United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-96/041
April 1996
EPA Project Summary
Assessing LIST Corrective Action
Technologies: Diagnostic
Evaluation of In situ SVE-Based
System Performance
R. L. Johnson, R. R. Dupont, and D. A. Graves
The objective of the report summa-
rized here is to present the data, meth-
ods, and tools required for evaluating
the performance of in situ systems for
cleaning up leaking underground stor-
age tanks sites. Soil vapor extraction
(SVE), in situ air sparging (IAS),
bioventing and intrinsic biodegradation
are in situ corrective action technolo-
gies that are being proposed and in-
stalled at an increasing number of un-
derground storage tank (LIST) sites that
are contaminated with petroleum prod-
ucts. It is often difficult to accurately
assess the performance of these sys-
tems for remediating soils and ground-
water. This is due in part to the com-
plexity and heterogeneous nature that
exist in the subsurface at each site. In
response to the need for accurate tests
and tools for evaluating the appropri-
ate application and remediation perfor-
mance of these corrective action tech-
nologies, the U.S. Environmental Pro-
tection Agency (EPA) Office of Re-
search and Development National Risk
Management Research Laboratory
(NRMRL) provided technical support to
EPA regions for evaluating in situ cor-
rective action technologies.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The five test procedures presented
herein can be used as diagnostic tools to
evaluate in situ remediation performance.
Three of the procedures (SVE air flow,
IAS air recovery, and IAS air distribution)
are tracer tests that can be used to evalu-
ate air flow in the subsurface. The tracer
tests are new procedures that have been
tested at a small number of sites and can
be expected to undergo revisions to im-
prove their diagnostic capabilities. The
other two procedures (bioventing and natu-
ral attenuation) are designed to evaluate
biodegradation in the subsurface. These
procedures have been demonstrated at a
much larger number of sites and are there-
fore likely to require fewer changes.
SVE Air Flow Tracer Tests
SVE is a remediation technique that
has been demonstrated to effectively re-
move volatile contaminants from a wide
variety of soil types. In many cases, SVE
has sufficiently remediated sites to allow
their closure. In other cases, however,
remediation has proved difficult. The rea-
son for failure in these cases can often be
traced to nonuniform air flow due to soil
characteristics (heterogeneity, high water
content, etc.). The procedures described
in this section of the manual provide a
means of assessing airflow pathways and,
as a consequence, evaluating the
remediation performance using SVE.
At most sites where SVE and/or
bioventing using vapor extraction (BV) is
used, it is difficult to relate measured soil
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vacuum data to the air flow field. Vacuum
data are frequently used to define the
radius of influence; however, the vacuum
data do not provide much insight into the
structure of the soil or the airflow path-
ways through the soil. Vacuum data tend
to present a picture of the flow field that is
much more uniform than is generally the
case. Small strata of lower or higher per-
meability can have profound effects on
flow patterns, and these effects may not
be reflected in the vacuum data. At many
sites, there is more flow from the surface
than is commonly assumed, and at many
sites there is less flow near the water
table than is commonly assumed. As a
consequence, at many sites the time re-
quired for soil cleanup using SVE/BV is
much longer than predicted, based on
simple calculations or analytical models.
Tracer tests to directly measure the air
flow field are easy to perform and have
the potential to significantly improve the
conceptual model of how air is actually
flowing at a site. Both naturally occurring
and introduced compounds can be used
as tracers. Oxygen and carbon dioxide
concentrations can be used to assess
where air is flowing in the subsurface.
Inert gases such as helium or sulphur
hexafluoride can be injected into the sub-
surface and tracked in situ and in SVE/BV
off-gas.
I AS Air Recovery Tests
IAS is a groundwater remediation tech-
nique in which air is injected directly into a
water-saturated medium to remove con-
taminants by volatilization and to enhance
aerobic degradation. IAS is used both to
remediate aqueous groundwater plumes
and to treat sources that contain non-
aqueous-phase liquids (NAPLs).
To prevent off-site migration of vapors
during IAS, combined IAS/SVE systems
are often designed in such a way that
extracted air flow exceeds air injection by
some multiplicative factor (e.g., 5X). In
addition, to demonstrate that the design is
working, soil gas vacuum surveys in the
vicinity of the IAS/SVE system are usually
conducted. It is generally concluded that if
no pressures greater than ambient are
observed, all of the IAS air is being cap-
tured by the SVE system. However, it is
generally difficult to relate vacuum data to
recovery of IAS air. This is the case be-
cause numerous potential air flow pat-
terns in the groundwater zone can exist.
For example, if IAS air is injected into
sand below a continuous clay layer, the
air may move laterally beyond the radius
of influence of the SVE well before it has
the opportunity to reach the water table.
In this case, the sparge air might not be
captured by the SVE system.
The previous example implies that un-
der some circumstances pressure mea-
surements alone will not conclusively dem-
onstrate that IAS air is being captured. As
a consequence, it is important to conduct
tests that can unambiguously determine if
all of the IAS air is being captured by the
SVE system.
The principle underlying the tracer re-
covery tests is simple. A tracer (e.g., he-
lium) is injected along with the IAS air into
the subsurface at a known rate and the
rate of recovery at the SVE system is
calculated from the observed tracer con-
centration in the SVE effluent and the
SVE flow rate.
IAS Air Distribution Tests
IAS air flow tests are conducted by in-
jecting a gas-phase tracer, such as SF6,
along with the IAS air and determining the
distribution of tracer in the subsurface by
collecting water samples from discrete lo-
cations and depths and determining the
concentration of the tracer in the water. In
the approach described in this manual,
tracer can be injected for a period of
one week, followed by groundwater sam-
pling in the vicinity of the IAS well.
Vertical groundwater profiling (VGP) is
a technique that allows water samples to
be collected at a number of discrete depths
in the subsurface. It is generally accom-
plished by driving a small (e.g., 1-inch) -
diameter pipe into the ground. The lead-
ing edge of the pipe usually consists of a
drive point followed by a screened interval
through which water can be drawn. The
pipe assembly can be advanced by ham-
mering, vibrating, or pushing.
Water samples can be drawn to the
surface using a variety of devices. If the
water table is within the suction limit, wa-
ter can be drawn to the surface through a
tube connected to a peristaltic pump. If
the water table is deeper, a small-diam-
eter bailer or bladder pump may be used.
Vertical profiles are generally made at a
number of locations and distances around
the IAS well to create a three-dimensional
picture of the air distribution.
Bioventing Field System Design
and Evaluation
Bioventing is a modification of the con-
ventional, gas based soil remediation tech-
nology that has been successfully applied
and documented for the remediation of
hydrocarbon contaminated soils either
used alone or for the "polishing" of re-
sidual, semivolatile contaminants remain-
ing in soil following high rate SVE.
Bioventing entails the use of SVE sys-
tems for the transport of oxygen to the
subsurface, where indigenous organisms
are stimulated to aerobically metabolize
contaminants located there. Bioventing
systems are designed and configured to
optimize oxygen transfer and oxygen utili-
zation efficiency and are operated at much
lower flow rates and with significantly dif-
ferent configurations from those of con-
ventional SVE systems.
This evaluation procedure has been de-
veloped to provide an integrated approach
for the evaluation of air flow/air permeabil-
ity (characteristics of gas phase
remediation systems common to both SVE
and bioventing systems), along with bio-
degradation rates (characteristic of the bio-
logically based bioventing technology)
quantified from respiration measurements
collected under field conditions for use in
the design and evaluation of field-scale in
situ bioventing systems. Both air flow data,
relating to oxygen supply, and biodegra-
dation rate data, relating to oxygen utiliza-
tion, are required for the rational design
and evaluation of bioventing systems, and
both types of data can be collected in the
field procedure described in the manual.
The bioventing test procedure presents
an approach to the site specific determi-
nation of the feasibility of bioventing tech-
nology that is integrated with system moni-
toring and performance evaluation from
initial site assessment activities through
final confirmatory soil core analyses. The
procedure is composed of five phases of
activity that include the following:
Assessment of the Potential for
Contaminant Biodegradation
Under Actual Field Conditions
In this first phase of the procedure res-
piration gas (O2/CO2) characterization is
incorporated into conventional soil gas sur-
vey activities to detect the magnitude and
extent of biological activity, and conse-
quently, oxygen depletion/carbon dioxide
enrichment of the soil gas at the site. If
bioactivity is evident from soil gas survey
results, the next phase of the procedure is
carried out.
Assessment of Air Flow and In
situ Respiration Rates Under
Actual Field Conditions
With biodegradation evident at the field
site, air flow/air permeability distribution
and actual oxygen uptake rates must then
be determined. The test procedure de-
scribes a combined air flow/tracer-/n situ
respiration test procedure that takes ad-
vantage of monitoring probes and subsur-
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face oxygenation provided during the air
flow test for collecting site wide respiration
data. Procedures are described to reduce
the respiration data to generate respira-
tion rates and to assess their statistical
significance relative to site background res-
piration rate levels. Finally, procedures for
converting respiration rates into equiva-
lent hydrocarbon degradation rates are
provided, along with estimation procedures
for the time to site remediation.
Bioventing System Design
Based on air flow and in situ respiration
rate result, the potential oxygen supply
rate (air flow) is matched with the oxygen
demand rate (in situ respiration rates) in
rational bioventing system design. The
nature of the respiration rate law observed
from the assessment-phase results are
used to recommend either a pulse operat-
ing mode system (zero order reactions) or
continuous mode system (first order reac-
tions) to optimize overall system perfor-
mance.
Full-Scale System Monitoring
and Performance Evaluation
The procedure describes the use of rou-
tine shutdown tests to monitor the changes
taking place in respiration rates over time
as contaminants are removed from the
site. These respiration rates are statisti-
cally compared to background respiration
levels so that when only background ac-
tivity is detected throughout the site, soil
core samples may be taken to confirm
system performance.
System Performance
Verification
The final phase of the procedure de-
scribes the use of soil core samples, col-
lected from locations near those used for
initial site characterization based on quar-
terly in situ respiration results, as the ulti-
mate proof that soil remediation has pro-
ceeded to the point where site closure is
possible.
Evaluation of Natural
Attenuation in Groundwater
Natural attenuation is a risk manage-
ment strategy that invokes intrinsic
bioremediation, dilution, dispersion, sorp-
tion, and other physical loss mechanisms
to control exposure to contaminants and
restore the environment.
Intrinsic bioremediation is the preferred
term to describe the natural biological pro-
cesses that lead to contaminant biodegra-
dation (Wiedemeier et al., 1994). Intrinsic
bioremediation can occur in any environ-
ment that supports microbiological activ-
ity; however, the rate of biodegradation
may be slow due to the lack of a suitable
respiratory substrate (such as oxygen) or
inorganic nutrients (such as fixed nitro-
gen), an extreme pH, low soil moisture, or
limited contaminant bioavailability. Accu-
rate delineation of contamination, under-
standing subsurface conditions and char-
acteristics, and contaminant migration rates
and direction are critical for evaluating the
success of natural attenuation and for es-
tablishing regulatory support for its use at
a site.
The procedure presents a logical pro-
gression of data collection, evaluation, and
interpretation for quantifying and applying
natural attenuation. The approach is high-
lighted in the stepwise process for evalu-
ating, selecting, and monitoring natural at-
tenuation for groundwater remediation pre-
sented below:
• Collect and evaluate existing site data.
• Identify exposure points, water use
practices, and receptors of the aqui-
fer (RBCA).
• Determine groundwater flow direction,
velocity, and distance to nearest re-
ceptor.
• Define the risk associated with the
current groundwater conditions
(RBCA).
• Assess potential for natural attenua-
tion using existing data and prelimi-
nary risk evaluation.
• Construct a conceptual model for
natural attenuation on site:
- If preliminary site data provide
evidence that natural attenuation is
occurring, proceed.
- If risk of human exposure or fur-
ther environmental damage is un-
acceptable or if adequate site data
indicate the natural attenuation is
not or cannot occur, evaluate other
remedial strategies.
• Conduct site characterization to spe-
cifically support natural attenuation:
- Contaminant mass
- Contaminant concentration
- Presence of source areas
- General groundwater monitoring
parameters, e.g., electron accep-
tors, respiration products, pH, alka-
linity, etc.
- Define abiotic mechanisms that
result in change in concentration,
e.g., dilution, dispersion, dissolution
from a source area, retardation, etc.
• Refine the conceptual model, incor-
porating new site data.
• Determine if supplemental treatment
technologies (e.g., NAPL recovery/
source removal) are required to en-
sure successful and expedient natu-
ral attenuation.
• Project performance of natural attenu-
ation using analytical or numerical
methods.
- Analytical modeling includes the
application of the calculations pre-
sented in this document and other
emerging analytical approaches
such as multivariate statistical
analysis.
• Compare natural attenuation model
predictions with long-term risk:
- If risk is acceptable, proceed.
- If risk is unacceptable, evaluate
a more protective remedial strat-
egy.
• Develop a long-term monitoring plan:
- Revise attenuation model as data
become available.
- Sample and analyze to verify con-
tinuing site remediation.
- Locate "sentry" wells to delimit
the maximum allowable extent of
contaminant migration before a con-
tingency plan is executed.
- Define a contingency plan in case
natural attenuation does not meet
expectations or otherwise fails to
protect human health and the envi-
ronment.
• Execute monitoring plan:
- Sample and analyze sentry wells.
- Sample and analyze groundwa-
ter from selected monitoring wells.
- Evaluate results and compare
with expectations.
- Close site when cleanup goals
are reached.
- Default to contingency if sentry
wells become contaminated, or if
natural attenuation otherwise fails
to protect human health and the
environment.
The full report was submitted in partial
fulfillment of Contract No. 68-C2-0108,
Work Assignment No. 2, by IT Corpora-
tion, under the sponsorship of the U.S.
Environmental Protection Agency.
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R. L. Johnson is with the Oregon Graduate Institute of Science and Technology,
Beaverton, OR 97006; R. R. Dupontis with the Utah Water Research Laboratory,
Utah State University, Logan, UT 84322; and D. A. Graves is with IT Corp.,
Knoxville, TN 37923.
Chi-Yuan Fan is the EPA Project Officer (see below).
The complete report, entitled "Assessing UST Corrective Action Technologies:
Diagnostic Evaluation of In Situ SVE-Based System Performance," (Order No.
PB96-163597; Cost: $35.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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