Environmental Protection
Agency
Underground Storage Tanks
_.,».._,,,, ,CTi ua, ,vie,-i,iw! Ing
Systems Laboratory
P.O. Box 93478
Us Vegas NV 89193-3478
February 1993
&EPA Tank Issues
Site Characterization for
External Leak Monitoring
A series of informative articles of
interest to tank ownan and consult-
ants concerned witfi management
of underground tanlts tor storage of
fuel.
Printed on Recycled Paper
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Introduction
Before designing an external monitoring system, information is needed about the natural
and man-made characteristics of the site. Site characteristics are important in selecting the
appropriate monitoring method. This paper discusses site characteristics with respect to
external monitoring by iiquid product monitoring and vapor monitoring.
Hydrologic Site
Characteristics
Water Table fluctuates in response to:
/; seasonal variations in recharge and
discharge
DEPTH TO THE WATER TABLE—The water table, also called the free surface, is the level
at which the liquid \k at atmospheric pressure. Liquids can enter wells that are perforated
below the fre« surface. The depth to the water table at a given time is thus relatively simple
to determine by measuring the water level in a well, The position of the water table and
fluctuations of the water table are the major factors in selection of the monitoring method..
The position of the water table is important in selecting a liquid product monitor or a vapor
monitor in design and construction of monitoring wells, and in placement of hydrocarbon
monitors. Although the depth to the water table is relatively easy to determine at any
specific time the range of fluctuations of the water table that will occur during the time that
the monitoring network is to be in operation is not so readily determined nor is it easily
predicted Natural fluctuations of the water table occur in response to seasonal cycles of
recharge such as by infiltration of rainfall and by changes in water stage of nearby lakes
and streams. Fluctuations of water level may also be caused by withdrawal of water by
nearby wells.
Depth
to
Water
Spring Summer
Fall
Winter
2) long term variations in recharge and
discharge
12 3 4 5 87
3) nearby ground-water withdrawal
Depth
to
Water
Pump On Pump OM
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4) changes in water level In nearby streams
and lakes
High Stage
Normal Stage
Low stage
The range of fluctuations at an underground storage tank site may be estimated from
the water-level measurements taken periodically over a long period of time In re-
sponse to natural factors that cause water table fluctuations, there is an annual cycle of
fluctuations. Water levels generally reach their highest levels in the year during the late
spring following or near the end of the annual period of recharge. The period of re-
charge is when there is an excess of water for infiltration from the surface to the water
table and when the stream and lake levels are high. Water levels decline as the
potential for evaporation and transpiration increases. In the late fall or early winter,
whon evaporation and transpiration are tew and precipitation increases, water levels
begin to rise. The annual cycle is similar in many parts of the country, but the seasonal
timing of climatic factors affects regional differences in timing of the annual cycle. Water
levals also respond to tang term periods of drought and above normal precipitation and
betow-normal temperature. Thus the seasonal cycles are superimposed on long term
water level changes. In urban areas, water level fluctuations may be influenced by
activities such as recharge from storm runoff and lawn watering.
Water levels measured monthly or recorded continuously over a period of several years
may provide a basis for an estimate of the range of water-level fluctuations. Such a long
record of water levels before establishing a tank monitoring system would be practicable
in few, if any cases. In addition to measurement of water levels at the site, the files of
State and Federal water agencies may provide water-level records in nearby wells in
the shallow water-table aquifer, or in shallow wells in similar hydrogeologic and climatic
saltings. A hydrogeotogist could estimate the probable range of water table fluctuations
on the basis of such records and site information including water levels at the site and
neamy production wells that may cause drawdown at the storage tank installation, local
hydrogeologic conditions, such as the potential for recharge and discharge from the
aquifer, and the interconnection between the aquifer and nearby streams and lakes.
DIRECTION OFGROUND-WATER FLOW-The natural water table gradient genially
slopes from high to tew topographic surfaces — commonly toward streams and lakes
During flood stages the gradient may be reversed. Changes in direction of ground water
flow may also be caused by noarby pumping wells.
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Pumping
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Sites that are contaminate by previous leaks or large surface spills mav not bo
amenaWeto Jiquid product monitoring because of hfgh bac^Jnd cement of
hydrocarbon and the difficulty of discriminating between prodW o?n~
and the product of old teaks or surface spite, The backfill "material
monrtprs are located must be relatively permeable to allow the
S? S * °nI0fL0flh8 W3ter tabte- ad tank ""***». may d
monrtor because of the bw permeability of the backfill or the presence ofhfch
content of residual hydrocarbons from previous spills or leaks
* fine-9fained sand. sit. and clay may be of such low
. ati°n °f hydrocarbons to *• ^er table is great*
fOVe^ZrofS*Urati0n'fin^raioedmaterNI may hold much of
i released in the saturated zone migrates very slowty toward the water tab/a
•n f me-grained material or may be trapped betew the water tabte
Vapor Monitoring
- Most suitable in soils with tow moisture content,
highly permeable backfill, and where
background hydrocarbon content is low or
negligible
• Must be located above water table
• Commonly not used where the water table is
within the tank excavation zone
contaminatad ^ Pilous leaks or surface spills may
similar problems that occur in liquid product monitoring
hydrocarbon
or old leaks. The backfill material in whfch'tfwTvaoor
montors are located must b» relatively permeable to allow the hydTocWn
vapors tomgrate rapidly by diffusion. Old tank installations maj toSta. to
montor because of the tew gas permeability of the backfill or the ^Sof
high content of residual hydrocarbons from previous spills or leak*
h b""qf;>u*
of hydrocarbon vapors is impeded or confined. Under such
^1^ to ^ dat8ct^- High moisture content
rm«ab,lrty and the rate of diffusion of hydrocarbon
.
the watertabte all the soil pores are saturated with water. Vapor
rtwaan a leak a^ its
h» P"*1^ *°UW have
* Withi° *•*' Z
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References
Johnson, Richard L, McCarthy, Kathleen A., Perratt, Matthew and Hinman, Nancy,
1989. Direct comparison of vapor-, free-product- and aqueous-phase monitoring for
gasoline leaks from underground storage systems: Report of Oregon Graduate
Center, Beaverton, Oregon, 11 p. Also in: National Water Well Association, Houston
Conference Proceedings, 1989.
Schroiber, Robert, Levy, Benjamin, Rosenberg, Myron, 1988, Modeling vapor phase
movement in relation to UST leak detection - Phase 1: Final Report, Environmental
Monterinp Systems Laboratory, Office of Research and Development, U.S. Environ-
mental Protection Agency, Las Vegas, Nevada, 81 p.
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
i •
Tank Issues* are short articles of information on the current state-of-the-art on
management of underground fuel tanks. These articles provide recommendations
but are not regulations; neither the U.S. Environmental Protection Agency nor the
Environmental Research Center, University of Nevada, Las Vegas may be held
responsible for consequences of following recommendations in these articles. All
appropriate state, local, and federal regulations should ba followed in installation and
operation of leak detection devices and in management of underground storage
tanks.
Harry Reid Center for Environmental Studies
University of Nevada, Las Vegas
4505 Maryland Partway
Las Vegas. NV 89154
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business . - •
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EPA
PERMIT No. G-35
EPA/600/9-90/046
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