United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
P.O. Box 93478
Las Vegas NV 89193-3478
EPA/600/9-90/044
October 1990
Underground Storage Tanks
&EPA Tank Issues
Design and Placement
of Vapor Monitoring Wells
A series of informative articles of
interest to tank owners and consult-
ants concerned with management of
underground tanks for storage of fuel.
-------
Location of
Vapor Monitors
or Sampling
Points
- Locate vapor monitoring wells in high
permeability backfill
- Use one vapor monitoring well for each
10 to 15 foot radius
- Use closer spacing in fine-grained soils
- Use additional vapor monitoring wells for
backup and to monitor piping systems
- Use additional vapor monitoring wells to
help discriminate between leaks and
surface spills
- Instrument each excavation separately
The number of vapor monitoring wells needed to monitor a storage system of one or mors
tanks in a single excavation can range from one to four or more. Hydrocarbon vapors move
relatively freely by diffusion in coarse-grained backfill. Although diffusion of hydrocarbon
vapors will distribute the vapors in measurable amounts through the vertical section,
because hydrocarbon vapors are heavier than natural soil gas
and also because leaks commonly occur near the bottom
of tanks, vapors will be detected earlier in the lower part
of the excavation. One
vapor monitor
in dry coarse-
grained backfill
of a single
excavation would
21tt4ia-
be expected to
detect a tank
leaking gasoline
within a distance of 10 feet in less than 30
days according to model studies (Schreiber, Levy, and
Rosenberg, 1988). State and local agencies have adopted a
variety of network design requirements for vapor monitoring.
Although the requirements vary, they tend to require wells to
be spaced no more than 10 to 17 feet apart. If the backfill is
wet or of fine-grained material, the spacing between vapor sensors may need to be
decreased. Additional detectors may be located to detect leaks from piping systems
associated with the tank systems.
Leak detection devices in one excavation may not be relied upon to detect leaks in an
adjacent, separate excavation. Leak monitors for each excavation should be considered
as a separate and independent system. If the backfill of the tank excavation is not known
or is of low permeability material, tracer tests can be made to determine if vapor monitors
will provide adequate detection and, if so, the spacing required for monitors at the
particular excavation.
Installation of
Vapor Monitoring
Wells
- Install vapor monitoring wells above water
table
- Slot vapor monitoring wells from bottom of
casing to the surface
- Install vapor monitoring wells in permeable
backfill of excavation
Vapor wells or probes are typically constructed from 2- to 4-
inch I.D. polyvinyl chloride (PVC) or stainless steel casing.
Other materials which are used include cast iron, galvanized
iron, polyethylene, polypropylene, fluorocarbon resins, and
Teflon. The casing is generally installed by auger or
hydraulic ram to a depth of about 1 foot below the bottom of
the tank excavation. The casing should be slotted from the
top of the plug in the bottom of the casing to the surface
seal of the casing. The casing is sealed within 2 to 5 feet of
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.
If the water table is within the zone of excavation, liquid
monitoring of the floating product is commonly done. If
vapor monitoring is used where the water table is within the
zone of excavation, the point of vapor withdrawal or
placement of the vapor detector must be above the water
table.
Cap
-------
Introduction
Vapor monitors are relatively easy to install at many sites and may be suitable devices for
monitoring underground storage tanks and piping systems for leaks. This paper discusses
the use of vapor monitors at new and old tank installations for detecting leaks from under-
ground hydrocarbon storage tanks. This paper discusses the site conditions under which
vapor monitors can be effectively used and conditions which can mitigate or prevent the
effective use of vapor monitors.
Migration and
Monitoring of
Hydrocarbon
Vapor from
Underground
Leaks
Vapor monitors are useful devices for
detection of leaks
Vapor monitors detect hydrocarbons in
soil air above water table
Vapor monitors may determine leaks,
usually not leak rates
Hydrocarbon vapors are readily detected by vapor monitors. Fuel hydrocarbons leaking from
underground storage tanks and piping are commonly released as liquid product. In the
unsaturated zone above the water table, the product volatilizes and the vapors migrate
rapidly in permeable soils, primarily by diffusion. Vapor monitoring may be the only suitable
means of external detection where the water table is greater than 20 or 30 feet below grade.
Vapor monitors employ devices which measure gaseous hydrocarbons in the soil air in the
vicinity of underground storage tanks. Vapor monitoring detectors are used to detect leaks,
but usually not to determine the rate of tank leak. The rate of tank leak may need to be
determined by other methods such as inventory or internal monitoring methods. However,
external vapor monitoring methods are capable of detecting small leaks that cannot be
measured by inventory methods.
Leaking fuels volatilize, diffuse in the soil air, and are detected by hydrocarbon vapor monitors.
Natural and
Engineered Site
Conditions for
Vapor Product
Monitoring
- Vapor monitors are most suitable where
background hydrocarbon content is low
or negligible
- Vapor monitors are located above water
table
- Vapor monitors are located in coarse-
grained, permeable backfill
Existing tank installations may be difficult to monitor because of the presence of high content
of residual hydrocarbons from previous spills or leaks. The problem of high background
hydrocarbon vapor content may be overcome by the removal of hydrocarbon gases by soil
vapor extraction methods (Hutzler, Murphy, and Gierke, 1989). Another solution may be by
the addition of compound-specific tracers to fuels to identify leaks from the tanks being
monitored.
The backfill material in which
the vapor monitors are located
must be relatively permeable to
allow the hydrocarbon vapors to
migrate rapidly by diffusion.
High moisture content of soils
reduces the gas permeability
and the rate of diffusion of
hydrocarbon vapors. Vapor
monitors must, of course, be i~
located above the water table
where hydrocarbon vapors are
free to migrate through the
unsaturated material.
i
3
s
\ 1
3 (
Ha/a ot dtffu*
content a/id
1
tion decreases v
decreasing gram
f
vtth mosturs
size.
Dry Gravel Dry Sand Moist Sand Wet Sand Wet Clay
-------
References
Hutzler, N. J., Murphy, B. E., and Gierke, J. S., 1989, Review of soil vapor extraction system
technology: Soil Vapor Extraction Technology for Underground Storage Tank Sites Work-
shop, U. S. Environmental Protection Agency Risk Reduction Engineering Laboratory,
Edison NJ, June 27-28, 1989, 25 p.
Schreiber, Robert, Levy, Benjamin, Rosenberg, Myron, 1988, Modeling vapor phase
movement in relation to UST leak detection - Phase 1: Final Report, Environmental Systems
Laboratory, Office of Research and Development, U.S. Environmental Protection Agency,
Las Vegas, Nevada, 81 p.
Technical Editor, M. S. Bedinger
Project Officer, Phil Durgin
" V
Prepared by Environmental Research Center, University of Nevada Las Vegas in
cooperation with U.S. Environmental Protection Agency
"Tank issues," are short articles on the management of underground fuel tanks.
These articles provide recommendations but are not regulations. All appropriate
state, local, and federal regulations should be followed in installation and
operation of leak detection devices and in management of underground storage
tanks.
Environmental Monitoring Systems Laboratory
P. 0. Box 93478
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/9-90/044
------- |