United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193
Research and Development
EPA/600/S6-87/005 Aug. 1987
AEPA Project Summary
Processes Affecting Subsurface
Transport of Leaking Underground
Tank Fluids
Scott W. Tyler, Michael R. Whitbeck, Marsha W. Kirk, John W. Hess,
Lome G. Everett, David K. Kreamer, Barbara H. Wilson, and Jeff van Ee
This document presents the funda-
mental theories and an understanding
of the processes controlling migration
and fate in the subsurface of material
released from an underground storage
tank.
Processes affecting the migration of
fluids from a leaking underground stor-
age tank and their effects on monitoring
methods are reviewed by experts. An
understanding of these processes is
critical to an understanding of the
methods that monitor underground
storage tanks for material released from
the tank in the environment surrounding
the tank.
Soil heterogeneities and the potential
for multiphase flow will lead to high
monitoring uncertainties if leak detec-
tion systems rely on liquid sampling
alone. Vapor transport is also affected
by these properties, although to a lesser
degree. More research is needed, how-
ever, to better understand the physics
of vapor transport. The processes of
adsorption, partitioning, and microbial
alteration of fluids in the subsurface
may have strong effects on the uncer-
tainty of monitoring systems. Fate pro-
cesses have received less attention than
liquid and vapor transport processes
and will require significantly more re-
search before the effects are fully
understood.
This Project Summary wag developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings ol the research
project that la fully documented In a
separate report ol the same title (tee
Project Report ordering Information at
back).
Introduction
Estimates of the number of under-
ground storage tanks in the United States
range from 3 to 5 million. Estimates of
the number of leaking tanks vary, but
increasingly more incidents of leaking
tanks are being reported. These tanks
may contaminate soil and ground water
to the extent that the environment and
human health is adversely impacted.
The Solid Waste Disposal Act was
amended by the Hazardous and Solid
Waste Amendments of 1984 (HSWA;
Public Law 98-616) to provide for the
development and implementation of a
comprehensive regulatory program for
underground storage tanks (USTs). Tanks
containing hazardous waste are subject
to regulation under Subtitle C of the
Resource Conservation Recovery Act of
1976. Tanks containing petroleum and
hazardous substances are subject to
regulation under Subtitle I of the Haz-
ardous and Solid Waste Amendments;
regulations for these will be proposed in
1987 and will become final in 1988.
Regulations for tanks containing haz-
ardous waste were promulgated by the
EPA (40 CFR Part 264) on July 14, 1986.
Many approaches exist in the preven-
tion, detection, and cleanup of leaked
product from USTs. One approach is to
monitor the environment outside the tank
to determine if product is entering the
environment from the tank, piping, or
surface spills. The Environmental Moni-
toring Systems Laboratory in Las Vegas
(EMSL-Las Vegas) is conducting research
to evaluate the feasibility of monitoring
outside the tank for leaks from the tank.
The environment outside a tank is sub-
ject to a number of factors that may
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affect and complicate detecting a leak
from a tank system (Figure 1). These
factors affect the transport and fate of
product from the tank to the external leak
detection system. Studies of these factors,
particularly factors affecting organic and
petroleum products, have been initiated
in recent years in response to an in-
creasing number of reported cases where
the environment or humans have been
exposed to material released from USTs.
In addition, recent state and Federal laws
and regulations on ground-water con-
tamination have served to increase the
need of researchers to understand the
processes affecting the subsurface trans-
port and fate of leaking underground tank
fluids.
Approach
The demand for technical guidance that
has been created by new regulations has
created a need to develop and disseminate
information quickly. Initial steps taken by
EMSL-Las Vegas to evaluate external leak
detection systems have included surveys
of the literature. Federal, state and local
agencies, industry, and academia. The
results from these surveys will be made
available in a variety of reports and
technical notes. The first effort to develop
information on the processes affecting
the subsurface transport of product from
leaking USTs consisted of several experts
being asked to summarize the existing
knowledge on: liquid flow, vapor flow,
soil surface and interfacial effects of
product in the environment around a
tank, and the implications of subsurface
biological activity in the monitoring of
USTs. Four experts in these areas covered
their respective fields, and a team of
hydrogeologists from the Desert Research
Institute coordinated the experts in their
examination of the problems posed by
external leak detection monitoring of
USTs.
Each expert was asked to refer to the
available literature and to describe in
layman's terms the present state of
knowledge in the subject area. Each ex-
pert was also asked to address those
areas where the understanding of the
processing affecting subsurface transport
of leaking undergound tank fluids was
weak and where further research was
required. The conclusions reached by each
expert were consolidated and summarized
in the trial section of the full report where
the advantages and complications of a
number of monitoring approaches were
listed against four fate and transport
processes. The monitoring approaches
were: active and passive liquid monitoring,
2
Top Soil •*•
Vadose Zone -+•
Capillary Fringe •*
Water Table
Sampling Points
\
Pea Gravel
«- Aquifer
*- Clay
«- Drinking
Water
Aquifer
Spill
Figure 1.
The hydrogeologic environment around underground storage tanks can be
complicated even under idealized circumstances.
Tab/* 1. Parameters Affecting the Transport and Fate of Organics in the Soil and
Ground Water
Soil
Contaminant
Environmental
Multiphase permeability
Residual saturation
Pore size distribution
Fracture density
Wettability
Soil texture
Porosity
Variability of soil properties
Porosity
Water content
Soil structure & variability
Permeability to air
Moisture content
Organic content
Clay content
Soil surface area
Pore water chemistry
Soil gas diffusion
Colonization potential
Oxygen concentration
Methane concentration
Contaminant velocity
Fluid Transport Parameters
Density
Viscosity
Solubility
Surface tension
Vapor Transport Parameters
Volatility
Vapor diffusivity
Distribution coefficients
Temperature
Precipitation
Depth to water table
Biological activity
Recharge
Temperature
Barometric changes
Water table fluctuations
Surface Chemistry Parameters
Solubility Temperature
Concentration Pressure
Microbiological Parameters
Nutrient loading
Toxicity
Solubility
pH
Temperature
Recharge & groundwater
transport
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active and passive vapor monitoring, and
surface and borehole geophysics.
Conclusions
The monitoring of leaks from outside
the tank is complicated by a number of
factors (Table 1), and no one monitoring
approach will be applicable for all ap-
plications. The assessment of those
factors in evaluating the'performance of
external leak detection monitoring sys-
tems is not easy. Uncertainty in flow
directions because of variations in con-
ductivity, water content, texture, etc. is a
major problem in obtaining predictable
performance from an external leak detec-
tion sensing system. Since an increase in
the distance from the leak to the sensor
also increases the number of hetero-
geneities encountered and, hence, in-
creases the uncertainty in flow direction,
it is important to locate the sensor close
to the leak source. Tank installations may
be engineered to reduce further the
heterogeneity near the tank, and this
would make the sensor location less
sensitive.
Active samplers, i.e., samplers that
pump the environment, appear to be less
affected by transient spills; however,
further research is needed to develop
sensor criteria.
The present state of knowledge is
limited in the transport and fate processes
affecting leaked organic products from
USTs. Further research is needed to
understand those processes.
Scoff W. Tyler, Michael R. Whitbeck, Marsha W. Kirk, and John W. Hess are
with the Desert Research Institute, Reno, NV 89506; Lome G. Everett is
with Kaman Tempo, Santa Barbara, CA 93102; David K. Krearner is with
Arizona State University, Tempe, AZ 85287; Barbara H. Wilson is with the
University of Oklahoma, Ada, OK 74820; and the EPA author Jeff van Ee
(also the EPA Project Officer, see below) is with the Environmental Monitoring
Systems Laboratory, Las Vegas, NV 89193-3478.
The complete report entitled "Processes Affecting Subsurface Transport of
Leaking Underground Tank Fluids," (Order No. PB 87-201 521/AS; Cost:
$13.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at'
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
P.O. Box 93478
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-87/005
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