Demonstration Results

Results showed that anaerobic biodegradation of chlorinated volatile organic compounds (VOC) was less efficient in the Colliers Mills (CM) wetland sediment than in those at Aberdeen Proving Ground (APG), Maryland. Unlike the APG site, the CM site responded rapidly to precipitation events, which altered groundwater flow and chemistry during the spring as compared to the drier summer and fall. When spring recharge was high, wetland groundwater flow was predominantly vertically downward. An influx of oxygenated rainwater was evident in the change from methanogenic to iron-reducing conditions in the shallow sediments, and anaerobic degradation products of TCE were not evident. Organic substrate concentrations did not limit TCE degradation at the CM site; instead, the less efficient solvent degradation in CM sediments likely was attributed partly to differences both in groundwater residence time in the wetland sediment and in active wetland microbial communities. TCE degradation in the CM microcosms was insignificant over a 30-day period as compared to the APG microcosms, indicating that microorganisms critical for rapid, complete biodegradation were lacking or inactive at the CM site. Although not formally validated as part of the project, pore-water samples collected from peepers (diffusion samplers) generally showed higher concentrations of ferrous iron, sulfide, and VOC degradation products than samples from the other devices and provided evidence supporting the occurrence of degradation in shallow wetland pore-water.