ŁEPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600/4-89/034
March 1991
Handbook of
Suggested Practices for the
Design and Installation of
Ground-Water Monitoring
Wells
Borehole
Casing Joint
Compressive Forces
Casing
Tensile (Pull-apart) Forces
Critical at Casing Joints
Collapse Forces
. (Critical at Greater Depths)
Well Intake (Screen)
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EPA/600/4-89/034
March 1991
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
-T Rebecca J. Petty
/\ Ohio Department of Natural Resources
Division of Groundwater
Columbus, Ohio 43215
Jay H. Lehr and Helen Sedoris
National Water Well Association
Dublin, Ohio 43017
David M. Nielsen
Blasland, Bouck and Lee
Westerville, Ohio 43081
Jane E. Denne (also the Project Officer)
Advanced Monitoring Systems Division
Environmental Monitoring Systems Laboratory
Las Vegas, Nevada 89193-3478
Environmental Monitoring Systems Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Las Vegas, Nevada 89193-3478
Printed on Recycled Paper
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Roof
Chicago, IL 60604-3590
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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 RCR A 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 informa-
tion 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, an-
thropogenic 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. Main-
tenance 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.
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Contents
Notice ii
Abstract iii
Figures viii
Tables x
Acknowledgments xi
Section
1. Introduction 1
Objectives and scope 1
Purpose and importance of proper ground-water monitoring well installation 1
Organization of the document 7
References 7
2. Factors influencing ground-water monitoring well design and installation 9
Geologic and hydrogeologic conditions 9
Hydrogeologic regions of the United States 9
Site-specific geologic and hydrogeologic conditions 18
Facility characteristics 18
Type of facility 19
Waste characteristics 20
Other anthropogenic influences 23
Equipment that the well must accommodate 23
Borehole geophysical tools and downhole cameras 24
Water-level measuring devices 26
Ground-water sampling devices 26
Aquifer testing procedures 26
References 27
3. Monitoring well planning considerations 29
Recordkeeping 29
Decontamination 29
Decontamination area 30
Types of equipment 32
Frequency of equipment decontamination 32
Cleaning solutions and/or wash water 32
Containment of residual contaminants and cleaning solutions and/or wash water 33
Effectiveness of decontamination procedures 34
Personnel decontamination 34
References 34
4. Description and selection of drilling methods 35
Introduction 35
Drilling methods for monitoring well installation 35
Hand augers 35
Driven wells 35
Jet percussion 36
Solid-flight augers 37
Hollow-stem augers 38
Direct mud rotary 40
Air rotary drilling 42
Air rotary with casing driver 44
Dual-wall reverse-circulation 44
Cable tool drilling 47
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Other drilling methods 49
Drilling fluids 49
Influence of drilling fluids on monitoring well construction 49
Drilling fluid characteristics 50
Mud-based applications 51
Air-based applications 52
Soil sampling and rock coring methods 53
Split-spoon samplers 54
Thin-wall samplers 55
Specialized soil samplers 56
Core barrels 57
Selection of drilling methods for monitoring well installation 58
Matrix purpose 58
Matrix description and development 58
How to use the matrices 61
How to interpret a matrix number 61
Criteria for evaluating drilling methods 61
Versatility of the drilling equipment and technology with respect to the hydrogeologic
conditions at the site 63
Reliability of formation (soil/rock/water) samples collected during drilling 64
Relative drilling costs 67
Availability of equipment 67
Relative time required for well installation and development 67
Ability of drilling technology to preserve natural conditions 68
Ability of the specified drilling technology to permit the installation of the proposed
casing diameter at the design depth 68
Ease of well completion and development 69
Drilling specifications and contracts 69
References 71
5. Design components of monitoring wells 73
Introduction 73
Well casing 73
Purpose of the casing 73
General casing material characteristics 73
Types of casing materials 75
Coupling procedures for joining casing 83
Well casing diameter 85
Casing cleaning requirements 86
Casing cost 86
Monitoring well intakes 86
Naturally-developed wells 87
Artificially filter-packed wells 88
Well intake design 93
Annular seals 96
Purpose of the annular seal 96
Materials used for annular seals 97
Methods for evaluating annular seal integrity 101
Surface completion and protective measures 101
Surf ace seals 101
Above-ground completions 101
Flush-to-ground surface completions 102
References 102
6. Completion of monitoring wells 105
Introduction 105
Well completion techniques 105
Well intake installation 105
Filter pack installation 106
Annular seal installation 107
Types of well completions 109
Single riser/limited-interval wells 109
Single riser/flow-through wells 109
Nested wells HO
vi
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Multiple-level monitoring wells Ill
General suggestions for well completions 112
References 112
7. Monitoring well development 115
Introduction/philosophy 115
Factors affecting monitoring well development 115
Type of geologic material 115
Design and completion of the well 116
Type of drilling technology 116
Well development 117
Methods of well development 117
Bailing 120
Surge block 121
Pumping/overpumping/backwashing 121
References 123
8. Monitoring well network management considerations 125
Well documentation 125
Well maintenance and rehabilitation 125
Documenting monitoring well performance 125
Factors contributing to well maintenance needs 127
Downhole maintenance 128
Exterior well maintenance 128
Comparative costs of maintenance 130
Well abandonment 130
Introduction 130
Well abandonment considerations 130
Well abandonment procedures 131
Grouting procedures for plugging 132
Clean-up, documentation and notification 133
References 133
Master References 134
Appendices
A. Drilling and constructing monitoring wells with hollow-stem augers 141
B. Matrices for selecting appropriate drilling equipment 165
C. Abandonment of test holes, partially completed wells, and completed wells
(American Waterworks Association, 1984) 207
Glossary 209
VII
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Figures
Number EagŁ
1 Ground-water regions of the United States 10
2a Location of the Western Mountain Ranges region 10
2b Topographic and geologic features in the southern Rocky Mountains part of the Western
Mountain Ranges region 10
3a Location of the Alluvial Basins region 11
3b Common ground-water flow systems in the Alluvial Basins region 11
4a Location of the Columbia Lava Plateau region 11
4b Topographic and geologic features of the Columbia Lava Plateau region 11
5a Location of the Colorado Plateau and Wyoming Basin region 12
5b Topographic and geologic features of the Colorado Plateau and Wyoming Basin region 12
6a Location of the High Plains region 12
6b Topographic and geologic features of the High Plains region 12
7a Location of the Nonglaciated Central region 13
7b Topographic and geologic features of the Nonglaciated Central region 13
7c Topographic and geologic features along the western boundary of the Nonglaciated
Central region 13
8a Location of the Glaciated Central region 14
8b Topographic and geologic features of the Glaciated Central region 14
9a Location of the Piedmont and Blue Ridge region 14
9b Topographic and geologic features of the Piedmont and Blue Ridge region 14
lOa Location of the Northeast and Superior Uplands region 15
lOb Topographic and geologic features of the Northeast and Superior Uplands region 15
1 la Location of the Atlantic and Gulf Coastal Plain region 15
lib Topographic and geologic features of the Gulf Coastal Plain 15
12a Location of the Southeast Coastal Plain region 16
12b Topographic and geologic features of the Southeast Coastal Plain 16
13a Location of the Alluvial Valleys ground-water region 17
13b Topographic and geologic features of a section of the alluvial valley of the
Mississippi River 17
14 Topographic and geologic features of an Hawaiian island 17
15 Topographic and geologic features of parts of Alaska 17
16 Migration of a high density, miscible contaminant in the subsurface 21
17 Migration of a low density, soluble contaminant in the subsurface 21
18 Migration of a low density, immiscible contaminant in the subsurface 22
19 Migration of a dense, non-aqueous phase liquid (DNAPL) in the subsurface 22
20 Sample boring log format 31
21 Format for an "as-built" monitoring well diagram 32
22 Typical layout showing decontamination areas at a hazardous materials site 33
23 Diagram of a hand auger 35
24 Diagram of a wellpoint 36
25 Diagram of jet-percussion drilling 37
26 Diagram of a solid-flight auger 37
27 Typical components of a hollow-stem auger 39
28 Diagram of a screened auger 40
29 Diagram of a direct rotary circulation system 42
30 Diagram of a roller cone bit 43
31 Diagram of a down-the-hole hammer 44
32 Range of applicability for various rotary drilling methods 45
33 Diagram of a drill-through casing driver 46
34 Diagram of dual-wall reverse-circulation rotary method 46
viii
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35 Diagram of a cable tool drilling system 48
36 Diagrams of two types of bailers 51
37 Practical drilling fluid densities 52
38 Viscosity-building characteristics of drilling clays 52
39 Schematic of the behavior of clay particles when mixed into water 53
40 Diagram of a split-spoon sampler 55
41 Diagram of a thin-wall sampler 56
42 Two types of special soil samplers 57
43 Internal sleeve wireline piston sampler 58
44 Modified wireline piston sampler 60
45 Clam-shell fitted auger head 60
46 Types of sample retainers 60
47 Diagram of a continuous sampling tube system 61
48 Diagram of two types of core barrels 62
49 Format for a matrix on drilling method selection 63
50 Forces exerted on a monitoring well casing and screen during installation 74
51 Static compression results of Teflon® screen 77
52 Types of joints typically used between casing lengths 84
53 Effect of casing wall thickness on casing inside and outside diameter 87
54 Envelope of coarse-grained material created around a naturally developed well 88
55 Plot of grain size versus cumulative percentage of sample retained on sieve 89
56 Determining effective size of formation materials 90
57 Determining uniformity coefficient of formation materials 91
58 Envelope of coarse-grained material emplaced around an artificially filter-packed well 91
59 Artificial filter pack design criteria 92
60 Selecting well intake slot size based on filter pack grain size 93
61 Types of well intakes 97
62 Cross-sections of continuous-wrap wire-wound screen 97
63 Potential pathways for fluid movement in the casing-borehole annulus 98
64 Segregation of artificial filter pack materials caused by gravity emplacement 106
65 Tremie-pipe emplacement of artificial filter pack materials 106
66 Reverse-circulation emplacement of artificial filter pack materials 107
67 Emplacement of artificial filter pack material by backwashing 107
68 Tremie-pipe emplacement of annular seal material (either bentonite or neat cement slurry) 108
69 Diagram of a single-riser/flow-through well 109
70 Typical nested well designs 110
71 Field-fabricated PVC multilevel sampler 111
72 Multilevel capsule sampling device installation 112
73 Multiple zone inflatable packer sampling installation 112
74 Diagrams of typical bailers used in monitoring well development 122
75 Diagram of a typical surge block 123
76 Diagram of a specialized monitoring well surge block 124
IX
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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 24
4 Descriptive information to be recorded for each monitoring well 30
5 List of selected cleaning solutions used for equipment decontamination 34
6 Applications and limitations of hand augers 38
7 Applications and limitations of driven wells 38
8 Applications and limitations of jet-percussion drilling 38
9 Applications and limitations of solid-flight augers 39
10 Applications and limitations of hollow-stem augers 41
11 Applications and limitations of direct mud rotary drilling 41
12 Applications and limitations of air rotary drilling 44
13 Applications and limitations of air rotary with casing driver drilling 47
14 Applications and limitations of dual-wall reverse-circulation rotary drilling 47
15 Applications and limitations of cable tool drilling 49
16 Principal properties of water-based drilling fluids 50
17 Approximate Marsh Funnel viscosities required for drilling in typical types of
unconsolidated materials 51
18 Drilling fluid options when drilling with air 52
19 Characteristics of common formation-sampling methods 54
20 Standard penetration test correlation chart 55
21 Index to matrices 1 through 40 59
22 Suggested areas to be addressed in monitoring well bidding specifications 69
23 Suggested items for unit cost in contractor pricing schedule 70
24 Trade names, manufacturers, and countries of origin for various fluoropolymer materials 76
25 Typical physical properties of various fluoropolymer materials 76
26 Hydraulic collapse and burst pressure and unit weight of stainless steel well casing 78
27 Typical physical properties of thermoplastic well casing materials at 73.4°F 81
28 Hydraulic collapse pressure and unit weight of PVC well casing 81
29 Hydraulic collapse pressure and unit weight of ABS well casing 81
30 Representative classes of additives in rigid PVC materials used for pipe or well casing 83
31 Chemical parameters covered by NSF Standard 14 83
32 Volume of water in casing or borehole 86
33 Correlation chart of screen openings and sieve sizes 95
34 Typical slotted casing slot widths 96
35 Intake areas (square inches per lineal foot of screen) for
continuous wire-wound well intake 96
36 Summary of development methods for monitoring wells 118
37 Comprehensive monitoring well documentation 126
38 Additional monitoring well documentation 126
39 As-built construction diagram information 126
40 Field boring log information 127
41 Regional well maintenance problems 129
42 Chemicals used for well maintenance 129
43 Well abandonment data 133
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Acknowledgments
This document presents a discussion of the design and installation of ground-
water monitoring wells without specific regulatory recommendations. The infor-
mation 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 acknowledgment
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, Blasland & 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 acknowledg-
ment 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 are 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
Tim De La Grange, De La Grange and Sons, California
Lauren Evans, Arizona 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
xi
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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 O. 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, California
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 Vegas
Joe DLugosz, U.S. EPA, EMSL - Las Vegas
Phil Durgin, U.S. EPA, EMSL - Las Vegas
Larry Eccles, U.S. EPA, EMSL - Las Vegas
Steven Gardner, U.S. EPA, EMSL - Las Vegas
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 Vegas
Kendrick Taylor also provided information contained in the borehole geophysical
tool section of the document
XII
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Section 1
Introduction
Objectives and Scope
The Handbook of Suggested Practices for the Design and
Installation of Ground-Water Monitoring Wells has been pre-
pared 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 the 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 situ-
ations.
Impetus for the development of the Handbook of Sug-
gested Practices 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 regulatory programs which establish ground-
water monitoring requirements for specific sources of con-
tamination. 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 signifi-
cantly. 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.
With such diverse statutes mandating ground-water
monitoring requirements, it is not surprising that the regula-
tions 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 direc-
tives issued by agencies responsible for implementation of the
regulations. Examples of guidance documents include the Of-
fice of Waste Programs Enforcement Technical Enforcement
Guidance Document (TEGD) (United States Environmental
Protection Agency, 1986), the OfficeofSolidWaste Documents
SW-846 (Wehran Engineering Corporation, 1977) and SW-
611 (United States Environmental Protection Agency, 1987).
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 supersede 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 rep-
resent 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 moni-
toring 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/orrepresentative 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.
1
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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)
Investigations/detection Correction Prevention
Ground-water Federally
Ambient monitoring Water funded Regulatory Regulate Standards for
Statutes Inventories ground-water related supply remedial requirements chemical new/existing Aquifer
of sources* monitoring to sources* monitoring actions for sources* production sources* protection Standards Other"
Atomic Energy Act
Clean Water Act X
Coastal Zone
Management Act
Comprehensive Environmental
Response, Compensation
and Liability Act . X
Federal Insecticide, Fungicide
and Rodenticide Act
Federal Land Policy and
Management Act (and
associated mining laws)
Hazardous Liquid Pipeline
Safety Act X
Hazardous Materials
Transportation Act X
National Environmental
Policy Act
Reclamation Act
Resource Conservation and
Recovery Act X
Safe Drinking Water Act . X
Surface Mining Control and
Reclamation Act
Toxic Substances Control Act
Uranium Mill Tailings
Radiation Control Act ..
Water Research and
Development Act
X X
XX 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
X X
X
X
X
X X
X
* Programs and activities under this heading relate directly to specific sources of ground-water contamination.
b This category includes activities such as research and development and grants to the states to develop ground-water related programs.
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Table 2. Federal Ground-Water Monitoring Provisions and Objectives (after Office of Technology Assessment, 1984)
Statutory authority
Monitoring provisions*
Monitoring objectives
Atomic Energy Act
CO
Clean Water Act
-Sections 201 and 405
-Section 208
Coastal Zone Management Act
Comprehensive Environmental
Response, Compensation,
and Liability Act
Federal Insecticide, Fungicide
and Rodenticide Act-
Section 3
Federal Land Policy and
Management Act (and
associated mining laws)
Hazardous Liquid Pipeline
Safety Act
Hazardous Materials
Transportation Act
National Environmental
Policy Act
Ground-water monitoring is specified in Federal regulations for low-level radioactive
waste disposal sites. The facility license must specify the monitoring requirements
for the source. The monitoring program must include:
- Pre-operational monitoring program conducted over a 12-month period. Parameters
not specified.
- Monitoring during construction and operation to provide early warning of releases of
radionuclides from the site. Parameters and sampling frequencies not specified.
- Post-operational monitoring program to provide early warning of releases of
radionuclides from the site. Parameters and sampling frequencies not specified.
System design is based on operating history, closure, and stabilization of the site.
Ground-water monitoring related to the development of geologic repositories will be
conducted. Measurements will include the rate and location of water inflow into
subsurface areas and changes in ground-water conditions.
Ground-water monitoring may be conducted by DOE, as necessary, as part of
remedial action programs at storage and disposal facilities for radioactive
substances.
Ground-water monitoring requirements are established on a case-by-case basis
for the land application of wastewater and sludge from sewage treatment plants.
No explicit requirements are established; however, ground-water monitoring studies
are being conducted by SCS under the Rural Clean Water Program to evaluate
the impacts of agricultural practices and to design and determine the effectiveness
of Best Management Practices.
The statute does not authorize development of regulations for sources. Thus, any
ground-water monitoring conducted would be the result of requirements established
by a State plan (e.g., monitoring with respect to salt-water intrusion) authorized
and funded by CZMA.
Ground-water monitoring may be conducted by EPA (or a State) as necessary to
respond to releases of any hazardous substance, contaminant, or pollutant
(as defined by CERCLA).
No monitoring requirements established for pesticide users. However, monitoring
may be conducted by EPA in instances where certain pesticides are contaminating
ground water.*"
Ground-water monitoring is specified in Federal regulations for geothermal
recovery operations on Federal lands for a period of at least one year prior to
production. Parameters and monitoring frequency are not specified.
Explicit ground-water monitoring requirements for mineral operations on Federal
lands are not established in Federal Regulations. Monitoring may be required
(as permit condition) by BLM.
Although the statute authorizes development of regulations for certain pipelines
for public safety purposes, the regulatory requirements focus on design and
operation and do not provide for ground-water monitoring.
Although the statute authorizes development of regulations for transportation for
public safety purposes, the regulatory requirements focus on design and
operation and do not provide for ground-water monitoring.
The statute does not authorize development of regulations for sources.
To obtain background water quality data
and to evaluate whether ground water
is being contaminated.
To confirm geotechnical and design parameters
and to ensure that the design of the
geologic repository accommodates actual field
conditions.
To evaluate whether ground water is being
contaminated.
To characterize a contamination problem and to
select and evaluate the effectiveness of corrective
measures.
To characterize a contamination problem (e.g., to
assess the impacts of the situation, to identify or
verify the source(s), and to select and evaluate the
effectiveness of corrective measures).
To characterize a contamination problem.
To obtain background water-quality data.
(Continued)
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Table 2. (Continued)
Statutory authority
Monitoring provisions*
Monitoring objectives
Reclamation Act
Resource Conservation and
Recovery Act
-Subtitle C
No explicit requirements established; however, monitoring may be conducted, as
necessary, as part of water supply development projects.
Ground-water monitoring is specified in Federal regulations for all hazardous
waste land disposal facilities (e.g., landfills, surface impoundments, waste piles,
and land treatment units).
Facilities in existence on the effective date of statutory or regulatory amendments
under the act that would make the facility subject to the requirements to have a
RCRA permit must meet Interim Status monitoring requirements until a final per-
mit is issued. These requirements specify the installation of at least one upgra-
dient well and three downgradient wells. Samples must be taken quarterly during
the first year and analyzed for the National Drinking Water Regulations, water
quality parameters (chloride, iron, manganese, phenols, sodium and sulfate), and
indicator parameters (pH, specific conductance, TOC and TOX). In subsequent
years, each well is sampled and analyzed annually for the six background water-
quality parameters and semi-annually for the four indicator parameters.
If contaminant leakage has been detected during detection monitoring, the owner or
operator of an interim status facility must undertake assessment monitoring. The
owner or operator must determine the vertical and horizontal concentration pro-
files of all the hazardous waste constituents in the plume(s) escaping from waste
management units.
Ground-water monitoring requirements can be waived by an owner/operator if a
written determination indicating that there is low potential for waste migration via
the uppermost aquifer to water supply wells or surface water is made and certified
by a qualified geologist or engineer. Ground-water monitoring requirements for a
surface impoundment may be waived if (1) it is used to neutralize wastes which
are hazardous solely because they exhibit the corrosivity characteristic under
Section 261.22 or are listed in Subpart D of Part 261 and (2) contains no other
hazardous waste. The owner or operator must demonstrate that there is no poten-
tial for migration of the hazardous wastes from the impoundment. The demonstra-
tion must be in writing and must be certified by a qualified professional.
The monitoring requirements for a fully permitted facility are comprised of a three-
part program:
-Detection Monitoring - Implemented when a permit is issued and there is
no indication of leakage from a facility. Parameters are specified in the
permit Samples must be taken and analyzed at least semi-annually for
active life of regulated unit and the post-closure care period. If there is a
statistically significant increase in parameters specified in permit, owner
or operator must notify Regional Administrator and sample ground water
in all monitoring wells for Appendix IX constituents.
-Compliance Monitoring - Implemented when ground-water
contamination is detected. Monitoring is conducted to determine whether
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 groundwater quality
standards are being met (compliance
monitoring), and to evaluate the effectiveness
of corrective action measures.
(Continued)
-------
Table 2. (Continued)
Statutory authority
Monitoring provisions*
Monitoring objectives
Resource Conservation and
Recovery Act (cont.)
-Subtitle C (cont.)
or not regulated units are in compliance with the ground-water protection
standard specified in facility permit. Samples must be taken and analyzed
at least quarterly for parameters specified in the permit. Samples must
also be analyzed for a specific list of constituents (Appendix IX to
Part 264).
-Corrective Action Monitoring - Implemented if compliance monitoring
indicates that specified concentration levels for specified parameters are
being exceeded and corrective measures are required. Monitoring must
continue until specified concentration levels are met. Parameters and
monitoring frequency not specified.
-Exemptions are provided from these regulations for owner or operator
exempted under Section 264.1, or if Regional Administrator finds unit is
engineered structure; does not receive or contain liquid waste or waste
containing free liquids; is designed and operated to exclude liquids
precipitation, and other run-on and run-off; has both inner and outer
containment layers; has a leak detection system built into each
containment layer; owner or operator will provide continuing operation
and maintenance of leak detection systems; and to a reasonable degree of
certainty will not allow hazardous constituents to migrate beyond the
outer containment layer prior to end of post-closure care period.
-Subtitle D
The 1984 Hazardous and Solid Waste Amendments require EPA to revise criteria
for solid waste management facilities that may receive household hazardous
waste or small quantity generator hazardous waste. At a minimum, the
revisions must require ground-water monitoring, establish location criteria and
provide for corrective action.
On August 30,1988, EPA published proposed rules requiring ground-water
monitoring at all new and existing municipal solid waste landfills.
-Subtitle I
Ground-water monitoring is one of the release detection options available for
owners and operators of petroleum underground storage tanks. It is also an
option at existing hazardous substance underground storage tanks until
December 22,1998. At the end of this period, owners and operators must upgrade
or replace this release detection method with secondary containment and intersti-
tial monitoring unless a variance is obtained.
Safe Drinking Water Act
-Part C-Underground
Injection Control Program
Ground-water monitoring requirements may be specified in a facility permit for
injection wells used for in-situ or solution mining of minerals (Class III wells)
where injection is into a formation containing less than 10,000 mg/l TDS.
Parameters and monitoring frequency not specified except in areas subject to
subsidence or collapse where monitoring is required on a quarterly basis.
Ground-water monitoring may also be specified in a permit for wells which inject
beneath the deepest underground source of drinking water (Class I wells).
Parameters and monitoring frequency not specified in Federal regulations.
To evaluate whether ground water is being
contaminated.
(Continued)
-------
Table 2. (Continued)
Statutory authority Monitoring provisions* Monitoring objectives
Surface Mining Control and Ground-water monitoring is specified in Federal regulations for surface and To obtain background water-quality data and
Reclamation Act underground coal mining operations to determine the impacts on the evaluate whether ground water is being
hydrologic balance of the mining and adjacent areas. A ground-water contaminated.
monitoring plan must be developed for each mining operation (including
reclamation). At 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 water-bearing stratum may be waived by the regulatory
authority if it can be demonstrated that it is not a stratum which serves as an
aquifer that significantly ensures the hydrologic balance of the cumulative
impact area.
Toxic Substance Control Act
-Section 6 Ground-water monitoring specified in Federal regulations requires monitoring To obtain background water-quality data.
prior to commencement of disposal operations for PCBs. Only three wells are
required if underlying earth materials are homogenous, impermeable and
uniformly sloping in one direction. Parameters include (at a minimum) PCBs,
pH, specific conductance, and chlorinated organics. Monitoring frequency not
specified.
No requirements are established for active life or after closure.
o>
-------
If proper monitoring well design and construction tech-
niques 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 poly vinyl chloride (PVC) casing together can
leach significant amounts of tetrahydrofuran, methylethyl ke-
tone, methylbutyl ketone, and other synthetic organic chemi-
cals 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 are
frequently sealed with neat cement grout, bentonite, or a ce-
ment-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 con-
tamination 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 impli-
cations of the impact of adhesion, including chemical bonding,
versus swell pressure have not been documented in the litera-
ture. 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 imme-
diately 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 aban-
donment. A discussion of the necessity of decontamination
procedures for drilling equipment used during monitoring well
installation is also included in this section. Section 4, "Descrip-
tion and Selection of Drilling Methods," includes a brief dis-
cussion of drilling and sampling methods used during monitor-
ing 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 repre-
sentative ground-water samples. Section 7, "Monitoring Well
Development," discusses the importance of proper develop-
ment and describes techniques used in monitoring wells. Sec-
tionS, "Monitoring WellNetworkManagement 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 dis-
cussion 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. andP.M. Buszka, 1986. Theeffectof three drilling
fluids on ground-water sample chemistry; Ground Water
Monitoring Review, vol. 6, no. 1, pp. 62-70.
Kurt, Carl E. andR.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 andEmergency Response, OS WER-9950.1,
United States Environmental Protection Agency, 317 pp.
United States Environmental Protection Agency, 1987. Test
methods for evaluating solid waste, physical/chemical
methods (S W-846); Office of Solid Waste and Emergency
Response, Government Printing Office, Washington, D.C.,
519 pp.
-------
Wehran Engineering Corporation, 1977. Procedures manual Wehrmann, H. Allen, 1983. Monitoring well design and
for ground-water monitoring atsolid waste disposal facilities construction; Ground Water Age, vol. 17, no. 8, pp. 35-38.
(SW-611); National Technical Information Service,
Springfield, Virginia, 269 pp.
-------
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 hydrogeo-
logic 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 form similar ground-water regions. The classification
system developed by Heath (1984) uses the type and interre-
lationship of the aquifers in an area as the major division for
regional designation. Additional factors including: 1) primary
versus secondary porosity, 2) 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 discus-
sion, 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 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 moun-
tains 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 meta-
morphic 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 and 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 the water
resources for this area. The Willamette Valley consists of
interbedded sands, silts and clays deposited by the Willamette
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.
-------
2. Alluvial Basin
1. Western
Mountain Ranges
15. Puerto Rico
and
Virgin Islands
800
n
9. Northeast and
Superior Uplands
1. Western
Mountain Ranges
9. Northeast and
Superior Uplands
6. Nonglaciated
Central Region
7. Glaciated
Central Region
6. Nonglaciated / f-
Central Region * '
6. Nonlaciated
ga
Central Region
4. Colorado Plateau
and Wyoming Basin
10. Atlantic
and Gulf
Coastal, Plain
11. Southeast
Coastal Plain
7. Glaciated
Central
Region
6. Nonglaciated
Central Region
8. Piedmont and
Blue Ridge
Figure 1. Ground-water regions of the United States (Heath, 1984).
(a)
Figure 2a. Location of the Western Mountain Ranges region
(Heath, 1984).
(b)
Figure 2b. Topographic and geologic features In the southern
Rocky Mountains part of the Western Mountain
Ranges region (Heath, 1984).
10
-------
(a)
Figure 3a. Location of the Alluvial Basins region (Heath, 1984).
(b)
Figure 3b. Common ground-water flow systems In the Alluvial
Basins region (Heath, 1984).
Figure 4a. Location of the Columbia Lava Plateau region
(Heath, 1984).
Older Mountains
Explanation
Present Soil Zone
Figure 4b. Topographic and geologic features of the Columbia
Lava Plateau region (Heath, 1984).
Columbia Lava Plateau —
The Columbia Lava Plateau consists of a sequence of lava
flows ranging in total thickness from less than ISO feet adjacent
to mountain ranges to over 3,000 feet in south-central Washing-
ton 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 con-
trolled 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 well-developed
interflow zones and extensive fracturing. These interflow
zones exhibit high hydraulic conductivities and are hydrauli-
cally 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 infiltra-
tion from streams that flow onto the plateau from adjacent
11
-------
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 char-
acterized 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 5b).
The sedimentary rocks in this region consist of Paleozoic-
to Cenozoic-age sandstones, limestones and shales. Evaporitic
rocks such as gypsum and 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 unconfmed conditions in outcrop areas
and confined conditions downdip. Aquifers in the region fre-
quently 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 depositedby streams andrivers 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 semi-consolidated allu-
vial 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 mate-
rial 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
(a)
Figure 5a. Location of the Colorado Plateau and Wyoming
Basin region (Heath, 1984).
(a)
Figure 6a. Location of the High Plains region (Heath, 1984).
Platte River
Dome
Explanation
[Uwatert—lSandstone
ILimestone
Water r^Metamorphic
QShale
(b)
Explanation
CDSand ^
ESGravel E3 Sandstone
(b)
Figure 5b. Topographic and geologic features of the Colorado Figure 6b. Topographic and geologic features of the High Plains
Plateau and Wyoming Basin region (Heath, 1984).
region (Heath, 1984).
12
-------
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 permeability calcium
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 evapotranspiration. 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 irri-
gation has led to long-term declines in water levels. Where
ground-water withdrawal rates have exceeded available re-
charge 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 the region is underlain by consoli-
dated sedimentary rocks, including sandstones, shales, car-
bonates and conglomerates that range from Paleozoic to Ter-
tiary 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 compo-
sition depending on the composition and structure of the under-
lying 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 rego-
lith. 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 character-
ized 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
Figure 7a. Location of the Nonglaciated Central region
(Heath, 1984).
Regolith
I I Fresh Water
SI Salty Water
(b)
Figure 7b. Topographic and geologic features of the
Nonglaciated Central region (Heath, 1984).
Explanation
HI] Water EZ3 Sandstone
Ł3 Shale
Metamorphic
Rocks
(c)
Figure 7c. Topographic and geologic features along the western
boundary of the Nonglaciated Central region (Heath,
1984).
13
-------
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 primarily vary with precipitation rates, evapo-
transpiration 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 un-
derlying bedrock has an insufficient amount of fractures or
solutioning. Because of the widespread occurrence of carbon-
ate rocks, ground water in these areas frequently exhibits high
hardness.
Piedmont and Blue Ridge —
The Piedmont lies between the coastal plain and the Appa-
lachian 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 sedi-
mentary rocks (Figures 9a and 9b).
Figure 8a. Location of the Glaciated Central region (Heath,
1984).
(a)
Figure 9a. Location of the Piedmont and Blue Ridge region
(Heath, 1984).
Moraine
Loess
Bedrock Outcrops
Best Well Sites
Fresh Water ind|Cated with X's
I I Salty Water
(b)
(b)
Figure 8b. Topographic and geologic features of the Glaciated F'8"™ 9b. Topographic and geologic features of the Piedmont
Central region (Heath, 1984). «nd Blue Ridge region (Heath, 1984).
14
-------
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 of
water to shallow wells and 2) the regolith serves as a storage
reservoir to 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 in the 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
density 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 meta-
morphic rocks that have been intruded by younger igneous
rocks and have been extensively folded and faulted (Figures
lOaandlOb).
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 produc-
ing moderate to large yields. Ground 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 south-
ward 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.
i •—<-i r\N
• i i • *
Figure 10a. Location of the Northeast and Superior Uplands
region (Heath, 1984).
(a)
Figure 11a. Location of the Atlantic and Gulf Coastal Plain
region (Heath, 1984).
Terraces
CZl Fresh Water ES3 Sally Water
(b)
Figure 10b. Topographic and geologic features of the Northeast Figure 11 b. Topographic and geologic features of the Gulf
and Superior Uplands region (Heath, 1984). Coastal Plain (Heath, 1984).
15
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Recharge to the aquifer occurs in outcrop areas from
precipitation and from infiltration along streams and rivers. In
some areas an increase do wndip 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 lime-
stone 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-consoli-
dated 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.
In southern Florida, water in the Floridan is typically
saline. In this area, water supplies are developed in the shal-
lower 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. Sedi-
ment-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. Re-
charge to the aquifer occurs from streams and from precipita-
tion. Withdrawals of ground water near a stream may cause a
reversal of hydraulic gradients; ground water previously flow-
ing 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 theeightmajor 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 composedprimarily 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
Discharge
-Recharge Area 1—Area-*-
(a)
Figure 12a. Location of the Southeast Coastal Plain Region.
Area of
Artesian
Flow
~~l *
-J
Potentiometric ,/r-
Surface _ ,' Limestone
- Spring
(b)
Figure 12b. Topographic and geologic features of the Southeast
Coastal Plain (Heath, 1984).
16
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Figure 13a. Location of the Alluvial Valleys ground-water region
(Heath, 1984).
(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 (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 perme-
able, approximately thirty percent of the precipitation infil-
trates 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-im-
pounded ground water is often withdrawn from horizontal
Gravel
Sand
Silt and Clay
Limestone
Figure 13b. Topographic and geologic features of a section of
the alluvial valley of the Mississippi River
(Heath, 1984).
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 through-
out much of the year, forming a zone of permafrost or perenni-
ally frozen ground. Permafrost occurs throughout the state
Lava Flows
Dikes
Dike Spring
>y/ CU Fresh Water
0 Salty Water
Gravel
Glacial Till
I | Water
I I Permafrost
Figure 14. Topographic and geologic features of an Hawaiian Figure 15. Topographic and geologic features of parts of Alaska
Island (Heath, 1984). (Heath, 1984).
17
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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 perma-
frost, 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 Hydrogeologic
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 used to correlate stratigraphic units across
the site. An understanding of the stratigraphy, including the
horizontal continuity and vertical thickness of formations be-
neath 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
bemonitored. 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 uncon-
solidated, also influence the type of well completion. In un-
consolidated 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 com-
pleted as a cased borehole with no screen intake or filter pack.
Where conduit-born fines are a problem in consolidated for-
mations, 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 karstic
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 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 materi-
als. 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 stage 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 poten-
tial 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 con-
tainment that impacts potential release of contaminants, both
onsite and offsite, and may require separate monitoring.
The physical and chemical characteristics of the contami-
nants, 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.
18
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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 antici-
pated 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 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. Land-
fills 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 pro-
tection 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 guide-
lines for "composite double liner systems" (including com-
pacted low permeability soils and two flexible synthetic mem-
branes) 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 loca-
tions 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 compar-
ing representative samples of background water quality to
samples taken from the downgradient margins of the waste
management area. The ground-water monitoring wells must be
properly cased, completed with an artificial filter pack, where
necessary, and grouted so that representative ground-water
samplescan becollected (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 facili-
ties 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, monitor-
ing 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 Occu-
pational 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-dis-
charging impoundments can either intentionally or uninten-
tionally 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
19
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depths of these impoundments reportedly range from 2 feet to
more than 30 feet below the ground surface (Office of Technol-
ogy 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" (includ-
ing 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 im-
poundments 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 im-
poundment below ground level and the depth of the first water-
bearing zone underlying the bottom of the impoundment.
Waste and Material Piles —
Large quantities of both wastes and materials may be
stockpiled for storage. Stockpiled material may include poten-
tially 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. Tail-
ings 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, SubpartL) 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 per-
colating 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 stockpiles of highway deicingsalt. 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 agricul-
tural, 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 mea-
sures 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 tominimum 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).
Underground Storage Tanks —
Underground storage tanks are used to store hazardous and
nonhazardous 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, main-
tenance and operation requirements for tanks containing haz-
ardous waste and commercial petroleum products (40 CFR,
Parts 264 and 265, Subpart J). These regulations include re-
quirements 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 process-
ing 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 (Of-
fice of Technology Assessment, 1984).
The radioactive waste disposal method depends on the
radiation levels and the waste characteristics. Low-level ra-
dioactive 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 monitor-
ing 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 con-
20
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figuration 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 compat-
ible 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, con-
taminants can be classified into categories that subsequently
influence monitoring well design: 1) compounds that are pri-
marily 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 MiscibtelSoluble 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 contami-
nant 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 trans-
ported 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 con-
taminants as they move with the ground water. Dispersion
occurs by mechanical mixingand 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 Mill,
1984; Mackay et al., 1985) and biodegradation (McCarty 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 must also be considered
in waste characterization (Bear, 1972). Figure 16 illustrates the
migration of a high density, miscible contaminant in the sub-
surface. 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 bound-
ary, possibly in opposition to the direction of regional ground-
water flow. Because the contaminant is also soluble, the con-
taminant 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 dis-
solved portion of the plume unless a specific monitoring pro-
gram 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 alow density, soluble
contaminant. The contaminant initially accumulates at the top
of the water table. Dissolution and dispersion of the contami-
nant 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 bound-
Figure 16. Migration of a high density, miscible contaminant in
the subsurface.
Figure 17. Migration of a low density, soluble contaminant in
the subsurface.
21
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aries 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, chemi-
cal attenuation and subsurface hydrogeology.
Relatively Immiscible/Insoluble Contaminants —
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
andbiological/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 thecontaminant affects the occurrence and movement
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, immis-
cible 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 typi-
cally 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
byBlakeandHaU(1984),Kovski(1984)>YanigaandWarburton
(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 con-
taminants 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 techniques including soil-gas
sampling and geophysical techniques can be utilized in plan-
ning 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
Germeroth, 1985; Lithland et al., 1985).
High density immiscible fluids are called dense non-
aqueous phase liquids (DNAPLs). DNAPLs include most ha-
logenated 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
(Mackayetal., 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 bound-
ary independent of regional ground-water flow. Residual ma-
terial 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 glob-
ules 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 contami-
Low Density
Immiscible Liquid
Small Dissolved Plume
7 7 7 7 77 7 77 77 7 7 7"
Small Dissolved Plume
Dense, Immiscible Liquid
Figure 18. Migration of a low density, Immiscible contaminant in Figure 19. Migration of a dense, non-aqueous phase liquid
the subsurface. (DNAPL) in the subsurface.
22
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nate 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
caudon 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.
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. How-
ever, 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 investigadon 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 cooperation. Another example is
where a presently regulated disposal facility is located on the
site of apreviously 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 natu-
rally-occurring constituents 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 direc-
tions 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 hydrau-
lic 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 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 re-
charge 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 cre-
ated 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 contamina-
tion 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 activi-
ties 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 de-
termine the diameter of the well that must be drilled. For
example, if the transmissivity of the monitored zone is to be
23
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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 repre-
sentative 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 monitor-
ing wells to obtain hydrogeologic information. Under appropriate
conditions, porosity.hydraulicconductivity.porefluidelectrical
conductivity and general stratigraphic logs can be obtained.
Unfortunately, borehole geophysical methods are frequently
limited by the materials and die drilling and completion meth-
ods used to construct the well. If it is anticipated that borehole
geophysical methods will be conducted in a well, it is neces-
sary 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 con-
ductivity of the formation will not operate in air-filled bore-
holes because of the lack of an electrical connection between
theelectrodes and the formation. Someindividuals 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 more 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 affected by the well fluid.
The casing material also influences which methods can be
used. No measurement of the electrical properties of the for-
mation 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 con-
ductivity 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 radiomet-
ric logs is affected by the thickness and material used in the
casing. This is particularly true when neutron moisture methods
arc used in PVC casing because the method is unable to
distinguish hydrogen in the PVC from hydrogen in the pore
fluid.
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
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 Radius of
Investigations
Water Open Metal Plastic Screen No Screen (cm) Comments
11444
11433
11411
22221
22221
22221
1 1 1
2 1 1
1 1 1
1 1 1
1 1 3
4 5-50
4 5-400
1 100-400
1 5-30
1 5-15
1 5-15 Big effect with PVC
1 0
1 0 Clear fluid only
1 0
4 0
4 2-6cm Strongly influenced
by screen
1 Works, this well property does not adversely affect the log
2 Works, but calibration affected
3 Works qualitatively
Doesn't work
24
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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 influ-
ence geophysical measurements. This occurs because all bore-
hole 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 afew 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 resistiv-
ity 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 radiomet-
ric 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 pack-
ers 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. Most 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 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 accommo-
dated 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 andMacCary, 1971; Senger, 1985). The useof 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 con-
tinuous record of well or borehole diameter that can be used to
detect broken casings, the location of fractures, solution devel-
opment, 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 in-
formation 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 (Keys, 1968; Keys and MacCary, 1971;
Kwader, 1985;Lindsey, 1985). Temperature logs have recently
been applied to the detection of anomalous fluid flow (Urban
and Diment, 1985). Induction, resistivity and temperature log-
ging 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., 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
25
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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 transduc-
ers, 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 above 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 sus-
pended on the end of a marked cable. When the sensor encoun-
ters 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 monitor-
ing program. A discussion of the advantages and disadvantages
of sampling devices is provided by Barcelona 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 con-
structed 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 be constructed
of various inert materials for wells with a diameter of 1.5 inches
or larger. The use of pressurized bladders ensures that the
sample does not contact the driving gas. Most bladder pumps
are capable of lifting samples from 300 to 400 feet, although
models capable of 1000 feet of Uft 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 150 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.
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 character-
istics 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 in-
jection or recovery tests, that add or remove smaller amounts of
water, are typically performed in formations with low trans-
missivity 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.
26
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Andres, K.G. and R. Canace, 1984. Use of the electrical
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coastal plain deposits of New Jersey; Proceedings of the
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Organic Chemicals in Ground Water. Prevention, Detection
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Barcelona, M.J., J.P. Gibb, J.A. Helfrich and E.E. Garske,
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Proceedings of the NWWA Conference on Surface and
Borehole Geophysical Methods in Ground-Water
Investigations; National Water Well Association, Dublin,
Ohio, pp. 387-397.
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., MJ.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, Niagara 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
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Hinchee, R.E. and HJ. Reisinger, 1985. Multi-phase transport
of petroleum hydrocarbons in the subsurface environment:
theory and practical application; Proceedings of the
NWW A/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 monit-
oring 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.
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geophysics to water-resources investigations, Book 2;
United States Department of the Interior, Washington,
D.C., 126pp.
Kovski, J.R., 1984. Physical transport process for hydrocarbons
in the subsurface; Proceedings of the Second International
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Kwader, T., 1985. Resistivity-porosity cross plots for
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Lindsey, G.P., 1985. Dry hole resistivity logging; Proceedings
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Lithland, ST., T.W. Hoskins and R.L. Boggess, 1985. A new
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27
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Mabey, W.R. and T. Mill, 1984. Chemical transformation in
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Chemicals in Ground Water: Prevention, Detection and
Restoration; National Water Well Association, Dublin,
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Ground-Water Monitoring; National Water Well
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between real and apparent accumulation of immiscible
hydrocarbons on the water table: a theoretical and empirical
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National Water Well Association, Dublin, Ohio, pp. 311-
315.
28
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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 mainte-
nance 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-qual-
ity 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, sub-
surface geology and hydrogeology should be recorded. Litho-
logic 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 li-
thology, 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
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 mea-
surements, 4) dates of sample collection (including type of
sampler, notations about sample collection and results of labo-
ratory 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 formation-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 sur-
face 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 contamina-
tion exists, decontamination measures are also employed in
uncontaminated areas as a quality control measure.
Planning a decontamination program for drilling and for-
mation 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
decontamination;
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
29
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Table 4. Descriptive Information to be Recorded for each Monitoring Well
General Information
Well Completion Information
Boring 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
Drilling Information
Type of drilling equipment
Type and design of drill bit
Any drilling fluid used
Diameter of drill bit
Diameter of hole
Penetration rate during drilling (fee/minute, minutes/foot, feet/hour, etc.)
Depth to water encountered during drilling
Depth to standing water
Soil/rock classification and description
Total well depth
Remarks on miscellaneous drilling conditions, including:
a) loss or gain of fluid
b) occurrence of boulders
c) cavities or voids
d) borehole conditions
e) changes in color of formation samples or fluid
f) odors while drilling
Sampling Information
Types of sampler(s) used
Diameter and length of sampler(s)
Number of each sample
Start and finish depth of each sample
Split spoon sampling:
a) size and weight of drive hammer
b) number of blows required for penetration of 6 inches
c) free fall distance used to drive sampler
Thin-walled sampling:
a) relative ease or difficulty of pushing sample OR
b) pounds per square inch (pal) necessary to push sample
Rock cores:
a) core barrel drill bit design
b) penetration rate (fee/minute, minutes/foot, fee/hour, etc.)
Percent of sample recovered
Elevation of top of casing (± .01 foot)
Casing:
a) material
b) diameter
c) total length of casing
d) depth below ground surface
e) how sections joined
f) end cap (yes or no)
Screen:
a) material
b) diameter
c) slot size and length
d) depth to top and bottom of screen
Filter pack:
a) type/size
b) volume emplaced (calculated and actual)
c) depth to top of filter pack
d) source and roundness
e) method of emplacement
Grout and/or sealant:
a) composition
b) method of emplacement
c) volume emplaced (where applicable)
(calculated and actual)
d) depth of grouted interval (top and bottom)
Backfill material:
a) depth of backfilled interval (top and bottom)
b) type of material
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)
Well development:
a) method
b) date/time; start/stop
c) volume and source water (if used)
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
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 exten-
sive 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 decon-
tamination 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 con-
tamination. 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 con-
tamination 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 con-
taminants 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 ad-
equate 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
30
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BORING NO. __2A_
SH_L_OF.
PROJECT AMI Manufacturing
BORING LOG
DATE START
LOCATION Sussex County
4 25"
CASING I.D.
CONTRACTOR Sprowls & Sons
GROUND ELEV.
. CORE SIZE NX
337.09'
Aug. 30. 1987 FINISH Aug. 31,1987
TOTAI nFPTH(FT) 23.50'
TYPE
Air Rotary w/Casing Hammer
BY
S. Smith
SOIL AND ROCK DESCRIPTION/COMMENTS
(Unified soil class system Rock description. Depth to water
table. Loss of drill fluid, etc.)
Gravelly SILT, little sand, trace clay
About 30% pebbles and granules.
Moderately moist. Moderate yellowish brown
(10 YR5/4. mottled 5Y5/2); drab Till.
[GM]
Gravelley SILT, little sand, trace clay
About 30% pebbles and granules. Dry to slightly
moist Moderate yellowish brown (10 YR5/4).
drab Till
[GM]
20'
Medium dark gray to dark gray SILTSTONE. sandy
SILTSTONE. with minor shale seams Fresh and
hard except at breaks along slightly to moderately
weathered shale seams. Jointed and broken approx-
imately as depicted. Coquma seam (15 V-15 2'). very
calcareous Generally only calcareous in sandy
SILTSTONE layers. Wet @17.5'
Medium dark gray to dark gray SILTSTONE. sandy
SILTSTONE. and minor shale seams, same as above
End of Bormq - Total Depth = 23 50'
25'
L_d
Piezometer 2A installed with screened interval of
18.0' to 230'
Overburden
Rock
13.0'
10.5'
Water Level
Date
Time
Total Depth 23.50'
Elevation Measuring Point
16.2'
8/30/87
1:00 p.m.
Top of Casing
16.35'
8/30/87
3:00 p.m.
Top of Casing
Comments Surface casing driven 8" into rock
Figure 20. Sample boring log format (after Electric Power Research Institute, 1985).
31
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Well Number 7H
Start 8/1 3/87 B:nn a.m. -1:00 p m
Finish fl/14/87 10 a.m. -12:00 p m
Drilling Method Hollow Stam Aufir
Steel (schedule 40) Protective
Casing with Hinged Cap
Drain
Concrete Pad (min. 4" thick on
undisturbed or compacted soil)
Elevation 856.03 feet
Frost Sleeve
Sand
Concrete Seal
' Sand
Granular Bentonite Seal
2% Bentonite-Cement Seal
2" PVC Casing (Schedule 40,
Flush Joint, Threaded
Granular Bentonite Seal
Silica Fine-Grained Sand
(Mortar Sand)
2" PVC Well Screen with
0.010 Inch Slot Open
Filter Pack (Clean Medium to
Coarse Silica Sand)
2" PVC Casing
10.25"
Figure 21. Format for an "as-built" monitoring well diagram.
should be discarded after each borehole is drilled. Clean equip-
ment 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 decontami-
nation procedures are employed as personnel and equipment
move from the hazardous material exclusion zone to a clean,
non-hazardous support zone.
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 decon-
tamination 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 de-
contamination 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 equip-
ment 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 por-
table, high-pressure steam cleaners equipped with pressure
hose and fittings.
Sometimes solutions other than water or steam are used for
equipment decontamination. Table 5 lists some of the chemi-
cals and solution strengths that have been used in equipment
decontamination programs. One commonly used cleaning so-
lution 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 demon-
strated that the environmental contaminant in question cannot
be removed from the surface of the equipment by detergents.
Acids or solvents should be used as cleaning solutions only
under exceptional circumstances because these cleaners are, in
32
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Heavy Equipment
Decontamination
Area
Exclusion
Zone
-O—O-
Auxiliary
Access
Control Path
Exit Path
Contamination
Reduction Zone
O
Legend
x—x- Hotline
O—O- Contamination
Control Line
Access Control
Point • Entrance
rJT-Ti Access Control
L2SJ Point • Exit
Support Zone
Dressout
Area
Redress
Area
Entry Path
Figure 22. Typical layout showing decontamination areas at a hazardous materials site (United States Environmental Protection
Agency, 1984).
and of themselves, hazardous materials and may serve as
contaminants if introduced into the borehole. When using
chemical solutions for equipment decontamination, water or
steam should always be used as a final rinse to remove any
residual chemical cleaner from the surface of the equipment and
thereby prevent contamination of the borehole by the cleaning
solution.
According to Moberly (1985), a typical sequence for
decontamination of low to moderately contaminated equipment
might include:
1) water or steam rinse to remove particulates;
2) steam wash with water or non-phosphate
detergent; and
3) steam or water rinse with potable water.
Additional wash/rinse sequences may be necessary to
completely remove the contaminants.
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 pro-
visions 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
33
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Table 5. List of Selected Cleaning Solutions Used for
Chemical Solution
Equipment Decontamination (Moberly, 1985)
Uses/Remarks
Clean Potable Water
Low-Sudsing Detergents
(Alconox)
Sodium Carbonate
(Washing Soda)
Sodium Bicarbonate-
faking Soda)
Trisodium Phosphate
(TSP Oakite)
None
Follow Manufacturer's
Directions
4#/10 Gal Water
4#/10 Gal Water
2#/1O Gal Water
4#/10 Gal Water
Calcium Hydrochloride (HTH) 8#/1O Gal Water
Hydrochloric Acid 1 Pt/10 Gal Water
Citric, Tartaric, Oxalic Acids 4#/10 Gal Water
(or their respective salts)
Organic Solvents (Acetone, Concentrated
Methanol, Methylene Chloride)
Used under high pressure or steam to remove heavy mud, etc., or to
rinse other solutions
General all-purpose cleaner
Effective for neutralizing organic acids, heavy metals, metal
processing wastes
Used to neutralize either base or neutral acid contaminants
Similar to sodium carbonate
Useful for solvents & organic compounds (such as Toluene,
Chloroform, Trichloroethylene). PBB's and PCB's
Disinfectant, bleaching & oxidizing agent used for pesticides,
fungicides, chlorinated phenols, dioxins, cyanides, ammonia & other
non-acidic inorganic wastes
Used for inorganic bases, alkali and caustic wastes
Used to clean heavy metal contamination
Used to clean equipment contaminated with organics or well casing to
remove surface oils, etc
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 stick cans. The contaminated cleaning fluids can be stored
temporarily in metal or plastic cans or drums until removed
from the site for proper disposal.
safety plan for field personnel should be of foremost concern
when drilling in known or suspected contaminated areas. Spe-
cific health and safety procedures necessary at the site depend
on the toxicity and physical and chemical properties of known
or suspected contaminants. Where hazardous materials are
involved or suspected, a site safety program should be devel-
oped by a qualified professional in accordance with the Occu-
pational Safety and Health Administration requirements in 29
CFR 1910.120. Field personnel at hazardous sites should re-
ceive medical screening and basic health and safety training, as
well as specific on-site training.
Effectiveness of Decontamination Procedures References
A decontamination program for drilling and formation
sampling equipment may need to include quality-control proce-
dures 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
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.
34
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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 dis-
cussion 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 15 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 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.
Figure 23. Diagram of a hand auger.
A more complete list of the applications and limitations of
hand augers is found in Table 6.
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
35
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- Drive cap
^f- Coupling
^—Casing
-Coupling
• Screen
Wellpoint
Figure 24. Diagram of a wellpolnt.
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 50 feet or more are
possible with hammers up to 1,000 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.
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.
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 mate-
rials or to break up rock at the bottom of the borehole. Concomi-
tantly, 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 posi-
tioned 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.
After the casing has been advanced to the desired monitor-
ing 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.
36
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Table 10. Applications and Limitations of Hollow-Stem Augers
Applications
Limitations
• All types of soil investigations
• Permits good soil sampling with split-spoon or thin-wall
samplers
• Water quality sampling
• Monitoring well installation in all unconsolidated formations
• Can serve as temporary casing for coring rock
• Can be used in stable formations to set surface casing
(example: drill 12-inch borehole; remove auger; set 8-inch
casing; drill 7 1/4-inch borehole with 3 1/4-inch ID augers to rock;
core rock with 3-inch tools; install 1-inch piezometer; pull augers)
1 Difficulty in preserving sample integrity in heaving formations
' Formation invasion by water or drilling mud if used to control
heaving
> Possible cross contamination of aquifers where annular
space not positively controlled by water or drilling mud or
surface casing
> Limited diameter of augers limits casing size
' Smearing of clays may seal off aquifer to be monitored
of the borehole. At the surface, the fluid discharges through a
pipe or ditch and enters into a segregated or baffled sedimen-
tation 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 drive. 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 inter-
fere 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 circu-
lated 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.
Table 11. Applications and Limitations of Direct Mud Rotary Drilling
Applications Limitations
Both split-spoon and thin-wall samples can be obtained in
unconsolidated 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.
For the rig sizes that are most commonly used for moni-
toring well installation, the maximum diameter borehole is
typically 12 inches. Unconsolidated deposits are sometimes
drilled with drag or fishtail-type bits, and consolidated forma-
tions 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 diam-
eters 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.
• Rapid drilling of clay, silt and reasonably compacted sand and
gravel
• Allows split-spoon and thin-wall sampling in unconsolidated
materials
• Allows core sampling in consolidated rock
• Drilling rigs widely available
• Abundant and flexible range of tool sizes and depth capabilities
• Very sophisticated drilling and mud programs available
• Geophysical borehole logs
• Difficult to remove drilling mud and wall cake from outer
perimeter of filter pack during development
' Bentonite or other drilling fluid additives may influence quality
of ground-water samples
1 Circulated (ditch) samples poor for monitoring well screen
selection
1 Split-spoon and thin-wall samplers are expensive and of
questionable cost effectiveness at depths greater than 150 feet
> Wireline coring techniques for sampling both unconsolidated
and consolidated formations often not available locally
1 Difficult to identity aquifers
' Drilling fluid invasion of permeable zones may compromise
validity of subsequent monitoring well samples
41
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Pump
suction
Borehole wall ^ | | /~ Cuttings circulated to surface
through annular space
Tncone bit
Figure 29. Diagram of a direct rotary circulation system
(National Water Well Association of Australia, 1984).
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 represen-
tative 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 en-
countered 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 continu-
ously and all of the cuttings are discharged, there is minimal
opportunity for recirculation and there is minimal contamina-
tion 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 pro-
ceeds. However, thin water-bearing zones often are not iden-
tifiable 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 may contribute water to the open borehole. To prevent
shallow zones from producing water or to prevent cross con-
tamination, the shallower zones must be cased off. Identifica-
tion of both thin and thick water-bearing zones is extremely
important because this information assists greatly in the place-
ment of well intakes and/or in the selection of isolated zones for
packer tests.
In hard, abrasive, consolidated rock, a down-the-hole ham-
mer 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 air 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 effi-
ciently.
Air rotary drilling is typically limited to drilling in consoli-
dated 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 uncon-
solidated materials, there is the tendency for the borehole to
collapse during drilling. Therefore, air rotary drilling in un-
consolidated formations is unreliable and poses a risk for
equipment. Where sufficient thicknesses of unconsolidated
deposits overlie a consolidated formation that will be drilled by
air rotary techniques, surface casing through the unconsoli-
dated 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.
42
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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 27. Typical components of a hollow-stem auger (after
Central Mine Equipment Company, 1987).
(ASTM, 1586) or thin-wall (ASTM, 1587) sampler isplaced 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
reinserted 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 bore-
holes can be drilled with a center plug that is installed on the
bottom of the drill rods and inserted during drilling. A small
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
Table 9. Applications and Limitations of Solid-Flight Augers
Applications
Limitations
Shallow soils investigations
Soil samples
Vadose zone monitoring wells (lysimeters)
Monitoring wells in saturated, stable soils
Identification of depth to bedrock
Fast and mobile
Unacceptable soil samples unless split-spoon or thin-wall
samples are taken
Soil sample data limited to areas and depths where stable
soils are predominant
Unable to install monitoring wells in most unconsolidated
aquifers because of borehole caving upon auger removal
Depth capability decreases as diameter of auger increases
Monitoring well diameter limited by auger diameter
39
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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 improve 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.
Thisarrangementequalizes 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 rods are raised
during the sampling process. Adding drilling mud may lessen
the heaving problem, but volume replacement of mud displace-
ment by removal of drilling rods must be fast enough to
maintain a positive head on the formation. Additionally, drill-
ing 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 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 con-
tamination 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 diam-
eter hollow stems. The equipment most frequently available to
• Continuous slot
screen
- Auger flighting
Auger head
Figure 28. Diagram of a screened auger.
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 disadvan-
tages of hollow-stem augers is found in Table 10. A more
comprehensive evaluation of this technology is presented in
Appendix A.
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
40
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—Jetting pipe
_ Cuttings washed up
annular space
-Bit
Drilling fluid discharged
through port in bit
Figure 25. Diagram of jet-percussion drilling (after Speedster
Division of Koehrlng Company, 1983).
A more complete listing of applications and limitations of
jet-percussion drilling is found in Table 8.
Solid-Flight Augers
Solid-flight augers (i.e. solid-stem, solid-core or continu-
ous 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 are
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 14 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,
Auger
connection"
Flighting •
Cutter head
Figure 26. Diagram of a solid-flight auger (after Central Mine
Equipment Company, 1987).
by being pushed into the borehole wall of the shallower forma-
tions. 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 remov-
ing the augers from the borehole. In a relatively stable forma-
tion, 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
sampleon the augers. Because the borehole often caves after the
saturated zone is reached, samples collected below the water
table are lessreliable. 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 particu-
larly true in heaving formations.
37
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Table 6. Applications and Limitations of Hand Augers
Applications
Limitations
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
Limited to very shallow depths
Unable to penetrate extremely dense or rocky
soil
Borehole stability difficult to maintain
Labor intensive
Table 7. Applications and Limitations of Driven Wells
Applications
Limitations
Water-level monitoring in shallow formations
Water samples can be collected
Dewatering
Water supply
Low cost encourages multiple sampling points
• Depth limited to approximately 50 feet (except in sandy
material)
• Small diameter casing
• No soil samples
• Steel casing interferes with some chemical analysis
• Lack of stratigraphic detail creates uncertainty regarding
screened zones and/or cross contamination
• Cannot penetrate dense and/or some dry materials
• No annular space for completion procedures
Table 8. Applications and Limitations of Jet-Percussion Drilling
Applications
Limitations
Allows water-level measurement
Sample collection in form of cuttings to surface
Primary use in unconsolidated formations, but may be used in
some softer consolidated rock
Best application is cinch borehole with 2-inch casing and
screen installed, sealed and grouted
Drilling mud may be needed to return cuttings to surface
Diameter limited to 4 inches
Installation slow in dense, bouktery day/till or similar
formations
Disturbance of the formation possible if borehole not
cased immediately
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 reinserted 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 visually 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.
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 neces-
sary. 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
38
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Figure 30. Diagram of a roller cone bit.
Monitoring wells drilled by air rotary methods are typi-
cally installed as open-hole completions. Because the borehole
is uncased, the potential exists for cross connection between
water-bearing zones within theborehole. 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 introducedduring 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 limits 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.
43
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Cuttings discharge
through pipe
Air to actuate
hammer and
remove cuttings
t
• Hammer
• Button bit
Figure 31. Diagram of a down-the-hole hammer (after Layne-
Western Company, Inc., 1983).
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.
Dual-Wall Reverse-Circulation
In dual-wall reverse-circulation rotary drilling, the circu-
lating 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).
Table 12. Applications and Limitations of Air Rotary Drilling
Applications
Limitations
Rapid drilling of semi-consolidated and consolidated rock
Good quality/reliable formation samples (particularly if small
quantities of water and surfactant are used)
Equipment generally available
Allows easy and quick identification of lithologic changes
Allows identification of most water-bearing zones
Allows estimation of yields in strong water-producing zones with
short "down time"
Surface casing frequently required to protect top of hole
Drilling restricted to semi-consolidated and consolidated
formations
Samples reliable but occur as small particles that are
difficult to interpret
Drying effect of air may mask lower yield water
producing zones
Air stream requires contaminant filtration
Air may modify chemical or biological conditions. Recovery
time is uncertain.
44
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Well Drilling Selection Guide
Type of Formation
Geolog c Origin »
Examples »
Hardness *
Drilling Methods
Diameter
Depth
Igneous and Metamorphic
Granite Quartzite
Basalt Gneiss Schist
Very hard to hard
, ft
\s
Downhole
hammer
Carbide
insert bit
*
C
• — Carbide tooth b
Small (4-8in)
Shallow (50-200ft)
Sedimentary
Limestone Sandstone Shale
Hard to soft
4
(
Ro
Is— »
n
\y
m
V •$,
m
lary c
ary
!
rill
Clay Sand Gravel
Unconsolidated
Small to medium (6-1 2in)
Shallow to deep (50- 1,000ft)
Figure 32. Range of applicability for various rotary drilling methods (Ingersoll-Rand, 1976).
The circulation fluid used in the dual-wall reverse circula-
tion 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 casing 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 cir-
culation 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
45
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A
Continuous sample discharge •
•Air supply
-Top-head drive
•Mast
• Casing driver
• Discharge for cuttings
Casing
Drill pipe
Drive shoe
Drill bit
Figure 33. Diagram of a drill-through casing driver (Aardvark
Corporation, 1977).
channel just above the hammer. When drilling with the ham-
mer, 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 devel-
oped 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
Top-head
drive
Figure 34. Diagram of dual-wall reverse-circulation rotary
method (Driscoll, 1986).
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 air-introduced contami-
nants 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 bore-
hole, the borehole must be very stable to permit the open-hole
completion.
46
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Table 13. Applications and Limitations of Air Rotary with Casing Driver Drilling
Applications Limitations
Rapid drilling of unconsolidated sands, silts and days
Drilling in alluvial material (including boulder formations)
Casing supports borehole thereby maintaining borehole integrity
and minimizing inter-aquifer cross contamination
Eliminates circulation problems common with direct mud rotary
method
Good formation samples
Minimal formation damage as casing pulled back (smearing of
clays and sits 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 pulverized as in all rotary drilling
• Air may modify chemical or biological conditions. Recovery
time is uncertain
A more complete list of applications and limitations of the
dual-wall reverse-circulation technique is found in Table 14.
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 re-
moving the drill bit and inserting a bailer (Figure 36). The bailer
is a bucket made from sections of thin-wall pipe with a valve on
the bottom that is actuated by the weight of 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 dis-
charged 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 redlining the same
material, and the drilling effort becomes very inefficient.
When drilling unconsolidated deposits comprised prima-
rily of silt and clay, the drilling action is very similar to that
described in the previous paragraph. Water m ust 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 prima-
rily of water-bearing sands and gravels, an alternate and more
effective drilling technique is available for cable tool opera-
tions. 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 unconsoli-
dated 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 ahigher 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
Table 14. Applications and Limitations of Dual-Wall Reverse-Circulation Rotary Drilling
Applications Limitations
Very rapid drilling through both unconsolidated and
consolidated formations
Allows continuous sampling in all types of formations
Very good representative samples can be obtained with minimal
risk of contamination of sample and/or water-bearing zone
In stable formations, wells with diameters as large as 6 inches
can be installed in open hole completions
Limited borehole size that limits diameter of monitoring wells
In unstable formations, well diameters are limited to
approximately 4 inches
Equipment availability more common in the southwest
Air may modify chemical or biological conditions; recovery
time is uncertain
Unable to install filter pack unless completed open hole
47
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Crown
sheave
Shock -
absorber
Casing and sand
line sheaves
Spudding beam
Heel sheave
Pitman
Swivel
socket
Drill
stem
Operating —f
levers
Drill bit
Figure 35. Diagram of a cable tool drilling system (Buckeye Drill Company/Bucyrus-Erie Company, 1982).
•Truck mounting
bracket
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;
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.
48
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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 expen-
sive. 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 alter-
native technologies. For example, smearing along sidewalls in
unconsolidated formations is generally less severe and is thin-
ner than with hollow-stem augering. Therefore, the prospect of
a successful completion in a thin water-bearing zone is gener-
ally enhanced.
A more complete listing of advantages and disadvantages
of cable tool drilling is found in Table 15.
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 build-
ing structures. Reverse-circulation rotary is used primarily for
the installation of large-diameter deep water wells.
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 extraor-
dinarily 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 moni-
toring wells. While either of these techniques have possible
application to monitoring well installation, they are not consid-
ered 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 sophisti-
cated, 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/oiI/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 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. At-
tempts to remove drilling fluids from the formation are made
during the well development process. Water is typically re-
moved 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.
Table 15. Applications and Limitations of Cable Tool Drilling
Applications
Limitations
Drilling in all types of geologic formations
Almost any depth and diameter range
Ease of monitoring well installation
Ease and practicality of well development
Excellent samples of coarse-grained materials
Drilling relatively slow
Heaving of unconsolidated materials must be controlled
Equipment availability more common in central, north central
and northeast sections of the United States
49
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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, tem-
perature 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.
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 wa-
ter being monitored will come in contact with the montmorillo-
nite. 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 struc-
ture. 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 subse-
quent 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 moni-
tored. 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.
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.
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 im-
pact 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 paniculate 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 montmo-
rillonite (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
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 Marsh Funnel viscos-
ity of clear water at 70°F is 26 seconds.
Table 16. Principal Properties of Water-Based Drilling Fluids
(Drlscoll, 1986)
Density (weight)
Viscosity
Yield point
Gel strength
Fluid-loss-control effectiveness
Lubricity (lubrication capacity)
50
-------
' Flapper
valve
(b)
Figure 36. Diagrams of two types of bailers:
a) dart valve and b) flat bottom.
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
Medium sand
Coarse sand
Gravel
Coarse gravel
35-45
45-55
55-65
65-75
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 be-
cause of its crystalline layered structure; its bonding character-
istics; and the ease of hydration of the sodium cation. Figure 38
demonstrates the variation in the viscosity building character-
istics 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 varia-
tion 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 paniculate 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 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.
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 paniculate 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.
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
51
-------
Weight of drilling fluid,lb/gal
10
15
20
Air
Mist
Stiff foam
Wet foam
Water
Maximum practical density using polymers
Polymeric drilling fluid saturated with NaCI
Maximum practical density using bentonite
Polymeric drilling fluid saturated with CaCI
Weighted bentonite drilling fluid using bante
J_
0 600 1,200
Weight of drilling fluid,kg/m3
Figure 37. Practical drilling fluid densities (Driscoll, 1986).
1,800
2,400
water to the borehole to minimize heaving of the formation
upward into the casing or hollow stem. When the zone imme-
diately 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.
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 contamina-
tion 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
Table 18. Drilling Fluid Options when Drilling with Air (after
Driscoll, 1986)
• Air Alone
• Air Mist
• Air plus a small amount of water/perhaps a small amount of
surfactant
• Air Foam
• Stable foam - air plus surfactant
• Stiff foam - air, surfactant plus polymer or bentonite
• Aerated mud/water base - drilling fluid plus air
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.
63.7
8.5
Weight, Ib/ft3
67.5 71.2 75.0 78.7 82.5 86.2 90.0
' Weight,'Ib/gal ' ' "~
9;5 10.0 10.5 11.0 11.5 12.0
9.0
60
,50
Ł40
05
o
~;30
!20
10
5 10 15 20 25 30 35 40 45 50
1QO Percent solids by weight
" 200 175 5040 30 25 201816 14 12 10 9 8
Yield (15-centipoise drilling fluid), barrels per ton
Figure 38. Viscosity-building characteristics of drilling clay*
(after Petroleum Extension Service, 1980).
52
-------
Bentonite
Complete
mixing
II
Incomplete
mixing
Higher viscosity per Ib of clay solids
a
u
of clay soh(
u u
Lower viscosity pel Ib ol day solids
0
Deflocculated
Low gel strength
> Lowest rate of filtration
> Firm filter cake
Flocculated
• Progressive gel strength (
• High rate of filtration
• Soft filter cake
Deflocculated
• Low gel strength
• Low rate of filtration
• Firm filter cake
Flocculated
• Sudden, non-progressive
gel strength
• Highest rate of filtration
• Soft filter cake
Figure 39. Schematic of the behavior of clay particles when mixed Into water (Driscoll, 1986).
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 forma-
tions when applied in conjunction with a casing hammer or a
dual-wall casing technique. For effective 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 5000 to 7000 feet per
minute is desirable for deep boreholes drilled at high penetra-
tion 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
53
-------
Table 19. Characteristics of Common Formation-Sampling Methods
Type of
Formation
Sample Collection
Method
Sample
Quality
Potential for
Continuous
Sample
Collection
Samples
Suitable
for Lab
Tests
Discrete
Zones
Identifiable
Increasing
Reliability
Unconsolidated
Consolidated
Solid core auger Poor No
Ditch (direct rotary) Poor Yes
Air rotary with casing driver Fair Yes
Dual-wall reverse circulation
rotary Good Yes
Piston samplers Good No
Split spoon and thin-wall
samplers Good Yes
Special samplers
(Dennison, Vicksburg) Good Yes
Cores Good Yes
Ditch (direct rotary) Poor Yes
Surface (dry air) Poor Yes
Surface (water/foam) Fair Yes
Cores (wireline or
conventional) Good Yes
No
No
No
No
Yes
Yes
Yes
Yes
No
No
No
Yes
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
I
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 unconsoli-
dated 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 obtain-
ing samples in unconsolidated materials by which other tech-
niques 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 un-
consolidated materials. Split-spoon and thin-wall sampling
techniques are the primary techniques that are used to obtain
data for monitoring well installation.
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 equip-
ment 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 Samplers
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 itrests on top of fresh undisturbed
formation. In order to obtain valid samples, the 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 con-
ducted 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
characteristics (i.e. rate of penetration, vibrations, stability,
etc.) of the formation being penetrated are also used to infer
54
-------
Head assembly
Split barrel
Spacer
Liner
Shoe
Figure 40. Diagram of a split-spoon sampler (Mobile Drilling
Company, 1982).
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)
Soil Type
Designation
Blows/Foot*
Sand
and
Silt
Clay
(Loose
Medium
Dense
Very Dense
{Very Soft
Soft
Medium
Stiff
Hard
0-10
11-30
31-50
>50
<2
3-5
6-15
1-25
>25
'Assumes: a) 2-inch outside diameter by 1 3/8-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 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 D1587 (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 fre-
quently 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 D1586 (American Society
for Testing and Materials, 1984).
55
-------
Cap screw
Head assembly
Tube
Figure 41. Diagram of a thin-wall sampler (Acker Drill Company,
Inc., 1985).
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 5.05-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 ce-
mented 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 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 clam 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.
56
-------
Drill rod
Air hose
N" rod coupling
Plug
Sampler head
Clamp
Wrench holes
Adapter
2B§« Allen set
TV«l screws
Rubber gasket
Sampling tube
Outer head
Outer tube
Check valve
Inner tube
Liner
• Sawtooth bit
- Basket retainer
•Bit
(b)
Figure 42. Two types of special soil samplers: a) Vicksburg sampler (Krynine and Judd, 1957) and b) Dennison sampler (Acker Drill
Company, Inc., 1985).
Core Barrels
When installing monitoring wells in consolidated forma-
tions 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 bar-
rels are available as illustrated in Figures 48a and 48b.
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 frac-
57
-------
Upper drive
head with left
threaded pin
Piston cable
Hardened drive
shoe
Schematic
Inner core barrel
(dedicated)
Outer core barrel
Piston with rubber
washers & brass
spacers
Figure 43. Internal sleeve wireline piston sampler (Zaplco et al.,
1987).
tured. In these formations very little or no core may be recov-
ered.
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 require-
ments that limit the applicability of the drilling techniques.
Each applicable drilling method that can be used in the de-
scribed 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 condi-
tions.
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 matri-
ces 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,
4) depth range of the monitoring well: 0 to 15 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 corre-
sponds to the combination of factors used to develop the
numbers on each matrix.
Each matrix provides a relative evaluation of the applica-
bility 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,
58
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/
/ General
/ Hydrogeologic
/ Conditions
/ and Well Design
/ Requirements
Unconsolidated
Consolidated
Saturated
Unsatu rated
Invasion Permitted
Invasion Prohibited
Depth 0-1 5 Feet
Depth 15-150 Feet
Depth > 150 Feet
<2-lnch Diameter Casing
2-4 Inch Diameter Casing
4-8 Inch Diameter Casing
I
I
-1
I
-------
Brass bushings
Teflon wiper disc
Swivel
Neoprene seals
Figure 44. Modified wireline piston sampler (Leach et al., 1988).
•Auger-head
Bit
(a) Basket
(c) Adapter ring
(b) Spring
(d) Flap valve
Figure 45. Clam-shell fitted auger head (Leach et al., 1988). Figure 46. Types of sample retainers (Mobile Drilling Company,
1982).
60
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Auger drill
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).
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 condi-
tions 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
for 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.
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 2 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 con-
siderations 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 evalu-
ation, it is necessary to reevaluate 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. Recog-
nizing 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 signifi-
cantly.
Criteria for Evaluating Drilling Methods
In determining the most appropriate drilling technology to
use at a specific site the following criteria must be considered.
61
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Inner tube-
Reaming
shell
Core lifter
Core lifter •
"Blank bit
(a)
(b)
Figure 48. Diagram of two types of core barrels:
a) single tube and b) double-tube (Mobile Drilling Company, 1982).
Core barrel
head outer
Ball
bearings
Dinner
Hanger
bearing
tube head assembly
Bearing
retainer
•Pin & nut
• Outer tube
'Reaming shell
•Blank bit
- Lifter case
62
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MATRIX NUMBER 1
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less; total
well depth Oto 15 feet.
\ Z (^
\ O ™
\ c ^3
\ ii
\ §
\ -°
\i°
\ 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
i
ty of Drilling
13
I
1
1
2
3
10
8
NA
7
7
9
Reliability
O
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= 3
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-------
monitoring program by drilling fluid invasion into the moni-
tored zone.
The interaction between the geologic formations, hydro-
logic 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 monitor-
ing wells in solid rock such as granite. It may be less obvious
that drilling through the saturated, unstable overburden overly-
ing 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, con-
versely, 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, judg-
ment 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) measure-
ments are made, and from which water samples can be obtained.
These water samples must accurately 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 par-
tially 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-repre-
sentative 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 satu-
rated 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 stratigraphic 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 entitled "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 method. 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 eitherof 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 possible to use
these techniques in saturated formations with the augers re-
moved 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 forma-
tions. 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 opti-
mize (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 contami-
nation. Shallow zones that may have been penetrated in the
upper portion of the borehole are also difficult to develop once
64
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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 zones 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 advan-
tage 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 encoun-
tered 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."
Direct Mud Rotary Drilling —
A variety of sampling technologies can be used in conceit
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 materi-
als. In directrotary 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 interpre-
tation 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. Corre-
sponding size limitations and sampling problems prevail.
As depths increase below about 150 feet, the time con-
sumed 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 stabi-
lized by the drilling mud for a sufficient period of time to
remove the drilling tools and run a complete suite of geophysi-
cal 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 uncon-
solidated materials, drilling by the mud rotary method offers
another advantage. Because the drilling mud maintains the
stability of the borehole, samples taken by split-spoon or thin-
wall methods ahead of the drill bit tend to be much 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 drill-
ing. 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 fil-
trate 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 montmorillo-
nite (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 mix 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 contami-
nants are being evaluated and where the potential reactions are
65
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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 param-
eters. When this occurs, the resultant quality is considered to be
representative (Barcelona, etal., 1985a). An additional discus-
sion 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."
Air Rotary —
Direct air rotary is restricted in application to consolidated
rock. Where the bedrock is overlain by unconsolidated materi-
als, 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 con-
trolled, 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 injec-
tion 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 gen-
erally 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 addi-
tional discussion on air rotary drilling can be found in the
section entitled "Drilling Methods for Monitoring Well Instal-
lation."
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 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 for-
mation 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 Instal-
lation."
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 them at
the surface. Because the borehole is cased, the samples col-
lected at the surface are very reliable and representative of the
formations penetrated. Sample collection using dual-wall ro-
tary has the following advantages: 1) third stratigraphic zones
often can be identified; 2) contamination of the borehole by
drilling fluid is minimized; 3) interaquifer cross-contamination
is minimized; 4) individual zones that are hydraulically distinct
can be identified with specific water levels, and discrete
samples of ten 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
66
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section entitled "Drilling Methods for Monitoring Well Instal-
lation."
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 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 sam-
pling tools inside the casing. This technique permits sampling
of fine-grained, unconsolidated formations.
The quality of cable tool samples from consolidated forma-
tions 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 require-
ments. 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 hollow-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 rela-
tive ratings reflect the total cost of drilling, sampling, casing,
screening, filter-packing, grouting, developing and surface
protecting the monitoring well. Equivalent costs of mobiliza-
tion and access are assumed. Relative ratings are based on
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 equip-
ment 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 availabil-
ity are based on the general availability of the drilling equip-
ment 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 unconsoli-
dated materials predominate. The portability of augering equip-
ment 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 broad-
ened 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 country.
Cable tool equipment availability is limited in many por-
tions 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 Well 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 drilling method. For example, if a relatively
deep hole drilled with cable tool techniques takes several days,
67
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weeks or longer, there may be significant scheduling disadvan-
tages. If longer-term supervision is required, then this addi-
tional 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 (pres-
sure 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 chemi-
cal interactions that might cause substantial or unpredictable
changes in the quality of the water being sampled. The follow-
ing 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 paniculate 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.
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 may 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 moni-
tored is preferable in any given setting. The matrices presented
indicate the relative impact of the various drilling methodolo-
gies 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 artifi-
cial filter packs and bentonite seals, a minimum annular space
4 inches greater in diameter than the maximum outside diam-
eter of the casing and screen is generally needed. A 2-inch
outside diameter monitoring well would then require a mini-
mum 6-inch: 1) outside diameter borehole, 2) auger inside
diameter or 3) casing inside diameter for reliable well installa-
tion. This need for a 4-inch annular space places a severe
limitation on the use of several currently-employed drilling
technologies.
For example, hollow-stem augers have been widely used to
install 2 3/8-inch outside diameter monitoring wells. A signifi-
cant portion of this work has been performed within 3 1/4-inch
inside diameter hollow-stem augers. At shallow depths, espe-
cially 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 Ap-
pendix A.
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
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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 calcula-
tions.
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, 4) 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 modification of
procedure may make the difference between success and fail-
ure. The ratings shown in the matrices are based on general
considerations. Their relative values expressed in the table vary
in specific circumstances. Most 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, 4) 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 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 includ-
ing: scheduling, materials, equipment, labor, permits, rights of
various parties, tests and inspections, safety, payments, con-
tracts, 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) perti-
nent subsurface information, 5) description of necessary per-
mits, 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 spe-
cial conditions appear to conflict, special conditions of the
contract prevail (Driscoll, 1986). Technical specifications con-
tain 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 non-competi-
tive. 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 ad-
dressed 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
• well logs
• depth of 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 of Equipment
procedures
materials
disposal of cuttings and liquids
Site Safety
equipment
training
Cond ions
permits
certificates
utility location
site clean up
procedures for drilling difficulties
non-functioning wells
government forms required
client's right 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 submit-
ting 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 bid-
ding 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 accord-
ing 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 accord-
ing 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 interpretation or imagination. This clarity can
best be obtained by listing individual pay items instead of
combining items into unspecified quantities in lump sum pric-
ing. 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 equip-
ment 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 theprocedures 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
Table 23. Suggested Items for Unit Cost in Contractor Pricing Schedule
Item
Pricing Basis
•Mobilization
•Site preparation
•Drilling to specified depth
•Sampling
•Material supply
surface casing
well casing
end caps
screen
filter material
bentonite seal(s)
grout
casing protector
•Support equipment
water truck and water
bulldozer
•Decontamination
•Standby
•Field expenses
•Material installation
•Well development
•Demobilization
•Drilling cost adjustment for variations in depths
lump sum
lump sum
per lineal foot or per hour
each
per lineal foot
per lineal foot
each
per lineal foot
per lineal foot or per bag
per lineal foot
per lineal foot or per bag
each
lump sum
per hour
lump sum
per hour
per man day or lump sum
per hour or lump sum
per hour or lump sum
lump sum
± per foot
70
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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 sched-
ule.
After the contract is signed and work is scheduled to begin,
a predrilling meeting between the supervising geologist and the
driller should be held to discuss operational details. This meet-
ing reduces the opportunity for misunderstanding of the speci-
fications and improves project relationships.
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: D1587; 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,SWSContractReport374,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, Lorne 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,
12pp.
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 reversecirculation;
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,
72pp.
UnitedStates Environmental Protection Agency, 1975. Manual
of water well construction practices; United States
Environmental Protection Agency, Office of Water
Supply, EPA-570/9-75-001, 156pp.
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.
71
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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 configura-
tions 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 (predomi-
nantly for water supply wells) focused on structural strength,
durability in long-term exposure to natural ground-water envi-
ronments and ease of handling. Different materials have dem-
onstrated versatility in well casing applications. In the late
1970s, questions about the potential impact that casing materi-
als 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 environ-
ment, 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) avail-
ability.
The most frequently evaluated characteristics that directly
influence the performance of casing materials in ground-water
monitoring applications are: 1) strength and 2) chemical resis-
tance/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 evalu-
ated, 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 sus-
pended 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 compres-
sive 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 compres-
sive 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 diam-
eter and wall thickness. Casing collapse strength is proportional
to the cube of the wall thickness. Therefore, a small increase in
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Borehole
Casing Joint
Casing
Tensile (Pull-apart) Forces
Critical at Casing Joints
Collapse Forces
. (Critical at Greater Depths)
Figure 50. Forces exerted on a monitoring well casing and screen during installation.
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.
A casing is most susceptible to collapse during installation
before placement of the filter pack or annular seal materials
around the casing. Although it may collapse during develop-
ment, once a casing is properly installed and therefore sup-
ported, collapse is otherwise seldom a point of concern (Na-
tional 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 filterpack 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
74
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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 resis-
tance 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 corro-
sion 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 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 forma-
tion water quality may be detected in samples collected from the
well. This "false positive" might be considered to be an indica-
tion 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 nega-
tive" 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 poly-
tetrafluoroethylene(PTFE), tetrafluoroethylene
(TFE), fluorinated ethylene propylene (FEP),
perfluoroalkoxy (PFA) and polyvinylidene
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 (P VC) and aery lonitrile butadiene styrene
(ABS).
In addition to the three categories that are widely used,
fiberglass-reinforced materials including fiberglass-reinforced
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 site-specific hydrogeologic and con-
taminant-related monitoring situations. These characteristics
for each of the three categories of materials are discussed below.
Fluoropolymer materials —
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 expan-
sion 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 proper-
ties are:
1) extreme temperature range - from -400°F to
+500°F in constant service;
2) outstanding electrical and thermal insulation;
75
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3) lowest coefficient of frictionof 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 other fluoropolymers. Polyvinylidene fluoride
(PVDF) is tougher and has a higher abrasion resistance than
other fluoropolymers and is resistant to radioactive environ-
ments. PVDF has a lower upper temperature limit than either
PTFE or PFA.
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 physi-
cal properties and different fabricating techniques. These ma-
terials may not always be interchangeable in service.
For construction of ground-water monitoring wells,
fluoropolymers possess several advantages over other thermo-
plastics and metallic materials. For example, fluoropolymers
are almost completely inert to chemical attack, even by ex-
tremely aggressive acids (i.e., hydrofluoric, nitric, sulfuric and
Table 24. Trade Names, Manufacturers, and Countries of Origin for Various Fluoropolymer Materials
Chemical Formulation
Trade Name
Manufacturer
Country of Origin
PTFE (or TFE)- Polytetrafluoroethylene
FEP- Fluorinated ethylene propylene
PFA- Perfluoroalkoxy
PVDF- Polyvinylidene fluoride
CTFE- Chlorotrifluoroethylene
Teflon
Halon
Fluon
Hostaflon
Polyflon
Algoflon
Soriflon
Neoflon
Teflon
Neoflon
Teflon
Kynar
Kel-F
Diaflon
DuPont
Allied
ICI
Hoechs
Daikin
Montedison
Ugine Kuhlman
Daikin
DuPont
Daikin
DuPont
Pennwalt
3M
Daikin
USA, Holland, Japan
USA
UK, USA
W. Germany
Japan
Italy
France
Japan
USA, Japan,
Japan
USA, Japan,
USA
USA
Japan
Holland
Holland
Table 25. Typical Physical
Properties
Properties of Various Fluoropolymer Materials (After Norton Performance Plastics, 1985)
Units ASTM Method TFE FEP PFA E-CTFE
CTFE
Tensile strength @73°F
Elongation @73°F
Modulus© 73°F
Tensile
Flexural
Elasticity in tension
Flexural strength@73°F
Izod impact strength
(1/2 X1/2-in. notched bar)
@+75°F
@-65°F
Tensile impact strength
@+73°F
@-65°F
Compressive stress @ 73°F
Specific gravity
psi
%
psi
psi
psi
ft. lbs./in.
of notch
ft. IbsVin.
of notch
ft. Ibs./sq. in.
ft. Ibs./sq. in.
psi
D638-D651
D638
D638
D790
D747
D790
D256
D1822
D695
D792
2500-6000
150-600
45,000-115,000
70,000-110,000
58,000
Does not break
3.0
2.3
320
105
1700
2.14-2.24
2700-3100
250-330
95,000
250,000
Does not break
No break
2.9
1020
365
-
2.12-2.17
4000-4300 7000
300-350 200
240,000
95,000-100,000240,000
-.
Does not break 7000
No break No break
-
-
-
-
2.12-2.17 1.68
4500-6000
80-250
206,000
238,000
1.5-3.0x1 0s
8500
5.0
-
-
-
4600-7400
2.10-2.13
Coefficient of friction
static & kinetic against
polished steel
Coefficient of linear
thermal-expansion
D696
0.05-0.08
5.5X105
0.06-0.09
5.5X105
0.05-0.06
6.7X105
0.15-0.65
14X10s
0.2-0.3
2.64X10-5
76
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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 ongo-
ing, 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;afifth 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 or-
ganic 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 ex-
ample, 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 etal.(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 sup-
ported 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 approxi-
mately 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 approxi-
mately 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 material. Addition-
ally, 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 conse-
quent reduction in well efficiency in deep fluoropolymer wells.
Dablow etal.(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
DuPont to determine the amount of deformation in PTFE 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.
100 200 300 400 500 600 700 800 900 1000
Compression Load - Lbs.
Note: Short Term Test -10 Minutes
Figure 51. Static compression results of Teflon* screen (Dablow
et al., 1988).
'Dupont's registered trademark for its fluorocarbon
resfn
According to Dablow et al. (1988), a recommended con-
struction 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 compres-
sive stress by supplying support on the outer wall of the casing.
This can only be accomplished successfully in relatively shal-
low 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. Addi-
tionally, 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 exist 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,
77
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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 en-
countered in a ground-water monitoring situation. However,
metallic materials are subject to corrosion during long-term
exposure to certain subsurface geochemical environments.
Corrosion of metallic well casings and well 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;
2) selective corrosion (dezincification) or loss of
one element of an alloy, leaving a structurally
weakened material;
3) bi-metallic corrosion, caused by the creation of a
galvanic cell ator 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
HjS in quantities as low as 1 milligram per liter
can cause severe corrosion;
4) total dissolved solids (TDS) - if TDS 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
CO2 content of the water exceeds 50 milligrams
per liter; and
6) chloride ion (Cl) content - if Cl content exceeds
500 milligrams per liter, corrosion canbeexpected.
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 in-
creased resistance to atmospheric corrosion. Achieving this
Table 26. Hydraulic Collapse and Burst Pressure and Unit Weight of Stainless Steel Well Casing (Dave Kill, Johnson Division, St.
Paul, Minnesota, Personal Communication, 1985)
Norn.
Size
Inches
2
2 1/2
3
31/2
4
5
6
Schedule
Number
5
10
40
80
5
10
40
5
10
40
5
10
40
5
10
40
5
10
40
5
10
40
Outside
Diameter,
Inches
2.375
2.375
2.375
2.375
2.875
2.875
2.875
3.500
3.500
3.500
4.000
4.000
4.000
4.500
4.500
4.500
5.563
5.563
5.563
6.625
6.625
6.625
Wall
Thickness
Inches
0.065
0.109
0.154
0.218
0.083
0.120
0.203
0.083
0.120
0.216
0.083
0.120
0.226
0.083
0.120
0.237
0.109
0.134
0.258
0.109
0.134
0.280
Internal
Inside Cross-Sectional
Diameter Area
Inches Sq. In.
2.245
2.157
2.067
1.939
2.709
2.635
2.469
3.334
3.260
3.068
3.834
3.760
3.548
4.334
4.260
4.026
5.345
5.295
5.047
6.407
6.357
6.065
3.958
3.654
3.356
2.953
5.761
5.450
4.785
8.726
8.343
7.389
11.54
11.10
9.887
14.75
14.25
12.72
22.43
22.01
20.00
32.22
31.72
28.89
Internal Pressure
psi
Test. Bursting
820
1.375
1.945
2.500
865
1.250
2.118
710
1.030
1.851
620
900
1.695
555
800
1.580
587
722
1.391
494
606
1.268
4.105
6.884
9.726
13.768
4.330
6.260
10.591
3.557
5.142
9.257
3.112
4.500
8.475
2.766
4.000
7.900
2.949
3.613
6.957
2.467
3.033
6.340
External
Pressure
psi
Collapsing
896
2.196
3.526
5.419
1.001
1.905
3.931
639
1.375
3.307
431
1.081
2.941
316
845
2.672
350
665
2.231
129
394
1.942
Weight
Pounds
per Foot
1.619
2.663
3.087
5.069
2.498
3.564
5.347
3.057
4.372
7.647
3.505
5.019
9.194
3.952
5.666
10.891
6.409
7.842
14.754
7.656
9.376
19.152
78
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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 occur-
rence 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 prod-
ucts include iron and manganese and trace metal oxides as well
as various metal sulfides (Barcelona et al., 1983). Under oxi-
dizing conditions, the principal products are solid hydrous
metal oxides; under reducing conditions, high levels of dis-
solved 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 cad-
mium species (Barcelona et al., 1983).
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 adsorp-
tion. 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 consid-
ered 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 avail-
able. The most common alloys used for 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 stand-
point. 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 chro-
mium 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 stain-
less 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 reduc-
ing 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. (1983) 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 accesso-
ries.
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 ma-
terials. 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). Thermo-
plastics 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 du-
rable 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 thermoplas-
tics 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 pro-
tected from breakage. Potential chemical problems are dis-
cussed in the following sections.
Polyvinyl chloride (PVC)—PVC plastics are produced by
combining PVC resin with various types of stabilizers, lubri-
cants, 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 etal., 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 ASTM standard
specification D-1785 that covers rigid PVC compounds (Ameri-
79
-------
can Society for Testing and Materials, 1986). This standard
categorizes rigid PVC by numbered cells designating value
ranges for certain pertinent properties and characteristics in-
cluding impact strength, tensile strength, rigidity (modulus of
elasticity), temperature resistance (deflection temperature) and
chemical resistance. ASTM standard specification F-480 cov-
ers 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.
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 contrib-
utes 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 arid Plastic Pipe Institute,
19181).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 (Ameri-
can Society for Testing and Materials, 1981). Minimum physi-
cal 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 rela-
tively low in comparison to metallic materials, but the devel-
oped string loading is not a limiting factor because the thermo-
plastic 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 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 cor-
rode 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 con-
centrations 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 solva-
tion. 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 (MIBK) 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 mat otherwise will
remain in the solid material. These chemical compounds in-
clude: 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 thermoplas-
tic 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 plasti-
cizers, stabilizers, fillers, pigments and lubricants. The propen-
sity of currently available information on potential contamina-
tion of water that cornes in contact with rigid thermoplastic
materials relates specifically to PVC; no information is cur-
rently 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 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
80
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Table 27. Typical Physical Properties of Thermoplastic Well Casing Materials at 73.4° (National Water Well Association and Plastic
Pipe Institute, 1981)
Property
Specific Gravity
Tensile Strength, Ibs. /in.2
Tensile Modulus of Elasticity, Ibs./in.2
Compressive Strength, Ibs./in.2
Impact Strength, Izod, ft. -Ib/inch notch
Cell Class,
per D-1788
ASTM Test Method 434 533
D-792
D-638
D-638
D-695
D-256
1.05
6,000*
350,000
7,200
4.0*
1.04
5,000*
250,000
4,500
6.0*
PVC
Cell Class,
per D-1784
12454-B&C 14333-C&D
1.40
7,000*
400,000*
9,000
0.65
1.35
6,000*
320,000*
8,000
5.0
Deflection Temperature Under Load
(264 psi), °F
Coefficent of Linear Expansion,
in./in. - °F
D-648
D-696
190*
5.5 X10'5
190*
6.0 X 10s
158*
3.0X10-6
140*
5.0 X105
* These are minimum values set by the corresponding ASTM Cell Class designation. All others represent typical values.
Table 28. Hydraulic Collapse Pressure and Unit Weight of PVC Well Casing (National Water Well Association and Plastic Pipe
Institute, 1981)
Outside Diameter
(inches) SCH*
Norn. Actual
2
21/2
3
31/2
4
41/2
5
6
2.375
2.875
3.500
4.000
4.500
4.950
5.563
6.625
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
-
-
SCH 80
SCH 40
SCH 80
SCH 40
Wall
Thickness
Min. (in.)
0.218
0.154
0.276
0.203
0.300
0.216
0.316
0.226
0.337
0.237
0.248
0.190
0.375
0.258
0.432
0.280
DR"
10.9
15.4
10.4
14.2
11.7
16.2
12.6
17.7
13.3
19.0
20.0
26.0
14.5
21.6
15.3
23.7
Weight in Air
(lbs/100 feet)
PVC 12454 PVC14333
94
69
144
109
193
143
235
172
282
203
235
182
391
276
538
358
91
66
139
105
186
138
227
176
272
196
226
176
377
266
519
345
Weight in Water
(lbs/100 feet)
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
84
171
62
* Schedule
Table 29. Hydraulic Collapse Pressure and Unit Weight of ABS Well Casing (National Water Well Association and Plastic Pipe
Institute, 1981)
Outside Diameter
(inches) SCH*
Nom. Actual
2
21/2
3
31/2
4
5
6
2.375
2.875
3.500
4.000
4.500
5.563
6.250
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
SCH 80
SCH 40
Wall
Thickness
Min. (in.)
0.218
0.154
0.276
0.203
0.300
0.216
0.318
0.226
0.337
0.237
0.375
0.258
0.432
0.280
DR"
10.9
15.4
10.4
14.2
11.7
16.2
12.6
17.7
13.3
19.0
14.8
21.6
15.3
23.7
Weight in Air
(lbs/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
(lbs/100 feet)
ABS 434 ABS 533
3.4
2.5
5.1
3.9
6.9
5.1
8.4
6.1
10.0
7.2
14.0
9.8
19.2
12.8
2.7
2.0
4.1
3.1
5.5
4.1
6.7
4.9
8.0
5.8
11.2
7.9
15.4
10.2
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
81
-------
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 (Barcelonaetal., 1985b; Barcelona, 1984; Barcelona
etal., 1983; Junk etal., 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 stabiliz-
ers, at levels approaching 5 percent by weight. Some represen-
tative 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 chemi-
cal constituents in PVC formulations. The purpose of these
specifications as outlined in NSF Standard 14 (National Sani-
tation 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. Most of these levels correspond to those set
by the Safe Drinking Water Act for chemical constituents
covered by the National Interim Primary Drinking Water Stan-
dards. 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 the speci-
fications. 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
etal., 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 inter-
fere with reversed-phase analysis for low micrograms per liter
levels of 2,4,6 trinitrotoluene (TNT), hexahydro-l,3,5-trinitro-
1,3,5-triazine (RDX), octahydro-l,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 con-
stituents 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, bromoform,
1,1,1-trichloroethane, 1,1,2-trichloroethane or chromium. In this
experiment, sorption was measured weekly for six weeks and
compared to acontrol; 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 deter-
mine 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.
In another laboratory study, Reynolds and Gillham (1985)
found that sorption of selected organics (specifically 1,1,1-
trichloroethane, 1,1,2,2-tetrachloroethane, bromoform, hexa-
chloroethane 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.
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 chemi-
cal constituents) has been conducted under laboratory condi-
82
-------
tions. Furthermore, in most of the laboratory work the P VC 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 gen-
erally 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).
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
Table 30. Representative Classes of Additives in Rigid PVC Materials Used for Pipe or Well Casing (Barcelona et al., 1983)
(Concentration in wt. %)
Heat stabilizers (0.2-1.0%) Fillers (1-5%)
Dibutyltin diesters of lauric and maleic acids CaCO,
Dibutyltin bis (laurylmercaptide)
Dibutyltin-B mercaptopropionate
di-n-octyltin maleate
di-n-octyltin-S,S'-bis isoctyl mercaptoacetate
di-n-octyltin-B mercaptopropionate
Various other alkyltin compounds
Various proprietary antimony compounds
diatomaceous earth
clays
pigments
Ti02
carbon black
iron and other metallic oxides
Lubricants (1-5%)
stearic acid
calcium stearate
glycerol monostearate
montan wax
polyethylene wax
Table 31. Chemical Parameters Covered by NSF Standard 14 (National Sanitation Foundation, 1988)
Parameter
Antimony (Sb)
Arsenic (As)
Barium(Ba)
Cadmium (Cd)
Chromium (Cr)
Lead(Pb)
Mercury (Hg)
Phenolic substances
Residual vinyl chloride monomer (RVCM)
Selenium (Se)
Total dissolved solids
Tin (Sn)
Total trihalomethanes (TTHM)
Taste and Odor Evaluations
Characteristic
Odor
Taste
Maximum Permissible Level mg/L (ppm)
0.050
0.050'
1.01
0.010'
0.0501
0.0201
0.0020'
0.050
2.0
0.010'
70.0
0.050
0.10
Permissible Level
Cold application 40
Hot application 50
Satisfactory
1 Established in the U.S. EPA National Primary Drinking Water Regulations.
*ln the finished product ppm (mg/kg).
83
-------
Coupling
a. Flush-joint Casing
(Joined by Solvent Welding)
b. Threaded, Rush-joint Casing
(Joined by Threading Casing
Together)
c Plain Square-end Casing
(Joined by Solvent Welding
with Couplings)
d. Threaded Casing
(Joined by Threaded Couplings)
e. Bell-end Casing
(Joined by Solvent Welding)
f. Plain Square-end Casing
(Joined by Heat Welding)
Figure 52. Types of joints typically used between casing lengths.
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 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
84
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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 inex-
pensive, 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 im-
proves 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 casing has a high initial tensile strength.
Thermoplastic Casing Joining —
There are two basic methods for joining sections of ther-
moplastic 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 prob-
lems 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
outerdiameter casing (Morrison, 1984).
Solvent cementing—In solvent cementing, a solvent primer
is generally used to clean the two pieces of casing to be joined
and a solvent cement 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 (MEK), methyl isobutyl ketone (MIBK), cyclohexanone
and dimethylformamide (Sosebee et al., 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 monitoring wells in which
PVC adhesives are used to join well casing sections. Barcelona
et al. (1983) noted that even minimal solvent cement applica-
tion 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 more than two years after the casing
was installed. In samples from adjacent 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 joining~The most common method of me-
chanical joining is by threaded connections. Molded and ma-
chined threads are available in a variety of thread configura-
tions including: acme, buttress, standard pipe thread and square
threads. Because most manufacturers have their own thread
type, threaded casing may not be compatible between manufac-
turers. 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 cas-
ing 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 molded directly onto the pipe (without use of
larger-diameter couplings) provides a flush joint between both
inner and outer diameters. Because the annular space is fre-
quently 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, varia-
tions 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 casings. 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 evaluat-
ing 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)
85
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Tabe 32. Volume of Water In Casing or Borehole (Driscoll, 1986)
Diameter Gallons Cubic Feet
of Casing per foot per Foot
or Hole of Depth of Depth
(In)
Liters Cubic Meters
per Meter per Meter
of Depth of Depth
1
1 1/2
2
21/2
3
31/2
4
41/2
5
51/2
6
7
8
9
10
11
12
14
16
18
20
22
24
26
28
30
32
34
36
0.041
0.092
0.163
0.255
0.367
0.500
0.653
0.826
1.020
1.234
1.469
2.000
2.611
3.305
4.080
4.937
5.875
8.000
10.44
13.22
16.32
19.75
23.50
27.58
32.00
36.72
41.78
47.16
52.88
0.0055
0.0123
0.0218
0.0341
0.0491
0.0668
0.0873
0.1104
0.1364
0.1650
0.1963
0.2673
0.3491
0.4418
0.5454
0.6600
0.7854
1.069
1.396
1.767
2.182
2.640
3.142
3.687
4.276
4.909
5.585
6.305
7.069
0.509
1.142
2.024
3.167
4.558
6.209
8.110
10.26
12.67
15.33
18.24
24.84
32.43
41.04
50.67
61.31
72.96
99.35
129.65
164.18
202.68
245.28
291.85
342.52
397.41
456.02
518.87
585.68
656.72
0.509 x 10^
1.142X10-3
2.024 x 10^
3.167x10^
4.558 x10J
6.209 x10J
8.1 10 x 10-3
10.26 x 10^
12.67x10J
15.33 X10-3
18.24x 10-3
24.84 X10J
32.43x10^
41.04x10^
50.67x10^
61.31 x10^
72.96 X10"3
99.35 x 10J
1 29.65 x 10-3
164.18 x 10"3
202.68 x10J
245.28x10^
291. 85 x 10J
342.52 x 10J
397.41 X 10J
456.02 x 10^
518.87 x10J
585.68 x 10J
656.72 x 10^
1 Gallon = .785 Liters
1 Meter = 3.281 Feet
1 Gallon Water Weighs 8.33 Ibs. = 3.785 Kilograms
1 Liter Water Weighs 1 Kilogram = 2.205 Ibs.
1 Gallon per foot of depth = 12.419 liters per foot of depth
1 Gallon per meter of depth = 12.419 x 10'3 cubic meters
per meter of depth
cost. For an additional discussion of casing diameter, refer to
the sections entitled "Equipment that the Well Must Accommo-
date" 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, machin-
ing 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.
Careful pre-installation cleaning of casing materials must
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) sug-
gest 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 materials. Cost is always a consideration for any
ground-water monitoring project and becomes increasingly
important as the number and/or depth of the wells increases.
However.iftheparticularcomponentsofinterestinamonitoring
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) open-
ings, 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.
86
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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
Sch5
0.065
0.083
0.083
0.109
0.109
Sen 5
2.245
3.334
4.334
5.345
6.407
SchIO
0.109
0.120
0.120
0.134
0.134
SchIO
2.157
3.260
4.260
5.295
6.357
Sch40
0.154
0.216
0.237
0.258
0.280
Sch40
2.067
3.068
4.026
5.047
6.065
Sch80
0.218
0.300
0.337
0.375
0.432
SchSO
1.939
2.900
3.826
4.813
5.761
Outside Diameter
(Standard)
2.375
3.500
4.500
5.563
6.625
Figure 53. Effect of casing wall thickness on casing inside and outside diameter.
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 diam-
eter generally should be filter-packed. For example, where 2-
inch diameter screens are installed in hollow-stem auger bore-
holes 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.
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 sedi-
ment-free water at maximum capacity (Figure 54).
The decision on whether or not a well can be naturally
developed is generally based on geologic conditions, specifi-
cally the grain-size distribution of natural formation materials
in the monitored zone. Wells can generally be naturally devel-
oped where formation materials are relatively coarse-grained
and permeable. Grain-size distribution is determined by con-
ducting 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 overempha-
sized.
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 the effective 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.
87
-------
Zone of
Coarsest
Natural
Material
Zone of
Medium-sized
Granular
Material
Original
Material of
Water-
bearing
Formation
\
\
Figure 54. Envelope of coarse-grained material created around a naturally developed well.
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 condi-
tions 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 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 mate-
rial 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
88
-------
1
I
a
3
1
o
100
90
80
70
60
50
40
30
20
10
\
10 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.
diameter of the well and in an accompanying increase in the
amount of water that flows toward and into the well (Figure 58).
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 paniculate 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 emplace-
ment 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 exten-
sively 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. Complet-
ing 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 unifor-
mity 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 con-
cerning the appropriateness of an artificial filter pack. Costs
associated with filter-packed wells are generally higher than
those associated with 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 perfo-
rated 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 inter-
ference; 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.
89
-------
I
100
90
80
70
60
50
100705040 30
20
U. S. Standard Sieve Numbers
16 12
30
CD
Q.
jo 40
Js
=>
o
20
10
The Effective Size of This Sand is
0.018-inch
10 20 30 40 50 60 70 80 90 100
Slot Opening and Grain Size, in Thousandths of an Inch
110 120
100
90
80
70
60
50
40
30
20
10
0
Figure 56. Determining effective size of formation materials.
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 mate-
rials and filter pack materials. Generally this ratio refers to
either the average (50 percent retained) grain size of the forma-
tion material or the 70 percent retained size of the formation
material. For example, Walker (1974) and Barcelona et al.
(1985a) recommend using a uniform filter pack grain size that
is 3 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 multi-
plied by a factor of 2 to exclude the entrance of fine silts, sands
and clays into the monitoring well. The United States Environ-
mental 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
90
-------
•o
s
a
<§
0>
CL
I
O
100
90
80
70
60
50
40
30
20
10
100 70 50 40 30
U.S. Standard Sieve Numbers
20 16 12
The Uniformity Coefficient of This
Sand Is:
0.026-Inch
0.010-Inch
:2.6
100
90
80
70
60
50
40
30
20
10
10 20 30 40 50 60 70 80 90 100
Slot Opening and Grain Size, in Thousandths of an Inch
110 120
130
Figure 57. Determining uniformity coefficient of formation materials.
Formation Materia
Well Intake-
Filter Pack
Material
- Borehole
Figure 58. Envelope of coarse-grained material em placed
around an artificially filter-packed well.
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 ap-
proach 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
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.
91
-------
U.S. Standard Sieve Numbers
•o
s
1
c
O
100
90
80
70
60
50
40
30
20
10
100705040 30
" Formation
Gradation
Filter Pack Ratio = 4
niformitv Coefficient = 2.2
Uniformity Coefficient = 2.5
Filter Pack Ratio = 4 to 6
D70 = 0.014 Inch
100
90
80
70
60
50
40
30
20
10
10 20 30 40 50 60 70 80 90 100 110 120 130
Slot Opening and Grain Size, in Thousandths of an Inch
Figure 59. Artificial filter pack design criteria.
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 Dimensions —
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 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 well-develop-
ment 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 sug-
gested 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."
92
-------
100 70 50 40 30 20
U.S. Standard Sieve Numbers
16 12
100
o
a.
80
70
60
50
ra 40
o
30
20
10
0
\
\
Dyo Formation = 0.014
D70 Filter Pack = 0.071
Filter Pack Ratio = 5
Uniformity Coefficient of
Filter Pack =1.3
Recommended Screen
Slot Opening = - 060 in.
(60 Slot)
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60 70 80 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.
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 mate-
rial (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 mate-
rial 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 devel-
opment, 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:
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
93
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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 50 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. Proce-
dures 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 Tech-
nology 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. Most 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 more spe-
cific 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 inte-
grated 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, mea-
surements of parameters such as pH, Eh, conductivity, dis-
solved 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 indig-
enous 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-dimen-
sional flow in a stratified but relatively homogeneous aquifer
such as fluvial sands and gravels.
Well Intake Type —
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 manufac-
turers. 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 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 percent-
age of open area is low, the opening sizes are highly variable and
opening sizes small enough to control fine materials are diffi-
cult 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
94
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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 Gauze
No. No.
.
_
-
.
-
_
.
.
6 90
7 80
8 70
10 60
12 50
14
16
18 40
20
23
25 30
28
31
33
35 20
39
47
56
62
66
79
93
94
111
125
132
157
187
223
250
263
312
375
438
500
Sieve
No.
400
325
270
250
200
170
150
115
100
80
65
60
48
42
35
-
32
28
_
24
-
20
-
16
14
12
-
10
9
8
-
7
-
6
5
4
31/2
-
3
21/2
0.371
0.441
0.525
Tyler
U.S Standard
Size of Openings
Inches mm
0.0015
0.0017
0.0021
0.0024
0.0029
0.0035
0.0041
0.0049
0.0058
0.0069
0.0082
0.0097
0.0116
0.0138
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
0.185
0.221
0.250
0.263
0.312
0.371
0.441
0.525
0.038
0.043
0.053
0.061
0.074
0.088
0.104
0.124
0.147
0.175
0.208
0.246
0.295
0.351
0.417
0.457
0.495
0.589
0.635
0.701
0.788
0.833
0.889
0.991
.168
.397
.590
.651
.981
2.362
2.390
2.794
3.180
3.327
3.962
4.699
5.613
6.350
6.680
7.925
9.423
11.20
13.33
Sieve
No.
400
325
270
230
200
170
140
120
100
80
70
60
50
45
40
-
35
30
.
25
-
20
-
18
16
14
-
12
10
8
-
7
-
6
5
4
31/2
1/4
-
5/16
3/8
7/16
1/2
Size of
Openings
Inches
0.0015
0.0017
0.0021
0.0024
0.0029
0.0035
0.0041
0.0049
0.0059
0.0070
0.0083
0.0098
0.0117
0.0138
0.0165(1/64)
0.0180
0.0197
0.0232
0.0250
0.0280
0.0310(1/32)
0.0331
0.0350
0.0394
0.0469
0.0555
0.062(1/16)
0.0661
0.0787
0.0931
0.094(3/32)
0.111
0.125(1/8)
0.132
0.157
0.187(3/16)
0.223
0.250(1/4)
0.263
0.312(5/16)
0.375(3/8)
0.438(7/16)
0.500(1/2)
extensively because they are the only types available with 2-
inch inside diameters.
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 imprac-
tical, 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 percent-
age 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
95
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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 machin-
ing tools. Slotted well casing can be manufactured from any
casing material although these intakes are most commonly
made from thermoplastic, fluoropolymer and fiberglass-rein-
forced 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)
0.006
0.007
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.025
0.030
0.035
0.040
0.050
0.060
0.070
0.080
0.100
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 ar-
ranged rods (Figure 62). At each point where the wire crosses
the rods, the two members are securely joined 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 (104
and 316), galvanized and low-carbon steel and 2) any thermo-
plastic that can be sonic-welded, including poly vinyl chloride
(PVC) and acrylonitrile butadiene styrene (ABS).
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 pre-
venting 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 outer face and widen
inwardly. This makes the intakes non-clogging because par-
deles slightly smaller than the openings can pass freely into the
well without wedging in the opening.
Well Intake Material Properties —
The intake is the part of the monitoring well that is most
susceptible to corrosion and/or chemical degradation and pro-
vides 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 deg-
radation, 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 deterio-
Table 35. Intake Areas (Square Inches per Lineal Foot of Screen) for Continuous Wire-Wound Well Intake (After Johnson Screens,
Inc., 1988)
Screen 6 Slot 8 Slot 10 Slot 12 Slot 15 Slot 20 Slot 25 Slot 30 Slot 35 Slot 40 Slot 50 Slot
Size (In.) (0.006") (0.008") (0.010") (0.012") (0.015") (0.020") (0.025") (0.030") (0.035") (0.04011) (0.050")
1 1/4 PS*
2 PS
1 1/2 PS
2 PS
3 PS
4 PS
4 Spec"
4 1/2 PS
5 PS
6 PS
8 PS
3.0
3.0
3.4
4.3
5.4
7.0
7.4
7.1
8.1
8.1
13.4
3.4
3.4
4.5
5.5
7.1
9.0
9.7
9.4
10.6
10.6
17.6
4.8
4.8
5.5
6.8
8.8
11.3
11.9
11.7
13.1
13.2
21.7
6.0
6.0
6.5
8.1
10.4
13.5
14.2
13.8
15.5
15.6
25.7
7.0
7.0
8.1
10.0
12.8
16.5
17.2
17.0
19.1
19.2
31.5
8.9
8.9
10.2
12.8
16.5
21.2
22.2
21.9
24.7
25.0
40.6
10.8
10.8
12.3
15.4
20.0
25.8
27.1
26.8
30.0
30.5
49.3
12.5
12.5
14.2
17.9
23.2
30.0
31.3
31.0
34.9
35.8
57.4
14.1
14.1
16.2
20.3
26.5
33.9
35.5
35.2
39.7
40.7
65.0
15.6
15.6
17.9
22.4
29.3
37.7
39.7
39.4
44.2
45.4
72.3
18.4
18.4
20.1
26.3
34.7
44.5
46.8
46.5
52.4
54.3
85.6
The maximum transmitting capacity of screens can be derived from these figures. To determine GPM per ft of screen, multiply the intake area
in square inches by 0.31. It must be remembered that this is the maximum capacity of the screen under ideal conditions with an entrance
velocity of 0.1 foot per second.
* PS means pipe size.
** Spec means special.
96
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lin 0
J I
Bridge Slot Screen
Figure 61. Types of well intakes.
Slotted Casing
Continuous Slot
Wire-wound Screen
>
>
>
>
x
>
>
>
>
>
Vertical Cross-section
Horizontal Cross-section
Figure 62. Cross-sections of continuous-wrap wire-wound
screen
rates after emplacement, the permeability in the vertical direc-
tion 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 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.
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 protec-
tion against infiltration of surface water and potential contami-
nants from the ground surface down the casing/borehole annu-
lus, 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 repre-
sentativeness 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 degrada-
tion. 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 calcu-
lations are the same as those performed to calculate filter pack
volume.
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;
97
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a) Between Casing and b) Through Seal Material c) By Bridging
Sea! Material
Figure 63. Potential pathways for fluid movement in the casing-borehole annulus.
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 includ-
ing 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 per-
meability than the natural formation materials from which they
are derived.
Bentonite —
Bentonite is a hydrous aluminum silicate comprised prin-
cipally of the clay mineral montmorillonite. Bentonite pos-
sesses 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 perme-
ability typically in the range of IxlO7 to IxlO'9 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) sodium
bentonite or 2) calcium bentonite. Sodium bentonite is the most
widely used because of its greater expandability and availabil-
ity, Calcium bentonite may be preferable in high-calcium
environments because shrinkage resulting from long-term cal-
cium-for-sodium ion exchange is minimized. Bentonite is
available in several forms including pellets, granules and
powder. Pellets are 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 ben-
tonite 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 in-
creases 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
98
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arc 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 bento-
nite. 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 moder-
ately 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. Hydra-
tion 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 gener-
ally not appropriate for use in the vadose zone because suffi-
cient moisture is not available to effect hydration of the bento-
nite. 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 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 or-
ganic 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, bento-
nite may not perform as an effective seal and another material
may be necessary.
In summary, factors that should be considered in evaluat-
ing 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 conti'nt 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 HI, 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; volumet-
ric shrinkage is approximately 17 percent and the porosity of the
set cement approximates 54 percent (Moehrl, 1964). The set-
ting 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:
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 6percent). 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) flyash(10percentto20percent).Flyashincreases
sulfate resistance and early compressive strength;
6) hydroxylated carboxylic acid. Hydroxylated
carboxylic acid retards setting time and improves
99
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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 manu-
ally 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 immedi-
ately pumped into the annulus. The types of pumps suggested
for use with grout include reciprocating (piston) pumps, dia-
phragm 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 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
final 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 geo-
logic 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 develop-
ment. 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 hydra-
tion 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 ma-
terials), 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
etal., 1980).
Molz and Kurt (1979) and Johnson et al. (1980) studied the
heat of hydration problem and concluded:
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 acelerators, such as calcium chlo-
ride, 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
100
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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 shrink-
age 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 be-
tween 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 10'5 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.
Methods for Evaluating Annular Seal Integrity
There are presently no fooproof 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 log-
ging 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 (acous-
tic, 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 effective-
ness 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 comple-
tion 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 condi-
tions 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. These stan-
dards 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 protec-
tive 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 expo-
sure 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.
101
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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, aprotective struc-
ture such as a utility vault or meter box is installed around well
casing that has 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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Dablow, John S. Ill, Grayson Walker and Daniel Persico,
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for monitoring wells cased with Teflon ® PTFE; Ground-
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103
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Section 6
Completion of Monitoring Wells
Introduction
Once a borehole has been completed to the desired moni-
toring 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 for-
mations, well intakes are installed as an integral part of the
casing string by lowering the entire unit into the open borehole
and placing the well intake opposite the interval to be moni-
tored. 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. I f the borehole
has 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 bore-
hole 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 de-
velop 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.
105
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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 through the bit. The only option for comple-
tion 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 environ-
ment 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 discus-
sion 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 (Driscoll, 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 periodi-
cally 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 11/2 inches; larger-diameter pipes
are advisable for filter pack materials that are coarse-grained or
characterized by uniformity coefficients that exceed 2.5 (Cali-
fornia 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
Fine portion
of filter pack
Coarse portion
of filter pack
Well intake
Figure 64. Segregation of artificial filter pack materials caused
by gravity emplacement.
xŁ~"
Sand
Casing
Well intake •
Tremie pipe
Borehole wall
"'.•^"T Filter pack material
Figure 65. Tremie-plpe emplacement of artificial filter pack
materials.
106
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method that employs a pump to pressure feed the materials into
the annulus is suggested by the California Department of Health
Services (1986).
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 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.
Backwashing filter pack material into place is accom-
plished by allowing filter pack material with a uniformity
coefficient of 2.5 or less to fall freely 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
Funnel
Filter pack
material
and water
6" Casing
(Casing pulled back during
filter pack installation)
Riser pipe
Centralizer >
Filter pack
Well intake
Water
Fine-grained
materials and
water
Filter pack
' material
Fine-grained
' materials and
water
Borehole wall
Well intake
Figure 66. Reverse-circulation emplacement of artificial filter
pack materials.
Figure 67. Emplacement of artificial filter pack material by
backwashing.
is paramount. An additional discussion on annular seals can be
found in the section entitled "Annular Seals. "
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 pelletized 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. Addi-
tional 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 bento-
nite 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
107
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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 de-
velop 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 pelletized bento-
nite 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 minimum 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-diam-
eter 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. Bento-
nite 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 (Figure 68).
The tremie pipe should be left at this position during the
emplacement procedure so that the slurry fills the annulus
Slurry
Annular seal material
Filter pack
Figure 68. Tremie-pipe emplacement of annular seal material
(either bentonite or neat cement slurry).
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.
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 adja-
cent 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 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 incorpo-
rate 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
108
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dilution of the slurry and the 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 pipe may be difficult to remove and/or a channel
may develop in the grout as the pipe is removed.
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 prin-
ciple 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 cement-
ing. 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 are 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 Wells
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.
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 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.
This type of well produces water samples that are a com-
posite of the water quality intercepted when the well is surged,
Surface protector
Casing or riser
Grout
Water table
Ground
water
flow
direction
Unconsolidated
aquifer
Bottom
of aquifer
Bottom cap
Figure 69. Diagram of a single-riser/flow-through well.
109
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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 perme-
able 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. Wells of these designs are
used to provide samples from 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 emplaced, and the seal added above the filter pack.
The filter pack provides 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 ex-
tended to the surface. This next well intake is filter-packed and
TBr-
7
7
^
<
V —
Fil
nprnsfV
- Surface seal
Grout seal
Filter sand
7
_
Grout seal — \^y/
- Filter sand
Filter pack
Filter sand
Screened interval
(a)
(b)
Figure 70. Typical nested well designs: a) series of single riser/limited interval wells in separate boreholes and b) multiple single
riser/limited interval wells in a single borehole (after Johnson, 1983).
110
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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 infil-
tration 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 through successive bentonite seals. A substan-
tial 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-con-
nected zones within the borehole. Of particular concern is
leakage along the borehole wall and along risers where overly-
ing 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 of these
wells is similar to the completion of nested wells in a single
borehole. Some of these samplers have individual tubing con-
nections 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 sophisti-
cated 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 sam-
pling 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 indi-
vidual 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 so-
phisticated sampling devices are available for very deep instal-
lations. These devices require durable, inflatable packer sys-
tems 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.
Ground
Water table
— End cap
Male & female
J couplings
1 Surface
PVC pipe
— Coupling
y Sampling points
End cap
PVC pipe
Screen
(a)
(b)
Figure 71. Field-fabricated PVC multilevel sampler: a) field installation and b) cross section of sampling point (Pickens et al., 1981).
111
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Protective
casing
Screened
interval
Sampling
tube
Backfill
Packer
Pumping port coupling
Measurement port coupling
End cap
Figure 72. Multilevel capsule sampling device installation
(Johnson, 1983).
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 isanon-aqueousphase 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-
Figure 73. Multiple zone Inflatable packer sampling installation
(Rehtlane and Patton, 1982).
dimensional aspectofcontaminantmigrationmust
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.
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.
References
California Department of Health Services, 1986. The
California site mitigation decision tree manual; California
Department of Health Services, Sacramento, California,
375pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson
Division, St. Paul, Minnesota, 1089 pp.
112
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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 O. 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-570/9-75-001,156pp.
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Section 7
Monitoring Well Development
Introduction/Philosophy
The objective of monitoring well development is fre-
quently 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 col-
lected 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. How-
ever, 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 sub-
stantiated. The United States Environmental Protection Agency
(1986) in the Technical Enforcement Guidance Document
(TEGD) states that, "a recommended acceptance/rejection
value of five nephelometric turbidity units (NTU) is based on
the need to minimize biochemical activity and possible interfer-
ence with ground-water sample quality." The TEGD also out-
lines 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 obtain-
ing a sample that is representative of conditions in the ground.
An evaluation of thedegree 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.
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 forma-
tion during the drilling process. 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
process. Other formation damage may be related to specific
installations. Some of this damage cannot be overcome satis-
factorily by the current capability to design and develop a
monitoring well. One important factor is the loss of stratifica-
tion in the monitored zone. Most natural formations are strati-
fied; 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 forma-
tion materials and ground water can vary between different
horizons. During the developmentprocess, those zones with the
highest permeability will be most affected by the development
of the well. Where a well intake crosses stratigraphic bound-
aries 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 develop-
ment 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 consoli-
dated 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
115
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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 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
clean, well-sorted sand and gravel with a permeability that
approximates 1 x 10'1 centimeters per second. However, a
monitoring well is much more difficult to install at a site where
the depth of the well will be 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 piezometric 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 un-
stable, it is difficult to achieve a good distribution of the filter-
pack material around the well intake during installation. Fur-
thermore, even if the filter pack installation is successful, it is
not possible to design a suf ficiently fine-grained filter pack that
will prevent the intrusion of the clays that are intimately
associated with the productive fine-grained sand. As a conse-
quence, 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 proce-
dures thatare 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 clogged by the clays that are being
removed. After a short operational period, insufficient quanti-
ties 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 paniculate 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 forma-
tion 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 ex-
ample, mechanical surging or bailing techniques that areeffective
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 paniculate matter typically builds up on the
borehole walls and plugs fissures, pore spaces, bedding planes
and other permeable zones. This paniculate 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 augeringprocedures, then
the development process must attempt to remove all of the
fluids that have infiltrated into the natural formation.
116
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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 re-
stricted 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 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 flow field and is
therefore more likely to yield a representative
sample;
6) collecting a non-turbid sample may 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 withoutexceedingacceptable
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 reestablish background levels in a
manner similar to that described in Barcelona et
al.(1985a)and/ortheUnitedStatesEnvironmental
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 able to understand the limitations of
the work.
Methods ofWell Development
Monitoring well development is an attempt to remove fine
paniculate matter, commonly clay and silt, from the geologic
formation near the well intake. If paniculate 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 be 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 paniculate movement. These blockages
may prevent the complete development of the well capacity.
This effect potentially impacts the quality of the water dis-
charged. Development techniques should remove such bridges
117
-------
Table 36. Summary of Development Methods for Monitoring Wells
oo
Reference
Gass(1986)
United States
Environmental
Protection
Agency (1986)
Barcelona et
al. ** (1983)
Scalf et al.
(1981)
National
Council of the
Paper Industry
for Air and
Overpumping
Works best in
clean coarse
formations and
some consolidated
rock; problems of
water disposal and
bridging
Effective develop-
ment requires flow
reversal or surges
to avoid bridges
Productive wells;
surging by alternat-
ing pumping and
allowing to equili-
brate; hard to create
must be
sufficient entrance
velocities; often
use with airlift
Applicable
drawback of flow in
one direction;
smaller wells hard
Backwashing
Breaks up
bridging, low
cost & simple;
preferientially
develops
Indirectly indicates
method applicable;
formation water
should be used
Suitable; periodic
removal of fines
Surge Block*
Can be effective;
size made for^"-
well; preferential
development where
screen >5'; surge
inside screen
Applicable; forma-
tion water should
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;
not easily used
on other rigs
Applicable; caution
against collapse of
intake or plugging
screen with clay
Bailer
Applicable
Productive wells;
more common than
surge blocks but
not as effective
Suitable; use
sufficiently
heavy bailer;
advantage of
removing fines;
may be custom
made for small
diameters
Jetting Airlift Pumping
Consolidated Replaces air surg-
and uncon- ing; filter air
solidated
application;
opens fractures,
develops discrete
zones; disadvantage
is external water
needed
Air should not
be used
Suitable
Air Surging
Perhaps most
widely used;
can entrain
air in form-
ation so as to
reduce per-
meability, affect
water quality;
avoid if possible.
Air should not
be used
Effectiveness
depends on
geometry of
device; air
filtered; crew
may be
exposed to
contaminated
water; per-
turbed Eh in
sand and
gravel not
persistent for
more than a
few weeks
Suitable;
avoid injecting
air into intake;
chemical
interference;
air pipe never
inside screen
Methods introducing foreign materials should be
avoided (i.e., compressed air or water jets)
Stream Im-
provement
(1981)
to pump if water
level below suction
-------
Table 36. (Continued)
Reference
Everett (1980)
Keely and
Boateng
(1987 a and b)
Overpumping
Development opera-
tion must cause
flow reversal to
avoid bridging; can
alternate pump off
and on
Probably most
desirable when
surged; second
series of
evacuation/
recovery cycles is
recommended after
resting the well for
24 hours; settlement
and loosening of
fines ocurs after
the first
development
attempt; not as
vigorous as
backwashing
Backwashing
Vigorous surging
action may not be
desirable due to
disturbance of
gravel pack
Surge Block* Bailer
Suitable; periodic
bailing to remove
fines
Method quite
effective in
loosening fines but
may be inadvisable
in that filter pack
and fluids may be
displaced to degree
that damages value
as a filtering media
Jetting Airlift Pumping Air Surging
High velocity jets of
water generally
most effective ;dis-
cret zones of
development
Popular but Air can become
less desirable; entrained behind
method dif- screen and reduce
ferent from permeability
water wells;
water displaced
by short down-
ward bursts of
high-pressure injection;
important not to jet
air or water across
screen because fines
driven into screen
cause irreversible
blockage; may substantially
displace native fluids
* Schalla and Landick (1986) report on special 2"- valved block
" For low hydraulic conductivity wells, flush water up annulus prior to sealing; afterwards pump
-------
and encourage the movement of particulates into the well.
These particulates can then be removed from the well by bailer
orpump and, in mostcases, the water produced will subsequently
be clear and non-turbid.
One of the major considerations in monitoring well devel-
opment 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 develop-
ment 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 is good
enough" or "it can't be done."
In most instances, monitoring wells installed in consoli-
dated 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 installa-
tion in sound rock or stable unconsolidated materials, and
where the addition of fluids during completion and develop-
ment is permissible, it is a good practice to precondition 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. Hushing 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 particulates 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 develop-
ment 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 disad-
vantages of using jetting even in "ideal conditions" are fourfold:
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 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 poten-
tial 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 four most suit-
able methods for monitoring well development are: 1) bailing,
2) surge block surging, 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 develop-
ment 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 with-
drawn, the drawdown created in the borehole causes the par-
ticulate matter outside the well intake to flow through the well
intake and into the well. Subsequent bailing removes the
paniculate matter from the well. To enhance the removal of
sand and other paniculate 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 refill 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 effective development tool because it
provides the same effects as both pumping and surging with a
surge block. The most effective equipment for bailing opera-
tions is generally available on cable tool rigs.
120
-------
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 oper-
ated 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 agita-
tion 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 satisfac-
tory 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 ihrough
the screened interval. The surge block is removed at regular
intervals and the fine material that has been loosened is re-
moved 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.
S urging within the well intake can result in serious difficul-
ties. 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 well, 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, italso reduces the effectiveness of the surging action.
In permeable 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: 1) the surge block
is 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.
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 paniculate 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 em-
ployed 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 en-
hances 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
121
-------
Standard
Bailer of
Teflon®
Standard
Bailer of
PVC
Top for Variable
Capacity Point Source
Bailer of PVC
Bottom
Emptying
Device
Retaining
Pin
Ball
Check
Sample
Chamber
1 Foot
Midsection
May Be Added
Here
Retaining
Pin
Ball Check
(a)
(b)
Figure 74. Diagrams of typical bailers used In monitoring well development: a) standard type and b) "point source" bailer
(Ttmco Manufacturing Company, Inc., 1982).
122
-------
Rubber
Flap
Pressure-relief
Hole
Figure 75. Diagram of a typical surge block (Driscoll, 1986).
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-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 devel-
oped directly to air, can be accomplished by properly imple-
mented 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 10 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 develop-
ment 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.
References
Barcelona, MJ., J.P. Gibb, J.A. Helfrich and E.E. Garske,
1985a. Practical guide for ground-water sampling; Illinois
State Water Survey,SWSContractReport374,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, Lorne 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,
39pp.
Scalf, M.R., J.F. McNabb, W.J. Dunlap, R.L. Cosby and J.
Fryberger, 1981. Manual of ground-water sampling
123
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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-
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.
[
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0.68 ID.)
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Cross Section H j M" NPTThreadins
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Bottom Fitting
JH- Stainless Steel Hex Nut (0.63")
° ^> Air Vent Ports (0.63")
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3 -4" Swage Block
Figure 76. Diagram of a specialized monitoring well surge block (Schalla and Landick, 1986).
124
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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 moni-
toring well construction and testing must frequently be pro-
vided 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 require-
ments. 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 refer-
enced 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 perma-
nent 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 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 pain t 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 thatreference 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 illus-
trating comparable items should be the same scale. In addition
to the unique monitoring well number, general well designa-
tions 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 com-
pleteness 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 qual-
ity is questioned. This section is designed to assist the user in
setting up a comprehensive maintenance schedule for a moni-
toring system.
Documenting Monitoring Well Performance
A monitoring well network should be periodically evalu-
ated to determine that the wells are functioning properly. Once
complete construction and "as-built" information is on file for
125
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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 casing, 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 grain size analysis)
• Seal and physical form
• Slurry or grout mix (percent cement, percent bentonite powder,
percent water)
Installation:
• Drilling method
• Drilling fluid (if applicable)
• Source of water (if applicable) and analysis of water
• Time period between the addition of backfill and construction of
well protection
Development:
• Date, time, elevation of water level prior to and after development
• Method used for 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
• Clarity 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 drillers log
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, grain size analysis, placement method, supplier)
Date of all work
Name, address of consultant, drilling company and stratigraphic log preparer(s)
Description and results of pump or stabilization test if performed
Methods used to decontaminate drilling equipment and well construction material
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 of bentonite seal
• Water table
• Earth materials stratigraphy throughout boring
• For rock wells, show details of bedrock seal
• For rock wells, indicate depths of water-bearing fractures, faults or fissures and approximate yield
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:
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
126
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Table 40. Field Boring Log Information (United States Environmental Protection Agency, 1966)
General:
• Project name
• Hole name/number*
• Date started and finished*
• Geologist's name*
• Driller's name*
• Sheet number
Information Columns:
• Depth*
• Sample location/number*
• Blow counts and advance rate
Narrative Description:
• Geologic observations:
• soil/rock type*
• color and stain*
• gross petrology*
• friability
• moisture content*
• degree of weathering*
• presence of carbonate*
- Drilling Observations:
• loss of circulation
• advance rates*
• rig chatter
• water levels*
• amount of air used, air pressure
• drilling difficulties*
• Other Remarks:
• equipment failures
• possible contamination*
• deviations from drilling plan*
• weather
• Hole location; map and elevation*
• Rig type
> Bit size/auger size*
' Petrologic lithologic classification scheme
used (Wentworth, unified soil classification system)
• Percent sample recovery*
• Narrative description*
• Depth to saturation*
• fractures*
• solution cavities*
• bedding*
• discontinuities*- e.g., foliation
• water-bearing zones*
• formational strike and dip*
• fossils
> changes in drilling method or equipment*
• readings from detective equipment, if any*
• amount of water yield or loss during drilling
at different depths*
• dapositional structures*
> organic content*
•odor*
> suspected contaminant*
> amounts and types of any liquids used*
• running sands*
> caving/hole stability*
* Indicates items that the owner/operator should record at a minimum.
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 Well 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 ca-
pacity, boundary conditions, stratification, sorting and fractur-
ing 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 develop-
ment, borehole instability and chemical, physical and/or bio-
logical 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 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 chemi-
cally 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.
127
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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 proce-
dures and/or difficulties may also cause mislocation of well
intakes and/or seals. Improperly connected or corroded casing
can separate at joints or collapse and cause interaquifer con-
tamination. 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 drill-
ing 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 fail-
ure 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 pro-
duction: 1) chemical, 2) physical, 3) biological or 4) a combi-
nation of the other three processes. Chemical incrustation may
be caused by carbonates, oxides, hydroxides or sulfate deposi-
tions 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 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 constitu-
ents 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 re-
moved. 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 tradi-
tional well rehabilitation: 1) acids, 2) biocides and 3) surfac-
tants. 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 applica-
tions in the water supply industry. Chemicals have very limited
application in the rehabilitation of monitoring wells because the
chemicals cause severechanges in the environmentof the wells.
These changes may last for a long time or may be permanent.
Before redevelopment with chemicals is considered, the nega-
tive 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 at-
tempted, parameters such as Eh, pH, temperature and conduc-
tivity should be measured. These measurements can serve as
values for comparison of water quality before and after well
maintenance.
Mechanical rehabilitation includes: overpumping, surg-
ing, 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 intro-
duction 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,
128
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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
1. Western Mountain Ranges
2. Alluvial Basins
3. Columbia Lava Plateau
4. Colorado Plateau,
Wyoming Basin
5. High Plains
7. Glaciated Central Region
8. Unglaciated Appalachians
9. Glaciated Appalachians
Alluvial
Sandstone
Limestone
Alluvial
Basaltic lavas
Alluvial
Interbedded sandstone
and shale
Alluvial
Interbedded sandstone,
limestone, shale
6. Unglaciated Central Region Alluvial
Sandstone
Limestone
Alluvial
Sandstone
Metamorphic
Limestone
Alluvial
Alluvial
Consolidated sedimentary
10. Atlantic and Gulf Coast Plain Alluvial and semiconsolidated
Consolidated sedimentary
Silt, day, sand intrusion, iron; scale deposition; biological fouling.
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 intrusion; scale deposition; iron; biological fouling.
Fissure plugging by clay and silt; casing failure; corrosion; salt water
intrusion; sand production.
Fissure plugging by clay, silt, carbonate 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 day; mineraliztion of fissures.
Predominantly cavernous production; fissure plugging by day 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 chemical fissure plugging; biological fouling; incrustation
of well intake structure.
* Excluding pumps and declining water table.
Table 42. Chemicals Used for Well Maintenance (Gass et al., 1980)
Chemical Name Formula Application
Concentration
Acids and biocides
Inhibitors
Chelating agents
Wetting agents
Hydrochloric acid
Sulfamic add
Hydroxyacetic acid
Chlorine
Diethyithiourea
Dow A-73
Hydrated ferric sulfate
Aldec 97
Polyrad 1 10A
Citric add
Phosphoric add
Rochelle salt
Hydroxyacetic acid
Plutonic F-68
Plutonic L-62
HCI
NH,SO,H
cA°3
CI2
(C2H6)NCSN (C2HS)
Fe2(S04)3- 2-3H20
Carbonate scale, oxides, hydroxides
Carbonate scale, oxides, hydroxides
Biocide, chelating agent, weak scale
removal agent
Biocide, sterilization, very weak acid
Metal protection
Metal protection
For stainless steel
With sulfamic acid
Metal protection
15%; 2-3 times zone volume
15%; 2-3 times zone volume
50-500 ppm
0.2%
0.01%
1%
2%
.375%
C6H,O7 Keeps metal ions in solution
H3PO. Keeps metal ions in solution
NaOOC (CHOH)2 COOK Keeps metal ions in solution
C2H403 Keeps metal ions in solution
Renders a surface non-repellent to a
wetting liquid
Renders a surface non-repellent to a
wetting liquid
Surfactants Dow F-33
Sodium Tripolyphosphate
Sodium Hexametaphosphate
Lowers surface tension of water thereby
increasing its cleaning power
129
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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 in-
spection 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, fungi or other evidence of
problems that may affect the representativeness
of water samples. If noorganismsand/orassociated
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 are used, the
lubricants and/or paint should be prevented from
entering the well;
4) 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 abandon-
ing andredrilling the well is an importantconsideration. Factors
that should be evaluated are: the construction quality of the
well, the accuracy of the well-intake placement and the preci-
sion of the documentation of the well. Capital costs of a new
well should also be considered. The actual "cost" of rehabilita-
tion 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 may 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 main-
tenance 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 qualities of water to mix.
Currently, many sites are being monitored for low concentra-
tions 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 aban-
donment 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 contami-
nation, 3) conserve aquifer yield and hydrostatic head and 4)
prevent intermixing of subsurface water (United States Envi-
ronmental 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. How-
ever, 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 avail-
ability of a rig type and site conditions may also be determining
130
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factors. The level of contamination and zone in which contami-
nation occurs may modify the choice of technique. If no cross-
contamination can occur between various zones and contami-
nation cannot enter from the surface, grouting the well from
bottom to top without removing the casing may be sufficient.
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 bore-
hole 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 cas-
ing 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 inad-
equate 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 annu-
lar 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 monitor-
ing 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 arig.
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 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
occurring. 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 re-
moved, 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 StatesEnvironmentalProtection 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) intermedi-
ate seals and 3) seals 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 elas-
tomers 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
usable and non-usable 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 well 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 (Driscoll, 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 through-
out 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 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
132
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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 a 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 back-
filled 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. Table43 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)
Name 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 casing
Depth to rock
Depth to water
Formation type
Material overlying rock (clay, sand, gravel, etc.)
Materials and quantities used to fill well in specific zones,
detailing in which formations and method used
Casing removed or left in place
Firm completing work
Signature of person doing work
Address of firm
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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 re-
sulted 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 monitor-
ing wells with hollow-stem augers, however, are neither stan-
dardized nor thoroughly documented in the published litera-
ture. Lack of standardization is partially due to variable
hydrogeologic conditions which significantly influence hol-
low-stem auger drilling techniques and monitoring well con-
struction 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 moni-
toring wells will be presented.
Auger Equipment
The equipment used for hollow-stem auger drilling in-
cludes 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 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
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).
141
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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 major-
ity 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 connection of the 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 connec-
tions 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 communi-
cation, 1987). When lubricants are used on the hollow-stem
auger threads, a nonreactive lubricant, such as a fluorinated
based grease, may be used to avoid introducing potential
contaminants that may affect the ground-water samples col-
lected 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 refer-
ences 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.
The hollow axis of the auger column facilitates the collec-
tion of samples of unconsolidated formations, particularly in
unsaturated cohesive materials. Two types of standard sam-
Key Way
Auger Bolt
• O-Ring
a. Keyed, Box and Pin Connection
Auger Bolt
Pin End
b. Box and Pin Connection
c. Threaded Connection
Figure 2. Three common methods for connecting hollow-stem
auger sections.
142
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Table 1. Typical Hollow-Stem Auger Sizes with Slip-Fit, Box and Pin Connections (from Central Mine Equipment Company, 1987)
Hollow-Stem
Inside Diameter (In.)
Flighting Diameter
(in.)*
Auger Head
Cutting Diameter (in.)
21/4
23/4
31/4
33/4
4 1/4
61/4
81/4
55/8
61/8
65/8
71/8
75/8
95/8
115/8
6 1/4
63/4
71/4
73/4
81/4
101/4
121/2
NOTE: Auger flighting diameters should be considered minimum manufacturing dimensions.
Table 2. Hollow-Stem Auger Sizes with Threaded Connections (from Mobile Drilling Company, 1982)
Hollow-Stem
Inside Diameter (in.)
Flighting Diameter
(in.)*
Auger Head
Cutting Diameter (In.)
21/2
33/8
4
6
61/4
81/4
81/2
11
8
9
11
131/4
NOTE: Auger flighting diameters should be considered minimum manufacturing dimensions.
piers which are used with hollow-stem augers are split barrel
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 represen-
tative 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 forma-
tion. These samplers are designed to recover relatively un-
disturbed 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 DI587-83, 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
Collectine, 1983; Gass, 1984).
In addition to these standard samplers, continuous sam-
pling tube systems are commercially available which permit the
collection of unconsolidated formation samples as the auger
column is rotated and axially advanced (Mobile Drilling Com-
pany, 1983; Central Mine Equipment Company, 1987). Con-
tinuous 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 rein-
serted 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 unconsoli-
dated material or rock.
Borehole Drilling
There are several aspects of advancing a borehole with
hollow-stem augers that are importantconsiderations 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 satu-
rated zones; and 3) potential vertical movement of contami-
nants 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 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 (1978a),
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., den-
143
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sity 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 maxi-
mum 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 influ-
enced 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 Colletine, 1983). By
eliminating or minimizing the use of drilling fluids, hollow-
stem auger drilling may alleviate concerns regarding the poten-
tial 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 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 au-
gers 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 there-
fore 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 diam-
eter of the auger flights as opposed to the cutting diameter of the
auger head. In noncohesive materials, the borehole diameter
may 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 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 may 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
lossely 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 —
When 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 column, and the center rod and pilot assembly are
removed from the hollow axis of the auger column (Figures 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 sam-
pling 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 sys-
tems 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.
144
-------
i;<5.. Column
6*;?^ ....o
;'Sampling
b-Rod
Open Axis of
Auger
Column with
Pilot
Assembly
Center i
Rod Removed
Split Barrel
or Thin- '.
Walled Tube
Sampler .#
Figure 3. Sequential steps showing borehole advancement with pilot assembly and collection of a formation sample
(after Riggs, 1983).
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 collec-
tion 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 col-
umn 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 sur-
face of the formation. If the potentiometric surface of the
formation rises above the ground elevation, however, the heav-
ing sand problem may be very difficult 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
145
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Auger Drill Rig
Auger Column
Barrel Sampler
— Non-rotating
Sampling Rod
Auger Head
Figure 4. Diagram of continuous sampling tube sytem (after
Central Mine Equipment Company, 1987).
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-outplate. The knock-out plate is wedged
inside the auger head and replaces the traditional pilot assembly
and center rod (Figure la). A major disadvantage of this
drilling technique is that the knock-out plate cannot be alter-
nately 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 con-
structed of inert materials when drilling a borehole for the
installation of a water-quality monitoring well. This will mini-
mize 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 prob-
lems with heaving sands (C. Harris, John Mathes and Associ-
ates, 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 main-
tained inside the auger column to counteract further the hy-
drostatic pressure in the heaving sand formation. Once drilling
is completed, the reverse flight center plug is slowly retracted
from the auger column so that movement of sand into the
hollow stem is not induced.
Although the use of clean water as 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 pro-
hibited at some ground-water monitoring sites. In these in-
stances, 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 ad-
vanced 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 forma-
tion 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.
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
146
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Sands Rising
Inside Hollow Center
Due to Hydrostatic
Pressure in Sand
Formation
Pilot
Assembly
a. Borehole Advanced into Saturated
Sand with Auger Column
Containing Pilot Assembly
Figure 5. Diagram showing heaving sand with hollow-stem auger drilling.
b. Movement of Loose Sands into the
Hollow Center of Auger as the Pilot
Assembly is Removed
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. Ver-
tical mixing of contaminants from different levels within a
single borehole may be a problem with several 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 con-
taminants 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
147
-------
Auger
Drill Rig
Water Swivel
Clean Water
Spindle Adapter Assembly Used for
Injecting Fluids Inside Auger Column
' . Sand
5 U . -J— ' : Water Table -
Open Auger Head
Figure 6. Injecting clean water through open drill spindle to
counteract heaving sand (after Central Mine
Equipment Company, 1987).
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 concnetrations. 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 con-
taminants 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 contami-
nants may serve as a persistent source of sampling bias.
Vertical movementof dissolved-phasecontaminants within
a borehole may also occcur 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 bore-
hole 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 prior to drill-
ing, during drilling or after installation of the monitoring well
may not easily be made. Keely and Boateng (1987b), however,
recount a case histroy 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 de-
veloped in the lower aquifer showed anomalous concentrations
forchromium. Although vertical ground-water gradients at the
site were generally downward, the areal distribution and con-
centrations of chromium in the lower aquifer were not indica-
tive 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 movementof contaminants
in the borehole or vertical movement of contaminants through
faulty seals along the casing of the monitoring wells. The
authors hypothesize that the vertical movement of the contami-
nants 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 site.
Where a shallow contaminated zone must be penetrated to
monitor ground-water quality at greater depths, a large-diam-
eter surface casing may be used to seal off the upper contami-
nated 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 conven-
tional 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 situa-
tion, 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 cohesive-
ness of the formation (Figure 14b). A large-diameter surface
148
-------
Clean Water Level
Within Auger Column
Knock-Out Plate
Positioned
Within Auger
Auger Column
Filled with Clean
a. Borehole Advanced into Saturated
Sand with Auger Column Containing
Nonretrievable Knock-Out Plate
b. Clean Water Added to Auger
Column Along with Removal of
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.
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 diamteter 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.
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 andBoateng(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 a
monitoring well. The following discussion will focus on tech-
niques 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
149
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Auger Column-*
Filled with Clean
Water as
Borehole is
Advanced
Water Level
Reverse Flight
Auger and
Center Rod
Saturated Sand
Formation
*Ł. Reverse Flight
$ Auger and
Center Rod
Slowly Retracted
from Auger
'•Column
^j^-'Auger Column
Filled with Clean
Water as Reverse
Flight Auger is
Retracted
a. Reverse Flight Auger Pushes
Cuttings Outwardly While Head
of Clean Water is Maintained
Inside Auger Column
b. Reverse Flight 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).
Nipple
r- Lock Nut
- Knock-Out Plate
• Slotted Coupling
-Plug
Figure 9. Diagram of a slotted coupling
(after Perry and Hart, 1985).
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 mate-
rials, 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
150
-------
Auger Column -
Filled with
Formation Water.
Knock-Out Plate
with Slotted
Coupling Permitting
Formation Water
to Enter Auger
Column as
Borehole is
Advanced
Auger Column
Water Level;
...jr.
Saturated Sand
Formation
Knock-Out Plate
with Slotted
Coupling
Removed from
Auger Head by
Ramrod
; Ramrod
Auger Column
Filled with
Formation Water
a. Borehole Advanced into
Saturated Sand with Auger
Column Containing Nonretrievable
Knock-Out Plate with
Slotted Coupling
b. Knock-Out Plate with Slotted
Coupling Removed from Auger
Head by Ramrod
Figure 10. Use of a nonretrievable knock-out plate with a slotted coupling to avoid a heaving sand problem
(after Perry and Hart, 1985).
Pipe Tee, ,r=
Pipe Flange
Supporting Pipe
I
J | SI. T Rubber
^ Gasket
Nipple -
-(-Brass Screen
Pipe Flange
Inside Diameter of
Hollow-Stem Auger
- Ball Valve
Nipple
Figure 11. Diagram of a screened well swab
(after Perry and Hart, 1985).
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 contrac-
tors. 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
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
151
-------
Auger Column
«" i"" • Ramrod with
Screened Well
Swab Attached
Water Level
Saturated Sand
Formation
Water Level
Rising Inside
Auger Column
After Removal of
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).
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 appropriate 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 nominal 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
152
-------
Table 3. Maximum Working Space Available Between Various Diameters of Threaded, Flush-Joint Casing and Hollow-Stem Augers
Nominal Outside
Diameter Diameter
of Casing of Casing
(in.) *(in.)
2 2.375
3 3.500
4 4.500
5 5.563
6 6.625
Working Space "A" (see Figure 17) for
Various Inside Diameter Hollow-Stem
Augers " (in.)
3 1/4 3 3/4 4 1/4 6 1/4
0875 1.375 1.875 3.875
0.250 0.750 2.750
1 750
0687
81/4
5.815
4.750
3.750
2.687
1.625
* Based on ASTM Standards D-1785 and F-480
** Inside diameters of hollow-stem augers taken from Table 1.
Flexible Center '•';;!
Plug Permitting :-:
Collection of
Water-Bearing
Sands, but
Preventing
Heaving Sands
from Entering
Hollow Stem
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).
153
-------
•' Shallow
Contaminant
_ Zone
'"* • • • • **'~
Large-Diameter
Auger Used to -
- Advance
Borehole in ~
. Cohesive
• Materials
~ Open Borehole -^ _
^T.' ' Shallow
^-' Contaminant
i •_ Zone
Protective _^_
Surface Casing ^
Set Below
Contaminant
Zone —
^'•7^- •• vV'' .- •: '. '.? ': T
T
__x~ Grouted Annular'
—^_ . Space
• — a. Large-Diameter Borehole — — •— •
T Advanced Below Known Depth~~_b- Auger Column Retracted from-- —-
-— • -— of Contamination • — • Borehole Which Remains Open.
• • — • • • — Due to Cohesive Materials
Ur
Auger Used to —
~ Advance ~
— Borehole to a —
Deeper Depth ~~
for Installation of _
' ~ Monitoring Well"
c. Surface Casing Installed Below
Known Depth of Contamination
with Drilling Continued Using
Smaller Diameter Auger
Figure 14. Sequence showing the installation of protective surface casing through a shallow contaminated zone in a cohesive
formation.
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 abarrier 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
well casing due to the nonuniform placement of the filter pack
or annular seal. Bridged material can lock the casing due to 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). Ac-
cording 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 com-
monly available at this time, would be limited to 4 inches or less.
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).
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 0.40 mm).
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
154
-------
Drop Hammer
Used to Drive
Casing
•>-' w.« •* r\ *••>'.
Protective
Surface Casing
Driven Flush
with Borehole
Wall in Non-
Cohesive
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 noncoheslve
formation (after Keely and Boateng, 1987a).
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 Environ-
mental 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 posi-
tioned 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 em-
placement 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 (Minning, 1982; Richter and Collentine, 1983; Gass,
1984; Schmidt, 1986; Keely and Boateng, 1987b).
155
-------
Locking Casing Cap
Vent Hole
Protective Casing
Ground Surface
Well Casing
Annular Seal
Filter Pack
Inner Casing Cap
Completion Depth
Figure 16. Typical design components of a ground-water
monitoring well.
The volume of filter pack required to fill the annular space
between the well intake and borehole wall should be predeter-
mined 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 infor-
mation 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 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 approxi-
mately 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 incre-
ments and pouring the filter pack down the working space
between the casing and auger column. Prior to filter pack em-
placement, 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 measur-
ing 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 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 re-
tracted 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 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
156
-------
Threaded, Flush-
Joint Casing
and Intake
Inside Diameter of
i Hollow-Stem Auger i
Maximum
Working Space
Hollow-Stem
Auger
Outside Diameter
of Casing
Maximum
Working Space
Auger Column
i— 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.
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 par-
ticles 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
asignificantproblem 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 segrega-
tion 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 50 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.
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 precalculated 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
157
-------
2-Inch Nominal Diameter -
Well Casing and Intake
4 1/4-Inch Diameter-
Hollow-Stem Auger
Design Length
of Filter Pack
12 Feet
T
Length of
Well Intake
10 Feet
I
Borehole
1 Diameter '
81/4 inches
Figure 18. Illustration for the sample calculation of a filter pack
as described in the text.
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 addtional filter pack emplacement is continued
until therequired length of filter pack is installed. Similar to the
free fall method of filter pack emplacement, careful measure-
ments 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 Mathes 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, com-
monly 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 be-
tween 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 rematin 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 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 lape
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
"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 verify-
ing the collapse of formation materials by measuring the depth
to the top of the caved materials continues until the coarse-
158
-------
Weighted
Measuring Tape
C-
Well Casing
-A-c'
Hollow-Stem
Auger
Plan View
Weighted
Measuring Tape
Weighted
Measuring Tape
Auger Column
Retracted
1 to 2 Feet
from Borehole
Weighted
Measuring Tape
C--
Hollow-Stem
Auger
Weighted -
Measuring Tape
Well Casing
-C'
Filter Pack
Pouring
Filter Pack
Cross-Sectional View
a. Placement of Weighted
Measuring Tape
b. Auger Column Retracted
Cross -Sectional View
c. Filter Pack Free -Falls Through
Working Space Between Casing
and Auger
Figure 19. Free fall method of filter pack emplacement with a hollow-stem auger.
grained sediments extend to a desired heigh t above the top of the
well intake. The finer-grained fraction of the collapsed forma-
tion 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, the final
phase of monitoring well construction typically involves the
installation of an annular seal. The annular seal is constructed
by emplacing 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 integ-
rity of the seal be maintained throughout the life of the monitor-
ing 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.
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
159
-------
Weighted
Measuring Tape
Plan View
Weighted
Measuring Tape
Auger Column
Retracted
1 to 2 Feet
from Borehole
Well Casing
- -C'
Tremie Pipe
Tremie Pipe
Positioned to
Bottom of
Borehole
Cross -Sectional View
a. Weighted Measuirng Tape and
Tremie Pipe in Retracted
Auger Column
Weighted
Measuring Tape "i
Filter Pack
Material Poured
Down Tremie
Tremie Pipe
Slowly Raised as
Filter Pack is
Filter Pack
tfzi&Z*
b. Filter Pack Poured Through Bottom-
Discharge Trernie Pipe
Figure 20. Tremie method of filter pack emplacement with a hollow-stem auger.
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 bento-
nite 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 betweeen 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 heightabove the filterpack. Actual depth measure-
ments of the emplaced pellets are recorded and compared with
the calculations for the volume of "bentonite pellets needed"
versus "bentonite pellets used."
The free fall of bentonite pellets through the working space
between the well casing and auger column provides the op-
portunity 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
160
-------
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 expand-
ing 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 agreater 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 clump-
ing 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 bento-
nite pellets are used, a suitable hydration period, as recom-
mended by the manufacturer, should be allowed prior to the
placement of the grout slurry. 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-dis-
charge 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 mate-
rial 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 opera-
tion 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 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-dis-
connect 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 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. Calcula-
tions 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 ad-
vantages and limitations for drilling and constructing monitor-
161
-------
ing 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 drill-
ing 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. Limita-
tions 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 Imitation 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.
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 bore-
hole 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 appropri-
ately-sized diameter hollow-stem auger for the installation of
the required-size well casing and intake. The maximum diam-
eter 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.
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 Tuchfield, 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. l,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.
Mcray, 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
162
-------
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 O., 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 O., 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 O., and Allen W. Hatneway 1988. Ground-
water 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, WJ. Dunlap, R.L. Cosby and J.
Fryberger, 1981. Manual of ground-water sampling
procedures; National Water Well Assocation, 93 pp.
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,
317pp.
163
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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 devel-
oped 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 develop-
ment of the matrices. These are detailed below:
1) Solid-flight auger and hollow-stem augerdrilling
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 tech-
niques are assumed to be from surface dis-
charge of the circulated sample;
b) during drilling with solid-flight augers, hol-
low-stem augers, mud rotary or cable tool
techniques are assumed to be taken by stan-
dard 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 dis-
charge; and
d) below 150 feet, during cable-tool drilling are
assumed to be taken by bailer.
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 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 drilling methods for the specified conditions.
165
-------
01
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XX XXX XXX XXX XXX
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1* /
I* /
./ General
/ Hydrogeologic
/ Conditions
/ and Well Design
/ Requirements
Unconsolidated
Consolidated
Saturated
Unsaturated
Invasion Permitted
Invasion Prohibited
Depth 0-15 Feet
Depth 15-150 Feet
Depth > 150 Feet
< 2-Inch Diameter Casing
2-4 Inch Diameter Casing
4-8 Inch Diameter Casing
2
0
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MATRIX NUMBER 1
General Hydrogeologic 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.
\ z w
\ 00
\ r~ Q
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2
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10
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7
7
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7
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6
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2
9
5
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10
9
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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 15 feet, and small completed well diameter of 2 inches or less allows maximum flexibility in equipment.
167
-------
MATRIX NUMBER 2
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less; total
well depth 15 to 150 feet.
\ Z8
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\ c ^5
\ i U
\ 11
\ i°
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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
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9
8
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TOTAL
NA
30
23
37
67
67
NA
60
63
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 permits the use of drilling fluid and additives in construction.
4. As the depth increases from 15 to 150 feet, the limit of hollow-stem auger equipment is approached. The actual limit varies 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 increases.
5. Where dual-wall air techniques are used, completion is through the bit.
168
-------
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3"
co
01
ET
s
6'
3
CD
CD
5'
CO
Cfl
CO
^;
s
3
^
3^
E"
CD
3
O
CD
O
3
CD
O
3"
O
o'
CD
O
m
X
PLANATORY
O
H
m
en
3
CO
O
01
cr
CD
o1
CD
O3
Ol
--J
-
CD
O
0
ro
O
c
01
1
33
0
oT
•2
o
o
00
-
o
0
o
o
en
CD
IAir Rotary with
Casing Hammer
00
0
Ol
-
-vl
o
o
o
en
o
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^" c ^*o !>o 2. 3. 01
33 Q-cQo"cOO^ 3' =' O-
| = « < ^I!10 <^ >
^ I | t |
-z. ->• -z. -z.~z.-z.-z.
-z. ^ -z. ~z.-z.-z.-z.
-z. -+ -z. -z.-z.-z.~z.
^> O J> J> J> J> J>
z - z -z.-z.-z.-z.
z z z z z z
^ CD ^ ^ ^ ^ ^
z z z z z z
^ CJ1 ^ ^ ^ ^ ^
Z -» Z Z Z Z Z
z z z z z z
^ O) ^ ^ ^ ^ ^
Z en Z Z Z Z Z
SE //
(/> O x'
^CRITERIA FOR EVALUATION
/ OF DRILLING METHODS
Versatility of Drilling Method
Sample Reliability
Relative Drilling Cost
Availability of Drilling Equipment
Relative Time Required for Well
Installation and Development
Ability of Drilling Technology to
Preserve Natural Conditions
Ability to Install Design Diameter
of Well
Relative Ease of Well Completion
and Development
O
r~
CD D
= O
Q.O
Sa
CD ~
~ Oi
3 Ł. O
—* (D (D
CJi O. 3
o-_ n
in' a
*8 i
§' ^
3 5"
Ł «Q
o o' S
1 ? ?
Ili
5 5' Z
^ S c
zs "• 2
Ł *" m
Q- g m
1 1 S
i. o
x m
CD W
P- «5'
O 3
Cd -n
OT S?
fi
11
CD m
CD
O
CD
to
I
-------
MATRIX NUMBER 4
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches total
well depth 0 to 15 feet.
\ oa
\ <°
\ ->uj
\ Ł5
\ z
\ o -*
\ "• oE
\ ~ D
\ *"0
\ 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
TD
O
.C
2
en
c
Q
O
z-
.—
i
2
NA
1
2
1
8
7
NA
8
10
10
>•
5
CO
"35
a:
0)
a.
E
CO
V)
NA
1
1
4
10
10
NA
5
8
10
CO
o
O
c
Q
CD
;>
03
CD
rr
NA
10
8
7
7
7
NA
6
5
5
C
E
Q.
'l3
CT
UJ
O)
Q
Q
"o
>-
i
_ca
CO
<
NA
10
10
9
9
10
NA
4
1
7
=
§ c
°|
~s ^
5|
OC -D
Ł S
P§
>l
to 2
0) W
a: Ł
NA
5
5
10
10
7
NA
6
6
4
o
O) VI
il
I -o
ID o
t-o
.S 2
SI
° &
^ CD
•^: co
-5 Ł
< a.
NA
5
2
4
8
4
NA
9
10
9
a;
IS
CO
Q
C
D)
W
CD
Q
to
c
o
11
< 0
NA
1
1
4
8
10
NA
10
8
10
c
o
0)
a.
o
o
5U
**_ ^~
o g
% Q-
CO O
UJ "CD
§> CD
|Q
(T ro
NA
4
1
2
8
5
NA
10
8
10
TOTAL
NA
37
30
41
68
60
NA
58
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. Four-inch casing diameter limits technique choices even though depths are shallow (15 feet or less) Large diameter (I D.)
hollow-stem augers required. Solid flight augers require open-hole completion in potentially unstable materials.
170
-------
MATRIX NUMBER 5
General Hydrogeologic 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.
\ ZOT
\ 00
\ c o
\ Zjfc
\ Ł2
\ Ul ^
\ cr 2
\ ^ Zj
\ UL —
^i rf ^
\ ™ ^
\ *0
\ —
\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
•D
Q)
O>
C
~-
'C
Q
.fi-
ll
I
NA
1
1
3
5
10
NA
8
10
9
^
«
in
CO
'53
DC
0)
Q.
CO
NA
1
1
3
10
10
NA
5
8
10
«
-P
*-'
D)
C
'C
o
I
to
CD
cr
NA
2
3
2
8
10
NA
5
8
5
_^
c
0)
Q.
'5
cr
Ul
O)
c
•^
Q
"5
~
XI
'5
>
NA
10
10
9
9
10
NA
4
1
7
~
i_ 5
•5 >
CT/-J
0) LJ
or -Q
d> ^-
I c
t — _o
*i CO
CO iii
ID »
DC Ł
NA
1
3
7
9
10
NA
9
8
5
0
O) CO
o c
II
-------
MATRIX NUMBER 6
General Hydrogeologic 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.
\ Z»
\ 2
\ =JL
\ M
\ c=3
\ 2 —
\ ** Q
\ |s
\0
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
5
D)
C
D
'o
1
NA
NA
NA
NA
NA
10
NA
7
10
g
Ł•
B
.2
"o>
CC
~5.
CO
W
NA
NA
NA
NA
NA
4
NA
8
10
8
CO
o
O
O>
c
Q
I
ro
0)
a:
NA
NA
NA
NA
NA
10
NA
5
6
3
c
o.
3
O>
C
"=
Q
'o
B
'5
>
NA
NA
NA
NA
NA
10
NA
4
1
7
5 ?
i_ a>
II
j= §
3 m
^Q
(T n
CD J^
1"
1- 0
>3
"^ CO
crS
NA
NA
NA
NA
NA
10
NA
7
6
4
o
!„
r" "O
0 C
0) O
H O
O) —
?™
Ł2 "^
I-ffl
§ »
«i:
NA
NA
NA
NA
NA
6
NA
g
10
g
a>
15
E
i5
c
$
Q
"co
In
c
o
? §
< 0
NA
NA
NA
NA
NA
10
NA
10
7
10
c
g
"OL
E
o
O
=
C
o 1
CO O
LU -^
-S
S -o
CC ffl
NA
NA
NA
NA
NA
6
NA
10
8
10
TOTAL
NA
NA
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 monitoring 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.
172
-------
MATRIX NUMBER 7
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches; total
well depth Oto 15 feet.
\ z
\ 2°
\ ^ Ł
\ 3 "*
\ ^
\ ^ /I
\ =1
\ 2 J
\ < tt
\ 5
\ t°
\ DC
\°
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
sz
"CD
2
0)
c
^
O
"o
™
03
(/)
m
>
NA
NA
NA
NA
NA
10
NA
8
NA
8
_>,
ii
CO
"53
OC
^=
Q
>
•*-*
CO
CD
tr
NA
NA
NA
NA
NA
10
NA
6
NA
4
^_,
c
E
Q.
ŁT
UJ
D)
C
^
Q
^
,|-
•^
_ra
CO
>
**
NA
NA
NA
NA
NA
10
NA
7
NA
7
—
s- •*•
? g
o E
±; Q.
^,2.
•- >
& r\
rr -D
0) c
I™
1- g
> _co
TO 2
rrS
NA
NA
NA
NA
NA
8
NA
10
NA
4
o
>,
c?Ł
*o ^
c ^
-^
s
< 0
NA
NA
NA
NA
NA
10
NA
10
NA
10
c
o
o3
"o.
E
o
0
> <-
"B ®
S E
CO CL
CO O
UJ o
CD >
•So
"S ^
rr
-------
MATRIX NUMBER 8
General Hydrogeologic 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.
\ ZOT
\ 00
\ t"* Q
\ ?J
\ §3
\ — Q
\ DC
\°
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
n
0)
c
Q
'o
co
NA
NA
NA
NA
NA
8
NA
NA
NA
10
>,
•S
"5
cc
0)
Q.
CO
(f)
NA
NA
NA
NA
NA
10
NA
NA
NA
10
8
O
O)
Q
o>
ra
0>
cc
NA
NA
NA
NA
NA
10
NA
NA
NA
6
C
'C
Q
O
'Ł
2
1
NA
NA
NA
NA
NA
10
NA
NA
NA
7
IE
i_
2. .
en in
O c
P =
E i
^8
0) •=
c iS
r= Z3
SI
"5 $
;E ">
NA
NA
NA
NA
NA
6
NA
NA
NA
10
B
0)
(0
b
c
CJI
s
Q
"c5
to
c
Q
11
NA
NA
NA
NA
NA
10
NA
NA
NA
10
c
g
J?
a.
E
o
O
1-
B Ł
l&
uj -5
Q) ^
ll
CC CQ
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.
174
-------
*• P°
0 H
01 =r
C/l CD
3 01
CQ U
9: 5'
" -6'
3 01
Ł. CD
CD Q.
^ C
Ł 8
0 0
00 ^
O CD
c? 3
cn o
1 f
=r u*
CD CO
W)
€|
O "^3
_k ^
•V §
o" 5
O" CD
0 c
|S
o o
® S
01 -i
D =
a 5-
(D (Q
1. ?
3 Q.
C7 g
» a
QJ
II
o ~
rr <
3 CD
_5- c/i
c 5'
CD
c/i n
CD §
X ^
o Ł
-S s
-* o
3 6"
C 3
o. •
O
oT
^
01
3
Q.
01
o-
CD
O
O
N
03
O
CD
rr
o
CD
oT
g-
—
•a
o
cr
CD
C/1
01
CD
•o
O
CD
W.
C/l
CD
CD
CD
—i
CD C
O D
consolida
hnology.
CD
Q.
6"
3
01
6'
_cn
•o
S
Q.
o
3
01
*<
C/l
Ł
I
c?
Q.
=.
w
01
c"
Ł
o'
CD
X
CD
^
CQ
CO
5.
S
5
c
ro
o
CD
O
E?
CD
O
o
o'
CD
O
""**
Q.
=r
3
m
X
Tl
1 —
Z
0
M
"^
Z
o
H
m
en
1 Cable Tool
0
o
f 0? * f >| >cf | | |
01 01 ._, -_, Q_ c ^: ^ — ^* < j
— C/l-O JO COO CQQ- ^ 5' o
5 5' 2. 2. ? ®S ^TlCQ ;Q -p
S'CQOJBO/n— r^
^ ^^ ^ oT S < § 2 Z ZZZZ
Z ZZ Z ZZZZ
Z Z Z - Z ZZZZ
Z Z Z - Z ZZZZ
ro
o
0
o
01
CO
Z ZZ-> Z ZZZZ
Z ZZ Z ZZZZ
Z ZZ Z ZZZZ
Z ZZ^ Z ZZZZ
Z ZZoi Z ZZZZ
m° //X^
sf X
W D >/
XCR'TERIA FOR EVALUATION
^X OF DRILLING METHODS
Versatility of Drilling Method
Sample Reliability
Relative Drilling Cost
Availability of Drilling Equipment
Relative Time Required for Well
Installation and Development
Ability of Drilling Technology to
Preserve Natural Conditions
Ability to Install Design Diameter
of Well
Relative Ease of Well Completion
and Development
r-
5 C
CD D
= O
Q-0
S
-» DJ
SŁ
o - •
«"§'
2.S
• c/>
O
(D
(D
S
S. I
O
^
o"
3
Si
6"
D
D"
Q.
=
03
C
Q.'
T3
CD
|
I
O
at
^
(Q
Q.
Ds'
3
CD
CD
O
<0
CD
O
S
n'
O
o
Q.
5'
tn
fi8
^
-------
MATRIX NUMBER 10
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 0 to 15 feet.
\ Z V)
\ 2°
\ ^UJ
\ >
\ §i
\ lg
\ W r>
\ ^~
\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
•o
o
Ł
1
CD
Q
Q
-^
Versatllit!
NA
1
NA
1
10
NA
NA
7
7
7
!Q
CO
rr
CD
a
CO
W
NA
1
NA
1
10
NA
NA
7
8
10
CO
O
0
CD
c
~
Q
Relative
NA
10
NA
7
9
NA
NA
5
5
5
'c
CD
Q.
3
CT
LJJ
O)
C
Q
o
^
Availabili
NA
10
NA
10
10
NA
NA
4
1
7
_
Ic
2. E
If
y
CC -o
Q) t-
E «
f Z
r— O
.11
5 s
cc Ł
NA
5
NA
5
10
NA
NA
6
6
2
o
CD cn
O c
•5.2
i T5
0 C
^8
'C "(5
Q ^
11
sS
5 s
< Q.
NA
5
NA
1
9
NA
NA
10
10
9
Ł
CD
CO
Q
c
O)
s
Q
to
.E
o
11
< 0
NA
1
NA
1
8
NA
NA
10
10
10
c
1
Q.
E
o
O
B g
fl» t
Si Q-
co O
UJ cu
•P
CD ^
OC CO
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 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. Jetting and mud rotary methods would require the addition of fluid.
5. 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.
176
-------
MATRIX NUMBER 11
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 15 to 150 feet.
\ z
\ 00
\ *~ I
\ 3 l"~
\ _l ^
\ ^^
\ "z
\ si
\ -°
\ 1L
\ I— O
\ tt
\°
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
0)
O)
Q
H—
O
1
CD
NA
1
NA
NA
8
NA
NA
8
10
10
Ł.
j§
CO
"55
rr
1
NA
1
NA
NA
10
NA
NA
5
8
10
to
<3
0)
c
• —
•c
O
CD
"Z
EC
NA
4
NA
NA
10
NA
NA
8
8
6
c
E
a
3
UJ
O)
c
O
'o
.5
1
NA
10
NA
NA
10
NA
NA
5
1
8
0)
k_ <1>
O Ł
if
to
cc -a
CD Ł
E *
Ł S
I— Q
-ft w
DC Ł
NA
1
NA
NA
8
NA
NA
10
8
2
o
m w
O c
^ •—
I1?
o c
CD O
HO
.i's
-c to
Q ~2i
51
-Q S.
NA
5
NA
NA
8
NA
NA
9
9
10
^_
S
E
Q
Q
C
O)
8
o
1
M
C
O
li
< 0
NA
1
NA
NA
8
NA
NA
10
10
10
0
«
a.
E
!„
o 1
oi Q-
BJ O
§>!
15 c
DC CO
NA
7
NA
NA
7
NA
NA
10
10
10
TOTAL
NA
30
NA
NA
69
NA
NA
65
64
66
EXPLANATORY NOTES:
1. Unconsolidated format'ons, 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. As depth increases the relative advantage of hollow-stem augering decreases.
5. 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.
177
-------
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
\ °0
\ Is
\ >
\ 3?
\ cc •—
\ n SJ
\ < (E
\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
o
.c
CD
^
D>
C
Q
B
2-
oj
w
CD
>
NA
NA
NA
NA
NA
NA
NA
8
10
9
>,
Reliabilil
Qi
Q.
CO
w
NA
NA
NA
NA
NA
NA
NA
6
10
6
(ft
O
o
O)
c
Q
>
ra
CD
CC
NA
NA
NA
NA
NA
NA
NA
10
9
7
c:
I
Q.
CT
111
C
~=
Q
~o
&
Availabi
NA
NA
NA
NA
NA
NA
NA
7
4
10
It
>_ CD
o ^
•ri 9-
^5
U- CD
•5 >
^Q
CD LJ
EC -D
a> 5
1 =
H- o
$s
aS
"CD p
tr Ł
NA
NA
NA
NA
NA
NA
NA
10
10
6
o
Is?
li
j^ "O
o c
CD o
HO
,c|
ll
••r m
o|
ts
5 Ł
< Q-
NA
NA
NA
NA
NA
NA
NA
8
10
8
ID
CO
i5
tz
O)
'w
1
O
2
c
Q
ll
< 0
NA
NA
NA
NA
NA
NA
NA
10
10
10
c
o
00
Q.
E
o
O
™
C
CC to
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 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.
178
-------
MATRIX NUMBER 13
General Hydrogeologic 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.
\ z w
\ 9°
\ 3 *"
\ "* ^
\ [2 u
\ U- —
\ •* Q
\ Ł
\ "* n
\ ^""
\ E
\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
o
J;
Ol
Q
Q
*o
>,
~
^ti
Q)
NA
1
NA
1
10
NA
NA
9
9
10
2?
15
CO
'CD
a:
CD
a
E
W
NA
1
NA
1
10
NA-
NA
8
8
10
w
o
O
Ol
c
^
O
u>
_
ra
CD
EC
NA
10
NA
7
10
NA
NA
5
5
6
_^
c
CD
a.
D
CT
LU
O)
C
E
Q
*Q
,-ti
"J5
CO
>
NA
10
NA
10
10
NA
NA
4
1
7
^
"a>
? Ł
Ł §.
^S 2
2- CD
s
;§ 2
a) J2
trŁ
NA
5
NA
5
10
NA
NA
6
5
4
2
O) «
o c
0-2
Ł T3
o c
(A
d>
a
2
V)
^
o
^* CD
I?
< 0
NA
1
NA
1
7
NA
NA
10
8
10
c
g
a>
CL
E
o
O
'c
B 1
CD 5^
CO O
UJ -5;
> a>
~ Q
•5 "c
tr co
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 monitoring well prohibits the use of drilling fluid and additives in construction.
4. Increasing diameter is influencing choice of equipment.
5. 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.
179
-------
MATRIX NUMBER 14
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; saturated; invasion of formation by drilling fluid not permitted; casing diameter 2 to 4 inches-
total well depth 15 to 150 feet.
\ Z Q
\ P°
\ 3*
\ > S
\ LJ
\ rr ^
\ Ł —
\ ^ U.
\ ^ o
\ ^
\ oc
\°
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
^
t^
co
to
>
NA
1
NA
1
5
NA
NA
9
9
10
>
J5
CO
13
DC
>
ra
0
OC
NA
2
NA
2
10
NA
NA
8
8
7
c
f^
cr
UJ
en
c
=
Q
^
•^
-=I
nj
CO
>
<
NA
10
NA
9
9
NA
NA
4
1
7
0
5 C
Q E
•o Q'
2. -0
5 >
Sa
rr -o
0 C
C!
I- 0
S —
co Ł
0 (O
cr Ł
NA
1
NA
3
10
NA
NA
9
9
5
o
C7> «
o c
0.2
J- -Q
O C
C7J —
C ^
C m
Q ^
"5 ŁŁ
^. g
5 Ł
< 0-
NA
5
NA
4
8
NA
NA
10
10
9
a>
0
CO
b
c
D)
'w
0
Q
~m
c
—
o
-^^
= ^
< "o
NA
1
NA
2
6
NA
NA
10
6
10
c
o
Q.
E
o
O
$ -
"5 g
% 0.
CO O
UJ "0
* s
5 -o
DC co
NA
4
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 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. Depth range is 15 to 150 feet
5 Increasing diameter and depth favor cable tool and air rotary with casing hammer techniques.
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.
180
-------
o>
Q.
IT
CU
c/l
3
o
[umes of drill
D
CQ
E"
Q.
03
CD
T3
CO
3
CO
CO
CT
CD
5
CD"
CO
CO
CD
i
CD
O3
cr
CD
Ł»
CD
^
03
C/3
CD
When using cable
i-tool drilling
in saturated f
ormal
o
w
^
co'
03
03
CO
C
3
1
03
0
Q.
^
5
CD
c"
Q.
D
CD
CD
Q.
CO
O
cr
CD
03
Q.
Q.
CD
Q.
5"
CD
3
CD
03
CT
CD
3
03
CD"
5
CO
pi
c_
CD
CD
03
13
D.
3
Q.
0
oT
<
CD
CT
O
Q.
CO
O
C
Q.
CD
n
c
CD
CT
CD
03
D.
Q.
6'
13
2.
c
Q.
^.
Increasing diame
CD
03
13
D.
Q.
CD
oT
q
O
03
cr
CD
O
g_
03
Q.
03
~*
oT
^
CT
03
CO
5
CQ
CT
03
3
3
CD
CD"
o
CT
.5
c
CD
O3
CO
The anticipated u
CO
CD
O_
CT
CD
0
D
5"
5
CQ
cp_
0
CT
Co
CD
C
CO
CD
g_
Q.
5'
CD
c"
CL
[U
cx
Q
Q.
D.
~.
CD
w
5"
o
^
w
c
^
o
fvj
Borehole stability
problems ar
•e potentially
CO
CD
CD
CD
technology.
—
Unconsolidated f
ormations, p
iredommantl'
*-^
CO
c;
03
f
S
CO
03
c"
S
6
CD
X
CD
5
CQ
CO
CD
13
O
-j
^-f
—
2,
c
CD
O
CD
O
-*
CD
O
o
o'
03
2,
Q.
^.
5'
CO
m
X
PLANATORY N
0
m
o?c?0^^|>o;>oŁ9o?
O" fl) OJ__ _ r\ Crr C— c^. < ^
» i is a ? 11 1" ^ 1 ^
§^^|^aT«^ I
— 03 T3 < •< S S 0>
o 3 Ł =
oT 3 CT
•5 to
- zz z zzzz
CD O CD ^ ^ ^ ^ ^ ^ ^
-* zz z zzzz
"^J O CO ^ ^ ^ J> ^ ^ ^
-zz z zzzz
O^ CD O ^ ^ ^ ^ ^ ^ ^
- zzzzzzz
-^ -zz z zzzz
O) O O ^ ^ ^ ^ ^ ^» ^
-^ ^ ~Z. ~Z. Z ZZZ2
CD O OJ>J> 5* ^^^"^
- -zz z zzzz
O CO O ^ ^ ^ ^ ^ ^ ^
- -zz z zzzz
O CO O ^ ^ ^ ^ ^ ^ ^
03g -^ZZ Z ZZZZ
— x/^
w O ./
/CRITERIA FOR EVALUATION
^X OF DRILLING METHODS
Versatility of Drilling Method
Sample Reliability
Relative Drilling Cost
Availability of Drilling Equipment
Relative Time Required for Well
Installation and Development
Ability of Drilling Technology to
Preserve Natural Conditions
Ability to Install Design Diameter
of Well
Relative Ease of Well Completion
and Development
O
O C
co o
si
CD CO
= O
II
:± CD
17 Q.
CQ ' '
-i CO
CD 03
Łc
CD 2
S CD
s|
|§
~2.
o1
3
Si
o'
=3
cr
Q.
5"
CQ
E"
o;
13
O
T3
CD
§
CD
Q.
O
CO
CQ
Q.
CD
CD"
— \
O
Ł>.
5'
o
3-
CD
CO
o
(D
3
sl
i
Q.
3
(D
O
O
(O
o'
O
0
Q.
5'
OT
Qa
|
O
-------
MATRIX NUMBER 16
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 0 to 15 feet.
\ z to
\ 2°
\ ^H
\ J ^
\ > s
\ fit ^
\ a3
\ LL —
\ *t S
\ — D
\ 6s
\
\ (j
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
Ł
5-
2:
0)
C,
"C
Q
0
>^
§
15
CO
CD
NA
NA
NA
4
NA
NA
NA
8
NA
10
jx.
7=
.O
CO
"33
tr
0)
Q.
E
ra
W
NA
NA
NA
5
NA
NA
NA
8
NA
10
"w
0
O
O)
c
s
Q
o>
•~
CO
0)
OC
NA
NA
NA
10
NA
NA
NA
8
NA
6
—
c
NA
NA
NA
10
NA
NA
NA
8
NA
8
_ CD
Ł E
°O o
2? "CD
3 CD
c?Q
cc -o
0 c
E ™
i- o
.il
15 S
0) «
cr Ł
NA
NA
NA
8
NA
NA
NA
8
NA
10
o
en to
o c
0.2
c •—
"5 !=
O) —
c 2
i= 3
ol
° 2
^* o
r1 ^
< ct
NA
NA
NA
1
NA
NA
NA
8
NA
10
,_
15
CO
b
c
en
C/)
Q
—
2
V)
o
^r d)
< 0
NA
NA
NA
4
NA
NA
NA
8
NA
10
c
o
'0
Q.
O
Q
5 •Ł
'o *^
E
CO O
HI "5
>l
•J O
"o> ?
CC ra
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 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.
182
-------
MATRIX NUMBER 17
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 15 to 150 feet.
\ z w
\ 2 °
\ "*H
\ ^ in
\ < 5
\ > (3
\ ^ Z
\ <* —.
\ ŁE
\ E ,.
\ *** ri
\, M
\ 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
-C
o
2
01
c
l_
Q
"o
>»
_^
CO
(5
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
>,
^
J3
CO
"CD
CC
CD
"5.
CO
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
_t_l
0)
O
o
O)
c
c
Q
,
J^
'n
I
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
Ic
i- CO
0 C
o -
.C T$
o c:
CD O
H O
-I
2^
>
i= co
— Q)
< ot
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
2
'cu
CO
b
c
O)
t/>
Q
—
CO
c
o
•*"*
< 0
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
c
o
5
Q.
E
o
o
"o
§ •Ł
"o ^
Q) E
(O °-
50 .2
gj
> Q)
^T3
Ł c
DC ca
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 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 of drilling fluid are permissible in less permeable materials.
183
-------
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.
\ 2
\ p O
\ 3 u
\ ^ s
\ in O
\ *^"
\ °-
\ ^ ^
\ — O
\ ^o
\ t
\°
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
"D
O
.C
CD
*—
D)
C
^
Q
"o
>:
~^
CO
CD
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
Ł
—
XI
CO
"CD
a:
JD
Q.
E
w
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
, .
«
o
O
a>
c
T=
Q
I
"CD
a:
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
c
CD
C
tz
g.
D
D"
LU
O)
c
Q
'o
.±;
^
CO
<
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
"CD
§ t:
i_ 0
o Ł
*~ Q.
"S °
Ł "CD
CT|
CD Q
en -o
CD c
h- O
> ™
ra 2
0 «
ac S
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
0
CJ) CO
o c;
"5.2
5 "O
0 C
CD O
HO
c 2
-H ^3
Qz
^ CD
^ CO
< i
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
CD
fc
CO
D
CL
CD
CO
CD
Q
—
CO
c
o
1^
< 0
NA
NA
NA
NA
NA
NA
NA
NA
NA
10
o
CD
H
E
o
O
5 -
"o ^
co Q-
ca o
UJ CD
IQ
~& T-
tr ra
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 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 of drilling fluid are permissible in less permeable materials.
184
-------
MATRIX NUMBER 19
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less;
total well depth to to 15 feet.
\ z«
\ "" O
\ '*I
\ ^ nj
\ < s
\ ^ <1
\ ^ z
\ oj
\ ffi
\ ^ Q
\ * u.
\ Ł
\°
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
x:
^
^
c
'C
Q
B
S
to
(O
(1)
>
4
7
3
8
10
8
5
9
9
6
>,
—
XI
1
V
"o.
w
CO
5
1
1
10
10
10
5
8
9
10
^
a>
O
0
0)
c
Q
Q)
~
ro
Q)
DC
9
10
8
10
10
7
8
6
6
3
c
0)
a
'D
O"
LU
Ol
^
D
B
5
ra
5
<
10
10
8
9
9
10
8
4
1
7
—
5 c
*^ Q.
•p o
SIB
"=; >
Is
EC -0
|!
t- 0
>I
"55 «
DC Ł
5
6
5
10
10
8
8
3
3
2
o
o c
0.2
^ ~
o c
CD O
1-0
c 2
— 13
11
*o g
•^ CD
IS?
< Q.
9
5
1
8
10
4
7
9
9
9
OJ
0)
ra
Q
c
0)
0)
Q
:=
CO
"to
Ł
o
^ CD
1?
< 0
6
1
1
10
10
10
8
10
10
10
c
g
IS
o.
E
o
O
§ -
B g
CD
S
i: Q
» c
GC ra
6
4
5
5
10
5
4
10
10
7
TOTAL
54
44
32
70
79
62
53
59
57
54
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 permits the use of drilling fluid and additives in construction.
185
-------
MATRIX NUMBER 20
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less-
total well depth 15 to 150 feet.
\ ZQ
\ il
\ Ł*
\ a 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
T3
O
Ł
o>
O)
c
Q
B
Versatility
NA
1
1
8
9
10
7
10
10
9
Ł•
'.5
eg
IB
CC
Q.
CO
CO
NA
1
1
10
10
10
5
8
9
10
8
O
O)
c
'C
Relative C
NA
7
4
10
10
10
8
6
6
3
C:
,
Availabllit
NA
10
10
9
9
10
8
4
1
7
Ic
>_ 0)
Ł E
*s
*"• fl)
^ fl)
S"Q
a: T3
m
.E 2
ll
0»
2- J5
^= (O
s a>
a>
^°
DC (0
NA
4
1
5
9
5
4
10
10
7
TOTAL
NA
35
24
69
76
67
56
64
61
57
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 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. Solid-flight and hollow-stem augers are favored to the limit of their depth capability.
186
-------
MATRIX NUMBER 21
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 inches or less;
total well depth greater than 150 feet.
\ z w
\ 00
\ rr v
\ *i
\ ^z
\ li
\ -°
\ *o
\ t
\ tt
\°
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
2
O)
c
6
"o
>•
™
CQ
0>
NA
NA
NA
NA
NA
10
5
9
10
9
>,
5
ttJ
"5
tr
0)
a
1
NA
NA
NA
NA
NA
2
5
9
10
5
^
8
D)
C
H
O
8>
>
ra
0)
tr
NA
NA
NA
NA
NA
10
B
6
6
5
^
c.
CD
g.
'3
cr
UJ
O)
c
Q
"o
—
]Q
5
2
<
NA
NA
NA
NA
NA
10
8
4
1
7
0
n
Ł E
Q-
|f
ID
c TJ
NA
10
10
9
9
10
8
4
1
7
-
i_ CD
O Ł
**" Q.
.? ^>
m~Q
oc n
r~ (T3
•i C
1- 0
.11
ro 2
6: Ł
NA
5
5
10
10
8
8
5
5
5
o
Is?
~Q ^
^ "D
o c
0) O
h- O
:= 3
•— •«-;
O ^
**" o
2. Ł
^ 0)
Q
s
w
c
o
•= S
< "o
NA
1
1
10
10
10
8
10
8
10
c
o
'•5
a.
E
o
O
5» -^
B "
^ Q_
CO O
HI "o
> s
Ł -o
DC co
NA
4
1
5
10
5
4
10
10
8
TOTAL
NA
37
24
70
79
62
53
60
57
60
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 sen/ere (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. Solid flight auger methods require open hole completion, which may or may not be feasible.
188
-------
MATRIX NUMBER 23
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 2 to 4 inches; total
well depth 15 to 150 feet.
\ 2 W
\ ^i
\ ™^ h~
\ Ij w
\ ^
\ LU «Ł
\ C --
\ °a
\ ~ °
\ Ło
\ — •
\ 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
TD
^
O)
c
Q
"o
^»
S
CO
to
>
NA
1
1
3
7
10
5
10
10
9
>,
J5
CO
"cu
DC
cu
Q.
E
CO
w
NA
1
1
10
10
10
5
8
9
10
-t-j
tn
o
0
01
c
"c
G
cu
CO
cu
oc
NA
1
1
10
10
10
8
6
6
4
c
0
E
Q.
Q-
LLJ
O)
C
Q
"5
,±i
5
CO
NA
10
10
9
9
10
8
4
1
7
=
3* •*"*
w S
Ł E
"^ O
QJ ~m
'§•»
DC -0
i!
H 0
>^2
is
CU w
DC Ł
NA
2
3
8
10
9
8
8
8
7
o
>,
O) CO
o c
0-9
C ™
-— TJ
o c
cu o
1-0
O) —
f i
ii
° Ł
—
-------
MATRIX NUMBER 24
General Hydrogeologic 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.
\ ZQ
\ P°
\ =U
\ <*
\ 53
\ rf *
\ ^
\ cc n
\ K°
\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
Ł.
1
To
w
c
o
ti
< 0
NA
NA
NA
NA
NA
10
10
10
10
10
o
"5.
E
o
O
Is
O Ł•
-------
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.
\ Z CO
\ 00
\ p ^
\ Dh
^^ i IU
\ 11
\ **• ~
\ - °
\ PC n
\ W X
\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
O)
Ł
o
o
Id
1
NA
NA
NA
4
7
10
5
8
NA
8
2?
j5
~&
cc
o>
Q.
1
NA
NA
NA
8
10
10
5
7
NA
10
«
o
O
D>
C
*c
Q
I
5
DC
NA
NA
NA
10
10
7
9
6
NA
3
^
c
1
Qu
'3
S
Ot
=
o
"o
'Ł
•Ł
'o *
§cx
o
UJ o>
II
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 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 permits the use of drilling fluid and additives in construction.
4. Diameter requirements limit the equipment that can be utilized.
5. Solid-flight augers require very difficult open-hole completion. Hollow-stem auger technique requires open-hole completion
for casing sizes greater than 4 inches.
191
-------
MATRIX NUMBER 26
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches- total
well depth 15 to 150 feet.
\ 2 W
\ 00
\ P ^
\ lli-
\ ^ !••
\ «t 2
\ UJ -!m
\ cc —
\ Ły
\ < ^
\ |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
•o
o
Ł
o>
2
en
c
•^
Q
"5
TO
O>
NA
NA
NA
NA
NA
10
NA
NA
NA
8
>,
~
m
ID
DC
Is
CC -D
Q) ^
Ec
H 0
>I
ra ™
DC Ł
NA
NA
NA
NA
NA
10
NA
NA
NA
5
o
O) w
O C
o 2
c ~
.C -D
OJ O
KO
c g
||
^ Q>
~ d)
< ol
NA
NA
NA
NA
NA
6
NA
NA
NA
10
o>
0)
CO
Q
C
en
'(/>
Q>
Q
"c5
Q
II
-Q ^
< 0
NA
NA
NA
NA
NA
10
NA
NA
NA
10
c
g
15
Q.
E
o
o
0)
•s S
o 2i
§c
Q.
O
LU "0
||
CC n
NA
NA
NA
NA
NA
4
NA
NA
NA
10
TOTAL
NA
NA
NA
NA
NA
70
NA
NA
NA
66
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 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. Diameter of borehole, and depth, eliminates most options.
5. Air rotary with casing hammer and dual-wall rotary are applicable for 4-inch casing.
192
-------
J,:
MATRIX NUMBER 27
General Hydrogeologic Conditions & 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.
\ z w
\ °s
\ P O
\ is
\ >o
\ HI *Ł
\ °-'
\ cc
\ — o
\ Ł"•
\ 1""
\ tt
\ 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
-o
f
0)
^
^
D)
C
"C
Q
"o
>,
iH
(0
NA
NA
NA
NA
NA
10
NA
NA
NA
8
>,
—
Jo
"o>
QC
0)
Q.
1
NA
NA
NA
NA
NA
8
NA
NA
NA
10
*^
o
O
D)
C
•=
O
|
n
o>
EC
NA
NA
NA
NA
NA
10
NA
NA
NA
6
c
g.
3
s
O)
c
Q
B
.•ti
^
(0
'5
>
NA
NA
NA
NA
NA
10
NA
NA
NA
7
=
l|
o E
Ł "55
= >
°-Q
g
Q
C
O)
Q
—
CO
«
( .
^* $
•— Q
CC ra
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 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 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. Diameter of borehole, and depth, eliminates most options.
193
-------
MATRIX NUMBER 28
General Hydrogeologic 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.
\ z w
\ c o
\ IE
\ ^
^^ 2 Q
\ 2E
\°
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
0>
c
Q
'o
=
to
0>
>
4
7
NA
8
8
NA
5
9
10
NA
Reliability
cr
9
10
NA
10
10
NA
8
6
6
NA
•Ł
Q)
O.
'D
s
O)
_c
Q
"o
;5
(0
1
10
10
NA
9
9
NA
8
4
1
NA
_
Ic
>. 0)
Ł E
•o§-
Ł|
'IS
Jo
cr n
<1) =
.ic
H g
•— ^
"nj «
crŁ
5
6
NA
10
10
NA
8
3
8
NA
o
O) ^ c
IS in
'B 2
? =
o 1
s!
(0 O
LLI "55
ID
cr n
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 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 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.
194
-------
MATRIX NUMBER 29
General Hydrogeologic 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.
\ Z «>
\ OQ
\ If
\ *"i
\ SB
\ <* S-
\ EC Q
\ UJ A
\ ^™
\ 2
\°
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
at
^
O
o
>s
~
"(3
2
>
NA
1
NA
9
10
NA
7
10
10
NA
>,
~
n
.2
"5
CC
OJ
0.
(0
CO
NA
1
NA
10
10
NA
5
8
9
NA
^
8
en
c
'C
D
>
ra
0)
CC
NA
7
NA
10
10
NA
8
6
6
NA
4-1
C
Q.
D
CT
UJ
O)
C
Q
"o
>4
~
15
nj
1
**
NA
10
NA
10
10
NA
8
4
1
NA
—
as
b
c
g>
Q
$
C
^_
^*"fl)
1^
< 0
NA
1
NA
9
9
NA
9
10
10
NA
c
g
"5.
E
o
O
|
"5 *
u ^
M Q.
n] O
uj-j
§ S
•S Q
•2 -a
CC en
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 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 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.
195
-------
MATRIX NUMBER 30
General Hydrogeologic Conditions & Well Design Requirements
Unconsolidated; unsaturated; invasion of formation by drilling fluid not permitted; casing diameter 2 inches or
less; total well depth greater than 150 feet.
\ z w
\ 2°
\ =t
\ < s
\ "z
\ si
\ ^ Q
\ ffi"-
\ t
\ cc
\o
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
E
Ł
2
D)
C
Q
"5
^
—
to
NA
NA
NA
NA
NA
NA
7
10
10
NA
>.
o
CO
"5
EC
CD
Q.
CO
CO
NA
NA
NA
NA
NA
NA
7
9
10
NA
o
O
O)
c
!=
o
Ł
>
to
CD
^ 0)
•— co
5 Ł
NA
NA
NA
NA
NA
NA
7
9
10
NA
2
CO
Q
c
D)
8
Q
eg
en
c
0
*^ __
^""5
I?
< 0
NA
NA
NA
NA
NA
NA
10
10
10
NA
g
"5.
e
o
o
!„
C
JQ.
O
"i
S Ł
•s Q
(T CD
NA
NA
NA
NA
NA
NA
4
10
8
NA
TOTAL
NA
NA
NA
NA
NA
NA
65
73
69
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. The depth requirement and the decision not to utilize drilling fluid limit equipment options.
5. 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.
196
-------
MATRIX NUMBER 31
General Hydrogeologic 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.
\ 11
\ UJ
\ Z* 5
\ ^ O
\ ^ z
\ si
\ -4" QC
\ 2°
\ H°
\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
JZ
0)
2
O)
c
^
Q
"o
(0
Q)
NA
1
NA
8
10
NA
5
9
9
NA
2-
!5
<8
"35
tr
DC
NA
10
NA
10
10
NA
8
6
6
NA
c:
^3 S.
< Q-
NA
5
NA
8
10
NA
7
9
9
NA
ai
ro
^
c
gi
1
—
R)
C
^ .
.fj Q)
i= >
< 0
NA
1
NA
7
9
NA
8
10
9
NA
g
"5.
E
o
Q
1^
"5 <"
u
(0 O
Hi a>
II
DC 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.
197
-------
MATRIX NUMBER 32
General Hydrogeologic 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 w
\ 00
\ EZ O
\ ^f
\ i IU
\ < 5
\ *z
\ ^
\ ^ o
\ t
\ 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
T3
o
1
O)
c
Q
o
>•
1
CD
NA
1
NA
3
7
NA
5
10
10
NA
>,
'.5
CO
"53
cc
CD
Q-
Ł
w
NA
1
NA
10
10
NA
5
8
9
NA
^
o
O
0)
'C
O
>
ra
0)
cc
NA
1
NA
10
10
NA
8
6
6
NA
•~
4)
o.
O"
111
0)
C
Q
Q
>-,
"?
CO
1
<
NA
10
NA
10
10
NA
8
4
1
NA
—
0) ^
k_ (p
o Ł
if
is
cc -o
0) c
P ra
.S c
H 0
J|
ra 2
CD <2
DC Ł
NA
2
NA
8
10
NA
8
6
6
NA
o
>,
O C
o.2
I?
0 C
CD O
I-O
c?5
= 3
D-_
^_
>,Ł
Ł «
Ł Ł
< Q.
NA
5
NA
8
10
NA
7
9
9
NA
V.
OJ
cu
CO
b
c
g>
D
2
to
_c
o
11
< 0
NA
1
NA
6
8
NA
8
10
8
NA
c
g
5
Q.
E
o
O
h
o 1
(O Q.
(Q O
UJ -5;
•-D
I13
CC co
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 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.
198
-------
MATRIX NUMBER 33
General Hydrogeologic 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.
\ ZQ
\ P?
\ ^ Ł
\ 3 »
\ < s
\ ui ^
\ Z
\ rf ^
\ E°
\ i°
\0
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
^
en
c
*c
O
|
*3
<5
>
NA
NA
NA
NA
NA
NA
5
9
10
NA
>,
—
J3
_co
cc
0)
Q.
CO
CO
NA
NA
NA
NA
NA
NA
5
9
10
NA
3
D)
=
Q
«
tr S
NA
NA
NA
NA
NA
NA
10
8
10
NA
0
^
O> CO
O C
°-B
J'-o
o c
CD O
HO
O) —
.Ł5
'C ^S
Qz
^»
C1 CD
•-= co
< ol
NA
NA
NA
NA
NA
NA
5
10
10
NA
o
2-
E
CO
b
c
O)
'Ł
Q
2
CO
c
o
^"55
I?
< 0
NA
NA
NA
NA
NA
NA
10
10
10
NA
0
^5
"5.
o
0
"CD
"o <"
si
to o
UJ 0
~ Q
•5-0
OC ra
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 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. No drilling fluid, increasing depth and diameter requirements eliminate many options.
5. Air rotary with casing hammer requires driving 8-inch or greater casing and completion by pullback.
199
-------
MATRIX NUMBER 34
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 0 to 15 feet.
\ 22
\ =lu
\ Sjo
\ o|
\ _*• flc
\ o
\ Ł "•
\ ^~
\ Ł
\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
•o
o
.c
a
O)
•c
O
"o
^
~
'•^
CO
!2
$
NA
NA
NA
NA
NA
NA
6
10
NA
NA
'—
la
"o
OC
CD
Q.
1
NA
NA
NA
NA
NA
NA
5
10
NA
NA
8
O)
c
•"E
Q
0)
.«
ra
CD
OC
NA
NA
NA
NA
NA
NA
10
6
NA
NA
c:
CD
g.
S
O)
Q
^
.t;
15
CO
1
NA
NA
NA
NA
NA
NA
10
6
NA
NA
o
5 c
i- CD
it
QJ Q
OC -D
•— c
1 — o
si
to 2
II
NA
NA
NA
NA
NA
NA
10
9
NA
NA
o
O) to
c i|
CD O
HO
rj>— -
C
:= 3
D ^
<4_ ^
o 5
^* QJ
< bt
NA
NA
NA
NA
NA
NA
6
10
NA
NA
o5
*Q)
CO
Q
C
O)
0)
Q
—
2
c
_
*• _
^ CD
II
NA
NA
NA
NA
NA
NA
6
10
NA
NA
c
g
IS
Q.
E
3
"55
"*r qj
m E
co Q-
co o
ui -5;
* CD
o
OC co
NA
NA
NA
NA
NA
NA
5
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/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. Diameter and no drilling fluid minimizes options
5. Jetting, mud rotary and cable tool methods would require the addition of fluid.
6. Air rotary with casing hammer requires driving 12-inch or greater diameter casing and completion by pullback.
7. Air rotary completion possible only if unsupported borehole is stable.
200
-------
MATRIX NUMBER 35
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 15 to 150 feet.
\ z w
\ 2°
\ "*H
\ "j UJ
\ ,
^
>
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
^
—
.0
ra
"CD
cc
CD
a.
|
w
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
"w
o
O
a>
c
:=
D
CD
>
15
CD
cr
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
.j_,
C
Ł^
^
UJ
O)
c
"c
Q
"o
;t±
1
'ra
"*
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
—
Q)
5 C
v. Q)
O Ł
•o CL
— >
m" Q
OC -o
E(0
_ j..
1 — O
j!
ra 2
cr Ł
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
o
O) f>
O C
o c
CD O
(— Q
C ^
~ 2
11
11
5 2
< D-
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
^_
0)
0)
CD
Q
C
0)
Q.
E
o
0
in
^ *-*
*o QJ
P
to Q-
co o
tu "ai
P
DC a
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 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. No drilling fluid, depth and diameter requirements have eliminated options
5. Oversize drillpipe 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-inch diameter outer casing that is required for 8-inch
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.
201
-------
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.
\ z w
\ -o
\ ^ 2z
\ UJ
\ < s
\ ^ 13
\ ^ ?
\ ^ j
\ Ł
\. ^ Q
\ Ł"•
\ H"
\ 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
.c
CD
^
2
O)
c
•'S
D
'o
>,
j=
CO
IB
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
>,
•^:
ri
CO
OC
0)
Q.
1
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
i .
CO
o
O
D)
C
==
D
CD
^
CO
CD
cr
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
c
CD
E
Q.
CT
111
D>
Q
"o
±i
;5
^5
ra
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
'5 S
DC T3
||
t- 0
>'*
0) «
oc Ł
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
o
>>
o c
"o 2
c —
o c
^3
i1™
— 3
Q|
•"^ ^-
o|
^•o
ll
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
^>
CO
b
c
D)
Q
3
(O
c
Q
3*1 "oi
I?
< 0
NA
NA
NA
NA
NA
NA
10
NA
NA
NA
c
.g
0)
Q.
E
Q
1-
**~ m
C
CO O
LU "a)
s s
•~ Q
1^
DC CO
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. 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. No drilling fluid, depth and diameter requirements have eliminated options.
5. Oversize drillpipe 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-inch diameter outer casing that is required for
8-inch diameter casing and screen completion.
9. If borehole is unstable, for 8-inch diameter casing there is no 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.
202
-------
en
co M -i m
ro
o
CO
g O CD
5 o o
CT
CD CD
(U _ V
l~* m ? ~"
| S n 5
g < <5 co
§ <» S <
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a> o 5T "o
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? - ° O
S >< D O.
ilti
11 1
2, 3
o
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o
S? ? o^ * 1 >? >2 Ł 9 5
O" 03 0> -n -n Q- C= C=^ S; < 3
-r- — 0)3 3 ""COO COO. D =• Q.
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rlM0)O^^— ^
§ 5 ^ ^ 3 | f |
o 3 s. 3 -1
a 33-
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-* 2 22222
-^JO J>CDO5 3> J>>J>J>
2 -» 2 2222
oi--j >ocn > >>>>
2 -L 2 2222
en — ^ j>too ^ 5">J>5>
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j^o j>ooo 3> >J>J>>
-^ 2 22222
^JO >CD~J > >>>>
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oo >oo > >>>>
-^ 2-^ 2 2222
00 O ^ O *^J ^ ^ ^ ^ ^
oics 2^JC3> 2 2222
CTcOn >O1CO > >>>>
11 //
./CRITERIA FOR EVALUATION
/ OF DRILLING METHODS
Versatility of Drilling Method
Sample Reliability
Relative Drilling Cost
Availability of Drilling Equipment
Relative Time Required for Well
Installation and Development
Ability of Drilling Technology to
Preserve Natural Conditions
Ability to Install Design Diameter
of Well
Relative Ease of Well Completion
and Development
0
r~
o
o
I
o!
SB-
CD
q.
5"
S
52
o"
D
O
Ł
o'
O
3
5
<<
_ O.
o- 3
•< g|
BO 00
o —
S
1 is
S z S3
10 1-1
a S
m >S
M
(5*
Q.
B)'
CD
? 3D
±5
o I-
7 m
s i
o §
« Sf
(0
(A
-------
MATRIX NUMBER 38
General Hydrogeologic Conditions & Well Design Requirements
Consolidated; invasion of formation by drilling fluid permitted; casing diameter 4 to 8 inches.
V 2
\ 00
\ c o
\ "* H
\ 3 "j
\ ^ "^
\ "'i
\ ^ ~
\ ^ ^
\ Ł°
\ t °
\ oc
\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
o
Ł
5
O)
c
c
O
Q
• —
1
CD
NA
NA
NA
NA
NA
9
8
NA
NA
10
2-
(0
ID
ra
0)
OC
NA
NA
NA
NA
NA
8
10
NA
NA
5
^
c
E
Q.
Q.
o
0
> "P
"o ^
« Q-
iS -§
o> =:
So
5n
^ =
cn co
NA
NA
NA
NA
NA
7
10
NA
NA
9
TOTAL
NA
NA
NA
NA
NA
64
77
NA
NA
65
EXPLANATORY NOTES:
1. 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.
204
-------
MATRIX NUMBER 39
General Hydrogeologic Conditions & Well Design Requirements
Consolidated; invasion of formation by drilling fluid not permitted; casing diameter 4 inches or less.
\ 2°
\ ^ ^
\ —) EJ
\ <Ł 2
\ UJ
\ oŁ —
\ 2 -^
\ < ""
\ 5°
\0
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
5
O>
c
Q
O
>-
1
Q)
NA
NA
NA
NA
NA
NA
8
NA
10
NA
>,
!5
.5
'35
DC
0)
Q.
E
(0
w
NA
NA
NA
NA
NA
NA
8
NA
10
NA
CO
0
Ol
c
~
Q
>
"oi
DC
NA
NA
NA
NA
NA
NA
10
NA
7
NA
c
0)
Ł
a
'5
ixi
O)
c
r::
Q
"o
Ł.
1
"•
NA
NA
NA
NA
NA
NA
10
NA
1
NA
a>
a5
o Ł
±| Q.
In °
Ł ^55
^ Q)
OC T3
C ^
.- C
ll
tr-
NA
NA
NA
NA
NA
NA
10
NA
10
NA
o
^*
o> w
0.2
'o c
d> o
HO
E1^
~W (Q
Qz
"5 ?;
li
•jQ 0)
< 0.
NA
NA
NA
NA
NA
NA
8
NA
10
NA
1
^
a)
b
c
-------
ro
o
01
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o
oT
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3 I ^<5 "«5 I
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- z z zzzz
-> Z Z ZZZZ
-> z z zzzz
-. Z Z ZZZZ
- z z zzzz
-. Z Z ZZZZ
o ^ ^ ^ ^ ^ ^
zz z-z z zzzz
z z z
z z z
-> Z Z ZZZZ
w Z Z ZZZZ
°i /
W O /
/CRITERIA FOR EVALUATION
/ OF DRILLING METHODS
Versatility of Drilling Method
Sample Reliability
Relative Drilling Cost
Availability of Drilling Equipment
Relative Time Required for Well
Installation and Development
Ability of Drilling Technology to
Preserve Natural Conditions
Ability to Install Design Diameter
of Well
Relative Ease of Well Completion
and Development
O
i—
O
O
O.
2.
5T
_a
5"
<
so
u>
O
« °
2, m
o ®
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03 ~~ O
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Q. (Q
3 3)
CD o
-
2, 3
03 (0
3 a
o. w
CD
CO
-------
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 1.2 — Wells in Unconsolidated Formations
Normally, abandoned wells extending only into consoli-
dated 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, disin-
fected sand or gravel may also be used as fill material opposite
the waterbearing formation. The remainder of the well, espe-
cially 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 may be used for fill material through the water-
producing horizon if limited vertical movement of water in the
formation will not 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 to20 ft (3.0 to 6.1 m) below and apoint 10 to 20ft (3.0 to6.1
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 1.4 —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
cenent, 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 advis-
able.
Section 1.5 —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 surfaces
or to the annular space outside the casing, it is recommended
that pressure cementing, using the minimum quantity of water
that will permit handling, be used. The use of pressure mudding
instead of this process is sometimes permissible.
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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 suit-
able to the geologic conditions can then be used.
Section 1.7— Seating 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 mis 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 in-
terstices. 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 for-
mations, 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, penetra-
tion, 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.
Reference
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.
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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.
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 hydro-
gen 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 bore-
hole and the casing of the well. Annular sealants should be
impermeable and resistant to chemical or physical deteriora-
tion.
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 contain-
ing 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).
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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 top 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
water 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 bio-
logical 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).
Borehole
A hole drilled or bored into the earth, usually for explor-
atory 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 acceler-
ate 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 log-
ging 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.
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 en-
trance into the well. Surface or temporary casing means a
temporary casing placed in soft, sandy or caving surface forma-
tion to prevent the borehole from caving during drilling. Pro-
tective 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.
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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-jointcasing (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.
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 facili tales 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 condi-
tion 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 crev-
ices or by infiltration into a porous media.
Clay
A plastic, soft, variously colored earth, commonly a hy-
drous 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.
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.
Confined Bed
The relatively impermeable formation immediately over-
lying or underlying a confined aquifer.
Contaminant
Any physical, chemical, biological or radiological sub-
stance or matter in water that has an adverse impact
Contamination
Contamination is the introduction into ground water of any
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chemical material, organic material, live organism or radioac-
tive material that will adversely affect the quali ty 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 trian-
gular-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 borehole. Such a sample preserves strati-
graphic contacts and structural features (United States Envi-
ronmental 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).
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-Pius 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 bore-
hole.
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 permeabil-
ity 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).
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 me-
chanical mixing and molecular diffusion.
Dissociation
The splitting up of a compound or element in to 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.
Down gradient
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 contami-
nants from a regulated unit. Regulations require the installation
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contaminant migration (United States Environmental Protec-
tion 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.
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 opera-
tion 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.
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 forma-
tion materials and the filter pack that typically refers to either
the average grain size (D50) or the 70-percent (D70) 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.
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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.
Flow-Through Well
The installation of a small-diameter well intake that pen-
etrates 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 unconsoli-
dated material characterized by a degree of lithologic homo-
geneity.
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 paniculate 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).
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.
Halogenated 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.
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Heaving Sand
Saturated sands encountered during drilling where the
hydrostatic pressure of the formation is greater than the bore-
hole 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).
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 ex-
pressed 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 bio-
logical reactions.
Induction Tool
A geophysical logging tool used to measure pore fluid
conductivity.
Inhibitor (Mud)
Substances generally regarded as drilling mud contami-
nants, 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).
Ketones
Class of organic compounds where the carbonyl group is
bonded to two alkyl 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 rieutron source is lowered
down the borehole, followed by a recorder, to determine mois-
ture 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
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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.
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 Environ-
mental 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 forma-
tions, 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 forma-
tion materials are allowed to collapse around the well intake and
fine formation materials are removed using standard develop-
ment 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.
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.
O-Ring Seal
A rubber seal emplaced between the threaded connections
of hollow-stem auger sections to prevent leakage and infiltra-
tion of fluids.
Overbank Deposits
Fine-grained sediment (silt and clay) deposited from sus-
pension 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
(Ingersoll-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 imper-
meable 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.
Polumeric 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 consoli-
dated 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.
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 degreeof 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.
Radioactive Logging
A logging process whereby a radioactive source is lowered
down a borehole to determine formation characteristics. Ra-
dioactive logging devices typically used for ground-water
investigations include gamma and neutron logging probes.
Radius of Influence (Cone of Depression)
The radial distance from thecenter of a well under pumping
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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 concur-
rently with a return flow of water. The water is pumped to the
surface through the casing.
In dual-wall reverse circulation rotary drilling, the circu-
lating 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 hydrogeochemical analysis.
Saturated Zone (Phreatic Zone)
The subsurface zone in which all pore spaces are filled with
water.
Scheduling
Standardization of casing diameters and wall thicknesses
where wall thickness increases as the scheduling number in-
creases.
Screen
See Well Intake.
Seal
The impermeable material, such as cement grout, bento-
nite or pudded clay, placed in the annular space between the
borehole wall and the permanent casing to prevent thedownhole
movement of surface water or the vertical mixing of water-
bearing zones.
Segregation
The differential settling of filterpackorothermaterials 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 sys-
tem 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 Envi-
ronmental 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.
Slotted Well Casing
Well intakes that are fabricated by cutting slots of prede-
termined width at regular intervals by machining tools.
Slug Test
A single well test to determine the in-situ hydraulic con-
ductivity of typically low-permeability formations by the in-
stantaneous addition or removal of a known quantity (slug) of
218
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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 par-
ticles (Driscoll, 1986).
Smectite
A commonly used name for clay minerals that exhibit high
swelling properties and a high cation exchange capacity.
Sodium Bentonite
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 sur-
face 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.
A type of clay added to drilling fluids to increase viscosity. Surfactant
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
m3 /day/m, and that varies with the duration of discharge
(Driscoll, 1986).
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 sample of the formation.
Spudding Beam
See Walking Beam.
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 (Driscoll, 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).
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 with-
out 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).
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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, transmissiv-
ity 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 grout-
ing 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 Protec-
tion Agency, 1975).
Turbidity
Solids and organic matter suspended in water.
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 perforned.
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 notaffected
by theregulatedfacility(UnitedStatesEnvironmental 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 Unconsolidated deposits.
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.
Water Swivel
See Swivel, Water.
Water Table
The upper surface in an unconfmed ground water body at
which the pressureis atmospheric (United StatesEnvironmental
Protection Agency, 1975).
Weight
Reference to the density of a drilling fluid. This is normally
expressed in either lb/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, arti-
ficial recharge, or acquisition of ground water or for conducting
220
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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 Pro-
tection 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 re-
stored; 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 forma-
tion.
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.
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 inter-
stices are filled with ground water.
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 StatesEnvironmental 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.
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.
221
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