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NGWA.org Ground Water Monitoring & Remediation 00, no. 0/ xxxx 0000/pages 00–00 1

Ground Water Monitoring & Remediation

© 2012, National Ground Water Association. Published 2012.This article is a U.S. Government work and is in the public domain in the USA.doi: 10.1111/j1745–6592.2012.01394.x

A Tracer Test to Characterize Treatment of TCE in a Permeable Reactive Barrierby Hai Shen, John T. Wilson, and Xiaoxia Lu

IntroductionInvestigations have identified hundreds of sites where

groundwater is contaminated with chlorinated solvents (AFCEE 2004). As one of the largest remediation liabilities in history, remediation of these sites provides an opportu-nity to advance biotic and abiotic remedial technologies. In the past 10 years, passive reactive barriers (PRBs) have been extensively used to treat chlorinated solvent contamina-tion in groundwater (Wilkin and Puls 2004). The traditional PRB commonly uses granular zero-valent iron or iron alloys as filling materials for treatment of chlorinated solvents. In recent years, a surge in the price of iron has made the search for alterative matrix materials for the PRBs attractive. Plant mulch, as a renewable and easily obtainable material, is becoming a favorable alternative for iron. A PRB con-structed with plant mulch is often called a biowall. It essen-tially functions like an in situ bioreactor in controlling and treating groundwater contamination. As a passive treatment

system, a biowall does not require a constant and intensive input of energy for operation, and thus is generally much less expensive than conventional technologies such as pump and treat. Construction of a biowall is also much less expensive than an iron-filled PRB because the mulch for the biowall can often be acquired for the cost of transportation to the site.

The biowall is constructed by excavating a trench across the plume perpendicular to groundwater flow, and then backfilling the trench with a mixture of woody plant tissue and sand to hold the plant tissue in place below the water table. As a result of installation of the biowall, a zone more permeable and uniformly packed than surrounding aquifer materials is created in the subsurface, potentially complicat-ing groundwater flow patterns surrounding the biowall.

In June 2002, a permeable reactive barrier filled with shredded tree mulch, cotton gin compost and sand was con-structed across the flow path of a trichloroethylene (TCE) plume at the OU-1 site in Altus Air Force Base, Oklahoma (AFCEE 2008). The biowall is 139-m long, 7.3-m deep, and 0.5-m wide, and was constructed to intercept the entire groundwater profile in a shallow aquifer (from 1.8 to 7.3 m below ground surface [bgs]).

Following construction of the biowall, 10 monitoring wells were installed along 2 lines perpendicular to the bio-wall to monitor groundwater geochemical conditions and

Abstract A tracer test was conducted to characterize the flow of groundwater across a permeable reactive barrier constructed with

plant mulch (a biowall) at the OU-1 site on Altus Air Force Base, Oklahoma. This biowall is intended to intercept and treat groundwater contaminated by trichloroethylene (TCE) in a shallow aquifer. The biowall is 139-m long, 7.3-m deep, and 0.5-m wide. Bromide was injected from an upgradient well into the groundwater as a conservative tracer, and was subsequently observed breaking through in monitoring wells within and downgradient of the biowall. The bromide breakthrough data demonstrate that groundwater entering the biowall migrated across it, following the slope of the local groundwater surface. The average seepage velocity of groundwater was approximately 0.06 m/d. On the basis of the Darcy velocity of groundwater and geometry of the biowall, the average residence time of groundwater in the biowall was estimated at 10 d. Assuming all TCE removal occurred in the biowall, the reduction in TCE concentrations in groundwater across the biowall corresponds to a first-order attenuation rate constant in the range of 0.38 to 0.15 per d. As an independent estimate of the degradation rate constant, STANMOD software was used to fit curves through data on the breakthrough of bromide and TCE in selected wells downgradient of the injection wells. Best fits to the data required a first-order degradation rate constant for TCE removal in the range of 0.13 to 0.17 per d. The approach used in this study provides an objective evaluation of the remedial performance of the biowall that can provide a basis for design of other biowalls that are intended to remediate TCE-contaminated groundwater.

2 H. Shen et al./ Ground Water Monitoring & Remediation 00, no. 0: 00–00 NGWA.org

and 1.5 m south of MP1 to intercept groundwater within the biowall at the intervals of 6.4-6.7, 4.0-4.3, and 2.1-2.4 m bgs, respectively.

Groundwater was continuously circulated between OU1-01 and UMP1 by pumping groundwater from UMP1 to OU1-01 for a total of 26 d at rate of approximately 0.9 L/min. A solution of sodium bromide was prepared that contained 15 g/L of bromide in distilled water. The bromide solution was pumped into the center of the screen of the injection well OU1-1 (3.7 m bgs) at a pumping rate of 0.01 L/min. During the injection period, groundwater in the injection well was constantly circulated to mix the bro-mide in the well by pumping groundwater from the bottom (7.3 m bgs) to the surface (1.8 m bgs) at a rate of 0.9 L/min. The mean and sample standard deviation of concentrations of bromide in OU1-1 over the 26 d of bromide injection was 177 and 65.8 mg/L (n = 47).

Two months after the beginning of bromide injection, core samples were collected between OU1-01 and UMP1 using the Geoprobe Macrocore® system. (Geoprobe System, Salina, Kansas) Core samples extended from the water table (1.5 m bgs) until auger refusal. One set of core samples were collected approximately 0.5 m south of OU1-1 and the other approximately 0.5 m north of UMP1. These locations are in the flow path between OU1-01 and UMP1. The core sam-ples were divided into separate portions with each at 30-cm long and each portion was then individually extracted with 200 mL of distilled water and analyzed for bromide. The measured bromide was corrected using soil moisture data and reported as the concentration in the pore water of the soil.

Groundwater Sampling and AnalysisFollowing the injection of bromide in OU1-01, ground-

water samples were collected from all wells in Figure 1

contaminant concentrations upgradient, within and immedi-ately downgradient of the biowall. Laboratory and field stud-ies show that the plant mulch is a long-term carbon source to sustain both biotic and abiotic transformation of TCE in the biowall (Kennedy and Everett 2004; Shen and Wilson 2007; Shen et al. 2010). The performance of the biowall in attenu-ating TCE was presented in another paper (Lu et al. 2008), and the monitoring data demonstrate that TCE concentra-tions were greatly reduced within the biowall, but increased again in the wells immediately downgradient of the biowall. This rebound in TCE concentrations downgradient of the biowall is not well understood, making it difficult to evaluate the remedial performance of the biowall objectively.

Instead of comparing concentrations of contaminant in wells upgradient, within the biowall, and downgradient of the biowall, we propose that a better description of the performance of the biowall is a first-order rate constant for transformation of TCE in the biowall. This description of treatment effectiveness can be applied to other geohydrolog-ical circumstances. To define the flow paths across the biow-all and facilitate a comprehensive evaluation of its remedial performance, a tracer test was conducted. The movement of the tracer was used directly to determine the seepage veloc-ity of the groundwater. The seepage velocity and effective porosity were used to estimate the Darcy velocity of ground-water entering the biowall. The Darcy velocity, the width of the biowall in the direction of groundwater flow, and the water-filled porosity of the biowall were used to estimate the residence time of groundwater in the biowall. Finally, the residence time and reduction in concentrations between wells upgradient and downgradient were used to estimate a first-order rate constant for TCE removal in the biowall.

The rate constants calculated from the reduction in concentrations between wells, and a mass balance of water entering and leaving the biowall was confirmed by fitting general equations that describe transport and degradation to the concentrations of bromide and TCE that broke through over time at individual monitoring wells downgradient of the injection well. There was useful agreement between the rate constants that were necessary to model the behavior of bromide and TCE in the tracer test and the rate constants that were extracted from concentration data, an estimate of the Darcy flow in the aquifer, and a simple mass balance of water through the biowall.

Experiment and Methods

Tracer Test and Monitoring NetworkThe bromide tracer test was started in April 2005.

Monitoring well OU1-01, located 7.6 m upgradient of the biowall, was used as to inject a solution of bromide (Figure 1). All wells in the figure are fully screened to intercept groundwater from 1.5 to 7.3 m bgs. Well OU1-01 has a diameter of 15 cm, and wells MP1 and MP4 have a diameter of 5 cm. All other wells in Figure 1 had a diam-eter of 2.5 cm. These wells are located upgradient, internal to and downgradient of the biowall, as shown in Figure 1. In addition, a nest of monitoring wells 111, 112, and 113 (not shown in Figure 1) were installed at locations 0.9, 1.2,

Figure 1. Plan view of the biowall and monitoring network. OU1-01 is a 15-cm diameter well. MP1 and MP4 are 5-cm diameter wells. All others are 2.5-cm diameter wells. All wells are screened from 1.5 to 7.3 m bgs, across the entire aquifer. The two dashed lines indicate the relative position of the biow-all, which is not drawn to scale.

U105

U106

UMP1

OU1-01

U107

U108

104

105

106

MP1

107

108

D104

D105

D106

MP4

D107

D108

-50

-40

-30

-20

-10

0

10

20

0 10 20 30

East, m

Nor

th, m

NGWA.org H. Shen et al./ Ground Water Monitoring & Remediation 00, no. 0: 00–00 3

California), a computer code accessible to the public (Simunek et al. 1999). To estimate the parameter values in Equation 2, the inverse approach in the STANMOD software was adopted by fitting Equation 2 to the bromide or TCE con-centrations measured in the monitoring wells. The nonlinear equations were solved by minimizing the sum of the squares of the residues (RSS) between observed and calculated concen-trations based on the Marquardt algorithm (Toride et al. 1995).

To solve Equation 2 using the STANMOD software, however, it is necessary to know the source information such as the bromide concentration and injection time (C

0 and T

in Equation 2). Although the monitoring data observed in injection well (OU1-01) and extraction well (UMP1) pro-vide reasonable bounds for estimating the source informa-tion via trial and error, another model was used to compare the parameter values estimated using Equation 2. Therefore, a different analytical solution to Equation 1 (Kinzelbach 1986) was used to analyze the bromide breakthrough curves obtained from selected monitoring wells:

)exp(

/4)/(

exp/2

2

ktRDt

Rvtx

RDtR

MC

a

−⎥⎦

⎤⎢⎣

⎡ −−=πη

(3)

where M (ML−2) is the solute mass assumed to be instan-taneously injected into the aquifer and η

a (=0.33) is the

porosity of the aquifer. All other parameters are the same as defined in Equation 1. To determine the parameter values in Equation 3, the Marquardt algorithm of nonlinear regres-sion analysis was used. M was considered as a variable and solved along with other parameters (D and v), by minimiz-ing RSS between the calculated concentrations and the observed data using SigmaPlot software (Systat Software, Inc., San Jose, California).

Knowing solute source parameters is a prerequisite for solving Equation 2. However, aquifer heterogeneity would make it impossible to know the source conditions applicable to breakthrough curves 5 m or longer from the source even in a sand aquifer (Devlin and Barker 1996). All the breakthrough wells listed in Table 1 have a distance 5 m or longer from the injection well, which may pose uncertainties in estimating the source data required for solving Equation 2. In addition, the circulation of groundwater through pumping from UMP1 and injecting to OU1-01 may generate a dipole plume of bromide by spreading the bromide east and west as well as between the two wells, and thus further complicate the application of source conditions to Equation 2. On the other hand, knowing source parameters are unnecessary to solve Equation 3 because the bromide mass can be estimated and solved directly by curve fittings of Equation 3 to breakthrough curves. Therefore, Equation 3 is applied to simulate breakthrough curves to estab-lish a basis for comparison of the results of Equation 2. Note that finding a solution for the solute mass (M) in Equation 3 cannot be guaranteed through the nonlinear curve fitting algor-ism, limiting its further use in modeling TCE fate and transport.

Results and Discussion

Bromide Transport and Groundwater Flow PatternsGroundwater appears to migrate predominantly in

a layer from 1.8 to 3.9 m bgs, as shown by the bromide

for chemical analysis monthly for the first year and then quarterly thereafter. Bromide was measured using a Lachat flow injection system with a method detection limit of 0.250 mg/L. TCE, cis-1,2-dichloroethylene (cis-DCE), trans-1,2-dichloroethylene (trans-DCE), and vinyl chloride were analyzed using gas chromatography/mass spectrom-etry (GC/MS) according to the procedures established in EPA Method 8260B. Samples were analyzed using a Varian Saturn (II) GC/MS system (Agilent Technologies, Santa Clara, California) equipped with a Tekmar 7000 headspace autosampler. Volatile organic compounds were separated on a J&W DB624 capillary column (30 m, 0.25 mm ID), and identified and quantified by the Ion Trap Detector (Agilent Technologies, Santa Clara, California). The method detec-tion limit for each compound was 0.3 µg/L.

Estimate of Water-Filled PorosityThe water-filled porosity of the aquifer material (η

a) was

estimated at 0.33 from the weight loss on drying of the core samples. The density of the aquifer solid is 2.64 g/cm3. The estimate of the water-filled porosity of the biowall material (η

w = 0.42) is described in supporting information for Shen

and Wilson (2007).

Breakthrough Curve AnalysisA one-dimensional convection-dispersion fate and trans-

port equation that includes terms accounting for first-order attenuation rate and linear equilibrium adsorption was used to analyze the breakthrough curves:

kC

x

Cv

x

CD

t

CR −

∂∂−

∂∂=

∂∂

2

2

(1)

where C is the solute concentration (ML−3) at time t (T) and the distance from the injection well of x (L); R is the retarda-tion factor (dimensionless); D is the dispersion coefficient (L2T−1); v is the average seepage velocity (LT−1); and k is the first-order degradation rate constant (T−1).

Assuming a tracer is injected for a period of T at a con-centration of C

0 under the boundary and initial conditions of

C(x, t = 0) = 0, ∂C ___ ∂x

(x = ∞, t) = 0, C(x = 0, t > T) = 0, C(x = 0,

0 < t ≤ T ) = C0, Van Genuchten and Alves (1982) obtained

the following analytical solution to Equation 1:

0

0

( , ) ( , ) for 0 ;2

( , ) [ ( , ) ( , )] for 2

CC x t A x t t T

CC x t A x t A x t T t T

= < ≤

= − − > (2)

where

⎟⎠⎞⎜

⎝⎛ +

⎟⎠⎞⎜

⎝⎛+⎟

⎠⎞⎜

⎝⎛ −=

Dt

Rvtxerfc

DR

vx

Dt

RvtxerfctxA

2

/exp

2

/),(

⎟⎠⎞⎜

⎝⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛

−−−=− exp

)(2

/)(),(

⎟⎟⎠

⎞⎜⎜⎝

⎛

−−+

)(2

/)(

TtD

RTtvxerfc

DR

vx

TtD

RTtvxerfcTtxA

Equation 2 has been programmed into the STANMOD software (U.S. Department of Agriculture, Riverside,


concentration profiles measured from the soil core samples (Figure 2). The concentration of bromide in the injected water averaged 177 mg/L. The bromide concentrations in pore water in the cores collected near injection well OU1-01 in the depth between 2.4 and 3.0 m bgs were in the range between 100 and 115 mg/L, which is in fairly good agree-ment with the concentration of bromide injected into the aquifer.

However, the concentrations of bromide in pore water dropped remarkably in the intervals deeper than 3.9 m or above 1.8 m bgs, indicating the presence of a preferential flow pathway in the aquifer at a depth near 1.8 to 3.9 m bgs (Figure 2A). The bromide concentrations measured in the core collected near extraction well UMP1 (Figure 2B) con-firm that bromide moved predominantly in the layer from 1.8 to 3.9 m bgs. At the time the cores were collected, con-centrations of bromide in the pore water near well UMP1 had reached approximately half the concentration injected in well OU1-01.

The 26 d of injection had created a mass of bromide-enriched groundwater that extended easterly and westerly as well as between wells OU1-01 and UPM1 predominantly in the layer of 1.8 to 3.9 m. A dipole plume of bromide was likely to be formed by spreading the introduced bromide mass east and west as well as northeast. The bromide break-through curves observed promptly in wells nearby OU1-01, including U107 (north), UMP1 (south), and 107 (northeast) (Table 1), in part defined the initial distribution of a dipole bromide plume. The transport of this dipole plume to down-gradient wells was followed as it moved with the natural flow of groundwater.

The natural flow of groundwater was from the west of the biowall, through the biowall toward the east (Figure 3). Bromide breakthrough curves observed in the monitoring network over a period of 3 years, along with the model simulations, are shown in Figure 4. Table 1 lists the hydrau-lic parameter values obtained by fitting Equations 2 and 3 to the bromide breakthrough curves. These parameter val-ues obtained from the bromide breakthrough curves were

Table 1 Hydraulic Parameters Estimated by Fitting Bromide Breakthrough Curves

Well NameDistance from

Injection Well, mDay When the Highest Conc.

of Bromide Was Observed

Hydraulic Parameter

Velocity, v (m/d) Dispersion Coefficient, D (m2/d)

By Equation 2 By Equation 3 By Equation 2 By Equation 3

OU1-01 0.0 Day 4 at 354 mg/L None None None None

UMP1 7.3 Day 33 at 38 mg/L 0.19 0.20 0.16 0.13

U107 5.0 Day 33 at 42 mg/L 0.17 0.17 0.055 0.077

107 9.6 Day 38 at 7.1 mg/L 0.17 0.18 0.088 0.096

MP1 11.1 Day 149 at 12 mg/L 0.068 0.072 0.070 0.070

106 21.3 Day 346 at 5.7 mg/L 0.058 0.060 0.033 0.035

D107 18.5 Day 149 at 2.6 mg/L 0.074 0.092 0.18 0.47

MP4 18.8 Day 260 at 3.2 mg/L 0.034 0.037 0.18 0.29

Average1 0.059 0.064 0.12 0.22

1UMP1, U107, and 107 were not included for calculations of the average values.

Figure 2. Vertical profiles of bromide concentrations in soil porewater. The cores were collected at locations 0.5 m south of OU1-1 (A) and 0.5 m north of UMP1 (B).

A - Soil core samples near OU1-01

1.5-1.8

1.8-2.1

2.1-2.4

2.4-2.7

2.7-3.0

3.0-3.3

3.3-3.6

3.6-3.9

3.9-4.2

4.2-4.5

Dep

th b

elow

gro

und

surf

ace,

met

er

Bromide in pore water, mg/L

B - Soil core samples near UMP1

0 20 40 60 80 100 120

0 20 40 60 80 100 120

1.8-2.1

2.1-2.4

2.4-2.7

2.7-3.0

3.0-3.3

3.3-3.6

3.6-3.9

3.9-4.2

4.2-4.5

4.5-4.8

Dep

th b

elow

gro

und

surf

ace,

met

er

Bromide in pore water, mg/L


biowall, and the measured porosity of the aquifer solids and the biowall matrix. Groundwater moved so slowly across the biowall that laminar flow took place. The groundwater residence time (τ) in the biowall was therefore estimated to be 10 d based on a plug flow configuration:

w w

a

0.46 0.4210

0.06 0.33

V LhW

Q Lhv

η ×τ = = = =η ×

(4)

where Q (m3/d) is the Darcy flow of groundwater into the biowall across an area with the length of L (m) and height of h (=2.1 m) which is best defined by the 2.1-m layer of the preferential flow pathway; V

w (m3) is the void volume

in the biowall to be filled by groundwater; W is the width of the biowall (=0.46 m); v is the groundwater average seepage velocity (=0.06 m/d); and η

w (=0.42) and η

a (=0.33) are the

porosity measured for the biowall and the aquifer materials, respectively.

Groundwater would be subject to some mechanical mix-ing upon entering the biowall because of the velocity changes created by the porosity difference. The mixing might result in a residence time in the biowall that is longer than the estimated 10 d. In principle, an estimate dependent on the bromide breakthrough curves could yield a more accurate residence time by accounting for the potential dispersion effect. In practice, however, the bromide breakthrough times are relatively insensitive to the biowall residence time under the site condition. This is because the bromide residence time (near 10 d) in the biowall only represents a tiny fraction (<7%) of the breakthrough time observed in the wells (149 to 260 d as shown in Table 1), and thus is extremely sensi-tive even to meaningless variations in hydraulic parameter values such as the groundwater seepage velocity.

To establish a constraint for the residence time, a fur-ther calculation was conducted to simulate one extreme flow scenario by assuming that groundwater from the more conductive depth interval, upon entering the biowall, spread to the entire saturation zone of the biowall (H = 7.3-1.8 = 5.5 m):

u w w

a

5.5 0.46 0.42 26.2.1 0.06 0.33

V LHWQ Lhv

η × ×τ = = = =η × ×

(5)

Although the groundwater under the condition of lami-nar flow could never get completely mixed in the biowall, the calculated value of 26 d set an upper bound for the groundwater residence time (τu) in the biowall.

The value of (τ) above was estimated with values for (v) as derived from a labor intensive and time-consuming tracer test, and values of (η

a)

as estimated by the weight lost on

drying core samples. These data will not be typically avail-able at other sites. The value of (v × η

a) in Equations 4 and 5

can also be estimated from Darcy’s Law as the product of the hydraulic conductivity and the hydraulic gradient. AFCEE (2008, F.2-4) provides a value for the hydraulic conductivity of the site at 2.7 m/d as determined by slug testing. A value for the hydraulic gradient was estimated at 0.006 based on the elevation data presented in Figure 3. Substituting 0.016 for (v × η

a) in Equations 4 and 5 yield estimates for (τ)

and (τu) of 12 and 31 d respectively, which are very close to the values estimated based on the tracer test result. This

considered representative of the groundwater flow condi-tion. The parameter values obtained from U107, UMP1, and 107 were excluded from the average value calculation to minimize potential influences of the pumping that had been conducted during the 26 d of bromide injection.

The velocity distribution patterns indicate that ground-water moved southwestward toward the biowall. Upon entering the biowall, the groundwater migrated across it, as evidenced by the observation that bromide moved to the downgradient wells D107 and MP4 at a velocity similar to that at MP1 and 106, two wells within the biowall (Figure 4; Table 1). The measured water table elevations (Figure 3) demonstrate that the hydraulic gradient was the force driv-ing groundwater movement through the biowall. The natural groundwater seepage velocity averaged 0.06 m/d.

The model simulations in Equation 2 are reproducible by Equation 3, and the two different equations generated consistent values of the groundwater seepage velocity with a relative error of 8% (Table 1). The relative error is 61% for the dispersion coefficient values produced by the two equations. The reasonable agreements of the model simula-tions with the bromide concentrations measured in all wells where the bromide breakthrough occurred (Figure 4) vali-date the applicability of Equation 2 in evaluating the fate and transport of solutes in groundwater at this site.

Groundwater Residence Time in the BiowallThe residence time of groundwater (τ) in the biowall was

estimated based on the seepage velocity, the geometry of the

Figure 3. Typical groundwater elevation observed in the moni-toring network during the tracer test period. The figure was generated using a single data set measured on February 3, 2006. The arrows indicate the groundwater flow directions.


Figure 4. Bromide breakthrough curves and model simulations using Equation 2 for the wells of (A) UMP1 (R2 = 0.890, RSS = 13.9), (B) U107 (R2 = 0.581, RSS = 38.7), (C) 107 (R2 = 0.709, RSS = 1.68), (D) MP1 (R2 = 0.867, RSS = 1.65), (E) 106 (R2 = 0.974, RSS = 0.10), (F) D107 (R2 = 0.650, RSS = 0.10), and (G) MP4 (R2 = 0.661, RSS=0.29). Model simulations were also conducted using Equation 3 for the wells of (A) UMP1 (R2 = 0.908, RSS = 3.42), (B) U107 (R2 = 0.566, RSS = 6.32), (C) 107 (R2 = 0.716, RSS = 1.31), (D) MP1 (R2 = 0.836, RSS = 5.12), (E) 106 (R2 = 0.954, RSS = 0.453), (F) D107 (R2 = 0.237, RSS = 1.59), and (G) MP4 (R2 = 0.621, RSS = 2.32).

0

5

10

15

20

25

30

35

40

Bro

mid

e C

once

ntra

tions

, mg/

L

Field Observations in UMP1

Simulations by Equation 2


0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 3500 50 100 150 200 250 300 350

Bro

mid

e C

once

ntra

tions

, mg/

L

Days since Injection

Field Observations in U107Simulations by Equation 2Simulations by Equation 3

0

2

4

6

8

Days since InjectionDays since Injection

Bro

mid

e C

once

ntra

tions

, mg/

L Field Observations in 107



0

2

4

6

8

10

12

0 100 200 300 400 500 600 0 100 200 300 400 500 600 700


Bro

mid

e C

once

ntra

tions

, mg/

L Field Observations in MP1Simulations by Equation 2Simulations by Equation 3

0

1

2

3

4

5

6

Bro

mid

e C

once

ntra

tions

, mg/

L


Field Observations in 106



1

2

3

Bro

mid

e C

once

ntra

tions

, mg/

L


Field Observations in D107Simulations by Equation 2Simulations by Equation 3

0

1

2

3

4

0 200 400 600 800 1000 1200 1400

0 200 400 600 800 1000 1200 1400

Bro

mid

e C

once

ntra

tions

, mg/

L


Field Observations in MP4Simulations by Equation 2Simulations by Equation 3

A B C

D E

G

F

comparison validates the use of widely available hydraulic parameters to estimate groundwater residence time in the biowall.

TCE Concentration Trend with Depth in the BiowallMonitoring data collected during the test period show

TCE concentrations in the wells downgradient of the bio-wall averaged 70 ± 20 µg/L, considerably higher than the wells within the biowall which averaged 21 ± 3 µg/L (indi-vidual well data not shown). This apparent anomaly may be related to the manner that monitoring wells sample ground-water from the system. Presumably, most of the ground-water sampled by monitoring wells in the aquifer will be contributed by the high flow depth interval. In contrast, the medium in the biowall is uniform across its depth, and water should be contributed equally from all depth intervals. The bulk of groundwater enters the biowall at a depth interval that corresponds to the more conductive interval (1.8 to

3.9 m bgs). Water that enters at this depth interval will tend to move directly across the biowall and have the minimum residence time in contact with the biowall matrix material (near10 d). Water that enters from the conductive interval and takes a longer flow path in the biowall, either above or below the most conductive interval, will have a longer residence time (between 10 and 26 d). Longer residence times result in lower concentrations of TCE. Because the monitoring wells in the biowall sampled water uniformly with depth, relatively more water should be contributed to the well from groundwater that is above or below the con-ductive interval, and as a result, has lower concentrations of TCE and degradation products.

The supposition that there might be a vertical profile in concentrations of TCE within the biowall was confirmed by field monitoring data. Figure 5 shows that the concentration of TCE was below the method detection limit (<0.3 µg/L) in groundwater at the bottom of the biowall (6.4 to 6.7 m bgs),


evaluation, while wells within the biowall may only be used to understand remedial mechanisms and to gain insight into performance issues.

TCE Degradation Rate in the BiowallTo estimate the rate constant of TCE degradation in the

biowall, the STANMOD software was run using the average value for groundwater velocity (0.06 m/d) and the average value for the dispersion coefficient (0.12 m2/d) as listed in Table 1. The TCE retardation factor was estimated previously as R = 3.8 from a column test that simulates the biowall operation (Shen and Wilson 2007). During the tracer injection period, the concentration of TCE in the upgradient wells adjacent to OU1-1 was 938 ± 334 µg/L. A constant TCE concentration of 938 µg/L was used as the initial and boundary conditions for simulating TCE degradation in the biowall. The rate con-stant for TCE removal was estimated by fitting Equation 2 to TCE concentrations measured in all the wells downgradient of the biowall, including MP4, D104, D105, D106, D107, and D108. The average concentration over the monitoring period ranged from 11 to 133 µg/L in the different wells. The rate constant yielded from the curve fitting is 0.14/d (R2 = 0.803, n = 63), which was in the same magnitude as the rate constants estimated by Shen and Wilson (2007) from their column study that simulated the construction and operation of the biowall at OU-1. The rate constant for the removal of TCE in the column study varied from 0.2 to 0.3 per day.

For comparison and confirmation purposes, the TCE degradation rate constant was estimated using the approach similar to that developed by Wiedemeier et al. (1999). The concentrations of TCE were normalized to the bromide concentrations. The first-order degradation rate constant (k) was calculated using Equation 6:

0,Br Br( / )Ln C C

k =τ (6)

where C0,Br

is the TCE concentration normalized by the bro-mide concentration observed at the time when the highest bromide concentration was measured in OU1-1; C

Br is the

TCE concentrations normalized by the bromide concentra-tions observed when the peak bromide concentrations were measured in the wells both within and downgradient of the biowall; and τ (=10 d) is the groundwater residence time within the biowall.

Table 2 lists the rate constant values calculated using the data obtained from each of the wells where bromide break-through has been observed. It should be pointed out that the data obtained from the wells within the biowall may also be used to calculate the TCE degradation rate constant using Equation 6 because the normalizing process using bromide automatically offsets the influence of dilution by the stag-nant or slowly-moving water at the bottom of the biowall. The rate constant estimated by Equation 6 was in the range from 0.13 to 0.17 per day with an average value of 0.14 per day, which is identical to the rate constant value estimated using Equation 2 and the TCE data observed in the down-gradient wells only.

The rate extracted for well MP4 was not included for the calculation of the average rate constant (Table 2). The

while TCE was detected at 10.5 µg/L in the interval of 4.0 to 4.3 m bgs, then the concentration of TCE declined to 0.7 µg/L at 2.1 to 2.4 m bgs. Similar concentration profiles were also observed for TCE degradation products, including DCE and VC (Figure 5). Water samples collected from the wells within the biowall are likely to exhibit a mixture of groundwater drawn evenly from all depths (1.5 to 7.3 m), including the stagnant or slowly-moving water at the top and bottom of the biowall.

Although all the wells in the aquifer intercepted ground-water at the same interval from 1.5 to 7.3 m, the wells downgradient of the biowall likely collected groundwater predominantly from the preferential flow pathway through which the majority of the groundwater exited the biowall. When this is the case, the concentrations of TCE in the wells downgradient of the biowall should be higher than concentrations in wells in the biowall itself.

A simple comparison of concentrations in wells upgra-dient of the biowall to wells in the biowall will overesti-mate the extent of degradation provided by the biowall. We believe that many permeable reactive barriers are subject to the same bias in monitoring. Typically, the design of biow-alls is based on a site-specific first-order degradation rate to calculate the biowall thickness without considerations for the potential impact of preferential flow pathways (Ahmad et al. 2007). However, the presence of a preferential flow pathway could dramatically shorten the residence time of groundwater in the biowall, and create a vertical contami-nant profile with concentration differences as high as one hundred fold. As shown in Figure 5, the concentrations observed in the preferential flow pathway for TCE, DCE, and VC are 22 to 556 times higher than the concentrations in the deeper zone of the biowall. The much higher concen-trations observed in the preferential flow path could com-promise the original design expectation. To increase the opportunity to meet performance requirements for a biowall or permeable reactive barrier in general, the potential impact of preferential flow paths needs to be incorporated into the system design. Generally speaking, monitoring wells down-gradient of a biowall are appropriate for the performance

Figure 5. TCE concentrations measured in a nest of wells (111, 112, and 113), which intercept groundwater in the biowall at the depths of 6.4–6.7, 4.0–4.3, and 2.1–2.4 m bgs, respectively.

0.0

2.5

5.0

7.5

0.1 1 10 100 1000

Concentration, μg/L

Dep

th, m

eter

bel

ow g

roun

d su

rfac

eTCECis-DCEVC


nonsense value that was extracted using Equation 6 is prob-ably due to the failure to catch the groundwater sample that contained the highest bromide concentration in this well, and include that true peak concentration in the calculation using Equation 6.

Despite the consistency of the rate constant values obtained using two different methods and the data sets observed in different wells, there are still several other fac-tors that might cause minor variations of the estimates of the rate constants for TCE removal. The dispersion coefficient obtained from the bromide tracer test might not fully rep-resent the behavior of TCE in the same water. It is known that dispersion is caused by tortuosity of flow paths through a porous medium and spatial disparity of solute concentra-tions. Undoubtedly the bromide tracer test well simulated the dispersive process resulting from tortuosity of the flow paths from the injection well to the biowall.

In terms of dispersion driven by concentration gradients, however, bromide and TCE may behave differently along the flow paths. The TCE in the tracer test was TCE already present in the aquifer. The TCE plume is decades old, and variations in concentrations along flow paths upgradient of the biowall are minimal. In contrast, the injected con-centrations of bromide were at least 100-fold higher than the ambient concentrations. For bromide, there was a huge gradient force driving rapid dispersion of bromide in the zone near the injection well. Then over time the concentra-tion gradient force became weaker and weaker with bro-mide migrating away from the injection well due to the continuous dilution effect by the surrounding groundwater. In contrast to the behavior of bromide, TCE acted with little dispersion driven by concentration gradients in the vicinity of the injection well because of the limited difference of TCE concentrations in this zone (938 ± 334 µg/L).

However, in the boundary between the aquifer and the biowall, a significant gradient of TCE concentrations was created because of the rapid degradation of TCE in the

biowall, therefore potentially increasing TCE dispersion in this zone. In order to determine if there is a value of the dis-persion coefficient that better describes TCE behavior along the different flow paths to the different wells, a curve fitting process was undertaken that treated the dispersion coeffi-cient as an unknown parameter in Equation 2. However, the modeling showed that the dispersion coefficient was highly correlated with the TCE degradation rate constant. Thus, no unique solution could be extracted by nonlinear regres-sion analysis. Regardless, the dispersion process attributable to spatial disparity of solute concentrations between TCE and bromide appear to have no effect on the estimate of the TCE degradation rate constant. This has been shown by the quantitative assessment results summarized in Table 2 based on the bromide-normalized TCE concentrations and their consistency with the rate constant value estimated using Equation 2.

Furthermore, a trend of increasing TCE concentrations (from 11 to 133 µg/L) in the wells downgradient of the bio-wall may be another factor that could affect value of the rate constant. The concentrations of TCE in most downgradient wells increased gradually during the period from Day 94 to Day 260 (July 2005 to February 2006) when the Altus area underwent a serious drought. The decreased infiltra-tion during the drought resulted in a continuous lowering of the groundwater table in all of the monitoring wells dur-ing the period (data not shown). Because the concentrations in the downgradient wells were higher than they might otherwise have been, the rate constants extracted from the tracer test were lower than the otherwise might have been under conditions of average of precipitation.

This study suggests an improved approach to describe the removal of contaminants in field-scale permeable reac-tive systems such as biowalls. Instead of comparing the con-centrations in monitoring wells upgradient of the biowall to concentrations inside the biowall, a better approach would be to estimate a rate constant for the removal of TCE in the biowall. The width of the biowall, the volumetric water content of the biowall, and the Darcy flow of groundwater into the biowall can be used to estimate the mean residence time (τ) of water in the biowall. Then the first-order rate constant for attenuation of TCE (K) can be estimated fol-lowing Equation 7 from (τ), from the concentration of TCE in an upgradient well and the concentration of TCE in a downgradient well along the flow path.

upgradient downgradient( / )Ln C CK =

τ (7)

Table 3 compares rate constants extracted using Equation 7 from monitoring data collected just before tracer test was initiated. Depending on the estimate of mean resi-dence time, the attenuation rate constants over various flow paths across the biowall vary from 0.07 to 0.61 per day. The means are 0.38 per day at 10-d residence time and 0.15 per day at 26-d residence time.

The values of the hydraulic and kinetic parameters cal-culated from the bromide breakthrough curves and TCE concentrations reasonably described the flow characteris-tics and the degradation capability in the biowall. Tracer

Table 2 TCE Attenuation Rate Constant Estimated from

TCE Concentrations Normalized by Bromide Concentrations

Well

TCE Concentration Normalized by

Maximum Bromide Concentration (µg/L

TCE/mg/L Br)Rate Constant

(per day)

Average Rate

Constant1 (per day)

OU1-1 3.19 Not calculated, bromide injec-

tion well

0.14 ± 0.02107 0.58 0.17

MP1 0.87 0.13

106 0.86 0.13

D107 0.88 0.13

MP4 37.7 −0.251The value obtained from MP4 was not included for calculation of the average rate constant.


experiments are labor intensive and time-consuming. The data used in Equation 7 are more generally available. The first-order rate constants that were extracted from the tracer test were similar to rate constants extracted from Equation 7. The first-order rate constants from the laboratory column study were also similar to rate constants extracted using Equation 7. The tracer study and the laboratory column study suggest the approach using Equation 7 to extract rate constants from field data.

Compared to evaluating a simple reduction in concentra-tions between monitoring wells, a field scale rate constant for removal is a more general description of the performance of the reactive barrier. Local variations in the hydraulic con-ductivity of the aquifer material that provides groundwater to a biowall will produce variations in the Darcy flow of water to adjacent portions of the biowall, resulting in varia-tions in residence time in the wall matrix and variations in the extent of treatment as groundwater crosses the biowall. If the heterogeneity in hydraulic conductivity is known, this information can be used in the design of the biowall. The biowall can be sized or scaled to provide adequate treatment of groundwater emanating from the most conductive regions of the aquifer. Local values for hydraulic conductivity, along with the geometry and water-filled porosity of the biowall matrix, can be used to estimate a local value for the resi-dence time (τ). If the width of the biowall is fixed, this local value can be used in Equation 7 to determine a value for the first-order rate constant (k) that must be attained to achieve the treatment goals at the site. If the biowall matrix (and thus the rate constant) is fixed, Equation 7 can be used to determine the width of the biowall, or the number of succes-sive biowalls that will be needed to meet the treatment goals.

Variations in hydraulic conductivity can be mapped vertically and horizontally using simple hydraulic tests that can be performed using push tools (Butler et al. 2002). If fully screened wells are available, a borehole flowmeter test can be used to identify vertical variations in contribution of water to a well, which can be normalized using the transmis-sivity of the well to estimate vertical variations in hydraulic conductivity (Molz et al. 1994; Young et al. 1998). Both of these approaches are illustrated and compared in Wilson et al. (2005).

If concentrations of contaminants in wells in a bio-wall are markedly lower than concentrations in wells

downgradient of the biowall, this suggests that variations in hydrologic conductivity may limit the performance of the biowall. A comparison of point concentrations of the contaminant with depth in the biowall, such as what was done in Figure 5, or with lateral distance along the biowall, can identify local areas in the biowall that will most benefit from remedy enhancement such as addition of vegetable oil.

AcknowledgmentsThe U.S. Environmental Protection Agency and the U.S.

Air Force funded the research described here through inter-agency agreements RW-57-92160901 Bark Mulch Trench Implementation and RW-57-92262201 Bark Mulch Trench Long-Term Evaluation. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agencies, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. We thank Cherri Adair, Ken Jewell, and the late Frank Beck for providing technical assistance and Shaw Environmental for performing the chemical analysis. We appreciate review comments from Woody Woodworth and site information provided by Altus Air Force Base. Scott Christenson pro-vided useful advice on the design of the study.

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Table 3 TCE Attenuation Rate Constant Estimated from TCE Concentrations and Estimated Residence Time of Water in

the Biowall (Wells Sampled on April 12, 2005)

Upgradient Downgradient Rate Constant for TCE Removal (Per Day)

Well TCE (µg/L) Well TCE (µg/L) 10-d Residence Time 26-d Residence Time

U108 615 D108 8 0.44 0.17

U107 1310 D107 3 0.61 0.23

UMP1 1210 MP4 178 0.19 0.07

U106 1520 D106 15 0.46 0.18

U105 842 D105 124 0.19 0.07

Average 0.38 0.15


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Biographical SketchesHai Shen, Ph.D., corresponding author, is at U.S. Department

of Energy, Los Alamos Site Office, NM 87545; (505) 665 5046; (505) 667 5948; [email protected].

John T. Wilson, Ph.D., is at Office of Research and Development, U.S. EPA, Robert S. Kerr Environmental Research Center, 919 Kerr Research Drive, Ada, OK 74820; [email protected].

Xiaoxia Lu, Ph.D., is at College of Urban and Environmental Sciences, Peking University, Beijing, China; [email protected].

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