EFFECT OF INCREASED INUNDATION ON BIOGEOCHEMICAL PROCESSES AND FATE OF CHLOROBENZENES IN A FRESHWATER WETLAND

Increased inundation area and frequency is a potential major factor associated with sea level rise that could affect contaminant fate in groundwater and sediments in coastal areas. We studied biogeochemical processes and fate of chlorobenzenes in a freshwater wetland at a Delaware Superfund site, where malfunctioning tide control gates caused inundation to increase after early 2011. The wetland groundwater is contaminated with trichlorobenzenes, dichlorobenzenes, monochlorobenzene (MCB), and benzene from sources in the wetland sediment and from discharge from the underlying aquifer. When wetland flooding was limited, iron reducing conditions were predominant in the wetland, while low to moderate methane and sulfide production was less widespread. Methane and sulfide production were associated with an increase in the molar percentages of MCB and benzene along flowpaths from the aquifer to the shallow wetland groundwater, indicating biodegradation of the polychlorinated benzenes through reductive dechlorination. In situ microcosms and microbial community analyses, however, indicated that some aerobic degradation of MCB and benzene also occurred throughout the wetland, limiting accumulation of these toxic intermediate products of reductive dechlorination. After wetland inundation increased, widespread highly reducing conditions were observed in the wetland groundwater, characterized by lower sulfate and higher methane concentrations compared to earlier measurements. Total volatile organic compound concentrations decreased and chloride concentrations increased in the wetland groundwater after 2011, indicating more rapid, complete degradation of the chlorobenzenes under the more highly reducing conditions. An order of magnitude increase in the percent abundance of dechlorinating Dehalococcoides bacteria in the wetland sediment supports reductive dechlorination as the prevalent degradation process under the highly reducing conditions associated with increased inundation. The molar percent MCB in the wetland groundwater doubled with the more highly reducing conditions, however, indicating a decrease in the aerobic degradation that previously limited MCB accumulation. In situ microcosms are underway to quantify the degradation mechanisms and compare to earlier tests.