EPA 510-K-92-813
FREE-PRODUCT RELEASE DETECTION FOR
• UNDERGROUND STORAGE TANK SYSTEMS
VOLUME 1
CAPABILITIES AND LIMITATIONS OF WELLS FOR
DETECTING AND MONITORING PRODUCT RELEASES
PREPARED FOR:
U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF UNDERGROUND STORAGE TANKS
WASHINGTON, D.C.
SUBMITTED TO:
MIDWEST RESEARCH INSTITUTE
FALLS CHURCH. VIRGINIA 22041
EPA CONTRACT NO. 68-01-7383
FEBRUARY 1988
^•^GERAGHTY
& MILLER, INC.
Ground- Water Consultants
Recycled/Recyclable
Printed on paper that contai
at least 50% recycled fiber
JO, X\ Printed on paper that contain:
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ACKNOWLEDGMENTS
Funding for this project was provided under EPA Contract
68-01-7383, Work Assignment 5 (Subtask 6), administered by
EPA's Office of Underground Storage Tanks (OUST). The
Technical Project Monitor for OUST was Mr. Thomas M. Young.
This document was prepared by Geraghty & Miller, Inc., (G&M)
for the EPA prime contractor, Midwest Research Institute
(MRI). Mr. Don A. Lundy served as Project and Program
Manager for G&M and Mr. Jeffrey S. Mahan was Officer-in-
Charge. They were assisted by Messrs. Tony Gogel, James
Gibb, Randy Senn, and Andrew Fuston. We gratefully
acknowledge the critical review and constructive comments by
Ms. Cecily Beall and Mr. H. Kendall Wilcox at MRI, and by Mr.
Young at EPA/OUST.
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TABLE OF CONTENTS
1. 0 INTRODUCTION 1
2 . 0 MONITOR-WELL CAPABILITIES/LIMITATIONS 4
2.1 Good Conditions/Capabilities of Wells 4
2.2 Poor Conditions Limiting Use of
Monitoring Wells 11
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1.0 INTRODUCTION
Free product detection with monitoring wells is one of
several methods of release detection that may be allowed
under the new U.S. Environmental Protection Agency (EPA)
regulations of underground storage tanks. Although several
other guidance documents are available concerning the
application of monitor wells for detecting and monitoring
ground-water contamination at RCRA and CERCLA sites, there is
little or no available information on the use of wells to
detect/monitor separate organic liquids, specifically
1ighter-than-water petroleum products.
The purpose of this document is to describe the capabil-
ities and limitations of free-product monitoring with wells.
Volume 1 deals primarily with the limitations, rather than
capabilities, of wells for detecting releases. Volume 2
presents a brief survey of the Florida UST program and how
wells are being used successfully to detect and monitor
releases. In developing both volumes, EPA's intent is to
encourage owner/operators and qualified professionals
involved with well design and installation to predict,
recognize, and avoid any potential problems associated with
using wells.
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The need to generate such information for owner/
operators and regulators has resulted in the preparation of
several new documents under the auspices of the Office of
Underground Storage Tanks (OUST). This particular document
is directed towards both technical and non-technical person-
nel in government or industry who will be making decisions
about the use of wells to detect and monitor free product.
To reach the widest audience and limit the use of technical
jargon, the writers have chosen to present the information in
a brief outline form with accompanying figures.
Those readers who are interested in more technical
information about well design and placement, mechanics of
product migration through the subsurface, interpretation of
free-product accumulation in wells, and practical aspects of
the recovery of free product from wells are referred to the
technical background document entitled "Corrective Action for
Petroleum Underground Storage Tanks," especially Chapter 3
and Appendix A, entitled "Technical Issues in Free Product
Monitoring and Removal." Most of the capabilities/limita-
tions discussed here are covered in that document in greater
detail. Those readers seeking help with how to recognize and
deal with these problems should consult the background
document.
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Some of the technical terminology discussed in great
detail in the technical background document and associated
appendices is used here. A petroleum product is defined as
liquid phase hydrocarbon that may range from unrefined crude
oil to highly refined gasoline. Free product is product that
saturates the voids of an earth material and will flow in
response to gravity. Residual product is product occupying a
small fraction of the void space which will not move in
response to gravity. Only free product can accumulate in
wells and other subsurface structures open to the atmosphere.
Residual product may exist in a pendular state of saturation
with air and water above the water table, or in an insular
state with water below the water table. The transfer of
product from one state to another with a moving water table
is discussed in Appendix A to the technical background
document.
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2.0 MONITOR WELL CAPABILITIES/LIMITATIONS
Table 1 is a list of site conditions under which monitor
wells are generally capable or significantly limited in
either detecting or monitoring product releases.
2.1 Good Conditions/Capabilities of Wells
Under some site conditions, the ideal environment for
release detection with a well is within an engineered
backfill around the UST system. The backfill should be
clean, well-sorted, coarse-grained sand or fine-grained
gravel. As shown in Figure 1, two other conditions are
needed to insure that the released product does not escape
into the adjoining earth materials before it can be detected
in the well. First, the excavation should either be
installed into low-permeability earth materials, or it should
be lined to seal off any escape pathways. Second, it is
highly beneficial if there is a shallow water table within
and near the base of the excavation on which the free product
can accumulate and spread. Figure 2 shows that installation
of a well through the base of the excavation to reach a
deeper water table is ill-advised. Even though this design
effectively provides a way to detect releases, it also
provides a pathway for contaminants to enter the local
ground-water system.
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TABLE 1.
CONDITIONS THAT AFFECT MONITOR-WELL
CAPABILITIES AND LIMITATIONS
Good Conditions for Monitoring — Capabilities
UST backfill materials
Granular earth materials outside the UST backfill
Absence of, or consideration for, perching horizons
. Knowledge of water-table depth and fluctuations
Poor Conditions for Monitoring — Limitations
Site hydrogeology
- Undefined fracture pattern and solution cavities
- Unknown perching horizon or confining unit
- Excessively deep or shallow water table
- Buried utilities, fill, or drainage control
structures
Well construction
- Well production zone placement relative to water
table
- Well screen design flaws
- Well casing materials and miscellaneous design
problems
Well location
- Consideration for other contaminant sources
- Spacing and proximity to USTs
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Pavement
Monitoring
Well
Tank Fill Port
Water Table ^ZTZJTn
Wells Can Readily Detect Free Product in The UST Excavation
Zone When: a." Native Materials Are Less Permeable Than
Backfill, b. A Shallow Water Table Exists Within Backfill
Water Table
Clay
Free Product
Residual Product
Backfill
Geraghty & Miller, Inc.
Figure 1
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Monitoring
Pavement Well Tank Fin Port
Water Tkble
Wells Completed Through the Excavation Zone Will Allow Free
Product Detection at the Expense of Aquifer Contamination
Water Table
Y/S,
Clay
Free- Product
Residual Product
Im Backfill
Geraghty .& Miller, Inc.
Figure 2
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When properly designed and installed outside the UST
excavation zone, monitor wells are applicable to free product
detection in granular earth materials such as sands, gravels,
silts, and some clays (Figure 3). The optimum hydrogeologic
condition is the presence of a shallow water table less than
20 feet deep in fairly permeable earth materials that exhibit
small variations in grain size and texture across the site.
A knowledge of local geology and hydrology can be very
helpful in anticipating these conditions and can often be
obtained from government publications.
Monitor wells are most directly applicable where there
are no laterally continuous, virtually impermeable zones
between the leak source and the water table to prevent free
product from reaching the regional water table (Figure 4) .
Special consideration must be given to monitor-well construc-
tion in areas where free product is perched on a low-per-
meability layer above the regional water table. For example,
consider a well screened as shown in Figure 5a, the purpose
of which is to detect free product that might have accumu-
lated on the regional water table. If the grout seal is
emplaced above the low-permeability layer, free product can
migrate downward through the gravel pack until it reaches the
regional water table.
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Storage Tank f£S::.;
Monitor Wells Can Be Used to Detect Free Product in
(a) Coarse Grained, and
(b) Fine Grained Granular Earth Materials.
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Residual Product (33 Silt and Clay
Free Product g| Sand And Gravel
Geraghty & Miller, Inc.
Figure 3
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Monitoring for Free Product is Feasible When Migration of
Free Product to the Water Table is Not Impeded
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Clay
Free Product
Residual Product
Sand And Gravel
Geraghty & Miller, Inc.
Figure 4
10
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Installation of two vertically nested monitor wells is
recommended in areas where fif-ee product is anticipated to
have accumulated above a low-permeability zone. One well
would be screened to detect free product associated with this
layer (Figure 5b) . A second well would be completed through
the layer and screened to monitor the regional water table
(Figure 5c) . In the latter case it is recommended that
surface casing be set and cemented in place prior to install-
ing the monitor well to preclude migration of free product
via the annular space. Detailed description of materials
encountered during drilling (e.g., use of continuous split
spooning) allows identification of low-permeability layers
and associated high-product saturation zones.
A properly constructed monitor well has a screened
interval that extends over the entire range of expected
water-table fluctuation. This is done to assure continuous
free product detection in the well (Figure 6). Defining this
range of fluctuation can be based on long-term hydrographs
from wells in the same aquifer and area and sometimes on the
basis of soil morphology and color.
2.2 Poor Conditions Limiting Use of Monitoring Wells
2.2.1 Hydrolocric Considerations
The use of individual monitor wells is not always
reliable for detecting free hydrocarbons in fractured rock,
11
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4-Surface Casing
Cemented
in Place
Wells Must Be Properly Constructed to Monitor Perched Free
Product Accumulations and Prevent Contamination of the Aquifer
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Glay
Free Product y/. Residual Product
Geraghty & Miller, Inc.
Figure 5
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. Range of
Water Tat>le Fluctuations
The Well Screen Should Be Positioned to Coincide with the
Expected Range of Water Table Fluctuations
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Residual-Insular
Product
Free Product
Geraghty & Miller, Inc.
Residual -Pendular
Product
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Figure 6
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karst, some metamorphic, and igneous terrains (Figure 7). If
the well does not intercept a fracture or solution zone
containing free product, no free product will be detected by
the well. Monitor wells screened below laterally continuous,
low-permeability layers would not normally be applicable for
monitoring free product. Free product floating on the layer
would not migrate to the screened interval if the well is
properly constructed (Figure 8).
The water table in some regions can be too deep to allow
detection of free product using monitoring wells (Figure 9).
Depending upon the volume of the spill or leak, the product
can be absorbed on earth materials above the water table in a
residual pendular saturation. Monitor wells completed and
screened in these contaminated materials will not detect free
product, but are capable of monitoring vapors. Wells
completed in the ground-water zone would not detect separate
liquid product, but may ultimately allow detection of
dissolved materials. Free product will most likely accumu-
late in monitor wells installed in areas with shallow water
tables and permeable earth materials.
In other regions, the water table can be too shallow to
allow detection of free product while maintaining typical
surface seal requirements around the well (Figure 10) . A
monitor well properly constructed with a surface seal would
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'w///r////////y////tn^^^
Note: No Free Product in Well
Fractured Rock
. Karstic Limestone
Wells That Do Not Intersect Fractures and Cavities Containing
Free Product Will Not Allow Free Product Detection
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Limestone
^ Free Product
Geraghty & Miller, Inc.
Figure 7
15
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Free Product Accumulated Above a Low Permeability Zone
Cannot Enter a Well Screened Below the Zone
Grout
Bentonite or
Grout Seal
Table
Gravel Pack
Clay
Free Product
Sand and Gravel
Geraghty & Miller, Inc.
Figure 8
16
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» \
Storage
Tank I
Product at Residual Saturation
Water Table
Wells Cannot Detect Product (as a Separate Liquid Phase) That
Does Not Accumulate Near the Water Table
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
17
Figure 9
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Product/Water Contact
Free Product May Be Sealed Off from a Well by the Surface
Grout Seal, Especially When the Water Table is Shallow.
Free Product
Bentonite or
Grout Seal
Geraghty & Miller, Inc.
Water Table
Gravel Pack
18
Figure 10
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prevent floating .product from entering the well through the
screen if the water table is within a foot or two of the land
surface. Attempts to construct the well to accommodate the
shallow conditions could result in surface contamination
entering the well.
After free product is discovered in a well, water-table
changes can mask the presence of product by trapping it below
the water table in an insular state of saturation (Figure
11) . Free product accumulations in wells usually decrease
with a significant rise in the water table and increase with
a decline in the water table. The sudden appearance of free
product in a well may not be due to a new release, but
rather, the transfer of product from one saturation state to
another in response to a falling water table. After initial
detection, fluid levels in a monitoring well can be measured
during different seasons to define the range of water-table
and product-thickness variations.
Utility lines, pipelines, and buried facilities (sewers,
water lines, drainage tiles, old trenches, etc.) and con-
struction fill can serve as preferential pathways for product
migration, making wells less effective as detection devices
(Figure 12) . These pathways can alter expected directions
and rates of free product movement. All underground lines
should be located as accurately as possible to account for
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a. Before Rise in Water Table
b. After Rise in Water Table
Free Product May Disappear During Rising Water Table
Conditions and Accumulate During Falling Water Table
Conditions
Grout
Bentonite or
Grout Seal
m
Water Table
Gravel Pack
Residual Product
Free Product
Geraghty & Miller, Inc.
20
Figure 11
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Direction of
Ground-Water Flow
'^^Z&SS^^^ •-TJ.TJ.TJL-U.-j. -•-
Preferential Flow of Product Through Buried Utilities Prevents
Detection With Wells.
• Monitoring Well
Free Product
Sewer Line
TILTH Water Line
Geraghty & Miller, Inc.
21
Figure 12
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this condition. Product that seeps into an underdrain
beneath a building can be detected and monitored by complet-
ing a well in the drain rock along the outside wall of a
building (Figure 13).
2.2.2 Well Construction Considerations
Free product will not be effectively detected if the
water table is significantly below or above the screened
interval (Figure 14 a,b) . Care must be taken in the selec-
tion of the proper screen/gravel pack depth interval.
Accurate measurement of the product thickness cannot be made
in the well if the screen length is not sufficient to
intercept the product/water interface (Figure 15). When the
screen/gravel pack interval is sufficiently long to cover the
entire product plume, the free product thickness inside the
well will exceed the free-product thickness outside the well.
Some petroleum products may not migrate through a screen
with slot openings that are too small (Figure 16).. Some
screen materials are preferentially wet to water in the
presence of free product. The interfacial capillary tension
that exists between the water and the product must be
overcome before free product can flow through the narrow slot
openings. Also, slot openings that are too narrow may
22
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First Floor
Monitoring Well Installation into Footing Drain
of a Building
F-'r- Grout -5
»$? bentomte or 3
^ Grout Seal st
£- Water Table W
g Drain Rock ^
^ Native Soils
^. Free Product ^
"
g Backfill
Geraghty & Miller, Inc.
23
Figure 13
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Note: Well Contains
Only Water
Incorrect Installation of Well Screen Below (a) or Above (b)
the Water Table Can Prevent Free Product Detection
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
24
Figure 14
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Product/Water Contact
The Well Screen Interval Must Extend Below the Product/Water
Contact in Order to Estimate the Product Thickness
Bentonite or
Grout Seal
Gravel Pack
%,, Free Product
Grout
Geraghty & Miller, Inc.
25
Figure 15
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Narrowly Slotted Well Screens that are Preferentially
Water Wet May Not Allow Free Product Entry
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
26
Figure 16
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prevent more viscous petroleum products from moving into the
well.
An open-ended, unscreened well restricts the entry of
free product. Free product may not be detected even though
it is present outside the well (Figure 17) . Also/ an open-
ended well will tend to allow fine-grained materials to enter
the casing, making further monitoring impractical as the well
fills with sediment.
Improper well-construction materials can affect the
viability of a monitoring well. For example, wells con-
structed with sections of loosely joined concrete culvert
will not allow detection of free product if the product is in
contact with a solid portion of the culvert (Figure 18a).
Well casing and screen constructed of materials that are
incompatible with free product can lose rigidity and col-
lapse, thereby ruining the well (Figure 18b). ABS plastic
pipe is an example of a material that is not compatible with
petroleum products. Generally, PVC, Teflon, and stainless
steel are adequate for monitoring-well materials. PVC is the
lowest cost material and is widely used for monitoring free-
product spills or leaks. Because PVC may swell in the
presence of free product, screen slot sizes may need to be
larger than in more stable materials.
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Free Product Layer
n
Product/Water Contact
Wells Open Only at the Bottom of an Unslotted Casing
are Not Suited for Free Product Detection
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
28
Figure 17
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ABS Plastic Pipe
• Well materials will disolve
Concrete Culvert
Selection of Improper Well Casing and Screen Materials Can
Prevent Free Product Detection
Grout
Bentonite or
Grout Seal
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
29
Figure 18
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Improper sealing of the near-surface annular space can
allow contaminated surface water or spilled petroleum product
to enter the gravel pack around the well (Figure 19) . This
can cause contamination of ground water and may lead to a
false alarm of a tank or piping leak. The materials used to
seal the top of the well generally consist of bentonite or a
cement grout.
2.2.3 Well-Location Considerations
The influence of nearby potential contaminant sources,
especially other UST systems, must be considered to properly
assess a site (Figure 20) . Monitoring wells should be
installed in areas which will allow detection of contamina-
tion migrating onto the property from other sources located
upgradient.
The proximity of monitor wells to a leak source must be
considered (Figure 21) . Wells might be located improperly to
detect a plume of liquid hydrocarbons from a leaking under-
ground storage tank near the property boundary. In this
instance, upon determination of the direction of ground-water
flow, additional wells should be located to detect releases
and/or to delineate the plume.
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Spilled Petroleum Product or
« Contaminated Water
Free Product Layer
Product/Water Contact
Water Table
Monitor Wells Should Be Sealed to Prevent Entry of Surface
Spills and Eliminate this Cause of "False Positives"
of UST Releases.
Water Table
Gravel Pack
Free Product
Geraghty & Miller, Inc.
31
Figure 19
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r
Direction of
Ground-Water Flow
Lack of Upgradient Well Implicates Station B
Monitor Wells Should Be Located
To Allow Detection of Contamination from Other Sources
• Monitoring Well
H Product Plume
Geraghty & Miller, Inc.
32
Figure 20
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Dissolved and Free
Product Plume
Direction of
Ground-Water Flow
Widely Spaced Monitor Wells May Not Detect an UST Release
• Monitoring Well
^ Product Plume
Geraghty & Miller, Inc.
33
Figure 21
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