A Tracer Test to Characterize Treatment of TCE in a Permeable Reactive Barrier
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NGWA.org Ground Water Monitoring & Remediation 00, no. 0/ xxxx 0000/pages 0000 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/j17456592.2012.01394.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: 0000 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.
0 10 20 30East, m
NGWA.org H. Shen et al./ Ground Water Monitoring & Remediation 00, no. 0: 0000 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 (C0 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:
where M (ML2) is the solute mass assumed to be instan-taneously injected into the aquifer and
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 g/L.
Estimate of Water-Filled PorosityThe water-filled porosity of the aquifer material (
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 (
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:
(1)where C is the solute concentration (ML3) at time t (T) and the distance from the injection well of x (L); R is the retarda-tion factor...