EFFICIENCY OF NATURAL ATTENUATION OF CHLORINATED SOLVENTS IN TWO FRESHWATER WETLANDS

摘要

Natural attenuation caused by anaerobic biodegradation is being investigated for chlorinated solvent plumes discharging from aerobic sand aquifers to two different freshwater wetlands―a tidal marsh at Aberdeen Proving Ground (APG), Maryland, and a cedar swamp in the New Jersey Pinelands (Collier Mills Wildlife Management Area). At the APG marsh, concentrations of parent contaminants, including trichloroethene (TCE) and 1,1,2,2-tetrachloroethane (PCA), and anaerobic daughter compounds generally decrease to below detection levels before land surface is reached along upward ground-water flowpaths in the wetland and creek-bottom sediments. At the Collier Mills swamp, concentrations of TCE and the daughter compound cis-l,2-dichloroethene (c12DCE) remain high along upward flowpaths in the wetland porewater in some areas. Surface-water concentrations of TCE average about 20 μg/L where the center of the plume discharges. Although mixed iron-reducing and methanogenic conditions are predominant in the wetland sediments at both sites, TCE degradation in the Collier Mills wetland sediment seems to be limited to production of c12DCE. Anaerobic laboratory microcosms showed that TCE degraded to below the detection level in less than 25 days in the APG wetland sediment, whereas degradation was insignificant over a 35-day period in the Collier Mills wetland sediment. The less efficient chlorinated solvent degradation in the wetland sediments at the Collier Mills site can be attributed partly to differences in ground-water residence time, and partly to differences in microbial communities in the wetland sediments.