Enhancing the Efficiency of a Permeable Reactive Barrier in Remediating Trichloroethylene in a Contaminated Groundwater Using an Amended Trench

Abstract:

A capped landfill at Beltsville, MD is contaminated with high levels of trichloroethylene (TCE) in the groundwater system that exceeds the 5 ppb maximum contaminant level. TCE is a toxic volatile organic compound with many health-associated problems. In 2013, a permeable reactive barrier (PRB) was installed perpendicular to the groundwater flow direction between the landfill and the downstream creek. The PRB was designed to provide beneficial conditions for the indigenous microbial community to reductively detoxify TCE to ethane. Monitoring results have shown that the PRB enhanced TCE degradation from 600 ppb up-gradient to <1 ppb down-gradient from the PRB. However, a buildup of TCE degradation products such as vinyl chloride (VC) has also occurred in that same area.

In order to promote the dechlolrination in the groundwater, a trench was designed to be installed between the plume and the PRB. The trench is consisted of biosolids, to increase the organic carbon content for the bacteria and crushed limestone, to increase the pH in groundwater to more favorable values for dechlorination to occur (the optimum pH is 6-8). Biochar was also added to the materials to adsorb TCE and provide a solid surface for the bacteria to attach to and form biofilms which can enhance TCE dechlorination.

DNA extraction from the biosolids was conducted followed by polymerase chain reactions (PCR) to study the microbial community present in the biosolids, methanogens and Dehalobacter (a genus of bacteria capable of dechlorinating TCE to DCE) were detected. The effect of biosolids on TCE degradation was evaluated by spiking mesocosms containing biosolids with TCE. No significant TCE dechlorination was observed, however, four months after setting up the mesocosms almost 90% of the headspace gas was consisted of methane. To study the effect of methanogenesis caused by biosolids, separate sets of mesocosms were set up with a mixture of biosolids and limestone (50-50 by weight) and a consortia of anaerobic bacteria capable of dechlorinating TCE all the way to ethene (WBC-2), with no added electron donor. One months later, dechlorination was not affected by the significantly higher concentration of methane, produced by the biosolids mixture. Therefore, the biosolids mixture can be used as an electron donor in the trench to promote dechlorination.