BIOGEOCHEMICAL PROCESSES AFFECTING GROUNDWATER DISCHARGE IN A CHLOROBENZENE-CONTAMINATED WETLAND

Many point sources along the Bay and its tributaries contain industrial organic compounds, such as polychlorinated benzenes, and have contaminated wetland sediments, bottom sediments, and discharging groundwater. We are studying the effect of tidal flooding on biogeochemical processes that control the quality of discharging groundwater in a freshwater wetland at the Standard Chlorine of Delaware Superfund Site. The wetland groundwater is contaminated with trichlorobenzenes, dichlorobenzenes, monochlorobenzene (MCB), and benzene from sources in the wetland sediment and from contaminated groundwater flowing from the underlying aquifer. When highly reducing conditions characterized by high methane or sulfide production and low sulfate concentrations existed, monochlorobenzene and benzene were predominant in the wetland groundwater, indicating biodegradation of the polychlorinated benzenes predominantly through reductive dechlorination. The extent of reductive dechlorination appeared to increase between 2011-15 after storms damaged the tide control gates, causing the frequency and extent of tidal inundation in the wetland to increase. The molar percentage of MCB doubled in the shallow groundwater across most of the wetland between 2011 and 2015. However, a decrease in benzene and in total concentrations of chlorinated benzenes indicated that complete degradation to non-aromatic compounds was occurring. Sulfate concentrations in the discharging groundwater were substantially lower in 2015, while pH, chloride, sulfide, ferrous iron, and methane concentrations increased compared to those concentrations prior to the increased inundation of the wetland. Chloride sources include release during reductive dechlorination and surface-water infiltration. Changes in the microbial community composition of the wetland sediment reflect the observed changes in biogeochemical processes and degradation of the chlorinated benzenes.