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RATE OF TCE DEGRADATION IN A PLANT MULCH PASSIVE REACTIVE BARRIER (BIOWALL)
Citation:
WILSON, J. T., X. LU, AND H. SHEN. RATE OF TCE DEGRADATION IN A PLANT MULCH PASSIVE REACTIVE BARRIER (BIOWALL). Presented at Ninth International Symposium, In-Situ and On-Site Bioremediation, Baltimore, MD, May 07 - 10, 2007.
Impact/Purpose:
To inform the public.
Description:
A passive reactive barrier was installed at the OU-1 site at Altus Air Force Base, Oklahoma to treat TCE contamination in ground water from a landfill. Depth to ground water varies from 1.8 to 2.4 meters below land surface. To intercept and treat the plume of contaminated ground water, a trench 140 meters long, 7.6 meters deep, and 0.46 meters wide was filled with a mixture of shredded tree mulch, composted cotton gin trash, and sand. As the plant mulch and compost decays, it provides conditions appropriate for biological reductive dechlorination of TCE. Because one of the mechanisms for removal of TCE is anaerobic biodegradation, this particular design for a passive reactive barrier is also called a Biowall. The Biowall was installed in June 2002. When monitored in 2005, it was still consistently reducing the concentrations of TCE in ground water leaving the Biowall by 80% or more. Presumably, the extent of treatment of TCE by the plant mulch in the Biowall will be a function of the residence time of contaminated ground water in the Biowall. To provide a basis for rational design of Biowalls at other sites, it would be useful to know the instantaneous rate of treatment of TCE during passage of ground water through the Biowall at the OU-1 site. To determine the rate of treatment, it is necessary to know the residence time water in the Biowall. In 2005 a bromide tracer test was conducted to determine the seepage velocity of ground water and the residence time of ground water in the Biowall. A series of fully-screened monitoring wells were installed in the Biowall on 12 meter intervals, a second series was installed 10 meters up gradient of the Biowall and a third series of wells was installed 10 meters down gradient of the Biowall. The most contaminated well up gradient of the Biowall contained 2,500 μg/L TCE. Over a 23 day period, approximately 22,000 liters of ground water was pumped from this well, amended with sodium bromide to an average concentration near 200 mg/L, and reinjected into an adjacent monitoring well up gradient of the Biowall. The concentrations of TCE and bromide were monitored in the wells in Biowall and down gradient of the Biowall. In four wells in or down gradient of the Biowall, the concentration of TCE, normalized to the concentration of bromide, was 18% to 35% of the up gradient concentration, with a mean of 27% remaining. The average seepage velocity of ground water was 0.07 meters per day. Based on the seepage velocity, the geometry of the Biowall, and the measured porosity of the sediment in the aquifer and the matrix of the biowall, the residence time of ground water in the Biowall would be 30 days. If all the TCE removal occurred in the Biowall, this corresponds to a pseudo-first order rate constant of 0.16 per day. Immediately after the pulse of bromide tracer was created, core samples were acquired from the aquifer, and the cores were analyzed for the concentration of bromide. The bromide tracer was moving through only 1.5 vertical meters of the aquifer, not the entire 5.9 vertical interval intercepted by the Biowall. If the contaminated ground water entering the wall from the 1.5 meter interval were uniformly distributed in the Biowall, the residence time would be 30 days, and the rate constant would be 0.043 per day. The instantaneous rate of removal of TCE in the Biowall is somewhere between 0.043 and 0.16 per day.