Draft_DE_Aquitard_Integrity_Analysis_Report_Oct2010.pdf

DRAFT MANUSCRIPT
X-RAY RADIOGRAPHY AND DIFFUSION ANALYSIS OF D/E CLAY
CONFINING UNIT, DUPONT CHAMBERS WORKS SITE
Andrew J.B. Cohen, PhD
NewJersey Institute ofTechnology
1 Introduction ..............................................................................................................1
2 D/E Clay Characteristics............................................................................................1
2.1 Field Observations.........................................................................................1
2.2 Geotechnical Results......................................................................................2
3 X-Ray Radiography...................................................................................................3
3.1 Method..........................................................................................................3
3.2 Results ..........................................................................................................3
4 Diffusion Analysis ....................................................................................................7
4.1 Basis of Analysis ...........................................................................................7
4.2 Concentration Data ........................................................................................8
4.3 Assessment of Retardation Factors ...............................................................11
4.4 Analytical Modeling of COC Diffusion ........................................................13
4.5 Analytical Modeling of Chloride Diffusion...................................................15
5 Other VOCs and SVOCs in the D/E Clay Porewater.................................................17
6 Conclusions ............................................................................................................18
7 References..............................................................................................................19

AndrewJ.B.Cohen,PhD,X-Ray Radiographyand Diffusion Analysis ofD/EConfining Unit
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1
1 Introduction
Some site-related chemical constituents are present in groundwater in the E Aquifer at the
DuPont Chambers Works site. The E Aquifer is separated from the upper aquifers by a
continuous, 14 to 64 foot thick D/E clay aquitard called the “D/E Confining Unit”. This
confining unit is composed of very stiff, oxidized paleosols that have undergone
extensive compaction and diagenesis. This characteristic is observed at the site and on a
regional scale. The presence of the groundwater constituents in the E Aquifer could be
due, in part, to the migration of DNAPL through the D/E Confining Unit via secondary
pathways such as root burrows or fractures. To test if this mechanism could be a cause of
elevated groundwater concentrations detected in the E Aquifer, D/E clay samples from
multiple depths were collected for detection of DNAPL,geotechnical analysis, x-ray
radiography, and measurement of chemical concentrations. The samples were collected
when well G08-R01D was installed in January 2008. This well is located in the central
portion of the Manufacturing Area.
X-ray images of the clay show signs of historical burrow-like structures filled with
diagenetic cement. No potential secondary flow pathways were evident in deeper
sections of the clay. No DNAPL was present on the top of the clay, but low levels of
chlorobenzene, benzene, and cis-1,2-dichloroethylene were detected in the clay. In the
absence of DNAPL, the presence of these chemicals can be due to diffusion of the
groundwater constituents in the overlying D Aquifer. The concentration profile in the
clay is a function of the time when the plume migrated to the sample location (i.e., “the
plume arrival time”) and properties of the clay. The diffusion analysis described in this
report below considers two different conceptual models of plume arrival. The analytical
models were fit to the data to estimate the time at which a groundwater plume in the D
Aquifer arrived at the point of measurement at the top of the D/E clay.
2 D/E Clay Characteristics
2.1 Field Observations
Based on D/E clay samples observed at multiple locations on-site, the D/E clay can be
characterized as follows:
 Grey or red clay; “very stiff”, “well compacted”, “dry”, “dense”
 Encountered Depth site-wide: 48-130 ft bgs
 Thickness: 14 to 64 ft
 Discontinuous and occasional sand lenses (average thickness ~ 7 ft)
 No visible fractures or rootholes/wormholes
 One sand filled 4” “rootlet” at top of unit (interface) observed at one location in
Carneys Point (location T29)
 Occasional gradation to silts/sand
 When sand lenses are present usually in upper part of clay

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Figure 1. Picture of D/E clay from G08-R01D boring.
2.2 Geotechnical Results
The D/E clay sample from the G08-R01D boring has the following geotechnical
properties:
 Water Content = 25.7%
 Liquid Limit = 50
 Plastic Limit = 25
 Plasticity Index = 25
 USCS symbol CH = 99.1% silt and clay/or sand = 0.9%
 Hydrometer (clay content less 2µm) = 41%
 Specific Gravity = 2.767
 Hydraulic Conductivity
 Average: 9.5 x 10-9 cm/sec
 Range 9.26 x 10-9 to 9.99 x 10-9 cm/sec
 Porosity = 41% (low for clay)
 TOC = 0.281%

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3 X-Ray Radiography
3.1 Method
 Collected 2-ft D/E clay core samples from four different depths:
o 110, 113, 116, 116-118 ft bgs
 Radiography performed by Dr. Harry Roberts of Louisiana State University
o Clay core cut into 10 cm high x 15 cm wide x 1 cm thick slab for analysis
 Complete methodology described in report “Evaluation of Consolidated Clay-
Rich Samples by X-Ray Radiography for DuPont Chambers Works” by Roberts
Geoservices.
3.2 Results
 Images shown are X-ray negatives and clays are usually medium to dark gray.
 Light areas represent zones of high absorption of X-radiation:
o Shells (calcium carbonate)
o Diagenetic inclusions (pyrite, carbonate, iron oxide, siderite)

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Six inches belowtop of clay
 Patterns of silt and some particle to particle diagenetic cement (probably iron
oxide) concentration that contrast distinctly with the clay-rich matrix
 Irregular and vertical distribution pattern of silt is suggestive of burrow structures
 The vertical trend of silt-rich sediment is interpreted as a root burrow which has
introduced slightly coarser sediment from a shallower stratigraphic horizon
 Silt-rich layers (light zones on the radiographs) occur at the bottoms of each core
section
Figure 2. X-ray radiograph negative of clay six inches below top of D/E Confining Unit.

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3.5 feet below top of clay
 Diagenetic products (probably iron oxide or iron carbonate) in upper part of the
sample
 Thin linear features concentrated in upper part of sample are suggestive of
“rootlets” originating from the larger silt-rich burrow structure
 Some of the small “rootlets” branch, increasing the probability of the root burrow
interpretation
 It is unclear as to whether this silt-rich root burrow structure could be a pathway
for the migration of fluids through this part of the sedimentary section
Figure 3. X-ray radiograph negative of clay 3.5 feet below top of D/E Confining Unit.

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6.5-8.5 feet below top of clay
 Homogeneous and featureless
 Small cross-core voids are associated with the core-cutting process
 No potential fluid migration pathways are obvious
 Samples are fine-grained with wispy light cross-core structures that do not appear
capable of being fluid migration pathways
Figure 4. X-ray radiograph negative of clay 6.5 to 8.5 feet below top of D/E Confining
Unit.

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7
4 Diffusion Analysis
Based on results of the X-ray radiography, there are no secondary flow pathways in the
D/E clay 6.5 feet below the top of the D/E Confining Unit. The analysis below provides
an additional line of evidence supporting this finding because the constituent
concentration in the clay exhibits a diffusion profile rather than an advection profile. In
the absence of DNAPL,which would enable direct assessment of the potential DNAPL
migration through the clay, the measured clay concentrations can be used to estimate how
long groundwater constituents have been present in the D Aquifer at the location of G08-
R01D.
4.1 Basis of Analysis
The dimensionless Peclet number, Pe, describes the effect of dispersion relative to
molecular diffusion (Bear,1972):
e
e
D
ud
P = (1)
where u is the porewater velocity defined earlier, d is the mean grain size (d50) of the
porous media, and De is the effective diffusion coefficient of the porous media. For
Pe<0.1, diffusion is the dominant transport mechanism (Bear,1972).
De is a function of the solute aqueous molecular diffusion coefficient (Do) and a
tortuosity factor (τ) that accounts for the irregular flow pathway around soil particles as
compared to flow in a straight line (Bear, 1972):
τ
o
e D
D = (2)
where τ can be approximated as follows (Dullien, 1992):
p
n
≅
τ , (3)
where n is the porosity, and p is a constant that is a function of the soil structure. Based
on a literature review, Parker et al. (2004) found that, for silty or clayey deposits, p
ranges from 0.4 to 2.0, and the average is 1.1. Using the average p and Do=1×10-9 m2/s,
De= 3.9×10-10 m2/s (3.9×10-6 cm2/s). Based on the range of p values, De ranges from
1.8×10-10 to 7.1×10-10 m2/s.
Based on the geotechnical analysis of the clay samples, d50 =0.0003 cm. Based on the
measured hydraulic conductivity (5x10-9 cm/s) and vertical hydraulic gradient, the
estimated maximum vertical porewater velocity through the D/E clay is approximately
1x10-8 cm/s. Using this velocity and the range of Devalues, Pe≈10-6. Therefore,
transport is diffusion dominated.

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8
For the case of migration of solutes into the D/E clay aquitard from a constant porewater
concentration at the clay-aquifer interface (i.e., the groundwater concentration at the base
of the D Aquifer), the porewater concentration at a depth, z, below the sand-clay interface
(z=0) and at time, t, may be approximated by the following analytical solution (Ogata,
1970):
















=
R
t
D
z
erfc
t
C
t
z
C
e
w
w
/
2
)
,
0
(
)
,
( (4)
Cw=porewater concentration
Cw(0,t)=porewater concentration at the base of the D Aquifer (also referred to as the
“clay-aquifer interface” below)
R=solute retardation factor (defined below)
Equation 4 applies to the case where the concentration in the D Aquifer at the sample
location increased relatively abruptly to a near-constant concentration. The increase
could be due to the migration of constituents from the shallower units to the D Aquifer
through discontinuities in the overlying C/D Aquitard and/or downward migration
through leaky well casings at other locations, e.g.
A best-fit of the analytical solution to the porewater concentration yields t, De, or R,
given fixed values for two of these variables.
4.2 Concentration Data
Samples of D/E clay were collected at ten (10) depths below the clay-aquifer interface on
January 24, 2008 (Table 1). These nine samples were analyzed for VOCs, SVOCs,
chloride and percent moisture. In addition, a composite sample was collected 137-198
cm below the interface (midpoint = 167.5 cm) and was analyzed for moisture content and
total organic carbon (TOC). The measured total concentration (sorbed plus aqueous), CT,
(laboratory results reported as mg/Kg), percent moisture, and the normalized
concentration profiles of the detected analytes are shown in Table 1 below. Figure 5
shows the concentration profiles.
FS/DUP Depth below Benzene Chlorobenzene cis-1,2-Dichloroethene 1,2-Dichloroethane Chloride Percent
interface (cm) mg/Kg mg/Kg mg/Kg mg/Kg mg/Kg Moisture (%)
±10.864% ±9.574% ±10.612% ±7.848% ±0.315%
FS 0.5 0.008 0.081 0.013 0.001 J 49.5 17.1
FS 7.6 0.004 J 0.044 0.008 <0.001 64.7 16.2
FS 15.2 0.003 J 0.033 0.005 <0.001 69.2 16.8
FS 22.9 0.003 J 0.031 0.006 <0.001 56.5 J 15.9
FS 30.5 0.001 J 0.016 0.005 J <0.001 54 J 17.2
FS 61.0 <0.0005 0.005 0.001 J <0.001 40.1 J 19.0
DUP 61.0 <0.0006 0.002 J <0.001 <0.001 44.5 J 19.1
FS 76.2 <0.0006 J <0.001 J <0.001 J <0.001 29.2 J 20.3
FS 91.4 <0.0005 0.001 J <0.001 <0.001 25.3 J 19.9
FS 121.9 <0.0005 0.002 J <0.001 <0.001 33.2 J 19.4
FS 167.5 -- -- -- -- -- 25.7

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9
Table 1. Laboratory analyticaldata ofmeasuredtotalconcentrations(on a dry weight basis)andpercent
moisture in clay. FS=field sample; DUP=duplicate sample,<=non-detect at statedreportinglimit;
J=analyte detectedabove MDLbut belowreportinglimit; concentration may not be accurateorprecise.
Laboratory reportedpercentmeasurement uncertaintyshownat topofeach column.
0
20
40
60
80
100
120
140
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
C/Co
Depth
below
clay-aquifer
interface
(cm)
Benzene (mg/Kg)
Chlorobenzene (mg/Kg)
cis-1,2-DCE (mg/Kg)
Chloride (mg/Kg)
Figure 5. Normalized concentration profiles based onmeasuredtotalconcentration.Normalized
concentrationis the measuredconcentrationdivided bythe measured concentrationat the top ofthe D/E
clay. Normalized concentrationsare not shown fornon-detectresults.
The concentrations of the VOCs generally decrease with depth. Due to the respective
detection limits of the constituents, the chlorobenzene profile extends deepest. The
profile is characteristic of diffusion in that the concentration gradient is steepest near the
interface and decreases with depth. The chloride concentration trend is similar to that of
the VOCs over most of the profile except near the top, where the trend is reversed. As
described later in this section, this trend may be due to a decrease in the porewater
chloride concentration at the top of the clay-aquifer interface after an initial increase.
The similar slope of the chloride concentration profile over most of the interval compared
to that of the VOCs suggests that chloride diffusion is similar to that of the VOCs. These
details are discussed in subsequent sections.
Porewater concentrations are not measured directly. Instead, the porewater concentration
is estimated based on the measured total concentration and using an equilibrium
partitioning model. For the case of a water-saturated soil in the absence of any separate

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10
phase liquid, and assuming linear equilibrium partitioning, porewater concentration, Cw,
can be estimated based on the well known relation (e.g., Pankow and Cherry, 1996):
b
oc
oc
T
W
n
K
f
C
C
ρ
+
= (5)
where
CT = total soil concentration (mg/Kg)
Koc = organic carbon partitioning coefficient of a solute (L/Kg)
foc = fraction organic carbon
ρb = soil dry bulk density (Kg/m3 or Kg/1000 L)
n = porosity
The solute retardation factor, R, is defined as follows (Freeze and Cherry, 1979):
n
f
K
R b
oc
oc ρ
+
=1 (6)
Combination of Equation 5 and 6 yields an alternate expression for porewater
concentration (Pankow and Cherry, 1996):
Rn
C
C b
T
W
ρ
= (7)
Bulk density and porosity were not measured, but the measured percent moisture, PM,
(by weight) is used based on the following relationship:
sample
of
mass
total
sample
in
water
of
mass
100
moisture
fraction =
÷
=
= PM
fm
(8)
For a water saturated sample:
)
( w
w
V
V
n
V
n
f
b
m
ρ
ρ
ρ
+
= (9)
where:
ρw = density of water
ρs = density of solids
V = volume of sample
Rearrangement of terms yields the following relationship:
)
1
( m
w
m
b f
f
n
−
=
ρ
ρ
(10)

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11
Where ρw = 1 Kg/L for all practical purposes. Therefore, the retardation factor for each
solute can be expressed as follows:
m
m
oc
oc
f
f
f
K
R
)
1
(
1
−
+
= (11)
This is the equation used herein.
4.3 Assessment of Retardation Factors
The average measured organic carbon from the composite sample collected at 137-198
cm, is deeper than the other samples, is 0.281% (foc=0.00281). This is an average of 4
replicate measurements, which ranged from 0.259% to 0.297%. The average percent
moisture of the clay is 17.3% in the interval wherein diffusion has occurred (0-80 cm;
Figure 3). Using the average foc and this average percent moisture, and a partitioning
coefficient (Koc) of 330 L/Kg and 60 L/Kg for chlorobenzene and benzene, respectively
(reported by others and referenced in Pankow and Cherry, 1996), the corresponding
retardation factors are R=5.4 and R=1.8. Similarly, R=1.6 for cis-1,2-DCE based on
Koc=49 L/Kg (Mackay et al., 1991). Prior to using these retardation factors to calculate
the porewater concentration (Cw) using the measured total concentrations (CT) and for use
in the analytical solution (Eq. 4), the applicability of these R values was evaluated by
comparing the normalized total concentration profile of the different solutes, as follows:
Substitution of Equation 5 into Equation 4 yields the analytical solution expressed in
terms of the total concentration, CT:
















=
R
t
D
z
erfc
C
t
z
C
e
o
T
T
/
2
)
,
( (12)
Where CTo is the measured total concentration in the clay at the top of the clay-aquifer
interface.
The normalized total concentration profile is therefore:
















=
=
R
t
D
z
erfc
C
t
z
C
t
z
C
e
o
T
T
/
2
)
,
(
)
,
( (13)
And the position of the advancing diffusion front for solute B relative to that of solute A
is therefore
B
A
e
A
B
e
A
B
R
t
D
R
t
D
z
z
A
B
= (14)

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12
The diffusion “front” is defined by the depth, z, where C is an arbitrary value less than 1
(e.g., C = 0.5). For example, diffusion of two solutes with the same t and De, but RA=5.4
and RB=1.8, the normalized diffusion front of solute B is 1.7 times deeper than the
diffusion front of solute B ( 8
.
1
/
4
.
5 =1.7).
As shown in Figure 1, the normalized total concentration profiles of benzene and
chlorobenzene are nearly the same. Cis-1,2-dichlorobenzene is also nearly the same in
the top three samples. The cause for the cis-1,2-DCE deviation below 20 cm is unclear.
It could be due to measurement error, as the concentration at 30.5 cm and 61 cm have ‘J’
qualifiers. Regardless, the profiles above 20 cm are essentially the same, and the overall
similarity indicates that the retardation factors are approximately equal. This assumes
that t are the same for each solute (i.e., the VOCs in the D Aquifer at G08-R01D have
been present for the same time), which is reasonable, and that De is the same. The latter
is also reasonable given that the differences in Do of the VOCs is only about 10%.
The possibility that sorption is exaggerated when using the R estimates above is
supported by that fact that the estimated porewater concentration at the interface using
R=1 for all VOCs more closely matches the measured groundwater concentration in the
overlying groundwater (Table 2). For example, the difference between the cis-1,2-DCE
groundwater concentration and the estimated porewater concentration at the interface
assuming R=1 is 4.9 ug/L, whereas the difference is 30.2 ug/L using R=1.8. The
estimated porewater concentration of chlorobenzene at the top of the clay using R=1 is
about 3 times less than the groundwater concentration, which is unexpected. However,
the estimated chlorobenzene porewater concentration using R>1 yields values that are
even more inconsistent with the measured overlying groundwater concentration. Liu and
Ball (2002), in a study of TCE diffusion into an aquitard, noted a similar discrepancy
between the measured groundwater concentrations and estimated pore water
concentrations at the interface, and also attributed the overestimation of sorption as a
possible cause.
VOA
(R=1.6, 1.8, 5.4) (R=1)
cis-DCE 68 37.8 (R=1.6) 63.1 (R=1) 30.2 4.9
Benzene 32 21.4 (R=1.8) 38.8 (R=1) 10.6 6.8
Chlorobenzene 1100 71.5 (R=5.4) 393.2 (R=1) 1028.5 706.8
Measured groundwater
concentration, Cgw (ug/L)
Absolute Difference (ug/L)
Estimated porewater
concentration, Cw (ug/L)
Table 2. Comparison ofmeasuredgroundwaterconcentrationto estimatedpore waterconcentration at
aquifer-clay interfacefordifferent R values.
There is a possibility that R≈1 if there is almost no organic carbon present in the depth
zone wherein concentrations have migrated, which is above the sample analyzed for
TOC. Since the clay is a mottled and overcompacted paleosol that has blotches of very
light grey to white clay, the latter of which is expected to have little to no organic carbon.
It is therefore possible that the measured foc is not applicable to the diffusion analysis, and
the corresponding retardation factor is unity because foc≈0. The fact that the moisture
content in the foc sample interval is considerably higher than in the upper intervals

DRAFT MANUSCRIPT
13
suggests that the composition of this deeper sample is indeed different than the upper
interval (see Table 1).
In addition to the possibility of R≈1, the discrepancy between measured groundwater
concentration and estimated porewater concentrations described above could also be due
to a stratified concentration profile in the aquifer and the location of the well screen, such
that the measured groundwater concentration is not representative of the groundwater
concentration at the clay-aquifer interface. In this case, the bottom of the well screen is
10 feet above the sand-clay interface. Additionally, the groundwater concentration was
measured nearly 5 months after the clay samples were collected.
Finally, there is a possibility that R values of the different VOCs are equal but greater
than one given the range of Koc values reported in the literature. For example,
chlorobenzene Koc as low as 83 L/Kg is reported by Mackay et al. (1991), although it is
not clear what type of soils this applies. Using this Koc would yield R=2.1 instead of 5.4.
Therefore, R for all three VOCs could be approximately the same value (R≈2), and the
resulting normalized concentration profiles would be the same as in Figure 1.
Overall, given the analyses described above, a retardation factor of one for all analytes
appears most appropriate, and R=1 was therefore used to estimate the pore water
concentration (Equation 5) and for curve fitting using Equation 4. Estimates based on
R≈2 are also provided.
4.4 Analytical Modelingof COC Diffusion
Figure 3 shows the porewater concentrations and the ‘best-fit’ simulated porewater
concentration (ug/L) profile using Equation 1 for benzene, chlorobenezene, and
cis-1,2-DCE.. For R=1 and De=3.9x10-10 m2/s, t=2 years, meaning that groundwater with
site-related VOCs migrated to the lower portion of the D Aquifer at G08-R01D in
January 2006, which is two years prior to the sampling date (January 2008). This finding
is unexpected given that VOCs have been detected in D Aquifer wells in the earliest
samples collected in 1989. A possible cause for this unexpected result is described
below.
An alternate conceptual model is that the groundwater concentration in the D Aquifer
increased gradually rather than abruptly, as modeled above. The analytical solution to
1-D diffusion with a linearly increasing concentration at z=0 is the solution to the
analogous heat-diffusion solution (Carslaw and Jeager, 1959):















 −
−
















+
=
R
t
D
x
t
D
x
R
t
D
x
erfc
R
t
D
x
kt
C
e
e
e
e
t
x
/
4
exp
/
4
/
2
1
2
2
,
π
, (15)

DRAFT MANUSCRIPT
14
where k is the rate change of the groundwater concentration at the aquifer-clay interface,
such that kt equals the porewater concentration at the interface when the sample was
collected.
For R=1 and De=3.9×10-10 m2/s, a reasonable, ‘best-fit’ is t=4 years, meaning that the
groundwater concentration was initially zero and increased steadily since January 2004.
-20
0
20
40
60
80
100
120
140
0 10 20 30 40 50
Benzene Conc (ug/L)
Depth
below
interface
(cm)
Measured Concentration < MDL
Measured Concentration > MDL
C(0,t)=kt
C(0,t)=Co; v=0
-20
0
20
40
60
80
100
120
140
0 200 400 600 800 1000 1200
Chlorobenzene Conc (ug/L)
Depth
below
interface
(cm)
C(0,t)=kt
C(0,t)=Co
-20
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80
cis-DCE Conc (ug/L)
Depth
below
interface
(cm)
C(0,t)=kt
C(0,t)=Co
Figure 6. Porewaterconcentrationprofile ofchlorobenzene,benzene,and cis-1,2-DCE,and associated
analyticalsolutions(Equations4and 15). The blue line above thexaxis is the groundwaterconcentration
measured in well G08-R01D in May 2008.

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15
The data can be fit equally well with other combinations of De and t, so long as the
quotient Det/R is constant. Considering the range of possible tortuosity factors (τ) and
the associated De, t ranges from 1.1 to 8.7 years, as shown below.
p τ D e (m2
/s)
Step Change of
Concentration at
z=0
Gradual Change
of Concentration
at z=0
0.4 0.71 7.1E-10 1.1 2.2
1.1 0.39 3.9E-10 2 4
2 0.18 1.8E-10 4.3 8.7
Time (in years) prior to sample
collection in January 2008
Table 3. Estimated time when the groundwater plume in D Aquifer arrived at top of D/E
Confining Unit at location G08-RO1D.
Further, assuming R may range from 1 to 2, t ranges from 1.1 to 17.4 years.
Differences in Do for specific solutes does not affect the interpretations significantly; the
range of Do deviates from the average Do by about 10%. Therefore, each of the estimated
times above is approximately ±10%.
Overall, analysis of the concentration profiles indicates that site-related VOCs in
groundwater at the base of the D Aquifer in the vicinity of G08-R01D began about one to
17 years prior to sampling (i.e., in the year 1991 or later). This estimated date does not
necessarily imply that this was the first time VOCs were present over the full thickness in
the D Aquifer at the location of G08. Historical data from other wells in the D Aquifer
show that VOCs were present when sampling first began in 1989. They could have also
been present at G08, but not uniformly throughout the thickness of the aquifer such that
the VOCs were not present immediately above the D/E clay-aquifer interface until 1991
or later.
4.5 Analytical Modelingof Chloride Diffusion
Figure 3 shows the chloride concentration profile. The profile appears at have at least
two trends. The trend near the top increases with depth to about 10 cm, followed by a
decreasing trend to about 90 cm. The three deepest samples could represent a near
constant concentration interval with some small scale variability, e.g., which appears to
be present given the difference between the field and duplicate sample collected at 60 cm.
The decreasing trend from 10 cm to 80 cm has a slope similar to that of the VOCs, which
suggests that an increase in chloride concentration occurred at the interface in a similar
fashion to that of the VOCs. The increasing trend with depth at the top is indicative of a
subsequent decrease in source concentration after some time. Figure 4 illustrates a
potential time-dependent boundary condition that can explain the observed profile. An
initial, near constant concentration profile of 125 mg/L (i.e., a “background”
concentration), followed by an increase in concentration at the interface at the same time
as that for the volatile VOCs, followed by a subsequent decrease approximately one year

DRAFT MANUSCRIPT
16
later. This decrease is inferred based on the model fit. It was not observed directly.
The corresponding analytical solution is obtained by superposition and is shown in Figure
5. The gradual change source term fits the data better.
Sample collected
Jan 2008
to
Jan 2004
t1
Feb 2007
ts
Jan 2008
125 mg/L
Cw
t
650 mg/L
240 mg/L
Gradual change source term
Step function source term
Sample collected
Jan 2008
to
Jan 2004
t1
Feb 2007
ts
Jan 2008
125 mg/L
Cw
t
650 mg/L
240 mg/L
Gradual change source term
Step function source term
Figure 7. Potentialchloride concentrationhistory in groundwaterat the clay-aquiferinterface forDe =
3.9x10-10
m2
/s.
-20
0
20
40
60
80
100
120
140
0 100000 200000 300000 400000
Chloride Conc (ug/L)
Depth
below
interface
(cm)
Measured
Concentration > MDL
step change
gradual change
gw data

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17
Figure 8. Chloride porewaterconcentrationdataand modeled profile fordifferent boundaryconditions.
The blue line above thexaxis is the groundwaterconcentrationmeasuredin well G08-R01D in May 2008.
The assumption of 650 mg/L as the initial concentration increase is consistent with the
concentration measured in well F07-M01D, which is located 850 feet southwest of
G08-R01D. The historical chloride concentrations there range from about 600 to 700
mg/L, and the first sample was collected in 1990. This is the nearest well to G08 where
chloride concentrations have been measured. The background concentration of 125 mg/L
is based on the concentrations below 60 cm (see Figure 5). . Whether these
concentrations represent an initial concentration prior to manufacturing activities began at
the site over 100 years ago is unknown. Chloride concentrations in the D Aquifer at
locations beneath the manufacturing area range from about 10 to 1,300 mg/L, and the
concentrations on the western and upgradient perimeter where no VOCs are present are
less than 100 mg/L. The lower concentrations near the site boundary may be
representative of pre-manufacturing conditions, and thus might be representative of the
typical historical chloride concentrations that would equilibrate with the clay porewater
and thus define the “background” concentration in the clay. Overall, the actual
background concentration in the clay is uncertain.
It is recognized that, although the fit to the data is good, the solution is not unique. That
is, other boundary conditions may also fit the data. For example, a similar solution to
that shown in Figure 5 is achieved assuming an initial concentration increase to 500 mg/L
that occurred in 2001 and that began to decrease in August 2007. Fitting additional
variations of boundary conditions in order to match the data does not provide additional
insight or reduction in uncertainty. The salient point is that the despite the differences,
the chloride concentration profile is not inconsistent with the inferred COC arrival time
based on the COC diffusion analysis. The chloride profile is representative of diffusion
from a change in concentration that occurred at the same time as the VOCs, with a
relatively simple, recent, and short-term variation in the chloride concentration at the
interface. Chloride concentration fluctuations on the order of 300 mg/L over a period one
year have been observed in other wells in the D Aquifer.
5 Other VOCs and SVOCs in the D/E Clay Porewater
Chlorobenzene, benzene, and cis-1,2-dichloroethene, and 1,2-dichloroethane were
detected in the clay samples and in the overlying groundwater, as described above. Other
VOCs such as aniline, trichloroethene, and 1,4-dichlorobenzene were detected in the
groundwater but not in the clay. However, it is likely that these VOCs are also present in
the clay, but the laboratory detection limit for the clay samples was such that the
corresponding estimated porewater concentrations had detection limits greater than the
groundwater concentrations, as shown below.

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18
Analyte
Measured Groundwater
Concentration (ug/L) in G08-
R01D (5/7/08)
Measured Total
Concentration (mg/Kg) in
Clay at D/E interface
(1/25/08)
Estimated Pore Water
Concentration in Clay at
D/E Interface (ug/L)
1,1-DICHLOROETHENE 1 J <0.001 <5
1,2-DICHLOROBENZENE ^630 <0.04 <194
1,3-DICHLOROBENZENE 66 <0.04 <194
1,4-DICHLOROBENZENE 67 <0.04 <194
ETHYLBENZENE 2 J <0.001 <5
TETRACHLOROETHYLENE 1 J <0.001 <5
TOLUENE 2 J <0.001 <5
TRANS-1,2-DICHLOROETHENE 11 <0.001 <5
TRICHLOROETHENE ^10 <0.001 <5
VINYL CHLORIDE ^18 <0.001 <5
XYLENES 3 J <0.001 <5
1,2,4-TRICHLOROBENZENE ^29 <0.04 <194
1-NAPHTHYLAMINE 11 J <0.2 <971
4-CHLOROANILINE 15 <0.08 <388
ANILINE ^79 <0.2 <971
BIS(2-CHLOROETHYL)ETHER 1 J <0.04 <194
NITROBENZENE 4 J <0.04 <194
N-NITROSODIPHENYLAMINE 3 J <0.04 <194
O-TOLUIDINE 1 J <0.24 <1165
Table 4. Groundwater, clay, and estimated clay porewater concentrations at G08-R01D.
6 Conclusions
Although DNAPL is not present at the base of the D Aquifer at G08-R01D, the analysis
of D/E clay samples suggests that DNAPL,if present, would not migrate through the
confining unit to the underlying E Aquifer. This conclusion is based on X-ray
radiography results which indicate there are no pathways for advective flow and the
observed VOC concentrations in the clay, which have diffusion profiles.
Using analytical solutions of diffusion into the clay from the overlying D Aquifer, the
estimated time at which VOCs were first present in the groundwater at the base of the D
Aquifer at G08-R01D is sometime after 1990 and as recent as 2007. This estimated time
range considers the uncertainty of the effective diffusion coefficient and retardation
factor. VOCs in the D Aquifer could also have been present prior to 1990 if the
groundwater concentrations are variable with depth.

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19
7 References
Bear, J., 1972. Dynamics of Fluids in Porous Media, Dover Publications, Mineola, New
York.
Carslaw, H.S. and J.C. Jeager,1952, Conduction of Heat in Solids, Second Edition,
Oxford University Press,London.
Dullien, F.A.L., 1992. Porous Media: Fluid Transport and Pore Structure, Second
Edition,, Academic Press,San Diego, California.
EPA, 1996. Soil Screening Guidance Technical Support Document, EPA Report
EPA/540/R-96/018.
Freeze, R.A. and J.A. Cherry, 1979. Groundwater, Prentice Hall, Englewood Cliffs, New
Jersey
Liu, C. and W.P. Ball, 2002, Back Diffusion of Chlorinated Solvent Contaminants from a
Natural Aquitard to a Remediated Aquifer Under Well-Controlled Field Condition:
Predictions and Measurements, Ground Water, 40, 175-184.
Mackay, D., Shiu, W.Y. and K.C.,Ma, 1991. Illustrated handbook of physical-chemical
properties and environmental fate for organic chemicals, Lewis Publishers, Inc., Chelsea,
Michigan.
Pankow, J.F. and J.A Cherry, 1996. Dense Chlorinated Solvents and other DNAPLs in
Groundwater, Waterloo Press,Portland, Oregon.
Parker, B. L., J.A. Cherry, S.W. Chapman, 2004, Field study of TCE diffusion profiles
below DNAPL to assess aquitard integrity, Journal of Contaminant Hydrology, 74, 197-
230.
White, R., M. Rivett, and J.H. Tellam, 2008, Paleo-roothole facilitated transport of
aromatic hydrocarbons through a Holocene clay bed. Environmental Science and
Technology, 42 (19). 7118-7124.

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