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