United States
Envtonnwntal Prataction
Agency
Underground Storage Tanks
Envjronmontal Monitoring
Systems Laboratory
P.O. Box 93478
LasVogas NV 89193-3478
March 1993
Tank Issues
Design and Placement
of Flpatjng Liquid
Monitoring Wells
A series of infcrmativo articles of
interest to tank omen and contort-
ants concerned with management
of underground tonka for storage of
fuel.
Printed on Recycled Paper
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Introduction
Migration of
Liquid Product
From
Underground
Leaks
- Uquld product monitors are useful
devices for measurement of leaks
- Liquid product monitors detect liquid
hydrocarbons floating oh the water
table
- Liquid product monitors detect leaks,
not leak rates
Liquid product monitoring js the predominant method of external leak detection where the
water table is within the zone of excavation. This paper discusses the use of liquid product
monitors at new and old tank installations for detecting leaks from underground hydrocar-
bon storage tanks. This paper discusses the site conditions under which liquid product
monitors can ba effectively used, conditions which may mitigate or prevent the effective use
of liquid product monitors, and the construction and placement of liquid product monitoring
wells. Liquid product monitors are not used to determine the rate of tank leak. The rate of
tank leak can be determined by other methods such as inventory or internal monitoring
methods. Effective use of liquid product monitors or any other method of leak detection
requires training and experience on the part of the user.
Natural and
Engineered
Site Conditions
for Liquid
Product
Monitors
- Liquid monitors are effective where
water table is loss than about 20 feet
betow grade arid greater than 2 to 5
feet below land surface •
• BaddM should be coarse-grained,
permeable material
• Liquid monitors detect product on the
water table
Fuel hydrocarbons leaking from an underground
storage tank are initially released as the product
liquid. The liquid product tends to migrate toward
the water table. If the product is released in the
vadose zone, it begins to infiltrate downward to
the water table and to partition between the
vapor, aqueous, and solid phases. As the liquid
product noars the water table, the plume of liquid
spreads out, initially entering the capillary zone
above the water table, then when the capillary
zone reaches capacity, the excess liquid
becomes part of the saturated zone. Fuel
leaking directly into the saturated zone from a
storage tank will tend to rise to the water table.
Fuel hydrocarbons in the saturated zone (below
the free surface or level of atmospheric pres-
sure) are free; to enter monitoring wells perfo-
rated at the water table. Floating product is
readily detected by specialized instruments.
Liquid monitors are relatively easy to install and
are suitable devices for monitoring underground
storage tanks in many situations.
Aleak above the water table infil-
trates downward to the capillary fringe \
and water table and migrates
downgradient with ground water. A
leak below the water table tends to
rise to the water table and move
downgradient with the ground water.
Liquid product monitoring is effective where the water level is below the bottom of the
tank excavation if the native material is coarse grained and very permeable, if the native
material is coarse grained and very permeable, the depth to water should not be greater
than about 20 feet below grade. At greater depths, the tirrw of percolation of product to
the water table is considered too long to provide timely detection of a leaking tank.
Because the well should be sealed to a depth of 2 feet or more to prevent infiltration of
liquids from the surface, it is not possible to detect liquid product within this depth of
water table.
Sites that aro contaminated by previous leaks or large surface spills may not be ame-
nable to liquid monitoring because of high background content of hydrocarbon and the
difficulty of detecting new leaks and discriminating between new leaks and surface spills.j
Liquid product monitoring wells can be located upgradient from the tank installation to
detect liquidjproduct migrating from off site. The backfill material in which the monitors
are located must be relatively permeable to allow the hydrocarbon to spread rapidly on
top of the water table. Old tank installations may be difficult to monitor because of both
or either the! low permeability of the backfill or the presence of high content of residual
hydrocarbons from previous spills or leaks.
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. _
SURFACE SPILLS AND ^FF S.TE CONTAM.NATION-Background levels of hydr
im m^8laSLa"d '*"" Product f™ °« - «~ ^y affect the Z
no mefood, the su^abHrty of vanous monitors, and the location of monitors, ft is impor-
ter^, taovy the ex.st.ng hydrocarbon levels at a new or existing site before the Sod
SSSSh"-^"1? °« IN instru™"'ati°" * installed. .n addition, the
detection of hydrocarbons from off site and the discrimination of surface spills from tank
leaks should be a factor in the design and selection of monitors for the art £1
£££££ overfil!ing If tank durin9 lkiuld transfer <*^™
Jl!S^*r^!^J-¥il S8nSOrS> dfy ""P""93 On transfsr lin«> *Pill
containment devices, and barriers to product migration around fill pipe manways.
s of secondary containment have been
b "^erground storage tank systems. Secondary containment refers to »ma,n.
mem of hydrocarbons that have leaked from an underground storage tank to pTevert
miration of th. product beyond the near-tank area before remedial Sn Sen
Secondary containment can be afforded by double-walled tanks and produces *
"" *«
in
Impermeable Uner
A ctonbto-wailed tank is a tank within a tank, with a very small space separating the two
tanks. Doubte-walled tanks can be monitored within the annularWbetween the^o
tanks and monrtonng beyond tha outside tank is generally not considered necessary.
Synthetic membrane liners consist of a rubber or polymer mat used to line the tank '
excavator,. Tanks and backfill are placed within the lined excavation. Liquid product
monrtonng wells are generally placed in the lined excavation to provide leak detection
Ltam are susceptible to chemical and physical degradation an^should not be conTd-
Str£ * \?*S?^ P"**"* teakage-Althouflh *•UMful lif« <*linere ™y
notb. tax,wn the We should be conwrvatlvelx estimated and other types of monitoring
should be implemented beyond the estimated useful life of the liner.
Some states require that, under certain conditions, underground storage tanks be
SS?'"?"*** VaUltS- BSCaUS* C°nCrate is "^ imf»™eable to liquid product
migratton and vapor transport, the vault should be lined or coated
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INSTALLATION OF EXTERNAL MONITORS AT NEW SITE VERSUS OLD SITE—
The successful operation of external monitors is greatly dependent upon the proper
selection and installation of monitors. The proper selection and installation is, in
turn, almost wholly dependent upon the natural and man-made characteristics of
the she. Site characteristics are most readily perceived and to a great extent can be
controlled during the construction and installation of a tank system.
Liquid Product
Versus Vapor
Monitoring
Fuel hydrocarbons leaking from an underground storage tank are commonly
released as the liquid product. The liquid product volatilizes in the soil air and the
gas migrates through the unsaturated zone by diffusion. The hydrocarbon gases in
the soil can be detected by vapor monitors. The liquid product tends to migrate to
the water table where, when the voids aro filled to capacity, the liquid product can
be detected in wells screened at the water table. *
Liquid Product
Monitoring
Depth to water should be leas than about 20
feet
Liquid product monitoring is useful when
seconduy containment is employed
Liquid product monitoring is not effective whisire the water level is below the bottom
of the tank excavation unless the native material is coarse grained and very
permeable. If the native material is coarso grained and very permeable, the depth
to water should be within about 20 to 30 feet of grade. At depths below about 20
feet, depending upon the rate of percolation of product to the water table, the time
delay in the product reaching the water table is considered too great to provide
timely detection of a leaking tank. As an aft amative, if the depth to water is greater
than 20 or 30 feet, the trench could be lined to provide secondary containment of
leaked product. If the water table is at shallow depths, less than 2 to 5 feet below
the surface, it is not possible to monitor liquid floating product where the water
table is above the perforated portion of the well casing. Because the well should be
sealed to below the frost line to prevent infiltration of liquids from the surface, it is
not possible to detect liquid product within this deplh of wator table. Because water
may enter the tank in excavations with high water levels, the external leak detec-
tion system should be augmented by monitoring the water layer in the tank as a
port of Inventory control.
Water T«bl«
_„--*---
~ **
Tho vapor monitor will detect hydrocarbon gases in the soil mar collected from above
the wstor table. The liquid monitor witt detect hydrocarbons floating on the water
table.
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Fins-grained backfill such as fine-grained sand, sift, and clay may be of such tow
permeability that the migration of hydrocarbons to the water table are greatly impeded.
Above the zone of saturation, fine-grained material may hold much of the fuel in the
capillary fringe above the water table (Johnson and others, 1989). Fuel released in the
saturated zone migrates very slowly toward the water table in fine-grained material or
may be trapped below the water table.
Liquid Product
Monitoring Wells
- Install liquid product monitoring wells to
tap water table
- Perforate well casing continuously from
1 bottom to surface seal
• Install in permeable backfill of excavation
Location of
Liquid Product
Monitoring
Wells
- Locate wells in high permeabiOty backfill
- Use two wells for one tank
- Use three or more wells for multiple tanks
• Use additional wells for backup, to locate
leaks, to monitor piping systems, and to
detect product from other tank installations
Materials'—Liquid monitoring wells constructed of
PVC (pofyvinyl chloride) provided with slots for hy-
draulic connection with product on the water table is
usually the cheapest material and is quite adequate.
However, many materials have bean used in construc-
tion of wells including stainless steel, PVC coated
steel, cast iron, galvanized iron, polyethylene, polypro-
pylene, fluorocarbon resins, and Teflon. Wells are
typically from 2- to 6-inch I.D. (inside diameter) pipe.
.Suite*
SMI
WMr
J.
Installation — The casing may be installed by hydrau-
lic ram or any of several drilling methods including
auger or hydraulic rotary to a depth of several feet
below the lowest fluctuation of the water table. If a
hydraulic rotary method is used, drilling mud should
not be added to the drilling fluid or a viscosity reverting
type of drilling fluid should be used to prevent clogging
of tho perforations in the well. Wells at new installa- T
tfons can be installed by backfilling around the pipe set
in place rather than by drilling after backfilling. The casing should be perforated continu-
ously from the top of the plug in the bottom of the casing to the surface seal of the
easing. The casing is sealed to below the frost line, about 2 to 5 feet below the surface.
to prevent entry of liquids through the well bore annulus. The top of the casing should
be fitted with a waterproof cap which is capable of being locked. When the monitoring
well is installed and before it is put into service, the well should be pumped to clear the
fine-grained particles clogging the perforations. Periodic maintenance should be
performed annually by pumping of the monitoring well to assure that the perforations
are dear and will admit fluid from the saturated zone.
The number of wells needed to monitor a storage system of one or more tanks in a
singte excavation may range from two to four or more. A minimum of two wells is
recommended for a single tank ex-cavation. Three or more wells are recommended for
an excavation of two or more tanks. Multiple wells at a tank excavation would provide
backup capability in case of a well failure and might provide a means of locating the
leak based on the time of arrival of hydrocarbons.
The liquid product leaking from an underground storage tank will migrate in the direction
of ground water flow. Where tho direction of ground water f tow is known, one or more
monitoring wells should be located within the backfill of the tank excavation on the
down-gradient side of the tanks. Monitor wells should also be located on the upgradient
side of the excavation, because the direction of the ground water gradient may change
in time. Monitor wells can be loeat-«d off site and upgradient from the excavation to
detect the migration of product from off site sources. Because the native soil in the
vicinity of a tank excavation may be of tow permeability, vapor leak detection devices in
each excavation should be considered as separate and independent systems.
•U.S.Gov«nmnt Printing Office 1993—750-071/60205
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?rfe*;
Vs&s .
- Instrument each excavation separately
• Locate walls on the downgradient side
of excavations
At sn axbting tank, thara may be insufficient space
in tha excavated zone to install monitor wells
without danger of drilling into the tank. At existing
installations, liquid monitor wells can be installed
near the excavation where native soils are
sufficiently permeable.
(a) Layout of monitor wells for detecting leaks
in a single tank excavation; (b) Layout of
monitor wells for detecting leaks in a 3-tank
excavation. \
t
3
C )
c )
Reference
Johnson, Richard L. McCarthy, Kathleen A., Perrott, Matthew and
Hinman, Nancy, 1989. Direct comparison of vapor-, free-product-,
and aqueous-phase monitoring for gasoline leaks from under-
ground storage systems: Oregon Graduate Center Report,
Beaverton, Oregon, 11 p. Also in: National Water Well Association,
Houston Conference Proceedings, 1989.
Technical Editor, M. S. Bedinger
Project Officer, Katrina E. Vamer
Prepared by Harry Reid Center,
University of Nevada Las Vegas in
cooperation with U. S. Environmental Protection Agency
Tank Issues" are short articles of information on the current state-of-the-art on management of undergroundfifel tanks.
These articles provide recommendations but are not regulations; neither the U.S. Ewironmentai Protection Agency nor the
Environm-iff'^ Research Center. University of Nevada, Las Vegas may be held responsible for consequences of following
recommendations in these articles. All appropriate stata. local, and federal regulatb^ shouid be folbwed in installation and
op<»ratkxi of leak detection devices and in man^omsht of underground.storage tanfea.
Harry Reid Canter for Environmental! Studies
University of Nevada, Las Vegas
4505 Maryland Parkway
Las Vegas, NV 89154
United States
Environmental Protection Agency
Cent&r for Environment Research Information
Cincinnati, OH 45268 •
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/9-90/045
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