borehole
if extremely difficult drilling conditions are encountered since there is
some structural weakness in the screened section. An additional discussion
of hollow-stem augers can be found in the section entitled "Drilling
Methods for Monitoring Well Installation."
130
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Direct Mud Rotary Drilling--
A variety of sampling technologies can be used in concert with mud
rotary drilling techniques. These include: 1) grab or ditch samples from
circulated cuttings, 2) split-spoon and thin- walled samples in
unconsolidated materials and 3) single and double-tube conventional core
barrels in consolidated materials. In direct rotary drilling, the
functions of the drilling fluid are to: 1) lubricate and cool the bit, 2)
remove fragmentary particles as they are loosened and 3) stabilize the
borehole. The cuttings are typically circulated up the borehole, through a
pipe or ditch, into a temporary settling tank or pit. The drilling fluid
is then circulated back down the drill pipe (Figure 29).
Samples taken from the ditch or settling pond (mud pit) are therefore
a composite of: 1) materials cut a few minutes earlier (time lag varies
with depth, borehole size, drill pipe and pump rate), 2) any unstable
materials that have washed or fallen into the borehole from a shallower
zone and 3) any re-circulated materials that failed to settle out during
earlier circulation. These materials are mixed with the drilling fluid and
any additives used during the drilling process. The interpretation of
these samples requires experience and even then the interpretation is
questionable. Ditch samples are frequently collected in the petroleum
industry, but have little practical value in the effective installation of
monitoring wells. Thin, stratified zones that require specific monitoring
are difficult to identify from ditch samples.
Both split-spoon (ASTM D1586) and thin-wall samples (ASTM D1587) can
be obtained while using direct rotary drilling methods in unconsolidated
materials. At shallow depths, samples are taken through the drill bit in
exactly the same manner as previously described for hollow-stem augers.
Corresponding size limitations and sampling problems prevail.
As depths increase below about 150 feet, the time consumed in taking
split-spoon and thin-wall samples becomes excessive and wireline sampling
devices are used to collect and retrieve samples. Samples can be taken
either continuously or intermittently. In unconsolidated materials,
wireline samplers can collect only disturbed samples and even then there
are recovery problems and limitations for both fine and coarse-grained
materials. In consolidated rock the best samples can be obtained by
coring.
A significant advantage of drilling with a good drilling mud program
is that typically the open borehole can be stabilized by the drilling mud
for a sufficient period of time to remove the drilling tools and run a
complete suite of geophysical logs in the open hole. This information is
used in concert with other data (i.e. the drilling time log, the sample
log, fluid loss or gain information and drilling characteristics) to
provide definitive evaluation of formation boundaries and to select screen
installation intervals.
When attempting to define the in-situ properties of unconsolidated
materials, drilling by the mud rotary method offers another advantage.
Because the drilling mud maintains the stability of the borehole, samples
131
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taken by split-spoon or thin-wall methods ahead of the drill bit tend to be
ouch more representative of indigenous formation conditions than those
samples taken, for example, during hollow-stem auger drilling. In auger
drilling it is sometimes very difficult to obtain a sample from below the
cutting head that has not been affected by the formation heaving upward
into the open borehole.
If the drilling fluid is clear water with no drilling additives, then
it may be difficult to maintain borehole stability because little mudcake
accumulates on the wall of the borehole. In this case, the loss or gain of
water while drilling is an indication of the location of permeable zones.
Because drilling fluid is used to drill the borehole and because this
fluid infiltrates into the penetrated formations, limited water-quality
information can be obtained while drilling. Drilling mud seals both high
and low-pressure zones if properly used. However, this sealing action
minimizes interaquifer cross-contamination while drilling. Before any zone
provides representative samples, all drilling mud and filtrate should be
removed from the formation(s) of interest by well development.
The most common additives to drilling mud are barite (barium sulfate)
for weight control and sodium montmorillonite (bentonite) for viscosity and
water loss control. Both can alter indigenous water quality.
Bentonite is extremely surface active and forms clay/organic complexes
with a wide range of organic materials. The water used to nix the drilling
mud is potentially interactive both with the drilling mud and with the
water in the formation. At the very least, the drilling fluid dilutes the
formation water that is present prior to the drilling activity. For these
reasons it is very difficult, if not impossible, to be confident that
sufficient development has been performed on a direct rotary-drilled
monitoring well, and that the water quality in a particular sample is truly
representative of the water quality in place prior to the construction of
the well.
Where very low concentrations of a variety of contaminants are being
evaluated and where the potential reactions are undefined, it is not
recommended that drilling fluid be used during monitoring well
installation. This same concept applies to boreholes drilled by cable tool
and/or augering techniques where drilling fluid Is necessary for borehole
stability. Where drilling mud is used, monitoring well development is
continued until such time as a series of samples provides statistical
evidence that no further changes are occurring in key parameters. When
this occurs, the resultant quality is considered to be representative
(Barcelona, et al., 1985a). An additional discussion of drilling fluids
can be found In the section entitled "Drilling Fluids."
Water-level measurements of different zones penetrated cannot be
determined while drilling with direct rotary methods. Accurate water
levels can only be determined by installing, screening and developing
monitoring wells in the specific zones of interest. An additional
discussion on direct mud rotary drilling can be found in the section
entitled "Drilling Methods for Monitoring Well Installation."
132
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Air Rotary--
Direct air rotary is restricted in application to consolidated rock.
Where the bedrock is overlain by unconsolidated materials, a borehole can
be drilled and sampled by alternative methods including: 1) roller-cone
bit with water-based fluid, 2) air with a casing driver, 3) cable tool or
4) augering. Formation samples are taken by the appropriate methods
discussed in the related sections of this discussion. Once surface casing
is installed and sealed into bedrock, the underlying bedrock can be
successfully drilled using air rotary methods.
When using air rotary drilling in semi-consolidated and consolidated
materials, air is circulated down the drill pipe and through the bit. The
air picks up the cuttings and moves the cuttings up through the annular
space between the drill pipe and the wall of the borehole. If the
formations drilled are dry, the samples reach the surface in the form of
dust. By injecting water or a mixture of water and surfactant (foam):
1) dust is controlled, 2) regrinding of samples is minimized and 3) the
sizes of individual particles are increased sufficiently to provide good
formation samples. Because the injected water/foam is constantly in motion
and supported by the air, there is only a slight possibility of water loss
or formation contamination during drilling.
After water is encountered in the borehole, further injection of water
from the surface can often be eliminated or minimized and good rock
fragments can be obtained that are representative of the formations
penetrated. Samples obtained in this manner are not affected by the
problems of recirculation, lag time and drilling fluid contamination that
plague sample evaluation when drilling mud'is used. Air may cause changes
in the chemical and biological activity in the area adjacent to the
borehole. Examples of quality changes include oxidation and/or stripping
of volatile organic chemicals. The time required for these changes to be
reversed varies with the hydrogeologic and geochemical conditions. Because
the rock boreholes are generally stable and penetration rates are high,
there is minimal contamination from previously-drilled upper zones.
Water-quality samples and water levels can be easily obtained from the
first saturated zone penetrated, but this zone must be cased if subsequent
zones are to be individually evaluated.
For monitoring well installation, the injected air must be filtered
prior to injection to prevent contamination of the borehole by oil
exhausted by the air compressor. Because a down-the-hole hammer requires
lubricating oil for operation, it has more limitations for monitoring well
installation. An additional discussion on air rotary drilling can be found
in the section entitled "Drilling Methods for Monitoring Well
Installation."
Air Rotary With Casing Driver--
Unconsolidated formations can be drilled and sampled by combining air
rotary drilling with a casing driver method. In this procedure the drill
bit is usually extended approximately one foot below the bottom of the open
133
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casing, and the casing is maintained in this position as the drill bit is
advanced (Figure 33). The casing is either large enough to permit
retraction of the bit, in which instance the casing must be driven through
the undergauge hole cut by the bit; or an underreamer is used, and the
casing moves relatively easily down into the oversized borehole.
Generally, the undergauge procedure is favored for sampling unconsolidated
formations, and the underreamer is favored for semi-consolidated
formations. Either technique allows good samples to be obtained from the
freshly-cut formation and circulated up the cased borehole. If chemical
quality of the formation sample is important, particularly with regard to
volatile organics or materials that can be rapidly oxidized, then air
drilling may not be appropriate. When the casing is advanced coincident
with the deepening of the borehole, the sample collection procedures and
the sample quality are very similar to those prevailing with the use of
direct air rotary. An additional discussion on air rotary with casing
driver can be found in the section entitled "Drilling Methods for
Monitoring Well Installation."
Dual-Wall Reverse Circulation Rotary--
In dual-wall reverse circulation rotary drilling, either water or air
can be used as the circulation medium. The outer wall of the dual-wall
system serves to case the borehole. Water (or air) is circulated down
between the two casing walls, picks up the cuttings at the bottom of the
borehole, transports the cuttings up the center of the inner casing and
deposits then at the surface. Because the borehole is cased, the samples
collected at the surface are very reliable and representative of the
formations penetrated. Sample collection using dual-wall rotary has the
following advantages: 1) thin stratigraphic zones often can be identified;
2) contamination of the borehole by drilling fluid is minimized;
3) interaquifer cross-contamination is minimized; ft) individual zones that
are hydraulically distinct can be identified with specific water levels,
and discrete samples often can be collected if sufficient time is allowed
for recovery; 5) in low hydraulic pressure formations, air pressure within
the borehole may prevent the formation water from entering the borehole and
6) sampling at the surface can be continuous. Split-spoon samples can also
be collected through the bit. One disadvantage is that because the outer
casing is removable and not sealed by grout, hydraulic leakage can occur
along the outside of the unsealed casing.
Water or foam can be injected to increase the penetration rate and
improve sample quality. An additional discussion of dual-wall reverse
circulation rotary drilling can be found in the section entitled "Drilling
Methods for Monitoring Well Installation."
Cable Tool--
When drilling in saturated, unconsolidated sand and gravel, good-
quality disturbed samples can be obtained by the cable tool "drive and
bail" technique. In this technique, casing is driven approximately 2 to 5
feet into the formation being sampled. The sample is then removed from the
casing by a bailer. For best sample quality, a flat-bottom bailer is used
134
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to clean the borehole (Figure 36). The entire sample is then collected at
the surface, quartered or otherwise appropriately split and made available
for gradation analyses. When drilling in unsaturated material, water must
be added to the borehole during drilling and sampling.
The drive and bail technique is often the best method for sampling
well-graded or extremely coarse-grained deposits because both coarse and
fine-grained fractions are collected during sampling. Large-diameter
casing can be driven and large bailers can be used. The most common size
range for casing is from 6 inches to 16 inches although larger sizes are
available. For the drive and bail technique to be effective, excessive
heaving of the formation upward into the casing during cleanout must be
prevented. This can usually be controlled by: 1) overdriving the casing,
thereby maintaining a "plug" of the next sample in the casing at all times,
2) careful operation of the bailer and 3) adding water to the borehole to
maintain positive hydraulic head within the borehole.
During drive and bail-type drilling, split-spoon (ASTM D1586) and
thin-wall (ASTM D1587) samples can be collected after cleaning out the
casing with the bailer. Samples are collected ahead of the casing by
inserting conventional sampling tools inside the casing. This technique
permits sampling of fine-grained, unconsolidated formations.
The quality of cable tool samples from consolidated formations varies
with drilling conditions. When the bedrock is saturated, good broken chips
of the formation can be obtained by bailing at frequent intervals. If the
chips remain in the borehole too long or if sufficient lubrication is
lacking, the samples are re-ground to powder.
When drilling by cable tool techniques and using a good casing
program, it is usually possible to identify and isolate individual water-
bearing units as they are drilled. This provides the opportunity to obtain
good water-level and water-quality data. An additional discussion on cable
tool drilling can be found in the section entitled "Drilling Methods for
Monitoring Well Installation."
Relative Drilling Costs
Drilling and completion costs vary for individual methods with each
set of general conditions and well design requirements. For example, the
cost of drilling and sampling with the hollow-stem auger method may be much
higher for a dense, bouldery till than it is for a similar depth in
saturated, medium-soft lake clays. The cost of installing nominal 2-inch
diameter casing and screen within hoilow-stem augers varies with depth and
borehole stability.
The relative drilling cost ratings shown on each matrix apply to the
broad range of conditions included within each set of general conditions
and well casing requirements. The relative ratings reflect the total cost
of drilling, sampling, casing, screening, filter-packing, grouting,
developing and surface protecting the monitoring well. Equivalent costs of
mobilization and access are assumed. Relative ratings are based on
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consideration of the average costs when compared to the other methods of
drilling throughout the continental United States. Local cost variations
can be significantly influenced by equipment availability and can cause
variation in these relative ratings. Where local costs vary from the
ratings shown, an adjustment should be made to the specific matrix so that
the actual costs are more accurately reflected.
Availability of Equipment
The ratings shown in the matrices for equipment availability are based
on the general availability of the drilling equipment throughout the United
States. The availability of specific equipment on a local basis may
necessitate the revision of the rating in the matrix to make the rating
more representative.
The type of equipment most generally available for monitoring well
installation is the direct mud rotary drilling rig. Direct mud rotary
techniques are applicable to water supply wells, gas and oil exploration
and development and soil testing. As a result, this equipment is widely
available throughout the country.
Solid-flight and hollow-stem augering equipment is also generally
available throughout all regions where unconsolidated materials
predominate. The portability of augering equipment and the prevalent use
of augers in shallow foundation investigations have increased auger
availability to almost all areas.
Air rotary drilling has primary application in consolidated rock.
Availability of equipment is greatest in those consolidated rock areas
where there are mining exploration, water-supply production activities or
quarrying applications. The availability of this equipment is greatest in:
1) the western mountainous area, 2) the northeast and 3) the northwest
parts of the country.
Casing drivers used in combination with direct air rotary drilling are
somewhat sparsely, but uniformly distributed throughout all regions.
Versatility in screen installation, casing pulling and application in
unconsolidated materials have broadened the use of air rotary with casing
driver techniques.
Dual-wall rotary drilling is becoming increasingly popular because the
technique can be used in a wide range of both consolidated and
unconsolidated formations. Availability is generally restricted to the
west-central and southwestern parts of the county.
Cable tool equipment availability is limited in many portions of the
south, southeast, southwest, west and northwest. It is generally available
in the north-central and northeastern portions of the country.
Relative Time Required for Veil Installation and Development
The time required for drilling the well, installing the casing and
screen and developing the well can be a significant factor when choosing a
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drilling method. For example, if a relatively deep hole drilled with cable
tool techniques takes several days, weeks or longer, there may be
significant scheduling disadvantages. If longer-term supervision is
required, then this additional cost factor must also be taken into account.
The excess cost of supervision is not included in the matrix evaluation.
Similarly, if a direct mud rotary technique is employed to make a fast
installation and an additional three weeks of development is required
before a valid sample can be obtained, the advantages of the rapid
installation need to be re-evaluated.
Ability of Drilling Technology to Preserve Natural Conditions
Assuming that the purpose of a monitoring well is to provide access to
a specific zone for which water-level (pressure head) measurements are to
be made, and from which water samples can be obtained to accurately
represent the quality of the water in place in the zone being monitored
then it is obviously important that the drilling methodology employed must
minimize the disturbance of indigenous conditions or offer a good
possibility that indigenous conditions can be restored. To achieve these
goals, the drilling methodology should result in minimal opportunity for
physical and/or chemical interactions that might cause substantial or
unpredictable changes in the quality of the water being sampled. The
following discussions present some of the problems and potential problems
related to the disturbance of the natural conditions as a consequence of
monitoring well drilling and installation:
1) When using drilling mud in the borehole, filtrate from the drilling
fluid invades the adjacent formations. This filtrate mixes with
the natural formation fluids and provides the opportunity for
chemical reaction between the mud filtrate and the formation fluid.
If chemical reactions occur, "false" water-quality readings may
result. The mixing effect is minimized by good development;
potential chemical reactions are more difficult to deal with in a
reasonably predictable manner. For example, if a high pH filtrate
invades a low pH formation and metals are present in either fluid,
precipitation of the metals can be anticipated in the vicinity of
the borehole. The metals may subsequently be re-dissolved at an
unknown rate, if chemical conditions are not constant. Thus, the
drilling fluid filtrate invasion can result in alternately low
and high readings of metals at different intervals of time.
2) When a monitoring well is drilled with augers, fine silts and clays
commonly smear along the borehole wall and frequently seal the
annular space between the augers and the borehole wall. This
sealing action can then minimize the cross-connection of
discrete zones. However, the fine-grained particulate matter that
is smeared into the zone of interest also reduces the flow from
that zone, introduces the possibility of cross-contamination from
another zone and presents the opportunity for the clays that are
smeared into the zone to sorb contaminants and consequently
generate non-representative water-quality results.
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In mud rotary drilling, a mudcake is deposited on the borehole
wall. This bentonitic mudcake serves to stabilize the borehole and
also has the capacity to sorb both organic and inorganic
constituents.
3) During any drilling process physical disruption of the formation
occurs and grain-to-grain relationships change. Regardless of
whether or not the well is completed with a natural or artificial
filter pack, the flow paths to the well are altered; tortuosity is
changed; Reynolds numbers are modified with flow path and velocity
variations; and equilibrium (if, in fact, the indigenous water
is at equilibrium) is shifted. If the formation is permitted to
collapse, as nay occur in sand and gravel materials, the removal of
the collapsed material exacerbates the problem.
With the changes that occur in the physical setting, it is very
difficult to be confident that the water samples subsequently
collected from the monitoring well truly reflect conditions in the
ground beyond the influence of the disturbed zone around the well.
The changes are of particular concern when analyzing for very low
concentrations of contamination.
It becomes apparent that a drilling technique that has the least
possible disruptive influence on the zone(s) being monitored is preferable
in any given setting. The matrices presented indicate the relative impact
of the various drilling methodologies for the designated circumstances.
Ability of the Specified Drilling Technology to Permit the Installation of
the Proposed Casing Diameter at the Design* Depth
The design diameter for the casing and well intakes(s) to be used in
any monitoring well depends on the proposed use of the monitoring well
(i.e. water-level measurement, high-volume sampling, low-volume sampling,
etc.). When installing artificial filter packs and bentonite seals, a
minimum annular space 4 inches greater in diameter than the maximum outside
diameter of the casing and screen is generally needed. A 2-inch outside
diameter monitoring well would then require a minimum 6-inch: 1) outside
diameter borehole, 2) auger inside diameter or 3) casing inside diameter
for reliable well installation. This need for a 4-inch annular space
places a severe limitation on the use of several currently-employed
drilling technologies.
Fox example, hollow-stem augers have been widely used to install
2 3/8-inch outside diameter monitoring wells. A significant portion of
this work has been performed within 3 L/4-inch inside diameter hollow-stem
augers. At shallow depths, especially less than fifteen feet, it has been
possible to install well intake and casing, filter pack, bentonite seal and
surface grout within the small working space. However, at greater depths,
it is very doubtful if many of these components are truly emplaced as
specified. There simply is not sufficient annular clearance to work
effectively. For a more complete discussion on filter pack and screen
emplacement in hollow-stem augers, refer to Appendix A.
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When drilling with direct air rotary with a casing hammer, the maximum
commonly-used casing size is 8 inches in diameter. The outside diameter of
the monitoring well casing should therefore be 4 inches or less to maintain
adequate working space. Because pipe sizes are classified by nominal
diameters, the actual working space will be somewhat less than the stated
annular diameter unless the actual pipe O.D. is used in calculations.
When drilling through unstable formations with dual-wall reverse
circulation methods, the monitoring well casing must be installed through
the bit. The hole in the bit barely permits the insertion of a nominal
2-inch diameter casing. This method does not allow the installation of an
artificial filter pack because there is no clearance between the bit and
2-inch casing.
The ratings presented in each matrix evaluate the relative ability of
the various methodologies to permit the installation of the design casing
diameters in the indicated hydrogeologic conditions.
Ease of Well Completion and Development
Well completion and development difficulty varies with: 1) well depth,
2) borehole diameter, 3) casing and well intake diameter, A) well intake
length, 5) casing and well intake materials, 6) drilling technique, 7) mud
program, 8) hydrostatic pressure of the aquifer, 9} aquifer transmissivity,
10) other hydrogeologic conditions and 11) geologic conditions that affect
the borehole. The relative ease of dealing with these variables by the
selected drilling equipment is shown in each matrix for the indicated
conditions. For example, where a relatively thin, low-yield aquifer has
been drilled with hollow-stem augers, the muddy clay/silt mixture from the
borehole tends to seal the zone where the well intake is to be set. The
development of this zone is very difficult. If a filter pack has been
installed, development becomes almost impossible. If direct mud rotary is
used to drill this same low transmissivity zone, and the mudcake from the
drilling fluid remains between the filter pack and the borehole wall, very
difficult development can be expected. If the borehole is drilled with
clear water, development might be easier.
For any given scenario a very subtle nodification of procedure may
make the difference between success and failure. The ratings shown in the
matrices are based on general considerations. Their relative values
expressed in the table vary in specific circumstances. Host importantly,
however, is that an experienced observer be able to make on-site
observations and to modify the procedures as the work progresses.
DRILLING SPECIFICATIONS AND CONTRACTS
The cost of installing a monitoring well depends on several factors
including: 1) site accessibility, 2) labor and material costs, 3) well
design, A) well use, 5) well development, 6) well yield and 7) local
geologic conditions (Everett, 1980). Because these factors are variable,
it is important to secure a well contract that addresses these items in a
concise and clear format. Proper formatting helps ensure that the well
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will be constructed as specified in the contract and for the agreed price.
In simple terms, a well-written contract is a quality control check on well
construction.
Monitoring well contracts are typically written in three major
sections including: 1) general conditions, 2) special conditions and 3)
technical specifications. General conditions address items related to the
overall project performance including: scheduling, materials, equipment,
labor, permits, rights of various parties, tests and inspections, safety,
payments, contracts, bonds and insurance (Driscoll, 1986). Special
conditions detail project-specific and site-specific items including: 1} a
general description of the purpose and scope of the work, 2) work schedule,
3) insurance and bond requirements, 4) pertinent subsurface information, 5)
description of necessary permits, 6) information on legal easements, 7)
property boundaries and utility location and 8) a description of tests to
be performed and materials to be used during the project (United States
Environmental Protection Agency, 1975). If general and special conditions
appear to conflict, special conditions of the contract prevail (Driscoll,
1986). Technical specifications contain detailed descriptions of
dimensions, materials, drilling methods and completion methods.
Most contracts are awarded as part of a bidding process. The bidding
process may be either competitive or nan-competitive. In a competitive
bidding process, contractors are asked to submit cost estimates based on a
set of specifications for drilling the monitoring wells. The
specifications are developed prior to the request for cost proposal by
either the client or a consultant to the client. Suggested areas that
should specifically be addressed in the specifications are listed in Table
22.
TABLE 22. SUGGESTED AREAS TO BE ADDRESSED IN MONITORING WELL BIDDING
SPECIFICATIONS
Scope of Work
Site Hydrogeology
• existing reports
• weHlogs
• depth ot wells
Schedule of Work Dates
Well Drilling Installation
materials
drilling method(s)
annular seal installation
development
protective equipment
disposal of cuttings
Record-Keeping and Requirements
Sampling Requirements and Procedures
Site Access
• road construction
• tree clearing
• drainage
• leveling
Decontamination ol Equipment
• procedures
• materials
• disposal of cuttings and liquids
Site Safety
• equipment
e training
Conditions
• permits
• certificates
• utility location
• site clean up
• procedures for drilling difficulties
• non-functioning wells
• government forms required
• chant's nghl to vary quantities or delete items
Payment Procedures
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After cost estimates are obtained, a contractor is selected based on
qualifications and pricing. Although some contracts are awarded by
choosing the lowest bidder, this practice is not suggested unless the
qualifications of the contractor indicate that a quality job can be
performed. It is good policy to meet with the selected bidder prior to
signing the contract and clarify every technical point and related unit
cost. This understanding, duly noted by minutes of the meeting, can
eliminate costly errors and misunderstandings. An inspection of the
contractor's equipment that will be used on the job should also be made.
Qualifications of contractors are often evaluated during a
prequalification process. A contractor prequalifies by submitting
information about previous job experience that is related to the scope of
work. The prequalification process allows the client to accept bids only
from contractors that demonstrate specific qualifications to perform the
job. This process helps to ensure that the monitoring wells will be
installed by competent contractors. When subcontractors for drilling or
supplies are to be employed, the list of subcontractors should also be
approved prior to the contract award.
Another way to avoid misunderstandings during the bidding process is
to hold a bidders meeting. In a bidders meeting, the potential contractors
meet in a group forum with the client to discuss the overall scope of the
proposed work and to discuss specifications for monitoring well
installation. Any questions about the specifications or problems with
performance according to the specifications can be discussed and resolved
prior to proposal submission. All information must be provided equally to
all prospective bidders.
In non-competitive bidding, cost estimates are provided by only one
contractor. Because the procedure may be less formal, the contractor may
play a more active role in developing the monitoring well specifications
and presenting a cost estimate. However, a less formal process may also
mean that written specifications for monitoring well installation may never
be developed. This situation should be avoided to help ensure that the
monitoring wells are constructed properly.
Cost proposals can be submitted in a variety of formats including:
1) fixed price, 2) unit price and 3) cost plus. Fixed-price contracts list
the manpower, materials and additional costs needed to perform the work and
specify a fixed price that will be paid upon completion of the work. Unit-
price contracts are similar, but establish a fixed price for each unit of
work that is performed. Cost-plus contracts list specific costs associated
with performing the work and include a percentage of those costs as an
additional amount that will be paid to perform a job. A percentage listed
in a cost-plus contract is typically viewed as the profit percentage being
proposed by the contractor. In fixed-price and unit-price proposals, the
profit percentage is included as part of the itemized pricing structure.
To ensure that the monitoring well is constructed according to the
intent of the specifications, the contract should be very specific and list
all necessary items and procedures so that nothing is left to
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interpretation or imagination. This clarity can best be obtained by
Listing individual pay items instead of combining items into unspecified
quantities in lump sum pricing. Suggested items that should specifically
be addressed in the contract on a unit price basis are listed in Table 23.
The bidder should also be required to supply information on: 1)
estimated time required for job completion, 2) date available to start
work, 3) type and method of drilling equipment to be used and 4) insurance
coverage. A pay item system may also reduce the need for change during the
drilling process by further clarifying the procedures to be used (Wayne
Westberg, M-W Drilling, Inc., personal communication, 1986). A change
order is a written agreement from the purchaser to the contractor
authorizing additions, deletions or revisions in the scope of work, or an
adjustment in the contract price or effective period of the contract
(United States Environmental Protection Agency, 1975). The contract should
specify what payment provisions will be made if the monitoring well cannot
be completed as specified. The contract should also define who bears the
costs and what the basis for payment will be when drilling difficulties are
encountered that were not anticipated in the pricing schedule.
TABLE 23. SUGGESTED ITEMS FOR UNIT COST IN CONTRACTOR PRICING SCHEDULE
Item Pricing Basic
• Mobilization lump sum
• Site preparation lump sum
• Drilling to specified depth per lineal foot or per hour
• Sampling each
• Material supply
surface casing per lineal loot
well casing per lineal foot
end caps each
screen per lineal foot
filter material per lineal foot or per bag
bentonite seal(s) per lineal foot
grout per lineal foot or per bag
casing protector each
• Support equipment
water truck and water lump sum
bulldozer per hour
• Decontamination lump sum
• Standby per hour
• Field expenses per man day or lump sum
• Material installation per hour or lump sum
• Well development per hour or lump sum
• Demobilization tump sum
• Drilling cost adiustment for variations m depths ± per loot
After the contract is signed and work is scheduled to begin, a pre-
drilling meeting between the supervising geologist and the driller should
be held to discuss operational details. This meeting reduces the
opportunity for misunderstanding of the specifications and improves project
relationships.
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REFERENCES
Aardvark Corporation, 1977. Product literature; Puyallup, Washington, 2
pp.
Acker Drill Company, Inc., 1985. Soil sampling tools catalog; Scranton,
Pennsylvania, 17 pp.
Acker, W.L., 1974. Basic procedures for soil sampling and core drilling;
Acker Drill Company, Inc., Scranton, Pennsylvania, 246 pp.
American Society for Testing and Materials, 1983. Standard practice for
thin-wall tube sampling of soils: D1S87; 1986 Annual Book of American
Society for Testing and Materials Standards, Philadelphia, Pennsylvania,
pp. 305-307.
American Society for Testing and Materials, 1984. Standard method for
penetration test and split barrel sampling of soils: D1586; 1986 Annual
Book of American Society for Testing and Materials Standards, Philadelphia,
Pennsylvania, pp. 298-303.
Barcelona, M.J., J.P. Gibb, J.A. Helfrich and E.E. Garske, 1985a.
Practical guide for ground-water sampling; Illinois State Water Survey, SWS
Contract Report 374, Champaign, Illinois, 93 pp.
Buckeye Drill Company/Bucyrus Erie Company, 1982. Buckeye drill operators
manual; Zanesville, Ohio, 9 pp.
Central Mine Equipment Company, 1987. Catalog of product literature; St.
Louis, Missouri, 12 pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson Division, St.
Paul, Minnesota, 1089 pp.
Everett, Lome G., 1980. Ground-water monitoring; General Electric Company
technology marketing operation, Schenectady, New York, 440 pp.
Hvorslev, M.J., 1949. Subsurface exploration and sampling of soils for
civil engineering purposes; United States Army Corps of Engineers,
Waterways Experiment Station, Vicksburg, Mississippi, 465 pp.
Ingersoll-Rand, 1976. The water well drilling equipment selection guide;
Ingersoll-Rand, Washington, New Jersey, 12 pp.
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Krynine, Dimitri P. and William R. Judd, 1957. Principles of engineering
geology and geotechnics; McGraw-Hill, New York, New York, 730 pp.
Layne-Western Company, Inc., 1983. Water, geological and mineral
exploration utilizing dual-wall reverse circulation; Product literature,
Mission, Kansas, 8 pp.
Leach, Lowell E., Frank P. Beck, John T. Wilson and Don H. Kampbell 1988
Aseptic subsurface sampling techniques for hollow-stem auger drilling-
Proceedings of the Second National Outdoor Action Conference on Aquifer
Restoration, Ground-Water Monitoring and Geophysical Methods, vol I-
National Water Well Association, Dublin, Ohio, pp. 31-51. ' '
Mobile Drilling Company, 1982. Auger tools and accessories; Catalog 182
Indianapolis, Indiana, 26 pp. '
National Water Well Association of Australia, 1984. Drillers training and
reference manual; National Water Well Association of Australia, St Ives
South Wales, 267 pp. '
Petroleum Extension Service, 1980. Principles of Drilling Fluid Control;
Petroleum Extension Service, University of Texas, Austin, Texas, 215 pp.
Speedstar Division of Koehring Company, 1983. Well drilling manual-
National Water Well Association, Dublin, Ohio, 72 pp.
United States Environmental Protection Agency, 1975. Manual of water well
construction practices; United States Environmental Protection Agency
Office of Water Supply, EPA-570/9-75-001, 156 pp.
Zapico, Michael M., Samuel Vales and John A. Cherry, 1987. A wireline
piston core barrel for sampling cohesionless sand and gravel below the
water table; Ground Water Monitoring Review, vol. 7, no. 3, pp. 74-82.
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SECTION 5
DESIGN COMPONENTS OF MONITORING WELLS
INTRODUCTION
It is not possible to describe a "typical" ground-water monitoring
well because each monitoring well must be tailored to suit the
hydrogeologic setting, the type of contaminants to be monitored, the
overall purpose of the monitoring program and other site-specific
variables. However, it is possible to describe the individual design
components of monitoring wells. These design components may be assembled
in various configurations to produce individual monitoring well
installations suited to site-specific conditions. Figure 21 illustrates
the monitoring well design components that are described in this chapter.
WELL CASING
Purpose of the Casing
Casing is installed in a ground-water monitoring well to provide
access from the surface of the ground to some point in the subsurface.
The casing, associated seals and grout prevent borehole collapse and
interzonal hydraulic communication. Access to the monitored zone is
through the casing and into either the open borehole or the screened
intake. The casing thus permits piezometric head measurements and
ground-water quality sampling.
General Casing Material Characteristics
Well casing can be made of any rigid tubular material. Historically,
the selection of a well casing material (predominantly for water supply
wells) focused on structural strength, durability in long-term exposure to
natural ground-water environments and ease of handling. Different
materials have demonstrated versatility in well casing applications. In
the late 1970s, questions about the potential impact that casing materials
may have on the chemical integrity or "representativeness" of a
ground-water sample being analyzed in parts per million or parts per
billion were raised. Today the selection of appropriate materials for
monitoring well casing must take into account several site-specific factors
including: 1) geologic environment, 2) natural geochemical environment, 3)
anticipated well depth, 4) types and concentrations of suspected
contaminants and 5) design life of the monitoring well. In addition,
logistical factors must also be considered including: 1) well drilling or
installation methods, 2) ease in handling, 3) cost and 4) availability.
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The most frequently evaluated characteristics that directly influence
the performance of casing materials in ground-water monitoring applications
are: 1) strength and 2) chemical resistance/interference. These
characteristics are discussed in more detail below.
Strength-Related Characteristics—
Monitoring well casing must be strong enough to resist the forces
exerted on it by the surrounding geologic materials and the forces imposed
on it during installation (Figure 50). The casing must also exhibit
structural integrity for the expected duration of the monitoring program
under natural and man-induced subsurface conditions. When casing strength
is evaluated, three separate yet related parameters are determined:
1) tensile strength, 2) compressive strength and 3) collapse strength.
The tensile strength of a material is defined as the greatest
longitudinal stress the substance can bear without pulling the material
apart. Tensile strength of the installed casing varies with composition,
manufacturing technique, joint type and casing dimensions. For a
monitoring well installation, the selected casing material must have a
tensile strength capable of supporting the weight of the casing string when
suspended from the surface in an air-filled borehole. The tensile strength
of the casing joints is equally as important as the tensile strength of the
casing. Because the joint is generally the weakest point in a casing
string, the joint strength will determine the maximum axial load that can
be placed on the casing. By dividing the tensile strength by the linear
weight of casing, the maximum theoretical depth to which a dry string of
casing can be suspended in a borehole can be calculated. When the casing
is in a borehole partially filled with water, the buoyant force of the
water increases the length of casing that can be suspended. The additional
length of casing that can be suspended depends on the specific gravity of
the casing material.
The compressive strength of a material is defined as the greatest
compressive stress that a substance can bear without deformation.
Unsupported casing has a much lower compressive strength than installed
casing that has been properly grouted and/or backfilled because vertical
forces are greatly diminished by soil friction. This friction component
means that the casing material properties are more significant to
compressive strength than is wall thickness. Casing failure due to
compressive strength limitation is generally not an important factor in a
properly installed monitoring well.
Equally important with tensile strength is the final strength-related
property considered in casing selection -- collapse strength. Collapse
strength is defined as the capability of a casing to resist collapse by any
and all external loads to which it is subjected both during and after
installation. The resistance of casing to collapse is determined primarily
by outside diameter and wall thickness. Casing collapse strength is
proportional to the cube of the wall thickness. Therefore, a small
increase in wall thickness provides a substantial increase in collapse
strength. Collapse strength is also influenced by other physical
properties of the casing material including stiffness and yield strength.
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Borehole
Casing joint
• Casing
Tensile (pull-apart) forces
critical at casing joints
Collapse forces
(critical at greater depths)
Figure SO. Forces exerted on a monitoring well casing and screen during installation.
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A casing is most susceptible to collapse during installation before
placement of the filter pack or annular seal materials around the casing.
Although it nay collapse during development, once a casing is properly
installed and therefore supported, collapse is otherwise seldom a point of
concern (National Water Well Association and Plastic Pipe Institute, 1981).
External loadings on casing that may contribute to collapse include:
1) net external hydrostatic pressure produced when the static water
level outside of the casing is higher than the water level on the
inside;
2) unsymmetrical loads resulting from uneven placement of backfill
and/or filter pack materials;
3) uneven collapse of unstable formations;
4) sudden release of backfill materials that have temporarily bridged
in the annulus;
5) weight of cement grout slurry and impact of heat of hydration of
grout on the outside of a partially water-filled casing;
6) extreme drawdown inside the casing caused by over pumping;
7) forces associated with well development that produce large
differential pressures on the casing; and
8) forces associated with improper installation procedures where
unusual force is used to counteract a borehole that is not
straight or to overcome buoyant forces.
Of these stresses, only external hydrostatic pressure can be predicted
and calculated with accuracy; others can be avoided by common sense and
good practice. To provide sufficient margin against possible collapse by
all normally-anticipated external loadings, a casing should be selected
such that resistance to collapse is more than required to withstand
external hydrostatic pressure alone. Generally, a safety factor of at
least two is recommended (National Water Well Association and Plastic Pipe
Institute, 1981). According to Purdin (1980), steps to minimize the
possibility of collapse include:
1) drilling a straight, clean borehole;
2) uniformly distributing the filter-pack materials at a slow, even
rate;
3) avoiding the use of quick-setting (high temperature) cements for
thermoplastic casing installation;
4) adding sand or bentonite to a cement to lower the heat of
hydration; and
5) controlling negative pressures inside the well during development.
Chemical Resistance Characteristics--
Materials used for well casing in monitoring wells must be durable
enough to withstand galvanic or electrochemical corrosion and chemical
degradation. Metallic casing materials are most subject to corrosion;
thermoplastic casing materials are most subject to chemical degradation.
The extent to which these processes occur depends on water quality within
the formation and changing chemical conditions such as fluctuations between
oxidizing and reducing states. Casing material must therefore be chosen
with a knowledge of the existing or anticipated ground-water chemistry.
When anticipated water quality is unknown, it is prudent to use
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conservative materials to avoid chemical or potential water quality
problems. If ground-water chemistry affects the structural integrity of
the casing, the products of casing deterioration may also adversely affect
the chemistry of water samples taken from the wells.
Chemical Interference Characteristics--
Materials used for monitoring well casing must not exhibit a tendency
to either sorb (take out of solution by either adsorption or absorption) or
leach chemical constituents from or into the water that is sampled from the
well. If a casing material sorbs selected constituents from the ground
water, those constituents will either not be present in any water-quality
sample (a "false negative") or the level of constituents will be reduced.
Additionally, if ground-water chemistry changes over time, the chemical
constituents that were previously sorbed onto the casing may begin to
desorb and/or leach into the ground water. In either situation, the
water-quality samples are not representative.
In the presence of aggressive aqueous solutions, chemical
constituents can be leached from casing materials. If this occurs,
chemical constituents that are not indicative of formation water quality
may be detected in samples collected from the well. This "false positive"
might be considered to be an indication of possible contamination when the
constituents do not relate to ground-water contamination per se, but rather
to water sample contamination contributed by the well casing material. The
selection of a casing material must therefore consider potential
interactions between the casing material and the natural and the
man-induced geochemical environment. It is important to avoid "false
positive" and especially "false negative" sample results.
Types of Casing Materials
Casing materials widely available for use in ground-water monitoring
wells can be divided into three categories
1) fluoropolymer materials, including polytetrafluoroethylene (PTFE),
tetrafluoroethylene (TFE), fluormated ethylene propylene (FEP),
perfluoroalkoxy (PFA) and polyvinylid«ne fluoride (PVDF);
2) metallic materials, including carbon steel, low-carbon steel,
galvanized steel and stainless steel (304 and 316); and
3) thermoplastic materials, including polyvinyl chloride (PVC) and
acrylonitrile butadiene styrene (ABS)
In addition to the three categories that are widely used, fiberglass-
reinforced materials including fiberglasss-re in forced epoxy (FRE) and
fiberglass-reinforced plastic (FRP) have been used for monitoring
applications. Because these materials have not yet been used in general
application across the country, very little data are available on
characteristics and performances. Therefore, fiberglass-reinforced
materials are not considered further herein.
Each material possesses strength-related characteristics and chemical
resistance/chemical interference characteristics that influence its use in
149
-------
site-specific hydrogeologic and contaminant-related monitoring situations.
These characteristics for each of the three categories of materials are
discussed below.
Fluoropolymer materia1s--
Fluoropolymers are man-made materials consisting of different
formulations of monomers (organic molecules) that can be molded by powder
metallurgy techniques or extruded while heated. Fluoropolymers are
technically included among the thermoplastics, but possess a unique set of
properties that distinguish them from other thermoplastics. Fluoropolymers
are nearly totally resistant to chemical and biological attack, oxidation,
weathering and ultraviolet radiation; have a broad useful temperature range
(up to 550°F); have a high dielectric constant; exhibit a low coefficient
of friction; have anti-stick properties; and possess a greater coefficient
of thermal expansion than most other plastics and metals.
There exist a variety of fluoropolymer materials that are marketed
under a number of different trademarks. Descriptions and basic physical
properties of some of the more popular fluoropolymers with appropriate
trademarks are discussed below.
Polytetrafluoroethylene (PTFE) was discovered by E.I. DuPont de
Nemours in 1938 and was available only to the United States government
until the end of World War II. According to Hamilton (1985), four
principal physical properties are:
1) extreme temperature range -- from -AOO°F to +550°F
in constant service;
2) outstanding electrical and thermal1 insulation;
3) lowest coefficient of friction of any solid material; and
4) almost completely chemically inert, except for some reaction with
halogenated compounds at elevated temperatures and pressures.
In addition, PTFE is flexible without the addition of plasticizers and
is fairly easily machined, molded or extruded. PTFE is by far the most
widely-used and produced fluoropolymer. Trade names, manufacturers and
countries of origin of PTFE and other fluoropolymer materials are listed in
Table 24. Typical physical properties of the various fLuoropolymer
materials are described in Table 25.
Fluorinated ethylene propylene (FEP) was also developed by E.I. DuPont
de Nemours and is perhaps the second most widely used fluoropolymer. It
duplicates nearly all of the physical properties of PTFE except the upper
temperature range, which is 100°F lower. Production of FEP-finished
products is generally faster because FEP is melt-processible, but raw
materials costs are higher.
Perfluoroalkoxy (PFA) combines the best properties of PTFE and FEP,
but the former costs substantially more than either of the the other
flouropolymers. Polyvinylidene fluoride (PVDF) is tougher and has a higher
abrasion resistance than other fluoropolymers and is resistant to
radioactive environments. PVDF has a lower upper temperature limit than
either PTFE or PFA.
150
-------
TABLE 24. TRADE NAMES, MANUFACTURERS AND COUNTRIES OF ORIGIN FOR VARIOUS
FLUOROPOLYMER MATERIALS
Chemical Formulation
PTFE (or TFE) — Poiytetrafluoroethylene
FEP — Fluonnaied elhytene propylene
PFA - Perfluoroalkoxy
PVDF — Polyvinylidene lluonoa
CTFE — Crilorotntluoroetriylene
Trade Name
Teflon
Halon
Fluon
Hostaflon
Polyfion
Algoflon
Sonflon
Neoflon
Teflon
Neotlon
TeHon
Kynar
Kel-F
Gallon
Manufacturer
DuPont
Allied
ICI
Hoecns
Oaikin
Montedison
Ugine Kuhlman
Oaikin
DuPont
Oaikin
DuPont
Pennwall
3M
Oaikin
Country of Origin
USA, Holland. Japan
USA
UK. USA
W Germany
Japan
Italy
France
Japan
USA. Japan. Holland
Japan
USA, Japan. Holland
USA
USA
Japan
TABLE 25. TYPICAL PHYSICAL PROPERTIES OF VARIOUS FLUOROPOLYMER MATERIALS
(AFTER NORTON PERFORMANCE PLASTICS, 1985)
Properties
snsiie strengcn qg rt r
Elongation @ 73* F
Modulus @73'F
Tensile
Ftourst
Etasbcrty in tension
Fiexurai strengtn @ 73*F
izod impact strength
( '.* x i*in notched oar)
@f75°F
@-65°F
Tensile impact strength
@+7PF
@-65'F
Compresswe stress @ 73*F
Specrfic 9^ivity
Coetheient et fnetion
static A kineMaoainsi
polished steel
Coefficient of linear
thefmat^xpansion
Units
psi
•b
DSI
P»
PSI
tt IbsVin
of notch
ft Iba/m
of notch
ft.lbsJsq in
rtlta^jq in.
psi
/•f
ASTM Method
D638-O651
0638
0638
O780
0747
0790
D2S6
01822
0696
D792
0696
TFE
2500-6000
150-600
45.000-115.000
70.000-1 10.000
5fl.OOO
Does not break
30
2.3
320
105
1700
214-2.24
005-0 OB
55M10^
FEP
2700-3100
250-330
95.000
250.000
Ooes not break
No break
2.9
1020
365
-
212-217
006409
55x10-*
PFA
4000-4300
300-350
—
95,000-100.000
-
Does not break
No break
—
—
—
-
2 12-2 17
005-006
67*10-*
E-CTFE
7000
200
240.000
240,000
—
7000
No break
—
—
—
-
168
015-065
14x10-*
CTFE
4500-6000
80-250
206.000
238.000
15-30x10*
8500
50
—
—
—
4600-7400
2 10-2 13
02-03
264x10r*
151
-------
Care should be exercised in the use of trade names to identify
fluoropolymers. Some manufacturers use one trade name to refer to several
of their own different materials. For example, DuPont refers to several of
its fluoropolymer resins as Teflon®, although the products referred to have
different physical properties and different fabricating techniques. These
materials may not always be interchangeable in service.
For construction of ground-water monitoring wells, fluoropolymers
possess several advantages over other thermoplastics and metallic
materials. For example, fluoropolymers are almost completely inert to
chemical attack, even by extremely aggressive acids (i.e., hydrofluoric,
nitric, sulfuric and hydrochloric) and organic solvents. In addition,
sorption of chemical constituents from solutions and leaching of materials
from the fluoropolymer chemical structure has been believed to be minimal
or non-existent. Although studies are still ongoing, Reynolds and Gillham
(1985) indicate that extruded tubing of at least one fluoropolymer (PTFE)
is prone to absorption of selected organic compounds, specifically
1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, hexachloroethane and
tetrachloroethane; a fifth organic compound studied, bromoform, was not
sorbed by PTFE. An observation of particular note made by Reynolds and
Gillham was that tetrachloroethane was strongly and rapidly sorbed by the
PTFE tubing such that significant reductions in concentration occurred
within minutes of exposure to a solution containing the aforementioned
organic compounds. These results indicate that PTFE may not be as inert as
previously thought. Barcelona and Helfrich (1988) provide a review of
laboratory and field studies of well casing material effects.
Although numerous such wells have been successfully installed, there
may be some potential drawbacks to using fluoropolymers as monitoring well
casing materials. For example, PTFE is approximately 10 times more
expensive than PVC. In addition, fluoropolymer materials are more
difficult to handle than most other well casing materials. Fluoropolymer
materials are heavier and less rigid than other thermoplastics and slippery
when wet because of a low coefficient of friction. Dablow et al. (1988)
discuss installation of fluoropolymer wells and address some of the
potential difficulties. As they point out, several strength-related
properties of fluoropolymers (PTFE in particular) must be taken into
consideration during the well design process, including: 1) pull-out
resistance of flush-joint threaded couplings (tensile strength);
2) compressive strength of the intake section; and 3) flexibility of the
casing string.
The tensile strength of fluoropolymer casing joints is the limiting
factor affecting the length of casing that can be supported safely in a dry
borehole. According to Dablow et al. (1988), experimental work conducted
by DuPont indicates that PTFE threaded joints will resist a pull-out load
of approximately 900 pounds. With a safety factor of two, 2-inch schedule
40 PTFE well casing with a weight of approximately 1.2 pounds per foot
should be able to be installed to a depth of approximately 375 feet.
Barcelona et al. (1985a) suggest that the recommended hang length not
exceed 320 feet. In either case, this is less than one tenth the tensile
strength of an equivalent-sized thermoplastic (i.e., PVC) well casing
152
-------
material. Additionally, because the specific gravity of PTFE is much
higher than that of thermoplastics (about 2.2), the buoyant force of water
is not great. However, the buoyant force is sufficient to increase the
maximum string length by approximately 10 percent for that portion of the
casing material in contact with water.
Compressive strength of fluoropolymer well casings and particularly
intakes is also a recognized problem area. A low compressive stress
when compared to other thermoplastics may lead to failure of the
fluoropolymer casing at the threaded joints where the casing is weakest and
the stress is greatest. According to Dablow et al. (1988), the "ductile"
behavior of PTFE has resulted in the partial closing of intake openings
with a consequent reduction in well efficiency in deep fluoropolymer wells.
Dablow et al. (1988) suggest that this problem can be minimized by
designing a larger slot size than is otherwise indicated by the sieve
analyses. In compressive strength tests conducted by DuFont to determine
the amount of deformation in PFTE well intakes that occurs under varying
compressive stresses, a linear relationship was demonstrated between
applied stress and the amount of intake deformation. This relationship is
graphically presented in Figure 51. From this graph, the anticipated
intake opening deformation can be determined and included in intake design
by calculating the load and adding anticipated intake opening deformation
to the intake opening size determined by sieve analysis.
30
20 -
= 10
100
TOO
—I—
300
—T"
400
—1—
500
—I—
600
700
800
—r-
900
1000
COMPRESSION LOAO-LBS.
Note Short term test — 10 minutes
Figure 51. Static compression results of Teflon' screen (Dablow et al., 1988).
•DuPonrs registered trademark for its tluorocarbon resin
153
-------
According to Dablow et al. (1988), a recommended construction
procedure to minimize compressive stress problems is to keep the casing
string suspended in the borehole so that the casing is in tension and to
backfill the annulus around the casing while it remains suspended. This
procedure reduces compressive stress by supplying support on the outer wall
of the casing. This can only be accomplished successfully in relatively
shallow wells in which the long-term tensile strength of the fluoropolymer
casing is sufficient to withstand tensile stresses imposed on the casing by
suspending it in the borehole. Additionally, continuous suspension of
casing in the borehole is not possible with hollow-stem auger
installations.
The third area of concern in fluoropolymer well casing installation is
the extreme flexibility of the casing string. Although easy solutions
exsist to avoid problems, the flexibility otherwise could cause the casing
to become bowed and non-plumb when loaded, and the resulting deformation
could cause difficulties in obtaining samples or accurate water levels from
these wells. Dablow et al. (1988) suggest three means of avoiding
flexibility problems: 1) suspending the casing string in the borehole
during backfilling (as discussed above); 2) using casing centralizers; or
3) inserting a rigid PVC or steel pipe temporarily inside the fluoropolymer
casing during backfilling.
Metallic Materials-
Metallic well casing and screen materials available for use in
monitoring wells include carbon steel, low carbon steel, galvanized steel
and stainless steel. Well casings made of any of these metallic materials
are generally stronger, more rigid and less temperature sensitive than
thermoplastics, fluoropolymer or fiberglass'reinforced epoxy casing
materials. Table 26 describes dimensions, hydraulic collapse pressure,
burst pressure and unit weight of stainless steel casing. The strength and
rigidity capabilities of metallic casing materials are sufficient to meet
virtually any subsurface condition encountered in a ground-water monitoring
situation. However, metallic materials are subject to corrosion during
long-term exposure to certain subsurface geochraicaL environments.
Corrosion of metallic well casings and w«ll intakes can both limit the
useful life of the monitoring well installation and result in ground-water
sample analytical bias. It is important, therefore, to select both casing
and screen that are fabricated of corrosion-resistant materials.
Corrosion is defined as the weakening or destruction of a material by
chemical action. Several well-defined forms of corrosive attack on
metallic materials have been observed and defined. In all forms, corrosion
proceeds by electrochemical action, and water in contact with the metal is
an essential factor. According to Driscoll (1986), the forms of corrosion
typical in environments in which well casing and well intake materials are
installed include:
1) general oxidation or "rusting" of the metallic surface, resulting
in uniform destruction of the surface with occasional perforation
in some areas;
154
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TABLE 26. HYDRAULIC COLLAPSE AND BURST PRESSURE AND UNIT WEIGHT OF STAINLESS STEEL WELL
CASING (AFTER ARMCO, INC., 1987)
Norn
Size
Inches
2
2M
3
31,
4
6
6
Schedule
Number
&
10
40
BO
s
10
40
6
10
40
5
10
40
5
10
40
S
10
40
6
10
40
Outside
Diameter,
Inches
2375
2375
2375
2375
2875
2875
2875
3500
3500
3500
4000
4000
4000
4500
4500
4500
5563
5563
5563
6625
6625
6625
Wall
Thickness
Inches
0065
0109
0154
0218
0063
0120
0203
0083
0120
0216
0083
0120
0226
0083
0120
0237
0109
0134
0258
0109
0134
0280
Inside
Diameter
Inches
2245
2157
2067
1939
2709
2635
2469
3334
3260
3068
3834
3760
3548
4334
4260
4026
5345
5295
5047
6407
6357
6065
Internal
Cross-Sectional
Area
Sq In
3958
3654
3356
2953
5761
5450
4785
8726
8343
7389
1154
11 10
9887
1475
1425
1272
2243
2201
2000
3222
31 72
2889
Internal Pressure
psi
Test
820
1375
1945
2500
865
1250
2118
710
1030
1851
620
900
1695
555
BOO
1 580
587
722
1391
494
606
1268
Bursting
4105
6884
9726
13768
4330
6260
10591
3557
5142
9257
3112
4500
8475
2766
4000
7900
2949
3613
6957
2467
3033
6340
External
Pressure
psi
Collapsing
896
2196
3526
5419
1001
1905
3931
639
1375
3307
431
1081
2941
316
845
2672
350
665
2231
129
394
1942
Weight.
Pounds
per Fool
1619
2663
3087
5069
2498
3564
5347
3057
4372
7647
3505
5019
9194
3952
5666
10891
6409
7842
14754
7656
9376
19152
-------
2) selective corrosion (dezincification) or loss of one element of an
alloy, leaving a structurally weakened material;
3) bi-metallie corrosion, caused by the creation of a galvanic cell at
or near the juncture of two different metals;
4) pitting corrosion, or highly localized corrosion by pitting or
perforation, with little loss of metal outside of these areas; and
5) stress corrosion, or corrosion induced in areas where the metal is
highly stressed.
To determine the potential for corrosion of metallic materials, the
natural geochemical conditions must first be determined. The following
list of indicators can help recognize potentially corrosive conditions
(modified from Driscoll, 1986):
1) low pH -- if ground water pH is less than 7.0, water is acidic and
corrosive conditions exist;
2) high dissolved oxygen content -- if dissolved oxygen content
exceeds 2 milligrams per liter, corrosive water is indicated;
3) presence of hydrogen sulfide (H.S) -- presence of H.S in quantities
as low as 1 milligram per liter can cause severe corrosion;
4) total dissolved solids (IDS) -- if IDS is greater than 1000
milligrams per liter, the electrical conductivity of the water is
great enough to cause serious electrolytic corrosion;
5) carbon dioxide (CO.) -- corrosion is likely if the C02 content of
the water exceeds 50 milligrams per_liter; and
6) chloride ion (Cl~) content -- if Cl~ content exceeds 500 milligrams
per liter, corrosion can be expected.
Combinations of any of these corrosive conditions generally increase the
corrosive effect. However, no data presently exist on the expected life of
steel well casing materials exposed to natural subsurface geochemical
conditions.
Carbon steels were produced primarily to provide increased resistance
to atmospheric corrosion. Achieving this increased resistance requires
that the material be subjected to alternately wet and dry conditions. In
most monitoring wells, water fluctuations are not sufficient in either
duration or occurrence to provide the conditions that minimize corrosion.
Therefore, corrosion is a frequent problem. The difference between the
corrosion resistance of carbon and low-carbon steels is negligible under
conditions in which the materials are buried in soils or in the saturated
zone; thus both materials may be expected to corrode approximately equally.
Corrosion products include iron and manganese and trace metal oxides as
well as various metal sulfides (Barcelona et al., 1983). Under oxidizing
conditions, the principal products are solid hydrous metal oxides; under
reducing conditions, high levels of dissolved metallic corrosion products
can be expected (Barcelona et al., 1983). While the electroplating process
of galvanizing improves the corrosion resistance of either carbon or
low-carbon steel, in many subsurface environments the improvement is only
slight and short-term. The products of corrosion of galvanized steel
include iron, manganese, zinc and trace cadmium species (Barcelona et al.,
1983).
156
-------
The presence of corrosion products represents a high potential for the
alteration of ground-water sample chemical quality. The surfaces on which
corrosion occurs also present potential sites for a variety of chemical
reactions and adsorption. These surface interactions can cause significant
changes in dissolved metal or organic compounds in ground water samples
(Marsh and Lloyd, 1980). According to Barcelona et al. (1983), even
flushing the stored water from the well casing prior to sampling may not be
sufficient to minimize this source of sample bias because the effects of
the disturbance of surface coatings or accumulated corrosion products in
the bottom of the well are difficult, if not impossible, to predict. On
the basis of these observations, the use of carbon steel, low-carbon steel
and galvanized steel in monitoring well construction is not considered
prudent in most natural geochemical environments.
Conversely, stainless steel performs well in most corrosive
environments, particularly under oxidizing conditions. In fact, stainless
steel requires exposure to oxygen in order to attain its highest corrosion
resistance; oxygen combines with part of the stainless steel alloy to form
an invisible protective film on the surface of the metal. As long as the
film remains intact, the corrosion resistance of stainless steel is very
high. Recent work by Barcelona and Helfrich (1986; 1988) and Barcelona et
al. (1988) suggest that biological activity may alter geochemistry near
stainless steel wells. Iron bacteria may induce degradation of the well
casing and screen.
Several different types of stainless steel alloys are available. The
most common alloys used fox well casing and screen are Type 304 and Type
316. Type 304 stainless steel is perhaps .the most practical from a
corrosion resistance and cost standpoint. It is composed of slightly more
than 18 percent chromium and more than 8 percent nickel, with about 72
percent iron and not more than 0.08 percent carbon (Driscoll, 1986). The
chromium and nickel give the 304 alloy excellent resistance to corrosion;
the low carbon content improves weldability. Type 316 stainless steel is
compositionally similar to Type 304 with one exception -- a 2 to 3 percent
molybdenum content and a higher nickel content that replaces the equivalent
percentage of iron. This compositional difference provides Type 316
stainless steel with an improved resistance to sulfur-containing species as
well as sulfuric acid solutions (Barcelona et al., 1983). This means that
Type 316 performs better under reducing conditions than Type 304.
According to Barcelona et al. (1983), Type 316 stainless steel is less
susceptible to pitting or pinhole corrosion caused by organic acids or
halide solutions. However, Barcelona et al. (1963) also point out that for
either formulation of stainless steel, long-term exposure to very corrosive
conditions may result in corrosion and the subsequent chromium or nickel
contamination of samples.
Thermoplastic Materials--
Thermoplastics are man-made materials that are composed of different
formulations of large organic molecules. These formulations soften by
heating and harden upon cooling and therefore can easily be molded or
extruded into a wide variety of useful shapes including well casings,
fittings and accessories.
157
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The most common types of thermoplastic weLl casing are polyvinyl
chloride (PVC) and acrylonitrile butadiene styrene (ABS). Casing made of
these materials is generally weaker, less rigid and more temperature-
sensitive than metallic casing materials. However, casing made of either
types of plastic can usually be selected where the strength, rigidity and
temperature resistance are generally sufficient to withstand stresses
during casing handling, installation and earth loading (National Water Well
Association and Plastic Pipe Institute, 1981). Thermoplastics also: 1)
offer complete resistance to galvanic and electrochemical corrosion; 2) are
light weight for ease of installation and reduced shipping costs; 3) have
high abrasion resistance; 4) have high strength-to-weight ratios; 5) are
durable in natural ground-water environments; 6) require low maintenance;
7) are flexible and workable for ease of cutting and joining and 8) are
relatively low in cost.
Long-term exposures of some formulations of thermoplastics to the
ultraviolet rays of direct sunlight and/or to low temperatures will cause
brittleness and gradual loss of impact strength that may be significant.
The extent of this degradation depends on the type of plastic, the extent
of exposure and the susceptibility of the casing to mechanical damage
(National Water Well Association and Plastic Pipe Institute, 1981). Many
thermoplastic formulations now include protection against degradation by
sunlight, but brittleness of casing, particularly during casing
installation remains a problem. Above-ground portions of thermoplastic
well casings should be suitably protected from breakage. Potential
chemical problems are discussed in the following sections.
Polwinyl chloride (PVC)--PVC plastics are produced by combining PVC
resin with various types of stabilizers, lubricants, pigments, fillers,
plasticizers and processing aids. The amounts of these additives can be
varied to produce different PVC plastics with properties tailored to
specific applications. PVC used for well casing is composed of a rigid
unplasticized polymer formulation (PVC Type 1) that is strong and generally
has good chemical resistance. However, several publications (e.g.,
Barcelona et al., 1983; Barcelona and Helfrich, 1988; and Nass, 1976)
raised questions of chemical resistance to low molecular weight ketones,
aldehydes and chlorinated solvents which may limit durability of the
casing.
PVC materials are classified according to ASTH standard specification
D-1785 that covers rigid PVC compounds (American Society for Testing and
Materials, 1986). This standard categorizes rigid PVC by numbered cells
designating value ranges for certain pertinent properties and
characteristics including impact strength, tensile strength, rigidity
(modulus of elasticity), temperature resistance (deflection temperature)
and chemical resistance. ASTM standard specification F-480 covers
thermoplastic water well casing pipe and couplings made in standard
dimension ratios. This standard specifies that PVC well casing can be made
from only a limited number of cell classification materials, predominantly
PVC 12454-B, but also including PVC 12454-C and PVC 14333-C and D (American
Society for Testing and Materials, 1981). Minimum physical property values
for these materials are given in Table 27. Hydraulic collapse pressure and
unit weight for a range of PVC well casing diameters is given in Table 28.
158
-------
TABLE 27. TYPICAL PHYSICAL PROPERTIES OF THERMOPLASTIC WELL CASING MATERIALS
@73.4° (NATIONAL WATER WELL ASSOCIATION AND PLASTIC PIPE INSTITUTE. 1981)
ABS PVC
Cell Class. Cell Class,
per D-1788 per D-17B4
Property ASTM Test Method 434 533 12454-B & C 14333-C & D
Specific Gravity
Tensile Strength, lbs/ms
Tensile Modulus of Elasticity. Ibs/m2
Compressive Strength. Ibs/in2
Impact Strength. Izod. ft-lb/mcn notch
Deflection Temperature Under Load
(264 psi). °F
Coefficient of Linear Expansion, in/in-' F
D-792
0638
0-638
0-695
0-256
0-648
0-696
105
6.000'
350.000
7.200
40'
190'
5 5 x 10-S
104
5.000-
250.000
4.500
SO'
ISO'
60X10"5
140
7.000-
400.000*
9.000
065
158'
30X10"5
135
6,000-
320000-
8.000
50
140'
50 x 10*
•These are minimum values set by the corresponding ASTM Cell Class designation All omen represent typical values
Acrylonitrile butadiene styrene (ABS)--ABS plastics are produced from
three different monomers: 1) acrylonitrile, 2) butadiene and 3) styrene.
The ratio of the components and the way in which they are combined can be
varied to produce plastics with a wide range of properties. Acrylonitrile
contributes rigidity, impact strength, hardness, chemical resistance and
heat resistance; butadiene contributes impact strength; styrene contributes
rigidity, gloss and ease of manufacturing (National Water Well Association
and Plastic Pipe Institute, 1981). ABS used for well casing is a rigid,
strong unplasticized polymer formulation that has good heat resistance and
impact strength.
Two ABS material types are used for well casings: 1) a higher
strength, high rigidity, moderate impact resistance ABS and 2) a lower
strength and rigidity, high impact strength ABS These two materials are
identified as cell class 434 and 533, respectively by ASTM standard
specification F-480 (American Society for Testing and Materials, 1981).
Minimum physical property values for ABS well casing are given in Table 27.
The high temperature resistance and the ability of ABS to retain other
properties better at high temperatures is an advantage in wells in which
grouting causes a high heat of hydration. Hydraulic collapse pressure for
a range of ABS well casing diameters is given in Table 29.
General strength/chemical resistance and/or interference
characteristics—The tensile strength of thermoplastics is relatively low
in comparison to metallic materials, but the developed string loading is
not a limiting factor because the thermoplastic well casing is lighter
weight than metallic materials. Table 27 shows the physical properties of
thermoplastic well casing materials. The tensile strength, which in part
determines the length of casing string that can be suspended in the
borehole is relatively large. According to calculations by the National
Water Well Association and Plastic Pipe Institute (1981), permissible
casing string lengths even in unsaturated boreholes exceed the typical
159
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TABLE 28. HYDRAULIC COLLAPSE PRESSURE AND UNIT WEIGHT OF PVC WELL CASING (NATIONAL WATER
WELL ASSOCIATION AND PLASTIC PIPE INSTITUTE, 1981)
Outside Diameter
(inches)
Nom Actual
2
2'fi
3
3'A
4
4V»
5
6
2375
2875
3500
4000
4500
4950
5563
6625
SCH-
SCHBO
SCH 40
SCH 80
SCH 40
SCHBO
SCH 40
SCHBO
SCH 40
SCHBO
SCH 40
—
—
SCHBO
SCH 40
SCHBO
SCH 40
Wall
Thickness
Mm (in)
0218
0154
0276
0203
0300
0216
0316
0226
0337
0237
0248
0190
0375
0258
0432
0280
OR"
109
154
104
142
117
162
126
177
133
190
200
260
145
216
153
237
Weight in Air
(Ibs / 100 feet)
PVC 12454 PVC 14333
94
69
144
109
193
143
235
172
282
203
235
182
391
276
538
35B
91
66
139
105
186
138
227
176
272
196
226
176
377
266
519
345
Weight in Water
(Ibs / 100 feel)
PVC 12454 PVC 14333
27
20
41
31
55
41
67
49
80
58
67
52
112
79
154
102
24
17
36
27
48
36
59
43
70
51
58
46
98
69
134
89
Hydraulic Collapse
Pressure (psi)
PVC 12454 PVC 14333
947
307
1110
400
750
262
589
197
494
158
134
59
350
105
314
78
758
246
885
320
600
210
471
158
395
126
107
47
280
B4
171
62
•Schedule
• Dimension ratio
-------
TABLE 29. HYDRAULIC COLLAPSE PRESSURE AND UNIT WEIGHT OF ABS WELL CASING (NATIONAL WATER
WELL ASSOCIATION AND PLASTIC PIPE INSTITUTE, 1981)
O>
Outside Diameter
(inches)
Norn Actual
a
2'-
3
3M
4
&
6
2375
2875
3500
4000
4500
5563
6250
SCH'
SCHBO
SCH 40
SCH BO
SCH 40
SCHBO
SCH 40
SCHBO
SCH 40
SCHBO
SCH 40
SCH 80
SCH 40
SCHBO
SCH 40
Wall
Thickness
Mm (in)
0218
0154
0276
0203
0300
0216
0318
0226
0337
0237
0375
0258
0432
0280
OR"
109
154
104
142
117
162
126
177
133
190
148
216
153
237
Weight in Air
(Ibs / 100 feet)
ABS 434 ABS 533
71
52
108
82
145
107
176
129
211
152
294
207
404
268
70
51
107
81
144
106
175
128
209
151
291
205
400
266
Weight in Water
(Ibs / 100 feet)
ABS 434 ABS 533
34
25
51
39
69
51
84
61
100
72
140
98
192
128
27
20
41
31
55
4 1
67
49
80
58
112
79
154
102
Hydraulic Collapse
Pressure (psi)
ABS 434 ABS 533
829
269
968
350
656
229
515
173
432
138
306
92
275
69
592
192
691
250
468
164
368
124
308
98
218
66
196
49
•Schedule
•Dimension ratio
-------
borehole depths of monitoring wells. In boreholes where the casing is
partially immersed, casing string length is even less of a problem because
the thermoplastics are low in density and therefore relatively buoyant.
With respect to chemical resistance, thermoplastic well casing
materials are non-conductors and therefore do not corrode either
electrochemically or galvanically like metallic materials. In addition,
thermoplastics are resistant to biological attack and to chemical attack by
soil, water and other naturally-occurring substances present in the
subsurface (National Water Well Association and Plastic Pipe Institute,
1981). However, thermoplastics are susceptible to chemical attack by high
concentrations of certain organic solvents, and long term exposure to lower
levels has as yet undocumented effects. This physical degradation of a
plastic by an organic solvent is called solvation. Solvent cementing of
thermoplastic well casings is based on solvation. Solvation occurs in the
presence of very high concentrations of specific organic solvents. If
these solvents, which include tetrahydrofuran (THF), methyl ethyl ketone
(MEK), methyl isobutyl ketone (HIBK) and cyclohexanone, are present in high
enough concentrations, the solvents can be expected to chemically degrade
thermoplastic well casing. However, the extent of this degradation is not
known. In general, the chemical attack on the thermoplastic polymer matrix
is enhanced as the organic content of the solution with which it is in
contact increases.
Barcelona et al. (1983) and the Science Advisory Board of the U.S. EPA
list the groups of chemical compounds that may cause degradation of the
thermoplastic polymer matrix and/or the release of compounding ingredients
that otherwise will remain in the solid material. These chemical compounds
include: 1) low molecular weight ketones, 2) aldehydes, 3) amines and 4)
chlorinated alkenes and alkanes. Recent reports of creosotes and petroleum
distillates causing disintegration of PVC casing support Barcelona's
findings. There is currently a lack of information regarding critical
concentrations of these chemical compounds at which deterioration of the
thermoplastic material is significant enough to affect either the
structural integrity of the material or the ground-water sample chemical
quality.
Among the potential sources of chemical interference in thermoplastic
well casing materials are the basic monomers from which the casing is made
and a variety of additives that may be used in the manufacture of the
casing including plasticizers, stabilizers, fillers, pigments and
lubricants. The propensity of currently available information on potential
contamination of water that comes in contact with rigid thermoplastic
materials relates specifically to PVC; no information is currently
available on ABS or on other similar thermoplastics. Therefore, the
remainder of this discussion relates to potential chemical interference
effects from PVC well casing materials.
Extensive research has been conducted in the laboratory and in the
field, specifically on water supply piping, to evaluate vinyl chloride
monomer migration from new and old PVC pipe. The data support the
conclusion that when PVC is in contact with water, the level of trace vinyl
chloride migration from PVC pipe is extremely low compared to residual
162
-------
vinyl chloride monomer (RVCM) in PVC pipe. Since 1976, when the National
Sanitation Foundation established an RVCM monitoring and control program
for PVC pipe used in potable water supplies and well casing, process
control of RVCM levels in PVC pipe has improved markedly. According to
Barcelona et al. (1983), the maximum allowable level of RVCM in
NSF-certified PVC products (less than or equal to 10 ppm RVCM) limits
potential leached concentrations of vinyl chloride monomer to 1 to 2
micrograms per liter. Leachable amounts of vinyl chloride monomer should
decrease as RVCM levels in products continue to be reduced. Although the
potential for analytical interference exists even at the low micrograms
per-liter level at which vinyl chloride monomer may be found in a solution
in contact with PVC, the significance of this interference is not currently
known.
With few exceptions, plasticizers are not added to PVC formulations
used for well casing because the casing must be a rigid material. Even if
plasticizers were added, levels would not be expected to exceed 0.01
percent (Barcelona et al., 1983). By contrast, flexible PVC tubing may
contain from 30 to 50 percent plasticizers by weight. The presence of
these high levels of plasticizers in flexible PVC tubing has been
documented to produce significant chemical interference effects by several
researchers (Barcelona et al., 1985b; Barcelona, 1984; Barcelona et al.,
1983; Junk et al., 1974). However, at the levels present in rigid well
casing, plasticizers were not reported to pose a chemical interference
problem.
Rigid PVC may contain other additives, primarily stabilizers, at
levels approaching 5 percent by weight. Some representative chemical
classes of additives that have been used in the manufacture of rigid PVC
well casing are listed in Table 30. Boettner et al. (1981) determined
through a laboratory study that several of the PVC heat stabilizing
compounds, notably dimethyltin and dibutyltin species, could potentially
leach out of rigid PVC at very low (low to sub micrograms per liter)
levels. These levels decreased dramatically over time. Factors that
influenced the leaching process in this study included solution pH,
temperature and ionic composition; and exposed surface area and surface
porosity of the pipe material. It is currently unclear what impact, if
any, the leaching of low levels of organotin compounds may have on
analytical interference.
In addition to setting a limit on RVCM, the National Sanitation
Foundation has set specifications for certain chemical constituents in PVC
formulations. The purpose of these specifications as outlined in NSF
Standard 14 (National Sanitation Foundation, 1988) is to control the amount
of chemical additives in both PVC well casing and pipe used for potable
water supply. The maximum contaminant levels permitted in a standardized
leach test on NSF-approved PVC products are given in Table 31. Host of
these levels correspond to those set by the Safe Drinking Water Act for
chemical constituents covered by the National Interim Primary Drinking
Water Standards. Only PVC products that carry either the "NSF we" (well
casing) or "NSF pw" (potable water) designation have met the specifications
set forth in Standard 14. Other non-NSF listed products may include in
their formulation chemical additives not addressed by the specifications or
may carry levels of the listed chemical parameters higher than permitted by
163
-------
TABLE 30. REPRESENTATIVE CLASSES OF ADDITIVES IN RIGID PVC MATERIALS USED FOR
PIPE OR WELL CASING (BARCELONA ET AL, 1983)
Heatftabi«Mrt
-------
the specifications. In all cases, the material used should be demonstrated
to be compatible with the specific applications. For example, even though
neither lead nor cadmium have been permitted as compounding ingredients in
United States-manufactured NSF-listed PVC well casing since 1970, PVC
manufactured in other countries may be stabilized with lead or cadmium
compounds that have been demonstrated to leach from the PVC (Barcelona et
al., 1983).
In other laboratory studies of leaching of PVC well casing material
chemical components into water, Curran and Tomson (1983) and Parker and
Jenkins (1986) determined that little or no leaching occurred. In the
former study, it was found when testing several different samples (brands)
of rigid PVC well casing that trace organics either were not leached or
were leached only at the sub-micrograms per liter level. In the latter
study, which was conducted using ground water in contact with two different
brands of PVC, it was concluded that no chemical constituents were leached
at sufficient concentrations to interfere with reversed-phase analysis for
low micrograms per liter levels of 2,4,6 trinitrotoluene (TNT),
hexahydro-l,3,5-trinitro-l,3,5-triazine (RDX), octahydro-1,3,5,7-
tetranitro-l,3,5,7-tetrazocine (HMX) or 2,4 dinitrotoluene (DNT) in
solution. The study by Curran and Tomson (1983) confirmed previous field
work at Rice University (Tomson et al., 1979) that suggested that PVC well
casings did not leach significant amounts (i.e. at the sub-micrograms per
liter level) of trace organics into sampled ground water.
Another potential area for concern with respect to chemical
interference effects is the possibility that some chemical constituents
could be sorbed by PVC well casing materials. Miller (1982) conducted a
laboratory study to determine whether several'plastics, including rigid PVC
well casing, exhibited any tendency to sorb potential contaminants from
solution. Under the conditions of his test, Miller found that PVC
moderately sorbed tetrachloroethylene and strongly sorbed lead, but did not
sorb trichlorofluoromethane, trichloroethylene, bromofonn, 1,1,1-tri-
chloroethane, 1,1,2-trichloroethane or chromium. In this experiment,
sorption was measured weekly for six weeks and compared to a control;
maximum sorption of tetrachloroethylene occurred at two weeks. While
Miller (1982) attributed these losses of tetrachloroethylene and lead
strictly to sorption, the anomalous behavior of tetrachloroethylene
compared to that for other organics of similar structure (i.e.,
trichloroethlyene) is not explained. In a follow-up study to determine
whether or not the tetrachloroethylene could be desorbed and recovered,
only a small amount of tetrachloroethylene was desorbed. Thus, whether or
not strong sorption or some other mechanism (i.e., enhanced biodegradation
in the presence of PVC) accounts for the difference is not clear (Parker
and Jenkins, 1986). In the laboratory study by Parker and Jenkins (1986),
it was found that significant losses of TNT and HMX from solution occurred
in the presence of PVC well casing. A follow-up study to determine the
mechanism for the losses attributed the losses to increased microbial
degradation rather than to sorption. These results raise questions
regarding whether or not losses found in other laboratory or even field
studies that did not consider biodegradation as a loss mechanism could be
attributed to biodegradation rather than to sorption.
165
-------
In another laboratory study, Reynolds and Gillham (1985) found that
sorption of selected organics (specifically 1,1,1-trichloroethane,
1,1,2,2-tetrachloroethane, bromoforn, hexachloroethane and
tetrachloroethylene) onto PVC and other polymeric well casing materials
could be a significant source of bias to ground-water samples collected
from water standing in the well. PVC was found to slowly sorb four of the
five compounds studied (all except 1,1,1-trichloroethane), such that
sorption bias would likely not be significant for the sorbed compounds if
well development (purging the well of stagnant water) and sampling were to
take place in the same day. B
It is clear that with few exceptions the work that has been done to
determine chemical interference effects of PVC well casing (whether by
leaching from or sorbing to PVC of chemical constituents) has been
conducted under laboratory conditions. Furthermore, in most of the
laboratory work the PVC has been exposed to a solution (usually distilled
deionized, or "organic-free" water) over periods of time ranging from
several days to several months. Thus the PVC had a period of time in which
to exhibit sorption or leaching effects. While this may be comparable to a
field situation in which ground water was exposed to the PVC well casing as
it may be between sampling rounds, few studies consider the fact that prior
to sampling, the well casing is usually purged of stagnant water residing
in the casing between sampling rounds. Thus, the water that would have
been affected by the sorption or leaching effects of PVC would ideally have
been removed and replaced with aquifer-quality water that is eventually
obtained as representative" of existing ground-water conditions. Because
the sample is generally taken immediately after purging of stagnant water
the sampled water will have had a minimum of time with which to come in '
contact with casing materials and consequently be affected by sorption or
leaching effects. Because of this, Barcelona et al. (1983) suggest that
the potential sample bias due to sorptive interactions with well casing
materials may be discounted. They point out that these effects are far
more critical in sample transfer and storage procedures employed prior to
sample separation or analysis. Nevertheless, other researchers do not
agree that purging avoids casing effects especially for wells that recover
slowly and thereby allow ample time for surface reactions to occur.
Composite Alternative Materials--
In certain conditions it may be advantageous to design a well using
more than one material for well components. For example, where stainless
steel or fluoropolymer materials are preferred in a specific chemical
environment, considerable cost savings may be realized by using PVC in
non-critical portions of the well. These savings may be considerable
especially in deep wells where only the lower portion of the well has a
critical chemical environment and tens of feet of lower-cost PVC may be
used in the upper portion of the well. In composite well designs the use
of dissimilar metallic components should be avoided unless an electrically
isolating design is incorporated (United States Environmental Protection
Agency, 1986).
166
-------
Coupling Procedures for Joining Casing
Only a limited number of methods are available for joining lengths of
casing or casing and screen together. The joining method depends on the
type of casing and type of casing joint. Figure 52 illustrates some common
types of joints used for assembling lengths of casing. Flush-joint,
threaded flush-joint, plain square-end and bell-end casing joints are
typical of joints available for plastic casing; threaded flush-joint,
bell-end and plain square end casing joints are typical of joints available
friT mat-nil-if r-adntr
for metallic casing.
Fluoropolymer Casing Joining--
Because fluoropolymers are inert to chemical attack or solvation even
by pure solvents, solvent welding cannot be used with fluoropolymers.
Similar to thermoplastic casing joining techniques, threaded joints wrapped
with fluoropolymer tape are preferred.
Metallic Casing Joining--
There are generally two options available for joining metallic well
casings: 1) welding via application of heat or 2) threaded joints. Both
methods produce a casing string with a relatively smooth inner and outer
diameter. With welding, it is generally possible to produce joints that
are as strong or stronger than the casing, thereby enhancing the tensile
strength of the casing string. The disadvantages of welding include-
1) greater assembly time, 2) difficulty in properly welding casing in the
vertical position, 3) enhancement of corrosion potential in the vicinity of
the weld and 4) the danger of ignition of potentially explosive gases that
may be present. Because of these disadvantages, threaded joints are more
commonly used with metallic casing and screen. Threaded joints provide
inexpensive, fast and convenient connections and greatly reduce potential
problems with chemical resistance or interference (due to corrosion) and
explosive potential. Wrapping the male threads with fluoropolymer tape
prior to joining sections improves the watertightness of the joint One
disadvantage to using threaded joints is that the tensile strength of the
casing string is reduced to approximately 70 percent of the casing
strength. This reduction in strength does not usually pose a problem
because strength requirements for small diameter wells typical of
monitoring installations are not as critical and because metallic casin*
has a high initial tensile strength.
Thermoplastic Casing Joining--
There are two basic methods for joining sections of thermoplastic well
casing: 1) solvent cementing and 2) mechanical Joining. Both methods
should maintain a uniform inner and outer casing diameter in monitoring
well installations. An inconsistent inner diameter causes problems when
tight-fitting downhole equipment (development tools, sampling or purging
devices, etc.) is used; an uneven outer diameter creates problems with
filter pack and annular seal placement. The latter problem tends to
promote water migration at the casing/seal interface to a greater degree
than is experienced with uniform outer diameter casing (Morrison, 1984).
167
-------
coupling
a Flush-joint casing b Threaded. Mush-joint casing c Plain square-end casing
domed by solvent welding) (joined by threading casing together) (,0med by solvent welding
with couplings)
d Threaded casing
domed by threaded couplings)
e. Bell-end casing
((oined by solvent welding)
I Plain square-end casing
domed by heat welding)
Figure 52. Types of joints typically used between casing lengths.
168
-------
Solvent cementing-In solvent cementing, a solvent primer is generally
used to clean the two pieces of casing to be joined and a solvent Lent is
then spread over the cleaned surface areas. The two sections are assembled
while the cement is wet. This allows the active solvent agent(s) to
penetrate and soften the two casing surfaces that are joined As the
cement cures, the two pieces of casing are fused together; a residue of
chemicals from the solvent cement remains at the joint. There are many
different formulations of solvent cement for thermoplastics, but most
cements consist of two or more of the following organic chemical
constituents: tetrahydrofuran (THF), methyl ethyl ketone (HER), methyl
isobutyl ketone (MIBK), cyclohexanone and dimethylformamide (Sosebee
et al.t 1983).
The cements used in solvent welding, which are themselves organic
chemicals, can have an impact on the integrity of ground-water samples
Sosebee et al. (1983) demonstrated that the aforementioned volatile organic
solvents do contaminate ground-water samples collected from raonitorina
wells in which PVC adhesives are used to join well casing sections
Barcelona et al. (1983) noted that even minimal solvent cement application
is sufficient to supply consistent levels of primer/cement components above
100 micrograms per liter in ground-water samples despite proper well
development and flushing prior to sampling. They further point out that
these effects may persist for months after well construction and even after
repeated attempts to develop the wells. Dunbar et al. (1985) cited a case
in which THF was found at levels ranging from 10 to 200 milligrams per
liter in samples taken from several PVC-cased monitoring wells in which PVC
solvent cement was used. These levels were found .ore than two years after
the casing was installed. In samples from adjac.nt monitoring wells In
which threaded PVC casing was used, no THF was found, prompting the
conclusion that the THF concentrations were a relict of solvent cement used
during well construction. The results of these studies point out that
solvent cementing is not appropriate for use in joining sections of
thermoplastic casing used in ground-water monitoring wells.
Mechanical 1oininfi--The most common method of mechanical joining is by
threaded connections. Molded and machined threads are available in a
variety of thread configurations including: aooe. buttress, standard pipe
thread and square threads. Because most manufacturers have their own
— •" ••" • ^•••^••^••^•^ iiu v o vUCX£ wWH
T*!! JyP*' th"aded ca8i"« ™«y not be compatible between manufacturers.
If the threads do not match and a joint is made, the joint can fail or leak
either during or after casing installation.
Because all Joints in a monitoring well casing must be watertight, the
extent of tightening of joints should comply with recommendations by the
manufacturer. Overtightening of casing joints can lead to structural
failure of the joint (National Water Well Association and Plastic Pipe
institute, 1981).
When using thermoplastic well casing, threaded Joints are preferred-
any problems associated with the use of solvent primers and cements can
thus be avoided. Casing with threads machined or oolded directly onto the
pipe (without use of larger-diameter couplings) provides a flush joint
169
-------
between both inner and outer diameters. Because the annular space is
frequently only minimal, casings that do not use couplings are best suited
to use in monitoring well construction. Although this type of joint
reduces the tensile strength of the casing string by about 30 percent as
compared to a solvent-cemented joint, in most monitoring well installations
this is not a critical concern. Where threaded joints are used,
fluoropolymer tape is often wrapped around the threads prior to joining
male and female sections to maximize the watertightness of the joint.
Well Casing Diameter
While casing outside diameters are standardized, variations in wall
thickness cause casing inside diameters to vary. In "scheduled" casing,
wall thickness increases as the scheduling number increases for any given
diameter of casing. As illustrated by Figure 53, nominal 2-inch casing is
a standard 2.375 inches outside diameter; wall thicknesses vary from 0.065
inch for schedule 5 to 0.218 inch for schedule 80. This means that inside
diameters for nominal 2-inch casing vary from 2.245 inches for schedule 5
thin-walled casing (typical of stainless steel) to only 1.939 inches for
schedule 80 thick-walled casing (typical of PVC). Wall thickness also
changes with pipe diameter in scheduling. Another method of evaluating
casing strength is by standard dimension ratios (SDR). A SDR is the ratio
of the wall thickness to the casing diameter. The ratio is referenced to
an internal pounds per square inch (psi) pressure rating such that all
casings with a similar SDR will have a similar psi rating. Where strength
of casing is important, scheduling and SDR numbers provide a means for
choosing casing.
Although the diameter of the casing for a monitoring well depends on
the purpose of the well, the casing size is generally selected to
accommodate downhole equipment. Additional casing diameter selection
criteria include: 1) drilling or well installation method used, 2)
anticipated depth of the well and associated strength requirements,
3) ease of well development, 4) volume of water required to be purged prior
to sampling (Table 32), 5) rate of recovery of the well after purging and
6) cost. For an additional discussion of casing diameter, refer to the
sections entitled "Equipment that the Well Must Accommodate" and
"Description and Selection of Drilling Methods."
Casing Cleaning Requirements
During the production of any casing material, chemical substances are
used to assist in the extrusion, molding, machining and/or stabilization of
the casing material. For example, oils and solvents are used in many
phases of steel casing production. In the manufacturing of PVC well
casing, a wax layer can develop on the inner wall of the casing;
additionally, protective coatings of natural or synthetic waxes, fatty
acids or fatty acid esters may be added to enhance the durability of the
casing (Barcelona et al., 1983). These substances are potential sources of
chemical interference and therefore must be removed prior to installation
of the casing in the borehole. If trace amounts of these materials still
adhere to the casing after installation, the chemical integrity of samples
taken from the monitoring well can be affected.
170
-------
Schedule 5
(stainless steel)
Schedule 10
(stainless steel)
Schedule 40
(stainless steel. PVC.
fluoropolymer)
Schedule 80
(PVC. fluoropolymer)
Wall thickness (inches)
Nominal 2
Nominal 3
Nominal 4
Nominal 5
Nominal 6
Inside Diameter
Nominal 2
Nominal 3
Nominal 4
Nominal 5
Nominal 6
SCHS
0065
0083
0083
0109
0109
SCHS
2245
3334
4334
5345
6407
SCH10
0109
0120
0120
0134
0134
SCH10
2157
3260
4260
5295
6357
SCH40
0154
0216
0237
0298
0280
SCH40
2067
3068
4026
5047
6065
SCH80
0218
0300
0.337
0375
0432
SCHBO
1939
2900
3826
4813
5761
Outside Diameter
(Standard)
2375
3500
4500
5563
6625
Figure 53. Effect of casing wall thickness on casing inside and outside diameter.
Careful pre-installation cleaning of casing materials oust be
conducted to avoid potential chemical interference problems from the
presence of substances such as cutting oils, cleaning solvents, lubricants,
threading compounds, waxes and/or other chemical residues. For PVC, Curran
and Tomson (1983) suggest washing the casing with a strong detergent
solution and then rinsing with water before installation. Barcelona et al.
(1983) and Barcelona (1984) suggest this same procedure for all casing
materials. To accomplish the removal of some cutting oils, lubricants or
solvents, it may be necessary to steam-clean casing materials or employ a
high-pressure hot water wash. Casing materials must also be protected from
contamination while they are on-site awaiting installation in the borehole.
This can be accomplished by providing a clean storage area away from any
potential contaminant sources (air, water or soil) or by using plastic
sheeting spread on the ground for temporary storage adjacent to the work
area. An additional discussion on decontamination of equipment can be
found in the section entitled "Decontamination."
Casing Cost
As Scalf et al. (1981) point out, the dilemma for the field
investigator often is the relationship between cost and accuracy. The
relative cost of PVC is approximately one tenth the cost of fluoropolymer
171
-------
TABLE 32. VOLUME OF WATER IN CASING OR BOREHOLE (DRISCOLL, 1986)
Diameter
of Casing
or Hole
(In)
1
I1/:
T
Gallons
per foot
of Depth
0041
0.092
Cubic Feet
per Foot
of Depth
0.0055
0.0123
Liters
per Meter
of Depth
0.509
1 142
0163 00218 ' 2.024
-"= ; 0.255 00341 \ 3 |67
3 1 0367 00491 4558
3": 0 500
4 0653
41/:
0.0668
6209
0.0873 8110
0826 01104 1026
'• 5 ' 1020 01364 ! 1267
Cubic Meters
per Metet
of Depth
0.509 x 10 '
I.l42x 10-J
2.024 x 10'
3.167 x 10'
4.558 x 10'
6.209 x 10'
8.110 x 10-'
10.26 .x 10'
12.67 x 10' :
5": 1 234 01650 15.33 ! 15.33 x 10'
6 1 469 ' Q 1963 1824 i 1824 x 10'
2000 02673 2484 2484 \ 10'
8 2611 03491 32.43 32.43 x 10'
9 , 3305 04418 ! 4104
10 4080 05454 : 5067
11 4937 0.6600 \ 61.31
12
14
5 875 0 7854 72.96
8.000 1.069
16 ] 1044 ' 1396
18
20
22
24
26
28
30
1322 1.767
1632 2.182
1975 2.640
2350
2758
3 142
3687
'9935
129.65
164.18
202.68
245.28
291.85
342.52
32.00 4.276 39741
3672 '' 4909 45602
32 4178 5585 51887
34
36
4716 6305 , 585.68
52.88
7.069 656.72
41.04 x 10'
50.67 x 10'
61.31 x 10-'
72.96 x 10-'
99.35 x 10'
129.65 x 10-'
16418 x lf>'
202.68 x 10'
245.28 x 10'
291.85 x 10'
342.52 x 10'
397.41 x 10'
456.02 x 10'
51887 x 10'
585.68 x 10'
656.72 x 10'
Gallon - 3.785 Lucre
Meter = 3.281 Feet
Gallon Water Weighs 8.33 Ibs. = 3.785 Kilograms
Liter Water Weighs I Kilogram = 2.205 Ibs.
Gallon per foot of depth = 12.419 liters per fool of depth
Gallon per meter of depth - 12.419 x 1&1 cubic meters per meter of depth
172
-------
materials. Cost is always a consideration for any ground-water raonitorin*
project and becomes increasingly important as the number and/or depth of
the wells increases. However, if the particular components of interest in
a monitoring program are also components of the casing, then the results
that are potentially attributable to the casing will be suspect If the
contaminants to be determined are already defined and they do not include
chemical constituents that could potentially leach from or sorb onto PVC
well casing (as defined by laboratory studies), it may be possible to use
PVC as a less expensive alternative to other materials.
MONITORING WELL INTAKES
Proper design of a hydraulically efficient monitoring well in
unconsolidated geologic materials and in certain types of poorly-
consolidated geologic materials requires that a well intake be placed
opposite the zone to be monitored. The intake should be surrounded by
materials that are coarser; have a uniform grain size; and have a higher
permeability than natural formation material. This allows ground water to
flow freely into the well from the adjacent formation material while
minimizing or eliminating the entrance of fine-grained materials (clay
silt, fine sand) into the well. When the well is properly designed and
developed, the well can provide ground-water samples that are free of
suspended solids. Sediment-free water reduces the potential for
interference in sample analyses and eliminates or reduces the need for
field sample filtration.
These purposes can be accomplished by designing the well in such a way
that either the natural coarse-grained formation materials or artificially
introduced coarse-grained materials, in conjunction with appropriately sized
intake (well screen) openings, retain the fine materials outside the well
while permitting water to enter (United States Environmental Protection
Agency, 1975). Thus, there are two types of wells and well intake designs
for wells installed in unconsolidated or poorly-consolidated geologic
materials: naturally developed wells and wells with an artificially
introduced filter pack. In both types of wells, the objective of a filter
pack is to increase the effective diameter of the well and to surround the
well intake with an envelope of relatively coarse material of greater
permeability than the natural formation material.
In the construction of a monitoring well it is imperative that the
natural stratigraphic setting be distorted as little as possible This
requires that the development of void space be minimized in unconsolidated
formations. As a consequence, boreholes that are over-sized with regard to
the casing and well intake diameter generally should be filter-packed For
example, where 2-inch diameter screens are installed in hollow-stem auser
ooreholes an artificial filter pack is generally recommended. This
prevents the collapse of the borehole around the screen with the subsequent
creation of void space and the loss of stratification of the formation
Collapse also frequently results in the failure of well seals emplaced on
top of the collapsed zone, although well development prior to seal
installation may help to minimize this potential problem.
173
-------
Naturally-Developed Wells
In a naturally-developed well, formation materials are allowed to
collapse around the well intake after it has been installed in the
borehole. The high-permeability envelope of coarse materials is developed
adjacent to the well intake in situ by removing the fine-grained materials
from natural formation materials during the well development process.
As described in Driscoll (1986), the envelope of coarse-grained,
graded material created around a well intake during the development process
can be visualized as a series of cylindrical zones. In the zone adjacent
to the well screen, development removes particles smaller than the screen
openings, leaving only the coarser material in place. Slightly farther
away, some medium-sized grains remain mixed with coarse materials. Beyond
that zone, the material gradually grades back to the original character of
the water-bearing formation. By creating this succession of graded zones
around the screen, development stabilizes the formation so that no further
movement of fine-grained materials will take place and the well will yield
sediment-free water at maximum capacity (Figure
The decision on whether or not a well can be naturally developed is
generally based on geologic conditions, specifically the grain-size
distribution of natural formation materials in the monitored zone. Wells
can generally be naturally developed where formation materials are
relatively coarse-grained and permeable. Grain -size distribution is
determined by conducting a sieve analysis of a sample or samples taken from
the intended screened interval. For this reason, the importance of
obtaining accurate formation samples cannot be overemphasized.
After the sample(s) of formation material is sieved, a plot of grain
size versus cumulative percentage of sample retained on each sieve is made
(Figure 55). Well intake opening sizes are then selected, based on this
grain size distribution and specifically on th« affective size and
uniformity coefficient of the formation materials. The effective size is
equivalent to the sieve size that retains 90 percent (or passes 10 percent)
of the formation material (Figure 56); the uniformity coefficient is the
ratio of the sieve size that will retain 40 percent (or pass 60 percent) of
the formation material to the effective size (Figure 57). A naturally-
developed well can be considered if the effective grain size of the
formation material is greater than 0.01 inch and the uniformity coefficient
is appropriate.
In monitoring well applications, naturally-developed wells can be used
where the maximum borehole diameter closely approximates the outside
diameter of the well intake. By maintaining a minimum space between the
well casing and the borehole face, the disturbance of natural stratigraphic
conditions is minimized. If these conditions are not observed, the radius
of disturbance reduces the probability that ambient flow conditions can be
restored.
Artificially Filter-Packed Wells
When the natural formation materials surrounding the well intake are
deliberately replaced by coarser, graded material introduced from the
174
-------
Zone of
coarsest
natural
material Zone ol
medium-sized
granular
material
Original
material
of water-
bearing
formation
Figure 54. Envelope of coarse-grained material created around a naturally-developed well.
175
-------
RETAINED
3
(J
10
'0 20 30 40 50 60 70 80 90 100
SLOT OPENING AND GRAIN SIZE. THOUSANDTHS OF AN INCH
Figure 55. Plot of grain size versus cumulative percentage of sample retained on sieve.
-------
too
100 70 SO 40 30 20
U S STANDARD SIEVE NUMBtHS
16 12
The effective size ol this sand is
20 30 40 50 60 70 80 90 100 110 120
SLOT OPENING AND GRAIN SIZE. IN THOUSANDTHS OF AN INCH
Figure 56. Determining effective size of formation materials.
-------
00
-------
surface, the well is artificially filter packed. The term "gravel pack" is
also frequently used to describe the artificial material added to the
borehole to act as a filter. Because the term "gravel" is classically used
to describe large-diameter granular material and because nearly all coarse
material emplaced artificially in wells is an engineered blend of coarse to
medium sand-sized material, the use of the terms "sand pack" or "filter
pack" is preferred in this document. Gravel-sized particles are rarely
used as filter pack material because gravel does not generally serve the
intended function of a filter pack in a monitoring well.
The artificial introduction of coarse, graded material into the
annular space between a centrally-positioned well intake and the borehole
serves a variety of purposes. Similar to naturally-developed filter pack,
the primary purpose of an artificial filter pack is to work in conjunction
with the well intake to filter out fine materials from the formation
adjacent to the well. In addition, the artificial filter pack stabilizes
the borehole and minimizes settlement of materials above the well intake.
The introduction of material coarser than the natural formation materials
also results in an increase in the effective diameter of the well and in an
accompanying increase in the amount of water that flows toward and into the
well (Figure 58).
I
Formation material
U/Alt intake
\' "•-••'
• •
•-."'.
• • *
. •
•...
• •• .
• * • . •
•'.• • •
. ...• . •
• • •
. • •
• (
. " »
• ^
• . • •
* . .
. •
• •
•-.•.• •
• .1
•« . f
. . • '
. . . •.
• • * .
. •
• * •
-
• • *
• . •
• • »
• •
• . •
. •
. •
• • * ^ •
m * •
-. ' •• •
.
• * • •
• •
•* * " • •
• • ' • .
•
1 • •
•
1 • . • •
• ; i |%
. . \
• » ( »
• • •
< Borehole
Figure S& Envelope of coarse-grained material emplaced around an artfficaHy filter-packed well.
179
-------
There are several geologic situations where the use of an artificial
filter pack material is recommended:
1) when the natural formation is uniformly fine-grained (i.e. fine
sand through clay-sized particles);
2) when a long screened interval is required and/or the intake spans
highly stratified geologic materials of widely varying grain
sizes;
3) when the formation in which the intake will be placed is a poorly
cemented (friable) sandstone;
4) when the formation is a fractured or solution-channeled rock in
which particulate matter is carried through fractures or
solution openings;
5) when the formation is shales or coals that will act as a constant
supply of turbidity to any ground-water samples; and
6) when the diameter of the borehole is significantly greater than
the diameter of the screen.
The use of an artificial filter pack in a fine-grained geologic
material allows the intake opening (slot) size to be considerably larger
than if the intake were placed in the formation material without the filter
pack. This is particularly true where silts and clays predominate in the
zone of interest and where fine opening sizes in well intakes to hold out
formation materials are either impractical or not commercially available.
The larger intake opening size afforded by artificial filter pack
emplacement thus allows for the collection of adequate volumes of
sediment-free samples and results in both decreased head loss and increased
well efficiency.
Filter packs are particularly well-suited for use in extensively
stratified formations where thin layers of fine-grained materials alternate
with coarser materials. In such a geologic environment, it is often
difficult to precisely determine the position and thickness of each
individual stratum and to choose the correct position and opening size for
a well intake. Completing the well with an artificial filter pack, sized
and graded to suit the finest layer of a stratified sequence, resolves the
latter problem and increases the possibility that the well will produce
water free of suspended sediment.
Quantitative criteria exist with which decisions can be made
concerning whether a natural or an artificial filter pack should be used in
a well (Campbell and Lehr, 1973; United States Environmental Protection
Agency, 1975; Williams, 1981; Driscoll, 1986). Generally the use of an
artificial filter pack is recommended where the effective grain size of the
natural formation materials is smaller than 0.010 inch and the uniformity
coefficient is less than 3.0. California Department of Health Services
(1986) takes a different approach and suggests that an artificial filter
pack be employed if a sieve analysis of formation materials indicates that
a slot size of 0.020 inches or less is required to retain 50 percent of the
natural material.
Economic considerations may also affect decisions concerning the
appropriateness of an artificial filter pack. Costs associated with
filter-packed wells are generally higher than those associated with
180
-------
naturally developed wells, primarily because specially graded and washed
sand must be purchased and transported to the site. Additionally, larger
boreholes are necessary for artificially filter-packed wells (e.g.,
suggested minimum 6-inch diameter borehole for a 2-inch inside diameter
well or 8-inch borehole for a 4-inch well).
An alternate design for the artificial filter pack is provided by the
"pre-packed" well intake. There are two basic designs that are
commercially available: 1) single-wall prepack and 2) double-wall prepack.
The single-wall prepack is fabricated by bonding well-sorted siliceous
grains onto a perforated pipe base. Epoxy-based bonds have been the most
commonly used, although other types of bonding materials have also been
employed. The double-wall prepack consists of an unbonded granular layer
of well-sorted silica grains between two perforated casings. The advantage
of the double-wall system is that it is extremely strong and should not
have chemical questions from bonding agent used in single wall.
The advantages of prepack well intakes are: 1) ease of installation
in either a stable borehole or within boreholes protected by auger flights
or casing (by the pullback method) and 2) the ability if properly sized to
provide filtration of even the finest formations, thereby effectively
minimizing turbidity in otherwise "difficult if not impossible to develop
formations." The disadvantages of this type of well intake are: 1) the
bonding material for the single-wall design may create chemical
interference; 2) wells with prepack screens are difficult to redevelop if
plugging occurs; and 3) commercial availability of this design has been
extremely variable through time. The single-wall epoxy-based well intake
is presently available only on an import basis; the double-wall well intake
is currently available from at least one domestic manufacturer.
Filter Pack Design--
Artificial filter pack design factors for monitoring wells include:
1) filter pack grain size; 2) intake opening (slot) size and length; 3)
filter pack length, 4) filter pack thickness and 5) filter pack material
type. When an artificial filter pack is dictated by sieve analysis or by
geologic conditions, the filter pack grain sizes and well intake opening
sizes are generally designed as a single unit.
The selection of filter pack grain size and well intake opening size
is a function of the formation. The filter pack is designed first because
it is the interface with the aquifer. The first step in designing the
filter pack is to obtain samples of the formation intended to be monitored
and perform sieve analyses on the samples. The filter pack material size
is then selected on the basis of the finest formation materials present.
Although design techniques vary, all use the filter pack ratio to
establish size differential between the formation materials and filter pack
materials. Generally this ratio refers to either the average (50 percent
retained) grain size of the formation material or the 70 percent retained
size of the formation material. For example, Walker (1976) and Barcelona
et al. (1985a) recommend using a uniform filter pack grain size that is 3
181
-------
to 5 times the 50 percent retained size of the formation materials.
Driscoll (1986) recommends a more conservative approach by suggesting that
for fine-grained formations, the 50 percent retained size of the finest
formation sample be multiplied by a factor of 2 to exclude the entrance of
fine silts, sands and clays into the monitoring well. The United States
Environmental Protection Agency (1975) recommends that filter pack grain
size be selected by multiplying the 70 percent retained grain size of the
formation materials by a factor between 4 and 6. A factor of 4 is used if
the formation is fine and uniform; a factor of 6 is used if the formation
is coarser and non-uniform. In both cases, the uniformity coefficient of
the filter pack materials should not exceed 2.5 and the gradation of the
filter material should form a smooth and gradual size distribution when
plotted (Figure 59). The actual filter pack used should fall within the
area defined by these two curves. According to Williams (1981), in uniform
formation materials, either approach to filter pack material sizing will
provide similar results; however in coarse, poorly sorted formation
materials, the average grain size method may be misleading and should be
used with discretion.
Two types of artificial filter packs are possible for use in
production wells: 1) the uniform, well-sorted grain size filter pack and 2)
the graded grain-size filter pack. Uniform filter packs are generally
preferred to graded packs for monitoring wells. Graded packs are more
susceptible to the invasion of formation materials at the formation-filter
pack interface. This invasion results in a partial filling of voids
between grains and a concomitant reduction in permeability. Graded packs
are also very difficult to install in the limited annular space available
without segregation of the filter pack material. With a uniform filter
pack, the fine formation materials can travel between the grains of the
pack and be pulled into the well during development. When this occurs, the
formation permeability is increased and the high permeability of the filter
pack is also retained.
The size of well intake openings can only be selected after the filter
pack grain size is specified. The opening (slot) size is generally chosen
on the basis of its ability to hold back between 85 percent and 100 percent
of the filter pack materials (United States Environmental Protection
Agency, 1975) (Figure 60).
Filter Pack Oimensions--
The filter pack should generally extend from the bottom of the well
intake to approximately 2 to 5 feet above the top of the well intake
provided the interval above the well intake does not result in
cross-connection with an overlying zone. If cross-connection is a
potential problem, then the design may need to be adjusted. The filter
pack placed above the intake allows for settlement of the filter pack
material that occurs during well development and allows a sufficient
"buffer" between the well intake and the annular seal above.
The filter pack must be at least thick enough to surround the well
intake completely but thin enough to minimize resistance caused by the
filter pack to the flow of water into the well during development. To
182
-------
100
US STANDARD SIEVE NUMBERS
100 70 SO 40 30 20
Filter Pack Ratio - 6
Uniformity Coefficienl - 2.1
Filter Pack Ratio = 4 '
Coefficient = 22
Uniformity Coefficient = 2.5
Filter Pack Ratio = 4 to 6
Range for
ilter Pack Gradation
10 20 30 40 SO 60 70 BO 90 100 110 120 130
SLOT OPENING AND GRAIN SIZE. IN THOUSANDTHS OF AN INCH
Figure 59. Artificial pack design criteria.
-------
I
1
tr
tr
LU
Q.
O
100 70 50 40 30
US STANDARD SIEVE NUMBERS
16 12
100
100
D70 formation = 0 014
D70 filler pack - 0.071
Filter Pack Ratio = 5
Uniformity Coefficient
of Filter Pack = 1.3
Recommended Screen
Slot Opening = 0.080 in
(60 slot)
20
10
20 30 40 50 80 70 60 90 100 110 120
SLOT OPENING AND GRAIN SIZE. IN THOUSANDTHS OF AN INCH
130
Figure 60. Selecting well intake slot size based on filter pack grain size.
-------
accommodate the filter pack, the well intake should be centered in the
borehole and the annulus should be large enough and approximately
symmetrical to preclude bridging and irregular placement of filter pack
material. A thicker filter pack neither increases the yield of the well
nor reduces the amount of fine material in the water flowing to the well
(Ahrens, 1957). Most references in the literature (Walker, 1974; United
States Environmental Protection Agency, 1975; Williams, 1981; Driscoll,
1986) suggest that a filter pack thickness of between 3 and 8 inches is
optimum for production wells. A thin filter pack is preferable from the
we11-development perspective, because it is difficult to develop a well
with a thick filter pack. Conversely, it is difficult to reliably
construct a well with a filter pack that is less than 2 inches thick.
Monitoring well filter pack thicknesses are commonly suggested to be at
least 2 to 4 inches. Methods to calculate the volume of filter pack
necessary are contained in Appendix A in the section entitled "Installation
of the Filter Pack."
Filter Pack Materials--
The materials comprising the filter pack in a monitoring well should
be chemically inert to alleviate the potential for alteration of
ground-water sample chemical quality. Barcelona et al. (1985b) suggest
that the filter pack materials should be composed primarily of clean quartz
sand or glass beads. The individual grains of the filter pack materials
should be well-rounded and consist of less than 5 percent non-siliceous
material (Driscoll, 1986). For natural materials, well rounded quartz is
preferred because quartz is nonreactive in nearly all ground-water
conditions and is generally available. A filter pack comprised of other
types of crushed stone should not be used because of potential chemical
alteration of ground water and problems from non-rounded material. If
crushed limestone is used, the alterations may be particularly significant
and pH modifications can be expected. Shale and carbonaceous material
should also be avoided.
Well Intake Design
Monitoring well intake design factors include: 1) intake opening
(slot) size, 2) intake length, 3) intake type and 4) corrosion and chemical
degradation resistance. Proper sizing of monitoring well intake openings
is one of the most important aspects of monitoring well design. There has
been in the past a tendency among some monitoring well designers to install
a "standard" or common slot size (e.g., 0.010 inch slots) in every well,
with no site-specific design considerations. As Williams (1981) points
out, this can lead to difficulties with well development, poor well
performance or, in some severe cases, well failure.
Well Intake Opening Sizes--
For artificially filter packed wells, the well intake opening sizes
are selected as previously discussed and illustrated in Figure 60. For
naturally packed wells, well intake opening sizes are generally selected
based on the following criteria that were developed primarily for
production wells:
185
-------
1) where the uniformity coefficient of the formation material is
greater than 6 and the material above the intended screened
interval is non-caving, the slot size should be that which retains
no less than 30 percent of formation material;
2) where the uniformity coefficient of the formation material is
greater than 6 and the material above the intended screened
interval is readily-caving, the slot size should be that which
retains no less than 50 percent of formation material;
3) where the uniformity coefficient of the formation material is
less than 3 and the material above the intended screened interval
is non-caving, the slot size should be that which retains no less
than 40 percent of formation material;
4) where the uniformity coefficient of the formation material is
less than 3 and the material above the intended screened interval
is readily-caving, the slot size should be that which
retains no less than 60 percent of formation material; and
5) where an interval to be monitored has layered formation material
of differing sizes and gradations, and where the SO percent
grain size of the coarsest layer is less than 4 times the 50
percent size of the finest layer, the slot size should be selected
on the basis of the finest layer. Otherwise, separate screened
sections should be sized for each zone.
Because these criteria were developed for production wells, those
factors that enhance yield are overemphasized. The objective of a
monitoring well is frequently to obtain a water quality sample that is
representative of the in-situ ground-water quality. Hence it is imperative
to minimize disturbance or distortion of flow lines from the aquifer into
the well. To achieve this objective, construction activities that result
in caving, void space or modification of the stratigraphy in the vicinity
of the wellbore must be avoided or minimized. Procedures for attaining
this objective have been discussed in this chapter in the section entitled
"Naturally-Developed Wells" and in Section 4 in the part entitled "Ability
of Drilling Technology to Preserve Natural Conditions."
The slot size determined from a sieve analysis is seldom that of
commercially available screen slot sizes (Table 33), so the nearest smaller
standard slot size is generally used. In most monitoring wells, because
optimum yield from the well is not as critical to achieve as it is in
production wells and because extensive development is more difficult to
accomplish in small-diameter monitoring wells, screens are usually designed
to have smaller openings than indicated by the above-stated design criteria
so that less formation material will be pulled into the well during the
development.
Well Intake Length Selection--
The selection of the length of a monitoring well intake depends on the
purpose of the well. Host monitoring wells function as both ground-water
sampling points and piezometers for a discrete interval. To accomplish
these objectives, well intakes are typically 2 to 10 feet in length and
only rarely equal or exceed 20 feet in length. Shorter intakes provide
186
-------
TABLE 33. CORRELATION CHART
OF SCREEN OPENINGS AND SIEVE SIZES (DRISCOLL, 1986)
Geologic
Material
Grain-size
Range
clay
&
silt
Fine
sand
medium
sand
coarse
sand
very
coarse
sand
very
fine
gravel
fine
gravel
Johnson
Slot
No.
—
6
7
8
ID
12
14
16
18
20
23
25
28
31
33
35
39
47
56
62
66
79
93
94
111
t:s
132
157
187
223
250
263
312
375
438
500
Gauze
No.
—
90
80
70
60
50
40
30
20
Tyler
Siere
No.
400
325
270
250
200
170
ISO
115
100
30
65
60
48
42
35
32
28
24
20
16
14
12
10
9
8
7
6
5
4
3'*
3
2":
0.371
0.441
0.525
Size of O
Inches
0.0015
0.0017
0.0021
0.0024
0.0029
0.0035
0.0041
0.0049
0.0058
0.0069
0.0082
00097
00116
00138
0.0164
0.0180
0.0195
0.0232
0.0250
0.0276
0.0310.
0.0328
0.035
0.039
0.046
0.055
0.062
0.065
0.078
0.093
0.094
0.110
0.125
0.131
0.156
0.1 85
0.221
0.250
0.263
0.312
0.371
0.441
0.525
penings
nun
0.038
0.043
0.053
0.061
0.074
0.088
0.104
0.124
0.147
0.175
0208
0246
0.295
0351
0417
0.457
0.495
0589
0.635
0.701
0.788
0.833
0.889
0.991
1 168
1.397
1.590
1.651
1.981
2.362
2.390
2.794
3.180
3.327
3.962
4699
5.613
6.350
6.680
7925
9.423
It. 20
13.33
US. Standard
Sieve
No.
400
325
270
230
200
170
140
120
100
SO
:o
60
50
45
40
35
30
25
20
18
16
14
12
10
3
7
6
5
4
3'/:
"«
/.
".
/.
'/:
Size of
Openings
Inches
00015
0.0017
0.0021
0.0024
0.0029
00035
0.0041
0.0049
0.0059
0.0070
0.0083
00098
0.0117
00138
0.0165<'U
0.0180
0.0197
0.0232
0.0250
0.0280
0.0310 CM
0.0331
0.0350
0.0394
0.0469
0.0555
0.062 ('/„)
0.0661
0.0787
00931
0.094
-------
more specific information about vertically-distributed water quality,
hydraulic head and flow in the monitored formation. However, if the
objective of the well is to monitor for the gross presence of contaminants
in an aquifer, a much longer screen can be selected to monitor a greater
thickness of the aquifer. This type of well can provide an integrated
water sample and an integrated hydraulic head measurement as well as access
for vertical profiling.
There are also situations where the "flow-through"-type well is
preferable. In a flow-through installation, a small-diameter screen of
2 inches diameter or less, is installed to fully penetrate an aquifer, or
to at least penetrate a significant portion of the aquifer. The diameter
of the screen is small so that minimal distortion of the flow field in the
aquifer is created. Borehole geochemical profiling is used to evaluate
vertical variations in contaminant flow; spot sampling can be used to
provide zone characterization with minimal vertical mixing. By slowly
lowering a geochemical probe into the borehole, measurements of parameters
such as pH, Eh, conductivity, dissolved oxygen and temperature can be taken
at close intervals (e.g. 1-foot, 2-foot or 5-foot intervals). These
measurements can be recorded successively from the top of the saturated
zone to the bottom of the screened interval with very slight disturbance to
the zone being measured. Measurements are taken as the probe is lowered
because vertical mixing in the borehole can be expected to occur as the
probe is withdrawn.
Once sufficient time has passed after sampling for indigenous
conditions to be reestablished, a grab sampler can be lowered to the
uppermost zone of interest and a water quality sample obtained. By slowly
and carefully sampling successively deeper.zones, a series of relatively
undisturbed water quality samples can be collected for laboratory analysis.
The laboratory results can subsequently be compared with the data obtained
from the geochemical probe. The method of geochemical evaluation is
particularly valuable for evaluating three-dimensional flow in a stratified
but relatively homogeneous aquifer such as fluvial sands and gravels.
Well Intake Tvpe--
The hydraulic efficiency of a well intake depends primarily on the
amount of open area available per unit length of intake. While hydraulic
efficiency is of secondary concern in monitoring wells, increased open area
in monitoring well intakes also permits easy flow of water from the
formation into the well and allows for effective well development. The
amount of open area in a well intake is controlled by the type of well
intake and opening size.
Many different types of intakes are available for use in production
wells; several of these are also suitable for use in monitoring wells.
Commercially-manufactured well intakes are recommended for use in
monitoring wells because stricter quality control measures are followed by
commercial manufacturers. Hand-slotted or drilled casings should not be
used as monitoring well intakes because there is poor control over the
intake opening size, lack of open area and potential leaching and/or
188
-------
chemical problems at the fresh surfaces exposed by hand sawing or drilling.
Similarly, casing that has been perforated either by the application of a
casing knife or a perforating gun after the casing is installed in the
borehole is not recommended because intake openings cannot be closely
spaced, the percentage of open area is low, the opening sizes are highly
variable and opening sizes small enough to control fine materials are
difficult or impossible to produce. Additionally, perforation tends to
hasten corrosion attack on metal casing because the jagged edges and rough
surfaces of the perforations are susceptible to selective corrosion.
Many commercially-manufactured well intakes have been used in
monitoring wells including: 1) the louvered (shutter-type) intake, 2) the
bridge-slot intake, 3) the machine-slotted well casing and 4) the
continuous-slot wire-wound intake (Figure 61). The latter two types of
intakes are used most extensively because they are the only types available
with 2-inch inside diameters.
ii n LI
tt Bn
cm cm
cm
Bridge slot screen
Shutter-type
screen
Stoned casing
Continuous slot
wire-wound screen
Figure 61. Types of w«N tnUke*.
189
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The louvered (shutter-type) screen has openings that are manufactured
in solid-wall metal tubing by stamping outward with a punch against dies
that limit the size of the openings (Helweg et al., 1984). The number and
sizes of openings that can be made depends on the series of die sets used
by individual manufacturers. Because a complete range of die sets is
impractical, the opening sizes of commercially-available screens are
somewhat limited. Additionally, because of the large blank spaces that
must be left between adjacent openings, the percentage of open area on
louvered intakes is limited. Louvered well intakes are primarily used in
artificially-packed wells because the shape of the louvered openings is
such that the shutter-type intakes are more difficult to develop in
naturally-packed wells. This type of intake, however, provides greater
collapse strength than most other intakes.
Bridge-slot screen is manufactured on a press from flat sheets or
plates of metallic material that are rolled into cylinders and seam-welded
after being perforated. The slot opening is usually vertical with two
parallel openings longitudinally aligned to the well axis. Five-foot
sections of bridge-slot screen that can be welded into longer screen
sections if desired are commonly available. The advantages of bridge-slot
screen include: a reasonably high intake opening area, minimal frictional
head losses and low cost. One important disadvantage is low collapse
strength that is caused by the presence of a large number of vertically-
oriented slots. The use of this type of intake is limited in monitoring
well application because it is only produced in diameters 6 inches and
larger.
Slotted well intakes are fabricated from standard well casing by
cutting horizontal (circumferential) or vertical (axial) slots of
predetermined widths at regular intervals with machining tools. Slotted
well casing can be manufactured from any casing material although these
intakes are most commonly made from thermoplastic, fluoropolymer and
fiberglass-reinforced epoxy materials. This type of intake is available in
diameters ranging from 3/4 inch to 16 inches (National Water Well
Association and Plastic Pipe Institute, 1981). Table 34 lists the most
common slot widths of slotted well casing.
TABLE 34. TYPICAL SLOTTED CASING SLOT WIDTHS (NATIONAL WATER WELL ASSOCIATION
AND PLASTIC PIPE INSTITUTE. 1981)
0006 0016 0.0*0
0.007 0018 0060
0.008 0030 0080
0.010 0025 0070
0.012 0030 0080
0014 0035 0100
The continuous slot wire-wound intake is manufactured by winding
cold-drawn wire, approximately triangular in cross section, spirally around
a circular array of longitudinally arranged rods (Figure 62). At each
point where the wire crosses the rods, the two members are securely joined
190
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by welding, creating a one-piece rigid unit (Driscoll, 1986).
Continuous-slot intakes can be fabricated of: 1) any metal that can be
resistance-welded, including bronze, silicon red brass, stainless steel
(304 and 316), galvanized and low-carbon steel and 2) any thermoplastic
that can be sonic-welded, including polyvinylchloride (PVC) and
acrylonitrile butadiene styrene (ABS).
0
\>
0
0
rs.
X
IX
<\
Vertical cross-section
Horizontal cross-section
Figure 62. Cross-sections of continuous-wrap wire-wound screen.
The slot openings of continuous-slot intakes are produced by spacing
the successive turns of the wire as desired. This configuration provides
significantly greater open area per given length and diameter than is
available with any other intake type. For example, for 2-inch inside
diameter well intake, the open area ranges from approximately 4 percent for
the smallest slot size (0.006 inch) to more than 26 percent for the largest
slot size (0.050 inch) (Table 35). Continuous-slot intakes also provide a
wider range of available slot sizes than any other type of intake and have
slot sizes that are accurate to within ±0.003 inch (Ahrens, 1970). The
slot openings are designated by numbers that correspond to the width of the
opening in thousandths of an inch. A number 10 slot, for example, refers
to an opening of 0.010 inch.
The continuous-slot intake also is more effective in preventing
formation materials from becoming clogged in the openings. The
triangular-shaped wire is wound so that the slot openings between adjacent
wires are V-shaped, with sharp outer edges; the slots are narrowest at the
191
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outer face and widen inwardly. This makes the intakes non-clogging because
particles slightly smaller than the openings can pass freely into the well
without wedging in the opening.
TABLE 35. INTAKE AREAS (SQUARE INCHES PER LINEAL FOOT OF SCREEN) FOR CONTINUOUS
WIRE-WOUND WELL INTAKE (AFTER JOHNSON SCREENS, INC., 1988)
Screen i S Slot
Size (in) i (0006")
v.PS- i 30
2 PS ; 30
nPS - 34
2PS ; 43
3 PS '54
« PS i 70
jSoee" j 74
4* PS i M
S PS J 81
6 PS I 91
8 PS i 134
3 Slot
(0008")
10 Slot
(0010")
34 j 48
34 j 43
45 1 53
55
71
90
97
94
106
106
176
S3
33
113
119
11 7
131
132
217
12 Slot
(0.01 2")
60
60
65
81
104
135
142
138
155
156
257
^^••••^MH^H
15 Slot
(0.015-)
70
70
81
100
128
165
172
170
191
192
315
20 Slot
(0.020")
39
89
102
ita
16.5
212
212
219
247
250
406
25 Slot
(0025")
108
108
tZ.3
U*
200
258
2T1
26.8
30.0
305
493
30 Slot
(0.030")
liS
125
142
179
232
300
313
310
349
358
574
35 Slot
(0.035")
141
141
162
203
265
33.9
355
35.2
397
40.7
65.0
40 Slot
(0.040")
156
156
179
224
293
377
397
394
442
45*
72.3
£0 Slot
(0050")
:ai
184
201
263
347
«S
463
465
52 «
543
3SS
The maximum iransmitting eaoaetiy ol screens can be aenved from inese figures. To determine GPM per It of screen, multiply me mteke area m
Muare mcnes oy 0 31 R must M rememoered mat mis is the maximum capacity ol We sown oncer ideal conditions wim an entrance velocity ol
01 foot per second
'°S means pipe iize
"Spec means special
Well Intake Material Properties-*
The intake is the part of the monitoring well that is most susceptible
to corrosion and/or chemical degradation and provides the highest potential
for sorption or leaching phenomena to occur. Intakes have a larger surface
area of exposed material than casing, are placed in a position designed to
be in contact with potential contaminants (the saturated zone) and are
placed in an environment where reactive materials are constantly being
renewed by flowing water. To avoid corrosion, chemical degradation,
sorption and leaching problems, the materials from which intakes are made
are selected using the same guidelines as for casing materials.
ANNULAR SEALS
Purpose- of the Annular Seal
Any annular space that is produced as the result of the installation
of well casing in a borehole provides a channel for vertical movement of
water and/or contaminants unless the space is sealed. In any
casing/borehole system, there are several potential pathways for water and
contaminants (Figure 63). One pathway is through the sealing material. If
the material is not properly formulated and installed or if it cracks or
deteriorates after emplacement, the permeability in the vertical direction
can be significant. These pathways can occur because of any of several
reasons, including: 1) temperature changes of the casing and sealing
material (principally neat cement) during the curing or setting of the
sealing material, 2) swelling and shrinkage of the sealing material while
192
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curing or setting or 3) poor bonding between the sealing material and the
casing (Kurt and Johnson, 1982). Another pathway may result if sealing
materials bridge in the annular space. All of these pathways can be
anticipated and usually avoided with proper annular seal formulation and
placement methods.
Filter pack
a) Between casing and seal material b) Through seal material
i By bridging
Figura 63. Potential pathway* for fluid movement in the casing-borehole annulus.
The annular seal in a monitoring well is placed above the filter pack
in the annulus between the borehole and the well casing. The seal serves
several purposes: 1) to provide protection against infiltration of surface
water and potential contaminants from the ground surface down the
casing/borehole annulus, 2) to seal off discrete sampling zones, both
hydraulically and chemically and 3) to prohibit vertical migration of
water. Such vertical movement can cause what is referred to as
"cross contamination." Cross contamination can influence the
representativeness of ground-water samples and can cause an anomalous
hydraulic response of the monitored zone, resulting in distorted data. The
annular seal increases the life of the casing by protecting it against
exterior corrosion or chemical degradation. A satisfactory annular seal
results in complete filling of the annular space and envelopes the entire
length of the well casing to ensure that no vertical migration can occur
within the borehole. Methods to calculate the volume of sealant necessary
to fill the annular space are contained in Appendix A in the section
entitled "Installation of the Filter Pack." Volume calculations are the
same as those performed to calculate filter pack volume.
193
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Materials Used for Annular Seals
According to Moehrl (1964), the material used for an annular seal
must:
1) be capable of emplacement from the surface;
2) hydrate or develop sufficient set strength within a reasonably
short time;
3) provide a positive seal between the casing and the adjacent
formations;
4) be chemically inert to formations or fluids with which it may come
in contact;
5) be permanent, stable and resist chemical or physical
deterioration; and
6) be sufficiently impermeable to fluids to ensure that the vertical
permeability of the casing/borehole system be lower than that of
surrounding formations.
The annular seal may be comprised of several different types of
permanent, stable, low-permeability materials including pelletized,
granular or powdered bentonite, neat cement grout and combinations of both.
The most effective seals are obtained by using expanding materials that
will not shrink away from either the casing or the borehole wall after
curing or setting. Bentonite, expanding neat cement or mixtures of neat
cement and bentonite are among the most effective materials for this
purpose (Barcelona et al., 1983; 1985a). If the casing/borehole annulus is
backfilled with any other material (e.g., recompacted, uncontaminated drill
cuttings; sand or borrow material), a low-permeability seal cannot be
ensured and the borehole may then act as a conduit for vertical migration.
This is particularly a problem when drill cuttings are used as a seal
because recompacted drill cuttings usually have a higher permeability than
the natural formation materials from which they are derived.
Bentonite--
Bentonite is a hydrous aluminum silicate comprised principally of the
clay mineral montmorillonite. Bentonite possesses the ability to expand
significantly when hydrated; the expansion is caused by the incorporation
of water molecules into the clay lattice. Hydrated bentonite in water
typically expands 10 to 15 times the volume of dry bentonite. Bentonite
forms an extremely dense clay mass with an In-place permeability typically
in the range of 1x10 to 1x10 cm/sec when hydrated. Bentonite expands
sufficiently to provide a very tight seal between the casing and the
adjacent formation material, thus making it a desirable sealant for the
casing/borehole annulus in monitoring wells.
Bentonite used for the purpose of sealing the annulus of monitoring
wells is generally one of two types: 1) sodiun bentonite or 2) calcium
bentonite. Sodium bentonite is the most widely used because of its greater
expandability and availability. Calcium bentonite may be preferable in
high-calcium environments because shrinkage resulting from long-term
calcium-for-sodium ion exchange is minimized. Bentonite is available in
several forms including pellets, granules and powder. Pellets are
194
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uniformly shaped and sized by compression of sodium montmorillonite powder.
Granules are irregularly shaped and sized particles of sodium
montmorillonite. Both pellets and granules expand at a relatively fast
rate when exposed to fresh water. The powdered form of bentonite is the
form produced by the processing plant after mining. While both pelletized
and granular bentonite may be emplaced in dry form, powdered bentonite is
generally made into a slurry to allow emplacement.
Bentonite slurry is generally prepared by mixing dry bentonite powder
into fresh water in a ratio of approximately 15 pounds of bentonite to 7
gallons of water to yield 1 cubic foot of bentonite slurry. The bentonite
and water are mixed by moderate agitation, either manually in a large tank
or with a paddle mixer. The use of high-shear mixing equipment increases
the viscosity development of the slurry and can reduce the ultimate working
time by as much as 20 percent. Thick bentonite slurries may swell quickly
into non-pumpable gel masses that cannot be emplaced. Pre-mix and/or
polymer (organic and inorganic) additives delay the wetting of the
bentonite and prevent premature hydration. Where additives are used, the
additives should be evaluated for potential effects on extant ground-water
quality. Once the slurry is mixed, it should remain workable for between
one-half and two hours. During this time, a positive displacement mud or
grout pump (typically a centrifugal, diaphragm, piston or moyno-type pump)
is used to emplace the seal at the desired depth.
Bentonite has a high cation exchange capacity. This high cation
exchange capacity allows the bentonite to exchange cations that are part of
the chemical structure of the bentonite (principally Na, Al, Fe and Mn)
with cations that exist in the aqueous solution (e.g., ground water) that
hydrates the bentonite. The bentonite may take up or release cations from
or into aqueous solution depending on: 1) the chemistry of both the
bentonite and the solution and 2) the pH and redox potential of the aqueous
solution. In addition to having a high cation exchange capacity, bentonite
generally sets up with a moderately high pH between 8.5 and 10.5. Thus,
bentonite may have an impact on the quality of ground water with which it
comes in contact. In particular, pH and metallic ion content may be
affected. If a bentonite seal is placed too close to the top of the well
intake, water-quality samples that are not representative of the aquifer may
be collected. The suggested practice is to place at least 1 foot of very
fine-grained sand on top of the filter pack and to place the bentonite
sealing material 2 to 3 feet above the top of the well intake, where
possible.
The effective use of bentonite pellets as a sealing material depends
on efficient hydration following emplacement. Hydration requires the
presence of water of both sufficient quantity and quality within the
geologic materials penetrated by the borehole. Generally, efficient
hydration will occur only in the saturated zone. Bentonitic materials by
themselves are generally not appropriate for use in the vadose zone because
sufficient moisture is not available to effect hydration of the bentonite.
Certain water-quality conditions inhibit the swelling of bentonite. For
example, bentonite mixed with water that has either a total dissolved
solids content greater than 500 parts per million or a high chloride
195
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content may not swell to occupy the anticipated volume and therefore may
not provide an effective seal. The degree of inhibition depends on the
level of chlorides or total dissolved solids in the water. Recent studies
conducted to determine the effects of some organic solvents and other
chemicals (i.e., xylene, acetone, acetic acid, aniline, ethylene glycol,
methanol and heptane) on hydrated clays including bentonite have
demonstrated that bentonite and other clays may lose their effectiveness as
low-permeability barrier materials in the presence of concentrated
solutions of selected chemical substances (Anderson et al., 1982; Brown et
al., 1983). These studies have shown that the hydraulic conductivity of
clays subjected to high concentrations of organic acids, basic and neutral
polar organic compounds and neutral non-polar organic compounds may
increase by several orders of magnitude due to dessication and dehydration
of the clay material. This dessication and dehydration can provide
conduits for vertical migration within boreholes in which bentonite is used
as sealing material. Villaume (1985) points to possible attack on and loss
of integrity of bentonite seals due to dehydration and shrinkage of the
clay by hydrocarbons in the free product phase. Thus, where these chemical
conditions exist in the subsurface, bentonite may not perform as an
effective seal and another material may be necessary.
In summary, factors that should be considered in evaluating the use of
bentonite as a sealant include:
1) position of the static water level in a given borehole (including
seasonal and other water-level fluctuations);
2) ambient water quality (particularly with respect to total
dissolved solids content and chloride content); and
3) types and potential concentrations of contaminants expected to be
encountered in the subsurface.
Cement--
Neat cement is a mixture of Portland cement (ASTM C-150) and water in
the proportion of 5 to 6 gallons of clean water per bag (94 pounds or 1
cubic foot) of cement. Five general types of Portland cement are produced:
Type I, for general use; Type II, for moderate sulfate resistance or
moderate heat of hydration; Type III, for high early strength; Type IV, for
low heat of hydration; and Type V, for high sulfate resistance (Moehrl,
1964). Of the five types of cement, Type I is the most widely used in
ground-water related work.
Portland cement mixed with water in the above-cited proportions
creates slurry that weighs approximately 14 to 15 pounds per gallon. A
typical 14 pounds per gallon neat cement slurry has a mixed volume of
approximately 1.5 cubic feet per sack and a set volume of approximately 1.2
cubic feet; volumetric shrinkage is approximately 17 percent and the
porosity of the set cement approximates 54 percent (Moehrl, 1964). The
setting time for such a cement mixture ranges from 48 to 72 hours depending
primarily on water content. A variety of additives may be mixed with the
cement slurry to change the properties of the cement. The more common
additives and associated effects on the cement include:
196
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1) bentonite (2 percent to 6 percent). Bentonite improves the
workability of the cement slurry, reduces the slurry weight and
density, reduces shrinkage as the cement sets and produces a lower
unit cost sealing material. Bentonite also reduces the set
strength of the seal, but this is rarely a problem because the seal
is seldom subject to high stress (Ahrens, 1970);
2) calcium chloride (1 percent to 3 percent). Calcium chloride
accelerates the setting time and creates a higher early strength;
these attributes are particularly useful in cold climates.
Calcium chloride also aids in reducing the amount of slurry
that enters into zones of coarse material;
3) gypsum (3 percent to 6 percent). Gypsum produces a quick-setting,
very hard cement that expands upon setting. However, the high
cost of gypsum as an additive limits the use to special
operations;
4) aluminum powder (less than 1 percent). Aluminum produces a
strong, quick-setting cement that expands on setting and
therefore provides a tighter seal (Ahrens, 1970);
5) fly ash (10 percent to 20 percent). Fly ash increases sulfate
resistance and early compressive strength;
6) hydroxylated carboxylic acid. Hydroxylated carboxylic acid
retards setting time and improves workability without compromising
set strength; and
7) diatomaceous earth. Diatomaceous earth reduces slurry density,
increases water demand and thickening time and reduces set
strength.
Water used to mix neat cement should be clean, fresh water free of oil
or other organic material and the total dissolved mineral content should be
less than 2000 parts per million. A high sulfate content is particularly
undesirable (Campbell and Lehr, 1975). If too much water is used, the
grout will be weakened and excessive shrinkage will occur upon setting. If
this occurs, the annulus will not be completely filled after the grouting
operation. The voids in the annulus may not be seen from the surface but
may still be present along the length of the casing (Kurt, 1983).
Mixing of neat cement grout can be accomplished manually or with a
mechanical mixer. Mixing must be continuous so that the slurry can be
emplaced without interruption. The grout should be mixed to a relatively
stiff consistency and immediately pumped into the annulus. The types of
pumps suggested for use with grout include reciprocating (piston) pumps,
diaphragm pumps, centrifugal pumps or moyno-type pumps. These pumps are
all commonly used by well drilling contractors.
Neat cement, because of its chemical nature (calcium carbonate,
alumina, silica, magnesia, ferric oxide and sulfur trioxide), is a highly
alkaline substance with a pH that typically ranges from 10 to 12. This
high pH presents the potential for alteration of the pH of water with which
it comes in contact. This alteration of pH in the ground water can
subsequently affect the representativeness of any water-quality samples
collected from the well. Because the mixture is emplaced as a slurry, the
coarse materials that comprise the filter pack around the intake portion of
197
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a monitoring well may be infiltrated by the cement if the cement is placed
directly on top of the filter pack. This is particularly true of thinner
slurries that are mixed with more than 6 gallons of water per sack of
cement. The cement infiltration problem also can be aggravated if well
development is attempted prior to the time at which the cement has reached
iinal set.
These problems can have a severe and persistent effect on the
performance of the monitoring well in terms of yield and sample integrity
If thin grout is placed on top of the filter pack and infiltrates, the
cement material can plug the filter pack and/or the well intake upon
setting. The presence of the high-pH cement within the filter pack can
cause anomalous pH readings in subsequent water samples taken from the
well. Dunbar et al. (1985) reported that wells completed in
low-permeability geologic materials with cement placed on top of the filter
pack consistently produced samples with a pH greater than 9 for two and
one-half years despite repeated attempts at well development. For these
reasons, neat cement should not be emplaced directly on top of the filter
pack of a monitoring well. Ramsey and Maddox (1982) have suggested that a
1 to 2-foot thick very fine-grained sand layer be placed atop the filter
pack material prior to emplacement of the neat cement grout to eliminate
the grout infiltration potential. A 2- to 5-foot thick bentonite seal will
accomplish the same purpose, but requires additional time to allow the
bentonite to hydrate prior to cement placement. Either or both of these
procedures serve to minimize well performance impairment and chemical
interference effects caused by the proximity of neat cement to the well
intake.
Another potential problem with the use of neat cement as an annular
sealing material centers around the heat generated by the cement as it
sets. When water is mixed with any type of Portland cement, a series of
spontaneous chemical hydration reactions occur. If allowed to continue to
completion, these reactions transform the cement slurry into a rigid solid
material. As the hydration reactions progress and the cement cures, heat
is given off as a by-product; this heat is known as the heat of hydration
(Troxell et al., 1968). The rate of dissipation of the heat of hydration
is a function of curing temperature, time, cement chemical composition and
the presence of chemical additives (Lerch and Ford, 1948). Generally, the
heat of hydration is of little concern. However, if large volumes of
cement are used or if the heat is not readily dissipated (as it is not in a
borehole because of the insulating properties of geologic materials),
relatively large temperature rises may result (Verbeck and Foster, 1950).
The high heats can cause the structural integrity of some types of well
casing, notably thermoplastic casing, to be compromised. Thermoplastics
characteristically lose strength and stiffness as the temperature of the
casing increases. Because collapse pressure resistance of a casing is
proportional to the material stiffness, if casing temperatures are raised
sufficiently this can result in failure of the casing (Johnson et al.
1980).
Molz and Kurt (1979) and Johnson et al. (1980) studied the heat of
hydration problem and concluded:
198
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1) peak casing temperatures increase as the grout thickness
increases. Temperature rises for casings surrounded by 1.5 inches
to 4 inches of Type I neat cement ranged from 16°F to 45°F;
temperature rises for casings surrounded by 12 inches of grout
(i.e. where washouts or caving or collapse of formation materials
into the borehole might occur) can be in excess of 170°F. In
the former case, plastic pipe retains a large fraction of
collapse strength, but in the latter case, some types of plastic
pipe lose a large fraction of the collapse strength (Gross, 1970);
2) the ratio of the grout-soil interface surface area to the volume
of grout significantly influences peak casing temperatures.
Additionally, peak temperature rise for any casing size is
nonlinear with respect to grout thickness. Lower peak
temperatures can thus be expected for smaller-diameter casings; and
3) peak temperatures are normally reached 8 to 10 hours after water
is added to the cement, and casing temperatures remain near their
peak for several hours before slowly returning to the original
temperature.
The use of setting time accelerators, such as calcium chloride, gypsum
or aluminum powder can increase the heat of hydration and cause casings to
overheat while the grout is curing. This temperature increase poses an
increased potential for casing failure. Both Molz and Kurt (1979) and
Johnson et al. (1980) attribute uncommon premature collapses of neat cement
grouted thermoplastic-cased wells to two factors: 1) that most grouting is
done by filling the annulus from the bottom upward and 2) that as the grout
cures, it gains strength and provides support to the casing.
Several methods can be used to minimize the heat of hydration. Adding
setting-time retardants, such as bentonite or diatomaceous earth, to the
grout mix tends to reduce peak temperatures. Other approaches include:
adding inert materials such as silica sand to the grout; circulating cool
water inside the casing during grout curing; and increasing the
water-cement ratio of the grout mix (Kurt, 1983). However, increasing the
water-cement ratio of the grout mix results in increased shrinkage and
decreased strength upon setting and more potential to move beyond where
expected or intended before setting.
Neat cement annular seals are subject to channeling between the casing
and the seal because of temperature changes during the curing process;
swelling and shrinkage of the grout while the mixture cures; and poor
bonding between the grout and the casing surface (Kurt and Johnson, 1982).
One method of ensuring a low-permeability grout seal in a monitoring well
is to minimize the shrinkage of the grout as it cures. Minimizing
shrinkage, lowering permeability and increasing the strength of cured grout
can be accomplished by minimizing water/cement ratios (Kurt and Johnson,
1982). Typical vertical permeabilities for casing/grout-systems were found
by Kurt and Johnson (1982) to range from 20 to 100 x 10 centimeters per
second. These permeabilities are higher than those determined for neat
cement grout only. This implies that the presence of casing is a factor
that increases the permeability of the system.
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Methods for Evaluating Annular Seal Integrity
There are presently no foolproof field tests that can be performed to
determine if a proper annular seal has been achieved. Of the most commonly
used field tests for checking seals in production wells, only one appears
to provide basic information on the integrity of an annular seal in a
monitoring well—geophysical logging. The accuracy of geophysical logging
techniques is often questioned because they are indirect sensing
techniques. The log most commonly used to check a seal composed of neat
cement grout is the cement bond (acoustic, sonic) log. A cement bond log
generally indicates bonded and non-cemented zones but cannot detect the
presence of vertical channels within the cement nor small voids in the
contact area with the casing. Cement bond logs are available for wells
with inside diameters of 2 inches or larger.
Where thermoplastic or fluorocarbon casing is installed, there is no
sound or sonic wave return recorded along the casing as is the case with
metallic pipe. As a consequence, the information derived is even more
difficult to interpret. Further, there are no good methods available to
evaluate the effectiveness of bentonite seals. This is an area in need of
further research.
SURFACE COMPLETION AND PROTECTIVE MEASURES
Two types of surface completions are common for ground-water
monitoring wells: 1) above-ground completion and 2) flush-to-ground surface
completion. An above-ground completion is preferred whenever practical,
but a flush-to-ground surface may be required at some sites. The primary
purposes of either type of completion are* to prevent surface runoff from
entering and infiltrating down the annulus of the well and to protect the
well from accidental damage or vandalism.
Surface Seals
Whichever type of completion is selected for a well, there should
always be a surface seal of neat cement or concrete surrounding the well
casing and filling the annular space between the casing and the borehole at
the surface. The surface seal may be an extension of the annular seal
installed above the filter pack or it may be a separate seal emplaced on
top of the annular seal. Because the annular space near the land surface
is large and the surface material adjacent to the borehole is disturbed by
drilling activity, the surface seal will generally extend to at least 3
feet away from the well casing at the surface; the seal will usually taper
down to the size of the borehole within a few feet of the surface. In
climates with alternating freezing and thawing conditions, the cement
surface must extend below the frost depth to prevent potential well damage
caused by frost heaving. A suggested design for dealing with heaving
conditions is shown in Figure 21. If cement is mounded around the well to
help prevent surface runoff from ponding and entering around the casing,
the mound should be limited in size and slope so that access to the well is
not impaired and to avoid frost heave damage. In some states, well
installation regulations were initially developed for water supply wells.
200
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These standards are sometimes now applied to monitoring wells, and these
may require that the cement surface seal extend to depths of 10 feet or
greater to ensure sanitary protection of the well.
Above-Ground Completions
In an above-ground completion, a protective casing is generally
installed around the well casing by placing the protective casing into the
cement surface seal while it is still wet and uncured. The protective
casing discourages unauthorized entry into the well, prevents damage by
contact with vehicles and protects PVC casing from degradation caused by
direct exposure to sunlight. This protective casing should be cleaned
thoroughly prior to installation to ensure that it is free of any chemicals
or coatings. The protective casing should have a large enough inside
diameter to allow easy access to the well casing and to allow easy removal
of the casing cap. The protective casing should be fitted with a locking
cap and installed so that there is at least 1 to 2 inches clearance between
the top of the in-place inner well casing cap and the bottom of the
protective casing locking cap when in the locked position. The protective
casing should be positioned and maintained in a plumb position. The
protective casing should be anchored below frost depth into the cement
surface seal and extend at least 18 inches above the surface of the ground.
Like the inner well casing, the outer protective casing should be
vented near the top to prevent the accumulation and entrapment of
potentially explosive gases and to allow water levels in the well to
respond naturally to barometric pressure changes. Additionally, the outer
protective casing should have a drain hole installed just above the top of
the cement level in the space between the protective casing and the well
casing (Figure 21). This drain allows trapped water to drain away from the
casing. This drain is particularly critical in freezing climates where
freezing of water trapped between the inner well casing and the outer
protective casing can cause the inner casing to buckle or fail.
A case-hardened steel lock is generally installed on the locking
casing cap to provide well security. However, corrosion and jamming of the
locking mechanism frequently occurs as the lock is exposed to the elements.
Lubricating the locks or the corroded locking mechanisms is not recommended
because lubricants such as graphite, petroleum-based sprays, silicone and
others may provide the potential for sample chemical alteration. Rather,
the use of some type of protective measure to shield the lock from the
elements such as a plastic covering may prove a better alternative.
In high-traffic areas such as parking lots, or in areas where heavy
equipment may be working, additional protection such as the installation of
three or more bumperguards" are suggested. Bumperguards are
brightly-painted posts of wood, steel or some other durable material set in
cement and located within 3 or 4 feet from the well.
Flush-to-Ground Surface Completions
In a flush-to-ground surface completion, a protective structure such
as a utility vault or meter box is instated around well casing that has
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been cut off below grade. The protective structure is typically set into
the cement surface seal before it has cured. This type of completion is
generally used in high-traffic areas such as streets, parking lots and
service stations where an above-ground completion would severely disrupt
traffic patterns or in areas where it is required by municipal easements or
similar restraints. Because of the potential for surface runoff to enter
the below-grade protective structure and/or well, this type of completion
must be carefully designed and installed. For example, the bond between
the cement surface seal and the protective structure as well as the seal
between the protective structure and removable cover must be watertight.
Use of an expanding cement that bonds tightly to the protective structure
is suggested. Installation of a flexible o-ring or gasket at the point
where the cover fits over the protective structure usually suffices to seal
the protective structure. In areas where significant amounts of runoff
occur, additional safeguards to manage drainage may be necessary to
discourage entry of surface runoff.
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guide for ground-water sampling; Illinois State Water Survey, SWS Contract
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Barcelona, M.J., J.P. Gibb and R. Miller, 1983. A guide to the selection of
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Hamilton, Hugh, 1985. Selection of materials in testing and purifying
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Harsh, J.M. and J.W. Lloyd, 1980. Details 'of hydrochemical variations in
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SECTION 6
COMPLETION OF MONITORING WELLS
INTRODUCTION
Once a borehole has been completed to the desired monitoring depth the
monitoring well must be properly installed. Although monitoring wells can
be completed in a variety of configurations,, successful completion of any
monitoring well must incorporate the following objectives:
1) the well completion must permit specific stratigraphic zones to be
sampled with complete confidence that the sample obtained is
representative of the in-situ water quality;
2) the well completion must permit contaminants with differing
physical properties to be sampled. For example, if the contaminant
is denser or lighter than water and therefore sinks or floats
accordingly, the well completion must allow collection of a
representative ground-water sample;
3) the well must be constructed to prevent cross contamination
between different zones. Cross contamination can occur if: a) the
intake and/or filter pack spans more than one hydraulic unit, b)
hydraulic communication between zones occurs along the
borehole/grout interface, the casing/grout interface or through
voids in the seal, c) fractures intersect the wellbore or d) if
loosely compacted soils are adjacent to the borehole;
4) the well completion should minimize any disturbance created during
the drilling process. For example, if the well was drilled by
hollow-stem augers, the completion techniques should eliminate the
void space created by the withdrawal of the augers; and
5) the well completion method should be cost effective; sample
integrity, of course, is of critical importance.
To achieve these objectives, the well intake, filter pack and annular seal
must be installed using appropriate techniques. The following discussion
addresses these techniques.
WELL COMPLETION TECHNIQUES
Well Intake Installation
In cohesive unconsolidated material or consolidated formations, well
intakes are installed as an integral part of the casing string by lowering
the entire unit into the open borehole andd placing the well intake opposite
the interval to be monitored. Centralizing devices are typically used to
center the casing and intake in the borehole to allow uniform installation
of the filter pack material around the well intake. If the borehole has
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been drilled by a technique that creates borehole damage, it is necessary to
develop the borehole wall. When the formation is sufficiently stable, this
development should be undertaken prior to setting the well intake. After
the filter pack has been installed it is very difficult to clean fractures
or to remove mudcake deposits that have been formed on the borehole wall.
If the borehole was drilled with the mud rotary technique, the borehole
should be conditioned and the wallcake removed from the borehole wall with
clean water prior to the installation of the well intake, if possible. An
additional discussion on well development is found in Section 7, entitled
"Monitoring Well Development."
In non-cohesive, unconsolidated materials when the borehole is drilled
by a drill-through casing advancement method such as a casing hammer or a
cable tool technique, the well intake should be centered inside the casing
at the end of the riser pipe and held firmly in place as the casing is
pulled back. When the well intake is being completed as a natural pack, the
outside diameter of the well intake should be between 1 and 2 inches smaller
than the outside diameter of the casing that is being retracted. If an
artificial filter pack is installed, the outside diameter of the well intake
should be at least 3 to 5 inches smaller than the outside diameter of the
casing that is being retracted. During artificial filter pack installation,
the filter pack material must be maintained above the lower-most level of
the casing as the casing is removed. This means that the filter pack is
being emplaced continually during the time that the casing is being pulled
back and the well intake is being exposed. This procedure minimizes the
development of excessive void space adjacent to the well intake as the
casing is pulled back.
When the casing is installed through the hollow stem of a hollow-stem
auger, an artificial filter pack generally should be emplaced because of the
disparity between the outside diameter of the auger flights and the usual
2-inch or 4-inch outside diameter of the casing and well intake that are
being installed within the auger flights. If the augers are withdrawn and
the formation allowed to collapse around the well intake without installing
an artificial filter pack to stabilize the borehole wall, the materials that
are adjacent to the well intake may be loose and poorly compacted.
Excessive void space adjacent to the well intake can provide an avenue for
cross contamination or migration of contaminants. This void or
loosely-compacted zone may also interfere with the placement of proper
seals.
Loosely-compacted material is difficult to adequately develop from
within a small-diameter borehole. The surging methods that are available
generally cannot recompact the materials adjacent to the well intake to
prevent bentonite or cement grout from migrating downward into the screened
zone. Additionally, where collapse is permitted, the collapsed zone around
the well intake is highly disturbed and is no longer stratified similar to
the stratification of the natural formation. As a consequence, there will
be mixing of horizontal zones, and the possibility exists that chemical
changes can be induced by the changes in the physical environment.
Where wells are installed in unconsolidated material by the dual-wall
reverse-circulation method, the well casing and well intake are installed
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through the bit. The only option for completion with this construction
method is to allow the materials to collapse around the screen. In this
instance, a greater sustained effort is suggested in well development
procedures than is normally required.
Filter Pack Installation
Several methods of emplacing artificial filter packs in the annular
space of a monitoring well are available, including: 1) gravity (free fall),
2) tremie pipe, 3) reverse circulation and 4) backwashing. The last two
methods involve the addition of clean water to the filter pack material
during emplacement. This addition of fluid can cause chemical alteration of
the environment adjacent to the well and pose long-term questions about the
representativeness of water samples collected from the well. As with other
phases of monitoring well construction, fluids (clean) should only be added
when no other practicable method exists for proper filter pack emplacement.
An additional discussion on choosing filter pack material size can be found
in the section entitled "Artificially Filter-Packed Wells."
Placement of filter packs by gravity or free fall can be successfully
accomplished only in relatively shallow wells where the probability of
bridging or segregation of the filter pack material is minimized. Bridging
causes unfilled voids in the filter pack and may prevent the filter pack
material from reaching the intended depth. Segregation of filter pack
material can result in a well that consistently produces sediment-laden
water samples. Segregation is a problem particularly in wells with a
shallow static water level. In this situation, the filter pack material
falls through the column of water at different rates. The greater drag
exerted on smaller particles due to their greater surface area-to-weight
ratio causes finer grains to fall at a slower rate than coarser grains.
Thus, coarser materials will comprise the lower portion of the filter pack
and finer materials will constitute the upper part (Figure 64). Segregation
may not be a problem when emplacing truly uniform filter packs where the
uniformity coefficient is less than 2.5, but placement by free fall is not
recommended in any other situation (Oriscoll, 1986).
With the tremie pipe emplacement method, the filter pack material is
introduced through a rigid tube or pipe via gravity directly into the
interval adjacent to the well intake (Figure 65). Initially, the end of the
pipe is positioned at the bottom of the well intake/borehole annulus. The
filter pack material is then poured down the tremie pipe and the tremie is
raised periodically to allow the filter pack material to fill the annular
space around the well intake. The minimum diameter of a tube used for a
tremie pipe is generally 1 1/2 inches; larger-diameter pipes are advisable
for filter pack materials that are coarse-grained or characterized by
uniformity coefficients that exceed 2.5 (California Department of Health
Services, 1986). When installing a filter pack with a uniformity
coefficient greater than 2.5 in wells deeper than 250 feet, a variation of
the standard tremie method that employs a pump to pressure feed the
materials into the annulus is suggested by the California Department of
Health Services (1986).
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In the reverse circulation method, a filter pack material and water
mixture is fed into the annulus around the well intake. Return flow of
water passes into the well intake and is then pumped to the surface through
the riser pipe/casing (Figure 66). The filter pack material should be
introduced into the annulus at a moderate rate to allow for an even
distribution of material around the well intake. Care must be exercised
when pulling the outer casing so that the riser pipe is not also pulled.
Figure 64. Segregation of artificial filter pack materials caused by gravity emplacement
Backwashing filter pack material into place is accomplished by allowing
filter pack material with a uniformity coefficient of 2.5 or less to fall
fxeely through the annulus while concurrently pumping clean fresh water down
the casing, through the well intake and back up the annulus (Figure 67).
Backwashing is a particularly effective method of filter pack emplacement in
cohesive, non-caving geologic materials. This method also minimizes the
formation of voids that tend to occur in tremie pipe emplacement of the
filter pack.
Annular Seal Installation
The two principal materials used for annular seals are bentonite and
neat cement. Often a combination of the two materials is used. Because the
integrity of ground-water samples depends on good seals, the proper
emplacement of these seals is paramount. An additional discussion on
annular seals can be found in the section entitled "Annular Seals."
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Casing
Well intake •
•Sand
-Tremie pipe
.Borehole wall
• Filter pack material
Figure 65. Tremte-pipe emplacement of artificial filter pack materials.
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Funnel
6" Casing
(Casing pulled back during
filter pack installation)
Well intake
Figure 66. Reverse-circulation emplacement of artificial filter pack materials.
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Water
Filter pack material
Fine-grained
materials and water
Fine-grained
materials and water
Rgure 67. Emplacement of artificial filter pack material by backwashing.
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Bentonite--
Bentonite may be emplaced as an annular seal in either of two different
forms: 1) as a dry solid or 2) as a slurry. Typically only palletized or
granular bentonite is emplaced dry; powdered bentonite is usually mixed with
water at the surface to form a slurry and then is added to the
casing/borehole annulus. Additional discussion on properties of bentonite
can be found in Chapter 5 in the section entitled "Materials Used For
Annular Seals."
Dry granular bentonite or bentonite pellets may be emplaced by the
gravity (free fall) method by pouring from the ground surface. This
procedure should only be used in relatively shallow monitoring wells that
are less than 30 feet deep with an annular space of 3 inches or greater.
When the gravity method is used, the bentonite should be tamped with a
tamping rod after it has been emplaced to ensure that no bridging of the
pellets or granules has occurred. Where significant thicknesses of
bentonite are added, tamping should be done at selected intervals during the
emplacement process. In deeper wells, particularly where static water
levels are shallow, emplacing dry bentonite via the gravity method
introduces both a very high potential for bridging and the likelihood that
sloughing material from the borehole wall will be included in the seal. If
bridging occurs, the bentonite may never reach the desired depth in the
well; if sloughing occurs, "windows" of high permeability may develop as the
sloughed material is incorporated into the seal. Either situation results
in an ineffective annular seal that may allow subsequent contamination of
the well.
In wells deeper than 30 feet, granular or pallatized bentonite can be
conveyed from the surface directly to the intended depth in the annulus by a
tremie pipe. Pelletized bentonite is sometimes difficult to work with in
small-diameter tremie pipes; a miniumum of 1 1/2-inch inside diameter pipe
should be used with 1/4-inch diameter pellets to minimize bridging and
subsequent clogging of the bentonite inside the tremie pipe. Larger-
diameter tremie pipes should be used with larger-diameter pellets. Where a
seal of either pelletized or granular bentonite must be placed at
considerable depth beneath the water surface, the tremie pipe can be kept
dry on the inside by keeping it under gas pressure (Riggs and Hatheway,
1986). A dry tremie pipe has a much lower potential for bridging in the
tremie because the material does not have to fall through a partially
water-filled pipe to reach the desired depth.
Bentonite slurry can be an effective well seal only if proper mixing,
pumping and emplacement methods are used. Bentonite powder is generally
mixed with water in a batch mixer and the slurry is pumped under positive
pressure through a tremie pipe down the annular space using some variety of
positive displacement pump (i.e., centrifugal, piston, diaphragm or
moyno-type pump). All hoses, tubes, pipes, water swivels and other
passageways through which the slurry must pass should have a minimum inside
diameter of 1/2 inch. A larger diameter (e.g., 1-inch) tremie pipe is
preferred. The tremie pipe should be placed just above the filter pack or
at the level where non-cohesive material has collapsed into the borehole
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(Figure 68). The tremie pipe should be left at this position during the
emplacement procedure so that the slurry fills the annulus upward from the
bottom. This allows the slurry to displace ground water and any loose
formation materials in the annular space. The tremie pipe can be raised as
the slurry level rises as long as the discharge of the pipe remains
submerged at least a foot beneath the top of the slurry. The tremie pipe
can be removed after the slurry has been emplaced to the intended level in
the annulus. The slurry should never be emplaced by free fall down the
annulus. Free fall permits the slurry to segregate thus preventing the
formation of an effective annular seal.
Slurry
Figure 68. Tremte-plpe emplacement of annular seal material (either bentonite or neat cement slurry).
Bentonite emplaced as a slurry will already have been hydrated to some
degree prior to emplacement, but the ability to form a tight seal depends on
additional hydration and saturation after emplacement. Unless the slurry is
placed adjacent to saturated geologic materials, sufficient moisture may not
be available to maintain the hydrated state of the bentonite. If the slurry
begins to dry out, the seal may dessicate, crack and destroy the integrity
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of the seal. Therefore, bentonite seals are not recommended in the vadose
zone.
Curing or hydration of the bentonite seal material occurs for 24 to 72
hours after emplacement. During this time, the slurry becomes more rigid
and eventually develops strength. Well development should not be attempted
until the bentonite has completely hydrated. Because of the potential for
sample chemical alteration posed by the moderately high pH and high cation
exchange capacity of bentonite, a bentonite seal should be placed
approximately 2 to 5 feet above the top of the well intake and separated
from the filter pack by a 1-foot thick layer of fine silica sand.
Neat Cement--
As with a bentonite slurry, a neat cement grout must be properly mixed,
pumped and emplaced to ensure that the annular seal will be effective.
According to the United States Environmental Protection Agency (1975), neat
cement should only be emplaced in the annulus by free fall when: 1) there is
adequate clearance (i.e., at least 3 inches) between the casing and the
borehole, 2) the annulus is dry and 3) the bottom of the annular space to be
filled is clearly visible from the surface and not more than 30 feet deep.
However, to minimize segregation of cement even in unsaturated annular
spaces, free fall of more than 15 feet should not be attempted in monitoring
wells. If a neat cement slurry is allowed to free fall through standing
water in the annulus, the mixture tends to be diluted or bridge after it
reaches the level of standing water and before it reaches the intended depth
of emplacement. The slurry also may incorporate material that is sloughed
from the borehole wall into the seal. If the sloughed material has a high
permeability, the resultant seal can be breached through the inclusion of
the sloughed material.
In most situations, neat cement grout should be emplaced by a tremie
pipe. The annular space must be large enough that a tremie pipe with a
minimum inside diameter of 1 1/2 inches can be inserted into the annulus to
within a few inches of the bottom of the space to be sealed. Grout may then
either be pumped through the tremie pipe or emplaced by gravity flow through
the tremie pipe into the annular space. The use of a tremie pipe permits
the grout to displace ground water and force loose formation materials ahead
of the grout. This positive displacement minimizes the potential for
contamination and/or dilution of the slurry and bridging of the mixture with
upper formation material.
In pressure grouting, the cement discharges at the bottom of the
annular space and flows upward around the inner casing until the annular
space is completely filled. A side discharge tremie may be used to lessen
the possibility that grout might be forced into the filter pack. Depending
on pressure requirements, the tremie pipe may be moved upward as the slurry
is emplaced or it may be left at the bottom of the annulus until the
grouting is completed. If the tremie pipe is not retracted while grouting,
the tremie pipe should be removed immediately afterward to avoid the
possibility of the grout setting around the pipe. If this occurs, the ?ipe
may be difficult to remove and/or a channel may develop in the grout a& .he
pipe is removed.
217
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In gravity emplacement, the tremie is lowered to the bottom of the
annular space and filled with cement. The tremie pipe is slowly retracted,
and the weight of the column forces the cement into the annular space. In
both gravity emplacement and pressure grouting, the discharge end of the
tremie pipe should remain submerged at least one foot below the surface of
the grout at all times during emplacement, and the pipe should be kept full
of grout without air space. To avoid the formation of cold joints, the
grout should be emplaced in one continuous pour before initial setting of
the cement or before the mixture loses fluidity. Curing time required for a
typical Type I Portland cement to reach maximum strength is a minimum of 40
hours.
Moehrl (1964) recommends checking the buoyancy force on the casing
during cementing with grout. Archimede's principle states that a body
wholly or partially immersed in a fluid is buoyed up by a force equal to the
weight of the fluid displaced by the body. Failure to recognize this fact
may result in unnoticed upward displacement of the casing during cementing.
This is particularly true of lighter thermoplastic well casings. Formulas
for computing buoyancy are provided by Moehrl (1964).
TYPES OF WELL COMPLETIONS
The ultimate configuration of a monitoring well is chosen to fulfill
specific objectives as stated at the beginning of this section. Monitoring
wells can be completed either as single wells screened in either short or
long intervals, single wells screened in multiple zones or multiple wells
completed at different intervals in one borehole. The decision as to which
type of monitoring well configuration to install in a specific location is
based on cost coupled with technical considerations and practicality of
installation.
In shallow installations it generally is more economical to complete
the monitoring wells as individual units that are in close proximity to each
other and avoid the complexity of multiple-zone completions in a single
borehole. In deeper installations where the cost of drilling is high
relative to the cost of the materials in the well and where cost savings can
be realized in improved sampling procedures, it may be better to install a
more sophisticated multilevel sampling device. The cost of these
completions is highly variable depending on the specific requirements of
the job. Cost comparisons should be made on a site-by-site basis.
Individual well completions will almost always be more economical at depths
of less than 80 feet. A discussion of the types of monitoring well
completions is presented below.
Single-Riser/Limited-Interval Veils
The majority of monitoring wells that are installed at the present time
are individual monitoring wells screened in a specific zone. Well intakes
are usually moderate in length, ranging from 3 to 10 feet. These wells are
individually installed in a single borehole with a vertical riser extending
from the well intake to the surface. Because the screened interval is
short, these are the easiest wells to install and develop. A typical
example of this design is shown in Figure 21.
218
-------
The intent of a well with this design is to isolate a specific zone
from which water-quality samples and/or water levels are to be obtained. If
the well intake crosses more than one zone of permeability, the water sample
that is collected will represent the quality of the more permeable zone. If
a pump is installed just above the well intake and the well is discharged at
a high rate, the majority of the sample that is obtained will come from the
upper portion of the well intake. If the pump is lowered to the mid-section
of the the well intake and pumped at a low rate, the bulk of the sample will
come from the area that is immediately adjacent to the zone of the pump
intake. At high pumping rates in both isotropic and stratified formations,
flow lines converge toward the pump so that the sample that is obtained is
most representative of the ground water moving along the shortest flow
lines. If the well is not properly sealed above the well intake, leakage
may occur from upper zones into the well intake.
Single-Riser/Flow-through Wells
Flow-through wells consist of a long well intake that either fully or
nearly fully penetrates the aquifer. The well intake is connected to an
individual riser that extends to the surface. Wells of this type are
typically small in diameter and are designed to permit water in the aquifer
to flow through the well in such a manner as to make the well "transparent"
in the ground-water flow field. An illustration of this type of well is
shown in Figure 69.
Water table
Ground
water
flow
direction
Well
intake
Unconsolidated
aquifer
Bottom cap
Bottom
of aquifer
Figure 69. Diagram of a single-riser/flow through well.
219
-------
This type of well produces water samples that are a composite of the
water quality intercepted when the well is surged, bailed or pumped heavily.
For example, if three or more well volumes are evacuated prior to sampling,
the sample obtained will be a composite sample representative of the more
permeable zones penetrated by the well intake; it will not be possible to
define the zone(s) of contribution. However, if the well is allowed to
maintain a flow-through equilibrium condition and if a sampler is lowered
carefully to the selected sampling depth, a minimally disturbed water sample
can be obtained by either taking a grab sample or by pumping at a very low
rate. This sample will be substantially representative of the zone in the
immediate vicinity of where the sample was taken. If the sampler is
successively lowered to greater depths and the water within the well intake
is not agitated, a series of discrete samples can be obtained that will
provide a reasonably accurate profile of the quality of the water that is
available in different vertical zones. Furthermore, if the flow-through
condition is allowed to stabilize after any prior disturbance and a downhole
chemical profiling instrument is lowered into the well, closely-spaced
measurements of parameters such as Eh, pH, dissolved oxygen, conductivity
and temperature can be made in the borehole. This provides a geochemical
profile of conditions in the aquifer. In specific settings, wells of this
design can provide water-quality information that is at least as reliable as
either the information obtained by multiple-zone samplers in a single well
or by information from multiple nested wells. In either application, the
described flow-through well design is lower in cost.
Nested Wells
Nested wells consist of either a series of: 1) single-riser/limited-
interval wells that are closely spaced so as to provide data from different
vertical zones in close proximity to each other or 2) multiple single-
riser/limited-interval wells that are constructed In a single borehole.
Illustrations of these designs are shown in Figures 70a and 70b. Veils of
these designs are used to provide samples froa different zones of an
aquifer(s) in the same manner as individual wells.
Multiple wells are constructed in a single borehole by drilling a
10-inch or larger diameter borehole, then setting one, two, or three 2-inch
single-riser/limited-interval wells within the single borehole. The deepest
well intake is installed first, the filter pack eaplaced and the seal added
above the filter pack. The filter pack provide* stabilization of the
deepest zone. After the seal is installed above the deepest zone, the next
succeeding (upward) well intake is installed and the individual riser
extended to the surface. This next well intake Is filter-packed and a
second seal is placed above the filter pack that is emplaced around the
second well intake. If there is a long vertical interval between successive
well intakes, neat cement grout is emplaced above the lower seal. Where
vertical separation permits, a 1-foot layer of fine silica sand should be
emplaced between the filter packs and sealants. This sand helps prevent
sealant infiltration into the filter pack and loss of filter pack into the
sealant. This procedure is repeated at all desired monitoring intervals.
Because each riser extends to the surface and is separate from the other
risers, a good seal must be attained around each riser as it penetrates
220
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Surface seal
Filler sand
= . \Groul seal
Filter pack
Screened interval
7
(a)
(b)
Figure 70. Typical nested well design*: a) series of single riser/limited interval wells in separate bore-
holes and b) multiple single riser/limited interval wells in a single borehole (After Johnson,
1983).
-------
through successive bentonite seals. A substantial problem with this type of
construction is leakage along the risers as well as along the borehole wall.
The primary difficulty with multiple completions in a single borehole
is that it is difficult to be certain that the seal placed between the
screened zones does not provide a conduit that results in interconnection
between previously non-connected zones within the borehole. Of particular
concern is leakage along the borehole wall and along risers where overlying
seals are penetrated. It is often difficult to get an effective seal
between the seal (e.g., bentonite or cement grout) and the material of the
risers.
Multiple-Level Monitoring Wells
In addition to well nests that sample at multiple levels in a single
location, a variety of single-hole, multilevel sampling devices are
available. These sampling devices range from the simple field-fabricated,
PVC multilevel sampler shown in Figure 71 to the buried capsule devices that
are installed in a single borehole as shown in Figure 72. The completion
— End cap
Ground ! -
Water table
Male & female
' couplings
Surface
— PVC pipe
Ih
H — Coupling
Sampling points
PVC pipe
—Screen
il
(a)
End cap
Figure 71. Field-fabricated PVC multilevel sampler, a) field installation and b) cross section of sampling
point (Pickens et al., 1981).
222
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Protective
casing
Sampling
tube
Screened
interval
Figure 72. Multilevel capsule sampling device installation (Johnson, 1983).
223
-------
of these wells is similar to the completion of nested wells in a single
borehole. Some of these samplers have individual tubing connections that
extend to the surface. Samples are collected from the tubing. With some
forms of instrumentation, water levels can also be obtained. There are
additionally, more sophisticated sampling devices available, such as shown
in Figure 73. These consist of multiple-zone inflatable packers that can be
installed in a relatively small borehole. They permit the sampling of
formation fluids at many intervals from within a single borehole.
Disadvantages of these devices are: 1) it is difficult, if not impossible,
to repair the device if clogging occurs, 2) it is difficult to prevent
and/or evaluate sealant and packer leakage and 3) these installations are
more expensive than single level monitoring wells.
Simple vacuum-lift multiple port devices can be used in shallow wells
where samples can be obtained from the individual tubing that extends to the
surface. With increasing depth, greater sophistication is required and a
variety of gas-lift sampling devices are available commercially. Still more
sophisticated sampling devices are available for very deep installations.
These devices require durable, inflatable packer systems and downhole tools
to open and close individual ports to obtain formation pressure readings and
take fluid samples. These can be used in wells that are several thousand
feet deep.
GENERAL SUGGESTIONS FOR WELL COMPLETIONS
1) Use formation samples, sample penetration logs, drilling logs,
geophysical logs, video logs and all other pertinent information
that can be obtained relating to the well installation to make
decisions on well completion. Make*every attempt to define the
stratigraphy before attempting to install well intakes.
2) Be aware of the control that stratigraphy exerts over flow-line
configuration when the sampling pump is and is not operating. In
an isotropic aquifer, the sample is representative of the quality
of formation water in the immediate vicinity of the pump. In a
fractured system or a stratified aquifer, flow can be highly
directional and confined.
3) Install the well intake in the exact zone opposite the desired
monitoring depth. If the well is designed to intercept "floaters,"
the well intake must extend high enough to provide for fluctuations
in the seasonal water table. If the well is designed to monitor
"sinkers," the topography of the bottom-most confining
layer must be sufficiently defined such that a well intake can be
installed at the topographical points where the sinkers can be
intercepted. If there is a non-aqueous phase present, the well
intake must intersect the appropriate pathways. Vertical
variations in hydraulic conductivity must be recognized as well as
horizontal variations. In consolidated rock, fracture zones
through which migration can occur must be intercepted. At all
times, the three-dimensional aspect of contaminant migration
must be taken into consideration.
4) Aquifer disruption must be minimized during the completion process.
Void space should not be unnecessarily created when pulling back
casing or augers. Non-cohesive material collapse around the well
intake should be minimized except where natural filter pack is used.
224
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Backfill
Packer
COuQknQ
Mo«iuf»m«nt port coupling
EndCJp
Figure 73. Multiple zone inflatable packer sampling Installation (Rehtiane and Patton, 1982).
225
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5) The depth and diameter limitations imposed by the type of equipment
and materials used in monitoring well construction must be
considered as an integral part of well completion. The filter pack
must be uniformly emplaced; bentonite and cement grout must be
emplaced by positive methods so that the zones that are supposed to
be isolated are truly isolated by positive seals. The design and
installation of a monitoring well are impacted by the constraints
of cost, but the errors resulting from a well that is improperly
constructed are much more expensive than a well that is properly
constructed. The extra time and cost of constructing a well
properly, and being as sure as possible that the information being
obtained is reliable, is well worth the extra cost of careful
installation.
226
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REFERENCES
California Department of Health Services, 1986. The California site
mitigation decision tree manual; California Department of Health Services,
Sacramento, California, 375 pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson Division,
St. Paul, Minnesota, 1089 pp.
Johnson, Thomas L., 1983. A comparison of well nests versus single-well
completions; Ground Water Monitoring Review, vol. 3, no. 1, pp. 76-78.
Moehrl, Kenneth E., 1964. Well grouting and well protection; Journal of the
American Water Works Association, vol. 56, no. 4, pp. 423-431.
Pickens, J.F., J.A. Cherry, R.M. Coupland, G.E. Grisak, W.F. Merritt and
B.A. Risto, 1981. A multilevel device for ground-water sampling; Ground
Water Monitoring Review, vol. 1, no. 1, pp. 48-51.
Rehtlane, Erik A. and Franklin D. Patton, 1982. Multiple port piezometers
vs. standpipe piezometers: an economic comparison; Proceedings of the Second
National Symposium on Aquifer Restoration and Ground-Water Monitoring,
National Water Well Association, Worthington, Ohio, pp. 287-295.
Riggs, Charles 0. and Allen W. Hatheway, 1988. Groundwater monitoring field
practice - an overview; Ground-Water Contamination Field Methods, Collins
and Johnson editors, ASTM Publication Code Number 04-963000-38,
Philadelphia, Pennsylvania, pp. 121-136.
United States Environmental Protection Agency, 1975. Manual of water well
construction practices; United States Environmental Protection Agency,
Office of Water Supply, EPA-570/9-75-001, 156 pp.
227
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SECTION 7
MONITORING WELL DEVELOPMENT
INTRODUCTION/PHILOSOPHY
The objective of monitoring well development is frequently
misconstrued to be merely a process that enhances the flow of ground water
from the formation into the well and that minimizes the amount of sediment
in the water samples collected from the well. These are the proper
objectives for the development of a production well but they do not fulfill
the requirements for a monitoring well. A monitoring well should
be a "transparent" window into the aquifer from which samples can be
collected that are truly representative of the quality of water that is
moving through the formation. This objective is difficult to attain and is
unattainable in some instances. However, the objective should not be
abandoned because of the difficulty.
The interpretation of any ground-water sample collected from a
monitoring well should reflect the degree of success that has been reached
in the development of the well and the collection of the sample. This
objective is frequently overlooked in the literature and in much of the
work that has been done in the field. Further research is required before
the reliability of samples taken from a monitoring well can be effectively
substantiated. The United States Environmental Protection Agency (1986) in
the Technical Enforcement Guidance Document states that, "a recommended
acceptance/rejection value of five nephelometric turbidity units (NTU) is
based on the need to minimize biochemical activity and possible
interference with ground-water sample quality." The TEGD also outlines a
procedure for determining the source of turbidity and usability of the
sample and well. There are instances where minimizing turbidity and/or
•biochemical activity will result in a sample that is not representative of
water that is moving through the ground. If the ground water moving
through the formation is, in fact, turbid, or if there is free product
moving through the formation, then some criteria may cause a well to be
constructed such that the actual contaminant that the well was installed to
monitor will be filtered out of the water. Therefore, it is imperative
that the design, construction and development of a monitoring well be
consistent with the objective of obtaining a sample that is representative
of conditions in the ground. An evaluation of the degree of success in
attaining this objective should always be included and considered in
conjunction with the laboratory and analytical work that is the final
result of the ground-water sample-collection process.
228
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If the ultimate objective of a monitoring well is to provide a
representative sample of water as it exists in the formation, then the
immediate objective and challenge of the development program is to restore
the area adjacent to the well to its indigenous condition by correcting
damage done to the formation during the drilling proccess. This damage may
occur in many forms: 1) if a vibratory method such as driving casing is
used during the drilling process, damage may be caused by compaction of the
sediment in place; 2) if a compacted sand and gravel is drilled by a
hollow-stem auger and then allowed to collapse around the monitoring well
intake, damage may be the resultant loss of density of the natural
formation; 3) if a drilling fluid of any type is added during the drilling
process, damage may occur by the infiltration of filtrate into the
formation; and 4) if mud rotary, casing driving or augering techniques are
used during drilling, damage may be caused by the formation of a mudcake or
similar deposit that is caused by the drilling proccess. Other formation
damage may be related to specific installations. Some of this damage
cannot be overcome satisfactorily by the current capability to design and
develop a monitoring well. One important factor is the loss of
stratification in the monitored zone. Most natural formations are
stratified; the most common stratigraphic orientation is horizontal. The
rate of water movement through different stratigraphic horizons varies;
sorption rates may differ as stratigraphy changes; and chemical interaction
between contaminants and the formation materials and ground water can vary
between different horizons. During the development process, those zones
with the highest permeability will be most affected by the development of
the well. Where a well intake crosses stratigraphic boundaries of varying
permeability, the water that moves into and out of the well intake will be
moving almost exclusively into and out of the high permeability zones.
FACTORS AFFECTING MONITORING WELL DEVELOPMENT
There are three primary factors that influence the development of a
monitoring well: 1) the type of geologic material, 2) the design and
completion of the well and 3) the type of drilling technology employed in
the well construction. From these factors it is also possible to estimate
the level of effort required during development so that the monitoring well
will perform satisfactorily.
Type of Geologic Material
The primary geologic consideration is whether or not the monitoring
well intake will be installed in consolidated rock or unconsolidated
material. If the intake is installed in consolidated rock or cohesive
unconsolidated material, the assumption can often be made that the borehole
is stable and was stable during the construction of the monitoring well.
In a stable borehole it is generally easier to: 1) install the well
intake(s) at the prescribed setting(s), 2) uniformly distribute and
maintain the proper height of a filter pack (if one was installed) above
the well intake(s), 3) place the bentonite seal(s) in the intended location
and 4) emplace a secure surface seal. However, if the well intake is
opposite unconsolidated material, the borehole may not be or may not have
been stable during well installation. Depending on the degree of borehole
instability during the well completion process, the well intake, filter
229
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pack, bentonite seal and/or surface seal may not have been installed as
designed. As a consequence, there is generally a greater degree of
difficulty expected in the development of wells that are installed in
unconsolidated formations.
The permeability of the formation also influences the ease of
development. Where permeability is greater, water moves more easily into
and out of the formation and development is accomplished more quickly. In
unconsolidated formations the ease or difficulty of development is less
predictable because there is considerable variation in the grain size,
sorting and stratification of many deposits. Zones that are developed and
water samples that are collected will be more representative of the
permeable portions of a stratified aquifer and may not be very
representative of the less permeable zones.
Design and Completion of the Well
A monitoring well can be installed relatively easily at a site where
the total depth of the well will be 25 feet; the static water level is
approximately 15 feet; and the monitored interval is a cleant we11-sorted
sand and gravel with a permeability that approximates 1 x 10 centimeters
per second. However, a monitoring well is much more difficult to install
at a site where the depth of the well will ba 80 feet; the well will be
completed in an aquifer beneath an aquitard; the water table in the shallow
aquifer is approximately 20 feet deep; the piazoaetric surface of the
semi-confined aquifer is approximately 10 feet deep; and the monitored
interval in the deeper zone is composed of fine-grained sand with silt.
Construction of the monitoring well in this scenario will be difficult by
any technique. No matter what construction method is used, a considerable
amount of time will be required for well completion and problems can be
anticipated during setting of the well intake, placement of the filter
pack, placement of the bentonite seal or placement of the grout.
Difficulties may also be experienced during the development process.
Another difficult monitoring well installation is where the well
intake is placed opposite extremely fine-grained materials. For example,
extremely fine-grained materials often occur as a series of interbedded
fine sands and clays such as might be deposited in a sequence of lake
deposits. A well intake set in the middle of these saturated deposits must
be completed with an artificial filter pack. However, because the deposits
are unstable, it is difficult to achieve a good distribution of the
filter-pack material around the well intake during installation.
Furthermore, even if the filter pack installation is successful, it is not
possible to design a sufficiently fine-grained filter pack that will
prevent the intrusion of the clays that are intimately associated with the
productive fine-grained sand. As a consequence, every time the well is
agitated during the sampling process, the clays are mobilized and become
part or all of the turbidity that compromises the value of the ground-water
samples. There currently is no design or development procedures that are
able to fully overcome this problem. The only way to minimize the
intrusion of the clays is to install an extremely fine-grained porous
filter. This filter has very limited utility because it rapidly becomes
230
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clogged by the clays that are being removed. After a short operational
period, insufficient quantities of samples are obtained and the filter can
no longer be used.
Where an artificial filter pack is installed, the filter pack must be
as thin as possible if the development procedures are to be effective in
removing fine particulate material from the interface between the filter
pack and the natural formation. Conversely, the filter pack must be thick
enough to ensure that during the process of construction it is possible to
attain good distribution of the filter pack material around the screen. It
is generally considered that the minimum thickness of filter pack material
that can be constructed effectively is 2 inches. Two inches is a desirable
thickness in situations where there is adequate control to ensure good
filter pack distribution. If there are doubts about the distribution, then
the filter pack must be thickened to assure that there is adequate
filtration and borehole support.
In natural filter pack installations where the natural formation is
allowed to collapse around the well intake, the function of development is
twofold: 1) to remove the fine particulate materials that have been
emplaced adjacent to the well intake and 2) to restore the natural flow
regime in the aquifer so that water may enter the well unimpeded.
It is easier to develop monitoring wells that are larger in diameter
than it is to develop small-diameter wells. For example, mechanical
surging or bailing techniques that are effective in large-diameter wells
are much less effective when used in wells that are less than 2 inches in
diameter because equipment to develop smaller-diameter wells has limited
availability. Further, in small-diameter wells when the depths become
excessive, it is difficult to maintain straightness and alignment of the
borehole because of the drilling techniques that are commonly used. It may
become imperative in this situation to use centralizers on the casing and
well intake that are being installed within these boreholes or to use other
methods to center the casing or ensure straight holes.
Type of Drilling Technology
The drilling process influences not only development procedures but
also the intensity with which these procedures must be applied. Typical
problems associated with special drilling technologies that must be
anticipated and overcome are as follows: 1) when drilling an air rotary
borehole in rock formations, fine particulate matter typically builds up on
the borehole walls and plugs fissures, pore spaces, bedding planes and
other permeable zones. This particulate matter must be removed and
openings restored by the development process; 2) if casing has been driven
or if augers have been used, the interface between the natural formation
and the casing or the auger flights are "smeared" with fine particulate
matter that must subsequently be removed in the development process; 3) if
a mud rotary technique is used, a mudcake builds up on the borehole wall
that must be removed during the development process; and 4) if there have
been any additives, as may be necessary in mud rotary, cable tool or
augering procedures, then the development process must attempt to remove
231
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all of the fluids that have infiltrated into the natural formation.
WELL DEVELOPMENT
Very little research has been performed that specifically addresses
movement of fluid, with or without contaminants present, through a
stratified aquifer into monitoring wells. Ground-water flow theory is
based on the primary assumptions of homogeneity and isotropism of the
formation. In production wells, these assumptions are acceptable because
the aquifer is stressed over a sufficient area for variations to be
"averaged." Most discussions of monitoring well flow characteristics are
based on the acceptance of these assumptions. However, these are not
always valid assumptions for attaining the objectives of monitoring wells.
Where it is intended to intercept a contaminant in a restricted zone
of a three-dimensional flow field, a monitoring well must be installed and
developed with a much greater precision than is normal for production
wells. The relative movement of fluid in specific zones becomes
significantly more important than the gross yield. Both installation and
development must be performed with a "spot precision" that preserves in-
situ conditions and permits the collection of a representative sample.
The methods that are available for the development of monitoring wells
have been inherited from production well development practices. These
methods include: 1) surging with a surge block, 2) bailing, 3) pumping,
overpumping and backwashing through the pump, 4) airlift pumping and 5)
air surging and jetting. A number of authors have written about these
available methods of development for monitoring wells. A summary of these
articles is contained in Table 36.
Based on a review of the literature and on a wide range of actual
field practices, a few generalizations about development of monitoring
wells can be made:
1) using air for well development can result in chemical alteration of
the ground water both as a result of chemical reaction with the air
and as a result of impurities introduced through the air stream;
2) adding water to the borehole for stabilization, surging,
backwashing, flushing or any other purpose has an unpredictable
effect on ground-water quality and at the very least causes
dilution. Even if the water added to the borehole was originally
pumped from the same formation, chemical alteration of the ground
water in the formation can occur if the water is reinjected. Once
water has been pumped to the surface, aeration can alter the
original water quality;
3) developing the formation at the interface between the outer
perimeter of an artificial filter pack and the inner perimeter of
the borehole is extremely difficult. Any mudcake or natural clay
deposited at this interface is very difficult to remove; incomplete
removal can have unquantifiable short- and long-range impacts on
the quality of the sampled ground water;
4) developing a well is relatively easy when the well intake is placed
in a clean homogenous aquifer of relatively high permeability. It
232
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TABLE 36. SUMMARY OF DEVELOPMENT METHODS FOR MONITORING WELLS
Reference
Gass (1986)
United Slates
Environmental
Overpumping
Works best n clean
coarse formations
and some consoli-
dated rock, prob-
lems ol water dis-
posal and bridging
Effective develop-
ment requires flow
Backwashmg
Breaks up bridging.
low cost A simple.
preferentially
develops
Indirectly indicates
method applicable.
Surge block* Bailer
Can be effective.
sue made for > 2"
well, preferential
development where
screen >S'. surge
inside screen
Applicable, forma- Applicable
lion water should
Jetting
Consolidated and
unconsolidated appli-
cation, opens frac-
tures, develops dis-
crete zones,
disadvantage is
external water
needed
Airlift Pumping
Replaces air surg-
ing, filter air
Air should nol be
used
Air Surging
Perhaps most
widely used, can
entrain air in
formation so as to
reduce permeability.
ailed water quality,
avoid il possible
Air should nol be
used
Protection reversal or surges to formation water
Agency (1986) avoid bridges should be used
Barcelona el
al" (1983)
Productive wells.
surging by alternat-
ing pumping and
allowing to equili-
brate, hard to create
sufficient entrance
velocities, often use
with airlift
Sciill ol al
(1981)
Suitable, periodic
removal ol fines
be used, in low-
yield formation.
outside water
source can be
used if analyzed
to evaluate impact
Productive wells.
use care to avoid
casing and screen
damage
Suitable, common
with cable-tool, nol
easily used on other
rigs
Productive wells.
more common than
surge blocks but
nol as effective
Suitable, use suffi-
ciently heavy bailer,
advantage of remov-
ing fines, may be
custom made for
small diameters
Suitable
Effectiveness de-
pends on geometry
ot device, air must be
filtered, crew may be
exposed lo contami-
nated waler. per-
turbed Eli in sand
and gravel nol per-
sistent lor moie than
a few weeks
Suitable, avoid
injecting air into
intake, chemical
interference, air pipe
never inside screen
National
Council of the
Paper Industry
lor Air and
Stream Im-
provement
(1981)
Applicable.
drawback of flow in
one direction.
smaller wells hard
lo pump if water
level below suction
against collapse ot
intake or plugging
screen with clay
Methods introducing foreign materials should be avoided
(i e. compressed air or waler jets)
-------
TABLE 36. SUMMARY OF DEVELOPMENT METHODS FOR MONITORING WELLS (CONTINUED)
Reference
Everetl (1960)
Keely and
Boateng
(1987 a and b(
Owarpumping Backwashmg
Oevetopment opera-
tion must cause
(tow reversal to
avoid bndgmg. can
alternate pimp oil
and on
Probably most Vigorous surging
desirable when action may not be
surged, second desirable due
-------
is very difficult to develop a representative well in an aquifer
that is stratified, slowly permeable and fine-grained, particularly
where there is substantial variation between the various stratified
zones;
5) developing a larger-diameter monitoring well is easier than
developing a smaller-diameter well. This is particularly true if
the development is accomplished by overpumping or backwashing
through the pump because suitable pumping capacity is not commonly
available for small-diameter wells with deep static water levels.
However, a smaller-diameter well is more "transparent" in the
aquifer flowfield and is therefore more likely to yield a
representative sample;
6) collecting a non-turbid sample nay not be possible because there
are monitoring wells that cannot be sufficiently developed by any
available technique. This may be the consequence of the existence
of turbid water in the formation or the inability to design and
construct a well that will yield water in satisfactory quantity
without exceeding acceptable flow velocities in the natural
formation;
7) applying many of the monitoring well development techniques in
small-diameter (2-inch) wells and using the design and
construction techniques discussed in the literature
are easiest in shallow monitoring situations with good
hydraulic conductivity. These techniques may be impractical
when applied to deeper or more difficult monitoring situations.
8) Adding clean water of known quality for flushing and/or jetting
should be done only when no better options are available. A record
must be kept of the quantities of water lost to the formation
during the flushing/jetting operation and every attempt must be
made to re-establish background levels in a manner similar to that
described in Barcelona et al. (1985a) and/or the United States
Environmental Protection Agency (1986) and
9) dealing objectively with the conditions and problems that exist for
every installation is essential. The problems encountered at each
site should be addressed and clearly presented in the final report.
Chemical analyses must be included in the final report so that
anyone evaluating these analyses is ible to understand the
limitations of the work.
METHODS OF WELL DEVELOPMENT
Monitoring well development is an attempt to remove fine particulate
matter, commonly clay and silt, from the geologic formation near the well
intake. If particulate matter is not removed, as water moves through the
formation into the well, the water sampled will be turbid, and the
viability of the water quality analyses will b« impaired. When pumping
during well development, the movement of water is unidirectional toward the
well. Therefore, there is a tendency for the particles moving toward the
well to "bridge" together or form blockages that restrict subsequent
particulate movement. These blockages may prevent the complete development
of the well capacity. This effect potentially impacts the quality of the
water discharged. Development techniques should remove such bridges and
encourage the movement of partlculates into the well. These particulates
235
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can then be removed from the well by bailer or pump and, in most cases, the
water produced will subsequently be clear and non-turbid.
One of the major considerations in monitoring well development is the
expense. In hard-to-develop formations it is not unusual for the
development process to take several days before an acceptable water quality
can be attained. Because development procedures usually involve a drilling
rig, crew, support staff and a supervising geologist, the total cost of the
crew in the field often ranges in cost from $100 to $200 per hour. Thus,
the cost of development can be the most expensive portion of the
installation of a monitoring well network. When this hourly cost is
compared to an often imperceptible rate of progress, there is a tendency to
prematurely say either, "that's good enough" or "it can't be done."
In most instances, monitoring wells installed in consolidated
formations can be developed without great difficulty. Monitoring wells
also can usually be developed rapidly and without great difficulty in sand
and gravel deposits. However, many installations are made in thin, silty
and/or clayey zones. It is not uncommon for these zones to be difficult to
develop sufficiently for adequate samples to be collected.
Where the borehole is sufficiently stable, due to installation in
sound rock or stable unconsolidated materials, and where the addition of
fluids during completion and development is permissible, it is a good
practice to pre-condition the borehole by flushing with clean water prior
to filter pack installation. When water is added to the well, the quality
of the water must be analyzed so that comparisons can be made with
subsequent water-quality data. Flushing of monitoring wells is appropriate
for wells drilled by any method and aids in the removal of mud cake (mud
rotary) and other finely-ground debris (air rotary, cable tool, auger) from
the borehole wall. This process opens clogged fractures and cleans thin
stratigraphic zones that might otherwise be non-productive. Flushing can
be accomplished by isolating individual open zones in the borehole or by
exposing the entire zone. If the entire zone is exposed, cross connection
of all zones can occur.
Where it is not permissible to add fluids during completion and
development, and the borehole is stable, mechanically scraping or
scratching the borehole wall with a scraper or wire brush, can assist in
removing particulates from the borehole wall. Dislodged partlculates can
be pumped or bailed from the borehole prior to filter pack, casing and well
intake installation.
Where the addition of fluid is permissible, the use of high pressure
jetting can be considered for screened intake development in special
applications. If jetting is used, the process should usually be performed
in such a manner that loosened particulates are removed (e.g. bailing,
pumping, flushing) either simultaneously or alternately with the jetting.
The disadvantages of using jetting even in "ideal conditions" are four-
fold: 1) the water used in jetting is agitated, pumped, pressurized and
discharged into the formation; 2) the fine (e.g., 10 slot, 20 slot) slotted
screens of most monitoring well intakes do not permit effective jetting and
development of the material outside the screen may be negligible or
236
-------
possibly detrimental; 3) there is minimal development of the interface
between the filter pack and the wall of the borehole (Table 36) and 4)
water that is injected forcibly replaces natural formation fluids. These
are serious limitations on the usefulness of jetting as a development
procedure.
Air development forcibly introduces air into contact with formation
fluids, initiating the potential for uncontrolled chemical reactions. When
air is introduced into permeable formations, there is a serious potential
for air entrainment within the formation. Air entrainment not only
presents potential quality problems, but also can interfere with flow into
the monitoring well. These factors limit the use of air surging for
development of monitoring wells.
After due consideration of the available procedures for well
development, it becomes evident that the three most suitable methods for
monitoring well development are: 1) bailing, 2) surge block surging and 3)
pumping/overpumping/backwashing and 4) combinations of these three methods.
Bailing
In relatively clean, permeable formations where water flows freely
into the borehole, bailing is an effective development technique. The
bailer is allowed to fall freely through the borehole until it strikes the
surface of the water. The contact of the bailer produces a strong outward
surge of water that is forced from the borehole through the well intake and
into the formation. This tends to break up bridging that has developed
within the formation. As the bailer fills and is rapidly withdrawn, the
drawdown created in the borehole causes the particulate matter outside the
well intake to flow through the well intake and into the well. Subsequent
bailing removes the particulate matter from the well. To enhance the
removal of sand and other particulate matter from the well, the bailer can
be agitated by rapid short strokes near the bottom of the well. This
agitation makes it possible to bail the particulates from the well by
suspending or slurrying the particulate matter. Bailing should be
continued until the water is free from suspended particulate matter. If
the well is rapidly and repeatedly bailed and the formation is not
sufficiently conductive, the borehole will be dewatered. When this occurs,
the borehole must be allowed to re-fill before bailing is resumed. Care
must be taken that the rapid removal of the bailer does not cause the
external pressure on the well casing to exceed the strength of the casing
and/or well intake thereby causing collapse of the casing and/or well
intake.
Bailing can be conducted by hand on shallow wells, although it is
difficult to continue actively bailing for more than about an hour. Most
drill rigs are equipped with an extra line that can be used for the bailing
operation. The most effective operation is where the bail line permits a
free fall in the downward mode and a relatively quick retrieval in the
upward mode. This combination maximizes the surging action of the bailer.
The hydraulic-powered lines on many rigs used in monitoring well
installation operate too slowly for effective surging. Bailing is an
237
-------
effective development tool because it provides the same effects as both
pumping and surging with a surge block. The most effective equipment for
bailing operations is generally available on cable tool rigs.
There are a variety of dart valve, flat bottom and sand pump bailers
available for the development of larger-diameter wells. These bailers are
typically fabricated from steel and are operated by using a
specially-designated line on the rig. For most monitoring well
applications, small-diameter PVC or fluoropolymer bailers are readily
available. When commercial bailers are not available, bailers can be
fabricated from readily-available materials. Bailers of appropriate
diameter, length, material and weight should be used to avoid potential
breakage of the well casing or screen. Figures 74a and 74b show a
schematic representation of typical commercially available small-diameter
bailers.
Surge Block
Surge blocks, such as are shown in Figures 75 and 76, can be used
effectively to destroy bridging and to create the agitation that is
necessary to develop a well. A surge block is used alternately with either
a bailer or pump so that material that has been agitated and loosened by
the surging action is removed. The cycle of surging-pumping/bailing is
repeated until satisfactory development has been attained.
During the development process, the surge block can be operated either
as an integral part of the drill rods or on a wireline. In either event
the surge block assembly must be of sufficient weight to free-fall through
the water in the borehole and create a vigorous outward surge. The
equipment that lifts or extracts the surge block after the downward plunge
must be strong enough to pull the surge block upward relatively rapidly
The surge block by design permits some of the fluid to bypass on the
downward stroke, either around the perimeter of the surge block or through
bypass valves.
The surge block is lowered to the top of the well intake and then
operated in a pumping action with a typical stroke of approximately 3 feet
The surging is usually initiated at the top of the well intake and
gradually is worked downward through the screened interval. The surge
block is removed at regular intervals and the fine material that has been
loosened is removed by bailing and/or pumping. Surging begins at the top
of the well intake so that sand or silt loosened by the initial surging
action cannot cascade down on top of the surge block and prevent removal of
the surge block from the well. Surging is initially gentle, and the energy
of the action is gradually increased during the development process. The
vigor of the surging action is controlled by the speed, length and stroke
of the fall and speed of retraction of the surge block. By controlling
these rates, the surging activity can range from very rigorous to very
gentle.
238
-------
Standard
bailer
of Teflon*
Standard
bailer
ofPVC
Bottom
emptying
device
Top for
variable capacity
point source
bailer of PVC
, Retaining
pin
Sample
chamber
1 Foot
midMCtion
may be added
hem
Retaining
pin
Ball check
(a)
(b)
Rgura 74. Diagrams of typical bailers used in monitoring well development a) standard type and
b) "point source" bailer (Timco Manufacturing Company, Inc., 1982).
239
-------
Rubber
flap
Figure 75. Diagram of a typical surge Mock (Driscoll, 1986).
Surging within the well intake can result in serious difficulties.
Vigorous surging in a well that is designed such that excessive sand can be
produced, can result in sand-locking the surge block. This should not
occur in a properly designed monitoring wall, nor should it occur if the
surge block of appropriate diameter is properly used. As in the case of
bailer surging, if excessive force is used, it is possible to cause the
collapse of the well intake and/or the casing.
An alternative to surging within the well intake is to perform the
surging within the casing above the well intake. This has the advantage of
minimizing the risk of sand locking. However, It also reduces the
effectiveness of the surging action. In peraeable material the procedure
of surging above the well intake is effective only for well intakes with
lengths of 5 feet or less.
If the well is properly designed, and if the surge block is: 1)
initially operated with short, gentle strokes above the well intake, 2)
sand is removed periodically by alternating sand removal with surging, 3)
the energy of surging is gradually increased at each depth of surging until
no more sand is produced from surging at that depth and 4) the depth of
surging is incrementally increased from top to bottom of the well intake,
then surging can be conducted effectively and safely.
240
-------
Water ports (025" 00.)
,0, -
Air vent passage
(0375"OD)
Cross Section
A-A1
Polypropylene tube (0 375" O D)
Stainless steel cable (0 063" 0 0)
Ferrule
Stainless steel hex nut (0 63")
Viton discs (0 05" thick. 21" 0 0 &
068ID)
SCH 80 PVC pipe (1 90" O 0 )
top fitting
> NPT threading
Stainless steel coupling
(1.325" O.O.)
Stainless steel pipe
(1.067" O.D.I
«- SCH 80 PVC pipe (1.90" O.O.)
bottom fining
IJ j I fr«- Stainless steel hex nut (0.63")
> Air vent ports (0.125" O.D)
Stainless steel tube (0.375" O D)
»Swage block
Figure 76. Diagram of a specialized monitoring well surge Mock (Schalla and Landick, 1986).
241
-------
Where there is sufficient annular space available within the casing
which is seldom the case with monitoring wells, it is effective to install
a low capacity pump above the surge block. By discharging from the well
concurrent with surging, a gradient is maintained toward the well. This
set-up assists in developing the adjacent aquifer by maintaining the
movement of particulate material toward the well.
Surging is usually most effective when performed by cable tool type
machines. The hydraulic hoisting equipment that is normally available on
most other types of drilling equipment does not operate with sufficient
speed to provide high-energy surging. Where properly used, the surge block
in combination with bailing or pumping may be the most effective form of
mechanical development.
Pumping/Overpumping/Backwashing
The easiest, least expensive and most commonly employed technique of
monitoring well development is some form of pumping. By installing a pump
in the well and starting the pump, ground-water flow is induced toward the
well. Fine particulate material that moves into the well is discharged by
the pump. In overpumping, the pump is operated at a capacity that
substantially exceeds the ability of the formation to deliver water. This
flow velocity into the well usually exceeds the flow velocity that will
subsequently be induced during the sampling process. This increased
velocity causes rapid and effective migration of particulates toward the
pumping well and enhances the development process. Proper design is needed
to avoid well collapse, especially in deep wells. Both pumping and
overpumping are easily used in the development of a well.
Where there is no backflow prevention valve installed, the pump can be
alternately started and stopped. This starting and stopping allows the
column of water that is initially picked up by the pump to be alternately
dropped and raised up in a surging action. Each time the water column
falls back into the well, an outward surge of water flows into the
formation. This surge tends to loosen the bridging of the fine particles
so that the upward motion of the column of water can move the particles
into and out of the well. In this manner, the well can be pumped,
overpumped and back-flushed alternately until such time as satisfactory
development has been attained.
While the preceding procedures can effectively develop a well, and
have been used for many years in the development of production wells,
pumping equipment suitable to perform these operations may not be available
that will fit into some small-diameter monitoring wells. To be effective
as a development tool, pumps must have a pumping capability that ranges
from very low to very high or be capable of being controlled by valving.
The sampling pumps that are presently designed to fit into small-diameter
boreholes commonly do not provide the upper range of capacities that often
are needed for this type of development. For shallow wells with water
levels less than 25 feet deep, a suction-lift centrifugal pump can be used
for development in the manner prescribed. The maximum practical suction
lift attainable by this method is approximately 25 feet. In practice,
bailing or bailing and surging is combined with pumping for the most
242
-------
efficient well development. The bailing or surging procedures are used to
loosen bridges and move material toward the well. A low-capacity sampling
pump or bailer is then used to remove turbid water from the well until the
quality is satisfactory. This procedure is actually less than completely
satisfactory, but is the best available technology with the equipment that
is currently available.
Air lifting, without exposing the formations being developed directly
to air, can be accomplished by properly implemented pumping. To do this
the double pipe method of air lifting is preferred. The bottom of the air
lift should be lowered to within no more than ten feet of the top of the
well intake, and in no event should the air lift be used within the well
intake. If the air lift is used to surge the well, by alternating the air
on and off, there will be mixing of aerated water with the water in the
well. Therefore, if the well is to be pumped by air lifting, the action
should be one of continuous, regulated discharge. This can be effectively
accomplished only in relatively permeable aquifers.
Where monitoring well installations are to be made in formations that
have low hydraulic conductivity, none of the preceding well development
methods will be found to be completely satisfactory. Barcelona et al
(1985a) recommend a procedure that is applicable in this situation- "In
this type of geologic setting, clean water should be circulated down the
well casing, out through the well intake and gravel pack, and up the open
borehole prior to placement of the grout or seal in the annulus
Relatively high water velocities can be maintained, and the mudcake from
the borehole wall will be broken down effectively and removed. Flow rates
should be controlled to prevent floating the gravel pack out of the
borehole. Because of the relatively low hydraulic conductivity of geologic
materials outside the well, a negligible amount of water will penetrate the
formation being monitored. However, immediately following the procedure,
the well sealant should be installed and the well pumped to remove as much
of the water used in the development process as possible."
All of the techniques described in this section are designed to remove
the effects of drilling from the monitored zone, and insofar as possible
to restore the formations penetrated to indigenous conditions. To this
end, proposed development techniques, where possible, avoid the use of
introduced fluids, including air, into the monitored zone during the
development process. This not only minimizes adverse impacts on the
quality of water samples, but also restricts development options that would
otherwise be available.
243
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REFERENCES
Barcelona, M.J., J.P. Gibb, J.A. Helfrich and E.E. Garske, 1985a.
Practical guide for ground-water sampling; Illinois State Water Survey, SWS
Contract Report 374, Champaign, Illinois, 93 pp.
Barcelona, M.J., J.P. Gibb and R. Miller, 1983. A guide to the selection
of materials for monitoring well construction and ground-water sampling;
Illinois State Water Survey, SWS Contract Report 327, Champaign, Illinois,
78 pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson Division, St.
Paul, Minnesota, 1089 pp.
Everett, Lome G., 1980. Ground-water monitoring; General Electric Company
technology marketing operation, Schenectady, New York, 440 pp.
Gass, Tyler E., 1986. Monitoring well development; Water Well Journal,
vol. 40, no. 1, pp. 52-55.
Keely, Joseph F. and Kwasi Boateng, 1987a. Monitoring well installation,
purging and sampling techniques part 1: conceptualizations; Ground Water,
vol. 25, no. 3, pp. 300-313.
Keely, Joseph F. and Kwasi Boateng, 1987b. Monitoring well installation,
purging, and sampling techniques part 2: case histories; Ground Water,
vol. 25, no. 4, pp. 427-439.
National Council of the Paper Industry for Air and Stream Improvement,
1981. Ground-water quality monitoring well construction and placement;
Stream Improvement Technical Bulletin Number 342, New York, New York,
39 pp.
Scalf, M.R., J.F. McNabb, W.J. Dunlap, R.L. Cosby and J. Fryberger, 1981.
Manual of ground-water sampling procedures; National Water Well
Association, 93 pp.
Schalla, Ronald and Robert W. Landick, 1986. A new valved and air-vented
surge plunger for developing small-diameter monitor wells; Ground Water
Monitoring Review, vol. 6, no. 2, pp. 77-80.
Timco Manufacturing Company, Inc., 1982. Geotechnical Products; product
literature, Prairie Du Sac, Wisconsin, 24 pp.
244
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United States Environmental Protection Agency, 1986. RCRA ground-water
monitoring technical enforcement guidance document; Office of Waste
Programs Enforcement, Office of Solid Waste and Emergency Response,
OSWER-9950.1, United States Environmental Protection Agency, 317 pp.
245
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SECTION 8
MONITORING WELL NETWORK MANAGEMENT CONSIDERATIONS
WELL DOCUMENTATION
Records are an integral part of any monitoring system. Comprehensive
records should be kept that document data collection at a specific site.
These data include boring records, geophysical data, aquifer analysis data,
ground-water sampling results and abandonment documentation. Armed with as
much data as possible for the site, an effective management strategy for the
monitoring well network can be instituted.
Excellent records of monitoring wells must be kept for any management
strategy to be effective. Documentation of monitoring well construction and
testing must frequently be provided as part of a regulatory program. Many
states require drillers to file a well log to document well installation and
location. Currently, some states have adopted or are adopting regulations
with unique reporting requirements specifically for monitoring wells. At
the state and federal level, guidance documents have been developed that
address reporting requirements. Tables 37,-38 and 39 illustrate some of the
items that various states have implemented to address monitoring well
recordkeeping. Table 40 shows the recommendations of the United States
Environmental Protection Agency (1986). An additional discussion on field
documentation can be found in the section entitled "Recordkeeping."
The most critical factor in evaluating or reviewing data from a
monitoring well is location. If a monitoring well cannot be physically
located in the field and/or on a map in relationship to other wells, only
limited interpretation of the data is possible. All monitoring wells should
be properly located and referenced to a datum. The degree of accuracy for
vertical and horizontal control for monitoring well location should be
established and held constant for all monitoring wells. In many cases, a
licensed surveyor should be contracted to perform the survey of the wells.
With few exceptions, vertical elevations should be referenced to mean sea
level and be accurate to 0.01 foot (Brownlee, 1985). Because elevations are
surveyed during various stages of well/boring installation, careful records
must be kept as to where the elevation is established. For example, if
ground elevation is determined during the drilling process, no permanent
elevation point usually can be established because the ground is disturbed
during the drilling process. A temporary pin can be established close to
the well location for use in later more accurate measurements, but the
completed well must be resurveyed to maintain the desired accuracy of
elevation. Each completed well should have a standard surveyed reference
point. Because the top of the casing is not always level, frequently the
246
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TABLE 37. COMPREHENSIVE MONITORING WELL DOCUMENTATION (AFTER WISCONSIN
DEPARTMENT OF NATURAL RESOURCES, 1985)
Well Design
• Length, schedule and diameter of casing
• Joint type (threaded, flush or solvent welded)
• Length, schedule and diameter of screen
• Percentage of open area in screen
• Slot size of screen
• Distance the filter pack extends above the screen
• Elevations of the top of well easing, bottom and top of protective
casing, ground surface, bottom of borehole, bottom of well
screen, and top and bottom of seal(s)
• Well location by coordinates or grid systems (example township
and range)
• Well location on plan sheet showing the coordinate system, scale.
a north arrow and a key
Materials
• Casing and screen
• Filter pack (including gram size analysis)
• Seal and physical form
• Slurry or grout mix (percent cement, percent bentomte powder.
percent water)
Installation
• Drilling method
• Drilling fluid (it applicable)
• Source of water (if applicable) and analysis of water
• Time penod between the addition of backfill and construction of
well protection
U0V6I opfnont
• Date. time, elevation of water level prior to and after development
• Method used tor development
• Time spent developing a well
• Volume of water removed
• Volume of water added (if applicable), source of water added.
chemical analyses of water added
• Clanty of water before and after development
• Amount of sediment present at the bottom of the well
• pH. specific conductance and temperature readings
Soils Information
• Soil sample test results
• Driller's observation or photocopied dnilef slog
Miscellaneous
• Water levels and dates
• Well yield
• Any changes made in well construction, casing elevation, etc
TABLE 38. ADDITIONAL MONITORING WELL DOCUMENTATION (AFTER NEBRASKA
DEPARTMENT OF ENVIRONMENTAL CONTROL, 1984)
Well identification number
Formation samples (depth and method of collection)
Water samples (depth, method of collection, and results)
Filter pack (depth, thickness, gram size analysis, placement method, supplier)
Date of all work
Name, address of consultant dnlling company and strattgraphic log preparer(s)
Descnption and resutts of pump or stabilization test if performed
Methods used to decontaminate dnlling equipment and well construction material
247
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TABLE 39. AS-BUILT CONSTRUCTION DIAGRAM INFORMATION (AFTER CONNECTICUT
ENVIRONMENTAL PROTECTION AGENCY, 1983)
Top of ground surface
Protective grouting and grading at ground surface
Well casing length and depth
Screen length and depth
Location and extent of gravel pack
Location and extent ot bentonite seal
Water table
Earth materials stratigraphy throughout boring
For rock wells show details of bedrock seal
For rock wells, indicate depths of water-beanng fractures, faults or fissures and approximate yield
TABLE 40. FIELD BORING LOG INFORMATION (UNITED STATES ENVIRONMENTAL
PROTECTION AGENCY. 1986)
General
• Protect name
• Hole name/number*
• Date started and finished'
• Geologist's name*
• Driller's name'
• Sheet number
• Hole location, map and elevation*
• Rig type
bit size/auger size*
e PetrotogK litnologic classification scheme
used {Wentworth. unified soil classification system)'
Information Columns:
• Depth'
• Sample location/number*
e Blow counts and advance rate
e Percent sample recovery'
• Narrative flesenntton*
e Oeptn to saturation'
Narrative Description:
• Geologic observations
-soil/rock type*
-color and stain*
-gross petrology'
-Inability
-moisture content*
-degree of weathering*
-presence of carbonate*
-fractures'
-solution cavities'
-boddtng*
-discontinuities* — eg, foliation
-water-oesnng zones'
-tormational stnke and dip'
-fossils
-depositional structures*
organic content*
-odor
-suspected contaminant*
Drilling Observations.
-loss of circulation
-advance rates*
-ng chattef
-water levels'
-amount of air used, ar pressure
-drilling difficulties*
-changes in drilling method or equipment*
-readings from detective equipment it any'
-amount of water yield or loss during dnlling
at different depths*
-amounts and types ol any liquids used*
-running sands'
-caving/hole stability*
• Other Remarks:
-equipment failures
-possible contamination'
-deviations from dnlling plan*
-weather*
•indicates items thai the c
r should record, at a minimum.
248
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highest point on the casing is used. Brownlee (1985) suggests that the
standard reference point should be consistent such that the north (or other)
side of all monitoring wells is the referenced point. Regardless of what
point is chosen, the surveyor should be advised before the survey is
conducted and the reference point clearly marked at each well. If paint is
used to mark the casing, the paint must not be allowed on the inside of the
casing. If spray paint is used, the aerosols can coat the inside of the
casing and may cause spurious water-quality results in subsequent samples.
An alternative way to mark the casing is to notch the casing so that a
permanent reference point is designated. The United States Environmental
Protection Agency (1986) recommends that reference marks be placed on both
the casing and grout apron.
Well locations should clearly be marked in the field. Each well should
have a unique number that is clearly visible on the well or protective
casing. To ensure good documentation, the well number may be descriptive of
the method used to install the well. For example, a well designated as C-l
could represent the first cored hole or HS-3 could be a hollow-stem auger
hole. If multilevel sampling tubes are being used, each tube should be
clearly marked with the appropriate depth interval.
Well locations should be clearly marked on a map. The map should also
include roads, buildings, other wells, property boundaries and other
reference points. In general, maps illustrating comparable items should be
the same scale. In addition to the unique monitoring well number, general
well designations may be desirable to include on the map. The Wisconsin
Department of Natural Resources (1985) suggests that PIEZ (piezometer), OW
(observation well), PVT (private well), LYS (lysimeter) and OTHER be used to
clarify the function of the wells.
Files should be kept on each monitoring well so that any suspected
problems with the monitoring well can be evaluated based on previous well
performance. The accuracy and completeness of the records will influence
the ability of the reviewer to make decisions based on historical data.
WELL MAINTENANCE AND REHABILITATION
The purpose of maintaining a monitoring well is to extend the life of
the well and to provide representative levels and samples of the ground
water surrounding the well. Maintenance includes proper documentation of
factors that can be used as benchmarks for comparison of data at a later
point. A scheduled maintenance program should be developed before sample
quality is questioned. This section is designed to assist the user in
setting up a comprehensive maintenance schedule for a monitoring system.
Documenting Monitoring Well Performance
A monitoring well network should be periodically evaluated to determine
that the wells are functioning properly. Once complete construction and
"as-built" information is on file for each well, the well should be
periodically re-evaluated to check for potential problems. The following
checks can be used as a "first alert" for potential problems:
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1) the depth of the well should be recorded every time a water sample
is collected or a water-level reading taken. These depths should
be reviewed at least annually to document whether or not the well is
filling with sediment;
2) if turbid samples are collected from a well, redevelopment of the
existing well should be considered or a new well should be installed
if necessary (Barcelona et al., 1985a);
3) hydraulic conductivity tests should be performed every 5 years or
when significant sediment has accumulated;
4) slug or pump tests should be performed every 5 years. Redevelopment
is necessary if the tests show that the performance of the well is
deteriorating;
5) piezometric surface maps should be plotted and reviewed at least
annually; and
6) high and low water-level data for each well should be examined at
least every 2 years to assure that well locations (horizontally and
vertically) remain acceptable. If the water level falls below the
top of the well intake, the quality of the water samples collected
can be altered.
Where serious problems are indicated with a well(s), geophysical logs
may be helpful in diagnosing maintenance needs. Caliper logs provide
information on diameter that may be used to evaluate physical changes in the
borehole or casing. Gamma logs can be used to evaluate lithologic changes
and can be applied to ascertain whether or not well intakes are properly
placed. Spontaneous potential logs can locate zones of low permeability
where siltation may originate. Resistivity logs identify permeable and/or
porous zones to identify formation boundaries. Television and photographic
surveys can pinpoint casing problems and well intake failure and/or
blockage. When used in combination, geophysical logs may save time and
money in identifying problem areas. An additional discussion of the
applicability and limitations of geophysical logging tools can be found in
the section entitled "Borehole Geophysical Tools and Downhole Cameras."
Factors Contributing to Veil Maintenance Needs
The maintenance requirements of a well are influenced by the design of
the well and the characteristics of the monitored zones. Water quality,
transmissivity, permeability, storage capacity, boundary conditions,
stratification, sorting and fracturing all can Influence the need for and
method(s) of well maintenance. Table 41 lists major aquifer types by
ground-water regions and indicates the most prevalent problems with
operation of the wells in this type of rock or unconsolidated deposit.
Problems with monitoring wells are typically caused by poor well design,
improper installation, incomplete development, borehole instability and
chemical, physical and/or biological incrustation. A brief description of
the major factors leading to well maintenance are discussed below.
Design--
A well is improperly designed if hydrogeologic conditions, water
quality or well intake design are not compatible with the purpose and use of
the monitoring well. For example, if water is withdrawn during the sampling
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TABLE 41. REGIONAL WELL MAINTENANCE PROBLEMS (GASS ET AL, 1980)
Ground Water Regions
Most Prevalent
Aquifer Types
•Most Prevalent Well Problems
i Western Mountain Ranges
2 Alluvial Basins
3 Columbia Lava Plateau
4 Colorado Plateau.
Wyoming Basin
5 High Plains
6 Unglaciated Central Region
7 Glaciated Central Region
8 Unglaciated Appalachians
9 Glaciated Appalachians
10 Atlantic and Gull Coast Plain
Alluvial
Sandstone
Limestone
Alluvial
Basaltic lavas
Alluvial
Interbedded sandstone
and snaie
Alluvial
Interbedded sandstone,
limestone, shale
Alluvial
Alluvial
Sandstone
Metamorphic
Limestone
Alluvial
Alluvial
Consolidated sedimentary
Alluvial and seen [consolidated
Consolidated sedimentary
Silt. clay, sand intrusion, iron: scale deposition, biological touting
Fissure plugging, casing failure, sand production
Fissure plugging by clay and silt mineralization of fissures
Clay. silt, sand intrusion: scale deposition, iron, biological fouling.
limited recharge, casing failure
Fissure and vesicle plugging by clay and silt, some scale deposition
Clay. silt, sand intrusion, iron: manganese, biological fouling
Low initial yields, plugging of aquifer during construction by
drilling muds and fines (clay and silt) natural to formations, fissure
plugging, limited recharge, casing failure
Clay, silt sand intrusion, scale deposition, iron, biological fouling.
limited recharge
Low initial yield: plugging of voids and fissures: poor development
and construction, limited recharge.
Clay, silt sand mtnnion; scale deposition: iron, biological fouling
Fissure plugging by clay and silt casing failure, corrosion, salt
water intrusion: sand production.
Fissure plugging by clay, silt caibonate scale: salt water intrusion.
Clay, silt sand intrusion: scale deposition, iron: biological fouling
Fissure plugging: sand intrusion: casing failure
Low initial yield: fissure plugging by silt and clay, mineralization
of fissures.
Predominantly cavernous production: fissure plugging by clay and
silt mineralization of fissures.
Clay, silt fine sand intrusion: iron, scale, biological fouling.
Clay, silt sand intrusion; scale deposition; biological fouling: iron
Fissure plugging; mineralization, low to medium initial yield
Clay, silt sand intrusion; mineralization of screens.
biological fouling.
Mechanical and cheimcal fissure plugging, biological fouling;
incrustation of well intake structure.
'Excluding pumps and declining water table.
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process and the well screen is plugged, the hydrostatic pressure on the
outside of the casing may be great enough to cause collapse of the well
intake if the strength of the material was not sufficient for the
application. This is particularly true if the well intake material was
chemically incompatible with the ground water and was weakened due to
chemical reactions. Another example is where the operational life of the
monitoring well exceeds the design life. If a well was installed for
short-term water level measurements and the well ultimately is used for
long-term sample collection, problems with material compatability may occur
Additionally, if the well intake openings are improperly sized and/or if the
filter pack is incorrectly designed or installed, siltation and turbid water
samples can result.
Installation--
If productive zones are not accurately identified during the well
drilling process, well intakes can be improperly located or zones can be
improperly sealed. Incorrect installation procedures and/or difficulties
may also cause mis location of well intakes and/or seals. Improperly
connected or corroded casing can separate at joints or collapse and cause
interaquifer contamination. Improperly mixed grout can form inadequate
seals. If casing centralizers are not used, grout distribution may be
inadequate. If the casing is corroded or the bentonite seal not properly
placed, grout may contaminate the water samples. Drilling mud filtrate may
not have been completely removed during the development process. The
surface seal could have been deteriorated or could have been constructed
improperly, and surface water may infiltrate along the casing/borehole
annulus. The intake filter pack must be properly installed.
Development—
Drilling mud, natural fines or chemicals used during drilling must be
removed during the development process. If these constituents are not
removed, water-sample quality may be compromised. Chemicals can also cause
screen corrosion, shale hydration or plugging of the well intake. In
general, the use of chemicals is not recommended and any water added during
the development process must be thoroughly tested.
Borehole stability--
Unstable boreholes contribute to casing failure, grout failure or
screen failure. Borehole instability can be caused by factors such as
improper well intake placement, excessive entrance velocity or shale
hydration.
Incrustation--
There are four types of incrustation that reduce well production:
1) chemical, 2) physical, 3) biological or 4) a combination of the other
three processes. Chemical incrustation may be caused by carbonates, oxides,
hydroxides or sulfate depositions on or within the intake. Physical
plugging of the wells is caused by sediments plugging the intake and
surrounding formation. Biological incrustation is caused by bacteria
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growing in the formation adjacent to the well intake or within the well.
The bacterial growth rate depends on the quantities of nutrients available.
The velocity at which the nutrients travel partially controls nutrient
availability. Examples of common bacteria found in reducing conditions in
wells include sulphur-splitting and hydrocarbon-forming bacteria;
iron-fixing bacteria occur in oxidizing conditions. Some biological
contamination may originate from the ground surface and be introduced into
the borehole during drilling. Nutrients for the organisms may also be
provided by some drilling fluids, additives or detergents.
Incrustation problems are most commonly caused by a combination of
chemical-physical, physical-biological or a combination of
chemical-physical-biological incrustations. Particulates moving through the
well intake may be cemented by chemical/biological masses.
Downhole Maintenance
Many wells accumulate sediment at the bottom. Sand and silt may
penetrate the screen if the well is improperly developed or screen openings
improperly sized. Rocks dropped by rock and bong technologists (Stewart,
1970), insects or waterlogged twigs can also enter the well through casing
from the surface. Sediment can also be formed by precipitates caused by
constituents within the water reacting with oxygen at the water surface
(National Council of the Paper Industry for Air and Stream Improvement,
1982).
If sediment build-up occurs, the sediment should be removed. A
sediment layer at the bottom of the well encourages bacterial activity that
can influence sample quality. In wells that are less than 25 feet deep,
sediment can be removed by a centrifugal pump, and an intake hose can be
used to "vacuum" the bottom of a well. In wells deeper than 25 feet, a hose
with a foot valve can be used as a vacuum device to remove sediment. In
some situations, bailers can also be used to remove sediment. Sediment
should be removed before purging and sampling to eliminate sample turbidity
and associated questions about sample validity.
More traditional maintenance/rehabilitation techniques used to restore
yields of water supply wells include chemical and mechanical methods that
are often combined for optimum effectiveness. Three categories of chemicals
are used in traditional well rehabilitation: 1) acids, 2) biocides and
3) surfactants. The main objectives of chemical treatment are: 1) to
dissolve the incrustants deposited on the well intake or in the surrounding
formation, 2) to kill the bacteria in the well or surrounding formation and
3) to disperse clay and fine materials to allow removal. Table 42 lists
typical chemicals and applications in the water supply industry. Chemicals
have very limited application in the rehabilitation of monitoring wells
because the chemicals cause severe changes in the environment of the wells.
These changes may last for a long time or may be permanent. Before
redevelopment with chemicals is considered, the negative aspects of chemical
alteration in an existing well with a long period of record must be
evaluated against negative aspects of replacing the old well with a new well
that may have new problems and no history. If chemical rehabilitation is
attempted, parameters such as Eh, pH, temperature and conductivity should be
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measured. These measurements can serve as values for comparison of water
quality before and after well maintenance.
TABLE 42. CHEMICALS USED FOR WELL MAINTENANCE (GASS ET AL, 1980)
Chemical Name
Formula
Application
Concentration
Acids and biocides
Hydrochloric acid HCI
Sulfamic acid NH,SOjH
Hydroxyacetic acid CsH403
Inhibitors
Chelatmg agents
Wenmg agents
Surfactants
Chlorine
Diethyithiourea
Dow A-73
Hydrated ferric sullate
Aldec97
Polyrad 110A
Citric acid
Phosphonc acid
Rochelle salt
Hydroxyacetic acid
Plutonic F-68
Plutonic L-62
DowF-33
Sodium Tnpolyphosphate
Sodium Hexametaphosphate
Cl,
(C,H,)NCSN (C,H,)
Fei(SO.)i • 2-3HiO
C.H.O,
HjPO.
NaOOC (CHOH)i COOK
CjH.0,
Carbonate scale oxides hydroxides
Carbonate scale oxides hydroxides
Biocide. chelatmg agent, weak scale
removal agent
Bncide, sterilization, very weak acid
Metal protection
Metal protection
For stainless steel
With sulfamic acid
Metal protection
Keeps metal ions in solution
Keeps metal ions in solution
Keeps metal ions in solution
Keeps metal ions in solution
Renders a surface non-repellent to a
wetting liquid
Renders a surface non-repellent to a
wetting liquid
Lowers surface tension of water thereby
increasing its cleaning power
I5ab 2-3 times zone volume
15%, 2-3 times zone volume
50-500 ppm
02%
001%
1%
2%
375%
Mechanical rehabilitation includes: overpumping, surging, jetting and
air development. These processes are the same as those used in well
development and are described in greater detail in the section entitled
"Methods of Well Development." Development with air is not recommended
because the introduction of air can change the chemical environment in the
well. Any type of rehabilitation for incrustation can be supplemented by
use of a wire brush or mechanical scraper with bailing or pumping to remove
the loose particles from the well.
Exterior Well Maintenance
Maintenance must also be performed on the exposed parts of the well.
Any well casing, well cap, protective casing, sampling tubing, bumper guard
and/or surface seal should be periodically inspected to ensure that
monitoring well sample quality will not be adversely affected. Suggested
routine inspection and maintenance options should be considered:
1) exposed well casing should be inspected. Well casing should be of
good structural integrity and free of any cracks or corrosion;
2) the well cap should be removed to inspect for spider webs, molds,
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fungi or other evidence of problems that may affect the
representativeness of water samples. If no organisms and/or
associated evidence are found, the upper portion of the casing
should be cleaned with a long-handled brush or other similar tool.
The cleaning should be scheduled after sample collection, and the
well should be completely purged after cleaning (National Council of
the Paper Industry for Air and Stream Improvement, 1982);
3) when metal casing is used as protective casing and a threaded cap
is used, the casing should be inspected for corrosion along the
threads. Corrosion can be reduced by lightly lubricating or
applying teflon tape to the threads to prevent seizing. Corrosion
of the casing can be reduced by painting. If lubricants and/or
paint is used, the lubricants and/or paint should be prevented from
entering the well;
A) where multilevel sampling tubes are used, the tubes should be
checked for blockages and labeling so that samples are collected
from the intended zones;
5) where exterior bumper guards are used, the bumper guards should be
inspected for mechanical soundness and periodically painted to
retain visibility; and
6) surface seals should be inspected for settling and cracking. When
settling occurs, surface water can collect around the casing. If
cracking occurs or if there is an improper seal, the water may
migrate into the well. Well seal integrity can best be evaluated
after a heavy rain or by adding water around the outside of the
casing. If the seal is damaged, the seal should be replaced.
Comparative Costs of Maintenance
Evaluating the cost of rehabilitating a well versus abandoning and
redrilling the well is an important consideration. Factors that should be
evaluated are: the construction quality of the well, the accuracy of the
well intake placement and the precision of the documentation of the well.
Capital costs of a new well should also be considered. The actual "cost" of
rehabilitation is hard to calculate. Different rehabilitation programs may
be similar in technique and price but may produce very different results.
In some situations different treatment techniques may be necessary to
effectively treat adjacent wells. Sometimes techniques that once improved a
well may only have a short-term benefit or nay no longer be effective.
However, the cost of not maintaining or rehabilitating a monitoring well may
be very high. The money spent through the years on man-hours for sample
collection and laboratory sample analyses may be wasted by the collection of
unrepresentative data. Proper maintenance and rehabilitation in the long
run is a good investment. If rehabilitation is not successful, abandonment
of the well should be considered.
WELL ABANDONMENT
Introduction
Unplugged or improperly plugged abandoned wells pose a serious threat
to ground water. These wells serve as a pathway for surface pollutants to
infiltrate into the subsurface and present an opportunity for various
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qualities of water to mix. Currently, many sites are being monitored for
low concentrations of contaminants. As detection limits are lowered it
becomes more important to have confidence in the monitoring system. An
improperly installed or maintained monitoring network can produce anomalous
sample results. Proper abandonment is crucial to the dependability of the
remaining or new installations.
The objectives of an abandonment procedure are to: 1) eliminate
physical hazards; 2) prevent ground-water contamination, 3) conserve aquifer
yield and hydrostatic head and 4) prevent intermixing of subsurface water
(United States Environmental Protection Agency, 1975; American Water Works
Association, 1984). The purpose of sealing an abandoned well is to prevent
any further disturbance to the pre-existing hydrogeologic conditions that
exist within the subsurface. The plug should prevent vertical movement
within the borehole and confine the water to the original zone of
occurrence.
Many states have regulations specifying the approved procedures for
abandonment of water supply wells. Some states require prior notification
of abandonment actions and extensive documentation of the actual abandonment
procedures. However, few states have specific requirements for abandonment
of monitoring wells.
Well Abandonment Considerations
Selection of the appropriate method for abandonment is based on the
information that has been compiled for each well. Factors that are
considered include: 1) casing material, 2) casing condition, 3) diameter of
the casing, 4) quality of the original seal, 5) depth of the well, 6) well
plumbness, 7) hydrogeologic setting and 8) the level of contamination and
the zone or zones where contamination occurs. The type of casing and
associated tensile strength limit the pressure that can be applied when
pulling the casing or acting as a guide when overdrilling. For example, PVC
casing may break off below grade during pulling. The condition of any type
of casing also may prohibit pulling. The diameter of the casing may limit
the technique that is selected. For example, hollow-stem augers may not be
effective for overdrilling large-diameter wells because of the high torque
required to turn large-diameter augers. The quality of the original annular
seal may also be a determining factor. For example, if a poor seal was
constructed, then pulling the casing may be accomplished with minimum
effort. The depth of the well may limit the technique applied. The
plumbness of a well may influence technique by making overdrilling or casing
pulling more difficult. The hydrogeology of the site may also influence the
technique selected. For example, hollow-stem augers may be used for
overdrilling in unconsolidated deposits but not in rock formations. The
availability of a rig type and site conditions may also be determining
factors. The level of contamination and zone in which contamination occurs
may modify the choice of technique. If no cross contamination can occur
between various zones and contamination cannot enter from the surface,
grouting the well from bottom to top without removing the casing may be
sufficient.
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Well Abandonment Procedures
Well abandonment procedures involve filling the well with grout. The
well may be filled completely or seals placed in appropriate zones and the
well only partially filled with grout. Completely filling the well
minimizes the possibility of borehole collapse and shifting of seals. The
material used to fill the well can be either carefully selected natural
material with a permeability that approximates the permeability of the
natural formation or a grout mixture with a lower permeability. If more
than one zone is present in the well, then either intermediate seals must be
used with natural materials or the well must be grouted. Monitoring wells
are most commonly abandoned by completely filling the well with a grout
mixture.
Wells can be abandoned either by removing the casing or by leaving all
or part of the casing in place and cutting the casing off below ground
level. Because the primary purpose of well abandonment is to eliminate
vertical fluid migration along the borehole, the preferred method of
abandonment involves casing removal. If the casing is removed and the
borehole is unstable, grout must be simultaneously emplaced as the casing is
removed in order to prevent borehole collapse and an inadequate seal. When
the casing is removed, the borehole can be sealed completely and there is
less concern about channeling in the annular space or inadequate
casing/grout seals. However, if the casing is left in place, the casing
should be perforated and completely pressure grouted to reduce the
possibility of annular channeling. Perforating small-diameter casings in-
situ is difficult, if not impossible.
Many different materials can be used to- fill the borehole. Bentonite,
other clays, sand, gravel, concrete and neat cement all may have application
in certain abandonment situations. Appendix C contains recommendations for
well abandonment that are provided by the American Water Works Association
(1984). These guidelines address the use of different materials for filling
the borehole in different situations. Regardless of the type of material or
combination of materials used for monitoring well abandonment, the sealant
must be free of contaminants and must minimize chemical alteration of the
natural ground-water quality. For example, neat cement should not be used
in areas where the pH of the ground water is acidic. The ground water will
attack the cement and reduce the effectiveness of the seal; the neat cement
also raises the pH and alters ground-water chemistry.
Procedures for Removing Casing—
If the well was not originally grouted, the casing may be pulled by
hydraulic jacks or by "bumping" the casing with a rig. A vibration hammer
also may be used to speed up the task. Casing cutters can be used to
separate the drive shoe from the bottom of the casing (Driscoll, 1986). If
the well intake was installed by telescoping, the intake may be removed by
sandlocking (United States Environmental Protection Agency, 1975).
A properly-sized pulling pipe must be used to successfully implement
the sandlocking technique. Burlap strips 2 to 4 inches wide and
approximately 3 feet long are tied to the pulling pipe. The pipe is lowered
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into the borehole to penetrate approximately 2/3 of the length of the well
intake. The upper portion of the well intake above the burlap is slowly
filled with clean angular sand by washing the sand into the well. The
pulling pipe is then slowly lifted to create a locking effect. Constant
pressure is applied and increased until the well intake begins to move. In
some instances, jarring the pipe may assist in well intake removal, but in
some cases this action may result in loss of the sand lock. As the well
intake is extracted from the well, the sand packing and pipe are removed.
Many contractors have developed variations of this sandlocking technique for
specific situations. For example, slots can be cut in the pulling pipe at
the level adjacent to the top of the well intake to allow excess sand to
exit through the pulling pipe. These slots prevent the well intake from
being overfilled and sandlocking the entire drill string. Slots can also be
cut in the pipe just above the burlap so that sand can be backwashed or
bailed from the inside pipe if the connection should need to be broken.
Right and left-hand couplings located between the drill pipe and pulling
pipe may be installed to disconnect the drill string if it becomes locked.
Well intakes that are 2 to 6 inches in diameter can be removed by latch-type
tools. For example, an elliptical plate cut in half with a hinge may be
used. The plate folds as it is placed in the well and unfolds when lifted.
If the well intake has a sump, the tool can be locked under the sump; if
there is no sump, the tool can be locked under the well intake (Driscoll
1986).
Another technique that may be used in conjunction with sandlocking
involves filling the borehole with a clay-based drilling fluid through the
pulling pipe while pulling the well intake and casing from the bottom. The
fluid prevents the borehole from collapsing. The level of the fluid is
observed to determine if the borehole is collapsing. Fluid rises if
collapse is occuring. If fluid is falling, it is an indication that fluid
is infiltrating into the surrounding formation. In this technique, the
borehole is grouted from the bottom to the surface.
Overdrilling can also be used to remove casing from the borehole. In
overdrilling, a large-diameter hollow-stem auger is used to drill around the
casing. A large-diameter auger is used because a larger auger is less
likely to veer off the casing during drilling. The hollow stem should be at
least 2 inches larger than the casing that is being removed. For example, a
3 1/4-inch inside diameter auger should not be used to overdrill a 2-inch
diameter casing. The augers are used to drill to the full depth of the
previous boring. If possible, the casing should be pulled in a "long"
string, or in long increments. If the casing sticks or breaks, jetting
should be used to force water down the casing and out the well intake. If
this technique fails, the augers can be removed one section at a time and
the casing can be cut off in the same incremental lengths. After all casing
has been removed, the hollow-stem augers are reinserted and rotated to the
bottom of the borehole. All the debris from the auger interior should be
cleaned out, the augers extracted and the borehole filled with grout by
using a tremie pipe (Wisconsin Department of Natural Resources, 1985). The
technique of overdrilling is not limited to hollow-stem augers.
Overdrilling can also be accomplished by direct rotary techniques using air,
foam or mud.
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Limiting factors in overdrilling are the diameter of the well and the
hydrogeology of the surrounding formation. When overdrilling, an attempt
should be made to remove all annular sealant so a good seal can be obtained
between the borehole wall and the grout. The plumbness of the original
installation is also very important; if the well was not installed plumb,
then overdrilling may be difficult.
A variation of overdrilling was used by Perrazo et al. (1984) to remove
4-inch PVC casing from monitoring wells. First the well was filled with a
thick bentonite slurry to prevent the PVC cuttings from settling in the
borehole. The auger was regularly filled with slurry to keep the casing
full and to form a mudcake on the wall. This mudcake served as a temporary
seal until a permanent seal was installed. A hollow-stem auger was used
with a 5 to 10-foot section of NW rod welded onto the lead auger for use as
a guide in drilling out the PVC casing. The auger was rotated, and the
casing was cut and spiraled to the surface. A 2-inch diameter roller bit
was threaded onto a drill rod and advanced to ensure the bottom area would
be sealed to the original depth. The grout mixture was pumped down the
drill stem and out the roller bit, displacing the bentonite slurry and water
to the surface. In wells where there was not sufficient pressure to
displace the bentonite slurry and standing water, the roller bit and drill
stem were removed, a pressure cap was threaded onto the top auger flight and
grout was pumped through the cap until increasing pressure forced the grout
to displace the bentonite slurry and water. The augers were then removed
and the grout was alternately "topped off" as each flight was removed.
Another technique involves jetting casing out of the well with water.
If the casing sticks or breaks off, a small-diameter fish tail-type bit is
connected to an A-rod to drill out the thermoplastic casing. The drilling
fluid flushes the cuttings to the surface. After the borehole is cleaned, a
tremie pipe is used to emplace grout from the bottom to the surface
(Wisconsin Department of Natural Resources, 1985).
Procedures for Abandonment Without Casing Removal--
If the casing is in poor condition, the interval adjacent to the
water-bearing zones can be ripped or perforated with casing rippers and then
the casing is filled and pressure grouted (United States Environmental
Protection Agency, 1975; Driscoll, 1986). A concern when using this method
is the accurate placement and effectiveness of the cuts (Perazzo et al.,
1984). Casing may be gun-perforated by using a device that fires steel
projectiles through the casing and into the formation. A jet-perforating
device may be used that is similar to the gun-perforator except that a
pre-shaped charge of high explosives is used to burn holes through the
casing (Ingersoll-Rand, 1985). The top portion of the casing is then pulled
so that a watertight plug in the upper 15 to 20 feet can be attained. This
step may be omitted where the annular space was originally carefully grouted
(Driscoll, 1986).
Using Plugs--
Three types of bridge plugs can be used to isolate hydraulic zones.
These include: 1) permanent bridge seals, 2) intermediate seals and 3) seals
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at the uppermost aquifer. The permanent bridge seal is the most deeply
located seal that is used to form a "bridge" upon which fill material can be
placed. Permanent bridge seals prevent cross contamination between lower
and upper water-bearing zones. Permanent seals are comprised of cement
Temporary bridges of neoprene plastic or other elastomers can provide
support for a permanent bridge during installation (United States
Environmental Protection Agency, 1975).
Intermediate seals are located between water-bearing zones to prevent
intermixing of different-quality water. Intermediate seals are comprised of
cement, sand/cement or concrete mixes and are placed adjacent to impermeable
zones. The remaining permeable zones are filled with clean disinfected
sand, gravel or other material (United States Environmental Protection
Agency, 1975).
The seal at the uppermost aquifer is located directly above the
uppermost productive zone. The purpose is to seal out surface water. An
uppermost aquifer seal is typically comprised of cement, sand/cement or
concrete. In artesian conditions, this seal prevents water from flowing to
the surface or to shallower formations (United States Environmental
Protection Agency, 1975). This plugging technique is generally used to
isolate useable and non-useable zones and has been used extensively in the
oil and gas industry.
If artesian conditions are encountered, several techniques can be used
to abandon the well. To effectively plug an artesian well, flow must be
stopped and the water level lowered during seal emplacement. The water
level can be lowered by: 1) drawing down the wall by pumping nearby wells,
2) placing fluids of high specific gravity in the borehole or 3) elevating
the casing high enough to stop the flow (Drlscoll, 1986). If the rate of
flow is high, neat cement or sand/cement grout can be piped under pressure,
or a packer can be located at the bottom of the confining formation above
the production zone (United States Environmental Protection Agency, 1975).
Fast-setting cement can sometimes be used in sealing artesian wells (Herndon
and Smith, 1984).
Grouting Procedures for Plugging
All materials used for grouting should be clean and stable; water used
should be free from oil and other contaminants (Driscoll, 1986). Grout
should be applied in one continuous grouting procedure from bottom to top to
prevent segregation, dilution and bridging of the sealant. The end of the
tremie pipe should always remain immersed in the slurry of grout throughout
the emplacement procedure. Recommendations for grout proportions and
emplacement procedures are discussed in the section entitled "Annular
Seals."
Many states permit or recommend a cement/bentonite mixture. The
bentonite possesses swelling characteristics that make it an excellent
plugging material (Van Eck, 1978). The grout mixture used should be
compatible with soil and water chemistry. For example, a salt-saturated
cement should be used for cementing in a salt-saturated area. The
cement/bentonite mixture should not extend through the vadose zone to the
260
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land surface or be used in areas of low soil moisture because cracking and
channeling due to dessication can allow surface water to infiltrate along
the casing (Driscoll, 1986). To ensure that the borehole was properly
grouted, records should be kept of the calculated volume of the borehole and
the volume of grout that was used; any discrepancy should be explained.
A concrete cap should be placed on the top of a cement/bentonite plug.
The concrete cap should be marked with a piece of metal or iron pipe and
then covered by soil. The metal allows for easy location of the well in the
future by metal detector or magnetometer.
Clean-up. Documentation and Notification
After abandonment is accomplished, proper site clean-up should be
performed. For example, any pits should be backfilled and the area should
be left clean (Fairchild and Canter, 1984). Proper and accurate
documentation of all procedures and materials used should be recorded. If
regulations require that abandonment of wells be reported, information
should be provided on the required forms and in compliance with the state
regulations. Table 43 shows information that is typically recorded on a
well abandonment form. The location of abandoned wells should be plotted on
a map and referenced to section lines, lot lines, nearby roads and buildings
as well as any outstanding geological features (Aller, 1984).
TABLE 43. WELL ABANDONMENT DATA (AFTER WISCONSIN DEPARTMENT OF NATURAL
RESOURCES. 1985)
Nairn of property owner
Address of owner/property
Well location (street section number, township and range)
Type of well installation method and date (drilled, driven, bored, dug), purpose of well (OW PIEZ. LYS)
Depth of well
Diameter of well
Depth of easing
Depth to rock
Depth to water
Formation type
Matenal overlying rock (clay. sand, gravel etc I
Materials ana quantities used to till well m specific zones, detailing in which formations and method used
Casing removed or left in place
Firm completing work
Signature of person doing work
Addiess of firm
261
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263
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APPENDIX A
DRILLING AND CONSTRUCTING MONITORING WELLS WITH
HOLLOW-STEM AUGERS
[This report was produced as a part of this cooperative agreement
and was published by Hackett (1987 and 1988).]
INTRODUCTION
Since the 1950's, hollow-stem augers have been used extensively by
engineers and exploration drillers as a practical method of drilling a
borehole for soil investigations and other geotechnical work. The
widespread use and availability of hollow-stem augers for geotechnical
investigations has resulted in the adaptation of this method to drilling
and installing ground-water monitoring wells. To date, hollow-stem augers
represent the most widely used drilling method among ground-water
professionals involved in constructing monitoring wells (McCray, 1986).
Riggs and Hatheway (1988) estimate that more than 90 percent of all
monitoring wells installed in unconsolidated materials in North America are
constructed by using hollow-stem augers.
The drilling procedures used when constructing monitoring wells with
hollow-stem augers, however, are neither standardized nor thoroughly
documented in the published literature. Lack of standardization is
partially due to variable hydrogeologic conditions which significantly
influence hollow-stem auger drilling techniques and monitoring well
construction practices. Many of these construction practices evolved in
response to site-specific drilling problems which are unique to hollow-stem
augers.
This report presents an objective discussion of hollow-stem auger
drilling and monitoring well construction practices. The drilling
equipment will be reviewed, and the advantages and limitations of the
method for drilling and installing monitoring wells will be presented.
AUGER EQUIPMENT
The equipment used for hollow-stem auger drilling includes either a
mechanically or hydraulically powered drill rig which simultaneously
rotates and axially advances a hollow-stem auger column. Auger drills are
typically mounted on a self-contained vehicle that permits rapid
mobilization of the auger drill from borehole to borehole. Trucks are
frequently used as the transport vehicle; however, auger drills may also be
mounted on all-terrain vehicles, crawler tractors or tracked carriers
(Mobile Drilling Company, 1983). These drilling rigs often have
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multi-purpose auger-core-rotary drills which have been designed for
geotechnical work. Multi-purpose rigs may have: 1) adequate power to
rotate, advance and retract hollow-stem augers; 2) adequate drilling fluid
pumping and tool hoisting capability for rotary drilling; and 3) adequate
rotary velocity, spindle stability and spindle feed control for core
drilling (Riggs, 1986).
The continuously open axial stem of the hollow-stem auger column
enables the borehole to be drilled while the auger column simultaneously
serves as a temporary casing to prevent possible collapse of the borehole
wall. Figure 1 shows the typical components of a hollow-stem auger column.
The lead end of the auger column is fitted with an auger head (i.e., cutter
head) that contains replaceable teeth or blades which break up formation
materials during drilling. The cuttings are carried upward by the flights
which are welded onto the hollow stem. A pilot assembly, which is commonly
comprised of a solid center plug and pilot bit (i.e., center head), is
inserted within the hollow center of the auger head (Figure 1). The
purpose of the center plug is to prevent formation materials from entering
the hollow stem of the lead auger, and the pilot bit assists in advancing
the auger column during drilling. A center rod, which is attached to the
pilot assembly, passes through the hollow axis of the auger column. Once
the borehole is advanced to a desired depth for either sampling the
formation or installing the monitoring well, the center rod is used to
remove the pilot assembly. After a sample of the formation has been
collected, the center rod is used to reinsert the pilot assembly into the
auger head prior to continued drilling. The top of the center rod is
attached to a drive cap (Figure 1). The drive cap is used to connect the
auger column to the spindle of the drill rig. This "double adapter" drive
cap ensures that the center rod and pilot assembly rotate along with the
auger column.
The auger column is comprised of a series of individual hollow auger
sections which are typically 5 feet in length. These individual 5-foot
auger sections are joined together by either slip-fit keyed box and pin
connections, slip-fit box and pin connections or threaded connections
(Figure 2). The majority of hollow-stem augers have keyed, box and pin
connections for transfer of drilling torque through the coupling and for
easy coupling and uncoupling of the auger sections (Riggs, 1987). Box and
pin connections use an auger bolt to prevent the individual auger sections
from slipping apart when the auger column is axially retracted from a
borehole (Figures 2a and 2b). Where contaminants are a concern at the
drilling site, an o-ring may be used on the pin end of the connection to
minimize the possible inflow of contaminants through the joint. Joints
with o-rings will leak as the o-rings become worn and it is difficult to
assess the degree of wear at each joint in the auger column when drilling.
Augers with watertight threaded connections are available; however, these
threaded connections commonly are used with commercial lubricants which may
contain hydrocarbon or metallic based compounds. When threaded hollow-stem
augers are used for the installation of water-quality monitoring wells, the
manufacturer recommends that no lubricants be used on the threads (H.E.
Davis, Vice President Mobile Drilling Pacific Division, personal
communication, 1987). When lubricants are used on the hollow-stem auger
threads, a nonreactive lubricant, such as a fluorinated based grease, may
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Drive Cap
Center Plug
Pilot Assembly
Components
Pilot Bit
Rod to Cap
Adapter
Auger Connector
Hollow Stem
Auger Section
Center Rod
Auger Connector
Auger Head
Replaceable
Carbide Insert
Auger Tooth
Figure 1. Typical components of a hollow-stem auger column (after Central Mine
Equipment Company, 1987).
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Key Way
Auger Bolt
, Pin End
0-Ring
a Keyed. Box and Pin Connection
Auger Bolt
Pin End
b. Box and Pin Connection
Square. Tapered Threads
c. Threaded Connection
Figure 2. Three common methods for connecting hollow-stem auger sections.
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be used to avoid introducing potential contaminants that may affect the
ground-water samples collected from the completed well.
The dimensions of hollow-stem auger sections and the corresponding
auger head used with each lead auger section are not standardized between
the various auger manufacturers. A typical range of hollow-stem auger
sizes with slip-fit, box and pin connections is shown in Table 1, and the
range of hollow-stem auger sizes with threaded connections is shown in
Table 2. Hollow-stem auger diameters are typically referenced by the
inside versus the outside (i.e., flighting) diameter. All references made
to the diameter of the hollow-stem auger in this report will refer to the
inside diameter, unless stated otherwise. Tables 1 and 2 also list the
cutting diameter of the auger heads which are mounted on the lead augers.
Common diameters of hollow-stem augers used for monitoring well
construction range from 3 1/4 to 8 1/4 inches for slip-fit, box and pin
connected augers and 3 3/8 to 6 inches for threaded augers.
TABLE 1. TYPICAL HOLLOW-STEM AUGER SIZES WITH SLIP-FIT,
BOX AND PIN CONNECTIONS
(from CENTRAL MINE EQUIPMENT COMPANY, 1987)
Hollow-Stem Flighting Diameter Auger Head
Inside Diameter (in.) (in.)* Cutting Diameter (in.)
2 1/4
2 3/4
3 1/4
3 3/4
4 1/4
6 1/4
8 1/4
5 5/8
6 1/8
6 5/8
7 1/8
7 5/8
9 5/8
11 5/8
6 1/4
6 3/4
7 1/4
7 3/4
8 1/4
10 1/4
12 1/2
*NOTE: Auger flighting diameters should be considered minimum manufacturing
dimens ions.
TABLE 2. HOLLOW-STEM AUGER SIZES WITH THREADED CONNECTIONS
(from MOBILE DRILLING COMPANY, 1982)
Hollow-Stem Flighting Diameter Auger Head
Inside Diameter (in.) (in.)* Cutting Diameter (in.)
2 1/2
3 3/8
4
6
6 1/4
8 1/4
8 1/2
11
8
9
11
13 1/4
*NOTE: Auger flighting diameters should be considered minimum
manufacturing dimensions.
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The hollow axis of the auger column facilitates the collection of
samples of unconsolidated formations, particularly in unsaturated cohesive
materials. Two types of standard samplers which are used with hollow-stem
augers are split-barrel samplers and thin-walled tube samplers.
Split-barrel samplers are typically driven 18 to 24 inches beyond the
auger head into the formation by a hammer drop system. The split-barrel
sampler is used to collect a representative sample of the formation and to
measure the resistance of the formation to penetration by the sampler. The
samples are used for field indentification of formation characteristics and
may also be used for laboratory testing. Thin-walled tube samplers may be
advanced a variable length beyond the auger head either by pushing or
driving the sampler into the formation. These samplers are designed to
recover relatively undisturbed samples of the formation which are commonly
used for laboratory testing. Standard practices for using split-barrel
samplers and thin-wall tube samplers are established under ASTM Standards
D1586-84 and D1587-83, respectively. The ability of hollow-stem augers to
accommodate these samplers, and thus to permit the collection of
undisturbed samples of the formation, is often cited as a major advantage
of the hollow-stem auger method of drilling (Minning, 1982; Richter and
Colleetine, 1983; Gass, 1984).
In addition to these standard samplers, continuous sampling tube
systems are commercially available which permit the collection of
unconsolidated formation samples as the auger column is rotated and axially
advanced (Mobile Drilling Company, 1983; Central Mine Equipment Company,
1987). Continuous sampling tube systems typically use a 5-foot barrel
sampler which is inserted through the auger head. The barrel sampler
replaces the traditional pilot assembly during drilling; however, the
sampler does not rotate with the augers. The open end of the sampler
extends a short but adjustable distance beyond the auger head, and this
arrangement allows sampling to occur simultaneously with the advancement of
the auger column. After the auger column has advanced a distance up to 5
feet, the loaded sampler is retracted from the auger column. The loaded
sampler is either immediately emptied and reinserted through the auger head
or exchanged for another empty sampler. Multi-purpose drill rigs, that are
capable of core drilling, can also use core barrels for coring either
unconsolidated material or rock.
BOREHOLE DRILLING
There are several aspects of advancing a borehole with hollow-stem
augers that are important considerations for ground-water monitoring. For
clarity and continuity, the topic of drilling a borehole with hollow-stem
augers will be presented under three subheadings: 1) general drilling
considerations; 2) drilling with hollow-stem augers in the unsaturated and
saturated zones; and 3) potential vertical movement of contaminants within
the borehole.
General Drilling Considerations
When drilling with hollow-stem augers, the borehole is drilled by
simultaneously rotating and axially advancing the auger column into
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unconsolidated materials or soft, poorly consolidated formations. The
cutting teeth on the auger head break up the formation materials, and the
rotating auger flights convey the cuttings upward to the surface. In
unconsolidated materials, hollow-stem auger drilling can be relatively
fast, and several hundred feet of borehole advancement per day is possible
(Keely and Boateng, 1987a). Drilling may be much slower, however, in dense
unconsoldiated materials and in coarse materials comprised primarily of
cobbles. A major limitation of the drilling method is that the augers
cannot be used to drill through consolidated rock. In unconsolidated
deposits with boulders, the boulders may also cause refusal of the auger
column. According to Keely and Boateng (1987a), this problem may be
overcome in sediments with cobbles by removing the pilot assembly from the
auger head and replacing the assembly with a small tri-cone bit. It is
then possible to drill through the larger cobbles by limited rotary
drilling, without the use of drilling fluids.
The depths to which a borehole may be advanced with a hollow-stem
auger depend on the site hydrogeology (i.e., density of the materials
penetrated and depth to water) and on the available power at the spindle of
the drill rig. Riggs and Hatheway (1988) state that, as a general rule,
the typical maximum drilling depth, in feet, with 3 1/4-inch to 4 1/4-inch
diameter hollow-stem augers is equivalent to the available horsepower at
the drill spindle, multiplied by a factor of 1.5. This general rule on
maximum drilling depths may be influenced by the types of formations being
drilled. Hollow-stem augers have been used to advance boreholes to depths
greater than 300 feet; however, more common depths of borehole advancement
are 75 to 150 feet (Riggs and Hatheway, 1988). The United States
Environmental Protection Agency (1986) generally recognizes 150 feet as the
maximum drilling depth capability of hollow-stem augers in unconsolidated
materials.
One significant advantage of using hollow-stem augers for ground-water
monitoring applications is that the drilling method generally does not
require the circulation of drilling fluid in the borehole (Scalf et al.,
1981; Richter and Co11entine, 1983). By eliminating or minimizing the use
of drilling fluids, hollow-stem auger drilling may alleviate concerns
regarding the potential impact that these fluids may have on the quality of
ground-water samples collected from a completed monitoring well. Without
the use of drilling fluids, the drill cuttings may also be more easily
controlled. This is particularly important where the cuttings are
contaminated and must be contained for protection of the drilling crew and
for disposal. In addition, subsurface contaminants encountered during the
drilling process are not continuously circulated throughout the borehole
via a drilling fluid.
The potential for formation damage from the augers (i.e., the
reduction of the hydraulic conductivity of the materials adjacent to the
borehole) varies with the type of materials being drilled. In homogeneous
sands and gravels, hollow-stem auger drilling may cause minimal damage to
the formation. Where finer-grained deposits occur, however, smearing of
silts and clays along the borehole wall is common. Keely and Boateng
(1987a) indicate that interstratified clays and silts can be smeared into
coarser sand and gravel deposits and can thereby alter the contribution of
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ground-water flow from various strata to the completed monitoring well.
Smearing of silts and clays along the borehole wall may also be aggravated
by certain drilling practices that are designed to ream the borehole to
prevent binding of the auger column (Keely and Boateng, 1987a). These
reaming techniques, which may be used after each few feet of borehole
advancement, include either rotating the auger column in a stationary
position or rotating the auger column while the column is alternately
retracted and advanced over a short distance in the borehole.
The diameter of the borehole drilled by hollow-stem augers is
influenced by the outside diameter of the auger head and auger flighting,
the type of formation material being drilled and the rotation of the
augers. As shown in Tables 1 and 2, the cutting diameter of the auger head
is slightly larger than the corresponding outside diameter of the flighting
on the hollow-stem auger. The cutting diameter of the auger head will
therefore initially determine the diameter of the borehole. However, as
the cuttings are conveyed up the flights during drilling, the diameter of
the borehole may also be influenced by the packing of the cuttings on the
borehole wall. Cuttings from cohesive formation materials with silts and
clays may easily compact along the borehole wall, whereas noncohesive sands
and gravels may not. Where cuttings are readily compacted on the
sidewalls, the borehole diameter may reflect the outside diameter of the
auger flights as opposed to the cutting diameter of the auger head. In
noncohesive materials, the borehole diameter nay be enlarged due to caving
of the side walls. In addition, reaming techniques used to prevent binding
of the auger column in the borehole often serve to enlarge the diameter of
the borehole beyond the outside diameter of the auger flights. The
diameter of the borehole may also be influenced by the eccentric rotation
of the augers which do not always rotate about a vertical axis. As a
result of these factors, the borehole diameter nay be variable over the
length of the borehole.
Drilling with Hollow-Stem Augers in the Unsaturated and Saturated Zones
The drilling practices used to advance a borehole with hollow-stem
augers in saturated materials and unsaturated materials are usually the
same when drilling in finer-grained deposits or compacted sands and
gravels. However, certain loosely compacted saturated sands, known as
"heaving sands" or "sandblows," may pose a particular drilling difficulty
(Minning, 1982; Perry and Hart, 1985; Keely and Boateng, 1987a). Heaving
sands can necessitate changes in basic drilling equipment and changes in
drilling practices. The following discussion focuses first on the drilling
procedures used to advance a borehole through the unsaturated zone. These
procedures are then contrasted with the drilling techniques used to advance
the auger column into saturated heaving sands.
Unsaturated Zones--
The drilling in the unsaturated zone, the hollow-stem auger column is
typically comprised of the components shown in Figure 1. A pilot assembly,
center rod and drive cap commonly are used, and the borehole is advanced
without the use of a drilling fluid. When the borehole has been advanced
to a desired sampling depth, the drive cap is detached from the auger
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column, and the center rod and pilot assembly are removed from the hollow
axis of the auger column (Figure 3a and 3b). A split barrel sampler or
thin-walled tube sampler, attached to a sampling rod, is then lowered
through the axis of the hollow-stem column. The sampler is advanced beyond
the auger head either by driving or pressing the sampler into the formation
materials (Figure 3c). The loaded sampler and sampling rod are removed
from'the auger column, and the pilot assembly and center rod are reinserted
prior to continued drilling. When formation samples are required at
frequent intervals during borehole advancement, the sequential removal and
reinsertion of the pilot assembly and center rod can be time consuming. In
order to minimize the time required to collect undisturbed formation
samples, continuous sampling tube systems can be used to replace the
traditional pilot assembly. Continuous samplers enable the collection of
formation samples simultaneously with the advancement of the borehole
(Figure 4). Driscoll (1986) states that the pilot assembly and center rod
may be omitted when drilling through some dense formation materials because
these cohesive materials usually form only a limited 2 to 4-inch thick
blockage of material inside the hollow center of the auger head. Drilling
with an open auger head in the unsaturated zone, however, is not a common
practice and is not recommended where detailed samples of the formation are
required.
Heaving Sands--
The drilling techniques used to advance the auger column within
heaving sands may vary greatly from those techniques used when drilling in
unsaturated materials. The problem may occur when the borehole is advanced
to a desired depth without the use of drilling fluids for the purpose of
either sampling the formation or installing*a monitoring well. As the
pilot assembly is retracted, the hydrostatic pressure within the saturated
sand forces water and loose sediments to rise inside the hollow center of
the auger column (Figure 5). Keely and Boateng (1987a) report that these
sediments can rise several tens of feet inside the lower auger sections.
The resulting "plug" of sediment inside the hollow auger column can
interfere with the collection of formation samples, the installation of the
monitoring well or even additional drilling.
The difficulties with heaving sands may be overcome by maintaining a
positive pressure head within the auger column. A positive pressure head
can be created by adding a sufficient amount of clean water or other
drilling fluid inside the hollow stem. Clean water (i.e., water which does
not contain analytes of concern to a monitoring program) is usually
preferred as the drilling fluid in order to minimize potential interference
with samples collected from the completed well. The head of clean water
inside the auger column must exceed the hydrostatic pressure within the
sand formation to limit the rise of loose sediments inside the hollow-stem.
Where the saturated sand formation is unconfined, the water level inside
the auger column is maintained above the elevation of the water table.
Where the saturated sand formation is confined, the water level inside the
auger column is maintained above the potentiometric surface of the
formation. If the potentiometric surface of the formation rises above the
ground elevation, however, the heaving sand problem may be very difficult
286
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858?**-*®
"-'• Auger -*"
Column
,.,
fSplil Barrel'-tf
or Thin-
Walled Tub*
Sampler
'^$:§^^i&*M
£$sS?*ofrf^>&yo ?*o
J-iUO-° -•»:«• O V> iJ 0 5V*-Q.«.'
-------
Auger Drill
Rig
Auger Column
Barrel Sampler
Non-rotating
Sampling Rod
, ' • t •: • v > v -
Auger Head
Figure 4. Diagram of continuous sampling tube system (after Central Mine Equipment
Company, 1987).
288
-------
Auger Column
Saturated Sand
Formation
Sands Rising
Inside Hollow Cen
Due to Hydrostatic
Pressure in Sand
Formation
Pilot
Assembly
a. Borahoto Advanced Into Saturated
Sand wNh Auoar Column
Containing PNol Aa»amMy
Movement of LOOM Sandt Into the
Ho»ow Center o< Augtr as Iht Pilot
AMtmMy it Rtntovtd
Figure 5. Diagram showing heaving sand with hollow-stem auger drilling.
289
-------
to counteract and may represent a limitation to the use of the drilling
method.
There are several drilling techniques used to maintain a positive
pressure head of clean water within the auger column. One technique
involves injecting clean water through the auger column during drilling.
This method usually entails removal of the pilot assembly, center rod and
drive cap. A special coupling or adapter is used to connect the auger
column to the spindle of the drilling rig. Clean water is then injected
either through the hollow-center coupling or through the open spindle of
the drill rig as the auger column is advanced (Figure 6). Large diameter,
side-feed water swivels are also available and can be installed between the
drive cap and the hex shank which connects the auger column to the spindle
of the drill rig. Clean water is injected through the water swivel and
into the auger column as the augers are advanced.
Another drilling technique used to overcome heaving sands is to first
advance the auger column by using a "nonretrievable" knock-out plate. The
knock-out plate is wedged inside the auger head and replaces the traditional
pilot assembly and center rod (Figure 7a). A major disadvantage of this
drilling technique is that the knock-out plate cannot be alternately
removed and reinserted from the auger column to permit the collection of
formation samples as the auger column is advanced. Once the auger column
is advanced to a desired depth, the column is filled to a sufficient height
with clean water. A ramrod commonly is used to strike and remove the
knock-out plate from the auger head (Figure 7b). The head of clean water
in the auger column must exceed the hydrostatic pressure in the sand
formation to prevent loose sediments from rising inside the auger column
once the knock-out plate is removed. The nonretrievable knock-out plate
should be constructed of inert materials when drilling a borehole for the
installation of a water-quality monitoring well. This will minimize
concerns over the permanent presence of the knock-out plate in the bottom
of the borehole and the potential effect the plate may have on ground-water
samples collected from the completed well.
Reverse flight augers represent another unique center plug design
which has had measured success in overcoming problems with heaving sands
(C. Harris, John Mathes and Associates, personal communication, 1987). The
flighting on the center plug and center rod rotates in an opposite
direction from the flighting on the auger column (Figure 8). As the auger
column advances through the heaving sands, the sand deposits are pushed
outward from the auger head by the reverse flighting on the center plug. A
sufficient head of clean water is maintained inside the auger column to
counteract further the hydrostatic pressure in the heaving sand formation.
Once drilling is completed, the reverse flight center plug is slowly
retracted from the auger column so that movenent of sand into the hollow
stem is not induced.
Although the use of clean water as a drilling fluid is recognized by
the United States Environmental Protection Agency as a proper drilling
technique to avoid heaving sand problems (United States Environmental
Protection Agency, 1986), the use of any drilling fluid may be undesirable
or prohibited at some ground-water monitoring sites. In these instances,
290
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• Water Swivel
Clean Water
Auger Drill
R.g
Spindle Adapter Assembly Used for
Injecting Fluids Inside Auger Column
Figure 6. Injecting dean water through open drill spindle to counteract heaving sand
(after Central Mine Equipment Company, 1987).
291
-------
Clean Water Level
Within Auger Column
:::iU>T^r Auger Column '.•»'.'•';'•!
? .•t>f«l«»i •_ •.!••!
Saturated Sand
Formation
Knock-Out Plate
Removed from
Auger Head by
Ramrod
Knock-Out Plate'
Positioned
Within Auger
Head
Auger Column
Filled with Clean
Water
Borehole Advanced Into Saturated
Sand with Auger Column Containing
NonraMwabte Knock-Out Plata
b. Clean Water Added to Auger
Column Along with Removal ol
Knock-Out Plate by Ramrod
Figure 7. Use of a nonretrievable knock-out plate and auger column filled with clean water
to avoid a heaving sand problem.
292
-------
§£. Reverse Flight
Auger and
Center Rod
H Slowly Retracted
$i from Auger
iHS Column
Auger Column
Filled with Clean iiSSft
Water as
Borehole Is
Advanced
Auger Column
Filled with Clean
Water as Reverse
Flight Auger Is
Retracted
Reverse Flight '*
Auger and
Center Rod
a. Reverse Flight Auger Pushes
Cuttings Outwardly While Head
of Clean Water Is Maintained
Inside Auger Column
b. Reverse Right Auger Slowly
Being Retracted from Auger
Column
Figure 8. Use of a reverse flight auger to avoid a heaving sand problem (after Central Mine
Equipment Company, 1987).
-------
the problem may be overcome by using commercial or fabricated devices that
allow formation water to enter the auger column, but exclude formation
sands. Perry and Hart (1985) detail the fabrication of two separate
devices that allow only formation water to enter the hollow-stem augers
when drilling in heaving sands. Neither one of these two devices permit
the collection of formation samples as the auger column is advanced through
the heaving sands. The first device consists of a slotted coupling
attached to a knock-out plate (Figure 9). As the auger column advances
below the water table, formation water enters the auger column through the
slotted coupling (Figure lOa). When the auger column is advanced to the
desired depth, a ramrod is used to dislodge the knock-out plate with
slotted coupling from the auger head (Figure lOb). Perry and Hart (1985)
report that the slotted coupling generally is successful in counteracting
heaving sand problems. However, where clays and silts are encountered
during drilling, the openings in the slotted coupling may clog and restrict
formation water from entering the auger column. To overcome this plugging
problem, Perry and Hart (1985) fabricated a second device to be used when
the slotted coupling became plugged. The second device is actually a
screened well swab (Figure 11). The swab is connected to a ramrod and is
lowered through the auger column once the column is advanced to the
desired depth. The ramrod is used to strike and remove the knock-out plate
from the auger head (Figure 12). The screened well swab filters the sand
and allows only formation water to enter the auger column (Perry and Hart,
1985). Once the water level rises inside the auger column to a height that
offsets the hydrostatic pressure in the formation, the screened well swab
is slowly removed so that movement of sand into the hollow stem is not
induced.
' ' L
Nipple
Lock Nut
Knock-Out Plate
Slotted Coupling
Plug
Figure 9. Diagram of a slotted coupling (after Perry and Hart, 1985).
294
-------
Auger Coium
Filled with
Formation
Knock-Oo
with Sioiteo
Coupling
Knock-Out
with Slotted
Coupling
Permitting
Formation Water
to Enter Auger
Column as
Borehole Is
Advanced
Auger Column
Filled with
Formation Water
Saturated Sand with Auger
Cokimn Containing Non-
feiriewMe Knock-Out Ptote
wHh Slotted Coupling
b. Knock-Oul Plate with Slotted
Coupling Removed Irom Auger
Head by Ramrod
Figure 10. Us* Of a nonretrievable knock-out plate wMh a slotted coupling to avoid a
heaving sand problem (after Perry and Hart, 1965).
295
-------
Pipe Flange
Nipple
Inside Diameter
of Hollow-Stem Auger
•Ball Valve
Nipple
Figure 11. Diagram of a screened well swab (after Perry and Hart, 1985).
Commercial devices that permit only formation water to enter the auger
column during drilling are also available. These devices include a variety
of patented designs, including nonwatertight flexible center plugs. These
devices replace the traditional pilot assembly in the auger head. Some
flexible center plugs are seated inside the auger head by means of a
specially manufactured groove in the hollow stem. These flexible center
plugs allow split-barrel samplers and thin-walled tube samplers to pass
through the center plug so that samples of the water bearing sands can be
collected (Figure 13). The flexible center plug, however, cannot be
retracted from the auger head and therefore severely restricts the ability
to install a monitoring well through the auger column. The monitoring well
intake and casing can be inserted through the flexible center plug, but the
plug eliminates the installation of filter pack and annular sealant (i.e.,
bentonite pellets) by free fall through the working space between the well
casing and auger column.
Potential Vertical Movement of Contaminants Within the Borehole
The potential for contaminants to move vertically within the borehole
during drilling is an important consideration when selecting a drilling
method for ground-water monitoring. Vertical mixing of contaminants from
different levels within a single borehole may be a problem with several
296
-------
%&
Screened Well
Xv"
!:^§gfgj? ocreened well
iwl$t$j&. Swab Attacheo
Sw!-s?SS;sK;S?J;ii)
a.~
•s Water Level
Rising Inside
Auger Column
After Removal ol
Knock-Out Plate
Screened Well
Swab, Attached
to Ramrod. Used
to Filter Out
Sand and Permit
Formation Water
to Enter Auger
Column
Knock-Out Plate
with Clogged
Slotted Coupling
Removed from
Auger Head by
Ramrod
Figure 12. Use of a screened well swab to avoid a heaving sand problem (after Perry and
Hart, 1985).
297
-------
Flexible Center
Plug Permitting
Collection of
Water-Bean ng
Sands, but
Preventing
Heaving Sanas
from Entering
Hollow Stem
m
' Auger Head i
f;S^r^S^^^M
... Formation ^v
X::-;::-.:-:-««M5.-.xt.vKS5;a:';.-Sl
Figure 13. Flexible center plug in an auger head used to overcome heaving sands and
permit sampling of formation materials (after Diedrich Drilling Equipment,
1986).
298
-------
different drilling methods, including hollow-stem augers. As the auger
column advances through deposits which contain solid, liquid or gas-phase
contaminants, there may be a potential for these contaminants to move
either up or down within the borehole. Where vertical movement of
contaminants occurs within the borehole, the cross contamination may be a
significant source of sampling bias (Gillham et al., 1983).
Vertical movement of contaminants within the borehole may occur when
contaminants from an overlying stratum are carried downward as residual
material on the augers. The potential for small amounts of contaminated
material to adhere to the auger head and lead auger is greatest in cohesive
clayey deposits (Gillham et al., 1983). Contaminants may also adhere to
split-barrel samplers and thin-walled tube samplers. If these sampling
devices are not adequately cleaned between usage at successive sampling
depths, contaminants from an overlying stratum may be introduced into a
lower stratum via the sampling device. Where reaming techniques have
enlarged the borehole beyond the outside diameter of the auger flights,
contaminants from an overlying stratum may slough, fall down the annular
space and come in contact with a lower stratum (Keely and Boateng, 1987a).
Even small amounts of contaminants that move downward in the borehole,
particularly to the depth at which the intake of the monitoring well is to
be located, may cause anomalous sampling results when analyzing samples for
contaminants at very low concentrations. According to Gillham et al.
(1983), this potential for sampling bias is greatest at monitoring sites
where shallow geological formations contain absorbed or immiscible-phase
contaminants.
Contaminants may also move upward within a borehole during hollow-stem
auger drilling. As the auger column is advanced through a stratum
containing contaminants, the contaminants may be carried upward along with
the cuttings. Contaminated material from a lower stratum may therefore be
brought into contact with an uncontaminated overlying stratum (Keely and
Boateng, 1987a). Cohesive materials within the contaminated cuttings may
smear and pack the contaminants on the sidewalls. Where contaminants are
displaced and smeared on the sidewall at the intended monitoring depth,
these contaminants may serve as a persistent source of sampling bias.
Vertical movement of dissolved-phase contaminants within a borehole
may also occur where two or more saturated zones with different heads are
penetrated by the auger column. When the water level in a contaminated,
overlying saturated zone is higher than the potentiometric surface of an
underlying uncontaminated zone, downward leakage of contaminated water
within the borehole may occur. This downward movement of water may occur
even if the augers are continually rotated in an attempt to maintain the
upward movement of cuttings (Gillham et al., 1983). Conversely, the upward
leakage of contaminants in the borehole may occur where the potentiometric
surface of an underlying contaminated zone is higher than the water level
in an overlying saturated zone.
The vertical movement of contaminants within the borehole drilled with
hollow-stem augers is not well documented in the published literature.
Lack of documentation is partially due to the difficulty of diagnosing the
problem in the field. The determination that an aquifer was contaminated
299
-------
prior to drilling, during drilling or after installation of the monitoring
well may not easily be made. Keely and Boateng (1987b), however, recount a
case history in which apparent vertical movement of contaminants in the
borehole occurred either during hollow-stem auger drilling and/or after
installation of the monitoring well. This case study involves a site at
which a heavily contaminated, unconfined clayey silt aquifer, containing
hard-chrome plating wastes, is underlain by a permeable, confined sand and
gravel aquifer. Water samples collected from monitoring wells developed in
the lower aquifer showed anomalous concentrations for chromium. Although
vertical ground-water gradients at the site were generally downward, the
areal distribution and concentrations of chromium in the lower aquifer were
not indicative of long-term leakage through the aquitard. Based on their
investigation of the site, Keely and Boateng (1987b) conclude that the
localized pattern of chromium values in the lower aquifer resulted from
either vertical movement of contaminants in the borehole or vertical
movement of contaminants through faulty seals along the casing of the
monitoring wells. The authors hypothesize that Che vertical movement of
the contaminants in the borehole may have occurred when contaminated solids
from the upper aquifer fell down the annular space during hollow-stem auger
drilling.
The potential for cross contamination during drilling may be reduced
if contamination is known or suspected at a sice. Where a shallow
contaminated zone must be penetrated to monitor ground-water quality at
greater depths, a large-diameter surface casing -nay be used to seal off the
upper contaminated zone before deeper drilling is attempted. Conventional
hollow-stem auger drilling alone, however, may not always be adequate for
installation of a larger diameter surface casing Depending on the
hydrogeological conditions at the site, a "hybrid" drilling method may be
necessary in which conventional hollow-stem auger drilling is combined with
a casing driving technique that advances the surface casing as the borehole
is advanced. Driving techniques used to advance and install surface casing
may include conventional cable tool drilling, rotary drilling with casing
hammer or a drop hammer system on an auger drill rig.
Conventional hollow-stem auger drilling may be used to set protective
surface casing where the shallow geological formations are comprised of
cohesive materials. In this situation, a large-diameter borehole may be
•advanced by the auger column to a depth below the known contamination
(Figure 14a). The auger column is then fully retracted from the borehole
at sites where the borehole will remain open due to the cohesiveness of the
formation (Figure 14b). A large-diameter surface casing is then set and
grouted into place. After grouting the large-diameter surface casing into
place a hollow-stem auger column of smaller outside diameter is used to
advance the borehole to the desired depth for installation of the
monitoring well (Figure 14c). Typical dimensions for augers used in this
scenario might be an 8 1/4-inch diameter hollow-stem auger with an auger
head cutting diameter of 12 1/2 inches to advance the borehole below the
contaminated zone. A nominal 10-inch diameter surface casing would
commonly be installed within the 12 1/2-inch diameter borehole. Four-and-
one-quarter-inch diameter augers with an eight-and-one-quarter-inch auger
head cutting diameter might then be used to continue drilling after the
surface casing is set.
300
-------
~*- • %<
!3£ Shallow •££
Contaminant
r- Shulluw
•rS' "*
'~ Coniaminanl
Large-Diameter
Auger Used lo
Advance
Borehole m
Cohesive
Materials
Grouted Annular
Space
Protective
Surlact! Casing
Set Beluw
Contanimanl
Zone
Open Borehole
Small-Diametei
Auger Used to
Advance
Borehole lo a
Deeper Depth
for Installation of
Monitoring Well
•. Large-Diameter Borehole
Advanced Below Known Depth
of Contamination
b. Auger Column Retracted from
Borehole Which Remain* Open
Due lo Cohesive Materials
Figure 14. Sequence showing the installation of protective surface casing through a
shallow contaminated zone in a cohesive formation.
c. Surface Casing Installed Below
Known Depth of Contamination
with Drilling Continued Using
Smaller Diameter Auger
-------
When the shallow geological formations are comprised of noncohesive
materials and the borehole will not stand open, a hybrid drilling technique
can be used in which the surface casing is advanced simultaneously with the
auger column. According to Keely and Boateng (1987a), this alternate
drilling technique is used to advance the auger column a few feet at a time
and then to drive the surface casing to the new borehole depth. The auger
column is telescoped inside the surface casing as the casing is driven
outside the augers (Figure 15). Five-foot lengths of casing typically are
used with this technique, and the casing is driven either by using the same
conventional 140-pound drop hammer that is used to advance split-barrel
samplers or a heavier 300-pound drop hammer. The sequential steps of
augering and casing advancement continue until the surface casing extends
below the depth of known contamination. Once the surface casing is set, a
smaller diameter hollow-stem auger column can be used to advance the
borehole to the desired depth for monitoring well installation.
MONITORING WELL INSTALLATION
Monitoring wells may be constructed for water-quality sampling, water-
level measurement or both. The intended purpose of the well influences the
design components of the well and, therefore, the installation method.
Figure 16 shows the typical design components of a monitoring well. The
following discussion will focus on techniques used to install water-quality
monitoring wells which consist of a well casing and intake, filter pack and
annular seal.
The methods used to construct water-quality monitoring wells with
hollow-stem augers depend primarily on site hydrogeology. In particular,
the cohesiveness of the formation materials penetrated by the auger column
may influence the well construction practices used. If the formation
materials are cohesive enough so that the borehole remains open, the entire
auger column may be retracted from the borehole prior to the installation
of the monitoring well casing and intake, filter pack and annular seal.
However, even in cohesive formation materials, drillers may refrain from
the practice of fully retracting the auger column from a completed borehole
to avoid unexpected caving of the borehole. The string of well casing and
attached intake may be centered in the open borehole by using casing
centralizers. The filter pack and annular sealant can then be emplaced
through the working annular space between the borehole and well casing.
When the auger column penetrates noncohesive materials and the
borehole will not remain open, the auger column is used as a temporary
casing during well construction to prevent the possible collapse of the
borehole wall. When the auger column is used as a temporary casing during
well construction, the hollow axis of the auger column facilitates the
installation of the monitoring well casing and intake, filter pack and
annular sealant. However, the practices that are used to emplace these
well construction materials through the working space inside the
hollow-stem augers are not standardized among contractors. Lack of
standardization has resulted in concerns about the proper emplacement of
the filter pack and annular seal in the monitoring well. To address these
concerns, the topic of monitoring well construction through hollow-stem
augers is presented in three separate discussions: 1) well casing diameter
302
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"Protective
Surface Casing
Driven Flush
with Borehole
Wall in Non-
Cohesive
Hammer
to Drive
a. Auger Column Advances
Borehole Slightly Beyond
Casing
b. Driving the Casing to the New
Borehole Depth
Figure 15. Sequence showing the installation of protective surface casing through a
shallow contaminated zone in a non cohesive formation (after Keely and
Boateng, 1987a).
303
-------
Inner Casing Cap
Locking Casing Cap
Vent Hole
Protective Casing
Completion Depth
Well Intake
Plug
Figure 16. Typical design components of a ground-water monitoring well.
304
-------
versus inside diameter of the hollow-stem auger; 2) installation of the
filter pack; and 3) installation of the annular seal.
Well Casing Diameter Versus Inside Diameter of the Hollow-Stem Auger
Once the borehole has been advanced to the desired depth for
installation of the monitoring well, the pilot assembly and center rod (if
used) are removed, and the depth of the borehole is measured. A measuring
rod or weighted measuring tape is lowered through the hollow axis of the
auger column. This depth measurement is compared to the total length of
the auger column in the borehole to determine whether loose sediments have
risen inside the hollow stem. Provided that the hollow stem is clear of
sediment, a string of well casing with attached intake is lowered inside
the auger column. Threaded, flush-joint casing and intake are commonly
used to provide a string of casing with a uniform outside and inside
diameter.
Although the well casing and intake may be centered inside the auger
column, many contractors place the well casing and intake toward one side
of the inner hollow-stem wall (Figure 17). The eccentric placement of the
casing and intake within the hollow-stem auger is designed to create a
maximum amount of working space (shown by the distance "A" in Figure 17)
between the outer wall of the casing and the inner wall of the auger. This
working space is used to convey and emplace the filter pack and the annular
sealant through the auger column. Table 3 lists the maximum working space
(A) that is available between various diameters of threaded, flush-joint
casing and hollow-stem augers, if the casing is set toward one side of the
inner hollow-stem wall.
The selection of an appropriately sized hollow-stem auger for drilling
and monitoring well construction should take Into account the nominal
diameter of the well casing to be installed and the working space needed to
properly convey and emplace the filter pack and annular sealant. The
smallest hollow-stem augers typically used for installing 2-inch nominal
diameter casing are 3 1/4-inch diameter augers; the smallest hollow-stem
augers typically used for installing 4-inch noainal diameter casing are 6
1/4-inch diameter augers (Riggs and Hatheway, 1988). Table 3 shows,
however, that the maximum working space available between a 2-inch nominal
diameter casing and a 3 1/4-inch diameter hollow-stem auger is less than 1
inch (i.e., 0.875 inch). This small working space can make the proper
emplacement of the filter pack and annular seal very difficult, if not
impossible. Too small a working space can either restrict the use of
equipment (i.e., tremie pipe) that may be necessary for the placement of
the filter pack and annular seal or inhibit the ability to properly measure
the actual emplacement of these materials in the borehole. A small working
space can also increase the possibility of bridging problems when
attempting to convey the filter pack and annular sealant between the
hollow-stem auger and well casing. Bridging occurs when the filter pack or
annular seal material spans or arches across the space between the inner
diameter of the auger and the outer diameter of the casing. The bridge of
filter pack or annular seal material forms a barrier which blocks the
downward movement of additional material through the working space. As a
result, gaps or large unfilled voids may occur around the well intake or
305
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Maximum
Working Space
Hollow-Stem
Auger
Threaded. Flush-
Joint Casing
and Intake
Inside Diameter of
Hollow-Stem Auger
— C1
Auger Col
Well Casing
Inserted Through
Hollow-Stem Auger
a. Plan View
b. Cross-Sectional View
Figure 17. Plan and cross-sectional views showing the maximum working space (A)
between the well casing and the hollow-stem auger.
306
-------
well casing due co the nonuniform placement of the filter pack or annular
seal. Bridged material can lock the casing and auger together and result
in the well casing being retracted from the borehole along with the augers.
Most contractors prefer to use 4 1/4-inch diameter augers to install 2-inch
nominal diameter casing, and 8 1/4-inch diameter augers to install 4-inch
nominal diameter casing to create an adequate working space that
facilitates the proper emplacement of the filter pack and annular seal (C.
Harris, John Mathes and Associates, personal communication, 1987).
According to United States Environmental Protection Agency (1986), the
inner diameter of the auger should be 3 to 5 inches greater than the outer
diameter of the well casing for effective placement of the filter pack and
annular sealant. Based on the United Sates Environmental Protection Agency
guideline for effective working space, 6 1/4-inch diameter hollow-stem
augers would be the recommended minimum size auger for installing a 2-inch
nominal diameter casing. In addition, the maximum diameter of a well which
could be installed through the hollow axis of the larger diameter augers,
which are commonly available at this time, would be limited to 4 inches or
less.
TABLE 3. MAXIMUM WORKING SPACE AVAILABLE BETWEEN VARIOUS
DIAMETERS OF THREADED, FLUSH-JOINT CASING AND HOLLOW-STEM AUGERS
Nominal Outside Working Space "A" (see Figure 17) for
Diameter Diameter Various Inside Diameter Hollow-Stem
of Casing of Casing Augers ** (in.)
(in.) * (in.) 3 1/4 3 3/4 4 1/4 6 1/4 8 1/4
2 2.375 0.875 1.375 1.875 3.875 5.815
3 3.500 — 0.250 0.750 2.750 4.750
4 4.500 — — — 1.750 3.750
5 5.563 — — --- 0.687 2.687
6 6.625 — — — — 1.625
* Based on ASTM Standards D-1785 and F-480
** Inside diameters of hollow-stem augers taken from Table 1.
Installation of the Filter Pack
After the well casing and intake are inserted through the hollow axis
of the auger column, the next phase of monitoring well construction
commonly involves the installation of a filter pack. The filter pack is a
specially sized and graded, rounded, clean silica sand which is emplaced in
the annular space between the well intake and borehole wall (Figure 16).
307
-------
The primary purpose of the filter pack is to filter out finer-sized
particles from the formation materials adjacent to the well intake. The
filter pack also stabilizes the formation materials and thereby minimizes
settlement of materials above the well intake. The appropriate grain size
for the filter pack is usually selected based on a sieve analysis of the
formation material adjacent to the well intake. The filter pack is usually
a uniform, well-sorted coarse to medium sand (i.e., 5.0 mm to 0.40mm).
However, graded filter packs may be used in a monitoring well which has an
intake installed in a fine-grained formation. The graded filter pack may
filter and stabilize silt and clay-sized formation particles more
effectively. The completion of a monitoring well with a properly sized,
graded and emplaced filter pack minimizes the extent to which the
monitoring well will produce water samples with suspended sediments.
The filter pack typically extends from the bottom of the well intake
to a point above the top of the intake (Figure 16). The filter pack is
extended above the top of the well intake to allow for any settlement of
the filter pack that may occur during well development and to provide an
adequate distance between the well intake and the annular seal. As a
general rule, the length of the filter pack is 10 percent greater than the
length of the intake to compensate for settlement. United States
Environmental Protection Agency (1986) recommends that the filter pack
extend from the bottom of the well intake to a maximum height of 2 feet
above the top of the intake, with the maximum height specified to ensure
discrete sample horizons.
The thickness of the filter pack between the well intake and borehole
wall generally will not be uniform because-the well casing and intake
usually are not centered in the hollow axis of the auger column. The
filter pack, however, should be at least thick enough to completely
surround the well intake. Tables 1 and 2 show that the cutting diameter of
the auger head ranges from 4 to 7 1/4 inches larger than the inside
diameter of the hollow-stem auger. When the well casing and intake are
positioned toward one side of the inner hollow-stem wall (Figure 17), the
annular space between the well intake and borehole wall may be as small as
2 to 3 5/8 inches. This annular space may still be adequate to preclude
bridging and irregular emplacement of the filter pack; however, there is
marginal tolerance for borehole sloughing or installation error. The
proper installation of a filter pack with hollow-stem augers can be
difficult if there is an inadequate working space between the casing and
the auger column through which the filter pack is conveyed (Hinning, 1982;
Richter and Collentine, 1983; Gass, 1984; Schmidt, 1986; Keely and Boateng,
1987b).
The volume of filter pack required to fill the annular space between
the well intake and borehole wall should be predetermined prior to the
emplacement of the filter pack. In order to determine the volume of filter
pack needed, three design criteria should be known. These three criteria
include: 1) the design length of the filter pack; 2) the diameter of the
borehole; and 3) the outside diameter of the well intake and casing. This
information is used to calculate both the volume of the borehole and the
volume of the well intake and casing over the intended length of the filter
pack. Once both volumes are calculated, the volume of the well intake and
308
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casing is subtracted from the volume of the borehole to determine the
volume of filter pack needed to fill the annular space between the well
intake and borehole wall. For example, Figure 18 illustrates a 2-inch
nominal diameter well casing and intake inserted through the hollow axis of
a 4 1/4-inch diameter hollow-stem auger. Based on the cutting diameter of
the auger head, the diameter of the borehole is shown as 8 1/4 inches and
the length of the well intake is 10 feet. The design length of the filter
pack is 12 feet to ensure that the filter pack extends 2 feet above the top
of the intake. The volume of the borehole over the 12 foot design length
of the filter pack will be 4.36 cubic feet. Using 2.375 inches as the
outside diameter of the well intake and casing, the volume of the intake
and casing over the 12 foot design length of the filter pack will be 0.38
cubic feet. By subtracting 0.38 cubic feet from 4.36 cubic feet, the
volume of filter pack needed to fill the annular space is determined to be
3.98 or approximately 4 cubic feet.
Once the theoretical volume of filter pack is calculated, this volume
is divided by the design length of the filter pack to determine the amount
of the material which should be needed to fill the annulus for each lineal
foot that the auger column is retracted. Referring again to the example
illustrated in Figure 18, 4 cubic feet divided by 12 feet would equal
approximately one-third cubic foot per foot. Therefore, for each foot that
the auger column is retracted, one-third cubic foot of filter pack should
be needed to fill the annular space between the well intake and borehole
wall.
The methods which are used to convey the filter pack through the
working space in the auger column and to emplace this material in the
annular space between the well intake and borehole wall depend on: 1) the
cohesiveness of the formation materials; 2) the height of a standing water
column in the working space between the casing and augers; and 3) the
grain-size and uniformity coefficient of the filter pack.
In cohesive formation materials in which the borehole stands open, the
filter pack commonly is emplaced by axially retracting the auger column
from the borehole in short increments and pouring the filter pack down the
working space between the casing and auger column. Prior to filter pack
emplacement, a measuring rod or weighted measuring tape is lowered to the
bottom of the borehole through the working space between the well casing
and auger column (Figure 19a) so that the total depth of the borehole can
be measured and recorded. The auger column is initially retracted 1 or 2
feet from the borehole (Figure 19b). A measured portion of the
precalculated volume of the filter pack is slowly poured down the working
space between the well casing and auger column (Figure 19c). The filter
pack is typically poured at a point diametrically opposite from the
measuring rod or weighted measuring tape. As the filter pack is being
poured, the measuring device is alternately raised and lowered to "feel"
and measure the actual placement of the filter pack. If a weighted
measuring tape is used as the measuring device, the tape is kept in
constant motion to minimize potential binding and loss of the weighted tape
as the filter pack is being poured. Continuous measurements of the depth
to the top of the emplaced filter pack are usually made as the filter pack
is slowly poured down the working space in order to avoid allowing the
309
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2-Inch Nominal Diameter
Well Casing and Intake
4'/«-lnch Diameter
Hollow-Stem Auger
T
Design Length
of Filter Pack
12 Feet
Length of
Well Intake
10 Feet
a? i
Borenoie
Diameter
8'/ incnes
Figure 18. Illustration for the sample calculation of a filter pack as described in the text
310
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Weighted •
Measuring
Casing
Hollow blem
Auger
Plan View
Weighuu ->~^
Measuring Tape
Cross-Sectional View
a. Placement ol Weighted
Measuring Tape
Weighted
Measuring Tape
Auger Column
Retracted
r«— 1 to 2 Feet
from Borehole
WeighU.-J
Measuring Tape
C --
Holluw Stem
Auger
Weighted —
Measunng Tape
Plan View
1 h
Well Casing
- C'
Fillet P.ILK
Pouriny
b. Auger Column Retracted
liter Pack
Cross-Secllonal View
c. Filler Pack Free-Falls Through
Working Space Between Casing
and Auger
Figure 19. Free fall method of filter pack emplacement with a hollow-stem auger.
-------
emplaced filter pack to rise up between the well intake/casing and the
inside of the hollow-stem auger. If the filter pack is permitted to rise
up between the casing and auger, the filter pack may lock the casing and
auger together and result in the casing being retracted from the borehole
along with the augers. Once the filter pack is emplaced to the bottom of
the auger column, the augers are retracted another 1 to 2 feet and a second
measured portion of the filter pack is added. These steps are repeated
until the required length of filter pack is emplaced. By knowing the
theoretical amount of filter pack needed to fill the annular space between
the well intake and borehole wall for each increment in which the auger
column is retracted, the emplacement of the filter pack may be closely
monitored. Calculations of the "filter pack needed" versus "filter pack
used" should be made and recorded for each increment that the auger column
is retracted. Any discrepancies should be explained.
Placement of filter pack by free fall through the working space
between the well casing and auger column can present the potential for
bridging or segregation of the filter pack material. As described earlier,
bridging can result in unfilled voids within the filter pack or in the
failure of the filter pack materials to be properly conveyed through the
working space between the well casing and auger column. Bridging problems,
however, may be minimized by: 1) an adequately sized working space between
the well casing and auger column; 2) slowly adding the filter pack in small
amounts; and 3) carefully raising and lowering the measuring rod or
weighted measuring tape while the filter pack is being added.
Segregation of graded filter pack material during free fall through
the working space between the well casing and auger column may still occur,
especially where the static water level between the casing and augers is
shallow. As the sand-sized particles fall through the standing column of
water, a greater drag is exerted on the smaller sand-sized particles due to
the higher surface area-to-weight ratio. As a result, coarser particles
fall more quickly through the column of water and reach the annular space
between the well intake and borehole wall first. The coarser particles may
therefore comprise the bottom portion of the filter pack, and the
smaller-sized particles may comprise the upper portion of each segment of
filter pack emplaced. Driscoll (1986) states that segregation may not be a
significant problem when emplacing uniform grain size, well-sorted filter
packs with a uniformity coefficient of 2.5 or less. However, graded filter
packs are more susceptible to segregation problems, and this could result
in the well consistently producing water samples with suspended sediment.
Potential bridging problems or segregation of graded filter packs may
be minimized by using a tremie pipe to convey and emplace the filter pack.
The use of a tremie pipe may be particularly important where the static
water level between the well casing and auger column is shallow. Schmidt
(1986) has suggested that at depths greater than SO feet, a tremie pipe
should be used to convey and emplace filter pack through hollow-stem
augers. A tremie pipe is a hollow, thin-walled, rigid tube or pipe which
is commonly fabricated by connecting individual lengths of threaded, flush-
joint pipe. The tremie pipe should have a sufficient diameter to allow
passage of the filter pack through the pipe'. The inside diameter of a
tremie pipe used for filter pack emplacement is typically 1 1/2 inches or
greater to minimize potential bridging problems inside the tremie.
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Emplacement of the filter pack begins by lowering a measuring rod or
weighted measuring tape to the bottom of the borehole, as previously
described in the free fall method of filter pack emplacement. The auger
column commonly is retracted 1 to 2 feet, and the tremie pipe is lowered to
the bottom of the borehole through the working space between the well
casing and auger column (Figure 20a). A measured portion of the pre-
calculated volume of filter pack is slowly poured down the tremie and the
tremie is slowly raised as the filter pack discharges from the bottom of
the pipe, filling the annular space between the well intake and borehole
wall (Figure 20b). Once the filter pack is emplaced to the bottom of the
auger column, the augers are retracted another 1 to 2 feet and a second
measured portion of the filter pack is added through the tremie pipe. This
alternating sequence of auger column retraction followed by additional
filter pack emplacement is continued until the required length of filter
pack is installed. Similar to the free fall method of filter pack
emplacement, careful measurements usually are taken and recorded for each
increment of filter pack which is added and emplaced.
During filter pack emplacement, whether by free fall or tremie
methods, the auger column may be retracted from the borehole in one of two
ways (C. Harris, John Matties and Associates, personal communication, 1987).
One method of retracting the augers is to use the drive cap to connect the
auger column to the drill head. The drill head then pulls back the auger
column from the borehole. This technique, however, commonly requires the
measuring rod, weighted measuring tape or tremie pipe (if used) to be
removed from the working space between the wall casing and auger column
each time the auger column is retracted. A second method of retracting the
augers is to hook a winch line onto the outside of the open top of the
auger column. The winch line is then used to pull the augers back. The
use of a winch line to pull the auger column from the borehole enables the
measuring rod, weighted measuring tape or tremie pipe to remain in the
working space between the well casing and auger column as the augers are
retracted. This latter auger retraction technique may provide greater
continuity between measurements taken during each increment of filter pack
emplacement. Retracting the auger column with the winch line can also
permit the option of adding filter pack while the auger column is
simultaneously withdrawn from the borehole. Bridging problems, which lock
the well casing and augers together and cause the casing to pull out of the
borehole along with the augers, may also be more readily detected when the
auger column is retracted by using a winch line. The use of a winch line,
however, may pull the auger column off center. If the auger column is
pulled off center, there may be an increased potential for the casing to
become wedged within the augers.
When the formation materials adjacent to the well intake are
noncohesive and the borehole will not remain open as the auger column is
retracted, the method for installing the filter pack may require the use of
clean water (C. Harris, John Mathes and Associates, personal communication,
1987). Similar to the other methods of filter pack emplacement, a
measuring rod or weighted measuring tape is first lowered to the bottom of
the borehole through the working space between the well casing and auger
column. Clean water is then added to the working space between the casing
and augers to maintain a positive pressure head in the auger column. As
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Weighted
Measuring Tape
C
Plan View
Weighted - ]
Measuring Tape
C - - -
Auger Column
Retracted
1 to 2 Feet
from Borehole
Well Casing
- -C'
Tremie Pipe
Tremie Pipe
Positioned to
Bottom of
Borehole
—C1
01
Cross-Sectional View
a. Weighted Measuring Tape and
Tremit Pip* in Retracted
Auger Column
Weighted -—_
Measuring Tape
Filter
Material Poureo
Down Tremie
Tremie Pipe
Slowly Raised as
Filter PacK Is
Poured
Filter PacK -J
b. Filter Pack Poured Through Bottom-
Discharge Tremie Pipe
Figure 20. Tremie method of filter pack emplacement with a hollow-stem auger.
314
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the auger column is slowly retracted using a winch line, a measured portion
of the precalculated volume of filter pack is poured down the working space
between the well casing and auger column. The head of clean water in the
working space between the casing and augers usually holds the borehole open
while the filter pack material is emplaced in the annular space between the
well intake and borehole wall. This procedure of slowly retracting the
auger column with a winch line while filter pack material is poured through
a positive pressure head of clean water in the working space continues
until the required length of filter pack is installed. Once again,
measurements of the emplaced filter pack usually are taken and recorded along
with calculations of "filter pack needed" versus "filter pack used."
If the formation materials adjacent to the well intake are noncohesive
and comprised of coarse-grained sediments, an artificial filter pack may
not have to be installed. The natural coarse-grained sediments from the
formation may instead be allowed to collapse around the well intake (with
appropriately sized openings) as the auger column is retracted from the
borehole. This procedure initially involves retracting the auger column 1
to 2 feet. A measuring rod or weighted measuring tape is then lowered
through the working space between the auger column and casing to verify the
collapse of formation material around the well intake and to measure the
depth to the top of the "caved" materials. Once the formation materials
collapse around the well intake and fill the borehole beneath the auger
column, the augers are retracted another 1 to 2 feet. This alternating
sequence of retracting the auger column and verifying the collapse of
formation materials by measuring the depth to the top of the caved
materials continues until the coarse-grained sediments extend to a desired
height above the top of the well intake. The finer-grained fraction of the
collapsed formation materials is later removed from the area adjacent to
the well intake during well development.
Installation of the Annular Seal
Once the well intake, well casing and filter pack are installed
through the hollow axis of the auger column, tha final phase of monitoring
well construction typically involves the installation of an annular seal.
The annular seal is constructed by eraplacing a stable, low permeability
material in the annular space between the well casing and borehole wall
(Figure 16). The sealant is commonly bentonite, expanding neat cement or a
cement-bentonite mixture. The annular seal typically extends from the top
of the filter pack to the bottom of the surface seal. The annular seal
provides: 1) protection against the movement of surface water or
near-surface contaminants down the casing-borehole annulus; 2) isolation of
discrete sampling zones; and 3) prevention of the vertical movement of
water in the casing-borehole annulus and the cross-contamination of strata.
An effective annular seal requires that the casing-borehole annulus be
completely filled with a sealant and that the physical integrity of the
seal be maintained throughout the life of the monitoring well. The sealant
should ideally be chemically nonreactive to minimize any potential impact
the sealant may have on the quality of ground-water samples collected from
the completed monitoring well.
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Although bentonite and cement are the two most widely used annular
sealants for monitoring wells, these materials have the potential for
affecting the quality of ground-water samples. Bentonite has a high cation
exchange capacity and may have an appreciable impact on the chemistry of
the collected ground-water samples, particularly when the bentonite seal is
in close proximity to the well intake (Gibb, 1987). Hydrated cement is
highly alkaline and may cause persistent, elevated pH values in
ground-water samples when the cement seal is near or adjacent to the well
intake (Dunbar et al., 1985). Raising the pH of the ground water may
further alter the solubility and presence of other constituents in the
ground-water samples.
An adequate distance between the well intake and the annular sealant
is typically provided when the filter pack is extended 2 feet above the top
of the well intake. Bentonite pellets are commonly emplaced on top of the
filter pack in the saturated zone (United States Environmental Protection
Agency, 1986). Water in the saturated zone hydrates and expands the
bentonite pellets thereby forming a seal in the casing-borehole annulus
above the filter pack. The use of bentonite pellets directly on top of the
filter pack generally is preferred because the pellet form of bentonite may
minimize the threat of the bentonite infiltrating the filter pack. United
States Environmental Protection Agency (1986) recommends that there be a
minimum 2-foot height of bentonite pellets in the casing-borehole annulus
above the filter pack. The bentonite pellets, however, should not extend
above the saturated zone.
Bentonite pellets are emplaced through the hollow-stem augers by free
fall of the pellets through the working space between the well casing and
auger column. Prior to emplacing the bentonite pellets, the theoretical
volume of bentonite pellets needed to fill the annular space between the
well casing and borehole wall over the intended length of the seal is
determined (see section on Installation of the Filter Pack for a discussion
on how to calculate the theoretical volume of material needed). A
measuring rod or weighted measuring tape is lowered to the top of the
filter pack through the working space between the casing and augers. A
depth measurement is taken and recorded. The auger column is then
retracted 1 or 2 feet from the borehole and a measured portion of the
precalculated volume of bentonite pellets is slowly poured down the working
space between the well casing and auger column. In some instances, the
bentonite pellets may be individually dropped, rather than poured, down
this working space. The bentonite pellets free fall through the working
space between the casing and augers and fill the annular space between the
well casing and borehole wall immediately above the filter pack. As the
bentonite pellets are being added, the measuring rod or weighted measuring
tape is slowly raised and lowered to lightly tamp the pellets in place and
to measure the depth of emplacement of the bentonite pellets. Once the
bentonite pellets are emplaced to the bottom of the auger column, the
augers are again retracted 1 or 2 feet from the borehole and more bentonite
pellets are added. This procedure continues until the bentonite pellets
are installed to the required height above the filter pack. Actual depth
measurements of the emplaced pellets are recorded and compared with the
calculations for the volume of "bentonite pellets needed" versus "bentonite
pellets used."
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The free fall of bentonite pellets through the working space between
the well casing and auger column provides the opportunity for bridging
problems to occur. Bridging problems are likely to occur particularly when
the static water level in the working space is shallow and the well is
relatively deep. As bentonite pellets fall through a column of standing
water, the bentonite on the outer surface of the pellet starts to hydrate
and the pellet surface expands and becomes sticky. Individual bentonite
pellets may begin sticking to the inside wall of the auger column or to the
outer surface of the well casing after having fallen only a few feet
through a column of water between the casing and augers. Bentonite pellets
may also stick together and bridge the working space between the casing and
augers. As a result, the pellets may not reach the intended depth for
proper annular seal emplacement. The bentonite pellets will continue to
expand as the bentonite fully hydrates. An expanding bridge of bentonite
pellets in the working space may eventually lock the well casing and auger
column together causing the casing to pull back out of the borehole as the
auger column is retracted.
Careful installation techniques can minimize the bridging of bentonite
pellets in the working space between the casing and augers. These
techniques include: 1) adequately sizing the working space between the
well casing and auger column; 2) slowly adding individual bentonite pellets
through the working space; and 3) frequently raising and lowering the
measuring device to break up potential bridges of pellets. Driscoll (1986)
reports that freezing the bentonite pellets or cooling the pellets with
liquid nitrogen to form an icy outer coating may enable the bentonite
pellets to free fall a greater depth through standing water before
hydration of the pellets begins. The frozen bentonite pellets should,
however, be added individually in the working space between the casing and
augers to avoid clumping of the frozen pellets as they contact the standing
water in the working space.
The potential problem of bentonite pellets bridging the working space
between the well casing and auger column may be avoided by using instead a
bentonite slurry, neat cement grout or cement-bentonite mixture pumped
directly into the annular space between the well casing and borehole wall
in the saturated zone. In the unsaturated zone, neat cement grout or a
cement-bentonite mixture commonly is used as the annular sealant. In
either instance, the slurry is pumped under positive pressure through a
tremie pipe which is first lowered through the working space between the
well casing and auger column. However, tremie emplacement of a bentonite
slurry or cement-based grout directly on top of the filter pack is not
recommended because these slurry mixtures may easily infiltrate into the
filter pack. Ramsey et al., (1982) recommend that a 1 to 2-foot thick fine
sand layer be placed on top of the filter pack prior to emplacement of the
bentonite slurry or cement grout. The fine sand layer minimizes the
potential for the grout slurry to infiltrate into the filter pack. If
bentonite pellets are initially emplaced on top of the filter pack, prior
to the addition of a bentonite slurry or cement-based grout, the pellets
serve the same purpose as the fine sand and minimize the potential for the
infiltration of the grout slurry into the filter pack. When bentonite
pellets are used, a suitable hydration period, as recommended by the
manufacturer, should be allowed prior to the placement of the grout slurry.
317
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Failure to allow the bentonite pellets to fully hydrate and seal the
annular space above the filter pack may result in the grout slurry
infiltrating into the filter pack.
A side-discharge tremie pipe, rather than a bottom-discharge tremie
pipe, should be used to emplace bentonite slurry or cement-based grouts
above the filter pack. A side-discharge tremie may be fabricated by
plugging the bottom end of the pipe and drilling 2 or 3 holes in the lower
1-foot section of the tremie. The pumped slurry will discharge laterally
from the tremie and dissipate any fluid-pumping energy against the borehole
wall and well casing. This eliminates discharging the pumped slurry
directly downward toward the filter pack and minimizes the potential for
the sealant to infiltrate into the filter pack.
Prior to emplacing a bentonite slurry or cement-based grout via the
tremie method, the theoretical volume of slurry needed to fill the annular
space between the well casing and borehole wall over the intended length of
the annular seal is determined (see section on Installation of the Filter
Pack for a discussion on how to calculate the theoretical volume of
material needed). An additional volume of annular sealant should be
prepared and readily available at the drill site to use if a discrepancy
occurs between the volume of "annular sealant needed" versus "annular
sealant used." The installation of the annular sealant should be completed
in one continuous operation which permits the emplacement of the entire
annular seal.
The procedure for emplacing a bentonite slurry or cement-based grout
with a tremie pipe begins by lowering a measuring rod or weighted measuring
tape through the working space between the well casing and auger column. A
measurement of the depth to the top of the fine sand layer or bentonite
pellet seal above the filter pack is taken and recorded. The auger column
is commonly retracted 2 1/2 to 5 feet and a side-discharge tremie pipe,
with a minimum 1-inch inside diameter, is lowered through the working space
between the casing and augers. The bottom of the tremie is positioned
above the fine sand layer or bentonite pellet seal. A measured portion of
the precalculated volume of bentonite slurry or cement-based grout is
pumped through the tremie. The grout slurry discharges from the side of
the pipe, filling the annular space between the well casing and borehole
wall. As the grout slurry is pumped through the tremie, the measuring rod
or weighted measuring tape is slowly raised and lowered to detect and
measure the depth of slurry emplacement. Once the slurry is emplaced to
the bottom of the auger column, the augers are retracted another 2 1/2 to 5
feet. If the auger column is retracted by using a winch line, the
measuring rod or tape and tremie pipe may remain inside the working space
between the casing and augers as the augers are pulled back from the
borehole. Retracting the auger column with the winch line may also permit
the option of pumping the grout slurry through the tremie while the auger
column is simultaneously withdrawn from the borehole. A quick-disconnect
fitting can be used to attach the grout hose to the top of the tremie pipe.
This fitting allows the grout hose to be easily detached from the tremie as
individual 5-foot auger sections are disconnected from the top of the auger
column. By successively retracting the auger column and pumping the
bentonite slurry or cement-based grout into the annular space between the
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well casing and borehole wall, the annular sealant is emplaced from the
bottom of the annular space to the top. The tremie pipe can be moved
upward as the slurry is emplaced or it can be left in place at the bottom
of the annulus until the annular seal is emplaced to the required height.
Measurements of the depths of the emplaced annular seal are taken and
recorded. Calculations of the theoretical volume of "annular sealant
needed" versus "annular sealant used" should also be recorded, and any
discrepancies should be explained.
SUMMARY
Hollow-stem augers, like all drilling methods, have advantages and
limitations for drilling and constructing monitoring wells. Advantages of
using hollow-stem auger drilling equipment include: 1) the mobility of the
drilling rig; 2) the versatility of multi-purpose rigs for auger drilling,
rotary drilling and core drilling; 3) the ability to emplace well casing
and intake, filter pack and annular seal material through the hollow-stem
auger; and 4) the utility of the hollow-stem auger for collecting
representative or relatively undisturbed samples of the formation. Other
advantages associated with hollow-stem augers relate to the drilling
procedure and include: 1) relatively fast advancement of the borehole in
unconsolidated deposits; 2) minimal formation damage in sands and gravels;
3) minimal, if any, use of drilling fluids in the borehole; and 4) good
control or containment of cuttings exiting from the borehole. Limitations
of the drilling procedure include: 1) the inability to drill through hard
rock or deposits with boulders; 2) smearing of the silts and clays along
the borehole wall; 3) a variable maximum drilling depth capability, which
is typically less than 150 feet for most rigs; and 4) a variable borehole
diameter.
The drilling techniques used to advance a borehole with hollow-stem
augers may vary when drilling in the unsaturated versus the saturated zone.
In the unsaturated zone, drilling fluids are rarely, if ever, used.
However, in a saturated zone in which heaving sands occur, changes in
equipment and drilling techniques are required to provide a positive
pressure head of water within the auger column. This may require the
addition of clean water or other drilling fluid inside the augers. If a
positive pressure head of water cannot be maintained inside the auger
column when drilling in heaving sands, the heaving sands may represent a
limitation to the use of hollow-stem augers for the installation of a
monitoring well.
The vertical movement of contaminants in the borehole may be a concern
when drilling with hollow-stem augers. When monitoring the quality of
ground water below a known contaminated zone, hollow-stem auger drilling
may not be advisable unless protective surface casing can be installed.
Depending on the site hydrogeology, conventional hollow-stem auger drilling
techniques alone may not be adequate for the installation of the protective
surface casing. A hybrid drilling method may be needed which combines
conventional hollow-stem auger drilling with a casing driving technique
that advances the borehole and surface casing simultaneously.
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The procedure used to construct monitoring wells with hollow-stem
augers may vary significantly depending on the hydrogeologic conditions at
the drill site. In cohesive materials where the borehole stands open, the
auger column may be fully retracted from the borehole prior to the
installation of the monitoring well. In noncohesive materials in which the
borehole will not remain open, the monitoring well is generally constructed
through the hollow axis of the auger column.
The procedures used to construct monitoring wells inside the
hollow-stem augers may also vary depending on specific site conditions and
the experience of the driller. The proper emplacement of the filter pack
and annular seal can be difficult, or impossible, if an inadequate working
space is available between the well casing and hollow-stem auger. An
adequate working space can be made available by using an
appropriately-sized diameter hollow-stem auger for the installation of the
required size well casing and intake. The maximum diameter of a monitoring
well constructed through the hollow-stem auger of the larger diameter
augers now commonly available will typically be limited to 4 inches or
less. Assurance that the filter pack and annular seal are properly
emplaced is typically limited to careful measurements taken and recorded
during construction of the monitoring well.
320
-------
REFERENCES
Central Mine and Equipment Company, 1987. Catalog of product literature;
St. Louis, Missouri, 12 pp.
Diedrich Drilling Equipment, 1986. Catalog of product literature; LaPorte,
Indiana, 106 pp.
Driscoll, Fletcher G., 1986. Groundwater and Wells; Johnson Division, St.
Paul, Minnesota, 1089 pp.
Dunbar, Dave, Hal Tuchfeld, Randy Siegel and Rebecca Sterbentz, 1985.
Ground water quality anomalies encountered during well construction,
sampling and analysis in the environs of a hazardous waste management
facility; Ground Water Monitoring Review, vol. 5, no. 3, pp. 70-74.
Gass, Tyler E., 1984. Methodology for monitoring wells; Water Well
Journal, vol. 38, no. 6, pp. 30-31.
Gibb, James P., 1987. How drilling fluids and grouting materials affect
the integrity of ground water samples from monitoring wells, opinion I;
Ground Water Monitoring Review, vol. 7, no. 1, pp. 33-35.
Gillham, R.W., M.L. Robin, J.F. Barker and J.A. Cherry, 1983. Groundwater
monitoring and sample bias; API Publication 4367, Environmental Affairs
Department, American Petroleum Institute, Washington D.C., 206 pp.
Hackett, Glen, 1987. Drilling and constructing monitoring wells with
hollow-stem augers, part I: drilling considerations; Ground Water
Monitoring Review, vol. 7, no. 4, pp. 51-62.
Hackett, Glen, 1988. Drilling and constructing monitoring wells with
hollow-stem augers, part II: monitoring well installation; Ground Water
Monitoring Review, vol. 8, no. 1, pp. 60-68.
Keely, Joseph F. and Kwasi Boateng, 1987a. Monitoring well installation,
purging and sampling techniques part 1: conceptualizations; Ground Water,
vol. 25, no. 3, pp. 300-313.
Keely, Joseph F. and Kwasi Boateng, 1987b. Monitoring well installation,
purging, and sampling techniques part 2: case histories; Ground Water,
vol. 25, no. 4, pp. 427-439.
321
-------
McCray, Kevin B., 1986. Results of survey of monitoring well practices
among ground water professionals; Ground Water Monitoring Review, vol. 6,
no. 4, pp. 37-38.
Minning, Robert C., 1982. Monitoring well design and installation;
Proceedings of the Second National Symposium on Aquifer Restoration and
Ground Water Monitoring; Columbus, Ohio, pp. 194-197.
Mobile Drilling Co., 1982. Auger tools and accessories product literature;
Indianapolis, Indiana, 26 pp.
Mobile Drilling Company, 1983. Mobile drill product catalog; Indianapolis,
Indiana,37 pp.
Perry, Charles A. and Robert J. Hart, 1985. Installation of observation
wells on hazardous waste sites in Kansas using a hollow-stem auger; Ground
Water Monitoring Review, vol. 5, no. 4, pp. 70-73.
Ramsey, Robert J., James M. Montgomery and George E. Maddox, 1982.
Monitoring ground-water contamination in Spokane County, Washington;
Proceedings of the Second National Symposium on Aquifer Restoration and
Ground Water Monitoring, Columbus, Ohio, pp. 198-204.
Richter, Henry R. and Michael G. Collentine, 1983. Will my monitoring
wells survive down there?: design and installation techniques for
hazardous waste studies; Proceedings of the Third National Symposium on
Aquifer Restoration and Ground Water Monitoring, Columbus, Ohio, pp
223-229.
Riggs, Charles 0., 1983. Soil Sampling in the vadose zone; Proceedings of
the NWWA/U.S. EPA Conference on Characterization and Monitoring of the
Vadose (Unsaturated) Zone, Las Vegas, Nevada, pp. 611-622.
Riggs, Charles 0., 1986. Exploration for deep foundation analyses;
Proceedings of the International Conference on Deep Foundations, Beijing,
China, volume II, China Building Industry Press, Beijing, China, pp.
146-161.
Riggs, Charles 0., 1987. Drilling methods and installation technology for
RCRA monitoring wells; RCRA Ground Water Monitoring Enforcement: Use of
the TEGD and COG, RCRA Enforcement Division, Office of Waste Programs
Enforcement, United States Environmental Protection Agency, pp. 13-39.
Riggs, Charles 0., and Allen W. Hatheway, 1988. Groundwater monitoring
field practice - an overview; Ground-Water Contamination Field Methods,
Collins and Johnston editors, ASTM Publication Code Number 04-963000-38,
Philadelphia, Pennsylvania, pp. 121-136.
Scalf, M.R., J.F. McNabb, W.J. Dunlap, R.L. Cosby and J. Fryberger, 1981.
Manual of ground-water sampling procedures; National Water Well Assocation,
93 pp.
322
-------
Schmidt, Kenneth D., 1986. Monitoring well drilling and sampling in
alluvial basins in arid lands; Proceedings of the Conference on
Southwestern Ground Water Issues, Tempe, Arizona, National Water Well
Association, Dublin, Ohio, pp. 443-455.
United States Environmental Protection Agency, 1986. RCRA Ground-water
monitoring technical enforcement guidance document; Office of Waste
Programs Enforcement, Office of Solid Waste and Emergency Response,
OSWER-99501.1, United States Environmental Protection Agency, 317 pp.
323
-------
APPENDIX B
MATRICES FOR SELECTING APPROPRIATE
DRILLING EQUIPMENT
The most appropriate drilling technology for use at a specific site
can only be determined by evaluating both the hydrogeologic setting and the
objectives of the monitoring program. The matrices presented here were
developed to assist the user in choosing an appropriate drilling
technology. These matrices address the most prevalent hydrogeologic
settings where monitoring wells are installed and encompass the drilling
technologies most often applied. The matrices have been developed to act
as guidelines; however, because they are subjective, the user is invited to
make site-specific modifications. Prior to using these matrices, the
prospective user should review the portion in Section 4 entitled "Selection
of Drilling Methods for Monitoring Well Installation."
Several general assumptions were used during development of the
matrices. These are detailed below:
1) Solid-flight auger and hollow-stem auger drilling techniques are
limited to a practical drilling depth of 150 feet in most areas
based on the equipment generally available;
2) Formation samples collected:
a) during drilling with air rotary, air rotary with casing hammer
and dual-wall air rotary techniques are assumed to be from
surface discharge of the circulated sample;
b) during drilling with solid-flight augers, hollow-stem augers,
mud rotary or cable tool techniques are assumed to be taken by
standard split-spoon (ASTM D1586) or thin-wall (ASTM D1587)
sampling techniques to a depth of 150 feet at 5-foot
intervals;
c) below 150 feet, during mud rotary drilling are assumed to be
circulated samples taken from the drilling mud at the surface
discharge and
d) below 150 feet, during cable-tool drilling are assumed to be
taken by bailer.
324
-------
If differing sampling methodologies are employed, the ratings for
reliability of samples, cost and time need to be re-evaluated.
(Wireline or piston sampling methods are available for use with
several drilling techniques; however, these methods were not
included in the development of the matrices);
3) Except for wells installed using driving and jetting techniques,
the borehole is considered to be no less than 4 inches larger in
diameter than the nominal diameter of the casing and screen used
to complete the well (e.g. a minimum 6-inch borehole is necessary
for completion of a 2-inch diameter cased well);
4) Artificial filter pack installation is assumed in all completions
except for wells installed using driving and jetting techniques;
5) The development of ratings in the matrices is based on the
largest expressed casing diameter in each range listed in the
"General Hydrogeologic Conditions & Well Design Requirements"
statement;
6) For purposes of the "General Hydrogeologic Conditions & Well
Design Requirements," air is not considered as a drilling fluid
and
7) In the development of the dual-wall rotary technique ratings in
the matrices, air is considered to be the circulation medium.
Each applicable drilling method that can b« used in the described
hydrogeologic setting and with the stated specific design requirements has
been evaluated on a scale of 1 to 10 with respect to the criteria listed in
the matrix. A total number for each drilling method was computed by adding
the scores for the various criteria. The totals represent a relative
indication of the desirability of drilling methods for the specified
conditions.
325
-------
INDEX TO MATRICES 1 THROUGH 40
Matrix
Numbw
i
! U
I
3
A
1
2
3
4
s
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
2?
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
I «
I *
326
-------
MATRIX NUMBER 1
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated. saturated: invasion of formation by drilling fluid permitted; casing diameter 2 inches or less: total
well depth 0 to 15 feet.
\ §8
\ P ^
\ * 5
\ 3 "*
\ ^
\ W2
\ |
\s°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Walt Rotary
Cable Tool
Q
£
I
I
8
"o
>>
S
1
1
1
2
3
10
3
NA
7
7
9
I
a
1
i
1
5
1
1
4
10
10
NA
5
a
10
c
S
Q.
3
PT
I
o
i
1
9
10
8
7
9
8
NA
6
6
5
UJ
c
O
"5
>
i
ii
<
10
10
10
9
9
10
NA
4
1
7
=
H —
* S
— E
ll
!!
1- 0
5 2
II
5
5
5
10
10
7
NA
6
G
4
o
>.
?£
fa
r "6
8c
»-6
11
||
11
||
9
5
1
4
8
4
NA
9
9
10
5
i
t
03
Q
1
i Install Desi
0
ii
6
'
1
5
10
10
NA
10
10
10
5
3!
E
u
Ease ol Wei
jlopmenl
{j
• B
x <5
4
4
1
2
9
5
NA
10
9
10
TOTAL
49
37
29
44
75
62
NA
57
56
65
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
4 The shallow depth of up to IS feet, and small completed well diameter of 2 inches or less allows maximum flexibility in equipment
327
-------
MATRIX NUMBER 2
General Hydrogeoiogte Conditions & Well Design Requirements
Unconsohdated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less; total
well depth 15 to 150 feet.
\ P
\ Is
\ £
\ O j
\ i*
\ £°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
•a
1
?
i
o
2-
Versallll
NA
1
1
3
8
10
NA
8
10
9
i
S
i
o
1
NA
1
1
3
9
10
NA
S
8
10
S
u
01
I
1 Relative
NA
4
S
2
8
10
NA
7
7
5
c
01
I
3
CT
IU
O)
c
I
~o
X
Availabi
NA
10
10
9
9
10
NA
4
1
7
f !
o g
If
f|
li
II
NA
1
3
7
8
9
NA
9
10
5
0
X
11
of
O
II
§1
ll
NA
S
1
4
8
4
NA
8
9
10
1
9
I
u
c
O)
i
Q
i
1
2-1
11
NA
1
1
3
8
10
NA
10
10
10
i
I
o
€
•i
UJo
Relative
and Oev
NA
7
1
1
9
4
NA
9
8
10
TOTAL
NA
30
23
37
67
67
NA
60
63
66
EXPLANATORY NOTES:
1 Uneonsolidated formations, predominantly saturated, wtth saturation exerting significant influence on the choice of drilling
technology
2. Borehole stability proWema are potentially severe.
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
4 As the depth increases from IS to 150feet, the limit of hollow-stem auger equipment is approached The actual limit vanes with
geologic conditions, specific equipment capability and borehole size (both outside diameter and inside diameter) requirements.
Hollow-stem auger techniques are favored for shallower depths, with mud rotary being favored as the depth men
5 Where dual-watt air techniques are used, completion is through the bit.
328
-------
MATRIX NUMBER 3
General Hydrageotogte Condition* ft Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less, total
well depth greater than 150 feet.
\ if
\ ^a
\ H
\ ®d
\ ^1
\ OC 11
\ Ul S
\i
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
«>
o>
c
Q
'o
1
1
NA
NA
NA
NA
NA
10
NA
8
10
9
1
"5
C
1
1
NA
NA
NA
NA
NA
1
NA
6
10
8
1
\j
o>
c
i
i
i
NA
NA
NA
NA
NA
10
NA
5
8
5
i
|
HI
o>
c
Q
o
i
I
NA
NA
NA
NA
NA
10
NA
4
1
7
2 6
if
Q)
§1
ff
l|
ll
NA
NA
NA
NA
NA
9
NA
7
10
4
0
||
|!
||
ii
'o $
ii
NA
NA
NA
NA
NA
5
NA
10
10
9
w
Q
C
-------
MATRIX NUMBER 4
General Hydrogeologte Conditions & Well Design Requirements
Unconsohdated: saturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches total
well depth 0 to 15 feet.
\ PS
\ 1
\°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
i
I
Q
"5
CB
i
NA
1
2
1
8
7
NA
8
10
10
pie Reliability
E
a
NA
1
1
4
10
10
NA
5
8
10
i
o>
0
i
S
I
NA
10
8
7
7
7
NA
6
S
5
c
1
ability ol Drilling
•B
CD
I
NA
10
10
9
9
10
NA
4
1
7
_
if
live Time Requin
illation and Deve
S S
£1
NA
5
5
10
10
7
NA
6
6
4
O
fl
ly ol Drilling Tec
srve Natural Cor
*2 O
a S
NA
5
2
4
8
4
NA
9
10
9
5
a>
5
c
O)
I
1
o
S >
ll
NA
1
1
4
8
10
NA
10
8
10
c
i
o
o
ive Ease of Well <
tevelopment
s
11
NA
4
1
2
8
5
NA
10
8
10
TOTAL
NA
37
30
41
68
60
NA
58
58
65
EXPLANATORY NOTES:
1. Unconsohdated formations, predominantly saturated, with saturation eienmg significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitonng well permits the use of drilling fluid and Mditives in construction.
4 Four-inch casing diameter limits technique choices even though aeoins are mallow (15 feet or less). Large diameter (1.0.)
hollow-stem augers required. Solid flight augers require open-hole completion m potentially unstable materials
330
-------
MATRIX NUMBER 5
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches- total
well depth 15 to 150 feet.
\ IS
\ ^T
\ *^ ^
\ ^S
\ «i
\ §3
\ ^ 5
\ |s
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
^
S
i
"o
2-
3
1
NA
1
1
3
5
10
NA
a
10
9
i
IT
t
i
NA
1
1
3
10
10
NA
5
8
10
8
I
Q
I
i
NA
2
3
2
8
10
NA
5
8
5
i
£
Q.
5
S
o>
c
Q
O
a
CO
1
NA
10
10
9
9
10
NA
4
1
7
0) _
o g
i!
II
Q) LJ
oc-o
So
E *
£ o
S^
_s
3£
Q} VI
NA
1
3
7
9
00
NA
9
8
5
o
X
II
II
u c
S o
i-O
o>—
s s
||
O ®
il
NA
5
1
4
8
4
NA
9
10
9
V
V
s
Q
1
Cfl
i
ti
o
11
NA
1
1
2
5
10
NA
10
8
10
o
••
Q.
5
1-
* 72
o|
ID O
Ul jj
4) £
-S
li
NA
4
1
2
5
S
NA
10
8
10
TOTAL
NA
25
21
32
59
69
NA
60
61
65
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction
4 Four-inch casing diameter limits technique choice even though depths are 15 to 150 feet Large diameter (I.D.) hollow-stems are
required. Solid flight augers require open-hole completion in potentially unstable matenals.
5. With increasing depth, mud rotary, dual-wall rotary and cable tool techniques become favored.
331
-------
MATRIX NUMBER 6
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches; total
well depth greater than 150 feet.
\ l§
\ ls=
\ 2
\ "" E
\5
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•o
o
ly of Drilling Me
ai
>
NA
NA
NA
NA
NA
10
NA
7
10
9
Reliability
0)
1
= 1
II
NA
£
i
Q
1
s
"a
1
o
il
< 0
NA
NA NA
NA
NA
NA
6
NA
9
10
9
NA
NA
NA
10
NA
10
7
10
S
I
E
Ease of Well Cc
•elopment
11
•jS-o
V c
CC ID
NA
NA
TOTAL
NA
NA
'
NA
NA
NA
6
NA
10
8
10
NA
NA
NA
66
NA
60
58
60
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2 Borehole stability problems are potentially severe.
3. The anticipated use of the monitonng well permits the use of drilling fluid and additives in construction
4 Four-inch casing diameter and depths greater than 150 feet limit technique choices
5 With increasing depth, mud rotary, dual-wall rotary and cable tool techniques become favored
332
-------
MATRIX NUMBER 7
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated: saturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches; total
well depth 0 to 15 feet.
\ P°
\ 3C
\ ^1
\ |
\ *
\°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
1
i
itility of Drilling
*
NA
NA
NA
NA
NA
10
NA
8
NA
8
pie Reliability
I
NA
NA
NA
NA
NA
10
NA
8
NA
10
CA
live Drilling Co
CD
0
oc
NA
NA
NA
NA
NA
10
NA
6
NA
4
c
a
5
S
a
^
lability of Drillu
2
NA
NA
NA
NA
NA
10
NA
7
NA
7
=
?!
if
w m
live Time Requi
Nation and Dev
« a
II
NA
NA
NA
NA
NA
8
NA
10
NA
4
o
ll
ll
« s
ly of Drilling Te
srve Natural Cc
- 2
Si
NA
NA
NA
NA
NA
6
NA
10
NA
8
5
CD
5
§,
y to Install Des
all
I!
NA
NA
NA
NA
NA
10
NA
10
NA
10
|
I
1
ive Ease of Wei
tevelopment
il
NA
NA
NA
NA
NA
3
NA
10
NA
10
TOTAL
NA
NA
NA
NA
NA
67
NA
69
NA
61
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well permits the use of dnlling fluid and additives in construction
4. Casing diameter 4 to 8 inches requires up to 12-inch borehole size and eliminates all techniques except mud rotary, cable tool and
air rotary with casing hammer (that can usually dnve large O.D. casing to shallow depth).
333
-------
MATRIX NUMBER 8
General Hydrogeotogte Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches; total
well depth 15 to 150 feet.
\ 2g
\ 5 ^
\ ^ "^
\ ^ ^
\ "i
\ 13
\ if
\
NA
NA
NA
NA
NA
10
NA
NA
NA
7
=
5 c
a £
it
5 g
m C
D 2
C *•
'§
II
if
«
2
M
o
ll
NA
NA
NA
NA
NA
10
NA
NA
NA
10
i
5
a
E
O
I-
0»
« a
IB O
ui "5
U
ll
ac a
NA
NA
NA
NA
NA
3
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
67
NA
NA
NA
67
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
4 Casing diameter 4 to 8 inches requires up to 12-inch borehole and eliminates all techniques except mud rotary and cable tool
334
-------
MATRIX NUMBER 9
General HydrogeoJogfe Conditions & Well Design Requirements
Unconsolidated: saturated: invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches; total
well depth greater than 150 feet.
\ p§
\ ^ ^
\ 3i
\ m O
\ o|
X III JK
\ r* iQ
\§
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
5
01
c
Q
"5
£
S
IB
i
NA
NA
NA
NA
NA
10
NA
NA
NA
10
X
Reliabihl
S.
i
NA
NA
NA
NA
NA
6
NA
NA
NA
10
a
1
i
^
NA
NA
NA
NA
NA
10
NA
NA
NA
4
|
e
|
in
I
o
0
2-
i
i
NA
NA
NA
NA
NA
10
NA
NA
NA
7
—
3* *"
5 i
||
|1
£ g
II
n S
"Z
I
S
Q
c
2"
£
a
1
•£.=
II
NA
NA
|
5
I
3
of
ii
ui -5
II
il
NA
NA
i
NA
NA
NA
9
NA
NA
NA
10
NA
NA
NA
6
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
66
NA
NA
NA
63
EXPLANATORY NOTES:
significant influence on the choice of drilling
1. Unconsolidated formations, predominantly saturated, with saturation
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well permits the use of drilling fluid «rx) additives in construction.
4 Casing diameter 4 to 8 inches requires up to 12-mch borehole ana ennnn*t«t an techniques except mud rotary and cable tool.
335
-------
MATRIX NUMBER 10
General Hydrogeologte Conditions & Weil Design Requirements
Unconsohdated; saturated; invasion of formation by drilling fluid not permitted; casing diameter 2 inches or less;
total well depth 0 to 15 feet
\ zo
\ "z
\ °^
\ ^
\ £o
\°
ORILUNG \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
j
i
i
<§
0
i
I
NA
1
NA
1
10
NA
NA
7
7
7
x
t;
i
s
It
Ife
I
NA
1
NA
1
10
NA
NA
7
8
10
_
o
I
O
1
TQ
i
tc
NA
10
NA
7
9
NA
NA
5
5
S
1
5
LU
Ok
c
O
o
1
CO
NA
10
Is
9 o
flj JE
3 |
01 C
ii
11
NA
S
i
NA
10
10
NA
NA
4
1
7
NA
S
10
•NA
NA
6
6
2
2
>,
If
i|
Hf3
fa
il
0|
ll
NA
S
NA
1
9
NA
NA
10
10
9
Diameter 1
§i
1
U
3
1
Q
ll
NA
1
NA
1
8
NA
NA
10
10
10
|
!
o
o
1-
"5 S
|o
||
ll
NA
4
NA
1
9
NA
NA
10
9
10
TOTAL
NA
37
NA
27
75
NA
NA
59
56
60
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on me choice of drilling
technology.
2. Borehole stability problems are potentially severe, so open-hole completion (i e., solid-flight auger) may not be possible.
3. The anticipated use of the monitoring well prohibits the use or drilling fluid and additives in construction
4 Jetting and mud rotary methods would require the addition of fluid.
S. When using cable-tool drilling in saturated formations, it is assumed that no drilling fluid needs to be added in permeable matenats
and that small volumes of drilling fluid are permissible in less permeable materials.
336
-------
MATRIX NUMBER 11
General Hydrogeotogte Conditions & Well Design Requirements
Unconsohdated; saturated: invasion of formation by drilling fluid not permitted; casing diameter 2 inches or less-
total well depth 15 to 150 feet.
\ p
\ 35]
\ w®
\ o3
\ gm
\ 1°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
£
i
f
=
Q
O
!
NA
1
NA
NA
8
NA
NA
8
10
10
9s
S
S
. 0)
o E
at
^ o
5 g
|l
li
>s
a S
£i
NA
1
NA
NA
8
NA
NA
10
8
2
o
>.
Q) |f>
s §
ll
^ §
II
11
"o 8
2 S
NA
S
NA
NA
8
NA
NA
9
9
10
£
£•
1
u
c
O)
V)
0}
Q
15
1
o
If
NA
1
NA
NA
8
NA
NA
10
10
10
§
o
a.
o
"3
"5 v
si
p
NA
7
NA
NA
7
NA
NA
10
10
10
TOTAL
NA
30
NA
NA
69
NA
NA
65
64
66
EXPLANATORY NOTES:
1. Unconsohdated formations, predominantly saturated, with saturation exerting significant influence on the choice of dnllina
technology. "
2. Borehole stability problems are potentially severe.
3 The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 As depth increases the relative advantage of hollow-stem augenng decreases.
S Jetting and mud rotary methods would require the addition of fluid
6 When using cable-tool dnlling in saturated formations, it is assumed that no drilling fluid needs to be added in permeable materials
and that small volumes of dnlling fluid are permissible in less permeable materials.
337
-------
MATRIX NUMBER 12
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid not permitted: casing diameter 2 inches or less'
total well depth greater than 150 feet.
\ z w
\ oQ
\ i£
\ *S
\ si
\ i°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Right
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
1
i
O)
c
2
5
"5
>
Versatilil
NA
NA
NA
NA
NA
NA
NA
8
10
9
£
Rehabil
1
NA
NA
NA
NA
NA
NA
NA
6
10
6
1
f
Q
Relative
NA
NA
NA
NA
NA
NA
NA
10
9
7
1
a
5
a
UJ
I
c
a
o
2-
Availabi
NA
NA
NA
NA
NA
NA
NA
7
4
10
h
w W
2 |
if
fi
li
if
NA
NA
NA
NA
NA
NA
NA
10
10
6
0
f*" «
ol
Jf
£5
<3>-=
C S
ii
<•• .».
o S
NA
NA
NA
NA
NA
NA
NA
8
10
8
o
o
5
Q
1
1
3
1
2
f!
< 0
NA
NA
NA
NA
NA
NA
NA
10
10
10
3
f"
O
i=
o S
0) 5
:
Relative
and Dev«
NA
NA
NA
NA
NA
NA
NA
10
9
10
TOTAL
NA
NA
NA
NA
NA
NA
NA
69
72
66
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction
4 Jetting and mud rotary methods would require the addition of fluid.
5. When using cable-tool dnlling in saturated formations, it is assumed that no drilling fluid needs to be added in permeable materials
and that small volumes of dnlling fluid are permissible in less permeable materials.
338
-------
MATRIX NUMBER 13
General Hydrogeologlc Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches-
total well depth 0 to 15 feet.
\ IS
\ ^ I
\ =c
\ *'
\ "§
\ K ••
\ ^ lj
\ si
\ *
\o
DRILUNG \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
T3
£
I
O)
C
Q
'5
1
$
NA
1
NA
1
10
NA
NA
9
9
10
>,
I
1 Sample Helis
NA
1
NA
1
10
NA
NA
8
a
10
to
3
Relative Dnlli
NA
10
NA
7
10
NA
NA
5
5
6
c
|
5
&
S
c
Q
Availability 01
NA
10
NA
10
10
NA
NA
4
1
7
oi
?!
B £
it
5 >
II
£*
m C
Relative Timi
Installation a
NA
5
NA
5
10
NA
NA
6
S
4 "
0
Is
f S
If
0 o
t-O
11
Ability of Dril
Preserve Nat
NA
1
NA
1
8
NA
NA
10
10
9
%
i
o
e
i
1
o
a *
< 0
NA
1
NA
1
7
NA
NA
10
8
10
c
g
S
^
>
31
Relative Ease
and Developi
NA
4
NA
2
7
NA
NA
10
8
10
TOTAL
NA
33
NA
28
72
NA
NA
62
54
66
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitonng well prohibits the use of drilling fluid and additives in construction
4 Increasing diameter n influencing choice of equipment
5. Jetting and mud rotary methods would require the addition of lluid.
6. When using cable-tool drilling m saturated formations, it is assumed that nodnlbng fluid needs to be added in permeable materials
and that small volumes of drilling fluid are permissible in less permeable materials
339
-------
MATRIX NUMBER 14
General Hydrogeotogte Conditions & Well Design Requirements
Unconsolidated; saturated: invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches-
total well depth IS to 150 feet.
\ ll
\ \l
\ <*
\ w 8
\ z
\ 2^
\ H
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
,3
£
£
*
I
Q
Versatility o
NA
1
NA
1
5
NA
NA
9
9
10
>.
1
S
1 Sample Rel
NA
1
NA
1
10
NA
NA
5
8
10
TS
0
U
O)
1 Relative Dn
NA
2
NA
2
10
NA
NA
8
8
7
c
a.
5
S
a
E
Q
"o
Availability
NA
10
NA
9
9
NA
NA
4
1
7
—
l|
o £
P-2
s ?
ll
1^
Relative Tir
Installation
NA
1
NA
3
10
NA
NA
9
9
S
0
««
•§1
c =
c P
u c
as
ll
Ability of D
Preserve Ni
NA
5
NA
4
a
NA
NA
10
10
9
5
1
5
c
i
c
o
If
NA
o
i
S
i-
o|
ll
"S c
tC a
NA
i ; 4
NA
2
6
NA
NA
10
6
10
NA
2
6
NA
NA
10
6
10
TOTAL
NA
25
NA
24
64
NA
NA
67
57
68
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly saturated, with saturation e*tnmg significant influence on the choice ol drilling
technology.
2. Borehole stability problems are potentially severe, so open-hole completion « • solid-flight auger) may not be possible.
3, The anticipated use of the monitoring well prohibits the use of drilling rimo ana additives in construction.
4 Depth range is 15 to 150 feet.
5. Increasing diameter and depth favor cable tool and air rotary with casing nammer techniques.
6. When using cable-tool drilling in saturated lormattons, it is assumed thai no anihng fluid needs to be added in permeable materials
and that small volumes of drilling fluid are permissible in less permeaine materials
340
-------
MATRIX NUMBER 15
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; saturated: invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches:
total well depth greater than 150 feet.
\ Ii
\ 3Q
\ « S
\ 3
\ "z
\ O •*
\ •*§
\ Is
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
v
5
0)
2
E
1
o
1
5
NA
NA
NA
NA
NA
NA
NA
9
10
9
2
a
-s
C
B
I
NA
NA
NA
NA
NA
NA
NA
8
10
7
o
?
Q
i
«5
£
NA
NA
NA
NA
NA
NA
NA
10
9
6
1
1
5
a-
Ul
o>
c
O
"o
S
S
5
1
NA
NA
NA
NA
NA
NA
NA
7
4
10
ic
o I
if
5 §
,Tfi
E TJ
If
if
11
NA
NA
NA
NA
NA
NA
NA
10
10
6
o
a w
o £
^ o
js
£,9
H O
O)-=
c 2
ii
"5 S
•5"«
1 S
.=
11
NA
NA
NA
NA
NA
NA
NA
10
9
10
e
o
"a
E
O
=
0 |
ll
B —
^ =
E a
NA
NA
NA
NA
NA
NA
NA
10
8
10
TOTAL
NA
NA
NA
NA
NA
NA
NA
74
70
67
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Increasing diameter and depth favor cable tool and air rotary with casing hammer techniques.
S Jetting and mud rotary methods would require the addition of fluid.
6 When using cable-tool drilling in saturated formations, it is assumed that no drilling fluid needs to be added in permeable materials
and that small volumes of drilling fluid are permissible in less permeable materials.
341
-------
MATRIX NUMBER 16
General Hydrogedoglc Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid not permitted; casing diameter 4 to 8 inches;
total well depth 0 to 15 feet
\ si
\ i?
\ i!
\ **• 5
\ -3
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
J
4>
I
i
Versatility of
NA
NA
NA
4
NA
NA
NA
8
NA
10
2
Sample Relia
NA
NA
NA
S
NA
NA
NA
8
NA
10
|
f
1 Relative Drill
NA
NA
NA
10
NA
NA
NA
8
NA
6
c
i
5
s
I
Q
Availability 01
NA
NA
NA
10
NA
NA
NA
8
NA
8
ic
\. »
il
II
*?
Relative Tinru
Installation a
NA
NA
NA
a
NA
NA
NA
8
NA
10
o
n ea
0 C
0 2
||
.= 1
Ability ot On!
Preserve Nat
NA
NA
NA
1
NA
NA
NA
8
NA
10
S
0
Q
C
I
i
Ability to Ins
Of Well
NA
NA
NA
4
NA
NA
NA
a
NA
10
c
o
i
1
o
U
of
Relative Ease
and Oevelopi
NA
NA
NA
4
NA
NA
NA
8
NA
10
TOTAL
NA
NA
NA
46
NA
NA
NA
64
NA
74
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe, so open-hole completion (i.e.. solid-flight auger) may not be possible.
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Maximum casing diameter exceeds practical equipment capability except for cable tool, air rotary with casing hammer and
possibly solid-flight augers.
5. Jetting and mud rotary methods would require the addition of fluid.
6 When using cable-tool dnllmg in saturated formations, it is assumed that no drilling fluid needs to be added m permeable materials
and that small volumes of drilling fluid are permissible m less permeable materials
342
-------
MATRIX NUMBER 17
General Hydrogeotogte Conditions A Well Design Requirements
Unconsolidated; saturated: invasion of formation by drilling fluid not permitted; casing diameter 4 to 8 inches-
total well depth 15 to 150 feet.
\ P§
\ 3fc
\ >
Rehabih
01
1
3>
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
S
o
O)
c
i
Relative
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
1
a.
5
(T
UJ
O)
S
~
Q
0
2-
Availabi
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
_
Jj ^
of
it
3 fj
K O
||
ii
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
o
at at
o c
1-
CJ C
0 0
*~ —
IS
Qz
o §
t|
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
L_
Q>
1
1
§>
i
Q
1
s_
11
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
c
o
i
§•
0
^
5 •"
si
5|
Is
f-o
_ c
O. a
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
NA
NA
NA
NA
80
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice of drilling
technology.
2. Borehole stability problems are potentially severe.
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Maximum diameter requiring 12-inch borehole exceeds practical equipment capability for depth range except tor cable tool
methods.
5 Jetting and mud rotary methods would require the addition of fluids.
6 When using cable-tool drilling in saturated formations, it is assumed that no dnllmg fluid needs to be added m permeable materials
and that small volumes of dnllmg fluid are permissible in less permeable matenals.
343
-------
MATRIX NUMBER 18
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid not permitted; casing diameter 4 to 8 inches;
total well depth greater than 150 feet.
§«
0
Tfc C X
\ § F
X etf 3
\ «z
\ oc 2
\ ~°
\5
ORIUING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
_
j^
i
at
Versatility of Drillin
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
Sample Reliability
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
*£
8
Relative Drilling Cc
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
c
01
B
Q.
5
s
9
Availability ol Drilli
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
91
3c
>. 0)
o E
If
Relative Time Reqi
Installation and Dt
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
o
ig
O
Jf
CD O
Ability ol Drilling T
Preserve Natural C
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
0
c
at
£
Ability to Install DC
ol Well
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
c
Q
5
Q.
O
?
Relative Ease of W
and Development
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
NA
NA
NA
NA
80
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly saturated, with saturation exerting significant influence on the choice ol drilling
technology.
2 Borehole stability problems are potentially severe.
3. The anticipated use ol the monitoring well prohibits the use of drilling lluid and additives in construction
4 Maximum diameter requiring 12-inch borehole exceeds practical equipment capability for depth range except for cable tool
methods.
5. Jetting and mud rotary methods would require the addition of fluids.
6. When using cable-tool drilling in saturated formations, it is assumed that no drilling fluid needs to be added in permeable materials
and that small volumes ol drilling fluid are permissible in less permeable materials.
344
-------
MATRIX NUMBER 19
General Hydrogedogte Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less
total well depth <0 to 15 feet.
V Z9
\ 2°
\ ^ ?
\ 3W
\ < S
\ ^ £3
\ lil *p
\ 8
\ Is
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
S
£
«j
2
ty of Drilling
s
4
7
3
8
10
8
5
9
9
6
Reliability
1
I
5
1
1
10
10
10
5
8
9
10
Drilling Cos
Relative
9
10
8
10
10
7
8
6
6
3
c
e
g
3
9
Ul
e
i
0
Availabil
10
10
8
9
9
10
8
4
1
7
—
% **
w i
5 1.
"S o
S«
11
tL-0
Is
l§
II
o
>>
g»s
o 2
5 5
0 C
^3
= 5
o 9
< £
5 9
6 5
5 i
10 a
10 10
8 4
8 7
3 9
3 9
2 9
w
S
w
1
5
c
O)
1
1
o
ti
< 0
6
1
1
10
10
10
8
10
10
10
c
9
u
JS
a
^ e
o f
fc
a
6
4
5
5
10
5
4
10
10
7
TOTAL
54
44
32
70
79
62
53
59
57
54
EXPLANATORY NOTES:
1. Unconsohdated formations, predominantly unsaturated. with monitoring conducted m individual, relatively isolated, saturated
zones. Drilling is through primarily unsaturated matenal. but completion « m a saturated zone.
2. Borehole stability problems vary from slight (e.g., dense, silt/clay) to sew* ie g coarse gravel and boulders)
3. The anticipated use of the monitonng well permits the use of dnllmg fluid *na additives in construction.
345
-------
MATRIX NUMBER 20
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated: unsaturated; invasion of formation by dnlling fluid permitted' casing diameter 2 inches or less-
total well depth 15 to ISO feet.
\ 90
\ 3
\ <*
\ 2*
\ o^
\ *%
\i*
\s
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
TJ
o
c.
<5
I
Q
•5
s
0
*
NA
1
1
8
9
10
7
10
10
9
a
S
a
i
to
NA
1
1
10
10
10
5
8
9
10
o
E
I
1
•3
*
NA
7
4
10
10
10
8
6
6
3
—
i
£
5.
a>
c
a
o
2-
i
s
<
NA
10
10
9
9
10
8
4
1
7
i -
3 c
ll
1}
5 >
s&
ac -o
£ o
||
2 3
"r-
NA
6
5
10
10
8
8
7
6
2
2
^
O) (A
% i
||
|j
o S
t«
n S
-------
MATRIX NUMBER 21
General Hydrogeotogto Conditions ft Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less;
total well depth greater than ISO feet.
\ 1§
\ 3?
^L J **
\ *
\ "* z
\ * 5
\ 9
\ "u.
\i
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
^
Q
i
s
o>
c
Q
O
f
NA
NA
NA
NA
NA
10
S
9
10
9
a
S
i
NA
NA
NA
NA
NA
2
5
9
10
5
O
£
~
Q
I Relative
NA
NA
NA
NA
NA
10
8
6
6
5
c
i
—
S
O>
c
Q
O
Availabil
NA
NA
NA
NA
NA
10
8
4
1
7
—
§ C
h_ 3)
fyf
f
— 3,
«1
H 0
J|
NA
NA
NA
NA
NA
10
7
8
8
3
o
>.
O> 0>
o c
11
ll
tn J?
11
~ *•
QZ
If
NA
NA
NA
NA
NA
4
8
9
10
8
S
1
<0
Q
c
Ol
i
S
n
c
o
ii
II
NA
NA
NA
NA
NA
10
10
10
10
10
§
^
|
O
s
*!
a S.
So
UJ 75
M
1 =
CC a
NA
NA
NA
NA
NA
5
4
10
9
7
TOTAL
NA
NA
NA
NA
NA
61
55
65
64
54
EXPLANATORY NOTES:
i Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through primanly unsaturated matenal. but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g., dense, silt/clay) to servere (e.g.. coarse gravel and boulders).
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
4 Dual-wall air completion is through the bit.
5 Air rotary with casing hammer and dual-wall air methods become relatively more advantageous under these conditions.
347
-------
MATRIX NUMBER 22
General Hydrogeologlc Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches' total
well depth 0 to 15 feet.
\ §8
\. a^ ^3
\ 3"
\ ii
\°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
w
2
&
Versatility of Dnllir
NA
1
1
8
10
8
5
9
9
8
Sample Reliability
NA
1
1
10
10
10
s
8
9
10
8
Relative Drilling C
NA
10
4
10
10
7
8
5
6
3
c
1
s
Qk
E
Availability of Drill
NA
10
10
9
9
10
8
4
1
7
=
5 c
5 E
if
si
Relative Time Req
Installation and DC
NA
5
5
10
10
•8
8
5
S
5
2
Ol in
f|
i? « ^
Ability of Drilling 1
Preserve Natural C
NA
5
1
8
10
4
7
9
9
9
S
en
5
c
Ol
a
Ability to Install Di
ol Well
NA
1
1
10
10
10
a
10
8
10
§
0>
0.
U
ai
Relative Ease ol W
and Development
NA
4
1
5
10
5
4
10
10
8
TOTAL
NA
37
24
70
79
62
53
60
57
60
EXPLANATORY NOTES:
1. Unconsolklated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through primarily unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g.. dense, silt/clay) to servero (e.g., coarse gravel and boulders).
3. The anticipated use of the monitonng well permits the use of dnlhng fluid and additives in construction.
4. Solid flight auger methods require open hole completion, which may or may not be feasible
348
-------
MATRIX NUMBER 23
General Hydrogeologie Conditions & Well Design Requirements
Unconsohdated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches; total
well depth 15 to 150 feet.
\ i°
\ Ii
\ II
\ •?
\ ~°
\ So
\g
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
E
™
Q
"5
s
I
NA
1
1
3
7
10
5
10
10
9
M
i
5
1
I
NA
1
1
10
10
10
5
8
9
10
8
"
1*
Q
1
i
NA
1
1
10
10
10
8
6
6
4
c
|
i
"5
Q
"5
S
1
NA
10
10
9
9
10
8
4
1
7
_
w 0>
If
||
so
S?
Is
"ii
<3 2
ii
NA
2
3
8
10
9
8
8
8
7
0
S|
s!
OJ~
•= 1
§1
s^
is
NA
5
1
8
10
4
7
9
9
9
!•.
1
C
Q
"a
1
O
11
NA
1
1
6
8
10
6
10
10
10
c
o
"a.
O
§
$ s
og
si
iii ^
Ii
il
NA
4
1
5
8
5
4
10
9
10
TOTAL
NA
25
19
59
72
68
51
65
62
66
EXPLANATORY NOTES:
1. Unconsolktated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones Drilling is through pnmanly unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g.. dense, silt/clay) to servers (e.g.. coarse gravel and boulders).
3. The anticipated use of the monitonng well permits the use of dnlling fluid and additives in construction.
4 Solid flight augers require open hole completion, which may or may not be feasible.
349
-------
MATRIX NUMBER 24
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches- total
well depth greater than 150 feet. '
\ -o
\ 3
\ M
\ si
\ «-
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•o
o>
c
Q
0
Versatility
NA
NA
NA
NA
NA
9
5
9
10
9
>.
1
1
1
1
NA
NA
NA
NA
NA
1
3
9
10
5
a
?
«
Q
I Relative
NA
NA
NA
NA
NA
10
10
6
6
4
c
Q.
1
I
w
Q
O
Availabil
NA
NA
NA
NA
NA
10
8
4
1
7
V
Se-
lf
II
u •••
tt-a
II
u
If
o
>.
oi in
0 C
o 2
C £
Is
c1"
Qz
'o S
NA NA
NA NA
NA NA
NA NA
NA NA
9 4
10 B
8 9
10 10
3 B
i
I
e
o>
=
c
Abilit/ to
of Well
NA
NA
NA
NA
NA
10
10
10
10
10
2
5
|
1-
o|
il
1?
OC a
NA
NA
NA
NA
NA
5
4
10
8
10
TOTAL
NA
NA
NA
NA
NA
58
58
65
65
56
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly unsaturated. with monttonng conducted in individual, relatively isolated saturated
zones. Drilling is through pnmanly unsaturated material, but completion « m • saturated zone
2. Borehole stability problems vary from slight (e.g., dense, silt/clay) to severe le g coaree gravel and boulders).
3 The anticipated use of the monitonng well permits the use of drilling fluid and additives m construction.
4 Air rotary method requires generally vary difficult open-Hole completion The bomnoie may. however, be stabilized with fluid after
drilling is complete.
350
-------
MATRIX NUMBER 25
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated: invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches; total
well depth 0 to 15 feet.
\ 2§
\ iS
\ ^ '
\ u "
\ «^ ^
\ o^
\ ^ *
\ «°
\ "o
\ '
V
DRILLING \
METHODS \
Hand Auger
Driving
E
.e
%
2
I
Q
"5
tz
n
I
NA
NA
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
NA
4
7
10
5
8
NA
a
^
i
i
s
a.
1
NA
NA
•«
3
u
f
I
S
^
2
0
C
NA
NA
1
NA
8
10
10
5
7
NA
10
NA
10
10
7
9
6
NA
3
c
o
1
3
III
O)
C
"c
O
'o
i
2
I
NA
NA
NA
9
9
10
8
4
NA
7
ic
j i
•a o
So
11
go
a-o
0 g
i g
S 5
^ s
II
NA
NA
NA
10
9
• 8
8
8
NA
4
o
Is
|2
c €
0 C
^3
01 75
j 2
QZ
II
il
NA
NA
NA
8
8
5
4
10
NA
10
«
«
Q
C
S>
S
Q
i
1
i
i"?
•5 $
< 0
NA
NA
NA
7
7
10
5
10
NA
10
c
ai
Q.
P
i
s?
0 g
0) J-
01 &
a O
UJ 75
Q
1'
CC a
NA
NA
NA
4
4
3
4
10
NA
9
TOTAL
NA
NA
NA
60
64
63
48
63
NA
61
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through pnmanly unsaturated matenal. but completion is in a saturated zone.
Z. Borehole stability problems vary from slight (e.g.. dense, silt/clay) to severe (e.g.. coarse gravel and boulders).
3. The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
4 Diameter requirements limit the equipment that can be utilized.
S Solid-Night augers require very difficult open-hole completion. Hollow-stem auger technique requires open-hole completion
for casing sizes greater than 4 inches.
351
-------
MATRIX NUMBER 26
General Hydrogeologic Conditions & Well Design Requirements
Unconsohdated: unsaturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches- total
well depth 15 to 150 feet.
\ §§
\ ^ ?
\ Is
\ "z
\ If
\ flc ••_
\ "o
\s
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
1
2
I
Q
'o
^
w
>
NA
NA
NA
NA
NA
10
NA
NA
NA
8
>•
•5
a
5
1
i
E
$
NA
NA
NA
NA
NA
10
NA
NA
NA
10
8
O
£
•
Q
I
<5
i
NA
NA
NA
NA
NA
10
NA
NA
NA
6
c
0>
e
I
3
ff
UJ
Q
"o
i
»
5
1
NA
NA
NA
NA
NA
10
NA
NA
NA
7
* *.
o|
if
ffi C
c a
•N
2!
« 2
£1
NA
NA
NA
NA
NA
10
NA
NA
NA
5
o
**
sg
If
II
~ ^f
02
^ A.
?
I i
A C
NA
NA
NA
NA
NA
6
NA
NA
NA
10
u
o>
CO
a
1
lo
NA
NA
NA
NA
NA
10
NA
NA
NA
10
§
w
1
3
_ 'c
o £
Q) ""
UJ ^J
73 ^
o c
(T eg
NA
NA
NA
NA
NA
4
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
70
NA
NA
NA
66
EXPLANATORY NOTES:
1. Unconsohdated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through primarily unsaturated matenal. but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g.. dense, silt/clay) to servere (e.g.. coarse gravel and boulders).
3. The anticipated use of the monitonng well permits the use of dnlhng fluid and additives in construction.
4 Diameter of borehole, and depth, eliminates most options.
5 Air rotary with casing hammer and dual-wall rotary are applicable for 4-mch casing.
352
-------
MATRIX NUMBER 27
General Hydrogeologic Conditions ft Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches- total
well depth greater than 150 feet.
\ §8
\ *~x
\ =5
\ *°
\1
\5°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•5
i
ly of Drilling K
1
>
NA
NA
NA
NA
NA
10
NA
NA
NA
8
Reliability
i
i
NA
NA
NA
NA
NA
8
NA
NA
NA
10
Drilling Cost
I
i
NA
NA
NA
NA
NA
10
NA
NA
NA
6
c
Q.
5
o-
UJ
O)
Q
0
i
s
I
NA
NA
NA
NA
NA
10
NA
NA
NA
7
"5 ^
3 c
o E
|l
H 0
ii
03 *
ll
2
O)0)
11
£ Tj
||
||
||
i!
NA NA
NA NA
NA NA
NA NA
NA NA
>0 6
NA NA
NA NA
NA NA
4 10
w
s
£
6
01
&
a
1
O
5-^
II
NA
NA
NA
NA
NA
8
NA
NA
NA
10
_
£
I
Ease of Well (
Jlopment
!i
S-o
S c
IT (B
NA
NA
NA
NA
NA
4
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
66
NA
NA
NA
65
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly unsaturated. with monitonng conducted in individual, relatively isolated, saturated
zones. Drilling is through pnmanly unsaturated matenal. but completion n in a saturated zone.
2. Borehole stability problems vary from slight (e.g.. dense, silt/clay) to ierv«r« le g. coarse gravel and boulders).
3. The anticipated use of the monitonng well permits the use of drilling fluid w additives in construction.
4 Diameter of borehole, and depth, eliminates most options.
353
-------
MATRIX NUMBER 28
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid not permitted; casing diameter 2 inches or
less; total well depth 0 to 15 feet.
\ !§
\ 3«
\ 4 S
\ w°
\ si
\ *?
\i°
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
1
I
Versatility of Dril
4
7
NA
8
8
NA
5
9
10
NA
^
Sample Reliabihl
5
1
NA
10
10
NA
5
8
9
NA
1
Relative Drilling
9
10
NA
10
10
NA
8
6
6
NA
i
E
g.
3
a
UJ
O)
=
Availability ol Dr
10
10
NA
9
9
NA
8
4
1
NA
«_
O £
If
Relative Time Re
Installation and I
5
6
NA
10
10
NA
8
3
8
NA
2
X
"3 O
55
i— Q
Ability of Drilling
Preserve Natural
9
5
NA
8
8
NA
7
9
10
NA
49
s
o
c
Ol
1
Ability to Install (
of Well
6
1
NA
10
10
NA
8
10
10
NA
i
I
E
6
!_
Relative Ease of \
and DevelopmenI
6
4
NA
5
10
NA
4
10
10
NA
TOTAL
54
44
NA
70
75
NA
53
59
64
NA
EXPLANATORY NOTES.
1 Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through pnmanly unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders).
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Jetting, mud rotary and cable tool methods would require the addition of fluid.
5. Air rotary with casing hammer requires driving 6-inch or greater diameter casing and completion by pullback.
6. Air rotary, hand auger and solid-flight auger completion possible only if unsupported borehole is stable.
354
-------
MATRIX NUMBER 29
General Hydrogedogic Conditions & Well Design Requirements
Unconsolidated. unsaturated: invasion of formation by drilling fluid not permitted; casing diameter 2 inches or
less; total well depth 15 to 150 feet.
\ p°
\ 3C
\ ^
\!1
\5
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
3
£
U
2
O)
c
i
"5
8
£
NA
1
NA
9
10
NA
7
10
10
NA
S
IB
i
I
1
NA
1
NA
10
10
NA
5
8
9
NA
s
U
f
=
G
I
1
NA
7
NA
10
10
NA
a
6
6
NA
c
5
UJ
I
i
"5
1
1
1
NA
10
NA
10
10
NA
a
4
1
NA
=
fl
£ t
«•§
w (0
11
Is
M
ll
NA
6
o
X
O) n
o S
o 2
c ~
II
oi
II
NA
5
I
NA
9
10
NA
8
6
9
NA
NA
8
10
NA
7
9
9
NA
a
CO
Q
i>
8
1
1
o
•= $
<"5
NA
1
NA
9
9
NA
9
10
10
NA
§
I
E
o
O
i,
o 2
Sc
Q.
n o
UJ ^
if
NA
4
NA
5
10
NA
4
10
10
NA
TOTAL
NA
35
NA
70
79
NA
56
63
64
NA
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through pnmanly unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders).
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Jetting, mud rotary and cable tool methods would require the addition of fluid.
5. Air rotary with casing hammer requires dnving 6-inch or greater diameter casing and completion by pullback
6. Air rotary and solid-flight auger completion possible only if unsupported borehole is stable.
355
-------
MATRIX NUMBER 30
General Hydrogeologic Conditions & Well Design Requirements
Unconsohdated; unsaturated; invasion of formation by dnlling fluid not permitted; casing diameter 2 inches or
less: total well depth greater than 150 feet.
\ 2g
\ i^
\ < s
\ uS
\ o3
\ UL «
\ *§
\ ^ MB
\s
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
3
£
I
£
S
O
'o
TD
i
NA
NA
NA
NA
NA
NA
7
10
10
NA
•5
S
"5
DC
o>
!
NA
NA
NA
NA
NA
NA
7
9
10
NA
Ol
c
Q
S
i
cc
NA
NA
NA
NA
NA
NA
10
8
8
NA
I
a
o-
LU
Ol
C
"c
a
"5
2-
i
1
NA
NA
NA
NA
NA
NA
10
8
4
NA
—
if
o E
at
w Q]
|«
§Q
e-o
0) jj=
^2
| «
il
NA
NA
NA
NA
NA
NA.
10
9
9
NA
2
o 2
o c
,23
O) —
c 2
— 3
= n
QZ
o ?
u
NA
NA
NA
NA
NA
NA
7
9
10
NA
w
1
a
a
c
gi
S
Q
^
1
O
2-i
1!
NA
NA
NA
NA
NA
NA
10
10
10
NA
S
1
E
U
I
c?
o £
8t
a
a o
Ul «
I "
11
NA
NA
NA
NA
NA
NA
4
10
8
NA
TOTAL
NA
NA
NA
NA
NA
NA
65
73
69
NA
EXPLANATORY NOTES:
1 Unconsohdated formations, predominantly unsaturated, with monitoring conducted in individual, relatively isolated, saturated
zones. Dnlling is through primarily unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders).
3. The anticipated use of the monitoring well prohibits the use of dnlling fluid and additives in construction.
4 The depth requirement and the decision not to utilize dnlling fluid limit equipment options.
S Jetting, mud rotary and cable tool methods would require the addition of fluid.
6 Air rotary with casing hammer requires driving 6-inch or greater diameter casing and completion by pullback.
7 Air rotary completion possible only if unsupported borehole is stable.
356
-------
MATRIX NUMBER 31
General Hydrogeotogic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid not permitted: casing diameter 2 to 4 inches'
total well depth 0 to 15 feet.
^ z to
\ P°
\ 3*
\ J 91
\ ii
\ ^
\is
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
„
5
i
I
Q
'5
3
I
NA
1
NA
8
10
NA
5
9
9
NA
i
i
1
1
NA
1
NA
10
10
NA
5
a
9
NA
8
I
i
i
I
NA
10
NA
10
10
NA
8
6
6
NA
c
0)
1
5
s
I
Q
0
i
l
NA
10
NA
10
10
NA
8
4
1
NA
—
Ic
Q P
II
5 2
fl
1 1
si
if
tr £
NA
5
NA
10
10
NA
8
6
6
NA
2
f i
gl
n
ii
ii
ii
is
s
Q
2
(O
£
O
fl
<0
NA
1
NA
7
9
NA
8
10
9
NA
s
«
a.
f
o §
8c
a.
i|
C a
NA
4
NA
5
8
NA
4
10
10
NA
TOTAL
NA
37
NA
68
77
NA
53
62
59
NA
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through primarily unsaturated material, but completion is in a saturated zone
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders).
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Jetting, mud rotary and cable tool methods would require the addition of fluid.
5. Air rotary with casing hammer requires driving 8-inch or greater casing and completion by pullback.
6. Air rotary and solid-flight auger completion possible only if unsupported borehole is stable.
357
-------
MATRIX NUMBER 32
General Hydrogedogic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches-
total well depth 15 to 150 feet.
\ Z<0
\ 2°
\ ^ P
\ <*
\ wS
\ flC ™
\ o d
\ fc 2
\ ^ Q
\fs
\5
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•o
o
£
2
OI
c
"E
Q
0
>•
Versatlll
NA
1
NA
3
7
NA
5
10
10
NA
j.
~
S
S
"3
tr
j»
NA
1
NA
10
10
NA
5
8
9
NA
"n
^
O)
C
i
i
«
s.
NA
1
NA
10
10
NA
8
6
6
NA
c
91
?
a-
LU
c
s
Q
'o
Availab
NA
10
NA
10
10
NA
8
4
1
NA
=
5 S
o I
ll
!«
So
X^B
o
II
Si
« 2
'g> «
NA
2
NA
8
10
NA
8
6
6
NA
2
tn
c
S
a? o
h- O
01 To
— 3
11
"S di
O
is
< 0.
NA
5
NA
8
10
NA
7
9
9
NA
£
0)
E
5
c
_
2
o
II
NA
1
NA
6
8
NA
8
10
8
NA
|
0)
"5.
o
s
5
_ 'c
o s
II
Relative
and Dew
NA
4
NA
3
8
NA
3
10
3
NA
TOTAL
NA
25
NA
58
73
NA
52
63
52
NA
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly unsaturated. with monitoring conducted m individual, relatively isolated saturated
zones. Drilling is through primarily unsaturated matenal. but compwion >» >n • saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe >• g coarse gravel and boulders).
3. The anticipated use of the monitonng well prohibits the use of dniiing fluid ino additives in construction.
4 Jetting, mud rotary and cable tool methods would require the addition o> MUKJ
5. Air rotary with casing hammer requires dnvmg B-mch or greater casing and completion by pullback.
6 Air rotary and solid-flight auger completion possible only if unsupported oonjnoie is stable.
358
-------
MATRIX NUMBER 33
General Hydrogeologlc Conditions & Well Design Requirements
Unconsolidated: unsaturated: invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches-
total well depth greater than 150 feet.
\ !§
\ 3«
\ ^ •
\ ^2
\ °3
\ M
\ So
\g
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
•j*
I
f
-=
Q
O
>•
Versalilr
NA
NA
NA
NA
NA
NA
5
9
10
NA
x
S
9
i
41
I
NA
NA
NA
NA
NA
NA
5
9
10
NA
8
\*
o>
c
Q
I
NA
NA
NA
NA
NA
NA
10
6
6
NA
1
a
5
o-
UJ
I
a
3
Availabi
NA
NA
NA
NA
NA
NA
10
6
4
NA
u
§c
Q C
if
5 g
||
ll
11
ss
||
NA
NA
NA
NA
NA
.NA
10
8
10
NA
o
01 n
0 =
o 9
if
So
n
= 3
QZ
'o 9
> g
? S
NA
NA
NA
NA
NA
NA
S
10
10
NA
I
I
eg
Q
f
i
«
c
Ability tc
of Well
NA
NA
NA
NA
NA
NA
10
10
10
NA
§
I
e
o
if
UJ TR
Relative 1
and Devt
NA
NA
NA
NA
NA
NA
5
10
10
NA
TOTAL
NA
NA
NA
NA
NA
NA
60
68
70
NA
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated saturated
zones. Drilling is through primarily unsaturated matenal. but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders).
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 No drilling fluid, increasing depth and diameter requirements eliminate many options.
S. Air rotary with casing hammer requires driving 8-inch or greater casing and completion by pullback.
359
-------
MATRIX NUMBER 34
General Hydrogeologto Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid not permitted; casing diameter 4 to 8 inches;
total well depth 0 to 15 feet.
\ is
\ Is
\ ^ 3E
\ flC *
\ O j
\ i*
\ So
\o
DRIUING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
CD
2
o>
c
a
o
3
NA
NA
NA
NA
NA
NA
6
10
NA
NA
^
i
2
1
S
NA
NA
NA
NA
NA
NA
5
10
NA
NA
--
n
.S
a
a
Relative
NA
NA
NA
NA
NA
NA
10
6
NA
NA
Equipment 1
en
£
™
O
2>
Availabil
NA
NA
NA
NA
NA
NA
10
6
NA
NA
d for Well
opment
= »
5%
erg
12
IS
li
ii
NA
NA
NA
NA
NA
NA
10
9
NA
NA
inology to
Jitions
o C
0> 0
H O
I!
c fS
Qz
*^ en
Z.
^1
NA
NA
NA
NA
NA
NA
6
10
NA
NA
n Diameter 1
S1
«
O
i
a
Ability ti
of Well
NA
NA
NA
NA
NA
NA
6
10
NA
NA
1
J
^
> ^
o S
So.
a 0
lil ai
Relative
andDev
NA
NA
NA
NA
NA
NA
S
10
NA
NA
TOTAL
NA
NA
NA
NA
NA
NA
58
71
NA
NA
EXPLANATORY NOTES:
1. Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated, saturated
zones. Drilling is through primarily unsaturated material, but completion is in a saturated zone.
2. Borehole stability problems vary from slight (e.g. dense, silt/day) to severe (e.g. coarse gravel and boulders)
3. The anticipated use of the monitoring well prohibits the use of drilling fluid and additives in construction.
4 Diameter and no dnlling fluid minimizes options
5 Jetting, mud rotary and cable tool methods would require the addition of fluid.
6 Air rotary with casing hammer requires dnving 12-inch or greater diameter casing and completion by pullback
7 Air rotary completion possible only if unsupported borehole is stable.
360
-------
MATRIX NUMBER 35
General Hydrogeologte Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by dnlling fluid not permitted; casing diameter 4 to 8 inches-
total well depth 15 to 150 feet.
\ ll
\ ^% ^J
\ =
\ So
\ *
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
I
™
e
I
0
=
4
i
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
£
a
0)
a
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
^
8
u
o>
I
f
9
i
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
Equipment I
Ol
c
i
o
£
S
I
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
?!
W ™J
5 £
|Q
IE TJ
Q) C
m ~
5 —
f 1
aci
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
inology to
ditions
o e
l|
f|
Q|
OS
> 5
1!
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
w
>^
il
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
i
1
VJ
=
H
a O
UJ ^
- Q
Ic
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
TOTAL
NA
NA
NA
NA
NA
NA
80
NA
NA
NA
EXPLANATORY NOTES:
1 Unconsolidated formations, predominantly unsaturated. with monitoring conducted in individual, relatively isolated saturated
zones. Dnlling is through pnmanly unsaturated material, but completion is in a saturated zone.
2 Borehole stability problems vary from slight (e.g. dense, silt/clay) to severe (e.g. coarse gravel and boulders)
3. The anticipated use of the monitoring well prohibits the use of dnlling fluid and additives in construction
4 No dnlling fluid, depth and diameter requirements have eliminated options.
5 Oversize dnllpipe and/or auxiliary air probably required.
6. Jetting, mud rotary and cable tool methods would require the addition of fluid.
7 Air rotary completion possible only if unsupported borehole is stable.
8. Air rotary with casing hammer unlikely to penetrate to specified depths with 12-mch diameter outer casing that is required for 8-mch
diameter casing and screen completion.
9 If borehole is unstable, for 8-inch diameter casing there is no currently available method that can be used to fulfill the requirements
as stated above. Therefore, fluid would be necessary to install the well and invasion-permitting matrices will apply.
361
-------
MATRIX NUMBER 36
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated: unsaturated; invasion of formation by drilling fluid not permitted: casing diameter 4 to 8 inches;
total well depth greater than 150 feet.
\ M
\ 3*"
\ <»
\ gs
\ *«
\ tX i«
\ Ul S
\g
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•a
£
ly ol Drilling Me
3
1
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
£
a
S
I
1
i
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
Drilling Cost
I
3
(C
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
1
e
a
5
a-
S
I
i
"o
E
S
1
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
*-
5 c
•Si
Time Required
ion and Develof
11
S S
oi <2
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
°
0)«
— c
I Drilling Techni
! Natural Condit
O S
£•§
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
S
o
E
i
c
Ol
I
1
—
if
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
c
o
i
Ease ot Well Co
alopment
M
i?
E
-------
MATRIX NUMBER 37
General Hydrageologte Conditions & Well Design Requirements
Consolidated; invasion of formation by drilling fluid permitted; casing diameter 4 inches or less.
\ 5i
\ 3 ui
\ <¥
\ ll
\ ^ flc
\ — Q
\ S ft
\ *^
\ 2
\o
DRILLING \
METHODS \
\
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
i
I
E
Q
"5
^
ts
is
I
NA
NA
NA
NA
NA
7
8
NA
10
8
£
i
&
o>
Q.
1
NA
NA
NA
NA
NA
6
9
NA
10
7
5
3
Ol
"E
O
0
>
a
i
NA
NA
NA
NA
NA
8
10
NA
7
5
1
E
Q.
5
s
at
c
<§
o
M
•=
S
i
NA
NA
NA
NA
NA
10
9
?
5c
w Q)
l}
Js
a. 73
4) c
I a
1" O
to ^
- =
II
o
at en
1
i
1
o
> =
<0
NA
NA
NA
NA
NA
10
10
NA
10
10
i
£
a
U
i.
o £
«I
a O
LU "55
M
il
cc a
NA
NA
NA
NA
NA
7
10
NA
10
8
TOTAL
NA
NA
NA
NA
NA
63
75
NA
68
55
EXPLANATORY NOTES:
1. Consolidated formations, all types
2. The anticipated use of trie monitonng well permits the use of drilling fiu«j ana additives in construction.
3. Boreholes are expected to be sufficiently stable to permit open-no^ comoMtion
4 Core sampling will improve the relative value of the mud rotary metnod
5. Where dual-wall air is available it becomes an equally preferred memoa win air rotary.
363
-------
MATRIX NUMBER 38
General Hydrogeologte Conditions & Well Design Requirements
Consolidated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches.
\ 2§
\ ?al
\ "1
\ o3
\ ^
\ 1°
\o
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
|
0)
Ol
c
Q
"5
Versatilil
NA
NA
NA
NA
NA
9
a
NA
NA
10
S
S
£
4)
I
NA
NA
NA
NA
NA
6
10
NA
NA
9
3
O
I
Q
I Relative
NA
NA
NA
NA
NA
8
10
NA
NA
5
1
a
5
Ul
O)
c
1
"5
Availabil
NA
NA
NA
NA
NA
10
9
NA
NA
8
=
s|
it
t§
T3
|j a
||
NA
NA
NA
NA
NA
8
10
NA
NA
5
o
^
g"S
o 9
O C
,25
= 5
Qz
II
NA
NA
NA
NA
NA
6
10
NA
NA
9
ai
Tit
W
i
Q
01
i
Q
I
I
ft
Ability l<
of Well
NA
NA
NA
NA
NA
10
10
NA
NA
10
c
g
0}
1
o
U
i
?c
o 2
CA G>
Relative
and Devi
NA
NA
NA
NA
NA
7
10
NA
NA
9
TOTAL
NA
NA
NA
NA
NA
64
77
NA
NA
65
EXPLANATORY NOTES:
V Consolidated formations, all types
2 The anticipated use of the monitoring well permits the use of drilling fluid and additives in construction.
3 Boreholes are expected to be sufficiently stable to permit open-hole completion.
4 Core sampling will improve the relative value of the mud rotary method.
5 Where dual-wall air is available it becomes an equally preferred method with air rotary, but borehole diameter is limited to
approximately 10 inches.
364
-------
MATRIX NUMBER 39
General Hydrogeologic Conditions & Well Design Requirements
Consolidated; invasion of formation by drilling fluid not permitted; casing diameter 4 inches or less.
\ li
\ l£
\ 2o
\ 03
\ ^
\ * Q
\ ^
\P
DRILLING \
METHODS \
Hand Auger
Driving
Jetting
Solid Flight
Auger
Hollow Stem
Auger
Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
•o
c.
1
O)
c
O
"5
>•
1
NA
NA
NA
NA
NA
NA
8
NA
10
NA
j.
.5
(0
"5
C
S.
a.
NA
NA
NA
NA
NA
NA
8
NA
10
NA
_
y
S
S£
£
a
i
s
I
NA
NA
NA
NA
NA
NA
10
NA
7
NA
"c
V
a
—
3
LU
s
a
o
ts
2
•=
NA
NA
NA
NA
NA
NA
10
NA
1
NA
3
5 C
w 9)
if
w Q)
S- »
»1
i"
K 2
|i
li
NA
NA
NA
NA
NA
NA
10
NA
10
NA
2
at M
II
.C 'O
K (J
fl
'E 7g
^ ~
o 9
Is
<£
NA
NA
NA
NA
NA
NA
8
NA
10
NA
w
£
i
s
a
c
g>
Z
S
Q
I
1
_
ll
NA
NA
NA
NA
NA
NA
10
NA
10
NA
c
o
jj
a.
0
2
li
o"
m ^>
Is
li
NA
NA
NA
NA
NA
NA
10
NA
10
NA
TOTAL
NA
NA
NA
NA
NA
NA
74
NA
68
NA
EXPLANATORY NOTES:
1. Consolidated formations, all types
2 The anticipated use of the monitoring well does not permit the use of drilling fluid and additives in construction
3 Boreholes are expected to be sufficiently stable to permit open hole completion.
4. Both mud rotary and cable tool methods are potentially invasive, thereby reducing options to air dnlling methods.
S. Air rotary may require extra air and/or special dnll pipe.
365
-------
MATRIX NUMBER 40
General Hydrogeologic Conditions A Well Design Requirements
Consolidated: invasion of formation by drilling fluid not permitted; casing diameter 4 to 8 inches.
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Driving
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Mud Rotary
Air Rotary
Air Rotary with
Casing Hammer
Dual Wall Rotary
Cable Tool
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EXPLANATORY NOTES:
1. Consolidated formations, all types
2. The anticipated use of the monitoring well does not permit the use of drilling fluid and additives in construction
3. Boreholes are expected to be sufficiently stable to permit open hole completion.
4 Both mud rotary and cable tool methods are potentially invasive, thereby reducing options to air drilling methods
5. Air rotary may require extra air and/or special drill pipe.
366
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APPENDIX C
(Supplement to Chapter 8)
ABANDONMENT OF TEST HOLES, PARTIALLY COMPLETED WELLS AND
COMPLETED WELLS
(AMERICAN WATER WORKS ASSOCIATION, 1984)
SECTION I.I--GENERAL
The recommendations contained in this appendix pertain to wells and
test holes in consolidated and unconsolidated formations. Each sealing job
should be considered as an individual problem, and methods and materials
should be determined only after carefully considering the objectives
outlined in the standard.
SECTION I.2--WELLS IN UNCONSOLIDATED FORMATIONS
Normally, abandoned wells extending only into consolidated formations
near the surface and containing water under water-table conditions can be
adequately sealed by filling with concrete, grout, neat cement, clay, or
clay and sand. In the event that the water-bearing formation consists of
coarse gravel and producing wells are located nearby, care must be taken to
select sealing materials that will not affect the producing wells.
Concrete may be used if the producing wells can be shut down for a
sufficient time to allow the concrete to set. Clean, disinfected sand or
gravel may also be used as fill material opposite the waterbearing
formation. The remainder of the well, especially the upper portion, should
be filled with clay, concrete, grout, or neat cement to exclude surface
water. The latter method, using clay as the upper sealing material, is
especially applicable to large-diameter abandoned wells.
In gravel-packed, gravel-envelope, or other wells in which coarse
material has been added around the inner casing to within 20 to 30 ft (6.1
to 9.1 m) of the surface, sealing outside the casing is very important.
Sometimes this sealing may require removal of the gravel or perforation of
the casing.
SECTION 1.3—WELLS IN CREVICED FORMATIONS
Abandoned wells that penetrate limestone or other creviced or
channelized rock formations lying immediately below the surface deposits
should preferably be filled with concrete, grout, or neat cement to ensure
permanence of the seal. The use of clay or sand in such wells is not
desirable because fine-grained fill material may be displaced by the flow
of water through crevices or channels. Alternate layers of coarse stone
and concrete say be used for fill material through the water-producing
horizon if limited vertical movement of water in the formation will not
367
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affect the quality or quantity of water in producing wells. Only concrete,
neat cement, or grout should be used in this type of well. The portion of
the well between a point 10 to 20 ft (3.0 to 6.1 m) below and a point 10 to
20 ft (3.0 to 6.1 m) above should be sealed and a plug of sealing material
formed above the creviced formation. Clay or sand may be used to fill the
upper part of the well to within 20ft (6.1 m) of ground level. The upper
20 ft (6.1 m) should be sealed with concrete or cement grout.
SECTION I.4--WELLS IN NONCREVICED ROCK FORMATIONS
Abandoned wells encountering non-creviced sandstone or other
water-bearing consolidated formations below the surface deposits may be
satisfactorily sealed by filling the entire depth with clay, provided there
is no movement of water in the well. Clean sand, disinfected if other
producing wells are nearby, may also be used through the sandstone up to a
point 10 to 20 ft (3.0 to 6.1 m) below the bottom of the casing. The
upper portion of this type of well should be filled with concrete, neat
cement, grout or clay to provide an effective seal against entrance of
surface water. If there is an appreciable amount of upward flow, pressure
cementing or mudding may be advisable.
SECTION I-5--MULTIPLE AQUIFER WELLS
Some special problems may develop in sealing wells extending into more
than one aquifer. These wells should be filled and sealed in such a way
that exchange of water from one aquifer to another is prevented. If no
appreciable movement of water is encountered, filling with concrete, neat
cement, grout, or alternate layers of these materials and sand will prove
satisfactory. When velocities are high, the procedures outlined in
Sec. 1.6 are recommended. If alternate concrete plugs or bridges are used,
they should be placed in known nonproducing horizons or, if locations of
the nonproducing horizons are not known, at frequent intervals. Sometimes
when the casing is not grouted or the formation is noncaving, it may be
necessary to break, slit, or perforate the casing to fill any annular space
on the outside.
SECTION 1.6—WELLS WITH ARTESIAN FLOW
The sealing of abandoned wells that have a movement between aquifers
or to the surface requires special attention. Frequently the movements of
water may be sufficient to make sealing by gravity placement of concrete,
cement grout, neat cement, clay or sand impractical. In such wells, large
stone aggregate (not more than one third of the diameter of the hole), lead
wool, steel shavings, a well packer, or a wood or cast-lead plug or bridge
will be needed to restrict the flow and thereby permit the gravity
placement of sealing material above the formation producing the flow. If
preshaped or precast plugs are used, they should be several times longer
than the diameter of the well, to prevent tilting.
Since it is very important in wells of this type to prevent
circulation between formations or loss of water to the surface or to the
annular space outside the casing, it is recommnded that pressure
cementing, using the minimum quantity of water that will permit handling,
368
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be used. The use of pressure mudding instead of this process is sometimes
permissible.
In wells in which the hydrostatic head producing flow to the surface
is low, the movement of water may be arrested by extending the well casing
to an elevation above the artesian-pressure surface. Previously described
sealing methods suitable to the geologic conditions can then be used.
SECTION I.7--SEALING MATERIALS
A number of materials that can be used for sealing wells
satisfactorily, including concrete, cement grout, neat cement, clay, sand,
or combinations of these materials, are mentioned in this appendix. Each
material has certain characteristics and distinctive properties; therefore,
one material may be especially suited for doing a particular job. The
selection of the material must be based on the construction of the well,
the nature of the formations penetrated, the material and equipment
available, the location of the well with respect to possible sources of
contamination, and the cost of doing the work.
Concrete is generally used for filling the upper part of the well or
water-bearing formations, for plugging short sections of casings, or for
filling large-diameter wells. Its use is cheaper than neat cement or
grout, and it makes a stronger plug or seal. However, concrete will not
penetrate seams, crevices, or interstices. Furthermore, if not properly
placed, the aggregate is likely to separate from the cement.
Cement grout or neat cement and water-are far superior for sealing
small openings, for penetrating any annular space outside of casings, and
for filling voids in the surrounding formation. When applied under
pressure, they are strongly favored for sealing wells under artesian
pressure or those encountering more than one aquifer. Neat cement is
generally preferred to grout because it does not separate.
Clay, as a heavy mud-laden or special clay fluid applied under
pressure, has most of the advantages of cement grout. Its use is preferred
by some competent authorities, particularly for sealing artesian wells.
Others feel that it may, under some conditions, eventually be carried away
into the surrounding formations.
Clay in a relatively dry state, clay and sand, or sand alone may be
used advantageously as sealing materials, particularly under water-table
conditions where diameters are large, depths are great, formations are
caving, and there is no need for achieving penetration of openings in
casings, liners, or formations, or for obtaining a watertight seal at any
given spot.
Frequently combinations of these materials are necessary. The more
expensive materials are used when strength, penetration, or watertightness
are needed. The less expensive materials are used for the remainder of the
well. Cement grout or neat cement is now being mixed with bentonite clays
and various aggregates. Superior results and lower cost are claimed for
such mixtures.
369
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REFERENCES
American Water Works Association, 1984. Appendix I: Abandonment of test
holes, partially completed wells and completed wells; American Water Works
Association Standard for Water Wells, American Water Works Association,
Denver, Colorado, pp. 45-47.
370
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GLOSSARY
ABANDONMENT
The complete sealing of a well or borehole with grout or other impermeable
materials to restore the original hydrogeologic conditions and/or to prevent
contamination of the aquifer.
ABSORPTION
The penetration or apparent disappearance of molecules or ions of one or
more substances into the interior of a solid or liquid. For example, in
hydrated bentonite, the planar water that is held between the mica-like
layers is the result of absorption (Ingersoll-Rand, 1985).
ACCELERATOR
Substances used to hasten the setting or curing of cement such as calcium
chloride, gypsum and aluminum powder.
ACRYLONITRILE BUTADIENE STYRENE (ABS)
A thermoplastic material produced by varying ratios of three different
monomers to produce well casing with good heat resistance and impact
strength.
ADAPTER
A device used to connect two different sizes or types of threads, also known
as sub, connector or coupling (Ingersoll-Rand, 1985).
ADSORPTION
The process by which atoms, ions or molecules are held to the surface of a
material through ion-exchange processes.
ADVECTION
The process by which solutes are transported with and at the same rate as
moving ground water.
AIR ROTARY DRILLING
A drilling technique whereby compressed air is circulated down the drill
rods and up the open hole. The air simultaneously cools the bit and removes
the cuttings from the borehole.
AIR ROTARY WITH CASING DRIVER
A drilling technique that uses conventional air rotary drilling while
simultaneously driving casing. The casing driver is installed in the mast
of a top-head drive air rotary drilling rig.
371
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ALIPHATIC HYDROCARBONS
A class of organic compounds characterized by straight or branched chain
arrangement of the constituent carbon atoms joined by single covalent bonds
with all other bonds to hydrogen atoms.
ALKALINITY
The ability of the salts contained in the ground water to neutralize acids.
Materials that exhibit a pH of 7 or greater are alkaline. High-pH materials
used in well construction may have the potential to alter ambient water
quality.
ALUMINUM POWDER
An additive to cement that produces a stronger, quick-setting cement that
expands upon curing.
ANISOTROPIC
Having some physical property that varies with direction (Driscoll, 1986).
ANNULAR SEALANT
Material used to provide a positive seal between the borehole and the casing
of the well. Annular sealants should be impermeable and resistant to
chemical or physical deterioration.
ANNULAR SPACE OR ANNULUS
The space between the borehole wall and the well casing, or the space
between a casing pipe and a liner pipe.
AQUIFER
A geologic formation, group of formations, or part of a formation that can
yield water to a well or spring.
AQUIFER TEST
A test involving the withdrawal of measured quantities of water from or
addition of water to a well and the measurement of resulting changes in head
in the aquifer both during and after the period of discharge or addition
(Driscoll, 1986).
AQUITARD
A geologic formation, or group of formations, or part of a formation of low
permeability that is typically saturated but yields very limited quantities
of water to wells.
AROMATIC HYDROCARBONS
A class of unsaturated cyclic organic compounds containing one or more ring
structures or cyclic groups with very stable bonds through the substitution
of a hydrogen atom for an element or compound.
ARTESIAN WELL
A well deriving water from a confined aquifer in which the water level
stands above the ground surface; synonymous with flowing artesian well
(Driscoll, 1986).
372
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ARTIFICIAL FILTER PACK
See Gravel Pack.
ATTENUATION
The reduction or removal of constituents in the ground water by the sum of
all physical, chemical and biological events acting upon the ground water.
AUGER FLIGHTS
Winding metal strips welded to the auger sections that carry cuttings to the
surface during drilling.
BACKWASH (WELL DEVELOPMENT)
The surging effect or reversal of water flow in a well that removes
fine-grained material from the formation surrounding the borehole and helps
prevent bridging (Driscoll, 1986).
BACKWASHING
A method of filter pack emplacement whereby the filter pack material is
allowed to fall freely through the annulus while clean fresh water is
simultaneously pumped down the casing.
BAILER
A long, narrow bucket-like device with an open cop and a check valve at the
bottom that is used to remove water and/or cuttings from the borehole.
BAILING (WELL DEVELOPMENT)
A technique whereby a bailer is raised and lowered in the borehole to create
a strong outward and inward movement of wat«r from the borehole to prevent
bridging and to remove fine materials.
BARIUM SULFATE
A natural additive used to increase the density of drilling fluids.
BENTONITE
A hydrous aluminum silicate available in powder, granular or pellet form and
used to provide a tight seal between the well casing and borehole.
Bentonite is also added to drilling fluid to impart specific characteristics
to the fluid.
BIODEGRADATION
The breakdown of chemical constituents through the biological processes of
naturally occuring organisms.
BIT
The cutting tool attached to the bottom of the drill stem. Bit design
varies for drilling in various types of formations and includes roller, cone
and drag-type bits.
BIT, AUGER
Used for soft formations with auger drill (Ingersoll-Rand, 1985).
373
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BOREHOLE
A hole drilled or bored into the earth, usually for exploratory or economic
purposes, such as a water well or oil well (United States Environmental
Protection Agency, 1986).
BOREHOLE GEOPHYSICS
Techniques that use a sensing device that is lowered into a borehole for the
purpose of characterizing geologic formations and their associated fluids.
The results can be interpreted to determine lithology, geometry resistivity,
bulk density, porosity, permeability, and moisture content and to define the
source, movement, and physical/chemical characteristics of ground water
(United States Environmental Protection Agency, 1986).
BRIDGE SEAL
An artificial plug set to seal off specific zones in the abandonment of a
well.
BRIDGE-SLOT INTAKE
A well intake that is manufactured on a press from flat sheets that are
perforated, rolled and seam welded where the slots are vertical and occur as
parallel openings longitudinally aligned to the well axis.
BRIDGING
The development of gaps or obstructions in either grout or filter pack
materials during emplacement. Bridging of particles in a naturally
developed or artificial gravel pack can also occur during development.
CABLE TOOL DRILLING
A drilling technique whereby a drill bit attached to the bottom of a
weighted drill stem is raised and dropped to crush and grind formation
materials.
CALCIUM CHLORIDE
A soluble calcium salt added to cement slurries to accelerate the setting
time, create higher early strength and to minimize movement of the cement
into zones of coarse material.
CALCIUM HYDROXIDE
A primary constituent of wet cement.
CALIPER LOGGING
A logging technique used to determine the diameter of a borehole or the
internal diameter of casing through the use of a probe with one to four
spring expanding prongs. Caliper logging indicates variations in the
diameter of the vertical profile.
CAPILLARY FRINGE
The pores in this zone are saturated but the pressure heads are less than
atmospheric.
374
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CASING
An impervious durable pipe placed in a well to prevent the borehole walls
from caving and to seal off surface drainage or undesirable water, gas, or
other fluids and prevent their entrance into the well. Surface or temporary
casing means a temporary casing placed in soft, sandy or caving surface
formation to prevent the borehole from caving during drilling. Protective
casing means a short casing installed around the well casing. Liner pipe
means a well casing installed without driving within the casing or open
borehole.
CASING, FLUSH-COUPLED
Flush-coupled casing is joined with a coupling with the same outside
diameter as the casing, but with two female threads. The inside diameter of
the coupling is approximately 3/16 inch smaller than that of the casing.
Flush-coupled casing has thinner walls than flush-joint casing
(Ingersoll-Rand, 1985).
CASING, FLUSH-JOINT
Flush-joint casing has a male thread at one end and a female thread at the
other. No coupling is used (Ingersoll-Rand, 1985).
CASING DRIVER
A device fitted to the top-head drive of a rotary rig that is used to
advance casing into the subsurface.
CATION EXCHANGE CAPACITY (CEC)
The measure of the availability of cations that can be displaced from sites
on surfaces or layers and which can be exchanged for other cations. For
geologic materials, CEC is expressed as the-number of milliequivalents of
cations that can be exchanged in a sample with a dry mass of 100 grams.
CEMENT
A mixture of calcium aluminates and silicates made by combining lime and
clay while heating and which is emplaced in the annular space to form a seal
between the casing and the borehole.
CEMENT BOND LOG
A logging device that uses acoustical signals to determine the integrity of
the cement bond to the casing.
CEMENT, QUICK-SETTING
Cement of special composition and fineness of grind that sets much quicker
than ordinary cement. This cement is used for deviating holes and plugging
cavities (Ingersoll-Rand, 1985).
CEMENTING
The emplacement of a cement slurry by various methods so that it fills the
space between the casing and the borehole wall to a predetermined height
above the bottom of the well. This secures the casing in place and excludes
water and other fluids from the borehole.
375
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CENTER PLUG
A plug within the pilot assembly of a hollow-stem auger that is used to
prevent formation materials from entering the stem of the lead auger during
drilling.
CENTER ROD
A rod attached to the pilot assembly that facilitates removal from the lead
end of the hollow-stem auger.
CENTRALIZER
Spring-loaded guides that are used to center the casing in the borehole to
ensure effective placement of filter pack or grout.
CHECK VALVE
Ball and spring valves on core barrels, rods and bailers that are used to
control water flow in one direction only.
CIRCULATE
To cycle drilling fluid through the drill pipe and borehole while drilling
operations are temporarily suspended to condition the drilling fluid and the
borehole before hoisting the drill pipe and to obtain cuttings from the
bottom of the well before drilling proceeds (Ingersoll-Rand, 1985).
CIRCULATION
The movement of drilling fluid from the suction pit through the pump, drill
pipe, bit and annular space in the borehole and back again to the suction
pit. The time involved is usually referred to as circulation time
(Ingersoll-Rand, 1985).
CIRCULATION, LOSS OF
The loss of drilling fluid into the formation through crevices or by
infiltration into a porous media.
CLAY
A plastic, soft, variously colored earth, commonly a hydrous silicate of
alumina, formed by the decomposition of feldspar and other aluminum
silicates (Ingersoll-Rand, 1985).
COLLAPSE STRENGTH
The capability of a casing or well intake to resist collapse by any or all
external loads to which it is subjected during and after installation.
COMPRESSIVE STRENGTH
The greatest compressive stress that a substance can bear without
deformation.
CONDUCTIVITY
A measure of the quantity of electricity transferred across unit area per
unit potential gradient per unit time. It is the reciprocal of resistivity.
376
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CONE OF DEPRESSION
A depression in the ground-water table or potentiometric surface that has
the shape of an inverted cone and develops around a well from which water is
being withdrawn. It defines the area of influence of a well (Driscoll.
1986).
CONE OF IMPRESSION
A conical mound on the water table that develops in response to well
injection whose shape is identical to the cone of depression formed during
pumping of the aquifer.
CONFINED AQUIFER
An aquifer which is bounded above and below by low-permeability formations.
CONFINING BED
The relatively impermeable formation immediately overlying or underlying a
confined aquifer.
CONTAMINANT
Any physical, chemical, biological or radiological substance or matter in
water that has an adverse impact.
CONTAMINATION
Contamination is the introduction into ground water of any chemical
material, organic material, live organism or radioactive material that will
adversely affect the quality of the ground water.
CONTINUOUS SAMPLING TUBE SYSTEM
Thin-wall sampling tube attached in advance -of the cutting head of the
hollow-stem auger that allows undisturbed samples to be taken continuously
while the augers are rotated.
CONTINUOUS SLOT WIRE-WOUND INTAKE
A well intake that is made by winding and welding triangular-shaped,
cold-rolled wire around a cylindrical array of rods. The spacing of each
successive turn of wire determines the slot size of the intake.
CORE
A continuous columnar sample of the lithologic units extracted from a
bprehole. Such a sample preserves stratigraphic contacts and structural
features (United States Environmental Protection Agency, 1986).
CORE BARREL
A reaming shell and length of tubing used during air or mud rotary drilling
to collect formation samples in both consolidated and unconsolidated
formations. Core barrels may be single or double walled and of a swivel or
rigid type.
CORE LIFTER
A tapered split ring inside the bit and surrounding the core. On lifting
the rods, the taper causes the ring to contract in diameter, seizing and
holding the core (Ingersoll-Rand, 1985).
377
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CORROSION
The adverse chemical alteration that reverts elemental metals back to more
stable mineral compounds and that affects the physical and chemical
properties of the metal.
COST-PLUS CONTRACT
Drilling contracts that list specific costs associated with performing the
work and include a percentage of those costs as an additional amount that
will be paid to perform a job.
COUPLING
A connector for drill rods, pipe or casing with identical threads, male
and/or female, at each end (Ingersoll-Rand, 1985).
CROSS CONTAMINATION
The movement of contaminants between aquifers or water-bearing zones through
an unsealed or improperly sealed borehole.
CUTTER HEAD
The auger head located at the lead edge of the auger column that breaks up
formation materials during drilling.
CUTTINGS
Formation particles obtained from a borehole during the drilling process.
DECONTAMINATION
A variety of processes used to clean equipment that has contacted formation
material or ground water that is known to be or suspected of being
contaminated.
DENNISON SAMPLER
A specialized sampler of a double-tube core design with a thin inner tube
that permits penetration in extremely stiff or highly cemented
unconsolidated deposits while collecting a thin-wall sample.
DENSITY
The weight of a substance per unit volume.
DEVELOPMENT
The act of repairing damage to the formation caused during drilling
procedures and increasing the porosity and permeability of the materials
surrounding the intake portion of the well (Driscoll, 1986).
DIATOMACEOUS EARTH
A cement additive composed of siliceous skeletons of diatoms used to reduce
slurry density, increase water demand and thickening time while reducing set
strength.
DIFFERENTIAL PRESSURE
The difference in pressure between the hydrostatic head of the drilling
fluid-filled or empty borehole and the formation pressure at any given depth
(Ingersoll-Rand, 1985).
378
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DIRECT MUD ROTARY
A drilling technique whereby a drilling fluid is pumped down the drill rod,
through the bit and circulates back to the surface by moving up the annular
space between the drill rods and the borehole.
DISPERSION
A process of contaminant transport that occurs by mechanical mixing and
molecular diffusion.
DISSOCIATION
The splitting up of a compound or element into two or more simple molecules,
atoms or ions. Applied usually to the effect of the action of heat or
solvents upon dissolved substances. The reaction is reversible and not as
permanent as decomposition; that is, when the solvent is removed, the ions
recombine (Ingersoll-Rand, 1985).
DNAPLS
Acronym for dense, nonaqueous-phase liquids.
DOWNGRADIENT
In the direction of decreasing hydrostatic head (United States Environmental
Protection Agency, 1986).
DOWNGRADIENT WELL
A well that has been installed hydraulically downgradient of the site and is
capable of detecting the migration of contaminants from a regulated unit.
Regulations require the installation of three or more downgradient wells
depending on the site-specific hydrogeological conditions and potential
zones of contaminant migration (United States Environmental Protection
Agency, 1986).
DOWN-THE-HOLE HAMMER
A pneumatic drill operated at the bottom of the drill pipe by air pressure
provided from the surface.
DRAWDOWN
The extent of lowering of the water surface in a well and water-bearing zone
resulting from the discharge of water from the well.
DRILL COLLAR
A length of heavy, thick-walled pipe used to stabilize the lower drill
string, to minimize bending caused by the weight of the drill pipe and to
add weight to the bit.
DRILL PIPE
Special pipe used to transmit rotation from the rotating mechanism to the
bit. The pipe also transmits weight to the bit and conveys air or fluid
which removes cuttings from the borehole and cools the bit (Driscoll, 1986).
DRILL ROD
Hollow flush-jointed or coupled rods that are rotated in the borehole that
are connected at the bottom to the drill bit and on the top to the rotating
or driving mechanism of a drilling rig.
379
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DRILL STRING
The string of pipe that extends from the bit to the driving mechanism that
serves to carry the mud down the borehole and to rotate the bit.
DRILLING FLUID
A water or air-based fluid used in the well drilling operation to remove
cuttings from the borehole, to clean and cool the bit, to reduce friction
between the drill string and the sides of the borehole and to seal the
borehole (Driscoll, 1986).
DRIVE BLOCK
A heavy weight used to drive pipe or casing through unconsolidated material.
DRIVE COUPLINGS
Heavy-duty couplings used to join sections of heavy-wall casing that are
specifically designed to withstand the forces during driving casing.
DRIVE HEAD
A component fastened to the top of pipe or casing to take the blow of the
drive block (Ingersoll-Rand, 1985).
DRIVE SHOE
A forged steel collar with a cutting edge fastened onto the bottom of the
casing to shear off irregularities in the hole as the casing advances. It
is designed to withstand drive pressures to protect the lower edge of the
casing as it is driven (United States Environmental Protection Agency,
1986).
DRIVEN WELL
A well that is driven to the desired depth, either by hand or machine; may
employ a wellpoint, or alternative equipment.
DROP HAMMER
A weighted device used to drive samplers during drilling and sampling.
DUAL-WALL REVERSE CIRCULATION
A drilling technique whereby the circulating fluid is pumped down between
the outer casing and the inner drill pipe, through the drill bit and up the
inside of the drill pipe.
EFFECTIVE GRAIN SIZE (EFFECTIVE DIAMETER)
The particle grain size of a sample where 90 percent represents coarser-size
grains and 10 percent represents finer-size grains, i.e., the coarsest
diameter in the finest 10 percent of the sediment.
ELECTRIC LOGGING
Logging techniques used in fluid-filled boreholes to obtain information
concerning the porosity, permeability and fluid content of the formations
drilled based on the dielectic properties of the aquifer materials.
ESTABLISHED GRADE
The permanent point of contact of the ground or artificial surface with the
casing or curbing of the well.
380
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ESTABLISHED GROUND SURFACE
The permanent elevation of the surface at the site of the well upon
completion.
FILTER CAKE (MUDCAKE)
The suspended solids that are deposited on the borehole wall during the
process of drilling.
FILTER CAKE THICKNESS (MUDCAKE)
A measurement, in 32nd of an inch, of the solids deposited on filter paper
during the standard 30-minute API filter test, or measurement of the solids
deposited on filter paper for a 7 1/2-minute duration (Ingersoll-Rand,
1985).
FILTER PACK
Sand, gravel or glass beads that are uniform, clean and well-rounded that
are placed in the annulus of the well between the borehole wall and the well
intake to prevent formation material from entering through the well intake
and to stabilize the adjacent formation.
FILTER PACK RATIO
A ratio used to express size differential between the formation materials
and the filter pack that typically refers to either the average grain size
(D ) or the 70-percent (D?Q) retained size of the formation material.
FILTRATE INVASION
The movement of drilling fluid into the adjacent formation that occurs when
the weight of the drilling fluid substantially exceeds the natural
hydrostatic pressure of the formation.
FIXED-PRICE CONTRACTS
Drilling contracts that list the manpower, materials and additional costs
needed to perform the work specified as a fixed cost payable upon
completion.
FLOATERS
Light-phase organic liquids in ground water capable of forming an immiscible
layer that can float on the water table (United States Environmental
Protection Agency, 1986).
FLOAT SHOE
A drillable valve attached to the bottom of the casing.
FLOCCULATION
The agglomeration of finely divided suspended solids into larger, usually
gelatinous particles through electrical charge alignment of particles.
FLOW METER
A tool used to monitor fluid flow rates in cased or uncased boreholes using
low-inertia impellers or through changes in thermal conductance as liquids
pass through the tool.
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FLOW-THROUGH WELL
The installation of a small-diameter well intake that penetrates all or a
significant portion of the aquifer. The well is designed to minimize
distortion of the flow field in the aquifer.
FLUID LOSS
Measure of the relative amount of fluid lost (filtrate) through permeable
formations or membranes when the drilling fluid is subjected to a pressure
differential (Ingersoll-Rand, 1985).
FLUOROPOLYMERS
Man-made materials consisting of different formulations of monomers molded
by powder metallurgy techniques that exhibit anti-stick properties and
resistance to chemical and biological attack.
FLUSH-COUPLED CASING
See Casing, Flush-coupled.
FLUSH-JOINT CASING
See Casing, Flush-joint.
FLY ASH
An additive to cement that increases sulfate resistance and early
compressive strength.
FORMATION
A mappable unit of consolidated material or unconsolidated material
characterized by a degree of lithologic homogeneity.
FORMATION DAMAGE
Damage to the formation resulting from drilling activities (e.g. the
invasion of drilling fluids or formation of mudcake) that alter the
hydraulic properties of formation materials.
FORMATION FLUID
The natural fluids present in the formation or aquifer.
FORMATION STABILIZER (FILTER PACK)
A sand or gravel placed in the annulus of the well between the borehole and
the well intake to provide temporary or long-term support for the borehole
(Driscoll, 1986).
GEL STRENGTH
A measure of the capability of the drilling fluid to maintain suspension of
particulate matter in the mud column when the pump is off.
GRAIN SIZE
The general dimensions of the particles in a sediment or rock, or of the
grains of a particular mineral that make up a sediment or rock. It is
common for these dimensions to be referred to with broad terms, such as
fine, medium, and coarse. A widely used grain size classification is the
Udden-Wentworth grade scale (United States Environmental Protection Agency,
1986).
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GRAVEL PACK (ARTIFICIAL FILTER PACK); see also Filter Pack
A term used to describe gravel or other permeable filter material placed in
the annular space around a well intake to prevent the movement of finer
material into the well casing, to stabilize the formation and to increase
the ability of the well to yield water.
GROUND WATER
Any water below the surface of the earth, usually referring to the zone of
saturation.
GROUT
A fluid mixture of neat cement and water with various additives or bentonite
of a consistency that can be forced through a pipe and emplaced in the
annular space between the borehole and the casing to form an impermeable
seal.
GROUTING
The operation by which grout is placed between the casing and the wall of
the borehole to secure the casing in place and to exclude water and other
fluids from moving into and through the borehole.
GYPSUM
An additive to cement slurries that produces a quick-setting, hard cement
that expands upon curing.
HALOGENATEO HYDROCARBONS
An organic compound containing one or more halogens (e.g., fluorine,
chlorine, bromine, and iodine) (United States Environmental Protection
Agency, 1986).
HAND AUGER
Any of a variety of hand-operated devices for drilling shallow holes into
the ground.
HEAD LOSS
That part of potential energy that is lost because of friction as water
flows through a porous medium.
HEAT OF HYDRATION
Exothermic or heat-producing reaction that occurs during the curing of
cement.
HEAVING SAND
Saturated sands encountered during drilling where the hydrostatic pressure
of the formation is greater than the borehole pressure causing the sands to
move up into the borehole.
HIGH-YIELD DRILLING CLAY
A classification given to a group of commercial drilling-clay preparations
having a yield of 35 to 50 bbl/ton and intermediate between bentonite and
low-yield clays. High-yield drilling clays are usually prepared by
peptizing low-yield calcium montmorillonite clays or, in a few cases, by
blending some bentonite with the peptized low-yield clay (Ingersoll-Rand,
1985).
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HOLLOW-STEM AUGER DRILLING
A drilling technique in which hollow, interconnected flight augers, with a
cutting head, are pressed downward as the auger is rotated.
HOMOGENEOUS
Exhibiting a uniform or similar nature.
HYDRAULIC CONDUCTIVITY
A coefficient of proportionality that describes the rate at which a fluid
can move through a permeable medium. It is a function of both the media and
of the fluid flowing through it (United States Environmental Protection
Agency, 1986).
HYDRAULIC GRADIENT
The change in static head per unit of distance in a given direction. If not
specified, the direction generally is understood to be that of the maximum
rate of decrease in head.
HYDROSTATIC HEAD
The pressure exerted by a column of fluid, usually expressed in pounds per
square inch (psi). To determine the hydrostatic head at a given depth in
psi, multiply the depth in feet by the density in pounds per gallon by 0.052
(Ingersoll-Rand, 1985).
IMMISCIBLE
Constituents that are not significantly soluble in water.
INCRUSTATION (ENCRUSTATION)
The process by which a crust or coating is formed on the well intake and/or
casing, typically through chemical or biological reactions.
INDUCTION TOOL
A geophysical logging tool used to measure pore fluid conductivity.
INHIBITOR (MUD)
Substances generally regarded as drilling mud contaminants, such as salt and
calcium sulfate, are called inhibitors when purposely added to mud so that
the filtrate from the drilling fluid will prevent or retard the hydration of
formation clays and shales (Ingersoll-Rand, 1985).
ISOTROPIC
A medium whose properties are the same in all directions.
JET PERCUSSION
A drilling process that uses a wedge-shaped drill bit that discharges water
under pressure while being raised and lowered to loosen or break up material
in the borehole.
KELLY
Hollow steel bar that is in the main section of drill string to which power
is directly transmitted from the rotary table to rotate the drill pipe and
bit (Driscoll, 1986).
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KETONES
Class of organic compounds where the carbony1 group is bonded to two aIkyI
groups (United States Environmental Protection Agency, 1986).
KNOCK-OUT PLATE
A nonretrievable plate wedged within the auger head that replaces the
traditional pilot assembly and center rod that is used to prevent formation
materials from entering the hollow auger stem.
LOGGING, RADIOACTIVE
The logging process whereby a neutron source is lowered down the borehole,
followed by a recorder, to determine moisture content and to identify water-
bearing zones.
LOST CIRCULATION
The result of drilling fluid escaping from the borehole into the formation
by way of crevices or porous media (Driscoll, 1986).
LOUVERED INTAKE
A well intake with openings that are manufactured in solid-wall metal tubing
by stamping outward with a punch against dies that control the size of the
openings.
LOW-SOLIDS MUDS
A designation given to any type of mud where high-performing additives have
been partially or wholly substituted for commercial or natural clays
(Ingersoll-Rand, 1985).
LOW-YIELD WELL
A relative term referring to a well that cannot recover in sufficient time
after well evacuation to permit the immediate collection of water samples
(United States Environmental Protection Agency, 1986).
MACHINE-SLOTTED INTAKE
Well intakes fabricated from standard casing where slots of a predetermined
width are cut into the casing at regular intervals using machining tools.
MALE AND FEMALE THREADS
Now called pin and box threads, as in the oil industry (Ingersoll-Rand,
1985).
MARSH FUNNEL
A device used to measure drilling fluid viscosity where the time required
for a known volume of drilling fluid to drain through an orifice is measured
and calibrated against a time for draining of an equal volume of water.
MARSH FUNNEL VISCOSITY
A measure of the number of seconds required for 1 quart (946 ml) of a given
fluid to flow through the Marsh funnel.
MECHANICAL JOINING
The use of threaded connections to join two sections of casing.
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MESH
A measure of fineness of a woven material, screen, or sieve; e.g. a 200-mesh
sieve screen with a wire diameter of 0.0021 inch (0.0533 mm) has an opening
of 0.074 mm, or will pass a particle of 74 microns (Ingersoll-Rand, 1985).
MISCIBLE
Materials that are soluble in water and are typically mobile in the
subsurface.
MONITORING WELL
An excavation that is constructed by a variety of techniques for the purpose
of extracting ground water for physical, chemical or biological testing or
for measuring water levels.
MONTMORILLONITE
A clay mineral commonly used as an additive to drilling muds; the main
constituent in bentonite.
MOUNDING
A phenomenon usually created by the recharge of ground water from a man-made
structure into a permeable geologic material. Associated ground-water flow
will be away from the man-made structure in all directions (United States
Environmental Protection Agency, 1986).
MUD ADDITIVE
Any material added to a drilling fluid to alter its chemical or physical
properties.
MUD BALANCE
A balance used to determine the density of a drilling mud. (Ingersoll-Rand,
1985).
MULTIPLE LEVEL MONITORING WELLS
A single-hole monitoring well that is installed with devices capable of
sampling at multiple levels within the formation(s).
NATURAL CLAYS
Clays that are encountered when drilling various formations, whose yield may
vary greatly, and that may or may not be purposely incorporated into the
drilling mud system.
NATURALLY DEVELOPED WELL
A well construction technique whereby the natural formation materials are
allowed to collapse around the well intake and fine formation materials are
removed using standard development techniques.
NEAT CEMENT
A mixture of Portland cement (ASTM C-150) and water in the proportion of 5
to 6 gallons of clean water per bag (94 pounds or 1 cubic foot) of cement.
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NESTED WELLS
Monitoring wells that consist either of a series of single-riser/limited-
interval wells that are closely spaced laterally, but with intakes at
different depths, or as multiple single-riser/limited-interval wells
constructed in a single borehole.
NOMINAL
A term used to describe standard sizes for pipe from 1/8 in to 12 in (3.2 mm
to 305 mm) in diameter specified on the basis of the inside diameter.
Depending on the wall thickness, the inside diameter may be less than or
greater than the number indicated (Driscoll, 1986).
OBSERVATION WELL
A well drilled in a selected location for the purpose of observing
parameters such as water level and pressure changes (Driscoll, 1986).
OPEN HOLE
The uncased portion of the well that is open to the formation.
ORGANIC POLYMERS
Drilling fluid additives comprised of log-chained, heavy organic molecules
used to alter the physical and chemical characteristics of the fluid.
0-RING SEAL
A rubber seal emplaced between the threaded connections of hollow-stem auger
sections to prevent leakage and infiltration of fluids.
OVERBANK DEPOSITS
Fine-grained sediment (silt and clay) deposited from suspension on a flood
plain by floodwaters that cannot be contained within the stream channel
(Bates and Jackson, 1987).
OXIDATION
The loss of electrons from a substance.
PACKER
A compressible cylinder of rubber and metal that is placed in or outside a
well to plug or seal the well or borehole at a specific point.
PARTIAL PENETRATION
the intake portion of the well is less than the full thickness of the
aquifer.
PARTS PER MILLION (PPM)
Unit weight of solute per million unit weights of solution (solute plus
solvent), corresponding to a weight-percent (Ingersoil-Rand, 1985).
PENETRATION, RATE OF
The rate at which the drill proceeds in the deepening of the borehole,
typically expressed in terms of feet per hour or in blow counts per foot
advanced.
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PERCHED GROUND WATER
Ground water in a saturated zone that is separated from the main body of
ground water by a less permeable unsaturated zone or formation.
PERCOLATE
The act of water seeping or filtering through materials without a definite
channel.
PERMEABILITY
A measure of the relative ease with which a porous medium can transmit a
liquid under a potential gradient (United States Environmental Protection
Agency, 1975).
PIEZOMETERS
Generally a small-diameter, non-pumping well used to measure the elevation
of the water table or potentiometric surface (United States Environmental
Protection Agency, 1986).
PILOT ASSEMBLY
The assembly placed at the lead end of the auger consisting of a solid
center plug and a pilot bit.
PLUGS, CASING
Plug made of drillable material to correspond to the inside diameter of the
casing. Plugs are pumped to bottom of casing to force all cement outside of
casing (Ingersoll-Rand, 1985).
PLUGGING
The complete filling of a borehole or well with an impermeable material
which prevents flow into and through the borehole or well.
PLUME
An elongated and mobile column or band of a contaminant moving through the
subsurface.
POLYMERIC ADDITIVES
The natural organic colloids developed from the guar plant that are used for
viscosity control during drilling.
POLYVINYL CHLORIDE (PVC)
Thermoplastics produced by combining PVC resin with various types of
stabilizers, lubricants, pigments, fillers and processing aids, often
formulated to produce rigid well casing.
POROSITY
The percentage of void spaces or openings in a consolidated or
unconsolidated material.
PORTLAND CEMENT
Cement specified as Type I or Type II under ASTM C-150 standards.
POTENTIOMETRIC DATA
Ground-water surface elevations obtained at wells and piezometers that
penetrate a water-bearing formation.
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POTENTIOMETRIC SURFACE
An imaginary surface representing the total head of ground water in a
confined aquifer that is defined by the level to which water will rise in a
well (Driscoll, 1986).
PRECIPITATE
Material that will separate out of solution or slurry as a solid under
changing chemical and/or physical conditions.
PRESSURE SEALING
A process by which a grout is confined within the borehole or casing by the
use of retaining plugs or packers and by which sufficient pressure is
applied to drive the grout slurry into and within the annular space or zone
to be grouted.
PROTECTIVE CASING
A string of casing set in the borehole to stabilize a section of the
formation and/or to prevent leakage into and out of the formation and to
allow drilling to continue to a greater depth.
PROTECTORS, THREAD
A steel box and pin used to plug each end of a drill pipe when it is pulled
from the borehole to prevent foreign matter or abrasives from collecting on
the greasy threads and to protect threads from corrosion or damage while
transporting or in storage (Ingersoll-Rand, 1985).
PUDDLED CLAY
Puddling clay is a mixture of bentonite, other expansive clays, fine-grained
material and water, in a ratio of not less than 7 pounds of bentonite or
expansive clay per gallon of water. It must be composed of not less than 50
percent expansive clay with the maximium size of the remaining portion not
exceeding that of coarse sand.
PULLING CASING
To remove the casing from a well.
PUMPING/OVERPUMPING/BACKWASHING
A well development technique that alternately starts and stops a pump to
raise and drop the column of water in the borehole in a surging action.
PUMP TEST
A test used to determine aquifer characteristics performed by pumping a well
for a period of time and observing the change in hydraulic head that occurs
in adjacent wells. A pump test may be used to determine degree of hydraulic
interconnection between different water-bearing units, as well as the
recharge rate of a well (United States Environmental Protection Agency,
1986).
PUMPING WATER LEVEL
The elevation of the surface of the water in a well or the water pressure at
the top of a flowing artesian well after a period of pumping or flow at a
specified rate.
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RADIOACTIVE LOGGING
A logging process whereby a radioactive source is lowered down a borehole to
determine formation characteristics. Radioactive logging devices typically
used for ground-water investigations include gamma and neutron logging
probes.
RADIUS OF INFLUENCE (CONE OF DEPRESSION)
The radial distance from the center of a well under pumping conditions to
the point where there is no lowering of the water table or potentiometric
surface (Driscoll, 1986).
REAMER
A bit-like tool, generally run directly above the bit, used to enlarge and
maintain a straight borehole. (After Ingersoll-Rand, 1985)
REAMING
A drilling operation used to enlarge a borehole.
REHABILITATION
The restoration of a well to its most efficient condition using a variety of
chemical and mechanical techniques that are often combined for optimum
effectiveness.
RESISTIVITY
The electrical resistance offered to the passage of a current, expressed in
ohm-meters; the reciprocal of conductivity. Fresh-water muds are usually
characterized by high resistivity; salt-water muds, by low resistivity
(Ingersoll-Rand, 1985).
REVERSE CIRCULATION
A method of filter pack emplacement where the filter pack material is fed
into the annulus around the well intake concurrently with a return flow of
water. The water is pumped to the surface through the casing.
In dual-wall reverse circulation rotary drilling, the circulating fluid is
pumped down between the outer casing and inner drill pipe, and then up and
out through the drill bit to the surface.
RIG
The machinery used in the construction or repair of wells and boreholes.
ROTARY TABLE DRIVE
Hydraulic or mechanical drive on a rotary rig used to rotate the drill stem
and bit.
RVCM
Residual vinyl chloride monomer.
SAMPLES
Materials obtained from the borehole during the drilling and/or formation
sampling process that provide geological information. May also refer to
water from completed well used for hydrogeochemlcal analysis.
SATURATED ZONE (PHREATIC ZONE)
The subsurface zone in which all pore spaces are filled with water.
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SCHEDULING
Standardization of casing diameters and wall thicknesses where wall
thickness increases as the scheduling number increases.
SCREEN
See Well Intake.
SEAL
The impermeable material, such as cement grout, bentonite or pudded clay,
placed in the annular space between the borehole wall and the permanent
casing to prevent the downhole movement of surface water or the vertical
mixing of water-bearing zones.
SEGREGATION
The differential settling of filter pack or other materials that occurs in
the annular space surrounding the intake during placement by gravity (free
fall).
SET CASING
To install steel pipe or casing in a borehole.
SHALE SHAKER
Vibratory screen connected in line to the circulation system of a mud rotary
rig through which the drilling fluid passes and where suspended material is
separated and samples are collected.
SHELBY TUBE
Device used in conjunction with a drilling rig to obtain an undisturbed core
sample of unconsolidated strata (United States Environmental Protection
Agency, 1986).
SIEVE ANALYSIS
Determination of the particle-size distribution of soil, sediment or rock by
measuring the percentage of the particles that will pass through standard
sieves of various sizes (Driscoll, 1986).
SINGLE-RISER/LIMITED-INTERVAL WELL
An individual monitoring well installed with a limited-length well intake
that is used to monitor a specific zone of a formation.
SINKERS
Dense-phase organic liquids that coalesce in an immiscible layer at the
bottom of the saturated zone (United States Environmental Protection Agency,
1986).
SLIP-FIT BOX AND PIN CONNECTIONS
A type of coupling used to join two hollow-stem auger sections.
SLOTTED COUPLINGS
A device attached to the knock-out plate at the base of the lead auger that
allows water to pass into the center of the auger during drilling while
preventing the entrance of sediment or sand into the hollow stem.
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SLOTTED WELL CASING
Well intakes that are fabricated by cutting slots of predetermined width at
regular intervals by machining tools.
SLUG TEST
A single well test to determine the in-situ hydraulic conductivity of
typically low-permeability formations by the instantaneous addition or
removal of a known quantity (slug) of water into or from a well, and the
subsequent measurement of the resulting well recovery (United States
Environmental Protection Agency, 1986).
SLURRY
A thin mixture of liquid, especially water, and any of several finely-
divided substances such as cement or clay particles (Driscoll, 1986).
SMECTITE
A commonly used name for clay minerals that exhibit high swelling properties
and a high cation exchange capacity.
SODIUM BENTONITE
A type of clay added to drilling fluids to increase viscosity.
SOLIDS CONCENTRATION or CONTENT
The total amount of solids in a drilling fluid as determined by distillation
that includes both the dissolved and the suspended or undissolved solids.
The suspended solids content may be a combination of high and low specific
gravity solids and native or commercial solids. Examples of dissolved
solids are the soluble salts of sodium, calcium and magnesium. Suspended
solids make up the mudcake; dissolved solids remain in the filtrate. The
total suspended and dissolved solids contents are commonly expressed as
percent by weight (Ingersoll-Rand, 1985).
SOLID-FLIGHT AUGER
A solid-stem auger with a cutting head and continuous flighting that is
rotated by a rotary drive head at the surface and forced downward by a
hydraulic pulldown or feed device.
SOLVATION
The degradation of plastic well casing in the presence of very high
concentrations of specific organic solvents.
SOLVENT CEMENTING
A method of joining two sections of casing where solvent is applied to
penetrate and soften the casing pieces and fuses the casing together as the
solvent cement cures.
SORPTION
The combined effect of adsorption and/or absorption.
SPECIFIC CAPACITY
The rate of discharge of water from a well per unit of drawdown of the water
level, commonly expressed in gpm/ft or m /day/m, and that varies with the
duration of discharge (Driscoll, 1986).
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SPECIFIC YIELD
The ratio of the volume of water that a given mass of saturated rock or soil
will yield by gravity to the volume of the mass expressed as a percentage
(Driscoll, 1986).
SPLIT-SPOON SAMPLER
A hollow, tubular sampling device driven by a 140-pound weight below the
drill stem to retrieve samples of the formation.
SPUDDING BEAM
See Walking Beam
STANDARD DIMENSION RATIO
A ratio expressed as the outside diameter of casing divided by the wall
thickness.
STATIC WATER LEVEL
The distance measured from the established ground surface to the water
surface in a well neither being pumped nor under the influence of pumping
nor flowing under artesian pressure.
SURFACE SEAL
The seal at the surface of the ground that prevents the intrusion of
surficial contaminants into the well or borehole.
SURFACTANT
A substance capable of reducing the surface tension of a liquid in which it
is dissolved. Used in air-based drilling fluids to produce foam, and during
well development to disaggregate clays (Driscoil. 1986).
SURGE BLOCK
A plunger-like tool consisting of leather or rubber discs sandwiched between
steel or wooden discs that may be solid or valved that is used in well
development.
SURGING
A well development technique where the surge block is alternately lifted and
dropped within the borehole above or adjacent to the screen to create a
strong inward and outward movement of water through the well intake.
SWIVEL, WATER
A hose coupling that forms a connection between the slush pumps and the
drill string and permits rotation of the drill string (Ingersoll-Rand,
1985).
TEFLON®
Trade name for fluoropolymer material.
TELESCOPING
A method of fitting or placing one casing inside another or of introducing
screen through a casing diameter larger than the diameter of the screen
(United States Environmental Protection Agency, 1975).
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TEMPERATURE SURVEY
An operation to determine temperatures at various depths in the wellbore,
typically used to ensure the proper cementing of the casing or to find the
location of inflow of water into the borehole (Ingersoll-Rand, 1985).
TENSILE STRENGTH
The greatest longitudinal stress a substance can bear without pulling the
material apart.
TEST HOLE
A hole designed to obtain information on ground-water quality and/or
geological and hydrological conditions (United States Environmental
Protection Agency, 1975).
THERMOPLASTIC MATERIALS
Man-made materials often used for well casing that are composed of different
formulations of large organic molecules that are softened by heating and
hardened by cooling and can be easily molded and extruded.
THIN-WALL SAMPLERS
A hollow tubular sampling device that is pressed into the formation below
the drill stem to retrieve an undisturbed sample.
TOP-HEAD DRIVE
A drive for the drill stem where the bottom sub of the hydraulic drive motor
is connected directly to the drill rod.
TOTAL DISSOLVED SOLIDS (TDS)
A term that expresses the quantity of dissolved material in a sample of
water.
TRANSMISSIVITY
The rate at which water is transmitted through a unit width of an aquifer
under a unit hydraulic gradient. Transmissivity values are given in gallons
per day through a vertical section of an aquifer one foot wide and extending
the full saturated height of an aquifer under hydraulic gradient of 1 in the
English Engineering System; in the International System, transmissivity is
given in cubic meters per day through a vertical section in an aquifer one
meter wide and extending the full saturated height of the aquifer under a
hydraulic gradient of 1 (Driscoll, 1986).
TREMIE METHOD
Method whereby filter pack is emplaced or bentonite/cement slurries are
pumped uniformly into the annular space of the borehole through the use of a
tremie pipe.
TREMIE PIPE
A device, usually a small-diameter pipe, that carries grouting materials to
the bottom of the borehole and that allows pressure grouting from the bottom
up without introduction of appreciable air pockets (United States
Environmental Protection Agency, 1975).
TURBIDITY
Solids and organic matter suspended in water.
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UNCONFINED AQUIFER
An aquifer not bounded above by a bed of distinctly lower permeability than
that of the aquifer and containing ground water below a water table under
pressure approximately equal to that of the atmosphere.
UNCONSOLIDATED FORMATION
Unconsolidated formations are naturally-occurring earth formations that have
not been lithified; they may include alluvium, soil, gravel, clay and
overburden, etc.
UNDERREAMER
A bit-like tool with expanding and retracting cutters for enlarging a drill
hole below the casing (Ingersoll-Rand, 1985).
UNIFIED SOIL CLASSIFICATION SYSTEM
A standardized classification system for the description of soils that is
based on particle size and moisture content.
UNIFORMITY COEFFICIENT
A measure of the grading uniformity of sediment defined as the 40-percent
retained size divided by 90-percent retained size.
UNIT-PRICE CONTRACTS
Drilling contracts that establish a fixed price for materials and manpower
for each unit of work performed.
UPGRADIENT WELL
One or more wells that are placed hydraulically upgradient of the site and
are capable of yielding ground-water samples that are representative of
regional conditions and are not affected by the regulated facility
(United States Environmental Protection Agency, 1986).
VADOSE ZONE (UNSATURATED ZONE)
A subsurface zone above the water table in which the interstices of a porous
medium are only partially filled with water (United States Environmental
Protection Agency, 1986).
VICKSBURG SAMPLER
A strong thin-walled sampler for use in stiff and highly cemented
unconso1idated depos its.
VISCOSITY
The resistance offered by the drilling fluid to flow.
VOLATILE ORGANICS
Liquid or solid organic compounds with a tendency to pass into the vapor
state (United States Environmental Protection Agency, 1986).
WALKING BEAM (SPUDDING BEAM)
The beam of a cable tool rig that pivots at one end while the other end
connected to the drill line is moved up and down, imparting the "spudding"
action of the rig.
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WATER SWIVEL
See Swivel, Water.
WATER TABLE
The upper surface in an unconfined ground water body at which the pressure
is atmospheric (United States Environmental Protection Agency, 1975).
WEIGHT
Reference to the density of a drilling fluid. This is normally expressed in
either Ib/gal., Ib/cu ft, or psi hydrostatic pressure per 1000 ft of depth.
WELL
Any test hole or other excavation that is drilled, cored, bored, washed,
fractured, driven, dug, jetted or otherwise constructed when intended use of
such excavation is for the location, monitoring, dewatering, observation,
diversion, artificial recharge, or acquisition of ground water or for
conducting pumping equipment or aquifer tests. May also refer to casing and
intake.
WELL CAP
An approved, removable apparatus or device used to cover a well.
WELL CLUSTER
Two or more wells completed (screened) to different depths in a single
borehole or in a series of boreholes in close proximity to each other. From
these wells, water samples that are representative of different horizons
within one or more aquifers can be collected (United States Environmental
Protection Agency, 1986).
WELL CONSTRUCTION
Water well construction means all acts necessary to obtain ground water from
wells.
WELL CONTRACTOR
Any person, firm or corporation engaged in the business of constructing,
altering, testing, developing or repairing a well or borehole.
WELL DEVELOPMENT
Techniques used to repair damage to the borehole from the drilling process
.so that natural hydraulic conditions are restored; yields are enhanced and
fine materials are removed.
WELL EVACUATION
Process of removing stagnant water from a well prior to sampling (United
States Environmental Protection Agency, 1986).
WELL INTAKE (WELL SCREEN)
A screening device used to keep materials other than water from entering the
well and to stabilize the surrounding formation.
WELL LOG
A record that includes information on well construction details,
descriptions of geologic formations and well testing or development
techniques used in well construction.
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WELL POINT
A sturdy, reinforced well screen or intake that can be installed by driving
into the ground.
WELL SEAL
An arrangement or device used to cover a well or to establish or maintain a
junction between the casing or curbing of a well and the piping or equipment
installed therein to prevent contaminated water or other material from
entering the well at the land surface.
WELL VENT
An outlet at or near the upper end of the well casing to allow equalization
of air pressure in the well.
YIELD
The quantity of water per unit of time that may flow or be pumped from a
well under specified conditions.
YIELD POINT
A measure of the amount of pressure, after the shutdown of drilling fluid
circulation, that must be exerted by the pump upon restarting of the
drilling fluid circulation to start flow.
ZONE OF AERATION
The zone above the water table and capillary fringe in which the interstices
are partly filled with air.
ZONE OF SATURATION
The zone below the water table in which all- of the interstices are filled
with ground water.
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REFERENCES
Bates, Robert L. and Julia A. Jackson, eds, 1987. Glossary of geology;
American Geological Institute, Alexandria, Virginia, 788 pp.
Driscoll, Fletcher, G. 1986. Ground water and wells; Johnson Division,
St. Paul, Minnesota, 1089 pp.
Ingersoll-Rand, 1985. Drilling terminology; Ingersoll-Rand Rotary Drill
Division, Garland, Texas, 120 pp.
United States Environmental Protection Agency, 1975. Manual of water well
construction practices; United States Environmental Protection, Office of
Water Supply, EPA-570/9-75-001, 156 pp.
United States Environmental Protection Agency, 1986. RCRA ground-water
monitoring technical enforcement guidance document; Office of Waste Programs
Enforcement, Office of Solid Waste and Emergency Response, Washington, D.C.,
OSWER-9950.1, 317 pp.
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