The Department of Defense (DoD) has many sites with chlorinated solvent groundwater contamination, some of which discharge into wetland environments. Traditional cleanup technologies are costly and may cause damage to wetland ecosystems since chlorinated solvents tend to be relatively resistant to degradation within most natural aquifer systems. However, beneficial transformation can occur within highly reducing wetland sediments, suggesting natural attenuation may be a viable, cost-effective wetlands remediation option. The objective of this project was to demonstrate methodologies for assessing natural attenuation of chlorinated solvent-contaminated groundwater plumes discharging into wetlands.
Natural attenuation comprises all processes that can reduce contaminant exposure concentrations. In most aquifers, trichloroethene (TCE) and other chlorinated solvents tend to be relatively resistant to degradation, leading to chlorinated solvent plumes that may reach surface water discharge points such as wetlands. Conditions conducive to transformation reactions (biotic and abiotic) tend to exist in the reduced organic-rich wetland sediment prior to discharge to the surface water, thus natural attenuation can occur. The wetlands’ organic-rich sediment zones are relatively thin, thus requiring a high-resolution groundwater sampling framework to assess natural attenuation. A key sampling strategy in this project was the use of multilevel piezometer transects.
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.
Chlorinated solvent natural attenuation at discharge wetland sites has the potential to provide a low-cost remedial alternative where traditional cleanup technologies are either expensive or threaten sensitive ecosystems. However, this project has shown that natural attenuation of chlorinated solvents may not be efficient at all wetland sites despite organic-rich sediments. Working with regulators and the Air Force, a protocol was developed for the assessment of natural attenuation as a remedial option at discharge wetlands. (Project Completed - 2006)