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A case study for demonstrating the application of
U.S. EPA’s monitored natural attenuation
screening protocol at a hazardous
waste site
T. Prabhakar Clement a,*, Michael J. Truex b, Peter Lee c
aDepartment of Environmental Engineering, University of Western Australia, Nedlands, WA 6009, AustraliabBattelle Pacific Northwest Division, Richland, WA 99352, USA
cNPC Services Inc., 2401 Brooklawn Drive, Baton Rouge, LA 70807, USA
Received 11 June 2001; received in revised form 12 February 2002; accepted 13 February 2002
Abstract
Natural attenuation assessment data, collected at a Superfund site located in Louisiana, USA, are
presented. The study site is contaminated with large quantities of DNAPL waste products. Source
characterization data indicated that chlorinated ethene and ethane compounds are the major
contaminants of concern. This case study illustrates the steps involved in implementing the U.S.
EPA’s [U.S. EPA, 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents
in ground water, by Wiedmeier, T.H., Swnason, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T.,
Wilson, B.H., Kampbell, D.H., Hass, P.E., Miller, R.N., Hansen, J. E., Chapelle, F.H., Office of
Research and Development, EPA/600/R-98/128] monitored natural attenuation (MNA) screening
protocol at this chlorinated solvent site. In the first stage of the MNA assessment process, the field
data collected from four monitoring wells located in different parts of the plume were used to
complete a biodegradation scoring analysis recommended by the protocol. The analysis indicates
that the site has the potential for natural attenuation. In the second stage, a detailed conceptual model
was developed to identify various contaminant transport pathways and exposure points. The U.S.
EPA model and BIOCHLOR was used to assess whether the contaminants are attenuating at a
reasonable rate along these transport paths so that MNA can be considered as a feasible remedial
option for the site. The site data along with the modeling results indicate that the chlorinated ethene
and chlorinated ethane plumes are degrading and will attenuate within 1000 ft down gradient from
0169-7722/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0169 -7722 (02 )00079 -7
* Corresponding author. Department of Civil Engineering, Auburn University, AL 36830, USA.
E-mail address: [email protected] (T.P. Clement).
www.elsevier.com/locate/jconhyd
Journal of Contaminant Hydrology 59 (2002) 133–162
the source, well before reaching the identified exposure point. Therefore, MNA can be considered as
one of the feasible remediation options for the site.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Modeling; Groundwater contamination; Bioremediation; Biodegradation; Natural attenuation;
Reactive transport
1. Introduction
Waste disposal operations began at the Petro-Processors Inc. (PPI) Brooklawn site
(known as Brooklawn site) between 1968 and 1970 and continued until 1980. During this
period, various types of hazardous waste material were disposed at the site. The material
included dense nonaqueous phase liquids (DNAPLs) that originated from chlorinated
solvent manufacturing plants and from other refineries. Based on a preliminary site
investigation, which was completed by NPC Services, a draft work plan for implementing
remedial activities was developed in 1984. A hydraulic containment system, and an active
source recovery system coupled with the treatment of the extracted water were selected as
remedial strategies.
Microcosm tests recently performed using the sediment samples collected at the
Brooklawn site indicated that the soil microbes have the potential to degrade various
chlorinated compounds (Acar et al., 1995; Constant et al., 1995; Clover et al., 1998; Pardue,
1999; Truex et al., 2001). Therefore, monitored natural attenuation (MNA) appears to be
one of the additional remedial alternatives available for managing the dissolved plumes at
the site. Previously published natural attenuation studies indicate that the biological activity
required for degrading many chlorinated organic compounds are ubiquitously present in
most anaerobic aquifers (Semprini et al., 1995; Bradley and Chappelle, 1997; Lorah and
Olsen, 1999). If sufficient natural or contaminant-derived organic carbon is available as a
substrate to support the growth of microbial populations, then MNA can be considered as
one of the feasible options for managing chlorinated solvent plumes (Wiedemeier et al.,
1999; Clement et al., 2000). A detailed protocol is now available for assessing the
attenuation processes for applying the technology at field sites (U.S. EPA, 1998, 1999;
Lu et al., 1999). According to the protocol, the first task in implementing MNA at a field
site involves completion of an initial screening assessment study. Subsequent tasks involve
modeling to determine exposures and better quantify fate. The objective of this case study is
to illustrate the efforts involved in implementing the MNA screening process, prescribed in
U.S. EPA (1998), at hazardous waste sites and the benefits of modeling. The natural
attenuation data set collected at the Brooklawn site is used for this purpose.
2. Site characterization data
The surficial features of the Brooklawn site and the location of various monitoring
wells used in the site characterization effort are shown in Fig. 1. The field site is located to
the north of Baton Rouge, LO, USA, approximately 5200 ft (about 1.5 km) away from the
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162134
Mississippi River. The land to the south of the site is largely undeveloped swampy
lowlands of the Mississippi River floodplain, known as the Devil’s Swamp, and in the
north there is a large industrial development site. A small stream known as the Bayou
Baton Rouge, which originates about 7 miles north of the site, runs along the western
boundary of the site and turns east at the southern site boundary and breaks into several
distributaries which discharge into the Swamp. The dissolved contaminant plume at the
site extends about 4000 ft in the east–west direction and about 1100 ft in the north–south
direction.
2.1. Geological data
The Brooklawn site is located on the interface between ancient Pleistocene sedimentary
deposits and the recent alluvial sediments deposited by the Mississippi River. The interface
is marked by a topographic bluff line that transverses in the east–west direction. The bluff
line approximately parallels the Mississippi River, and is about 30 ft higher than the
adjacent floodplain. This line is an erosional feature carved by the Mississippi River. To
the north of the bluff line, the upland Pleistocene sedimentary deposits predominantly
consist of clayey material, and to the south of the line the floodplain alluvium sediments
predominantly consist of sandy material. Fig. 2 shows a geologic cross section of the site
through the primary contaminant source area. As shown in the figure, the upland
Pleistocene deposits consist of clay from the surface (or from the erosional contact point
with the alluvium) to a depth of about �160 ft MSL. The conductivity values of the clay
Fig. 1. Site details and well locations.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 135
Fig.2.Representativegeological
cross
sectionacross
thesourcearea.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162136
ranges from 10�6 to 10�4 ft/day. Some interlayers of silt and sand are present within the
Pleistocene clay unit. One of the interlayered silt zones, the ‘‘�40 ft silt’’ layer, seems to
be ubiquitously present across the entire site. The hydraulic conductivity of the silt zone is
about 0.1 to 1 ft/day. Additionally, an ‘‘intermediate sand unit’’, which is a 40-ft-thick
sandy silt layer, also runs across the site starting at a depth of about �80 ft MSL. The
‘‘400-ft’’ aquifer, which is a highly permeable sand aquifer (with conductivity values in
the range 10 to 100 ft/day), underlies the entire site starting at a depth of �160 ft MSL.
The alluvial sediments in the floodplain region are generally sandy-to-silty in texture
near the surface. With depth, the sediments become interlayered and intermixed with fine
sandy silts to medium sand separated by thin discontinuous clay layers which are generally
less than 5 ft thick. Clay mixed sands with appreciable amounts of organic material were
found in numerous soil borings within the floodplain region. The hydraulic conductivity of
the alluvium sand ranges from 1 to 10 ft/day. The thickness of the top alluvial unit
increases with distances away from the site. As shown in Fig. 2, along the clay–alluvium
interface, the alluvial unit pinches out and intersects with the Pleistocene clay unit; and
with distances away from the site, towards the river, the alluvial unit become thicker and
intersects the ‘‘�40 ft silt’’ layer and later intersects the ‘‘intermediate sand’’ layer.
2.2. Background geochemical data
Water samples collected from wells P-0936-1 and P-1620-1, which are located outside
the plume (see Fig. 1), were selected to assess the background geochemistry of Brooklawn
groundwater. Note that one of the selected wells (P-0936-1) is located upstream in the
Pleistocene clay unit and the other well (P-1620-1) is located downstream in the alluvium
unit. The measured geochemical constituents of the Pleistocene and alluvium units are
summarized in Table 1. The geochemical data indicate that the alluvium, which is
Table 1
Background geochemical characteristics of the groundwater
Constituent Alluviuma Pleistocene clay unitb Units
Chloride 29 34 mg/l
Specific conductance 616 604 Amho
Dissolved methane not detected not detected mg/l
Dissolved oxygen not detected 2 mg/l
Dissolved hydrogen 5.2 2 nM
Inorganic carbon 81 26.7 mg/l
Iron, ferrous 7.9 0.1 mg/l
Nitrite not detected 0.04 mg/l
Nitrate 0.3 0.8 mg/l
Oxidation/reduction potential �40 �46 mV
pH 6.6 7.3 standard units
Sulfide not detected 0.1 mg/l
Sulfate not detected 8 mg/l
Temperature 20 18 jCTotal organic carbon 11 1.5 mg/l
a Average concentration from wells P-1620-1 and P-2953-1.b Concentration from well P-0936-1.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 137
expected to be the primary transport pathway, can be characterized as an anaerobic system
with neutral pH and moderate ionic strength and hardness. The redox characteristics of the
Pleistocene clay zone also indicate reducing conditions. Although low levels of dissolved
oxygen were measured in P-0932-1, which is located outside the site boundary, several
other wells located within the plume in the Pleistocene clay were devoid of oxygen, thus
indicating anaerobic conditions. Measured organic carbon content of the alluvium sedi-
ments ranged from 0.39% to 1.13% (Valsaraj et al., 1999).
2.3. Source locations
Based on historic site characterization data, the spatial extents of various DNAPL
contamination zones were delineated. The boundaries of the source zones are shown in
Figs. 1 and 2. The bulk of the DNAPL mass is present in the eastern portion of the site.
However, some minor isolated pits and disposal drains were also present in the western
region. Historical data show that most of the waste products were initially disposed into
several earthen pits located in the Pleistocene clay unit along the upland portion of the site.
During remedial investigations, these upland waste NAPLs were found to be confined
with only minimal NAPL mass migration beyond the pits. However, large amounts of
NAPL were later disposed into excavated pits in the lower floodplain area, which are
surrounded by constructed levees. The NAPL products disposed in this area have migrated
beyond the limits of the pits. Further, during a flood event in the 1970s, a portion of the
levee failed and NAPL products were directly discharged into the small drainage channels
along the southwest corner of the site. This event created a ‘‘spill area’’ (see Fig. 1), which
was delineated based on soil core data collected from the channels. Further site
investigations revealed that the presence of NAPL was mostly limited to shallow regions,
with isolated deeper occurrences in the northern area of the channel adjacent to the
Brooklawn site.
2.4. Contaminant characterization data
The contamination currently present at the site consists of large quantities of pooled or
trapped NAPL-phase products and various dissolved plumes that emanate from the free
phase. The DNAPL present at the site is a complex mixture of several organic compounds.
Table 2 shows the groundwater concentrations of various contaminant species measured in
two monitoring wells located within the dissolved plume region. The first well, P-1426-6,
is located down gradient from the primary DNAPL source area, and the second well,
PBB21-1N, is located near the edge of a secondary spilled source region. The data show
that dissolved chlorinated ethene and chlorinated ethane compounds are at concentration
levels of one to two orders of magnitude greater than all other compounds. High values of
chloroform and 1,2-dichloropropane were also measured in some of the groundwater
samples. Chloroform can be biodegraded and is itself an anaerobic biotransformation
product of carbon tetrachloride (Beelen and Keulen, 1990; Picardal et al., 1995).
Researchers have shown that chloroform can be degraded by anaerobic methanogenic
enrichments and by other non-methanogenic anaerobic cultures (Bagley and Gossett,
1995). Literature information also suggests that 1,2-dichloropropane can be completely
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162138
dechlorinated under anaerobic conditions (Loffler et al., 1997). Since the dissolved levels
of chlorinated ethene and ethane compounds were good indicators of the status of overall
contamination, further analyses of chloroform and dichloropropane were not included in
this screening study. Non-chlorinated contaminants such as the BTEX (benzene, toluene,
ethylbenzene and xylene) compounds were also not included because the measured BTEX
levels in down gradient wells were much lower than chlorinated compounds. In this initial
MNA screening study, we primarily focus on the fate and transport of the following
chlorinated ethane and ethene components: 1,1,2,2-tetrachloroethane (TeCA), 1,1,2-
trichloroethane (TCA), 1,2-dichloroethane (DCA), chloroethane (CA), tetrachloroethene
(PCE), trichloroethene (TCE), dichloroethene (DCE), and vinyl chloride (VC). These
components were previously identified as the contaminants of concern for the site based
on their chemical, physical, and other transport properties (NPC, 1996). Since chlorinated
solvents are prevalent at most DNAPL waste sites, the U.S. EPA (1998) MNA guidelines
also primarily focus on analyzing these contaminants.
3. Biodegradation assessment
The MNA assessment process involves a six-step screening method (U.S. EPA, 1998).
As shown in Fig. 3, the first step in the MNA assessment is to use the site data to answer
the important question ‘‘Is biodegradation occurring at the site?’’ In order to address this
Table 2
Concentration of chlorinated constituents in the alluvium
Compound Concentration at well P-1426-6 (Ag/l) Concentration at well PBB21-1N (Ag/l)
1,2,4-Trichlorobenzenea <10 <20
1,2-Dichlorobenzenea,b <5 <5
1,2-Dichloropropanea 4640 24000
Bis(2-Chloroisopropyl)ethera 40.7 20.9
Carbon Tetrachloridea <5 <5
Chlorobenzenea,c <5 53
Chloroforma 70.1 4040
Hexachlorobenzenea <10 <20
Hexachlorobutadienea <10 <20
Hexachloroethanea <10 <20
1,1,2,2-Tetrachloroethane 153 6960
1,1,2-Trichloroethane 9710 98100
1,1-Dichloroethene 683 2670
1,2-Dichloroethane 15700 99600
Tetrachloroethene 117 3420
Trichloroethene 3710 13200
cis-1,2-Dichloroethene 2790 13100
trans-1,2-Dichloroethene 763 5
Vinyl chloride 25700 58200
a Compound is not included for further study in this initial screening for MNA.b The concentration of all isomers of dichlorobenzene were similar.c Chlorobenzene is included in the table because it is a biotransformation product of other chlorinated
benzenes.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 139
question, the average concentrations of various geochemical parameters measured in
alluvium wells P-1620-1 and P-2953-1, shown in Table 1, were first used to set the
background levels. Groundwater geochemical data collected from four wells including P-
1223-2, P-1426-6, PBB21-1N, and P-1535-2, which are expected to represent different
types of biochemical and/or hydraulic regimes present at the site, were used to assess the
biodegradation conditions. It can be seen from Fig. 1 that the first well, P-1223-2, is
located to the south of the primary NAPL source area. This well is adjacent to the highly
contaminated source area and was (prior to pumping, between 1970 and 1995) down
gradient from the source. Currently, the well is within the hydraulic capture zone of the
Fig. 3. Monitored natural attenuation screening process flow sheet [adapted from U.S. EPA, 1998].
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162140
extraction wells and contains significant concentrations of chlorinated solvents. The
second monitoring well, P-1426-6, is located about 300 ft south of the primary NAPL
source area. This well location was also previously (prior to pumping) down gradient from
the source, but is now outside the hydraulic capture zone of the pumping wells. The
hydraulic gradient at this location changes seasonally due to fluctuations in the Mississippi
River stage. The third monitoring well, PBB21-1N, is located at the edge of a NAPL
source area in a bayou channel in the ‘‘spill zone’’. This well is close to the NAPL source
region. The hydraulic gradient in this region has significant seasonal variations and is
influenced by the water levels in the bayou channel. The fourth well, P-1535-2, is located
in the swampy area down gradient from the previous monitoring location. The hydraulic
gradient at this location also has considerable fluctuations.
Using the U.S. EPA (1998) MNA framework, a specified number of ‘‘points’’ were
assigned depending on the concentration of the geochemical indicators observed in the
wells. Points were awarded only if the concentration of a geochemical indicator was within
the range specified in the screening criteria and if the indicator was not a constituent of the
original contaminant source. The points were added and interpreted based on U.S. EPA
guidelines to determine whether biodegradation is occurring at the selected location. If the
total score was above 15 points, the location was deemed to have a good potential for
natural attenuation. Further details of this scoring scheme are discussed in U.S. EPA
(1998).
The results of the biodegradation assessment for the four selected Brooklawn site wells
are summarized in Table 3a and b. As shown in the table, the biodegradation assessment
scores for all four locations are found to be greater than 15, indicating a good potential for
natural biodegradation. Most of the points assigned to each location were due to the
detection of geochemical indicators of the presence of anaerobic environments—such
environments can support biological activities required for mediating dechlorination
processes. A few points were also assigned for the presence of certain degradation
products. For example, in wells P-1426-6 and PBB21-1N, cis-DCE, a daughter product of
chlorinated ethene and chlorinated ethane biotransformation, was present at concentration
levels much greater than the concentrations of the other dichloroethene isomers. The
concentration of cis-DCE was below detectable levels at the other two monitoring
locations.
Points could not be assigned for the presence of several other biodegradation products
(other than cis-DCE) even though they were present at very high levels. This is because
several of these possible biodegradation products were already present in the NAPL
source, although at very low levels. For example, VC, which is a by-product of the
reductive dechlorination process, was measured as 25.7 mg/l in well P1426-6 and 58.2
mg/l in PBB21-IN. However, the measured VC values were consistently low near the
source region. Further, the measured weight percentage of VC in the NAPL source was
about 0.026%, which yields a maximum effective VC solubility of 0.9 mg/l. Therefore, the
high VC concentration levels observed in some of the down gradient wells could have
been due to biodegradation. However, because VC is contained within the original NAPL
source, points cannot be awarded for the presence of VC according to the U.S. EPA (1998)
guidelines. This demonstrates the conservative nature of the scoring strategy, which could
result in underestimation of the overall biodegradation potential.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 141
Table 3
Bioattenuation screening parameters and scoring
Contaminant/geochemical
indicator
Criteria In NAPL
source?
PBL-1223-2
(shallow)
concentration
PBL-1223-2
(shallow) score
P-1426-6 (deep)
concentration
P-1426-6
(deep) score
Dissolved oxygen (mg/l) <0.5 N/A 0 3 0.92 0
Nitrate (mg/l) <1 N/A 0.4 2 1 2
Iron(II) (mg/l) >1 N/A 44.2 3 3.09 3
Sulfate (mg/l) <20 N/A <5 2 <1.0 2
Sulfide (mg/l) >1 N/A 0.006 0 0.003 0
Methane >0.5 N/A no data 0 4 3
Oxidation/reduction
potential (mV)
<50 or <�100 N/A �58 1 no data 0
pH 5<pH>9 N/A 6.4 N/A 6.8 N/A
Total organic carbon (mg/l) >20 N/A no data 0 21.2 2
Temperature >20 N/A 22.3 1 no data 0
Carbon dioxide >2� N/A no data 0 no data 0
Alkalinity >2� N/A 9.9 0 38.8 0
Chloride (mg/l) >2� N/A 87 2 355 2
Hydrogen >1 nM N/A <0.08 0 4 3
Volatile fatty acids >0.1 N/A no data 0 no data 0
BTEX (mg/l) >0.1 Yes 0.02 0 0.065 0
TCE (Ag/l) Yes 5.32 0 3710 0
1,1,2-TCA (Ag/l) Yes <5 0 9710 0
1,2-DCA (Ag/l) Yes <5 0 15700 0
trans-1,2-DCE (Ag/l) Yes <5 0 763 0
cis-1,2-DCE (Ag/l) No no data 0 2790 2
1,1-DCE (Ag/l) No 21.8 2 683 2
VC (Ag/l) Yes 1010 0 25700 0
Chloroethane (Ag/l) No <5 0 <5 0
Ethene (mg/l) >0.01 or >0.1 No no data 0 16 3
Ethane (mg/l) >0.01 or >0.1 No no data 0 0.001 2
1,1,2,2-Tetrachloroethane (Ag/l) Yes <5 0 153 0
Tetrachloroethene (Ag/l) Yes <5 0 117 0
Total 16 26
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142
Contaminant/geochemical
indicator
Criteria In NAPL
source?
PBB21-1N
concentration
PBB21-1N
score
P-1535-2
concentration
P-1535-2
score
Dissolved oxygen (mg/l) <0.5 N/A 0.02 3 0 3
Nitrate (mg/l) <1 N/A 0.8 2 0.4 2
Iron(II) (mg/l) >1 N/A 88.5 3 39 3
Sulfate (mg/l) <20 N/A 21 0 0 2
Sulfide (mg/l) >1 N/A 0.001 0 0.001 0
Methane >0.5 N/A 2.35 3 0 0
Oxidation/reduction potential (mV) <50 or <�100 N/A no data 0 �53 1
pH 5<pH>9 N/A 6.15 N/A 7.6 N/A
Total organic carbon (mg/l) >20 N/A 111 2 30.8 2
Temperature >20 N/A no data 0 24 1
Carbon dioxide >2� N/A no data 0 no data 0
Alkalinity >2� N/A 69.5 0 146 1
Chloride (mg/l) >2� N/A 440 2 110 2
Hydrogen >1 nM N/A 3.7 3 >8 3
Volatile fatty acids >0.1 N/A no data 0 no data 0
BTEX (mg/l) >0.1 Yes 0.2 2 0.035 0
TCE (Ag/l) Yes 13200 0 413 0
1,1,2-TCA (Ag/l) Yes 98100 0 3400 0
1,2-DCA (Ag/l) Yes 99600 0 2770 0
trans-1,2-DCE (Ag/l) Yes <5 0 84.8 0
cis-1,2-DCE (Ag/l) No 13100 2 no data 0
1,1-DCE (Ag/l) No 2670 2 170 2
VC (Ag/l) Yes 58200 0 4080 0
Chloroethane (Ag/l) No <5 0 <5 0
Ethene (mg/l) >0.01 or >0.1 No 16 3 0 0
Ethane (mg/l) >0.01 or >0.1 No 0.385 2 0 0
1,1,2,2-Tetrachloroethane (Ag/l) Yes 6960 0 25.4 0
Tetrachloroethene (Ag/l) Yes 3420 0 <5 0
Total 29 22
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143
4. Development of conceptual models
As shown in Fig. 3, in the second stage of the MNA screening process, a conceptual
model must be developed to provide the context for understanding the transport processes
occurring at the site. Fig. 4 shows the details of the source area, major contaminant
transport pathways, and various other surficial features. In our conceptual model develop-
ment effort, contaminant transport from the major source area where the bulk of DNAPL
mass currently resides is only considered. Fig. 4 also shows the location of various
monitoring wells and boreholes; data from these wells were used to build the computer
simulation model. All model simulations completed in this study focused on predicting the
fate-and-transport of contaminants under natural gradient conditions. In order to be
consistent with this scenario, the data collected prior to the start of groundwater extraction
activities were only used.
4.1. Conceptual model for contaminant transport pathways and exposure points
Fig. 5 is a conceptual cross section model of the site, which shows the relative locations
of potential exposure points along various transport pathways. The Mississippi River and
the ‘‘400-ft’’ aquifer were identified as the exposure points of concern. As shown in the
figure, the conceptual model considers two distinct transport pathways: (1) a horizontal
Fig. 4. Conceptual model for contaminant transport and other site details.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162144
Fig.5.Descriptionofsourcelocationsandtransportpaths.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 145
path towards the Mississippi River and (2) a vertical path towards the deep ‘‘400-ft’’
aquifer. The total distance along the horizontal transport path, via the alluvium, from the
leading edge of the source to the Mississippi River is about 5200 ft. The total distance
along the vertical transport path, via the Pleistocene clay, from the bottom of the source to
the 400-ft aquifer, is approximately 140 ft. In addition to these two major transport
pathways, it is possible that the plumes might reach the 400-ft aquifer through an inclined
pathway via the alluvium where the clay layer might be discontinuous. Since little
characterization information is available beyond the site boundary, it is difficult to
determine the transport characteristics of this inclined pathway, which would include
transport via the alluvial zone and some portions of discontinuous clay layers. Preliminary
analyses suggest that such a lengthy transport pathway may not be critical because alluvial
sediments are naturally less permeable in the vertical direction as compared to the
horizontal direction and, hence, are expected to yield more resistance to vertical transport.
Also, the observed vertical hydraulic gradient between the alluvium and 400-ft aquifer is
very small. Therefore, within the context of this initial screening effort, transport along this
inclined pathway was not considered.
The model shown in Fig. 5 was conceptualized based on the borehole data shown in
Fig. 2 and the time series of groundwater levels shown in Fig. 6. As shown in Fig. 6, the
transient groundwater levels in the alluvium and in the ‘‘400-ft’’ aquifer were similar, and
the trend also closely followed the seasonal variations in the Mississippi River. Water
levels in the Pleistocene clay unit do not respond to the river variations. These field
observations are consistent with our conceptual model.
The water levels observed in the alluvium (P-1620-2) and the 40-ft-silt zone (S-UG-1)
wells showed an average hydraulic gradient towards the river. The interrelationships
between the water level response patterns observed in the alluvium, silt layer, and
Fig. 6. Seasonal variations in groundwater levels observed in different hydraulic units.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162146
Pleistocene clay unit were used to develop a conceptual framework for the transfer of
contaminant mass from the source zone to the alluvium unit. Note that the site character-
ization data, summarized in Fig. 2, reveals that a portion of the DNAPL source disposed in
the alluvium pits was in close contact with the silt layer. Therefore, in our conceptual
contaminant mass transfer model, it is assumed that the contaminants from the DNAPL
source seep into the 40-ft silt layer and later are transported by the horizontal groundwater
level gradient between the silt and alluvium zones.
4.2. Estimation of contaminant transport velocity
Groundwater levels observed in the silt (S-UG-1) and alluvium (P-1620-2) wells were
used to estimate a yearly averaged horizontal transport velocity. Table 4 provides the water
levels measured in these wells over a 12-month period. As shown in the table, the yearly
average of head differences between these two wells is 2.15 ft and the distance between the
wells is 800 ft; this yields an average groundwater gradient of 0.003. Using an estimated
porosity value of 0.3 and conductivity value of 5 ft/day for the alluvium, the average
transport velocity for the horizontal pathway, towards the river via the alluvium aquifer, is
0.05 ft/day.
Similarly, the vertical hydraulic gradient was estimated based on the average head
difference between the �40-ft silt well (S-UG-1) and the ‘‘400-ft’’ aquifer (well D-UG-1).
While the vertical transport path has a much shorter length than the horizontal transport
path, the material between the contamination and the ‘‘400-ft’’ aquifer is low permeable
clay. Based on measured hydraulic conductivity value of 0.0001 ft/day for the clay
material, an estimated vertical hydraulic gradient of 0.019 ft/ft and an estimated porosity of
0.4, the vertical transport velocity is approximated as 4.75�10�6 ft/day. Contaminants
Table 4
Calculation of average horizontal hydraulic gradient for the alluvium
Month Hydraulic head (ft),
Well S-UG-1
(Pleistocene interface)
Hydraulic head (ft),
Well P-1620-2
(Alluvium)
Hydraulic head
difference (ft)
April 31.48 35.15 �3.67
May 32.56 33.93a �1.37
June 31.54 30.71 0.83
July 31.06 31.25 �0.19
August 30.14 24.53 5.61
September 28.02 18.4 9.62
October 26.56 18.43 8.13
November 26.27 19.61 6.66
December 26.04 22.55 3.49
January 26.47 23.87 2.6
February 27.24 30.79 �3.55
March 30.41 32.86 �2.45
Average head difference (ft) 2.15
Approximate gradient (based on 800 ft distance between wells) (ft/ft) 0.003 (toward river)
a Estimated value since no data is available for the month.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 147
traveling at this velocity would require over 80,000 years to traverse the 140 ft of clay
zone present between the current location of the contaminant source and the top of the
‘‘400-ft’’ aquifer. Moreover, monitoring data from the 400-ft aquifer also indicated that the
aquifer is uncontaminated. Therefore, further analysis of the vertical pathway was not
considered in this initial screening assessment.
5. Biodegradation and reactive transport
The U.S. EPA screening model, BIOCHLOR, was used to model the attenuation
processes occurring at site. BIOCHLOR is an analytical computer code that is intended for
use as a screening-level model to determine if remediation by natural attenuation is
feasible at a chlorinated solvent site (Aziz et al., 2000). The code uses a novel analytical
solution strategy to solve the multi-species sequential reactive transport problem (Sun et
al., 1999; Sun and Clement, 1999; Clement, 2001). BIOCHLOR has the ability to simulate
uniform flow with three-dimensional dispersion, linear adsorption, and biodegradation via
reductive dechlorination reactions. The model can predict migration patterns of a parent
chlorinated solvent species (either TCA or PCE) and its daughter products.
BIOCHLOR assumes first-order kinetics to model the biological decay reactions. The
use of first-order kinetics is appropriate when the biodegradation rate is primarily a
function of the concentration of the contaminant, when the number of microorganisms that
can degrade the contaminant is constant over time within the region of interest, and when
all other nutrients critical to the biodegradation processes are in abundance. For most field-
scale natural attenuation modeling applications, the first-order assumption may be
considered as a reasonable approximation (U.S. EPA, 1998; Aziz et al., 2000) provided
the electron donor is not limiting. Moreover, the assumption of first-order kinetics is often
acceptable for biodegradation at low pollutant concentration levels (Schmidt et al., 1985),
which is typically encountered in most groundwater remediation problems.
The disposal operations at the site started between 1968 and 1970, and the field data
used for this paper were collected nominally between 1992 and January 1995. Therefore,
the simulations were completed for 25 years so that direct comparison to the field data is
possible. The physical dimensions of the contaminant source were determined based on
the scale of the primary source shown in Figs. 4 and 5. The BIOCHLOR model requires
several basic transport parameters as input values, which include advection velocity,
dispersion coefficients, and retardation factors for the contaminant species. Further, the
model requires first-order degradation rate coefficients for the selected chlorinated solvent
reductive dechlorination sequence (either chlorinated ethenes or chlorinated ethanes). In
the following sections, the methods used for selecting appropriate retardation values,
biodegradation rate constants, source zone concentration levels, and other flow and
transport parameters are summarized.
5.1. Retardation parameters
The effective transport velocity of the contaminants is greatly influenced by the
adsorption characteristics of the porous medium. A linear equilibrium-partitioning model
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162148
was assumed to quantify the adsorption characteristics of Brooklawn sediments. Estimates
for the partition coefficient Kd (and subsequently the retardation factor R) were obtained
using an empirical correlation function. Valsaraj et al. (1999) performed sorption experi-
ments using three types of Brooklawn sediments and developed the following function:
Kd ¼ 100:81K0:56ow foc
where Kow is the octanol–water coefficient and foc is the fraction of organic carbon of the
soil. Table 5 presents the value of Kow and the calculated values for Kd and R values for the
chemicals considered in this initial MNA screening study. An foc value of 0.39%, which
was the value measured by Valsaraj et al. (1999) for the sandy alluvial material, was used
to compute the retardation parameters summarized in Table 5.
5.2. Biodegradation rate parameters
The Brooklawn site contamination includes a mixture of both chlorinated ethene and
chlorinated ethane compounds. The BIOCHLOR model is capable of simulating the
degradation of either the chlorinated ethene reaction chain (starting with PCE) or the
chlorinated ethane reaction chain (starting with TCA), not the mixture. However, when
both ethene and ethane species are present at the site, then it is difficult to interpret ethene
daughter products because some of the chlorinated ethenes can be produced from
chlorinated ethane decay reactions. Fig. 7 shows possible reaction pathways for degrada-
tion of PCE and TeCA and the subsequent chlorinated ethane and chlorinated ethene
daughter products (Lorah and Olsen, 1999). In this work, the BIOCHLOR simulations
were first completed for each individual ethene and ethane reaction chain. Subsequently,
approximations were made to quantify the influence of chlorinated ethenes produced from
chlorinated ethane decay reactions.
First-order kinetic models are assumed to be sufficient to describe all the reaction steps
represented in Fig. 7, which encompasses both chlorinated ethane and chlorinated ethene
Table 5
Estimated value of retardation coefficients
Compound Log10 of octanol–water
coefficient log Kowa
Partition
coefficient Kd (l/kg)b
Retardation coefficient
(1+qKd/n)c
TeCA 2.39 0.55 3.9
TCA 2.12 0.62 4.3
DCA 1.47 0.17 1.9
CA – – 1 (estimate)
PCE 2.88 1.0 6.5
TCE 2.42 0.57 4.0
DCEd 1.48 0.25 2.4
VC 0.6 0.05 1.3
a Values from Schwarzenbach et al. (1993).b Estimated from Valsaraj et al. (1999) correlation.c Assuming q=1.6 kg/l and n=0.3.d Value listed is for 1,1-dichloroethene.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 149
decay reactions. In addition, since many of the biotic reactions need organic carbon as
substrate to support the direct dechlorination or co-metabolic reactions, it is assumed that
sufficient organic carbon is available at the site. This assumption is reasonable at
Brooklawn site because the site is located down gradient from a swamp that has large
amounts of decaying material that can supply large amounts of carbon. In addition, carbon
sources could also be derived from other waste organic material co-disposed with
chlorinated compounds.
To assess the biodegradation kinetics of a mixture of chlorinated ethane and chlorinated
ethene compounds, the degradation rate coefficients must be considered in conjunction
with the fraction of each potential product produced from different parent compounds. As
shown in Fig. 7, chlorinated ethane species can degrade and produce less-chlorinated
ethane species or chlorinated ethene species. Table 6 presents the estimated fraction of
each daughter product that could be expected to be formed from different parent
compound. These estimates are based on laboratory data (Lorah and Olsen, 1999; Chen
et al., 1996).
Table 7 summarizes the assumed values of first-order decay rate coefficients for
modeling anaerobic destruction of chlorinated ethane and chlorinated ethene species.
These values were used in the BIOCHLOR model to perform the initial fate-and-transport
analysis for the Brooklawn site. Note that rate coefficients are only presented for reductive
dechlorination along one class of daughter products (e.g., only ethanes or only ethenes)
because these are the only sequences that the BIOCHLOR model can describe. Also, to
match the required inputs for the BIOCHLOR model, TCA is assumed to be the parent
Fig. 7. Anaerobic transformation pathways for the chlorinated ethane and ethene compounds.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162150
compound of the chlorinated ethane series. Further, all DCE isomers are combined
together and assumed to be the daughter product of biodegradation.
5.3. Source concentrations
Dissolved concentration levels of chlorinated ethane and chlorinated ethene species
measured or estimated to be present near the source zone are listed in Table 8. The first
column in the table lists field measured concentration data. These data are averages of
dissolved-phase concentrations in the groundwater samples collected from wells W-0822-
3, W-1025-2, and W-0626-1, where the water is in direct contact with NAPL products (see
Fig. 2 for well locations). The concentrations in the second column were estimated based
on a solubility analysis using measured NAPL composition data (Truex et al., 1999). It is
interesting to note that in Table 8 the measured concentration values and the calculated
effective solubility values compare favorably whenever the field data are above the
detection limit.
As shown in Fig. 7, chlorinated ethenes can be produced from biodegradation of the
chlorinated ethanes present in the NAPL source. The source area concentration of the
‘‘parent’’ chlorinated ethanes (TeCA and TCA), which can potentially produce chlorinate
ethenes, are approximately 60 and 337 mg/l, respectively. Clearly, these high concen-
trations of ethane parent compounds can yield significant amounts of chlorinated ethenes
Table 6
Estimated fractional percent conversion for chlorinated ethanes and chlorinated ethenes
Reaction pathway Estimated yield
(mol/mol)
Based on data from:
PCE!TCE 1.0 Lorah and Olsen (1999)
TCE!DCE 1.0 Lorah and Olsen (1999)
DCE!VC 1.0 Lorah and Olsen (1999)aTeCA!TCA 0.35 Lorah and Olsen (1999)
TeCA!TCE 0.02 Lorah and Olsen (1999)
TeCA!DCE 0.63 Lorah and Olsen (1999)aTCA!DCA 0.2 Chen et al. (1996)
TCA!VC 0.8 Chen et al. (1996)aDCA!CA 0.7 Chen et al. (1996)
DCA!ethane 0.3 Assumed
a As expected, sum of all TeCA or TCA or DCA yields is equal to unity.
Table 7
First-order rate coefficients
Reaction Value (1/day) Basis
TCA!DCA 0.013 One tenth of the rate estimated for Lorah and Olsen (1999) microcosm data
DCA!CA 0.001 One tenth of the rate estimated for Lorah and Olsen (1999) microcosm data
CA!ethane 0.014 One tenth of the rate estimated for Lorah and Olsen (1999) microcosm data
PCE!TCE 0.005 Set to be equal to TCE conversion rate
TCE!DCE 0.005 One tenth of the rate measured in microcosms using Brooklawn sediments
DCE!VC 0.005 Set to be equal to TCE conversion rate
VC!ethene 0.0006 One tenth of the rate measured in microcosms using Brooklawn sediments
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 151
down gradient from the source. The conversion fractional percent data presented in Table 6
can be used to estimate the maximum possible concentration of each chlorinated ethenes
produced from the ethane compounds present in the source. These estimates are
summarized in the third column of Table 8. Note if the biodegradation rates of ethane
reactions are high (when compared to ethene reactions), the mass of ethene produced from
ethane may simply be added to the original ethene source levels. This approximation is
employed in this study to indirectly simulate the combined ethane–ethene reactions.
5.4. Transport parameters
The transport parameters used in the model simulations are summarized in Table 9. The
transport properties of the alluvium were estimated based on field measurements. Methods
used for estimating the hydraulic gradient values were discussed in Section 4. The values
of retardation factors for different chlorinated compounds were estimated based on site-
specific sorption data shown in Table 5. The value for porosity was estimated from
literature values for similar types of geologic materials. The longitudinal dispersivity value
was estimated based on the guidelines presented in Gelhar et al. (1992). The ratio of the
longitudinal to transverse dispersivity was assumed to be 0.1, and ratio of the longitudinal
to vertical dispersivity was assumed to be 0.01.
Table 8
Contaminant concentrations (in mg/l) near the source zone
Constituent Measured values Estimated valuesa As decay productsb
TeCA 57 52 N/A
TCA 337 210 N/A
DCA 600 184 N/A
CA <25 0 N/A
PCE <25 3.5 N/A
TCE <25 6.2 1
DCE <25 0.8 21
VC <25 1.0 133
a Estimated based on solubility analysis.b Calculated using the chlorinated ethane concentrations from the first column and yield data from Table 6.
Table 9
Parameters used in BIOCHLOR simulations
Property Value
Hydraulic conductivity (ft/day) 5
Hydraulic gradient 0.003
Longitudinal dispersivity (ft) 50
Porosity 0.3
Average retardation for ethenes 3.6
Average retardation for ethanes 2.4
Model area (zone-1) length (ft) 5000
Model area width (ft) 2000
Source thickness (ft) 45
Source width (ft) 1000
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162152
6. BIOCHLOR simulations
Three sets of BIOCHLOR simulations were completed to assess whether adequate
bioattenuation is occurring at the Brooklawn site, and to evaluate the potential that a
steady-state condition may be established such that the contaminant plumes will not reach
the receptors. Wherever possible, the simulation results were compared against observed
data collected from the test boreholes drilled during a site investigation effort completed in
between September 1994 and January 1995. These test boreholes, BL-1225-1, BL-1426-1,
BB-1429-1, and BB-1626-1 (see Fig. 4), are located approximately 150, 300, 400, and 500
ft, respectively, from the source.
Simulation 1 predicted chlorinated ethane transport for a transport period of 25 years so
that the model results can be compared to the measured contaminant levels. The source-
zone concentrations used in this simulation were set based on field-measured values (see
Table 8, column 1). Both Simulations 2 and 3 predicted chlorinated ethene transport using
two types of source conditions. Since measured (above detection limit) source-zone
concentrations are unavailable for chlorinated ethenes, the first set of ethene simulation
(Simulation 2) was performed using source concentration levels computed based on the
concentration estimate from a solubility analysis (see Table 8, column 2).
In the second set of ethene simulations (Simulation 3), it was assumed that all
chlorinated ethane degradation reactions would occur at a rapid rate that would allow
complete ethane degradation close to the source zone. To reflect this condition, the source-
zone concentrations of chlorinated ethenes in Simulation 3 were set equal to the sum of
concentrations used in Simulation 2 and the maximum amount of ethene concentrations
that could be produced as products of ethane biodegradation (i.e., add ethene concentration
levels in column 2 and column 3 of Table 8). Analysis of field data along with model
simulations showed that this was a reasonable approximation for the Brooklawn site; this
point is discussed in more detail in the following section. The ultimate objective of
Simulation 3 was to assess the impact of chlorinated ethane degradation on predicted
chlorinated ethene levels. Since BIOCHLOR cannot be used to simulate coupled ethane–
ethene degradation reactions, this simplified approximation was made to assess the
coupled degradation condition.
6.1. Simulation 1—chlorinated ethane reaction chain
Fig. 8 presents the model-predicted TCA, DCA, and CA concentration profiles after 25
years of migration. The figure shows the predicted contaminant profiles down gradient
from the source zone along the plume centerline under three transport conditions: (1) no
attenuation, (2) attenuation by adsorption, and (3) attenuation by adsorption and biode-
gradation. The results show that the attenuation mechanisms including sorption and
degradation considerably retard plume migration. Also, all the biodegradation reactions
including production and destruction of intermediate compounds, such as CA, occur
within 500 ft down gradient of the source.
Field data for TCA and DCA indicate that the concentration of each species is less than
1 mg/l at distances of 150 and 300 ft, respectively, down gradient from the source. With
only adsorption-related attenuation, the model predicts the TCA and DCA concentrations
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 153
Fig. 8. Comparison of field data against BIOCHLOR simulations TCA and its daughter products (t=25 years).
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162154
as 50 and 450 mg/l, respectively, at these locations. This comparison indicates that the
TCA and DCA field data more closely match the concentration profiles predicted by
BIOCHLOR under the condition where both biodegradation and adsorption are occurring.
Since field data are not available for comparison to predicted CA concentration profiles,
these profiles must only be considered as preliminary estimates.
The BIOCHLOR model assumes TCA as the parent ethane contaminant and predicts
sequential dechlorination of TCA to DCA and later to CA. Thus, TCA produced from its
parent ethane compound TeCA, and also the possibility of partial conversion of TCA and
DCA compounds to VC (see Fig. 7), which has been measured in microcosm tests (Chen
et al., 1996; Lorah and Olsen, 1999), were not considered in this simulation.
6.2. Simulation 2—chlorinated ethene reaction chain
Fig. 9 presents model-predicted PCE, TCE, DCE, and VC concentration profiles after
25 years of migration. Similar to the previous analysis, these simulations were also
completed under different conditions including no attenuation, attenuation by adsorption,
and attenuation by adsorption and biodegradation. Source zone concentration levels used
in this simulation are based on the estimates from a solubility analysis (Table 8, column 2).
Under the assumed conditions, the model predicted that VC would be produced and
degraded within 300 ft down gradient from the source when biodegradation was assumed
to occur in the aquifer. With only adsorption-related attenuation, the VC concentration was
predicted to continuously reduce toward zero within 300 ft down gradient from the source.
The concentration of PCE measured in the field was less than 0.1 mg/l at a distance of
150 ft down gradient from the source. With only adsorption-related attenuation, Simu-
lation 2 predicted 2.5 mg/l of PCE at this location. The predicted PCE profile seems to
match the field data only when biodegradation and adsorption were assumed to simulta-
neously occur in the aquifer. The field data for TCE, DCE, and VC were more difficult to
interpret from Simulation 2 because each of the ethene species can also be produced from
degradation of chlorinated ethane species. Fig. 9c and d clearly indicates that the field-
measured DCE and VC values significantly diverge from the model profiles. In particular,
the field-measured VC concentration levels are much higher than those predicted by the
model either with or without biodegradation. As illustrated in Fig. 7, chlorinated ethene
compounds, particularly VC, can be produced as a by-product of chlorinated ethane
degradation. These by-product effects are quantified in the next set of simulations.
6.3. Simulation 3—coupled chlorinated ethane and ethene reactions
BIOCHLOR model results for the ethane series (shown in Fig. 8) indicates that TCA
concentration levels decrease from over 300 to less than 1 mg/l within 150 ft down
gradient of the source. Field data also confirm that the TCA concentration reduces to less
than 1 mg/l close to the source zone. Since degradation of TCA can yield ethene
compounds, it is possible that significant amounts of chlorinated ethenes are being
produced through biological processes within the 300-ft region from the source.
The NAPL present at the Brooklawn site contains large amounts of TeCA. As shown in
Fig. 7, degradation of TeCA would yield chlorinated ethene compounds. At biologically
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 155
Fig. 9. Comparison of field data against BIOCHLOR simulations PCE and its daughter products (t=25 years).
T.P.Clem
entet
al./JournalofContaminantHydrology59(2002)133–162
156
active field sites, it is reasonable to assume that TeCAwill be transformed within the same
area of the aquifer as TCA because typical degradation rates of TeCA transformation are
equal to or greater than the rates observed for TCA transformation (Lorah and Olsen,
1999). Field measured TeCA and TCA concentration levels at the Brooklawn site also
appear to support this assumption.
To evaluate the impacts of transformation of TeCA and TCA on the transport of
chlorinated ethene species, Simulation 3 was conducted using an ‘‘effective’’ ethene
source. Simulation 3 assumes rapid ethane degradation close to the NAPL zone. By
invoking this assumption, the concentration levels of the source are set to be equal to the
sum of concentrations used in Simulation 2 and the maximum amount of ethene
concentrations that could be produced as byproducts of ethane biodegradation (Table 8,
column 2+colum3 3). Results of this simulation are summarized in Fig. 10. Since the
source concentration of the first ethene species, PCE, was identical to the value used in
Simulation 2, PCE results are not discussed here.
Comparison of TCE profiles shown in Fig. 10 against the profiles shown in Fig. 9
indicates that the Simulation 3 profiles are not significantly different from those predicted
by Simulation 2. This result is as expected because, as illustrated in Fig. 7, TCE
concentrations are not significantly impacted by chlorinated ethane biodegradation
products. The predicted profiles of DCE and VC compounds, however, are impacted
considerably. As shown in Fig. 10, use of the ‘‘effective’’ ethane by-product-based ethene
sources yielded model results that closely match DCE and VC field data.
6.4. Determination of steady-state plume conditions
Additional simulations using conditions similar to those used in Simulations 1 through
3 were completed for 50- and 100-year periods. The purpose of these simulations was to
assess whether the contaminant plumes would reach steady-state conditions and cease
migrating prior to reaching the exposure points. Although results are available for all the
species and for all three simulation conditions (Truex et al., 1999), the results for critical
contaminant species are only presented in this paper. Fig. 11 shows the predicted DCA
(using simulation conditions 1) and VC plume profiles (using simulation conditions 3)
after 25, 50, and 100 years of transport. The predicted profiles with and without
biodegradation are also shown in Fig. 11. The results indicate that under active
biodegradation conditions, chlorinated solvent plumes at this site would reach steady
state after about 25 years of transport. In addition, the model also predicted that the plumes
would degrade to very low non-detectable concentration levels within 1000 ft from the
source area.
6.5. Model limitations
It is important to note that the tools and the assessment process used in this study have
several limitations. The U.S. EPA model BIOCHLOR has two inherent limitations: (1) it
averages the retardation effects and (2) it can only model a single sequential reaction
chain. These approximations add considerable uncertainly to the modeling results, and
therefore, the modeling step should simply be considered as a part of the conceptualiza-
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 157
Fig. 10. Comparison of field data against BIOCHLOR simulations ethene simulation with adjusted source
concentrations (t=25 years).
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162158
tion process. Also, it is essential to recognize that Simulation 3 only provides an estimate
of the impact of chlorinated ethane degradation on the transport of the chlorinated
ethenes.
Since BIOCHLOR assumes uniform flow, only an averaged one-dimensional velocity
can be used in the model. However, over the modeled domain, the groundwater head
distribution will vary nonlinearly, particularly near the down gradient transient boundary
close to the Mississippi River. Therefore, the averaged velocity estimate used in the study
Fig. 11. Steady-state analysis using BIOCHLOR.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162 159
could be a lower estimate and may only reflect the transport conditions closer to the
source. However, since the contaminants are degraded within a short distance from the
source (close to well P-1620-2, which was used for estimating the head gradient), this
should be a reasonable approximation. Use of a detailed numerical reactive transport code,
such as RT3D (Clement, 1997; Clement et al., 2000) coupled with a transient MOD-
FLOW-based flow model, would help perform more realistic simulations under these
transient, coupled reactive transport conditions.
7. Summary and conclusions
A field study was completed to demonstrate the steps involved in implementing the
U.S. EPA (1998) monitored natural attenuation screening protocol at a chlorinated solvent
contaminated field site. A natural attenuation assessment data set collected at a Superfund
site located in Louisiana was used in the study. Data from four monitoring wells, located
in different parts of the contaminated area, were selected to complete the natural
attenuation scoring analysis recommended by the protocol. The estimated scores are
above the acceptable minimum score proposed in the protocol, thus indicating that the
contaminants in the aquifer have the potential for natural attenuation. The background
geochemical data also indicate that the site is highly anaerobic and has large amounts of
natural carbon and, hence, has the potential for supporting chlorinated solvent degrada-
tion reactions.
A recent report by the National Research Council (NRC, 2000 p. 244) concluded,
‘‘scoring systems are susceptible to misuse and because approaches to natural attenuation
have been advanced in recent years, the committee recommends the abandonment of
scoring systems in screening site for natural attenuation. Instead, the committee recom-
mends site-specific conceptual models and footprints.’’ Clearly, the U.S. EPA (1998)
scoring process employed in the study is an empirical assessment method. Despite this
limitation, if applied carefully, the method does provide a means for systematically
assembling a diverse set of bioremediation footprints in a compact format that can be
communicated with regulators and stakeholders. However, as recommended by NRC
(2000) and U.S. EPA (1998) reports, a detailed site-specific conceptual model should
always be included to further support the result of the scoring process.
A detailed conceptual framework was developed and was used to build a computer
model for the site. The U.S. EPA’s screening tool BIOCHLOR was employed for this
purpose. As per the U.S. EPA (1998) guidelines, two MNA evaluation criteria must be
applied to assess the modeling results. These two criteria pose the following questions
(U.S. EPA, 1998): (1) Has the plume moved a shorter distance than would be expected
based on the known (or estimated) time since the contaminant release and the contaminant
velocity in groundwater, as calculated from site-specific measurements of hydraulic
conductivity and hydraulic gradient, and estimates of effective porosity and contaminant
retardation? (2) Is it likely that site contaminants are attenuating at rates sufficient to meet
remediation objectives for the site in a time period that is reasonable compared to other
alternatives? If the answers to both these questions are affirmative, then the site can
proceed with full-scale evaluation of natural attenuation.
T.P. Clement et al. / Journal of Contaminant Hydrology 59 (2002) 133–162160
Based on the MNA screening data and modeling results presented in this paper, the
following conclusions can be made.. Comparison of field data against model predictions for contaminant transport showed
that the dissolved plume would appear to migrate slower than would be predicted if only
adsorption-related attenuation mechanisms or if no attenuation mechanisms were acting on
the plume constituents. This result meets the first MNA evaluation criterion.. Using the available biodegradation rate information, the screening-level reactive
transport model (BIOCHLOR) results indicated that the dissolved plume would reach a
steady-state condition such that contaminants would stop migrating through the aquifer
well before reaching the identified exposure points. This result meets the second MNA
evaluation criterion.
Thus, the MNA screening criteria outlined in U.S. EPA (1998) were sufficiently met for
the Brooklawn site; therefore, MNA can be considered as one of the feasible remediation
options.
Acknowledgements
This project work was supported by the NPC Services. The authors wish to thank the
NPC Services staff members including Robert Bolger, James Spencer, and Carl Douglas
for providing project support and necessary site characterization data. This paper, in part,
was completed when Dr. Clement was at the Centre for Water Research, University of
Western Australia. We like to thank Prof. Norman Jones, Dr. Greg Davis, Mr. Colin
Johnston, and the two anonymous reviewers for their constructive comments.
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