4>EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600/R-96/087
August 1996
BIOSCREEN
Natural Attenuation Decision
Support System
User's Manual
Version 1.3
Surface
Top of Water-
BearingUnit
Bottom of Water-
BearingUnit
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EPA/600/R-96/087
August 1996
BIOSCREEN
Natural Attenuation Decision Support System
User's Manual
Version 1.3
by
Charles J. Newell and R. Kevin McLeod
Groundwater Services, Inc.
Houston, Texas
James R. Gonzales
Technology Transfer Division
Air Force Center for Environmental Excellence
Brooks AFB, San Antonio, Texas
IAG#RW57936164
Project Officer
John T. Wilson
Subsurface Protection and Remediation Division
National Risk Management Research Laboratory
Ada, Oklahoma 74820
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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NOTICE
The information in this document was developed through a collaboration between the U.S.
EPA (Subsurface Protection and Remediation Division, National Risk Management Research
Laboratory, Robert S. Kerr Environmental Research Center, Ada, Oklahoma [RSKERC]) and the
U.S. Air Force (U.S. Air Force Center for Environmental Excellence, Brooks Air Force Base,
Texas). EPA staff contributed conceptual guidance in the development of the BIOSCREEN
mathematical model. To illustrate the appropriate application of BIOSCREEN, EPA contributed
field data generated by EPA staff supported by ManTech Environmental Research Services Corp,
the in-house analytical support contractor at the RSKERC. The computer code for BIOSCREEN
was developed by Ground Water Services, Inc. through a contract with the U.S. Air Force. Ground
Water Services, Inc. also provided field data to illustrate the application of the model.
All data generated by EPA staffer by ManTech Environmental Research Services Corp were
collected following procedures described in the field sampling Quality Assurance Plan for an in-
house research project on natural attenuation, and the analytical Quality Assurance Plan for ManTech
Environmental Research Services Corp.
An extensive investment in site characterization and mathematical modeling is often necessary
to establish the contribution of natural attenuation at a particular site. BIOSCREEN is offered as a
screening tool to determine whether it is appropriate to invest in a full-scale evaluation of natural
attenuation at a particular site. Because BIOSCREEN incorporates a number of simplifying
assumptions, it is not a substitute for the detailed mathematical models that are necessary for making
final regulatory decisions at complex sites.
BIOSCREEN and its User's Manual have undergone external and internal peer review
conducted by the U.S. EPA and the U.S. Air Force. However, BIOSCREEN is made available on
an as-is basis without guarantee or warranty of any kind, express or implied. Neither the United
States Government (U.S. EPA or U.S. Air Force), Ground Water Services, Inc., any of the authors
nor reviewers accept any liability resulting from the use of BIOSCREEN or its documentation.
Implementation of BIOSCREEN and interpretation of the predictions of the model are the sole
responsibility of the user.
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FOREWORD
The U. S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency strives
to formulate and implement actions leading to a compatible balance between human activities and
the ability of natural systems to support and nurture life. To meet these mandates, EPA's research
program is providing data and technical support for solving environmental problems today and
building a science knowledge base necessary to manage our ecological resources wisely, understand
how pollutants affect our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency' s center for investigation
of technological and management approaches for reducing risks from threats to human health and
the environment. The focus of the Laboratory's research program is on methods for the prevention
and control of pollution to air, land, water, and subsurface resources; protection of water quality in
public water systems; remediation of contaminated sites and ground water; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze development and
implementation of innovative, cost-effective environmental technologies; develop scientific and
engineering information needed by EPA to support regulatory and policy decisions; and provide
technical support and information transfer to ensure effective implementation of environmental
regulations and strategies.
This screening tool will allow ground water remediation managers to identify sites where
natural attenuation is most likely to be protective of human health and the environment. It will also
allow regulators to carry out an independent assessment of treatability studies and remedial
investigations that propose the use of natural attenuation.
Clinton W. Hall, Director
Subsurface Protection and Remediation Division
National Risk Management Research Laboratory
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IV
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TABLE OF CONTENTS
BIOSCREEN Natural Attenuation Decision Support System
Air Force Center for Environmental Excellence Technology Transfer
Division
INTRODUCTION 1
INTENDED USES FOR BIOSCREEN 1
FUNDAMENTALS OF NATURAL ATTENUATION 2
Biodegradation Modeling 2
The Air Force Natural Attenuation Initiative 3
Relative Importance of Different Electron Acceptors 4
Preferred Reactions by Energy Potential 4
Distribution of Electron Acceptors at Sites 5
Kinetics of Aerobic and Anaerobic Reactions 6
Biodegradation Capacity 10
BIOSCREEN CONCEPTS 12
BIOSCREEN Model Types 12
Which Kinetic Model Should One Use in BIOSCREEN? 14
BIOSCREEN DATA ENTRY 14
1. HYDROGEOLOGIC DATA 15
2 . DISPERSIVITY 17
3 . ADSORPTION DATA 19
4 . BIODEGRADATION DATA 21
5. GENERAL DATA 26
6 . SOURCE DATA 27
7 . FIELD DATA FOR COMPARISON 33
ANALYZING BIOSCREEN OUTPUT 33
Centerline Output 33
Array Output 33
Calculating the Mass Balance 34
BIOSCREEN TROUBLESHOOTING TIPS 37
Minimum System Requirements 37
Spreadsheet-Related Problems 37
Common Error Messages 37
REFERENCES 39
APPENDICES
A.1 DOMENICO ANALYTICAL MODEL 41
A.2 INSTANTANEOUS REACTION - SUPERPOSITION ALGORITHM 43
A.3 DERIVATION OF SOURCE HALF-LIFE 45
A.4 DISPERSIVITY ESTIMATES 47
A.5 ACKNOWLEDGMENTS 50
A.6 BIOSCREEN EXAMPLES 51
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BIOSCREEN User's Manual June 1996
INTRODUCTION
BIOSCREEN is an easy-to-use screening model which simulates remediation through natural
attenuation (RNA) of dissolved hydrocarbons at petroleum fuel release sites. The software,
programmed in the Microsoftฎ Excel spreadsheet environment and based on the Domenico
analytical solute transport model, has the ability to simulate advection, dispersion, adsorption,
and aerobic decay as well as anaerobic reactions that have been shown to be the dominant
biodegradation processes at many petroleum release sites. BIOSCREEN includes three different
model types:
1) Solute transport without decay,
2) Solute transport with biodegradation modeled as a first-order decay process (simple, lumped-parameter
approach),
3) Solute transport with biodegradation modeled as an "instantaneous" biodegradation reaction (approach used
by BIOPLUME models).
The model is designed to simulate biodegradation by both aerobic and anaerobic reactions. It
was developed for the Air Force Center for Environmental Excellence (AFCEE) Technology
Transfer Division at Brooks Air Force Base by Groundwater Services, Inc., Houston, Texas.
INTENDED USES FOR BIOSCREEN
BIOSCREEN attempts to answer two fundamental questions regarding RNA:
1. How far will the dissolved contaminant plume extend if no
engineered controls or further source zone reduction measures are
implemented?
BIOSCREEN uses an analytical solute transport model with two options for simulating
in-situ biodegradation: first-order decay and instantaneous reaction. The model will
predict the maximum extent of plume migration, which may then be compared to the
distance to potential points of exposure (e.g., drinking water wells, groundwater
discharge areas, or property boundaries). Analytical groundwater transport models have
seen wide application for this purpose (e.g., ASTM 1995), and experience has shown such
models can produce reliable results when site conditions in the plume area are relatively
uniform.
2. How long will the plume persist until natural attenuation
processes cause it to dissipate?
BIOSCREEN uses a simple mass balance approach based on the mass of dissolvable
hydrocarbons in the source zone and the rate of hydrocarbons leaving the source zone to
estimate the source zone concentration vs. time. Because an exponential decay in source
zone concentration is assumed, the predicted plume lifetimes can be large, usually
ranging from 5 to 500 years. Note: This is an unverified relationship as there are few
data showing source concentrations vs. long time periods, and the results should be
considered order-of-magnitude estimates of the time required to dissipate the plume.
BIOSCREEN is intended to be used in two ways:
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BIOSCREEN User's Manual June 1996
1. As a screening model to determine if RNA is feasible at a site.
In this case, BIOSCREEN is used early in the remedial investigation to determine if an
RNA field program should be implemented to quantify the natural attenuation occurring
at a site. Some data, such as electron acceptor concentrations, may not be available, so
typical values are used. In addition, the model can be used to help develop long-term
monitoring plans for RNA projects.
2. As the primary RNA groundwater model at smaller sites.
The Air Force Intrinsic Remediation Protocol (Wiedemeier, Wilson, et al., 1995) describes
how groundwater models may be used to help verify that natural attenuation is
occurring and to help predict how far plumes might extend under an RNA scenario. At
large, high-effort sites such as Superfund and RCRA sites, a more sophisticated model
such as BIOPLUME is probably more appropriate. At less complicated, lower-effort sites
such as service stations, BIOSCREEN may be sufficient to complete the RNA study.
(Note: "Intrinsic remediation" is a risk-based strategy that relies on RNA).
BIOSCREEN has the following limitations:
1. As an analytical model, BIOSCREEN assumes simple groundwater flow
conditions.
The model should not be applied where pumping systems create a complicated flow
field. In addition, the model should not be applied where vertical flow gradients affect
contaminant transport.
2. As an screening tool, BIOSCREEN only approximates more complicated
processes that occur in the field.
The model should not be applied where extremely detailed, accurate results that closely
match site conditions are required. More comprehensive numerical models should be
applied in these cases.
FUNDAMENTALS OF NATURAL ATTENUATION
Biodegradation Modeling
Naturally occurring biological processes can significantly enhance the rate of organic mass
removal from contaminated aquifers. Biodegradation research performed by Rice University,
government agencies, and other research groups has identified several main themes that are
crucial for future studies of natural attenuation:
1. The relative importance of groundwater transport vs. microbial kinetics is a key consideration for
developing workable biodegradation expressions in models. Results from the United Creosote site
(Texas) and the Traverse City Fuel Spill site (Michigan) indicate that biodegradation is better
represented as a macro-scale wastewater treatment-type process than as a micro-scale study of
microbial reactions.
2. The distribution and availability of electron acceptors control the rate ofin-situ biodegradation for
most petroleum release site plumes. Other factors (e.g., population of microbes, pH, temperature,
etc.) rarely limit the amount of biodegradation occurring at these sites.
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BIOSCREEN User's Manual June 1996
These themes are supported by the following literature. Borden et al. (1986) developed the
BIOPLUME model, which simulates aerobic biodegradation as an "instantaneous" microbial
reaction that is limited by the amount of electron acceptor, oxygen, that is available. In other
words, the microbial reaction is assumed to occur at a much faster rate than the time required for
the aquifer to replenish the amount of oxygen in the plume. Although the time required for the
biomass to aerobically degrade the dissolved hydrocarbons is on the order of days, the overall
time to flush a plume with fresh groundwater is on the order of years or tens of years. Borden et
al. (1986) incorporated a simplifying assumption that the microbial kinetics are instantaneous into
the USGS two-dimensional solute transport model (Konikow and Bredehoeft, 1978) using a
simple superposition algorithm. The resulting model, BIOPLUME, was able to simulate solute
transport and fate under the effects of instantaneous, oxygen-limited in-situ biodegradation.
Rifai and Bedient (1990) extended this approach and developed the BIOPLUME II model, which
simulates the transport of two plumes: an oxygen plume and a contaminant plume. The two
plumes are allowed to react, and the ratio of oxygen to contaminant consumed by the reaction is
determined from an appropriate stoichiometric model. The BIOPLUME II model is documented
with a detailed user's manual (Rifai et al, 1987) and is currently being used by EPA regional
offices, U.S. Air Force facilities, and by consulting firms. Borden et al. (1986) applied the
BIOPLUME concepts to the Conroe Superfund site; Rifai et al. (1988) and Rifai et al. (1991) applied
the BIOPLUME II model to a jet fuel spill at a Coast Guard facility in Michigan. Many other
studies using the BIOPLUME II model have been presented in recent literature.
The BIOPLUME II model has increased the understanding of biodegradation and natural
attenuation by simulating the effects of adsorption, dispersion, and aerobic biodegradation
processes in one model. It incorporates a simplified mechanism (first-order decay) for handling
other degradation processes, but does not address specific anaerobic decay reactions. Early
conceptual models of natural attenuation were based on the assumption that the anaerobic
degradation pathways were too slow to have any meaningful effect on the overall natural
attenuation rate at most sites. Accordingly, most field programs focused only on the distribution
of oxygen and contaminants, and did not measure the indicators of anaerobic activity such as
depletion of anaerobic election acceptors or accumulation of anaerobic metabolic by-products.
The Air Force Natural Attenuation Initiative
Over the past several years, the high cost and poor performance of many pump-and-treat
remediation systems have led many researchers to consider RNA as an alternative technology for
groundwater remediation. A detailed understanding of natural attenuation processes is needed
to support the development of this remediation approach. Researchers associated with the U.S.
EPA's R.S. Kerr Environmental Research Laboratory (now the Subsurface Protection and
Remediation Division of the National Risk Management Laboratory) have suggested that
anaerobic pathways could be a significant, or even the dominant, degradation mechanism at
many petroleum fuel sites (Wilson, 1994). The natural attenuation initiative, developed by the
AFCEE Technology Transfer Division, was designed to investigate how natural attenuation
processes affect the migration of plumes at petroleum release sites. Under the guidance of Lt.
Col. Ross Miller, a three-pronged technology development effort was launched in 1993 which will
ultimately consist of the following elements:
1) Field data collected at over 30 sites around the country (Wiedemeier, Miller, et al., 1995)
analyzing aerobic and anaerobic processes.
2) A Technical Protocol, outlining the approach, data collection techniques, and data analysis
methods required for conducting an Air Force RNA Study (Wiedemeier, Wilson, et al., 1995).
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BIOSCREEN User's Manual
June 1996
3) Two KNA modeling tools: the BIOPLUME III model being developed by Dr. Hanadi Rifai at Rice
University (Rifai et al., 2995), and the BIOSCREEN model developed by Groundwater Services,
Inc. (BIOPLUME III, a more sophisticated biodegradation model than BIOSCREEN, employs
particle tracking of both hydrocarbon and alternate electron acceptors using a numerical solver.
The model employs sequential degradation of the biodegradation reactions based on zero order, first
order, instantaneous, or Monod kinetics).
Relative Importance of Different Electron Acceptors
The Intrinsic Remediation Technical Protocol and modeling tools focus on evaluating both
aerobic (in the presence of oxygen) and anaerobic (without oxygen) degradation processes. In the
presence of organic substrate and dissolved oxygen, microorganisms capable of aerobic
metabolism will predominate over anaerobic forms. However, dissolved oxygen is rapidly
consumed in the interior of contaminant plumes, converting these areas into anoxic (low-oxygen)
zones. Under these conditions, anaerobic bacteria begin to utilize other electron acceptors to
metabolize dissolved hydrocarbons. The principal factors influencing the utilization of the
various electron acceptors by fuel-hydrocarbon-degrading bacteria include: 1) the relative
biochemical energy provided by the reaction, 2) the availability of individual or specific electron
acceptors at a particular site, and 3) the kinetics (rate) of the microbial reaction associated with the
different electron acceptors.
Preferred Reactions by Energy Potential
Biologically mediated degradation reactions are reduction/oxidation (redox) reactions, involving
the transfer of electrons from the organic contaminant compound to an electron acceptor.
Oxygen is the electron acceptor for aerobic metabolism, whereas nitrate, ferric iron, sulfate, and
carbon dioxide can serve as electron acceptors for alternative anaerobic pathways. This transfer
of electrons releases energy which is utilized for microbial cell maintenance and growth. The
biochemical energy associated with alternative degradation pathways can be represented by the
redox potential of the alternative electron acceptors: the more positive the redox potential, the
more energetically favorable the reaction. With everything else being equal, organisms with
more efficient modes of metabolism grow faster and therefore dominate over less efficient forms.
Electron
Acceptor
Oxygen
Nitrate
Ferric Iron
(solid)
Sulfate
Carbon Dioxide
Type of
Reaction
Aerobic
Anaerobic
Anaerobic
Anaerobic
Anaerobic
Metabolic
By-Product
C02
N2, C02
Ferrous Iron
(dissolved)
H2S
Methane
Reaction
Preference
Most Preferred
U
U
U
Least Preferred
Based solely on thermodynamic considerations, the most energetically preferred reaction should
proceed in the plume until all of the required electron acceptor is depleted. At that point, the next
most-preferred reaction should begin and continue until that electron acceptor is consumed,
leading to a pattern where preferred electron acceptors are consumed one at a time, in sequence.
Based on this principle, one would expect to observe monitoring well data with "no detect" results
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BIOSCREEN User's Manual June 1996
for the more energetic electron acceptors, such as oxygen and nitrate, in locations where evidence
of less energetic reactions is observed (e.g. monitoring well data indicating the presence of ferrous
iron).
In practice, however, it is unusual to collect samples from monitoring wells that are completely
depleted in one or more electron acceptors. Two processes are probably responsible for this
observation:
1. Alternative biochemical mechanisms exhibiting very similar energy potentials (such as aerobic
oxidation and nitrate reduction) may occur concurrently when the preferred electron acceptor is
reduced in concentration, rather than fully depleted. Facultative aerobes (bacteria able to utilize
electron acceptors in both aerobic and anaerobic environments), for example, can shift from aerobic
metabolism to nitrate reduction when oxygen is still present but at low concentrations (i.e. 1 mg/L
oxygen; Snoeyink and Jenkins, 1980). Similarly, the nearly equivalent redox potentials for sulfate
and carbon dioxide (see Wiedemeier, Wilson, et al., 1995) indicate that sulfate reduction and
methanogenic reactions may also occur together.
2. Standard monitoring wells, with 5- to 10- foot screened intervals, will mix waters from different
vertical zones. If different biodegradation reactions are occurring at different depths, then one
would expect to find geochemical evidence of alternative degradation mechanisms occurring in the
same well. If the dissolved hydrocarbon plume is thinner than the screened interval of a
monitoring well, then the geochemical evidence of electron acceptor depletion or metabolite
accumulation will be diluted by mixing with clean water from zones where no degradation is
occurring.
Therefore, most natural attenuation programs yield data that indicate a general pattern of
electron acceptor depletion, but not complete depletion, and an overlapping of electron
acceptor/metabolite isopleths into zones not predicted by thermodynamic principles. For
example, a zone of methane accumulation may be larger than the apparent anoxic zone.
Nevertheless, these general patterns of geochemical changes within the plume area provide
strong evidence that multiple mechanisms of biodegradation are occurring at many sites. The
BIOSCREEN software attempts to account for the majority of these biodegradation mechanisms.
Distribution of Electron Acceptors at Sites
The utilization of electron acceptors is generally based on the energy of the reaction and the
availability of the electron acceptor at the site. While the energy of each reaction is based on
thermodynamics, the distribution of electron acceptors is dependent on site-specific
hydrogeochemical processes and can vary significantly among sites. For example, a study of
several sites yielded the following summary of available electron acceptors and metabolic by-
products:
Measured Background Electron Acceptor/By-Product Concentration (mg/L)
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BIOSCREEN User's Manual
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Base Facility
POL Site,
Hill AFB, Utah*
Hangar 10 Site,
Elmendorf AFB,
Alaska*
SiteST-41,
Elmendorf
AFB,Alaska*
Site ST-29,
Patrick AFB, Florida*
Bldg. 735,
Grissom AFB, Indiana
SWMU66Site,
Keesler AFB, MS
POL B Site,
Tyndall AFB, Florida
Background
Oxygen
6.0
0.8
12.7
3.8
9.1
1.7
1.4
Background
Nitrate
36.2
64.7
60.3
0
1.0
0.7
0.1
Maximum
Ferrous
Iron
55.6
8.9
40.5
2.0
2.2
36.2
1.3
Background
Sulfate
96.6
25.1
57.0
0
59.8
22.4
5.9
Maximum
Methane
2.0
9.0
1.5
13.6
1.0
7.4
4.6
*Data collected by Parsons Engineering Science, Inc.; all other data collected by Groundwater Services, Inc.
At the Patrick AFB site, nitrate and sulfate are not important electron acceptors while the oxygen
and the methanogenic reactions dominate (Wiedemeier, Swanson, et al., 1995). At Hill AFB and
Grissom AFB, the sulfate reactions are extremely important because of the large amount of
available sulfate for reduction. Note that different sites in close proximity can have quite
different electron acceptor concentrations, as shown by the two sites at Elmendorf AFB. For data
on more sites, see Table 1.
Kinetics of Aerobic and Anaerobic Reactions
As described above, aerobic biodegradation can be simulated as an "instantaneous" reaction that
is limited by the amount of electron acceptor (oxygen) that is available. The microbial reaction is
assumed to occur at a much faster rate than the time required for the aquifer to replenish the
amount of oxygen in the plume (Wilson et al., 1985). Although the time required for the biomass
to aerobically degrade the dissolved hydrocarbons is on the order of days, the overall time to
flush a plume with fresh groundwater is on the order of years or tens of years.
For example, microcosm data presented by Davis et al. (1994) show that microbes in an
environment with an excess of electron acceptors can degrade high concentrations of dissolved
benzene very rapidly. In the presence of surplus oxygen, aerobic bacteria can degrade ~1 mg/L
dissolved benzene in about 8 days, which can be considered relatively fast (referred to as
"instantaneous") compared to the years required for flowing groundwater to replenish the plume
area with oxygen.
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BIOSCREEN User's Manual
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TABLE 1
BIODEGRADATION CAPACITY (EXPRESSED ASSIMILATIVE CAPACITY) AT AFCEE NATURAL ATTENUATION SITES
BIOSCREEN Natural Attenuation Decision Support System
Maximum
Total BTEX Biodegradation Capacity/Expressed Assimilative Capacity (mg/L)
Site
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Concentration
Base
Hill AFB
Battle Creek ANGB
Madison ANGB
Elmendorf AFB
Elmendorf AFB
King Salmon AFB
King Salmon AFB
Plattsburgh AFB
EglinAFB
Patrick AFB
MacDill AFB
MacDill AFB
MacDill AFB
Offutt AFB
Offutt AFB
Westover AFRES
Westover AFRES
Myrtle Beach
Langley AFB
Griffis AFB
Rickenbacker ANGB
Wurtsmith AFB
Travis AFB
Pope AFB
Seymour Johnson
AFB
Grissom AFB
Tyndall AFB
Keesler AFB
State
Utah
Michigan
Wisconsin
Alaska
Alaska
Alaska
Alaska
New York
Florida
Florida
Florida
Florida
Florida
Nebraska
Nebraska
Massachusetts
Massachusetts
South Carolina
Virginia
New York
Ohio
Michigan
Califonia
North Carolina
North Carolina
Indiana
Florida
Mississippi
Site Name
Hangar 10
ST-41
FT-001
Naknek
Site 56
Site 57
Site OT-24
FPT-A3
FT-03
FT-08
SS-42
Bldg. 735
POLE
SWMU 66
Average
Median
Maximum
Minimum
(mg/L)
21.5
3.6
28.0
22.2
30.6
10.1
5.3
6.0
3.7
7.3
29.6
0.7
2.8
3.2
103.0
1.7
32.6
18.3
0.1
12.8
1.0
3.1
8.2
13.8
0.3
1.0
141
14.2
7.3
103.0
0.1
Observed Chatty
OZ
6.0
5.7
7.2
0.8
12.7
9.0
11.7
10.0
1.2
3.8
2.4
2.1
1.3
0.6
8.4
10.0
9.9
0.4
6.4
4.4
1.5
8.5
3.8
7.5
8.3
9.1
1.4
1.7
5.6
5.8
12.7
0.4
Nitrate
36.2
5.6
45.3
64.7
60.3
12.5
0
3.7
0
0
5.6
0.5
0
0
69.7
8.6
17.2
0
23.5
52.5
35.9
25.4
15.8
6.9
4.3
1.0
0.1
0.7
17.7
6.3
69.7
0
ye in Concentration (mg/L)
Iron
55.6
12.0
15.3
8.9
40.5
2.5
44.0
10.7
8.9
2.0
5.0
20.9
13.1
19.0
0
599.5
279.0
34.9
10.9
24.7
17.9
19.9
8.5
56.2
31.6
2.2
1.3
36.2
49.3
16.6
599.5
0
Sulfate
96.6
12.9
24.2
25.1
57.0
6.8
0
18.9
49
0
101.2
62.4
3.7
32.0
82.9
33.5
11.7
20.7
81.3
82.2
93.2
10.6
109.2
9.7
38.6
59.8
5.9
22.4
39.5
24.6
109.2
0
Methane
2.0
8.4
11.7
9.0
1.5
0.2
5.6
0.3
11.8
13.6
13.6
15.4
9.8
22.4
0
0.2
4.3
17.2
8.0
7.1
7 7
1.4
0.2
48.4
2.7
1.0
46
7.4
8.4
7.2
48.4
0
Aerobic
Respiration
1.9
1.8
2.3
0.3
40
2.9
3.7
3.2
0.4
1.2
0.8
0.7
0.4
0.2
2.7
3.2
3.1
0.1
2.0
1.4
0.5
2.7
1.2
2.4
2.6
2.9
0.5
0.5
1.8
1.9
4.0
0.1
Denitrification
7.4
1.1
9.2
13.2
12.3
2.6
0
0.7
0
0
1.1
0.1
0
0
142
1.8
3.5
0
48
10.7
7.3
5.2
3.2
1.4
0.9
0.2
0
0.1
3.6
1.3
14.2
0
Iron
Reduction
2.6
0.6
0.7
0.4
1.9
0.1
2.0
0.5
0.4
0.1
0.2
1.0
0.6
0.9
0
27.5
12.8
1.6
0.5
1.1
0.8
0.9
0.4
2.6
1.5
0.1
0.1
1.7
2.3
0.8
27.5
0
Sulfate
Reduction
21.0
2.8
5.3
5.5
12.4
1.5
0
41
1.1
0
22.0
13.6
0.8
7.0
18.0
7.3
2.6
4.5
17.7
17.9
20.3
2.3
23.7
2.1
8.4
13.0
1.3
4.9
8.6
5.4
23.7
0
Methanogenesis
2.6
10.8
15.0
11.6
1.9
0.2
7.2
0.4
15.2
17.4
17.4
19.7
12.6
28.8
0
0.2
5.5
22.0
10.2
9.1
9.8
1.8
0.3
62.0
3.5
1.2
5.9
9.5
10.8
9.3
62.0
0
Total
Biodegradation
Capacity (mg/L)
35.4
17.1
32.5
30.9
32.5
7 2
12.9
8.9
17.0
18.7
41.5
35.0
144
36.8
34.9
40.0
27.5
28.2
35.3
40.2
38.7
12.9
28.9
70.5
16.8
17.4
7.7
16.7
27.0
28.5
70.5
7.2
Source of
Data
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
PES
GSI
GSI
GSI
Note:
1. Utilization factors of the electron acceptors/by-products are as follows (mg of electron acceptor or by-product/mg of BTEX): Dissolved Oxygen: 3.14, Nitrate: 4.9, Iron: 21.8, Sulfate: 4.7, Methane: 0.78.
2. - = Data not available.
3. PES = Parsons Engineering Science (Wiedemeier, Miller, et al. 1995). GSI = Groundwater Services, Inc.
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BIOSCREEN User's Manual
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Recent results from the AFCEE Natural Attenuation Initiative indicate that the anaerobic
reactions, which were originally thought to be too slow to be of significance in groundwater, can
also be simulated as instantaneous reactions (Newell et al, 1995). For example, Davis et al. (1994)
also ran microcosm studies with sulfate reducers and methanogens that indicated that benzene
could be degraded in a period of a few weeks (after acclimation). When compared to the time
required to replenish electron acceptors in a plume, it appears appropriate to simulate anaerobic
biodegradation of dissolved hydrocarbons with an instantaneous reaction, just as for aerobic
biodegradation processes.
This conclusion is supported by observing the pattern of anaerobic electron acceptors and
metabolic by-products along the plume at RNA research sites:
If microbial kinetics were
limiting the rate of
biodegradation:
If microbial kinetics were
relatively fast (instantaneous):
Anaerobic electron acceptors (nitrate and
sulfate) would be constantly decreasing in
concentration as one moved downgradient
from the source zone, and
Anaerobic electron acceptors (nitrate and
sulfate) would be mostly or totally
consumed in the source zone, and
Anaerobic by-products (ferrous iron and
methane) would be constantly increasing
in concentration as one moved
downgradient from the source zone.
Anaerobic by-products (ferrous iron and
methane) would be found in the highest
concentrations in the source zone.
Observed
Cone.
Cone.
Cone.
O2, NO3, SO4
FE2+,CH4
Observed
Cone.
Cone.
Cone.
O2, NO3, SO4
FE2+,CH4
V
x
x
The second pattern is observed at RNA demonstration sites (see Figure 1), supporting the
hypothesis that anaerobic reactions can be considered to be relatively instantaneous at most or
almost all petroleum release sites. From a theoretical basis, the only sites where the instantaneous
reaction assumption may not apply are sites with very low hydraulic residence times (very high
groundwater velocities and short source zone lengths).
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BIOSCREEN User's Manual June 1996
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BIOSCREEN User's Manual
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1.0
0.5
0.0
40
20
LA
Tyndall
0 ปปปป * M * AA4
-T-' ,^| ,
200 400 600 800
BTEX
Sulfate
Nitrate
D. Oxygen
Methane
Iron
500
1500 2000
"LA
oi-^t k
Patrick
Jป
10
200
400
600
800
BTEX
Sulfate
Nitrate
D. Oxygen
Methane
Iron
10 ฐ
Elmendorf
ST-41
o.o J_$
10-
a
<4^*
"N.
^-^
25-c.
oil ป
200
400
ป ปi
600
800
10 -I
5
25 -I
0
4
2 .
0
C
Keesler
,^/V.
V
) 100 200 300
BTEX
Sulfate
A Nitrate
* D. Oxygen
* Methane
02
01
0.0
n
Elmendorf
HG-10
1000
2000
3000 4000
Distance along plume centerline
Distance along plume centerline
Figure 1. Distribution of BTEX, Electron Acceptors, and Metabolic By-Products vs. Distance Along
Centerline
of Plume.
10
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BIOSCREEN User's Manual
June 1996
Sampling Date and Source of Data: Tyndall 3/95, Keesler 4/95 (Groundwater Services, Inc.), Patrick
3/94 (note: one NO3 outlier removed, sulfate not plotted). Hill 7/93, Elmendorf Site ST41 6/94,
Elmendorf Site HG 106/94, (Parsons Engineering Science).
Kinetic-limited sites, however, appear to be relatively rare as the instantaneous reaction pattern is
observed even at sites such as Site 870 at Hill AFB, with residence times of a month or less. As
shown in Figure 1, this site has an active sulfate reducing and methane production zone within
100 ft of the upgradient edge of plume. With a 1600 ft/yr seepage velocity is considered, this
highly anaerobic zone has an effective residence time of 23 days. Despite this very short
residence time, significant sulfate depletion and methane production were observed in this zone
(see Figure 1). If the anaerobic reactions were significantly constrained by microbial kinetics, the
amount of sulfate depletion and methane production would be much less pronounced. Therefore
this site supports the conclusion that the instantaneous reaction assumption is applicable to
almost all petroleum release sites.
Biodegradation Capacity
To apply an electron-acceptor-limited kinetic model, such as the instantaneous reaction, the
amount of biodegradation able to be supported by the groundwater that moves through the
source zone must be calculated. The conceptual model used in BIOSCREEN is:
1. Groundwater upgradient of the source contains electron acceptors.
2. As the upgradient groundwater moves through the source zone, non-aqueous phase
liquids (NAPLs) and contaminated soil release dissolvable hydrocarbons (in the case of
petroleum sites, the BTEX compounds benzene, toluene, ethylbenzene, xylene are
released).
3. Biological reactions occur until the available electron acceptors in groundwater are
consumed. (Two exceptions to this conceptual model are the iron reactions, where the
electron acceptor, ferric iron, dissolves from the aquifer matrix; and the methane
reactions, where the electron acceptor, CC>2 is also produced as an end-product of the
reactions. For these reactions, the metabolic by-products, ferrous iron and methane, can
be used as proxies for the potential amount of biodegradation that could occur from the
iron-reducing and methanogenesis reactions.)
4. The total amount of available electron acceptors for biological reactions can be estimated
by a) calculating the difference between upgradient concentrations and source zone
concentrations for oxygen, nitrate, and sulfate; and b) measuring the production of
metabolic by-products (ferrous iron and methane) in the source zone.
5. Using stoichiometry, a utilization factor can be developed showing the ratio of the
oxygen, nitrate, and sulfate consumed to the mass of dissolved hydrocarbon degraded in
the biodegradation reactions. Similarly, utilization factors can be developed to show the
ratio of the mass of metabolic by-products that are generated to the mass of dissolved
hydrocarbon degraded in the biodegradation reactions. Wiedemeier, Wilson, et al, (1995)
provides the following utilization factors based on the degradation of combined BTEX
constituents:
Electron Acceptor/By-
Product
Oxygen
Nitrate
Ferrous Iron
BTEX Utilization Factor
gm/gm
3.14
4.9
21.8
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BIOSCREEN User's Manual June 1996
Sulfate
Methane
4.7
0.78
6. For a given background concentration of an individual electron acceptor, the potential
contaminant mass removal or "biodegradation capacity" depends on the "utilization
factor" for that electron acceptor. Dividing the background concentration of an electron
acceptor by its utilization factor provides an estimate (in BTEX concentration units) of the
assimilative capacity of the aquifer by that mode of biodegradation.
Note that BIOSCREEN is based on the BTEX utilization provided above. If other
constituents are modeled, the utilization factors in the software (scroll down from the
input screen to find the utilization factors) should be changed or the available oxygen,
nitrate, iron, sulfate, and methane data should be adjusted accordingly to reflect alternate
utilization factors.
When the available electron acceptor/by-product concentrations (No. 4) are divided by
the appropriate utilization factor (No. 5), an estimate of the "biodegradation capacity" of
the groundwater flowing through the source zone and plume can be developed. The
biodegradation capacity is then used directly in the BIOSCREEN model to simulate the
effects of an instantaneous reaction. The suggested calculation approach to develop
BIOSCREEN input data is:
Biodegradation Capacity (mg/L) =
{ (Average Upgradient Oxygen Cone.) - (Minimum Source Zone Oxygen Cone)} / 3.14
+ {(Average Upgradient Nitrate Cone.) - (Minimum Source Zone Nitrate Cone)} /4.9
+ {(Average Upgradient Sulfate Cone.) - (Minimum Source Zone Sulfate Cone)} / 4.7
+ { Average Observed Ferrous Iron Cone, in Source Area) / 21.8
+ { Average Observed Methane Cone, in Source Area } / 0.78
Biodegradation capacity is similar to "expressed assimilative capacity" described in the
AFCEE Technical Protocol, except that expressed assimilative capacity is based on the
maximum observed concentration observed in the source zone for iron and methane,
while the biodegradation capacity term used in BIOSCREEN is based on the average
concentration in the source zone for iron and methane. BIOSCREEN uses the more
conservative biodegradation capacity approach to provide a conservative screening tool
to users. Calculated biodegradation capacities (from Groundwater Services sites) and
expressed assimilative capacities (from Parsons Engineering-Science sites) at different
U.S. Air Force RNA research sites have ranged from 7 to 70 mg/L (see Table 1). The
median capacity for 28 AFCEE sites is 28.5 mg/L.
Note that one criticism of this lumped biodegradation capacity approach is that it
assumes that all of the various aerobic and anaerobic reactions occur over the entire area
of the contaminant plume, and that the theoretical "zonation" of reactions is not
simulated in BIOSCREEN (e.g. typically dissolved oxygen utilization occurs at the
downgradient portion and edges of the plume, nitrate utilization a little closer to the
source, iron reduction in the middle of the plume, sulfate reduction near the source, and
methane production in the heart of the source zone). A careful inspection of actual field
data (see Figure 1) shows little or no evidence of this theoretical zonation of reactions; in
12
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BIOSCREEN User's Manual June 1996
fact all of the reactions appear to occur simultaneously in the source zone. The most
common pattern observed at petroleum release sites is that ferrous iron and methane
seems to be restricted to the higher-concentration or source zone areas, with the other
reactions (oxygen, nitrate, and sulfate depletion), occurring throughout the plume.
BIOSCREEN assumes that all of the biodegradation reactions (aerobic and anaerobic)
occur almost instantaneously relative to the hydraulic residence time in the source area
and plume. Because iron reduction and methane production appear to occur only in the
source zone (probably due to the removal of these metabolic by-products) it is
recommended to use the average iron and methane concentrations observed in the source
zone for the calculation of biodegradation capacity instead of maximum concentrations.
In addition, the iron and methane concentrations are used during a secondary calibration
step (see below). Beta testing of BIOSCREEN indicated that the use of the maximum
concentration of iron and methane tended to overpredict biodegradation at many sites by
assuming these reactions occurred over the entire plume area. Use of an average value
(or some reduced value) helps match actual field data.
7. Note that at some sites the instantaneous reaction model will appear to overpredict the
amount of biodegradation that occurs, and underpredict at others. As with the case of
the first-order decay model, some calibration to actual site conditions is required. With
the first-order decay, the decay coefficient is adjusted arbitrarily until the predicted
values match observed field conditions. With the instantaneous reaction model, there is
no first-order decay coefficient to adjust, so the following procedure is recommended:
A) The primary calibration step (if needed) is to manipulate the model's dispersivity
values. As described in the BIOSCREEN Data Entry Section below, values for
dispersivity are related to aquifer scale (defined as the plume length or distance to
the measurement point) and simple relationships are usually applied to estimate
dispersivities. Gelhar et al. (1992) cautions that dispersivity values vary between 2-3
orders of magnitude for a given scale due to natural variation in hydraulic
conductivity at a particular site. Therefore dispersivity values can be manipulated
within a large range and still be within the range of values observed at field test sites.
In BIOSCREEN, adjusting the transverse dispersivity alone will usually be enough to
calibrate the model.
B) As a secondary calibration step, the biodegradation capacity calculation may be
reevaluated. There is some judgment involved in averaging the election acceptor
concentrations observed in upgradient wells; determining the minimum oxygen,
nitrate and sulfate in the source zone; and estimating the average ferrous iron and
methane concentrations in the source zone. Although probably not needed in most
applications, these values may be adjusted as a final level of calibration.
BIOSCREEN CONCEPTS
The BIOSCREEN Natural Attenuation software is based on the Domenico (1987) three-
dimensional analytical solute transport model. The original model assumes a fully-penetrating
vertical plane source oriented perpendicular to groundwater flow, to simulate the release of
organics to moving groundwater. In addition, the Domenico solution accounts for the effects of
advective transport, three-dimensional dispersion, adsorption, and first-order decay. In
BIOSCREEN, the Domenico solution has been adapted to provide three different model types
representing i) transport with no decay, ii) transport with first-order decay, and iii) transport with
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BIOSCREEN User's Manual June 1996
"instantaneous" biodegradation reaction (see Model Types). Guidelines for selecting key input
parameters for the model are outlined in BIOSCREEN Input Parameters. For help on Output, see
BIOSCREEN Output.
BIOSCREEN Model Types
The software allows the user to see results from three different types of groundwater transport
models, all based on the Domenico solution:
1. Solute transport with no decay. This model is appropriate for predicting the movement
of conservative (non-degrading) solutes such as chloride. The only attenuation
mechanisms are dispersion in the longitudinal, transverse, and vertical directions, and
adsorption of contaminants to the soil matrix.
2. Solute transport with first-order decay. With this model, the solute degradation rate is
proportional to the solute concentration. The higher the concentration, the higher the
degradation rate. This is a conventional method for simulating biodegradation in
dissolved hydrocarbon plumes. Modelers using the first-order decay model typically use
the first-order decay coefficient as a calibration parameter, and adjust the decay
coefficient until the model results match field data. With this approach, uncertainties in a
number of parameters (e.g., dispersion, sorption, biodegradation) are lumped together in
a single calibration parameter.
Literature values for the half-life of benzene, a readily biodegradable dissolved
hydrocarbon, range from 10 to 730 days while the half-life for TCE, a more recalcitrant
constituent, is 10.7 months to 4.5 years (Howard et al., 1991). Other applications of the
first-order decay approach include radioactive solutes and abiotic hydrolysis of selected
organics, such as dissolved chlorinated solvents. One of the best sources
of first-order decay coefficients in groundwater systems is The Handbook of Environmental
Degradation Rates (Howard et al., 1991).
The first-order decay model does not account for site-specific information such as the
availability of electron acceptors. In addition, it does not assume any biodegradation of
dissolved constituents in the source zone. In other words, this model assumes
biodegradation starts immediately downgradient of the source, and that it does not
depress the concentrations of dissolved organics in the source zone itself.
3. Solute transport with "instantaneous" biodegradation reaction. Modeling work
conducted by GSI indicate first-order expressions may not be as accurate for describing
natural attenuation processes as the instantaneous reaction assumption (Connor et al.,
1994). Biodegradation of organic contaminants in groundwater is more difficult to
quantify using a first-order decay equation because electron acceptor limitations are not
considered. A more accurate prediction of biodegradation effects may be realized by
incorporating the instantaneous reaction equation into a transport model. This approach
forms the basis for the BIOSCREEN instantaneous reaction model.
To incorporate the instantaneous reaction in BIOSCREEN, a superposition method was
used. By this method, contaminant mass concentrations at any location and time within
the flow field are corrected by subtracting 1 mg/L organic mass for each mg/L of
biodegradation capacity provided by all of the available electron acceptors, in accordance
with the instantaneous reaction assumption. Borden et al. (1986) concluded that this
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BIOSCREEN User's Manual
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simple superposition technique was an exact replacement for more sophisticated oxygen-
limited expressions, as long as the oxygen and hydrocarbon had the same transport rates
(e.g., retardation factor, R = 1). Connor et al. (1994) revived this approach for use in
spreadsheets and compared the results to those from more sophisticated but difficult to
use numerical models. They found this approach to work well, even for retardation
factors greater than 1, so this superposition approach was incorporated into the
BIOSCREEN model (see Appendix A.2).
Which Kinetic Model Should One Use in BIOSCREEN?
BIOSCREEN gives the user three different models to choose from to help see the effect of
biodegradation. At almost all petroleum release sites, biodegradation is present and can be
verified by demonstrating the consumption of aerobic and anaerobic electron acceptors.
Therefore, results from the No Biodegradation model are intended only to be used for
comparison purposes and to demonstrate the effects of biodegradation on plume migration.
Some key factors for comparison of the First-order Decay model and the Instantaneous Reaction
model are presented below:
FACTOR
Able to Utilize Data from
AFCEE Intrinsic Remediation
Protocol?
Simple to Use?
Simplification of Numerical
Model?
Familiar to Modelers?
Key Calibration Parameter
Over - or Underestimates
Source Decay Rate?
First-Order Decay
Model
No - Does not account for
electron acceptors/by-products
Yes
Yes - many numerical models
include first-order decay
More commonly used
First-Order Decay Coefficients
May underpredict rate of source
depletion (see Newell et al,
1995)
Instantaneous
Reaction Model
Yes - Accounts for availability of
electron acceptors and by-products
Yes
Yes - Simplification of
BIOPLUME III model
Used less frequently
Source Term/Dispersivity
May be more accurate for
estimating rate of source depletion
(see Newell et al, 1995)
A key goal of the AFCEE Natural Attenuation Initiative is to quantify the magnitude of RNA
based on field measurements of electron acceptor consumption and metabolic by-product
production. Therefore, the Instantaneous Reaction model is recommended either alone or in
addition to the first-order decay model (if appropriate calibration is performed) for most sites
where the Intrinsic Remediation Technical Protocol (Wiedemeier, Wilson, et al., 1995) has been
applied. For a more rigorous analysis of natural attenuation, the BIOPLUME III model (to be
released in late 1996) may be more appropriate.
BIOSCREEN DATA ENTRY
Three important considerations regarding data input are:
1 To see the example data set in the input screen of the software, click on the "Paste
Example Data Set" button on the lower right portion of the input screen.
2) Because BIOSCREEN is based on the Excel spreadsheet, you have to click outside of the
cell where you just entered data or hit "return" before any of the buttons will work.
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BIOSCREEN User's Manual
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3) Several cells have data that can be entered directly or can be calculated by the model
using data entered in the grey cells (e.g., seepage velocity can be entered directly or
calculated using hydraulic conductivity, gradient, and effective porosity). If the
calculation option does not appear to work, check to make sure that there is still a
formula in the cell. If not, you can restore the formula by clicking on the "Restore
Formulas" button on the bottom right hand side of the input screen. If there still
appears to be a problem, click somewhere outside of the last cell where you entered
data and then click on the "Recalculate" button on the input screen.
Parameter
Seepage Velocity (Vs)
Units
ft/yr
Description
Actual interstitial groundwater velocity, equaling Darcy velocity
divided by effective porosity. Note that the Domenico model and
BIOSCREEN are not formulated to simulate the effects of chemical
diffusion. Therefore, contaminant transport through very slow
hydrogeologic regimes (e.g., clays and slurry walls) should
probably not be modeled using BIOSCREEN unless the effects of
chemical diffusion are proven to be insignificant. Domenico and
Schwartz (1990) indicate that chemical diffusion is insignificant for
Peclet numbers (seepage velocity times median pore size divided
by the bulk diffusion coefficient) > 100.
Typical Values
0.5 to 200 ft/yr
Source of Data
Calculated by multiplying hydraulic conductivity by hydraulic
gradient and dividing by effective porosity. It is strongly
recommended that actual site data be used for hydraulic
conductivity and hydraulic gradient data parameters; effective
porosity can be estimated.
How to Enter Data
1) Enter directly or 2) Fill in values for hydraulic conductivity,
hydraulic gradient, and effective porosity as described below and
have BIOSCREEN calculate seepage velocity. Note: if the
calculation option does not appear to work, check to make sure that
the cell still contains a formula. If not, you can reincarnate the
formula by clicking on the "Restore Formulas" button on the
bottom right hand side of the input screen. If there is still a
problem, make sure to click somewhere outside of the last cell
where you entered data and then click on the "Recalculate" button
on the input screen.
Parameter
Units
Description
Typical Values
Hydraulic Conductivity (K)
cm/ sec
Horizontal hydraulic conductivity of the saturated porous medium.
Clays:
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BIOSCREEN User's Manual
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Source of Data
How to Enter Data
Clean sands: 1x10 ^ -1 cm/s
Gravels: > 1 cm/s
Pump tests or slug tests at the site. It is strongly recommended
actual site data be used for most RNA studies.
Enter directly. If seepage velocity is entered directly,
parameter is not needed in BIOSCREEN.
that
this
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Hydraulic Gradient (i)
ft/ft
The slope of the potentiometric surface. In unconfined aquifers,
this is equivalent to the slope of the water table.
0.0001 - 0.05 ft/ft
Calculated by constructing potentiometric surface maps using static
water level data from monitoring wells and estimating the slope of
the potentiometric surface.
Enter directly. If seepage velocity is entered directly, this
parameter is not needed in BIOSCREEN.
Parameter
Effective Porosity (n)
Units
unitless
Description
Dimensionless ratio of the volume of interconnected voids to the
bulk volume of the aquifer matrix. Note that "total porosity" is the
ratio of all voids (included non-connected voids) to the bulk
volume of the aquifer matrix. Difference between total and
effective porosity reflect lithologic controls on pore structure. In
unconsolidated sediments coarser than silt size, effective porosity
can be less than total porosity by 2-5% (e.g. 0.28 vs, 0.30) (Smith
and Wheatcraft, 1993).
Typical Values
Values for Effective Porosity:
Clay 0.01 - 0.20
Silt 0.01 - 0.30
Fine Sand 0.10 - 0.30
Medium Sand 0.15-0.30
Coarse Sand 0.20 - 0.35
Gravel 0.10 - 0.35
(From Wiedemeier, Wilson,
et si., 1995; originally from
Domenico and Schwartz, 1990
and Walton, 1988).
Sandstone 0.005 - 0.10
Unfract. Limestone 0.001- 0.05
Fract. Granite 0.00005 - 0.01
(From Domenico and Schwartz, 1990)
Source of Data
Typically estimated. One commonly used value for silts and sands
is an effective porosity of 0.25. The ASTM RBCA Standard (ASTM,
1995) includes a default value of 0.38 (to be used primarily for
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BIOSCREEN User's Manual
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How to Enter Data
unconsolidated deposits).
Enter directly. Note that if seepage velocity is entered directly, this
parameter is still needed to calculate the retardation factor and
plume mass.
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Parameter
Longitudinal Dispersivity (alpha x)
Transverse Dispersivity (alpha y)
Vertical Dispersivity (alpha z)
Units
ft
Description
Dispersion refers to the process whereby a plume will spread out in a
longitudinal direction (along the direction of groundwater flow),
transversely (perpendicular to groundwater flow), and vertically
downwards due to mechanical mixing in the aquifer and chemical
diffusion. Selection of dispersivity values is a difficult process, given
the impracticability of measuring dispersion in the field. However,
simple estimation techniques based on the length of the plume or
distance to the measurement point ("scale") are available from a
compilation of field test data. Note that researchers indicate that
dispersivity values can range over 2-3 orders of magnitude for a given
value of plume length or distance to measurement point (Gelhar et al.,
1992). In BIOSCREEN, dispersivity is used as the primary calibration
parameter (see pg 12). For more information on dispersivity, see
Appendix A.4, pg 47).
Typical Values
Typical dispersivity relationships as a function of Lp (plume length or
distance to measurement point in ft) are provided below. BIOSCREEN
is programmed with some commonly used relationships representative
of typical and low-end dispersivities:
Longitudinal Dispersivity
Alpha x = 3 98 0 83 floe
L B1
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BIOSCREEN User's Manual
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How to Enter
Data
Appendix A.4, pg 47).
1) Enter directly or 2) Fill in value of the estimated plume length and
have BIOSCREEN calculate the dispersivities.
Parameter
Units
Description
Typical Values
Source of Data
How to Enter
Data
Estimated Plume Length (Lp)
ft
Estimated length (in feet) of the existing or hypothetical groundwater
plume being modeled. This is a key parameter as it is generally used to
estimate the dispersivity terms (dispersivity is difficult to measure and
field data are rarely collected).
For BTEX plumes, 50 - 500 ft. For chlorinated solvents, 50 to 1000 ft.
To simulate an actual plume length or calibrate to actual plume data,
enter the actual length of the plume. If trying to predict the maximum
extent of plume migration, use one of the two methods below.
1) Use seepage velocity, retardation factor, and simulation time to
estimate plume length. While this may underestimate the plume length
for a non-degrading solute, it may overestimate the plume length for
either the first-order decay model or instantaneous reaction model if
biodegradation is significant.
2) Estimate a plume length, run the model, determine how long the
plume is predicted to become (this will vary depending on the type of
kinetic expression that is used), reenter this value, and then rerun the
model. Note that considerable time and effort can be expended trying
to adjust the estimated plume length term to match exactly the
predicted modeling length. In practice, most modelers make the
assumption that dispersivity values are not very precise, and therefore
select ball-park values based on estimated plume lengths that are
probably ฑ25% of the actual plume length used in the simulations.
Note that BIOSCREEN is very sensitive to the dispersion estimates,
particularly for the instantaneous reaction model.
Enter directly. If dispersivity data are entered directly, this parameter
is not needed in BIOSCREEN.
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BIOSCREEN User's Manual
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3. ADSORI
ON DATA
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Retardation Factor (R)
unitless
The rate at which dissolved contaminants moving through an
aquifer can be reduced by sorption of contaminants to the solid
aquifer matrix. The degree of retardation depends on both aquifer
and constituent properties. The retardation factor is the ratio of the
groundwater seepage velocity to the rate that organic chemicals
migrate in the groundwater. A retardation value of 2 indicates that
if the groundwater seepage velocity is 100 ft/yr, then the organic
chemicals migrate at approximately 50 ft/yr.
BIOSCREEN simulations using the instantaneous reaction
assumption at sites with retardation factors greater than 6 should
be performed with caution and verified using a more sophisticated
model such as BIOPLUME III (see Appendix A.2).
1 to 2 (for BTEX in typical shallow aquifers)
Usually estimated from soil and chemical data using variables
described below (pb = bulk density, n = porosity, Koc = organic
carbon-water partition coefficient, Kd = distribution coefficient, and
foe = fraction organic carbon on uncontaminated soil) with the
following expression:
R = 1 + where Kd = Koc foe
n
In some cases, the retardation factor can be estimated by comparing
the length of a plume affected by adsorption (such as the benzene
plume) with the length of plume that is not affected by adsorption
(such as chloride). Most plumes do not have both types of
contaminants, so it is more common to use the estimation technique
(see data entry boxes below).
1) Enter directly or 2) Fill in the estimated values for bulk density,
partition coefficient, and fraction organic carbon as described
below and have BIOSCREEN calculate retardation.
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Soil Bulk Density (pj
kg/L or g/cm3
Bulk density, in kg/L, of the aquifer matrix (related to porosity and
pure solids density).
Although this value can be measured in the lab, in most cases
estimated values are used. A value of 1.7 kg/L is used frequently.
Either from an analysis of soil samples at a geotechnical lab or more
commonly, application of estimated values such as 1.7 kg/L.
Enter directly. If the retardation factor is entered directly, this
parameter is not needed in BIOSCREEN.
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Organic Carbon Partition Coefficient (Koc)
(mg/kg) / (mg/L) or (L/kg) or (mL/g)
Chemical-specific partition coefficient between soil organic carbon
and the aqueous phase. Larger values indicate greater affinity of
contaminants for the organic carbon fraction of soil. This value is
chemical specific and can be found in chemical reference books.
Note that many users of BIOSCREEN will simulate BTEX as a single
constituent. In this case, either an average value for the BTEX
compounds can be used, or it can be assumed that all of the BTEX
compounds have the same mobility as benzene (the constituent with
the highest potential risk to human health).
Benzene 38 L/kg Ethylbenzene 95 L/kg
Toluene 135 L/kg Xylene 240 L/kg
(ASTM, 1995)
(Note that there is a wide range of reported values; for example,
Mercer and Cohen (1990) report a Koc for benzene of 83 L/kg.
Chemical reference literature or relationships between Koc and
solubility or Koc and the octanol-water partition coefficient (Kow).
Enter directly. If the retardation factor is entered directly, this
parameter is not needed in BIOSCREEN.
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Fraction Organic Carbon (foe)
unitless
Fraction of the aquifer soil matrix comprised of natural organic
carbon in uncontaminated areas. More natural organic carbon
means higher adsorption of organic constituents on the aquifer
matrix.
0.0002 - 0.02
The fraction organic carbon value should be measured if possible by
collecting a sample of aquifer material from an uncontaminated zone
and performing a laboratory analysis (e.g. ASTM Method 2974-87 or
equivalent). If unknown, a default value of 0.001 is often used (e.g.,
ASTM 1995).
Enter directly. If the retardation factor is entered directly, this
parameter is not needed in BIOSCREEN.
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4. BIODEGRADATION DATA
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
First-Order Decay Coefficient (lambda)
1/yr
Rate coefficient describing first-order decay process for dissolved
constituents. The first-order decay coefficient equals 0.693 divided
by the half-life of the contaminant in groundwater. In BIOSCREEN,
the first-order decay process assumes that the rate of biodegradation
depends only on the concentration of the contaminant and the rate
coefficient. For example, consider 3 mg/L benzene dissolved in
water in a beaker. If the half -life of the benzene in the beaker is 728
days, then the concentration of benzene 728 days from now will be
1.5 mg/L (ignoring volatilization and other losses).
Considerable care must be exercised in the selection of a first-order
decay coefficient for each constituent in order to avoid significantly
over-predicting or under-predicting actual decay rates. Note that
the amount of degradation that occurs is related to the time the
contaminants spend in the aquifer, and that this parameter is not
related to the time it takes for the source concentrations to decay by
half.
0.1 to 36 yr1 (see half-life values)
Optional methods for selection of appropriate decay coefficients are
as follows:
Literature Values: Various published references are available listing
decay half-life values for hydrolysis and biodegradation (e.g., see
Howard et al., 1991). Note that many references report the half-lives;
these values can be converted to the first-order decay coefficients
using k = 0.693 / ti/2 (see dissolved plume half-life).
Calibrate to Existing Plume Data: If the plume is in a steady-state
or diminishing condition, BIOSCREEN can be used to determine
first-order decay coefficients that best match the observed site
concentrations. One may adopt a trial-and-error procedure to derive
a best-fit decay coefficient for each contaminant. For still-expanding
plumes, this steady-state calibration method may over-estimate
actual decay-rate coefficients and contribute to an under-estimation
of predicted concentration levels.
1) Enter directly or 2) Fill in the estimated half-life values as
described below and have BIOSCREEN calculate the first-order
decay coefficients.
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Dissolved Plume Solute Half-Life (t1/2)
years
Time, in years, for dissolved plume concentrations to decay by one
half as contaminants migrate through the aquifer. Note that the
amount of degradation that occurs is related to the time the
contaminants spend in the aquifer, and that the degradation IS NOT
related to the time it takes for the source concentrations to decay by
half.
Modelers using the first-order decay model typically use the first-
order decay coefficient as a calibration parameter, and adjust the
decay coefficient until the model results match field data. With this
approach, uncertainty in a number of parameters (e.g., dispersion,
sorption, biodegradation) are lumped together in a single calibration
parameter.
Considerable care must be exercised in the selection of a first-order
decay coefficient for each contaminant in order to avoid significantly
over-predicting or under-predicting actual decay rates.
Benzene 0.02 to 2.0 yrs
Toluene 0.02 to 0.17 yr
Ethylbenzene 0.016 to 0.62 yr
Xylene 0.038 to 1 yr
(from ASTM, 1995)
Optional methods for selection of appropriate decay coefficients are
as follows:
Literature Values: Various published references are available listing
decay half-life values for hydrolysis and biodegradation (e.g., see
Howard et al, 1991).
Calibrate to Existing Plume Data: If the plume is in a steady-state
or diminishing condition, BIOSCREEN can be used to determine
first-order decay coefficients that best match the observed site
concentrations. A trial-and-error procedure may be adopted to
derive a best-fit decay coefficient for each contaminant. For
expanding plumes, this steady-state calibration method may over-
estimate actual decay-rate coefficients and contribute to an under-
estimation of predicted concentration levels.
Enter directly. If the first-order decay coefficient is entered directly,
this parameter is not needed in BIOSCREEN.
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Delta Oxygen (02)
mg/L
This parameter, used in the instantaneous reaction model, is one
component of the total biodegradation capacity of the groundwater
as it flows through the source zone and contaminant plume. The
model assumes that 3.14 mg of oxygen are required to consume 1
mg of BTEX (Wiedemeier, Wilson, et al, 1995). Note that this
parameter is used for the instantaneous reaction model, which is
appropriate only for readily biodegradable compounds such as
BTEX that degrade according to the assumed BIOSCREEN
utilization factors, and is not appropriate for more recalcitrant
compounds such as the chlorinated solvents.
Data from 28 AFCEE sites (see Table 1):
Median = 5.8 mg/L Maximum = 12.7 mg/L Minimum = 0.4
mg/L
For planning studies, typical values taken from Table 1 can be used.
For actual RNA studies, the Air Force Intrinsic Remediation
Technical Protocol (Wiedemeier, Wilson, et al., 1995) should be
applied. Enter the average background concentration of oxygen
minus the lowest observed concentration of oxygen in the source
area. BIOSCREEN automatically applies the utilization factor used
to compute a biodegradation capacity.
Enter directly.
Parameter
Units
Description
Typical Values
Source of Data
Delta Nitrate (N03)
mg/L
This parameter, used in the instantaneous reaction model, is one
component of the total biodegradation capacity of the groundwater
as it flows through the source zone and contaminant plume. The
model assumes that 4.9 mg of nitrate are required to consume 1 mg
of BTEX (Wiedemeier, Wilson, et al, 1995). Note that this parameter
is used for the instantaneous reaction model, which is appropriate
only for readily biodegradable compounds such as BTEX that
degrade according to the assumed BIOSCREEN utilization factors,
and is not appropriate for more recalcitrant compounds such as the
chlorinated solvents.
Data from 28 AFCEE sites (see Table 1):
Median = 6.3 mg/L Maximum = 69.7 mg/L Minimum = 0
mg/L
For planning studies, typical values taken from Table 1 can be used.
For actual RNA studies, the Air Force Intrinsic Remediation
Technical Protocol (Wiedemeier, Wilson, et al., 1995) should be
applied. Enter the average background concentration of nitrate
minus the lowest observed concentration of nitrate in the source
area. BIOSCREEN automatically applies the utilization factor to
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How to Enter Data
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
compute a biodegradation capacity.
Enter directly.
Observed Ferrous Iron (Fe2+)
mg/L
This parameter, used in the instantaneous reaction model, is one
component of the total biodegradation capacity of the groundwater
as it flows through the source zone and contaminant plume. Ferrous
iron is a metabolic by-product of the anaerobic reaction where solid
ferric iron is used as an electron acceptor. The model assumes that
21.8 mg of ferrous iron represents the consumption of 1 mg of BTEX
(Wiedemeier, Wilson, et al, 1995). Note that this parameter is used
for the instantaneous reaction model, which is appropriate only for
readily biodegradable compounds such as BTEX that degrade
according to the assumed BIOSCREEN utilization factors, and is not
appropriate for more recalcitrant compounds such as the chlorinated
solvents.
Because ferrous iron reacts with the sulfide produced from the
reduction of sulfate, some or most of the ferrous iron may not be
observed during groundwater sampling. Some researchers suspect
that the observed ferrous iron concentration is much less (10% or
less) than the actual amount of ferrous iron that has been generated
due to the sorption of ferrous iron onto the aquifer matrix (Lovely,
1995). If this is the case, then the value used for this parameter
should be much higher than the observed maximum concentration
of ferrous iron in the aquifer.
Data from 28 AFCEE sites (see Table 1):
Median = 16.6 mg/L Maximum = 599.5 mg/L Minimum = 0
mg/L
For planning studies, typical values taken from Table 1 can be used.
For actual RNA studies, the Air Force Intrinsic Remediation
Technical Protocol (Wiedemeier, Wilson, et al., 1995) should be
applied. Enter the average observed concentration, in mg/L, of
ferrous (dissolved) iron found in the source area (approximately the
area where ferrous iron has been observed in monitoring wells).
BIOSCREEN automatically applies the utilization factor to compute
a biodegradation capacity.
Enter directly.
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Delta Sulfate (SOJ
mg/L
This parameter, used in the instantaneous reaction model, is one
component of the total biodegradation capacity of the groundwater
as it flows through the source zone and contaminant plume. The
model assumes that 4.7 mg of sulfate are required to consume 1 mg
of BTEX (Wiedemeier, Wilson, et al., 1995). Note that this parameter
is used for the instantaneous reaction model, which is appropriate
only for readily biodegradable compounds such as BTEX that
degrade according to the assumed BIOSCREEN utilization factors,
and is not appropriate for more recalcitrant compounds such as the
chlorinated solvents.
Data from 28 AFCEE sites (see Table 1):
Median = 24.6 mg/L Maximum = 109.2 mg/L Minimum = 0
mg/L
For planning studies, typical values taken from Table 1 can be used.
For actual RNA studies, the Air Force Intrinsic Remediation
Technical Protocol (Wiedemeier, Wilson, et al., 1995) should be
applied. Enter the average background concentration of sulfate
minus the lowest observed concentration of sulfate in the source
area. BIOSCREEN then computes a biodegradation capacity.
Enter directly.
Parameter
Observed Methane (CH )
Units
mg/L
Description
This parameter, used in the instantaneous reaction model, is one
component of the total biodegradation capacity of the groundwater
as it flows through the source zone and contaminant plume.
Methane is a metabolic by-product of methanogenic activity. The
model assumes that 0.78 mg of methane represents the consumption
of 1 mg of BTEX (Wiedemeier, Wilson, et al, 1995). Note that this
parameter is used for the instantaneous reaction model, which is
appropriate only for readily biodegradable compounds such as
BTEX that degrade according to the assumed BIOSCREEN
utilization factors, and is not appropriate for more recalcitrant
compounds such as the chlorinated solvents.
Typical Values
Data from 28 AFCEE sites (see Table 1):
Median = 7.2 mg/L Maximum = 48.4 mg/L Minimum = 0.0
mg/L
Source of Data
For planning studies, typical values taken from Table 1 can be used.
For actual RNA studies, the Air Force Intrinsic Remediation
Technical Protocol (Wiedemeier, Wilson, et al., 1995) should be
applied. Enter the average observed concentration of methane
found in the source area (approximately the area where methane is
observed in monitoring wells). BIOSCREEN automatically computes
a biodegradation capacity.
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How to Enter Data
Enter directly.
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Model Area Length and Width (L and W)
ft
Physical dimensions (in feet) of the rectangular area to be modeled.
To determine contaminant concentrations at a particular point along
the centerline of the plume (a common approach for most risk
assessments), enter this distance in the "Modeled Area Length" box
and see the results by clicking on the "Run Centerline" button.
If one is interested in more accurate mass calculations, make sure
most of the plume is within the zone delineated by the Modeled
Area Length and Width. Find the mass balance results using the
"Run Array" button.
10 to 1000 ft
Values should be slightly larger than the final plume dimensions or
should extend to the downgradient point of concern (e.g., point of
exposure). If only the centerline output is used, the plume width
parameter has no effect on the results.
Enter directly.
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Simulation Time (t)
years
Time (in years) for which concentrations are to
steady-state simulations, enter a large value (i.e
be sufficient for most sites).
be calculated. For
, 1000 years would
1 to 1000 years
To match an existing plume, estimate the time between the original
release and the date the field data were collected. To predict the
maximum extent of plume migration, increase the simulation time
until the plume no longer increases in length.
Enter directly.
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fRCE DATA
Parameter
Source Thickness In Saturated Zone (z)
Units
ft
Description
The Domenico (1987) model assumes a vertical plane source of
constant concentration. For many fuel spill sites the thickness of this
source zone is only 5-20 ft, as petroleum fuels are LNAPLs (light
non-aqueous phase liquids) that float on the water table. Therefore,
the residual source zones that are slowly dissolving, creating the
dissolved BTEX plume, are typically restricted to the upper part of
the aquifer.
Surface
Top of Water-
Bearing Unit
Source Thickness
Bottom of Water-
Bearing Unit
Typical Values
5-50 ft
Source of Data
This value is usually determined by evaluating groundwater data
from wells near the source zone screened at different depths. If this
type of information is not available, then one could estimate the
amount of water table fluctuation that has occurred since the time of
the release and use this value as the source zone thickness (equating
to the smear zone). Otherwise, a simple assumption of 10 feet would
probably be appropriate for many petroleum release sites. Note that
if DNAPLs are present at the site (e.g., a chlorinated solvent site), a
larger source zone thickness would probably be required.
How to Enter Data
Enter directly.
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Source Zone Width
ft
The Domenico (1987) model assumes a vertical plane source of
constant concentration. BIOSCREEN expands the simple one source-
zone approach by allowing up to five source zones with different
concentrations to account for spatial variations in the source area.
10 - 200 ft
To define a varying source concentration across the site:
1) Draw a line perpendicular to the groundwater flow direction in the
source zone. The source zone is typically defined as being the area
with contaminated soils having high concentrations of sorbed
organics, free-phase NAPLs, or residual NAPLs. If the source zone
covers a large area, it is best to choose the most downgradient or
widest point in the source area to draw the perpendicular-to-flow line.
2) Divide the line into 1, 3, or 5 zones. A total of 5 zones is shown on
the input screen.
3) Determine the width and corresponding average concentration of
Zones 1, 2, and 3. Typically Zone 3 will contain the highest
concentration. Note that the model assumes the source zone is
symmetrical and will automatically define source zones 4 and 5 to be
identical to Zones 2 and 1. Therefore, it is not necessary to specify all 5
zones. For simpler problems, you can either use three zones to define
varying source concentrations across the site (enter information in
Zones 2 and 3, and the model will define Zone 4) or just use a single
zone (enter data for Zone 3 only).
4) Enter the width and source concentration into the appropriate
zones on the spreadsheet For example, if a total source width of 100 ft.
is divided into five zones, enter 20 ft for each zone width. Enter the
average concentration observed across each zone.
| 1 / of Source Zones
-"= h^KiS"
_~ Jf?r-^
ซh
"~ N
\
N
s
Bearing Unit ^-^^^^^
Enter directly.
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Parameter
Units
Description
Typical Values
Source of Data
Source Zone Concentration
mg/L
BIOSCREEN requires source zone concentrations that correspond to
the source zone width data (see previous page). Suggested rules of
thumb regarding how to handle multiple constituents are:
1) If the maximum plume length is desired, model lumped
constituents (such as BTEX). If a risk assessment is being performed,
data on individual constituents are needed.
2) If lumped constituents are being modeled (BTEX all together), use
either average values for the chemical-specific data (Koc and lambda)
or the worst-case values (e.g., use the lowest of the Koc and lambda
from the group of constituents being modeled) to overestimate
concentrations. Most modeling will be performed assuming that the
ratio of BTEX at the edge of the plume is the same as at the source.
For more detailed modeling studies, Wilson (1996) has proposed the
following rules of thumb to help account for different rates of
reaction among the BTEX compounds:
If the site is dominated by aerobic degradation (most of the
biodegradation capacity is from oxygen, a relatively rare
occurrence) assume that the benzene will degrade first and that the
dissolved material at the edge of the plume is primarily TEX.
If the site is dominated by nitrate utilization (most of the
biodegradation capacity is from nitrate, a relatively rare occurrence)
assume that benzene will degrade last and that the dissolved
material at the edge of the plume is primarily benzene.
If the site is dominated by sulfate reduction (most of the
biodegradation capacity is due to sulfate utilization, a more
common occurrence) assume that the benzene will degrade at the
same rate as the TEX constituents and that the dissolved material at
the edge of the plume is a mixture of BTEX.
If the site is dominated by methane production (most of the
biodegradation capacity is due to methanogenesis, a more common
occurrence) assume that benzene will degrade last and that the
dissolved material at the edge of the plume is primarily benzene.
3) If individual constituents are being modeled with the
instantaneous reaction assumption, note that the total biodegradation
capacity must be reduced to account for electron acceptor utilization
by other constituents present in the plume. For example, in order to
model benzene as an individual constituent using the instantaneous
reaction model in a BTEX plume containing equal source
concentrations of benzene, toluene, ethylbenzene and xylene, the
amount of oxygen, nitrate, sulfate, iron, and methane should be
reduced by 75% to account for utilization by toluene, ethylbenzene,
and xylene.
0.010 to 120 mg/L
Source area monitoring well data (see figure on previous page).
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How to Enter Data Enter directly
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Parameter
Source Half-Life (Value Calculated by Model;
Units
years
Descriptio
n
The Domenico (1987) model assumes the source is infinite, i.e. the source
concentrations are constant. In BIOSCREEN, however, an approximation for a
declining source concentration has been added. Note that this is an
experimental relationship, and it should be applied with caution. The
declining source term is based on the following assumptions:
There is a finite mass of organics in the source zone present as a free-phase
or residual NAPL. The NAPL in the source zone dissolves slowly as fresh
groundwater passes through.
The change in source zone concentration can be approximated as a first-
order decay process. For example, if the source zone concentration "half-life"
is 10 years and the initial source zone concentration is 1 mg/L, then the
source zone concentration will be 0.5 mg/L after 10 years, and 0.25 mg/L
after 20 years.
Note that the assumption that dissolution is a first-order process is only an
approximation, and that source attenuation is best described by first-order
decay when concentrations are relatively low (< 1 mg/L). For more
information on dissolution, see Newell et al, (1994). The source half-life IS
NOT related to lambda, the biodegradation half-life for dissolved
constituents. Lambda is used to calculate the amount of biodegradation of
dissolved organics after they leave the source zone and travel through the
plume area. The source half-life is related to the rate of dissolution occurring
in the source zone, and describes the change in source concentrations over
time.
The BIOSCREEN software automatically calculates the source zone
concentration half-life if the user enters a best estimate for the mass of
dissolvable organics zone (soluble organic constituents sorbed on the soil,
residual NAPLs, and free product) in the source. The half-life of the
dissolution process can be approximated if one knows the mass of
dissolvable organics in the source zone (in mg or kg), the flow rate through
the source zone, and the average concentration of dissolved organics that
leave the source zone. The equation is based on integrating the
concentration vs. time relationship (first-order decay) and using the
relationship that the mass in the source zone over time is proportional to the
source concentration over time. This yields the following expression for the
half-life of the concentration of dissolved organics in the source zone (see
Appendix A.3):
half s
ace= (0.693*M0)/(Q*C0)
where:
MO
H:
c r^--.:.^.----..:.
t, ,, = Half-life of source
halt source
concentration (yrs)
Q = Groundwater flow through
source zone (L/yr)
CQ = Effective source zone cone.
(observed concentration + biodeg
capacity for inst. react.
assumption) at t = 0 (mg/L)
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MQ = Mass of dissolvable orj
source zone at t = 0 (mg)
panics in
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Parameter
Description
(cont'd)
Key Questions:
Typical Values
Source of Data
How to Enter
Source Half -Life (Value Calculated by Model) (Cont'd)
Why are there two source half-lives reported? Note that BIOSCREEN
automatically selects the correct source half-life value depending on
which kinetic model is being used (see Which Model Should One Use?
under BIOSCREEN Concepts).
Two source half-lives are reported by the model in the source half-life
cell: the smaller number will be the source half -life from dissolution if
Instantaneous Reaction kinetics are used, and the larger value will be
for No Degradation or First-order Decay kinetics. The first-order
decay model assumes biodegradation starts immediately
downgradient of the source, and that the rate of dissolution is reflected
by the concentration of dissolved organics actually measured in
monitoring wells. In other words, the first-order decay model assumes
CQ is equal to the observed source concentration.
The instantaneous reaction model assumes biodegradation is
occurring directly in the source zone, and that the effective source
zone concentration CQ is equal to the measured concentration in the
source zone plus any "missing" concentration due to biodegradation.
For example, if the source zone concentration in monitoring wells is 5
mg/L, and the biodegradation capacity is 10 mg/L, the effective
source concentration CQ (concentration before biodegradation) is 15
mg/L. In other words, CQ is equal to the measured source
concentration plus the biodegradation capacity provided by the
electron acceptor concentration. This means use of the instantaneous
reaction assumption will result in higher dissolution rates and shorter
source lifetimes ( see Newell et al, 1995).
Does BIOSCREEN account for travel time away from the declining
source? With the declining source option in BIOSCREEN, the
concentration for any location and any time is calculated using a
source concentration determined by the first-order decay calculations
shown above. The time used to determine the source concentration is
adjusted to account for the travel time between the source and
measurement point.
For example, consider the case where a declining source term is used
with a source half-life of 10 years and a solute velocity of 100 ft/yr. To
calculate the concentration at a point 2000 ft away at time = 30 years,
BIOSCREEN foUows these steps
1) Calculates travel time from point to source: 2000/100 = 20 years
2) Subtracts travel time from simulation time: 30 yrs - 20 yrs = 10 yrs
3) Calculates source decay coeff.: ksource = 0.693/ (source half -life)
4) Calculates source cone, at t = 10 yr: Cio= Co exp(-ksourcexlฐyrs)
1 to 10,000 years
Calculated by model from soluble mass in NAPL and soil (see below),
source concentrations, and groundwater velocity.
Calculated directly by model. Change by changing soluble mass.
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Data
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Parameter
Units
Description
Typical Values
Source of Data
How to Enter
Soluble Mass in NAPL, Soil
kg
The best estimate of dissolvable organics in the source zone is
obtained by adding the mass of dissolvable organics on soils, free-
phase NAPLs, and residual NAPLs. This quantity is used to estimate
the rate that the source zone concentration declines. Note that this is
an experimental and unverified model that should be applied with
care (the model probably underpredicts removal rate).
For gasoline or JP-4 spills, BTEX is usually assumed to comprise the
bulk of dissolvable organics in the source zone. To simulate a
declining source, use the method described below. For constant-
source simulations, either enter a very large number for soluble mass
in the source zone (e.g., 1,000,000 kg) or type "Infinite".
0.1 to 100,000 kg
This information will most likely come from either:
1) Estimates of the mass of spilled fuel (remember to convert the total
mass of spilled fuel to the dissolvable mass; for example BTEX
represents only 5-15% of the total mass of gasoline).
2) Integration of maps showing contaminated soil zones (data in
mg/kg) and/ or NAPL zones (usually product thickness). The user
should estimate the volume of contaminated soil, convert to kg of
contaminated soil, and multiply by the average soil concentration. To
make the estimate more accurate, the user might have to divide the
soil into different zones of soil concentrations, into unsaturated vs.
saturated soil, and/ or into different depths. (One standard approach
is to divide into a vertically averaged unsaturated zone map and a
vertically averaged saturated zone map.) If the user is making
estimates from NAPL data, remember the thickness of product in a
aquifer is only 10-50% of the actual product thickness in the well
(Bedient et al, 1994).
Note that the data is to be entered in kg, and the model will convert
the results to estimate the source half-life. An example is provided
below assuming a bulk density of 1.7 kg/L (e.g., 100 ft2 x 20 ft x 28.3
L/ft3 x 1.7 kg/L x 600 mg/Kg x 1Q-6 kg/mg = 58 kg):
SOLUBLE
Model ^ฃ^\ MASS
Source v JS~\ ll -Soil Area 1: 100 sq. ft Depth 20 ft
Zone \ /a S. i-TI c.ซ V
>/l A^fc-*-T|| A C -1 f^ ' ?ฐ ^-S
yWA 1 I Jl II Average Soil Concentration b
PI / VW7/ =600 mg/Kg BTEX
Plume . / \^2vSL
/x^ ^*\y ^^ Soil Zone 2: 22ฐ sq- ft Depth 20 ft
/ / \ Average Soil Concentration
/ / \ =50 mg/Kg BTEX
1 / ^ Soil Zone 3: 400 sq. ft Depth 20 ft
^f Average Soil Concentration
\-^S =10 mg/Kg BTEX
TOTAL SOLUBLE MASS 73 Kg
Enter directly.
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Data
Parameter
Units
Description
Typical Values
Source of Data
How to Enter Data
Field Data for Comparison
mg/L
These parameters are concentrations of dissolved organics in wells
near the centerline of the plume. These data are used to help
calibrate the model and are displayed with model results in the "Run
Centerline" option.
0.001 to 50 mg/L
Monitoring wells located near the centerline of the plume.
Enter as many or as few of these points as needed. The data are used
only to help calibrate the model when comparing the results from the
centerline option. Note that the distance from source values cannot
be changed; use the closest value possible.
ANALYZING BIOSCREEN OUTPUT
The output shows concentrations along the centerline (for all three kinetic models at the same
time) or as an array (one kinetic model at a time). Note that the results are all for the time entered
in the "Simulation Time" box.
Centerline Output
Centerline output is displayed when the "Run Centerline" button is pressed on the input screen.
The centerline output screen shows the average concentration at the top of the saturated zone
(Z=0) along the centerline of the plume (Y=0). Clicking on "Animate" divides the simulation into
10 separate time periods and shows the movement of the plume based on the three BIOSCREEN
models (red: no degradation, blue: first-order decay, green: instantaneous reaction). Note that
all concentrations are displayed in units of mg/L.
Array Output
The array output is displayed when the "Run Array" button is pressed on the Input screen. The
user is asked to select one of the three model types (no degradation, first-order decay, or
instantaneous reaction). A 3-D graphic shows results on a 10-point-long by 5-point-wide grid. To
alter the modeled area, adjust the Model Area Length and Width parameters on the input screen.
To see the plume array that exceeds a certain target level (such as an MCL or risk-based cleanup
level), enter the target level in the box and push "Plot Data > Target". Only sections of the plume
exceeding the target level will be displayed. To see all the data again, push "Plot All Data". Note
that BIOSCREEN automatically resets this button to "Plot All Data" when the "Run Array"
button is pressed on the input screen. An approximate mass balance is presented on the array
output screen as described below.
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BIOSCREEN User's Manual June 1996
Calculating the Mass Balance
Plume Mass if No Biodegradation(kg)
The model calculates the total amount of dissolved contaminant that has left the source zone. If
the source is an infinite source, then the calculation is based on the discharge of groundwater
through the source zone (Darcy velocity for groundwater times the total source width times the
source depth) times the average concentration of the source zone (a weighted average of
concentration and source length for each of the different source zones) times the simulation
time.
If the source is a declining source, an exponential source decay term is used to estimate the
mass of organics that have left the source zone (see Source Data: Varying Concentrations Over
Time). Note that the source decay term is for dissolution of soluble organics from the source
zone and is not related to the first-order decay term for the dissolved constituents.
Note that the total mass in the plume is the same for the No Degradation and First-order Decay
models but is different for the Instantaneous Reaction model. The source zone dissolution rate
is calculated to be much higher if the instantaneous reaction model is selected. The
instantaneous reaction assumes that active biodegradation reactions occur in the source zone,
and that the observed concentrations of organics in source zone monitoring wells reflect
conditions after biodegradation. In this case, the actual concentration of organics coming off
the source zone is equal to the measured concentration plus the biodegradation capacity of the
upgradient groundwater. The resulting higher effective dissolution rate equates to a greater
amount of mass leaving the source area, leading to different mass values for the Instantaneous
Reaction model.
Actual Plume Mass(kg)
BIOSCREEN calculates the mass of organics in the 5x10 plume array for the three models:
1) No Degradation 2) 1st Order Decay 3) Instantaneous Reaction
The mass is calculated by assuming that each point represents a cell equal to the incremental
width and length (except for the first column which is assumed to be half as long as the other
columns because the source is assumed to be in the middle of the cell). The volume of affected
groundwater in each cell is calculated by multiplying the area of each cell by the source depth
and by porosity (the mass balance calculation assumes 2-D transport). The mass of organics in
each cell is then determined by multiplying the volume of groundwater by the concentration
and then by the retardation factor (to account for sorbed constituents).
How BIOSCREEN Estimates Actual Plume Mass for Biodegradation Models
If the mass of organics in the 5x10 plume array is within 50% to 150% of the mass of organics
that have left the source (see box above), then two values are calculated:
% Biodegraded, 1st order decay = (Plume Mass, 1st order decay) * 100 / (Plume mass, no
biodeg)
% Biodegraded, inst. react. = (Plume Mass, inst. react) * 100 / (Plume mass, no biodeg)
These percentages are multiplied against the Plume Mass if No Biodegradation Value (first
box) to estimate the actual plume mass for the two biodegradation models. If the No
Degradation model has been selected, there is no biodegradation, and the Actual Plume Mass
(second box) will equal the Plume Mass if No Biodegradation (first box).
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BIOSCREEN User's Manual June 1996
If BIOSCREEN Says " Can't Calc"
If the mass of organics in the plume does not fall within 50% to 150% of the mass of organics
that have left the source (first box), then the model concludes that the modeled area (see Input
Screen, Section 5: General Data) is not sized correctly to capture enough mass in the 5X10 array
and writes "Can't Calc" in the box. The user is encouraged to adjust the modeled length and
width to capture most of the No Degradation plume in the 5x10 array. In addition, sometimes
source conditions with variable concentrations and widths (see input screens) can make it
difficult to accurately capture the plume mass. If the user has problems obtaining a mass
balance even after changing the modeled area, change the source term to a single source zone
(instead of 3 or 5 zones) to improve the accuracy of the mass balance.
If problems still exist, ensure that the vertical dispersivity term (Section 2 on the Input Screen)
is set to 0 (the default value). The mass balance calculations are less accurate for three-
dimensional simulations.
Plume Mass Removed by Biodegradation (kg)
An estimate of the mass of contaminants that are biodegraded is provided in BIOSCREEN. The
model subtracts the Actual Plume Mass (second box) from the Plume Mass if No
Biodegradation (first box). For the No Degradation model, the first box equals the second box,
and Plume Mass Removed by Biodeg is zero. For the other two cases, the 2 boxes will differ,
and the amount of biodegradation will be calculated. The value beneath the third box shows
the % of organics that have left the source and have been biodegraded.
Change in Electron Acceptor/Byproduct Masses (kg)
BIOSCREEN uses the Plume Mass Removed by Biodegradation to back-calculate the amount of
measurable electron acceptors consumed and the amount of measurable metabolic by-products
that have been produced.
For example, the amount of oxygen consumed is calculated by:
Oxygen Consumed (kg) = (Plume Mass Removed by Biodeg) * (Delta O2/Util. Fact.)
(Biodeg. Capacity)
(see Biodegradation Capacity section to see how this term is calculated)
Note that the total sum of consumed electron acceptors does not equal the Plume Mass
Removed by Biodegradation. This is because the stoichiometry of the biodegradation reactions
do not represent a 1:1 relationship between the mass of hydrocarbon and electron acceptor
consumed (see Utilization Factor section).
Original Mass in Source (kg)
Equal to the Soluble Mass in NAPL and Soil entered by the user on the Input Screen. If the
user has selected an "Infinite" mass to simulate a non-declining source, this box will show
"Infinite."
Mass in Source Now (kg)
The amount of mass remaining in the source zone at the end of the simulation period is
calculated and displayed in this box. This calculation is performed as follows:
(Mass in the Source Now) =
(Original Mass in Source) - (Actual Plume Mass + Plume Mass Removed by Biodeg)
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BIOSCREEN User's Manual June 1996
Current Volume of Groundwater in Plume (ac-ft)
If the mass of organics in the plume falls within 50% to 150% of the mass of organics that have
left the source (first box), then the model concludes the modeled area (see Input Screen, Section
5: General Data) is appropriately sized to estimate the volume of the plume. In this case
BIOSCREEN counts the number of cells in the 5 x 10 array with concentration values greater
than 0, and multiplies this by the volume of groundwater in each cell (length * width * source
thickness * porosity).
If the user wishes to estimate the volume of the plume above a certain target level, enter the
target level in the appropriate box and press the appropriate model to display the result (No
Degradation, 1st Order Decay, or Instantaneous Reaction).
Note that the model does not account for the effects of any vertical dispersion.
Flowrate of Water Through Source Zone (ac-ft/yr)
Using the Darcy velocity, the source thickness, and the source width, BIOSCREEN calculates
the rate that clean groundwater moves through the source zone where it will pick up dissolved
hydrocarbons. Note that the groundwater Darcy velocity is equal to the groundwater seepage
velocity multiplied by porosity.
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BIOSCREEN User's Manual June 1996
BIOSCREEN TROUBLESHOOTING TIPS
Minimum System Requirements
The BIOSCREEN model requires a computer system capable of running Microsoftฎ Excel 5.0 for
Windows. Because of the volume of calculations required to process the numerical data
generated by the model, GSI recommends running the model on a system equipped with a 486
DX or higher processor running at 66 MHz or faster. A minimum of 8 Megabytes of system
memory (RAM) is strongly recommended.
The model's input and output screens are optimized for display at a monitor resolution of
640x480 (Standard VGA). If you are using a higher resolution, for example 800x600 or 1024x768,
see Changing the Model's Display.
For best results, Start Excel and Load the BSCREEN.XLS file from the File / Open menu.
Spreadsheet-Related Problems
The buttons won't work: BIOSCREEN is built in the Excel spreadsheet environment, and
to enter data one must click anywhere outside the cell where you just entered data. If you can see
the numbers you just entered in the data entry part of Excel above the spreadsheet, the data has
not yet been entered. Click on another cell to enter the data.
#### is displayed in a number box: The cell format is not compatible with the value, (e.g. the
number is too big to fit into the window). To fix this, select the cell, pull down the format menu,
select "Cells" and click on the "Number" tab. Change the format of the cell until the value is
visible. If the values still cannot be read, select the format menu, select "Cells" and click on the
"Font" tab. Reduce the font size until the value can be read.
#DIV/0! is displayed in a number box: The most common cause of this problem is that some
input data are missing. In some cases, entering a zero in a box will cause this problem. Double
check to make certain that all of the input cells required for your run have data. Note that for
vertical dispersivity, BIOSCREEN will convert a "0" into the data entry cell into a very low
number (1x10-") to avoid #DIV/0! errors.
There once were formulas in some of the boxes on the input screen, but they were accidentally
overwritten: Click on the "Restore Formulas for Vs, Dispersivities, R, and lambda" button on the
bottom right-hand side of the input screen. Note that this button will also restore the formulas
that make the Source Width and Source Concentrations for source zones 4 and 5 equal to source
zones 2 and 1, respectively.
The graphs seem to move around and change size: This is a feature of Excel. When graph scales
are altered to accommodate different plotted data, the physical size of the graphs will change
slightly, sometimes resulting in a graph that spreads out over the fixed axis legends. You can
manually resize the graph to make it look nice again by double-clicking on the graph and resizing
it (refer to the Excel User's Manual).
Common Error Messages
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BIOSCREEN User's Manual June 1996
Unable to Load Help File: The most common error message encountered with BIOSCREEN is
the message "Unable to Open Help File" after clicking on a Help button. Depending on the
version of Windows you are using, you may get an Excel Dialog Box, a Windows Dialog Box, or
you may see Windows Help load and display the error. This problem is related to the ease with
which the Windows Help Engine can find the data file, BIOSCRN.HLP. Here are some
suggestions (in decreasing order of preference) for helping WinHelp find it:
If you are fortunate enough to be asked to find the requested datafile, do so. It's called
BIOSCRN.HLP, and it was installed in the same directory/folder as the BIOSCRN.XLS
file.
Use the File/Open menus from within Excel instead of double-clicking on the filename
or Program Manager icon to open the BIOSCRN.XLS file. This sets the "current
directory" to the directory containing the Excel file you just opened.
Change the WinHelp call in the VB Module to "hard code" the directory information.
That way, the file name and its full path will be explicitly passed to WinHelp. Hints for
doing this are in the VBA module. Select the Macro Module tab and search for the text
"Helpfile".
As a last resort, you can add the BIOSCREEN directory to your path (located in your
AUTOEXEC.BAT file), and this problem will be cured. You will have to reboot your
machine, however, to make this work
The BIOSCREEN system was designed to be used on a PC with Windows configured to a
standard VGA resolution of 640x480 pixels. If you are using a larger monitor and your video
resolution is set to 800x600 pixels or greater, you will need to change the zoom factor in the
Visual Basic code.
In the first three lines in the Macro Module of the BIOSCREEN spreadsheet, change the number
after the equals sign in the following line:
Const ZoomValue = 65
If your display resolution is standard VGA (640x480), use 65 for the zoom value. If your
resolution is 800x600, use a zoom value of 82. If your resolution is not 640x480 or 800x600, if your
video performance is seriously degraded, or if you experience display problems, you may need to
change your video resolution (see the on-line help for Windows Setup or consult your Windows
installation manuals) and experiment with other values for ZoomValue.
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REFERENCES
American Society for Testing and Materials, 1995, "Standard Guide for Risk-Based Corrective Action
Applied at Petroleum Release Sites," ASTM E-1739-95, Philadelphia, PA.
Bedient, P. B., H.S. Rifai, and C.J. Newell, 1994. Groundwater Contamination: Transport and Remediation,
Prentice-Hall.
Borden, R. C, P. B. Bedient, M. D. Lee, C. H. Ward and J. T. Wilson, 1986. "Transport of Dissolved
Hydrocarbons Influenced by Oxygen Limited Biodegradation: 2. Field Application," Water Resour. Res.
22:1983-1990.
Connor, J. A., C.J. Newell, J.P. Nevin, and H.S. Rifai, 1994. "Guidelines for Use of Groundwater Spreadsheet
Models in Risk-Based Corrective Action Design," National Ground Water Association, Proceedings of
the Petroleum Hydrocarbons and Organic Chemicals in Ground Water Conference, Houston, Texas,
November 1994, pp. 43-55.
Connor, J.A., J. P. Nevin, R. T. Fisher, R. L. Bowers, and C. J. Newell, 1995a. RBCA Spreadsheet System and
Modeling Guidelines Version 1.0, Groundwater Services, Inc., Houston, Texas.
Connor, J.A., J. P. Nevin, M. Malander, C. Stanley, and G. DeVaull, 1995b. Tier 2 Guidance Manual for
Risk-Based Corrective Action, Groundwater Services, Inc., Houston, Texas.
Davis J.W., NT. Kliker, and C.L. Carpenter, 1994, Natural Biological Attenuation of Benzene in Ground
Water Beneath a Manufacturing Facility, Ground Water, Vol. 32, No. 2., pg 215-226.
Domenico, P. A. 1987. An Analytical Model for Multidimensional Transport of a Decaying Contaminant
Species. Journal of Hydrology, 91 (1987) 49-58.
Domenico, P. A. and F. W. Schwartz, 1990. Physical and Chemical Hydrogeology, Wiley, New York, NY.
Gelhar, L.W., Montoglou, A., Welty, C., and Rehfeldt, K.R., 1985. "A Review of Field Scale Physical Solute
Transport Processes in Saturated and Unsaturated Porous Media," Final Proj. Report., EPRI EA-4190,
Electric Power Research Institute, Palo Alto, Ca.
Gelhar, L.W., C. Welty, and K.R. Rehfeldt, 1992. "A Critical Review of Data on Field-Scale Dispersion in
Aquifers." Water Resources Research, Vol. 28, No. 7, pg 1955-1974.
Howard, P. H., R. S. Boethling, W. F. Jarvis, W. M. Meylan, and E. M. Michalenko, 1991. Handbook of
Environmental Degradation Rates, Lewis Publishers, Inc., Chelsea, MI.
Lee, M.D. V.W. Jamison, and R.L. Raymond, 1987, "Applicability of In-Situ Bioreclamation as a Remedial
Action Alternative," in Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Ground
Water Conference, Houston, Texas, November 1987, pp. 167-185.
Lovely, D. Personal Communication. 1995.
Mercer, J. W., and R. M. Cohen, 1990. "A Review of Immiscible Fluids in the Subsurface: Properties,
Models, Characterization and Remediation," Journal of Contaminant Hydrology, 6 (1990) 107-163.
Newell, C. J., R. L. Bowers, and H. S. Rifai, 1994. "Impact of Non-Aqueous Phase Liquids (NAPLs) on
Groundwater Remediation," American Chemical Society Symposium on Multimedia Pollutant
Transport Models, Denver, Colorado, August 1994.
Newell, C.J., J.W. Winters, H.S. Rifai, R.N. Miller, J. Gonzales, T.H. Wiedemeier, 1995. "Modeling Intrinsic
Remediation With Multiple Electron Acceptors: Results From Seven Sites," National Ground Water
Association, Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Ground Water
Conference, Houston, Texas, November 1995, pp. 33-48.
Rifai, H.S., personal communication, 1994.
Rifai, H. S. and P.B. Bedient, 1990, "Comparison of Biodegradation Kinetics With an Instantaneous Reaction
Model for Groundwater," Water Resources Research, Vol. 26, No. 4, pp. 637-645, April 1990.
Rifai, H. S., P. B. Bedient, R. C. Borden, and J. F. Haasbeek, 1987, BIOPLUME II - Computer Model of Two-
Dimensional Transport under the Influence of Oxygen Limited Biodegradation in Ground Water,
User's Manual, Version 1.0, Rice University, Houston, TX, 1987.
Rifai, H. S., P. B. Bedient, J. T. Wilson, K. M. Miller, and J. M. Armstrong, 1988, "Biodegradation Modeling at
Aviation Fuel Spill Site," /. Environ. Engineering 114(5):1007-1029,1988.
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BIOSCREEN User's Manual June 1996
Rifai, H. S., G. P. Long, P.B. Bedient, 1991. "Modeling Bioremediation: Theory and Field Application,"
Proceedings, In Situ Bioreclamation, Applications and Investigations for Hydrocarbon and
Contaminated Site Remediation, Ed. by R. E. Hinchee and R. F. Olfenbuttel, Battelle Memorial Institute,
Butterworth-Heinemann, Boston, March 1991.
Rifai, H. S., C. J. Newell, R. N. Miller, S. Taffinder, and M. Rounsavill, 1995. "Simulation of Natural
Attenuation with Multiple Electron Acceptors," Intrinsic Remediation, Edited by R. Hinchee, J. Wilson,
and D. Downey, Battelle Press, Columbus, Ohio, p 53-65.
Pickens, J.F., and G.E. Grisak,1981. "Scale-Dependent Dispersion in a Stratified Granular Aquifer," J. Water
Resources Research, Vol. 17, No. 4, pp 1191-1211.
REFERENCES (Cont'd)
Smith, L. and S.W. Wheatcraft, 1993. "Groundwater Flow" in Handbook of Hydrology, David Maidment,
Editor, McGraw-Hill, New York.
Snoeynik, V., and D. Jenkins, 1980. Water Chemistry. John Wiley and Sons, New York, New York.
U.S. Environmental Protection Agency, 1986, Background Document for the Ground-Water Screening
Procedure to Support 40 CFR Part 269 Land Disposal. EPA/530-SW-86-047, January 1986.
Walton, W.C., 1988. Practical Aspects of Groundwater Modeling: National Water Well Association,
Worthington, Ohio.
Wiedemeier, T.H., M. A. Swanson, J. T. Wilson, D. H. Kampbell, and R. N. Miller, 1995. "Patterns of
Intrinsic Bioremediation at Two United States Air Force Bases", Proceedings of the 1995 Battelle
Conference on Bioremediation, San Diego, California.
Wiedemeier, T.H., R.N Miller, J.T. Wilson, and D.H. Kampbell, 1995. "Significance of Anaerobic Processes
for the Intrinsic Bioremediation of Fuel Hydrocarbons", 1995. National Ground Water Association,
Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Ground Water Conference,
Houston, Texas, November 1995.
Wiedemeier, T. H., Wilson, J. T., Kampbell, D. H, Miller, R. N, and Hansen, J.E., 1995. "Technical Protocol
for Implementing Intrinsic Remediation With Long-Term Monitoring for Natural Attenuation of Fuel
Contamination Dissolved in Groundwater (Revision 0)", Air Force Center for Environmental
Excellence, April, 1995.
Wilson J. T., 1994. Presentation at Symposium on Intrinsic Bioremediation of Ground Water, Denver,
Colorado, August 1-Sept. 1,1994, EPA 600/R-94-162.
Wilson, J. T., J. F. McNabb, J. W. Cochran, T. H. Wang, M. B. Tomson, and P. B. Bedient, 1985. "Influence Of
Microbial Adaptation On The Fate Of Organic Pollutants In Groundwater," Environmental Toxicology
and Chemistry, v. 4, p. 721-726.
Wilson, J. T., 1996. Personal communication. He may be reached at the Subsurface Protection and
Remediation Division of the National Risk Management Laboratory, Ada, Oklahoma.
Xu, Moujin and Y. Eckstein, 1995, "Use of Weighted Least-Squares Method in Evaluation of the
Relationship Between Dispersivity and Scale," Journal of Ground Water, Vol. 33, No. 6, pp 905-908.
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APPENDIX A.I DOMENICO ANALYTICAL MODEL
The Domenico (1987) analytical model, used by BIOSCREEN, is designed for the multidimen-
sional transport of a decaying contaminant species. The model equation, boundary conditions,
assumptions, and limitations are discussed below.
Domenico Model with Instantaneous Reaction Superposition Algorithm
C(x,y,o,t) I \ x
v * / = -exp.
(Co + BC) 8 [ax
erfc\
. \ii2\\
Iv) \\
' )\
2(avt)
where:
f\ ฑ Yjf | - erf\ -^ ^12 \ (
Definitions
BC Bi ode gradation capacity (mg/L)
C(x,y,z,t) Concentration at distance x downstream of
source and distance y off centerline of plume
at time t (mg/L)
Cs Concentration in Source Zone (mg/L)
Co Concentration in Source Zone at t=0 (mg/L)
x Distance downgradient of source (ft)
y Distance from plume centerline of source (ft)
z Distance from surface to measurement point
(assumed to be 0; concentration is always
assumed to be at top of water table).
C(ea)n Concentration of electron acceptor n in
groundwater (mg/L)
UFn Utilization factor for electron acceptor n (i.e., mass ratio
of electron acceptor to hydrocarbon consumed in
biodegradation reaction)
0^ Longitudinal groundwater dispersivity (ft)
Oy Transverse groundwater dispersivity (ft)
az Vertical groundwater dispersivity (ft)
9e Effective Soil Porosity
K First-Order Degradation Rate (day'l)
u Groundwater Seepage Velocity (ft/yr)
K Hydraulic Conductivity (ft/yr)
R Constituent retardation factor
i Hydraulic Gradient (cm/cm)
Y Source Width (ft)
Z Source Depth (ft)
The initial conditions are:
1) c(x, y, z, 0) = 0 (Initial concentration = 0 for x, y, z, > 0)
2) c(0/ Y, Z, 0) = Co (Source concentration for each vertical plane source = Co at time 0)
The key assumptions in the model are:
1) The aquifer and flow field are homogenenous and isotropic.
2) The groundwater velocity is fast enough that molecular diffusion in the dispersion
terms can be ignored (may not be appropriate for simulation of transport through
clays).
3) Adsorption is a reversible process represented by a linear isotherm.
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The key limitations to the model are:
1) The model should not be applied where pumping systems create a complicated flow
field.
2) The model should not be applied where vertical flow gradients affect contaminant
transport.
3) The model should not be applied where hydrogeologic conditions change dramatically
over the simulation domain.
The most important modifications to the original Domenico model are:
1) The addition of "layer cake" source terms where three Domenico models are
superimposed one on top of another to yield the 5-source term used in BIOSCREEN
(see Connor et al., 1994; and the Source Width description in the BIOSCREEN Data
Entry Section).
2) Addition of the instantaneous reaction term using the superposition algorithm (see
Appendix A.2, below). For the instantaneous reaction assumption, the source
concentration is assumed to be an "effective source concentration" (Coe) equal to the
observed concentration in the source zone plus the biodegradation capacity (see
"Source Concentration" on the BIOSCREEN Data Entry section).
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BIOSCREEN User's Manual June 1996
APPENDIX A. 2 INSTANTANEOUS REACTION - SUPERPOSITION ALGORITHM
Early biodegradation research focused on the role of dissolved oxygen in controlling the rate of
biodegradation in the subsurface (Borden et al., 1986; Lee et al, 1987). Because microbial
biodegradation kinetics are relatively fast in comparison to the rate of oxygen transport in the
groundwater flow system, Borden demonstrated that the biodegradation process can be
simulated as an instantaneous reaction between the organic contaminant and oxygen. This
simplifying assumption was incorporated into the BIOPLUME I numerical model which
calculated organic mass loss by superposition of background oxygen concentrations onto the
organic contaminant plume. In BIOPLUME II, a dual-particle mover procedure was incorporated
to more accurately simulate the separate transport of oxygen and organic contaminants within
the subsurface (Rifai et al, 1987; Rifai, et al, 1988).
In most analytical modeling applications, contaminant biodegradation is estimated using a first-
order decay equation with the biodecay half-life values determined from research literature or
site data. However, by ignoring oxygen limitation effects such first-order expressions can
significantly overestimate the rate and degree of biodegradation, particularly within low-flow
regimes where the rate of oxygen exchange in a groundwater plume is very slow (Rifai, 1994). As
a more accurate method of analysis, Newell recommended incorporation of the concept of oxygen
superposition into an analytical model (Connor et al., 1994) in a manner similar to that employed
in the original BIOPLUME model (Borden et al. 1986). By this method, contaminant mass
concentrations at any location and time within the flow field are corrected by subtracting 1 mg/L
organic mass for each 3 mg/L of background oxygen, in accordance with the instantaneous
reaction assumption. Borden et al (1986) concluded this simple superposition technique was an
exact replacement for more sophisticated oxygen-limited models, as long as the oxygen and the
hydrocarbon had the same transport rates (e.g., retardation factor, R = 1).
In their original work, Borden et al. (1986) noted that for highly sorptive contaminants the
oxygen-superposition method might erroneously characterize biodegradation due to the differing
transport rates of dissolved oxygen and the organic contaminant within the aquifer matrix.
However, as demonstrated by Connor et al. (1994), the oxygen superposition method and
BIOPLUME II (dual particle transport) are in reasonable agreement for contaminant retardation
factors as high as 6. Therefore, the superposition method can be employed as a reasonable
approximation in BIOSCREEN regardless of contaminant sorption characteristics.
BIOSCREEN employs the same superposition approach for all of the aerobic and anaerobic
biodegradation reactions (based on evaluation of O^, NOs, SO4, Fe2+, and CfU). Based on work
reported by Newell et al. (1995), the anaerobic reactions (nitrate, ferric iron, and sulfate reduction
and methanogenesis) are amenable to simulation using the instantaneous reaction assumption.
The general approach is presented below:
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BIOSCREEN User's Manual
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Run model with no decay
(but with source zone
concentration equal to
measured source zone
concentration +
biodegradation capacity BC)
Subtract Biodegradation
Capacity (BC) from No
Decay Concentrations
Predict biodegraded plume
concentrations assuming
Instantaneous Reaction
Assumption
*->no decay
- BC
Co = Cmeasured +
BC / BC / BC / BC / BC I
BC / BC / BC / BC / BC / BC
- Cmst
Based on the biodegradation capacity of electron acceptors present in the groundwater system,
this algorithm will correct the non-decayed groundwater plume concentrations predicted by the
Domenico model (Appendix A.I) for the effects of organic constituent biodegradation.
To summarize:
1) The original BIOPLUME model (Borden et al. 1986) used a superposition method to simulate
the fast or "instantaneous" reaction of dissolved hydrocarbons with dissolved oxygen in
groundwater.
2) Borden et al. (1986) reported that this version of BIOPLUME was mathematically exact for the
case where the retardation factor of the contaminant was 1.0.
3) Rifai and Bedient (1990) developed the BIOPLUME II model with a dual-particle tracking
routine that expanded the original BIOPLUME model to handle contaminants with retardation
factors other than 1.0, in addition to other improvements.
4) Connor et al. (1994) compared the superposition method with the more sophisticated
BIOPLUME II model and determined that the two approaches yielded very similar results for
readily biodegradable contaminants with retardation factors between 1.0 and 6.0.
5) BIOSCREEN was developed using the superposition approach to simulate the "instantaneous"
reaction of aerobic and anaerobic reactions in groundwater. The biodegradation term in
BIOSCREEN is mathematically identical to the approach used in the original BIOPLUME
model. This mathematical approach (superposition) matches the more sophisticated
BIOPLUME II model very closely for readily biodegradable contaminant retardation factors of
up to 6.0. BIOSCREEN simulations using the instantaneous reaction assumption at sites with
retardation factors greater than 6.0 should be performed with caution and verified using a
more sophisticated model such as BIOPLUME III.
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APPENDIX A. 3 DERIVATION OF SOURCE HALF-LIFE
Purpose :
Given :
Determine the source half-life relationship used in BIOSCREEN (see Source Half-
Life discussion in BIOSCREEN Data Entry Section, pg 30).
1) There is a finite amount of soluble organic compounds in source zone (the
area with contaminated soils and either free-phase or residual NAPL.
2) These organics dissolve slowly as fresh groundwater passes through source
zone. Assume the change in mass due to dissolution can be approximated as
a first order process:
Procedure : 1) Calculate initial mass of dissolvable organics in source zone, Mo
2) Determine initial source concentration from monitoring well data, Co
3) Apply conservation of mass to a control surface containing source zone.
4) Set the expressions for mass at time t D 0 based on dissolution and
conservation of mass equal to each other and solve for an expression
describing the concentration at time t 00.
5) Apply initial conditions for concentration at time t=0 and solve for the first
order decay constant, ks.
Assumptions : 1) Groundwater flowrate is constant, Q(t)=Qo
2) Groundwater flowing through the source zone is free of organic compounds.
This implies that no mass is added to the system, only dissolution occurs.
Calculations: 1) Calculate initial mass of dissolved/soluble organic compound, Moby
using procedure described under "Soluble Mass in NAPL, Soil" page in
BIOSCREEN Data Input section.
2) Determine initial concentration, Co of organic compound in groundwater
leaving the source zone. This may be a spatial average, maximum value, or
other value representative of the groundwater concentration leaving the
source area. (Note that for the instantaneous reaction assumption, Co equals
the concentration observed in monitoring wells plus the biodegradation
capacity to account for rapid biodegradation reactions in the source zone.
See "Soluble Mass in NAPL, Soil" page in BIOSCREEN Data Input section).
C(t=0) = Co
3) Apply conservation of mass to a control surface that contains the source
zone. The mass present in the source zone at time t 0 0 is the initial mass
minus the change in mass.
(2)
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BIOSCREEN User's Manual June 1996
M(t)=M0+ Dc-s- Ot Q(t) C(t) dtdA (3)
DERIVATION OF SOURCE HALF-LIFE, Cont'd
Applying the assumptions equation (3) simplifies to
M(t)= Mo - Dt Qo C(t) dt (4)
4) Set the two expressions for mass of organic compound in the source zone at
time t D 0 (equations (1) and (4)) equal to each other and solve for an
expression describing the concentration leaving the source zone.
M0e-kst = Mo-DtQoC(t)dt (5)
(6)
QQ C(t) = ks Mo e-k
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APPENDIX A.4 DISPERSIVITY ESTIMATES
Dispersion refers to the process whereby a plume will spread out in a longitudinal direction
(along the direction of groundwater flow), transversely (perpendicular to groundwater flow), and
vertically downwards due to mechanical mixing in the aquifer and chemical diffusion. Selection
of dispersivity values is a difficult process, given the impracticability of measuring dispersion in
the field. However, dispersivity data from over 50 sites has been compiled by Gelhar el al. (1992)
(see figures A.I and A.2, next page).
The empirical data indicates that longitudinal dispersivity, in units of length, is related to scale
(distance between source and measurement point; the plume length; Lp in BIOSCREEN). Gelhar
et al. (1992) indicate 1) there is a considerable range of dispersivity values at any given scale (on
the order of 2 - 3 orders of magnitude), 2) suggest using values at the low end of the range of
possible dispersivity values, and 3) caution against using a single relationship between scale and
dispersivity to estimate dispersivity. However, most modeling studies do start with such simple
relationships, and BIOSCREEN is programmed with some commonly used relationships
representative of typical and low-end dispersivities:
Longitudinal Dispersivity
Alpha x =3.28 -0.83
Transverse Dispersivity
Alpha y = 0.10 alpha x
(Lp in ft)
Vertical Dispersivity
Alpha z = very low (i.e. 1 x e-99 ft)
(Xu and Eckstein, 1995)
(Based on high reliability
points from Gelhar et al., 1992)
(Based on conservative estimate
Other commonly used relationships include:
Alpha x = 0.1 Lp
Alpha y =0.33 alpha x
Alpha z =0.05 alpha x
Alpha z = 0.025 alpha x to 0.1 alpha x
(Pickens and Grisak, 1981)
(ASTM, 1995) (EPA, 1986)
(ASTM, 1995)
(EPA, 1986)
The BIOSCREEN input screen includes Excel formulas to estimate dispersivities from scale.
BIOSCREEN uses the Xu and Eckstein (1995) algorithm for estimating longitudinal dispersivities
because 1) it provides lower range estimates of dispersivity, especially for large values of Lp, and
2) it was developed after weighting the reliability of the various field data compiled by Gelhar et
al.. (1992) (see Figure A.I). BIOSCREEN also employs low-end estimates for transverse and
vertical dispersivity estimates (0.10 alpha x and 0, respectively) because: 1) these relationships
better fit observed field data reported by Gelhar et al. to have high reliability (see Figure A.2),
2) Gelhar et al. recommend use of values in the lower range of the observed data, and 3) better
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BIOSCREEN User's Manual
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results were realized when calibrating BIOSCREEN to actual field sites using lower dispersivities.
The user can override these formulas by directly entering dispersivity values in the input screen
cell.
Note that the Domenico model and BIOSCREEN are not formulated to simulate the effects of
chemical diffusion. Therefore, contaminant transport through very slow hydrogeologic regimes
(e.g., clays and slurry walls) should probably not be modeled using BIOSCREEN unless the
effects of chemical diffusion are proven to be insignificant. Domenico and Schwartz (1990)
indicate that chemical diffusion is small for Peclet numbers (seepage velocity times median pore
size divided by the bulk diffusion coefficient) greater than 100.
10
103
10
Longitudinal Dispersivity
= 10% of scale
Q.
(0
'
15
.E 10ฐ
10-2
10-3
(Pickem and Grisok, 1981)
Longitudinal Dispersivity
= 0.83 [Log10 '
(Xu and Eckstein, 1995)
RELIABILITY _
- O
O Low
O Intermediate
O High
Data Source: Gelharetal, 1992
i i null i i i n ill i i i n ill i i i n ml i i in ml i i mini i innii
101 10ฐ 101 102 103 104 105 10s
Scale (m)
Figure A.I. Longitudinal dispersivity vs. scale data reported by Gelhar
et al. (1992). Data includes Gelhar's reanalysis of several dispersivity
studies. Size of circle represents general reliability of dispersivity
estimates. Location of 10% of scale linear relationship plotted as dashed
line (Pickens and Grisak, 1981). Xu and Eckstein's regression (used in
BIOSCREEN) shown as solid line. Shaded area defines ฑ 1 order of
magnitude from the Xu and Eckstein regression line and represents
general range of acceptable values for dispersivity estimates. Note that
BIOSCREEN defines scale as Lp, the plume length or distance to
measurement point in ft, and employs the Xu and Eckstein algorithm
with a conversion factor.
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Data Source: Gelhar et al, 1992
Data Source: Gelhar et al, 1992
Transverse
Dispersivity =
10% of alpha x
Figure A. 2 Ratio of transverse dispersivity and vertical dispersivity to longitudinal dispersivity
data vs. scale reported by Gelhar et al. (1992). Data includes Gelhar's reanalysis of several
dispersivity studies. Size of symbol represents general reliability of dispersivity estimates. Location
of transverse dispersivity relationship used in BIOSCREEN is plotted as dashed line.
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BIOSCREEN User's Manual
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APPENDIX A.5 ACKNOWLEDGMENTS
BIOSCREEN was developed for the Air Force Center for Environmental Excellence, Brooks AFB,
San Antonio, Texas by Groundwater Services, Inc.
AFCEE Technology
Transfer Division
Chiefs:
AFCEE Project Officer:
BIOSCREEN Developers:
BIOSCREEN Manual :
Contributors to
BIOSCREEN:
BIOSCREEN Review Team:
Lt. Col. Ross Miller
Mr. Marty Faile
Mr. Jim Gonzales
Charles J. Newell, Ph.D., P.E. and R. Kevin McLeod
Groundwater Services, Inc.
5252 Westchester, Suite 270
Houston, Texas 77005
Charles J. Newell, Ph.D., P.E.
Groundwater Services, Inc.
phone: 713663-6600
fax: 713663-6546
cjnewell@gsi-net. com
rkmcleod@gsi-net. com
R. Todd Fisher, Xiaoming Liu, Tariq Kahn, Mat Ballard, Jackie
Winters, Phil Bedient, Anthony Holder, Hanadi Rifai
Gilberto Alvarez
Mike Barden
James Barksdale
Kathy Grindstaff
Robin Jenkins
Tim R. Larson
Luanne Vanderpool
Dr. Jim Weaver
Todd Wiedemeier
Todd Herrington
Matt Swanson
Kinzie Gordon
Joe R. Williams
Dr. John Wilson
Ying Ouyang
Rashid Islam
USEPA Region V, Chicago, 111.
Wisconsin Dept. of Natural Resources
US EPA Region IV, Atlanta, GA.
Indiana Dept. of Environmental
Management (IDEM)
Utah DEQ, Lust Program
Florida Dept. of Environmental
Protection
US EPA Region V, Chicago, 111.
US EPA National Risk Management
Research Laboratory
Parsons Engineering Science, Inc.
US EPA National Risk Management
Research Laboratory
US EPA National Risk Management
Research Laboratory
Computer Data Systems
The Air Force Center for Environmental Excellence is distributing BIOSCREEN via:
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BIOSCREEN User's Manual
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EPA Center for Subsurface Modeling
Support (CSMoS)
NRMRL/SPRD
P.O. Box 1198
Ada, Oklahoma 74821-1198
Phone: (405)436-8594
Fax: (405)436-8718
Bulletin Board: (405) 436-8506 (14,400 baud-
8 bits -1 stop bit -no parity).
Web: http://www.epa.gov/ada/kerrlab.html
(Electronic manuals will be in .pdf format; must
download Adobe Acrobat Reader to read and
print pdf files.)
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BIOSCREEN
Natural Attenuation
Decision Support System
Version 1.4
July 1997
VERSION 1.4 REVISIONS
Surface
Top of Water-
Bearing Unit
Bottom of Water-
Bearing Unit
by
Charles J. Newell, Ph.D., P.E. and R. Kevin McLeod
Groundwater Services, Inc.
Houston, Texas
James R. Gonzales
Technology Transfer Division
Air Force Center for Environmental Excellence
Brooks AFB, San Antonio Texas
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BIOSCREEN 1.4 Revisions July 1997
INTRODUCTION
BIOSCREEN is an easy-to-use screening model which simulates remediation
through natural attenuation (RNA) of dissolved hydrocarbons at petroleum fuel
release sites. The software, programmed in the Microsoftฎ Excel spreadsheet
environment and based on the Domenico analytical solute transport model, has
the ability to simulate advection, dispersion, adsorption, and aerobic decay as
well as anaerobic reactions, which have been shown to be the dominant
biodegradation processes at many petroleum release sites. BIOSCREEN includes
three different model types:
1) Solute transport without decay,
2) Solute transport with biodegradation modeled as a first-order decay process (simple, lumped-parameter
approach),
3) Solute transport with biodegradation modeled as an "instantaneous" biodegradation reaction (approach used
by BIOPLUME models).
The model is designed to simulate biodegradation by both aerobic and anaerobic
reactions. It was developed for the Air Force Center for Environmental
Excellence (AFCEE) Technology Transfer Division at Brooks Air Force Base by
Groundwater Services, Inc., Houston, Texas.
Version 1.3 of BIOSCREEN was released in October 1996. Version 1.4 of BIOSCREEN includes a
new mass flux calculation feature, a modification to the vertical dispersion term in the Domenico
model, a revised description of the Domenico analytical model equation, and a minor change to
the input display. This document describes these updates and provides new biodegradation
modeling information for BIOSCREEN users. Continue to refer to the existing BIOSCREEN
version 1.3 User's Manual as the primary source of information about BIOSCREEN.
NEW MASS FLUX CALCULATION FEATURE IN VERSION 1.4
Version 1.4 of BIOSCREEN includes a new feature to assist users in estimating the mass flux of
contaminants entering surface water bodies via groundwater plume discharge. This feature,
included on the "Run Array" Output, provides an estimate of the mass flux of contaminants in
units of mg/day computed at specific distances away from the source (see Figure 1).
Example Application
Set up BIOSCREEN to simulate the Keesler AFB SWMU 66 plume (Example 1 in the Version 1.3
User's Manual, page 52). Assume that the plume at Keesler AFB discharges into a hypothetical
stream located 210 ft away from the source zone as shown in Figure 1 (note that no such stream
actually exists at this location). Using BIOSCREEN 1.4 with the Instantaneous Reaction model,
calculate the mass flux of contaminants discharging into the stream (see Example 1 in Appendix
A).
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BIOSCREEN 1.4 Revisions July 1997
As shown in the attached Figure 4 (see Example 1 in Appendix A), the computed mass flux of
BTEX constituents within the groundwater plume at 224 ft away from the source is 1500 mg/day.
Therefore, in order to achieve a target concentration in the stream of < 0.001 mg/L total BTEX, a
minimum naturally-occurring flowrate of 1.5 x 106 L/day (0.61 cubic feet per second) is required.
Obtaining Streamflow Data
Two types of stream flowrates can be used for estimating exposure concentrations, depending on
the nature of the contaminant. For contaminants with acute effects on human or aquatic receptors
(such as ammonia), a minimum flowrate such as the 2-year 7-day average low flow value may be
appropriate. For contaminants with chronic effects on human or aquatic receptors (such as the
BTEX compounds), a harmonic mean or other form of average flow could be used.
The harmonic mean is defined as:
n where Qi = daily average discharge data
O, = n = number of days with data
z^hm i = n \ >
X
tTQ,
Calculation of 10-year 7-day average low flow values is discussed in several hydrology texts,
including the Handbook of Hydrology, David R. Maidment, ed. McGraw-Hill, 1993. Daily average
discharge data are often available through state or local agencies which regulate wastewater
treatment discharges. Streamflow data are also available through the U.S. Geological Survey
(USGS) for many larger streams (see the USGS World-Wide Web page:
http://water.usgs. go v/swr/).
For smaller, ungaged streams, or for locations not near a gaging station, data from an alternative
location having similar watershed characteristics (i.e., landuse, land cover, topography, channel
type, drainage area, etc.) may be used. For two locations that differ in size of the drainage area,
but are otherwise similar, Streamflow data from the gaged location may be adjusted by the ratio
of drainage areas to provide an estimate of the flow at the ungaged location.
Description of Calculation
The contaminant mass flux is determined using a simple calculation technique. The
concentration in each cell of the array is multiplied by: 1) the Darcy velocity, 2) the width
associated with each cell in the array, and 3) the thickness of the source zone. The plume mass
flux for a particular cross section is then determined by summing the five values in the array for
that cross section. The calculation technique is disabled when vertical dispersion is used, as the
vertical concentration profile is no longer uniform. In addition, the mass flux calculation should
only be used for gaining streams (streams where groundwater discharges into surface water) and
should not used for losing streams (streams that recharge groundwater).
The calculation approach is approximate, and other averaging techniques (use of geometric
means, etc.) might provide different results. Because the model defines the plume cross section
with only 5 points, the computed plume mass flux may appear to be slightly higher for a
downgradient point than an upgradient point in some instances. As illustrated in the example,
the mass flux estimates are sensitive to the model width, and for best results users should adjust
the model width so that the contaminant plume covers most of the calculated array (compare
mass flux results from a simulation using a 200 ft model width, Figure 4, to mass flux results from
a simulation using a 50 ft model width, Figure 6). Users should assume that the mass flux
estimates are probably accurate to ฑ 50%.
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BIOSCREEN 1.4 Revisions July 1997
NEW KILOGRAM TO GALLONS CONVERSION FEATURE IN VERSION 1.4
Version 1.4 of BIOSCREEN also includes a new feature to show users how much volume the
mass of contaminants displayed in the Array Output screen represents. For example, if
BIOSCREEN estimates that the Actual Plume Mass is 7.8 Kg (see Figure 4), the model will convert
this into an effective contaminant volume of 2.4 gallons of organic, using a density value of 0.87
g/mL (representative of the density of a BTEX mixture). The following mass values will be
converted to volumes: i) Plume Mass if No Biodegradation, ii) Actual Plume Mass, iii) Plume
Mass Removed by Biodegradation, iv) Original Mass in Source (Time = 0 Years), and v) Mass in
Source Now (Time = X Years).
To display the data converted into gallons, the user should click the iSee Gallons! button in the
iPlume and Source Masses! region of the Array Output screen. A dialog box appears with several
common fuel constituents (average BTEX, benzene, toluene, ethylbenzene, and para-xylene) and
their densities in g/mL. If an alternative value for constituent densities is available, this number
can be entered into the iDensityi box. When the iOKi button is pressed, the dialog box disappears
and the plume and source mass calculations in Kg are replaced with volume information in
gallons. To convert back to mass values, click on the "See Kg" button.
RELATED REFERENCES FOR BIOSCREEN MODELING
Ollila (1996) provides a good comparison of the Domenico model with the instantaneous reaction
superposition method against BIOPLUME II. Rifai et al. (1997) summarize the theory and use of
AFCEE's BIOPLUME III model. Nevin et al. (1997) describe software for deriving first-order
decay coefficients for steady-state plumes from actual site data.
Nevin, J. P., J.A. Connor, C.J. Newell, J.B. Gustafson, K.A. Lyons, 1997. "FATE 5: A Natural Attenuation
Calibration Tool for Groundwater Fate and Transport Modeling,", Petroleum Hydrocarbons and
Organic Chemicals in Groundwater, NWWA, Houston, Texas, Nov. 1997.
Ollila, P.W., 1996. Evaluating Natural Attenuation With Spreadsheet Analytical Fate and Transport
Models. Ground Water Monitoring and Remediation, Vol. XVI, No. 24, pp. 69-75.
Rifai, H.S., C.J. Newell, J.R. Gonzales, S. Dendrou, L. Kennedy, and J. Wilson, 1997. BIOPLUME III
Natural Attenuation Decision Support System Version 1.0 User's Manual. Air Force Center for
Environmental Excellence, Brooks AFB, Texas (in press).
IMPACT OF NON-BTEX CONSTITUENTS ON BIOSCREEN MODELING
BTEX constituents only comprise a small percentage of the total organic mass in
gasoline and JP-4 mixtures. However, the best available information suggests
that most JP-4 and gasoline plumes will be dominated by BTEX components, and
that only a small fraction of the plumes contain dissolved non-BTEX compounds.
This is due to the BTEX compounds having very high solubilities relative to the
remaining fraction of organic mass in these fuel mixtures. In other words, most
of the non-BTEX constituents of gasoline and JP-4 are relatively insoluble,
creating dissolved-phase plumes that are dominated by the BTEX compounds.
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BIOSCREEN 1.4 Revisions
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The following calculations support this conceptual model of BTEX-dominated
plumes from JP-4 and gasoline. For additional supporting data and calculations,
see Section 3.3.2 of Weidemeier et al., 1995.
Gasoline composition data presented by Johnson et al. (1990a and 1990b), and JP-
4 composition data presented by Stelljes and Watkin (Stelljes and Watkin, 1993;
data adapted from Oak Ridge National Laboratory, 1989) were used to
determine the effective solubility of these hydrocarbon mixtures in equilibrium
with water (effective solubility = mole fraction x pure phase solubility; see
Bedient, Rifai, and Newell 1994). The total effective solubility of all the
constituents was then compared to the effective solubility of the BTEX
constituents. The following tables show this calculation for fresh gasoline, two
weathered gasolines, and JP-4:
FRESH GASOLINE
(data from Johnson et al., 1990)
Constituent
Benzene
Toluene
Ethylbenzene
Xylenes
TOTAL BTEX
58 Compounds
TOTAL
Mass
Fraction
0.0076
0.055
0.0
0.0957
0.16
0.84
1.00
Mole
Fraction
0.0093
0.0568
0.0
0.0858
0.15
0.85
1.00
Pure-Phase Solubility Effective
(mg/L) Solubility (mg/L)
1780
515
152
198
152 - 1780 (range)
0.004 - 1230 (range)
-
17
29
0
17
63
30
93
% BTEX = (63 mg/L) -s- (93 mg/L) = 68
WEATHERED GASOLINE # 1
(data from Johnson et al., 1990a)
Constituent
Benzene
Toluene
Ethylbenzene
Xylenes
TOTAL BTEX
58 Compounds
Mass
Fraction
0.01
0.1048
0.0
0.1239
0.24
0.76
Mole
Fraction
0.0137
0.1216
0.0
0.1247
0.26
0.74
Pure-Phase Solubility
(mg/L)
1780
515
152
198
152 - 1780 (range)
0.004 - 1230 (range)
Effective
Solubility
(mg/L)
24
63
0
25
112
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TOTAL
1.00
1.00
126
% BTEX = (112 mg/L) + (126 mg/L) = 89
WEATHERED GASOLINE #2
(data from Johnson et al., 1990b)
Constituent
Benzene
Toluene
Ethylbenzene
Xylenes
TOTAL BTEX
64 Compounds
TOTAL
Mass
Fraction
0.0021
0.0359
0.013
0.080
0.13
0.87
1.00
Mole
Fraction
0.003
0.043
0.014
0.084
0.14
0.86
1.00
Pure-Phase Solubility
(mg/L)
1780
515
152
198
152 - 1780 (range)
0.004 - 1230 (range)
-
Effective
Solubility
(mg/L)
5
22
2
15
44
21
65
% BTEX = (44 mg/L) + (65 mg/L) = 68
VIRGIN JP-4
(data from Stalljes and Watkin, 1993; Oak Ridge N. Lab, 1989)
Constituent
Benzene
Toluene
Ethylbenzene
Xylenes
TOTAL BTEX
13 Compounds
TOTAL
Mass
Fraction
0.005
0.0133
0.0037
0.0232
0.045
(4.5%)
0.27
(27%)
0.315
(31.5)%
Mole
Fraction
0.023
0.053
0.013
0.080
0.168
0.832
1.000
Pure-Phase Solubility
(mg/L)
1780
515
152
198
152 - 1780 (range)
0.004 - 1230 (range)
-
Effective
Solubility
(mg/L)
42
27
2
16
87
4
91
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BIOSCREEN 1.4 Revisions July 1997
% BTEX = (87 mg/L) + (91 mg/L) = 95
In each of these four fuel samples, BTEX compounds comprise the majority of the
dissolved organic mass in equilibrium with water. The non-BTEX components
represent a much smaller portion of the dissolved mass. As expected, the
theoretical dissolved-phase concentrations from these samples are much higher
than what is typically observed in groundwater samples due to factors such as
dilution, the heterogeneous distribution of non-aqueous phase liquids, and the
low level of mixing occurring in aquifers (see Bedient, Rifai, and Newell, 1994
for a more complete discussion).
Note that the total effective solubility of weathered gasoline #1 (126 mg/L) is
greater than the total effective solubility of the fresh gasoline (93 mg/L). A
comparison of the two samples indicates that the fresh gasoline includes a
significant mass of light, volatile compounds that have pure-phase solubilities
that are much lower than that of the BTEX compounds (e.g., isopentane with a
vapor pressure of 0.78 atm and a solubility of 48 mg/L, compared to solubilities
of 152 -1780 mg/L for the BTEX compounds). When these light compounds are
weathered (probably volatilized), the mole fractions of the BTEX components
(the only remaining components with any significant solubility) increase, thereby
increasing the total effective solubility of the weathered gasoline. On the other
hand, weathered gasoline #2 has a total effective solubility that is significantly
lower than fresh gasoline (65 mg/L vs. 93 mg/L), suggesting that this gasoline
has weathered to the point where there has been significant removal of both
volatile and soluble components from the gasoline.
In their analysis, Stelljes and Watkin (1993) identified only 17 compounds
representing 31% by mass of a complete JP-4 mixture. However, a comparison of
the relative make-up of the quantified mixture to the reported make-up of JP-4
(also from Stelljes and Watkin, 1993) shows the various classes of organic
compounds to be equivalently represented in both mixtures. The quantified
mixture appears to be generally representative of the complete JP-4 mixture.
% benzenes, alkylbenzenes in identified compounds: 14% (note: equals 4.5% of 31.E
% benzenes, alkylbenzenes in complete JP-4 mixture: 18% (from Stelljes and Watkin, 1993)
% branched alkanes in all identified compounds: 26%
% branched alkanes in complete JP-4 mixture: 31 %
% cycloalkanes in all compounds identified: 7%
% cycloalkanes in complete JP-4 mixture: 16%
% naphthalenes in all compounds identified: 6%
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BIOSCREEN 1.4 Revisions July 1997
% naphthalenes in complete JP-4 mixture: 3%
% normal alkanes in all compounds identified: 47%
% normal alkanes in complete JP-4 mixture: 32%
Finally, it is important to note that there is considerable variability among
different fresh fuels, and even more variation among weathered fuels. Therefore,
these results should only be used as a general indicator that the BTEX
compounds comprise the majority of the soluble components in plumes
originating from JP-4 and gasoline releases. These results should not be used as
absolute, universal values for all sites.
With regard to biodegradation modeling, however, it is probably appropriate to
assume that BTEX compounds exert the majority (i.e. ~ 70% or greater) of the
electron acceptor demand at JP-4 and gasoline sites. To make modeling BTEX
using the instantaneous reaction approach more accurate, however, the total
concentrations of available electron acceptors can be reduced by some fraction to
account for the electron acceptor demand posed by biodegradable non-BTEX
organics in groundwater. Two examples of how to account for the impact for
non-BTEX components is to multiply all electron acceptor/by-product
concentrations used in the model by either i) the ratio of BTEX/TOC
concentrations, or ii) the ratio of BTEX/BOD concentrations (if TOC and BOD
data are available). If these data are not available, a conservative approach
would be to reduce all available electron acceptor/by-product concentrations
used in the model by 30% to account for the possible impacts of non-BTEX
organics in groundwater.
References for BTEX-Dominated Plumes
Bedient, P. B., H.S. Rifai, and C.J. Newell, Groundwater Contamination: Transport and Remediation,
Prentice-Hall, 1994.
Johnson, P.C., M.W. Kemblowski, and J.D. Colthart. 1990a. Quantitative Analysis of Cleanup of
Hydrocarbon-Contaminated Soils by In-Situ Soil Venting. Ground Water, Vol. 28, No. 3. May - June,
1990, pp 413-429.
Johnson, P.C., C.C. Stanley, M.W. Kemblowski, D.L. Byers, and J.D. Colthart. 1990b. A Practical Approach
to the Design, Operation, and Monitoring of In Site Soil-Venting Systems, Ground Water Monitoring
and Remediation, Spring, 1990, pp 159-178.
Oak Ridge National Laboratory, 1989. The Installation Restoration Program Toxicology Guide, DOE
Interagency Agreement No. 1891-A076-A1, Volumes III and IV, July, 1989.
Stelljes, M.E., and G.E. Watkin, 1993. "Comparison of Environmental Impacts Posed by Different
Hydrocarbon Mixtures: A Need for Site Specific Composition Analysis,", in Hydrocarbon
Contaminated Soils and Groundwater, Vol. 3, P.T. Kostecki and E.J. Calabrese, Eds., Lewis Publishers,
Boca Raton.
Wiedemeier, T. H, Wilson, J. T., Kampbell, D. H, Miller, R. N, and Hansen, J.E., 1995. "Technical Protocol
for Implementing Intrinsic Remediation With Long-Term Monitoring for Natural Attenuation of Fuel
-------
BIOSCREEN 1.4 Revisions July 1997
Contamination Dissolved in Groundwater (Revision 0)", Air Force Center for Environmental
Excellence, Brooks AFB, Texas, Nov., 1995.
CHANGES FROM BIOSCREEN 1.3
Display of Source Half-Life Values
The input screen for Version 1.4 has been modified to emphasize that BIOSCREEN generates two
different source half-lives when a value for "Soluble Mass in Source NAPL, Soil" is entered. As
discussed on page 31 of the Version 1.3 User's Manual, two half-lives are reported, one for the
Instantaneous Reaction model and one for the No Degradation or First Order Decay models.
Version 1.3 of BIOSCREEN presented both half-lives in one black box (black input boxes designate
intermediate values calculated by the model). As part of the Version 1.4 modifications, the single
box for source half-lives has been replaced with two boxes, one showing the source half-life
calculated using the instantaneous reaction model and one showing the source half-life calculated
using the No Degradation or First Order Decay models. The change was made to emphasize that
two different values are calculated by BIOSCREEN depending on which biodegradation model is
employed (see page 31 of the Version 1.3 User's Manual).
Vertical Dispersion Term
As explained in the Version 1.3 User's Manual, BIOSCREEN has been configured so that the
default vertical dispersivity is set to zero (see Appendix A.4 in the Version 1.3 User's Manual). In
BIOSCREEN 1.3, however, if the user opts to use a non-zero vertical dispersivity estimate, the
software may overestimate the effects of vertical dispersion in some cases, as described below.
BIOSCREEN 1.3 was coded so that vertical dispersion is assumed to occur in both directions as
the contaminants travel away from the source zone (i.e., downwards and upwards). For source
zones located in the middle of a thick aquifer, or in cases where recharge produces a clean zone
on top of the plume, this would be an appropriate approach. For source zones located at the top
of an aquifer (the case at most petroleum release sites), upward vertical dispersion above the
water table does not occur (unless recharge is significant), and therefore the model could
overestimate the effects of dispersion. While the vertical dispersion term in the Domenico
analytical model expression in the Version 1.3 User's Manual was correct, showing vertical
dispersion in only one direction (see Appendix A.I), the Version 1.3 model actually simulates
vertical dispersion in both directions.
In BIOSCREEN 1.4, the default approach of no vertical dispersion is still recommended. The
software code has been changed, however, so that there is vertical dispersion is modeled in the
downward direction only. (If a user would like to use BIOSCREEN 1.4 with dispersion in both
directions, multiply the vertical dispersivity estimate by a factor of 4 and enter the result as the
vertical dispersivity. This will have the effect of simulating vertical dispersion occurring in two
directions).
Most users will not notice any effect with this change, as BIOSCREEN's default vertical
dispersivity is set near zero corresponding to no vertical dispersion. BIOSCREEN 1.3 only
overestimates the effects of vertical dispersion if: 1) the default dispersivity value of zero is
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BIOSCREEN 1.4 Revisions
July 1997
replaced with a non-zero vertical value and 2) the source zone is located at the top of an aquifer
that does not have significant recharge.
Appendix A.I Domenico Analytical Model Equation
The Domenico analytical model expression provided in Appendix A.I of the BIOSCREEN Version
1.3 User's Manual incorrectly showed how the superposition term was employed, was unclear
about the separation of the first order decay model and the instantaneous reaction model, and did
not include the source decay term. Revised equation descriptions are provided below and
replace the single equation shown on page 41 of the Version 1.3 User's Manual. Note that the
equations encoded in the software were not in error and have not been modified (except as
described above with regard to vertical dispersion).
Domenico Model with First Order Decay Algorithm
C source
,C(x,y,0)
C(x,y,o,t) = C0exp[-ks(t-x/v)]
8 \_ax2 ^
erfc
L
CXy
*
where: v
K-i
B.R
Domenico Model with Instantaneous Reaction Superposition Algorithm
, C source
(Xy
ซz
C(x,y,o,i)=
i r (x - vt ) i
eric i - - J^T- i
8 [2(axvt)112 J
(Z)
i r
where:
K-i
OR
BC =E
C/F
Definitions
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BIOSCREEN 1.4 Revisions
July 1997
BC Bi ode gradation capacity (mg/L)
C(x,y,z,t) Concentration at distance x downstream of
source and distance y off centerline of plume
at time t (mg/L)
Cs Concentration in Source Zone (mg/L)
Co Concentration in Source Zone at t=0 (mg/L)
x Distance downgradient of source (ft)
y Distance from centerline of source (ft)
z Vertical Distance from groundwater surface to
measurement point (assumed to be 0;
concentration is always assumed to be at top
of water table).
C(ea)n Concentration of electron acceptor (or by-
product equivalent) n in groundwater (mg/L)
UFn Utilization factor for electron acceptor n (i.e., mass ratio
of electron acceptor/by-product to hydrocarbon consumed
in biodegradation reaction)
Ox Longitudinal groundwater dispersivity (ft)
OCy Transverse groundwater dispersivity (ft)
az Vertical groundwater dispersivity (ft)
X First-order decay coefficient for dissolved contaminants (yr~l)
9e Effective soil porosity
u Contaminant velocity in groundwater (ft/yr)
K Hydraulic conductivity (ft/yr)
R Constituent retardation factor
i Hydraulic gradient (ft/ft)
Y Source width (ft)
Z Source depth (ft)
t Time (yr)
kg First-order decay term for source concentration (yr )
10
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BIOSCREEN 1.4 Revisions
July 1997
ACKNOWLEDGMENTS
BIOSCREEN was developed for the Air Force Center for Environmental
Excellence, Brooks AFB, San Antonio, Texas by Groundwater Services, Inc.
AFCEE Technology
Transfer Division
Chief:
AFCEE Project
Officer:
BIOSCREEN Developers
BIOSCREEN Manual:
Contributors to
BIOSCREEN Version 1.4
Mr. Marty Faile
Mr. Jim Gonzales
Charles J. Newell, Ph.D., P.E. and R. Kevin McLeod
Groundwater Services, Inc.
2211 Norfolk Suite 1000
Houston, Texas 77005
Charles J. Newell, Ph.D., P.E.
Groundwater Services, Inc.
R. Todd Fisher
phone: 713522-6300
fax: 713522-8010
cjnewell@gsi-net. com
rkmcleod@gsi-net. com
The Air Force Center for Environmental Excellence is distributing BIOSCREEN
1.4 via:
EPA Center for Subsurface Modeling
Support (CSMoS)
NRMRL/SPRD
P.O. Box 1198
Ada, Oklahoma 74821-1198
Phone: (405)436-8594
Fax: (405)436-8718
Bulletin Board: (405) 436-8506 (14,400 baud-
8 bits -1 stop bit -no parity).
Internet:
http://www.epa.gov/ada/kerrlab.html
(Electronic manuals will be in .pdf format;
must download Adobe Acrobat Reader to
Note that first-time users should download:
1) The BIOSCREEN 1.4 software,
2) The BIOSCREEN 1. 3 User's Manual, and
3) The BIOSCREEN 1.4 Revisions document.
11
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BIOSCREEN 1.4 Revisions July 1997
APPENDIX 1. BIOSCREEN Version 1.4 EXAMPLE
Example 1: SWMU 66, Keesler AFB, Mississippi
Input Data
Fig. 1 Source Map
BIOSCREEN Modeling Summary
Fig. 2 BIOSCREEN Input Data
Fig. 3 BIOSCREEN Centerline Output
Fig. 4 BIOSCREEN Array Output
Fig. 5 BIOSCREEN Input Data, 50 ft Model Width
Fig. 4 BIOSCREEN Array Output, 50 ft Model Width
A-l
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BIOSCREEN 1.4 Revisions
July 1997
BIOSCREEN EXAMPLE 1
Keesler Air Force Base, SWMU 66, Mississippi
DATA TYPE
Hydrogeology
Dispersion
Adsorption
Biodegradatio
n
General
Source Data
Actual Data
OUTPUT
Parameter
Hydraulic Conductivity:
Hydraulic Gradient:
Porosity:
Original:
Longitudinal Dispersivity:
Transverse Dispersivity:
Vertical Dispersivity:
After Calibration:
Longitudinal Dispersivity:
Transverse Dispersivity:
Vertical Dispersivity:
Retardation Factor:
Soil Bulk Density pb:
foe:
Koc:
Electron Acceptor:
Background Cone. (mg/L):
Minimum Cone. (mg/L):
Change in Cone. (mg/L):
Electron Acceptor:
Max. Cone. (mg/L):
Avg. Cone. (mg/L):
Modeled Area Length:
Modeled Area Width:
Simulation Time:
Source Thickness:
Source Concentration:
Distance From Source (ft):
BTEX Cone. (mg/L):
Centerline Concentration:
Array Concentration:
Value
l.lxlO-2 (cm/ sec)
0.003 (ft/ ft)
0.3
13.3 (ft)
1.3 (ft)
0(ft)
32.5 (ft)
3.25 (ft)
0(ft)
1.0
1.7(kg/L)
0.0057%
B: 38 T: 135
E: 95 X: 240
02 N03 S04
2.05 0.7 26.2
Fe CH4
Note: Boxed values are
BIOSCREEN input values.
320 (ft)
200 (ft), 50 (ft)
6 (yrs)
10 (ft)
(See Figure 1)
30 60 180 280
5.0 1.0 0.5 0.001
See Figure 3
See Figure 4, 6
Source of Data
Slug-tests results
Static water level
measurements
Estimated
Based on estimated plume
length of 280 ft and
Xu/ Eckstein relationship
Based on calibration to
plume length (Note this is
well within the observed
range for long, dispersivity;
see Fig. A.I in Appendix
A..3. Remember to convert
from feet to meters before
using the chart).
Calculated from
R = 1+Koc x foe x pb/n
Estimated
Lab analysis
Literature - use Koc = 38
Based on March 1995
groundwater sampling
program conducted by
Groundwater Services, Inc.
Based on area of affected
groundwater plume
Steady-state flow
Based on geologic logs and
lumped BTEX monitoring
data
Based on observed
concentrations at site
A-2
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BIOSCREEN 1.4 Revisions
July 1997
Atfected
Groin crater
Zone
Affectec
Soil Zone
Sou nee Zone Assumption]
cooc in
14
3O
20
2.2
2.S
aoe
-L -I-Jfrl. t-^FL-d -ifi
LBGEM)
Mortal nq wel loos Boo
"Rmpofsrv QDO* pซratronHtซr (CFH ptaocvwlBf
Total BTCX cKtodsdin cjoundwolBf wmp+i,
BT1X ooroertnBc
detected
BIOSCREB4 SOURCE ZONE
ASSUhPTIOHS
FIGURE 1
A-3
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BIOSCREEN 1.4 Revisions July 1997
BIOSCREEN Modeling Summary, Keesler Air Force Base, SWMU
66, Mississippi:
BIOSCREEN was used to try to reproduce the movement of the plume from
1989 (the best guess for when the release occurred) to 1995.
The soluble mass in soil and NAPL was estimated by integrating BTEX soil
concentrations contours mapped as part of the site soil delineation program.
An estimated 2000 Kg of BTEX was estimated to be present at the site based on
GC/MS analysis of soil samples collected from both the vadose and saturated
zone. This value represented a source half-life of 60 years with the
instantaneous reaction model (the first value shown in the source half-life box
in Figure 2), a relatively long half-life, so the 2000 Kg measured in 1995 was
assumed to be representative of 1989 conditions.
The instantaneous reaction model was used as the primary model to try to
reproduce the plume length (~ 280 ft).
Because a decaying source was used, the source concentration on the input
screen (representing concentrations 6 yrs ago) were adjusted so the source
concentration on the centerline output screen (representing concentrations
now) were equal to 12 mg/L. Because the source decay term is different for
the first order decay and instantaneous reaction models, this simulation
focused on matching the instantaneous reaction model. The final result was a
source concentration of 13.68 mg/L in the center of the source zone (note on
the centerline output the source concentration is 12.021 mg/L).
The initial run of the instantaneous reaction model indicated that the plume
was too long. This indicates that there is more mixing of hydrocarbon and
electron acceptors at the site than is predicted by the model. Therefore the
longitudinal dispersivity was adjusted upwards (more mixing) until
BIOSCREEN matched the observed plume length. The final longitudinal
dispersivity was 32.5 ft.
As a check the first-order decay model was used with the BIOSCREEN default
value of 2 yrs. This run greatly overestimated the plume length, so the
amount of biodegradation was increased by decreasing the solute half-life. A
good match of the plume was reached with a solute half-life of 0.15 years. This
is within observed ranges reported in the literature (see solute half-life section,
page 22).
As shown in Figure 3, BIOSCREEN matches the observed plume fairly well.
The instantaneous model is more accurate near the source while the first
order decay model is more accurate near the middle of the plume. Both
models reproduce the actual plume length relatively well.
A-4
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BIOSCREEN 1.4 Revisions July 1997
As shown in Figure 4, the current plume is estimated to contain 7.8 kg of
BTEX. BIOSCREEN indicates that the plume under a no-degradation scenario
would contain 126.3 kg BTEX. In other words BIOSCREEN indicates that 94%
of the BTEX mass that has left the source since 1989 has biodegraded.
Most of the source mass postulated to be in place in 1989 is still there in 1996
(2000 kg vs. 1837 kg, or 92% left).
The current plume contains 1.0 ac-ft of contaminated water, with 1.019 acre-
ft/yr of water being contaminated as it flows through the source. Because the
plume is almost at steady state, 1.019 ac-ft of water become contaminated per
year with the same amount being remediated every year due to in-situ
biodegradation and other attenuation processes. This indicates that a long-
term monitoring approach would probably be more appropriate for this site
than active remediation, as the plume is no longer growing in size.
A hypothetical stream is assumed to be located approximately 210 ft
downgradient of the source (note no such stream exists at the actual site).
Using an estimated model width of 200 ft (see Figure 2), a mass flux of 1500
mg/day is calculated (see Figure 4) at a distance of 224 ft away from the
source (the closest point calculated by BIOSCREEN).
Users should be aware that the mass flux calculation is sensitive to the model
width assigned in Section 6 of the input screen (see Figure 2). A model width
of 200 ft was used in the original example so that most of the "no
degradation" plume was in the array, allowing calculation of the plume and
source masses (see pg. 34-35 of the BIOSCREEN Ver. 1.3 Manual for a more
detailed explanation).
For the mass flux calculation, however, a more accurate result will be obtained
by selecting a width where most of the plume of interest (in this cased the
instantaneous reaction plume) appears across the array. As shown in Figures
5 and 6, a model width of 50 ft was selected so that the instantaneous reaction
plume covered most of the BIOSCREEN array. With this width, a mass flux
value of 860 mg/day was calculated. This is a more accurate estimate of the
mass flux than the 1500 mg/day calculated above.
A-5
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BIOSCREEN 1.4 Revisions
July 1997
BIOSCREEN Natural Attenuation Decision Support System
Air Force Center for Environmental Excellence
Version 1.4
Keesier AFB
SWMU66
1. HYDROGEOLOGY
Seepage Velocity*
or
Hydraulic Conductivity
Hydraulic Gradient
Porosity
Vs
K
i
n
}(fi/yr)
(cm/sec)
(ft/ft)
(-)
2. DISPERSION
Longitudinal Dispersivity* alph
Transverse Dispersivity* alpha
Vertical Dispersivity* alpha z
or
Estimated Plume Length Lp
^ADSORPTION
Retardation Factor*
or
Soil Bulk Density
Partition Coefficient
FractionOrganicCarbon
47 IBibbEGifwbATION
1st Order Decay Coeff*
or
Solute Half-Life t-half
or Instantaneous Reaction Model
Delta Oxygen* DO
Delta Nitrate* NO3
Observed Ferrous Iron* Fe2+
Delta Sulfate* SO4
Observed Methane* CH4
0.15 ~](year)
; (mg/L)
Data input Instructions:
1115 I 1. Enter value directs.... or
5. GENERAL
Modeled Area Length*
Modeled Area Width*
Simulation Time*
320
200
6
Run Name
w f^~*
(ft) w J^^
(yr) *
IT or
| 0.02 |
Variable*
Hi
2. Calculate by filling in grey
cells below. (To restore
formulas, hit button below).
--' Data used directly in model.
Value calculated by model.
(Dont enter any data).
6. SOURCE DATA
Source Thickness in SatZpne*
Source Zones:
Vertical Plane Source: Look at Plume Cross-
Section and Input Concentrations & Widths
for Zones 1, 2, and 3
Source Half life (see Help
~
In Source NAPL, Soil
~"""
View of Plume Looking Down
Observed Centerline Concentrations at Monitoring WeHs
Sf No Data Leave Blank or Enter "0"
Concentration (mg/L) I
Dist. from Source (ft)|
8. CHOOSE TYPE OF OUTPUT TO SEE:
RUN
CENTERLINE
(mg/L) Q
(mg/L)
(mg/L)
View Output
RUN ARRAY
View Output
Recalculate This]
Sheet
Paste Example Dataset
Restore Formulas for Vs,
Dispersivities, R, lambda, other
Figure 2 . BIOSCREEN Input Screen. Keesier Air Force Base, Mississippi. (Note: longitudinal dispersivity has been changed from the
original computed value of 13.3 ft. to 32.5 ft. during calibration.)
A-6
-------
BIOSCREEN 1.4 Revisions
July 1997
at Z=0)
Distance /ram Source (ft)
TYPE OF MODEL
0
32
64
96
128
160
192
224
256 288
20
13.544 I 3.117 | 1.186 ! 0.488 | 0.208 ! 0.090 ! 0.040 I 0.018 ! 0.008 ! 0.004 j 0.002
Field Data from Site 12.000 ! 5.000 j 1.000
0.500
0.001
f st Order Decay
Instantaneous Reaction ....... ฐ ........ Wo Degradation
FfeW Date from Site
24,000
12.000
.2 10.000 -_-
4^
I *d 8.000 \
C c
S S 6.000
U 4.000 j-
2,000 3
0
50
Calculate
Animation
100
Time:
150 200
Distance From Source (ft)
6 Years
250
Return to
Input
300
Recalculate This
Sheet
350
Figure 3 . Centerline Output. Keesler Air Force Base, Mississippi.
A-7
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BIOSCREEN 1.4 Revisions
July 1997
Transverse
Distance (f$
100
50
-50
-100
IN at
Distance from Source (ft)
0
32
64
96
128
160
192
224
256
288
320
0,000
0,000
0,000
0,000
0,000
0,000
0,000
0,000
0,000
0,000
0.000
.oo
gggg
._._.
g.ggo
"54163"
g'ggg"
_._.
g.ggo
'1-248"
g'ggg"
._._.
.o
gggg
_._.
g.ggo
I'Pl
g'ggg"
__
.o
gggg
__
g.ggo
F'ill
g'ggg
_._.
g.ggo
Fill
g'ggg
_._.
.o
gogg
_._.
g.ggo
"g""g"g"4"
"g'ogg
_._.
g.ggo
"g""g"g"g"
"g'ogg
_._.
MASS \ 9.1E+3 I 7.2E+3 I 5.6E+3 I 4.6E+3 I 3.8E+3 I 3.0E+3 I 2.2E+3 I 1.5E+3 I 7.4E+2 I 5.2E+0 I O.OE+0
FLUX
(mgWay)
Ti
6 Years
Target Level: | 0.005 || mg/L
Displayed Model: jlnst. Reaction
Model to Display:
No Degradation
Model
1st Orcfer Decay
Mgcfe/
instantaneous
Reaction Model
Plume and Source Masses (Order-of-Magnifude Accuracy)
-100
256
100
See
Gallons
Plume Mass If No Blodegradatlon| 126.3 \(Kg)
- Actual Plume Massj 7.8 \(Kg)
= Plume Mass Removed by Blodeg
Contam. Mass In Source (t=0 Year
Contarn. Mass in Source Now (t=6Years)
Current Volume of Groundwater in Plume
Flowrate of Water Through Source Zone
288
1
Return to Input
(ao-n)
(ac-Wyr)
Recalculate
Figure 4 . Array Concentration Output. Keesler Air Force Base, Mississippi.
A-8
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BIOSCREEN 1.4 Revisions
July 1997
BIOSCREEN Natural Attenuation Decision Support System
Air Fores Center for Environmental Excellence
1. HYDROGEOLOGY
Seepage Velocity* Vs
or
Hydraulic Conductivity K
Hydraulic Gradient /
Porosity n
](fi/yr)
2. DISPERSION
Longitudinal Dispersivity* alphax
Transverse Dispersivity* alpha y
Vertical Dispersivity* alpha z
or
Estimated Plume Length Lp
SO4
CH4
(cm/sec)
(ft/ft)
(-)
3. ADSORPTION
Retardation Factor R
or
Soil Bulk Density rho
Partition Coefficient Koc
FractionOrganicCarbon foe
(K&l)
(v*g)
4. BIODEGRADATION
1 st Order Decay Coeff*
or
Solute Half-Life t-hatf
or Instantaneous Reaction Model
Delta Oxygen* DO
Delta Nitrate* A/O3
Observed Ferrous Iron* Fe2+
Delta Sulfate*
Observed Methane*
(mg/L)
(mg/L)
(mg/L)
(mg/L)
(mg/L)
Version 1.4
Keesler AFB
SWMU68
Run Name
5. GENERAL
Modeled Area Lengt!
Modeled Area Width'
Simulation Time*
6.
Data Input Instructions:
Enter value directlv....or
Calculate by filling in grey
cells below. (To restore
Mi
Variable* Data used direct Sy in model.
Value calculated by model.
SOURCE DATA
oyfpe I.hickness in Sat .Zone*
Source Zones:
Width* (ft) | Cone, f mg/L)* ^
Vertical Plane Source: Look at Plume Cross-
- Section and Input Concentrations & Widths
2. and 3
SourceHalflifefseeH e I p j?
nis^H
IstOrden
Inst. React.
Soluble Mass
In Source NAPL, Soil
(Kg)
View of Plume Looking Down
Observed Centerline Concentrations at Monitoring Wells
if No Data Leave Blank or Enter "0"
Concentration (mg/L)
Dist. from Source (ft)|
8. CHOOSE TYPE OF OUTPUT TO SEE:
RUN
CENTERLINE
View Output
RUN ARRAY
View Output
Help
Recalculate This
Sheet
Paste Example Dataset
Restore Formulas for Vs,
Dispersivities, R, lambda, other
Figure 5. BIOSCREEN Input Screen. Keesler Air Force Base, Mississippi, with 50 ft. modeled area width.
A-9
-------
BIOSCREEN 1.4 Revisions
July 1997
TransversQ
Distance fR)
IN
Distance from Source ffQ
"I
25
13
0
-13
-25
MASS
FLUX
(nig/day)
o i
1 .554 I
1 .554 !
12.021 I
1 .554 I
1 .554 !
9. 1E+3 I
Time:[
32
2.024
4.231
5.463
4.231
2.024
5.9E+3
6
64
1.962
3.603
4.248
3.603
1.962
5. f E+3
Years |
96
1.526
2.984
3.500
2.984
1.526
4. fE+3
128
1.036
2.391
2.860
2.391
1.036
3.2E+3
Target Level:
160
0.556
1.821
2.257
1.821
0.556
2.3E+3
| 0.005
192 i
0.096 I
1.272 i
1.678 !
1.272 i
0.096 I
1.5E+3 I
mg/L
224 |
0.000 I
0.737 !
1.114 !
0.737 i
0.000 I
256
0.000
0.210
0.559
0.210
0.000
8.6E+2 I 3.2E+2
Displayed
Model:
288
0.000
0.000
0.004
0.000
0.000
1.3E+0
320
0.000
0.000
0.000
0.000
0.000
O.OE+0
In st. Reaction
Mode! to Display:
No Degradation
1st Order Decay
Mode/
Instantaneous
Reaction Model
Plume and Source Masses (Order-of-Magnitude Accuracy)
(ft)
See
Gallons
Plume Mass if No Biodegradatlon|can't Calc.|fKg)
- Actual Plume Massjcan't Calc.|fKfl)
= Plume Mass Removed by Biodeg
Change in Electron Acceptor/Bvproduct Masses
Contam. Mass in Source (t=0 Years)
Contam. Mass in Source Now (t=6Years)
Current Volume of Groundwater in Plume
Flowrate of Water Through Source Zone
288
r
Return to Input
Jfac-ftJ
(ac-Wyr)
Recalculate
Figure 6. Array Concentration Output. Keesler Air Force Base, Mississippi, with 50 ft. modeled area width.
A-10
-------
BIOSCREEN User's Manual June 1996
APPENDIX A. 6 BIOSCREEN EXAMPLES
Example 1: SWMU 66, Keesler AFB, Mississippi
Input Data
Fig. 1 Source Map
BIOSCREEN Modeling Summary
Fig. 2 BIOSCREEN Input Data
Fig. 3 BIOSCREEN Centerline Output
Fig. 4 BIOSCREEN Array Output
Example 2: UST Site 870, Hill AFB, Utah
Input Data
Fig. 5 Source Map
BIOSCREEN Modeling Summary
Fig. 6 BIOSCREEN Input Data
Fig. 7 BIOSCREEN Centerline Output
Fig. 8 BIOSCREEN Array Output
51
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BIOSCREEN User's Manual
June 1996
BIOSCREEN EXAMPLE 1
Keesler Air Force Base, SWMU 66, Mississippi
DATA TYPE
Hydrogeology
Dispersion
Adsorption
Biodegradatio
n
General
Source Data
Actual Data
OUTPUT
Parameter
Hydraulic Conductivity:
Hydraulic Gradient:
Porosity:
Original:
Longitudinal Dispersivity:
Transverse Dispersivity:
Vertical Dispersivity:
After Calibration:
Longitudinal Dispersivity:
Transverse Dispersivity:
Vertical Dispersivity:
Retardation Factor:
Soil Bulk Density pb:
foe:
Koc:
Electron Acceptor:
Background Cone. (mg/L):
Minimum Cone. (mg/L):
Change in Cone. (mg/L):
Electron Acceptor:
Max. Cone. (mg/L):
Avg. Cone. (mg/L):
Modeled Area Length:
Modeled Area Width:
Simulation Time:
Source Thickness:
Source Concentration:
Distance From Source (ft):
BTEX Cone. (mg/L):
Centerline Concentration:
Array Concentration:
Value
l.lxlO-2 (cm/ sec)
0.003 (ft/ ft)
0.3
13.3 (ft)
1.3 (ft)
0(ft)
32.5 (ft)
3.25 (ft)
0(ft)
1.0
1.7(kg/L)
0.0057%
B: 38 T: 135
E: 95 X: 240
_O2 NO3 SO4
2.05 0.7 26.2
- 0.4 - 0 - 38
L-LffiJ G3 \-^2A\
Fe CH4
1 36.1 1 1 7.4 1
U&&J U^J
Note: Boxed values are
BIOSCREEN input values.
320 (ft)
200 (ft)
6 (yrs)
10 (ft)
(See Figure 1)
30 60 180 280
5.0 1.0 0.5 0.001
See Figure 3
See Figure 4
Source of Data
Slug-tests results
Static water level
measurements
Estimated
Based on estimated plume
length of 280 ft and
Xu/ Eckstein relationship
Based on calibration to
plume length (Note this is
well within the observed
range for long, dispersivity;
see Fig. A.I in Appendix
A. .3. Remember to convert
from feet to meters before
using the chart).
Calculated from
R = 1+Koc x foe x pb/n
Estimated
Lab analysis
Literature - use Koc = 38
Based on March 1995
groundwater sampling
program conducted by
Groundwater Services, Inc.
Based on area of affected
groundwater plume
Steady-state flow
Based on geologic logs and
lumped BTEX monitoring
data
Based on observed
concentrations at site
52
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BIOSCREEN User's Manual
June 1996
Source Zone Assumptionl
Qone In
14
SO
20
2.2
13.T
Zฃ
aoe
LEGEIO
MontaiirK) ml kxnfiart
oone pcndromiter ilCFTi piซiori>atflr lou&ori
_ _ BlEX
iMptotK mqlL. MrdHปe
BIOSCREBI SOURCE ZONE
ASSUhPTlOHS
FIGURE 1
53
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BIOSCREEN User's Manual June 1996
BIOSCREEN Modeling Summary, Keesler Air Force Base, SWMU 66,
Mississippi:
BIOSCREEN was used to try to reproduce the movement of the plume from 1989 (the best
guess for when the release occurred) to 1995.
The soluble mass in soil and NAPL was estimated by integrating BTEX soil concentrations
contours mapped as part of the site soil delineation program. An estimated 2000 Kg of BTEX
was estimated to be present at the site based on GC/MS analysis of soil samples collected from
both the vadose and saturated zone. This value represented a source half-life of 60 years with
the instantaneous reaction model (the first value shown in the source half-life box in Figure 2),
a relatively long half-life, so the 2000 Kg measured in 1995 was assumed to be representative
of 1989 conditions.
The instantaneous reaction model was used as the primary model to try to reproduce the
plume length (~ 280 ft).
Because a decaying source was used, the source concentration on the input screen
(representing concentrations 6 yrs ago) were adjusted so the source concentration on the
centerline output screen (representing concentrations now) were equal to 12 mg/L. Because
the source decay term is different for the first order decay and instantaneous reaction models,
this simulation focused on matching the instantaneous reaction model. The final result was a
source concentration of 13.68 mg/L in the center of the source zone (note on the centerline
output the source concentration is 12.021 mg/L).
The initial run of the instantaneous reaction model indicated that the plume was too long.
This indicates that there is more mixing of hydrocarbon and electron acceptors at the site than
is predicted by the model. Therefore the longitudinal dispersivity was adjusted upwards
(more mixing) until BIOSCREEN matched the observed plume length. The final longitudinal
dispersivity was 32.5 ft.
As a check the first-order decay model was used with the BIOSCREEN default value of 2 yrs.
This run greatly overestimated the plume length, so the amount of biodegradation was
increased by decreasing the solute half-life. A good match of the plume was reached with a
solute half-life of 0.15 years. This is within observed ranges reported in the literature (see
solute half-life section, page 22).
As shown in Figure 3, BIOSCREEN matches the observed plume fairly well. The
instantaneous model is more accurate near the source while the first order decay model is
more accurate near the middle of the plume. Both models reproduce the actual plume length
relatively well.
As shown in Figure 4, the current plume is estimated to contain 7.8 kg of BTEX. BIOSCREEN
indicates that the plume under a no-degradation scenario would contain 126.3 kg BTEX. In
other words BIOSCREEN indicates that 94% of the BTEX mass that has left the source since
1989 has biodegraded.
Most of the source mass postulated to be in place in 1989 is still there in 1996 (2000 kg vs. 1837
kg, or 92% left).
The current plume contains 1.0 ac-ft of contaminated water, with 1.019 acre-ft/yr of water
being contaminated as it flows through the source. Because the plume is almost at steady
state, 1.019 ac-ft of water become contaminated per year with the same amount being
remediated every year due to in-situ biodegradation and other attenuation processes. This
indicates that a long-term monitoring approach would probably be more appropriate for this
site than active remediation, as the plume is no longer growing in size.
54
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BIOSCREEN User's Manual June 1996
55
-------
BIOSCREEN Natural Attenuation Decision Support System
A ir Force Center for Environmental Excellence
1. HYDRQGEQLQGY
Seepage Velocity* Vs
or
Hydraulic Conductivity K
Hydraulic Gradient i
Porosity n
(crnfyec}
fft/ffj
2, DISPERSION
Longitudinal Dispersivity* alphax
Transverse Dispersivity* alpha y
Vertical Dispersivity* alpha z
or
Estimated Plume Length Lp
3, ADSORPTION
Retardation Factor* R
or
Soil Bulk Density rho
Partition Coefficient Koc
FractionOrganicCarbon foe
4. BIODEGRADATION
1st Order Decay Coefl*
or
Solute Half-Life t-haff
or instantaneous Reaction Model
Delta Oxygen* DO
Delta Nitrate* NQ3
Observed Ferrous Iron* Fe2+
Delta Sulfate* SO4
Qbsewed Methane* CH4
(peryr)
(mgfQ
fmg/LJ
(mgfQ
(mgfQ
{mg/L}
Version 1 , 3
KessierAFB
SWtfL/66
Run Name
Data Input Instructions:
5, GENERAL
Modeled Area Length*
Modeled Area Width*
Simulation Time*
320
200
(ft) r
Variabe*
Enter value directlv. , , , or
2. Calculate by filling in gr&/
below. (To restore
fy.siMiss.' M! ^,My.
Date wssrf dif&ct^ in_
Value calculated by mode/.
6. SOURCE DATA
Vertical Plane Source: Look at Plume Cross-
Section and fnput Concentrations & Widths
for Zones i, 2, and 3
Source Decay (see Hel
SourceHaMjte
Soluble Mass!T or
JnNAPLJBQil
?r FMLDDATA FOR COMPARISON
? View of Plume Looking Down
Observed Center/me Concentrations at Monitoring Wells
: if No Data Leave Blank or Enter "0"
Concentration (mg/L) 12.0
Diet, from Source (ft)j
B. CHOOSE TYPE OF OUTPUT TO SEE:
RUN ARRAY
RUN
CENTERLINE
View Output
View Output
Recalculate This
Sheet
Eiample
Formulas for Vs,
DispersMties, R, lambda, other
Figure 2 . BIOSCREEN Input Screen. Keesler Air Force Base, Mississippi.
56
-------
Distance from Source (ft)
TYHEQF/WGOe.
32
64
96
128
160
192
224
256
288
320
1st
13.544
.117 | 1.186 I I I I I 0.018 I I I
F/eW Date from Site
12.000 ! 5.000
.000
0.500
0.001
1st Order Decay * Instantaneous Reaction
No Degradation Q Field Data from Site
Calculate
Animation
50
200 250 200
Distance From Source (ft)
Time:
6 Years
250
Return to
Input
300
Recalculate This
Sheet
350
Figure 3 . Centerline Output. Keesler Air Force Base, Mississippi.
57
-------
Transverse
Oistewce (ft)
100
50
-50
-100
IN
Distance from Source (ft)
0
32
64
96
128
160
192
224
256
288
320
0000
aco
12.021
.
OQQQ
g.ggg
"agog"
llif
"gTgocF
"0.060"
g.ggg
I-100
"4548"
"gTggg"
"0.000"
.
g.ggg
"ao"'
g.ggg
"T"'
g.ggg
I'M
"1578"
ooo
agog
"0.000"
iioo
g.ggg
"g'ggg'
1111
"gTggg"
"o"ooo"
g.ggg
"ao"'
.
agog
"gTggg"'
"o"oob""
g.ggg
"gTggig"'
"gjggg"
"gTggg"
"o'oob""
Model to Display:
Mo Degradation
Model
Time:
6 Years
Target Level:
256
288
mg/L
Displayed Model:
Inst. Reaction
1st Order Decay
Model
Instantaneous
Reaction Model
Plume and Source Masses (Order-of-Magnitude Accuracy)
-100
Plume Mass if No Biodegradation| 126.3 \(Kg)
- Actual Plume
= Plurne Mass Removed by Biodeg|| 118.6 \(Kg)
Change in Electron Acceptor/Byproduct Masses:
Original Mass In Source (Time = 0 Years)
Mass in Source Now (Time = 6Years)
Current Volume of Groundwater in Plume
Flowrate of Water Through Source Zone
Return to Input
r.
fac-ftj
{ac-ft/yr}
Recalculate
Figure 4 . Array Concentration Output. Keesler Air Force Base, Mississippi.
58
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BIOSCREEN User's Manual
June 1996
EXAMPLE 2
Hill Air Force Base, UST Site 870, Utah
DATA TYPE
Hydrogeology
Dispersion
Adsorption
Biodegradation
General
Source Data
Actual Data
OUTPUT
Parameter
Hydraulic Conductivity:
Hydraulic Gradient:
Porosity:
Original
Longitudinal
Dispersivity:
Transverse Dispersivity:
Vertical Dispersivity:
Retardation Factor:
Soil Bulk Density pb:
foe:
Koc:
Electron Acceptor:
Background Cone. (mg/L):
Minimum Cone. (mg/L):
Change in Cone. (mg/L):
Electron Acceptor:
Max. Cone. (mg/L):
Avg. Cone. (mg/L):
Modeled Area Length:
Modeled Area Width:
Simulation Time:
Source Thickness:
Source Concentration:
Distance from Source (ft):
BTEX Cone. (mg/L):
Centerline Concentration:
Array Concentration:
Value
8.05xlO-3 (cm/ sec)
0.048 (ft/ ft)
0.25
28.5 (ft)
2.85 (ft)
0(ft)
1.3
1.7(kg/L)
0.08%
B: 38 T: 135
E: 95 X: 240
O2 NO3 SO4
6.0 17.0 100
Fe CH4
Note: Boxed values are BIOSCREEN
input values.
1450 (ft)
320 (ft)
5 (yrs)
10 (ft)
(See Figure 5)
340 1080 1350 1420
8.0 1.0 0.02 0.005
See Figure 7
See Figure 8
Source
Slug-tests results
Static water level
measurements
Estimated
Based on estimated plume
length of 1450 ft and Xu's
dispersivity formula
Note: No calibration was
necessary to match the
observed plume length.
Calculated from
R = 1+Koc x foe x pb/n
Estimated
Lab analysis
Literature - use Koc = 38
Based on July 1994
groundwater sampling
program conducted by
Parsons Engineering
Science, Inc.
Based on area of affected
groundwater plume
Steady-state flow
Based on geologic logs
and lumped BTEX
monitoring data
Based on observed
concentration contour at
site (see Figure 5)
59
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BIOSCREEN User's Manual
June 1996
MW-1
MW-14
9.756
EPA-82-E
<0.001
EPA-82-N
<0.001
SCALE (ft.)
0 200 400
LEGEND
Monitoring well location
g July 1994 Geoprobe sampling location
BTEX concentration Isopleth, mg/L, July 1994
Affected Soil Zone
BIOSCREEN SOURCE ZONE
ASSUMPTIONS
UST Site 870, Hill AFB, Utah
FIGURES
60
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BIOSCREEN User's Manual June 1996
BIOSCREEN Modeling Summary Hill Air Force Base, UST Site 870, Utah:
BIOSCREEN was used to try to reproduce the movement of the plume.
An infinite source was assumed to simplify the modeling scenario because no estimates of the
source mass were available from soil sampling data. The source was assumed to be in the
high concentration zone of the plume area (see Figure 5). Note that the zone of affected soil
was quite large; however much of the affected soil zone downgradient of the source was
relatively low concentration.
Two modeling approaches could be applied: 1) assuming the source zone is just
downgradient of the affected soil area (near well EPA-82-C) and ignoring the area upgradient
of the this point, and 2) modeling most of the plume with source near MW-1. Alternative 1 is
theoretically more accurate, as BIOSCREEN cannot account for the contributions from any
affected soil zone downgradient of the source. At the case of Hill AFB, however, it was
assumed that the contributions from this downgradient affected soil were relatively minor and
that the main process of interest was the length of the plume from the high-concentration
source zone. Therefore Alternative 2 was modeled, with the note that the middle of the actual
plume may actually have higher concentrations than would be expected due to the
contaminants in the downgradient affected soil zone.
The instantaneous reaction model was used as the primary model to try to reproduce the
plume length (~ 280 ft) as shown in Figure 7.
The initial run of the instantaneous reaction model reproduced the existing plume without any
need for calibration of dispersivity.
As a check the first-order decay model was used with the BIOSCREEN default value of 2 yrs.
This run greatly overestimated the plume length, so the amount of biodegradation was
increased by decreasing the solute half-life. A half-life value of 0.1 years was required to
match the plume length, although the match in the middle in the plume was much poorer.
As shown in Figure 7, BIOSCREEN matches the observed plume fairly well. The
instantaneous model is more accurate near the source while the first order decay model is
more accurate near the middle of the plume. Both models reproduce the actual plume length
relatively well.
As shown in Figure 8, the model was unable to calculate the mass balances. A quick
evaluation shows the reason: with a seepage velocity of 1609 ft/yr and a 5 year simulation
time, the undegraded plume should be over 8000 ft long. Because the mass balance is based
on a comparison of a complete undegraded plume vs. a degraded plume, a model area length
of 8000 ft would be required for BIOSCREEN to complete the mass balance calculation.
Therefore two runs would be needed to complete the simulation: 1) a run with a modeled
length of 1450 feet to calibrate and evaluate the match to existing data, and 2) a run with a
modeled length of 8000 ft to do the mass balance. The results of the second run (change of
model area length from 1450 ft to 8000 ft) indicate that over 99% of the mass that has left the
source has biodegraded by the time groundwater has traveled 1450 ft.
Because the plume is no longer moving, a long-term monitoring approach is probably more
appropriate for this site than active remediation.
61
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BIOSCREEN Natural Attenuation Decision Support System
Air FofMf Center for Environmental Exce/ience
1. HYDROGEOLOGY
Seepage Velocity*
or
Hydraulic Conductivity
Hydraulic Gradient
Porosity
Vs
2. DISPERSION
Longitudinal Dispersivity* alphax
Transverse Dispersivity*
Vertical Dispersivity*
or
Estimated Plume Length Up
3, ADSORPTION
Retardation Factor* R
or
Soil Bulk Density rho
Partition Coefficient Koc
F ra cti o nOrg a nic Carbon foe
....._____......._
1st Order Decay Coeff &**<&
or
Solute Half-Life t-half
of Instantaneous Reaction Model
Delta Oxygen* DO
Delta Nitrate* W03
Observed Ferrous Iron* Fe2+
Delta Sulfate* SO4
Observed Methane* CH4
OZHf-J
I
(perj/r)
Version 1.3
HiltAFB
USTSite 379
5. GENERAL
Modeled Area Length*
Modeled Area Width*
Simulation Time*
rffunjyame_
.L _
w C^L u
(ft) w I
(yr) * r
Data Input Instructions:
Ente
Calc
cells below. (To restore
*.f. Enter value directi/....or
2. Calculate bv fillinQ in
6. SOURCE DATA
ง0.yEe.. I!?.!?!!.?.? '0 .?.3!-l:SDI
Source Zones:
Width* (ft) [Cone. fmti/Lf,
Value calculated by mode
an/ efef aj .
m
Vertical Plane Source: Look at Plume Cross-
.- Section and Input Concentrations & Widths
for Zones 1, 2, and 3
source
SgurceHalflife
Soluble Mass I
In NAPL, Son
f. FIELD DATA FOR COMPARISON
Concentration (mg/L)
Dist. from Source (ft)
View of Plume Looking Down
Observed Centerline Concentrations at Monitoring Wells
I If Wo Data Leave Blank or Enter "0"
0 145 290 435 580 725 870 101511160 1305 1450
8. CHOOSE TYPE OF OUTPUT TO SEE;
HUN
CENTERLINE
View Output
RUN
View Output
Help
Recalculate
This
Restore Formulas forVs,
Figure 6. BIOSCREEN Input Screen. Hill Air Force Base, Utah.
62
-------
Distance from Source (ft)
TYPE OF MODEL
'- i --
1st
::";_::;, f '- O -."::"." X : ST;
Ffefef Data from Site
0
9.000
r :.::. J
9.000
145 290 435 . 580 725
.-! , -. : . r. - ; . . , . . -;
4.348 \ 1.969 | 0.905 j 0.424 | 0.201
ii\
. - :: - ; "/ .' -' -'. - :.' ..--. : - i n _
i 8-ฐฐฐ i i i
870 1 01 5
j : i i J - i -I i .
! i 0.047
: :
: .: : _
i | 1 .000
1160 1305
. ; . K. _ ; . .
! 0.023 | 0.011
1 ; .4-. ; U ,-.:v
1 j 0.020
1450
, ... . :
I 0.005
:
j 0.005
' 1st Order Decay =*= instantaneous Reaction
No Degradation Q Fieid Dsta from Site
',000
200
Calculate
Animation
400 600 SOO 1000
Distance From Source (ft)
Time:
1200
5 Years
Return to
Input
MOO 1600
Recalculate This
Sheet
Figure 7. Centerline Output. Hill Air Force Base, Utah.
63
-------
Trawsverat
Distance (ft)
f
160
80
-80
-160
IN
Distance from Source (ft)
0
145
.oo
-oo
p. goo
"gjfff"
"4e6
o
omo
OBOO
290
435
g.ggo
".oo"
00 o
giro
"b"bbb"
ooop
"ojfib"
580
725
870
1015
g.ggg
"g'ggg"
"5-268"
"o'p'pp""
"oJobb"
g.ggg
"oJggjiii'"
"4492'
"o'ligo"
-
.
g.ggg
"."
oioo
gggg
"b"J6bo"
1160
g.ggg
"'J"
gjggg
"6'Jooo"
1305
1450
g.ggg
"g.ggg"
"OJ385"
"gjgigg"
"oifio""
g.ggg
0-000
-Opg
"b!boo"
Display:
Wo Degradation
Time:
5 Years
Target Level: | 0.005 | rngfl.
Displayed Model;
Inst. Reaction
1st Order Decay
Mottet
tn&tantmeom
ftesction
P/time and Source Masses
-160
(ft)
1160
160
1305
Plume Mass If No Blodegradatlon[Cant Calc.|(Kg)
- Actual Plume Mass|Can't Calc.|(K.g)
= Plume Mass Removed by Biodeq
Change in Electron ACceptorByproauct Masses:
Oxygen Nitrate
Original Mass In bource (Time = 0 Years)
Mass in Source Now (Time = SYears)
Current Volume of Groundwater in Plume
Flowrate of Water Through Source Zone
Return to input
Figure 8 . Array Concentration Output. Hill Air Force Base, Utah.
64
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BIOSCREEN User's Manual June 1996
65
------- |