BIOGEOCHEMICAL CONTROLS ON DEGRADATION OF CHLOROBENZENES IN A WETLAND

At a Delaware Superfund site, the biogeochemical controls on biodegradation of tri-, di-, and mono-chlorobenzenes and benzene are being evaluated in a wetland impacted by past spills and continuing groundwater discharge. Samples were collected from passive diffusion samplers, piezometers, sediment cores, in situ microcosms, and fixed-film bioreactor experiments. Shallow groundwater in the western wetland area that was impacted by a hydrochloric acid spill had high chloride concentrations, low pH (3.8 to 5.5), and predominantly iron-reducing conditions. In the eastern area, groundwater had high sulfate concentrations (200-1,000 milligrams per liter) as a result of a sulfuric acid spill, but more moderate pH than the western wetland. Although development of strongly reducing conditions was inhibited near the upland boundary at some sites with high sulfate, sulfate-reducing or mixed sulfate-reducing and methanogenic conditions were predominant in the eastern area. Higher sulfur and lower ferric iron concentrations in sediment cores collected in the eastern wetland than those in the western area indicated depletion of bioavailable iron and precipitation of iron sulfides as a result of the high sulfate concentrations in the groundwater. Molar ratios of the contaminants indicated that anaerobic biodegradation of tr- and dichlorobenzenes to monochlorobenzene and benzene was more prevalent in the eastern (high sulfate) area than in the western wetland area, where the low pH may inhibit degradation. In situ microcosms with ¹³C-monochlorobenzene showed removal of monochlorobenzene and incorporation of ¹³C in carbon dioxide and biomass in the eastern and western wetland areas, under conditions that ranged from suboxic to sulfate-reducing or mixed sulfate-reducing and methanogenic. Experiments with a continuous flow bioreactor seeded with native microorganisms in groundwater from the high sulfate area showed both aerobic and anaerobic biodegradation of dichlorobenzenes, monochlorobenzene, and benzene, although monochlorobenzene and benzene degradation rates decreased under sulfate-reducing and methanogenic conditions compared to aerobic conditions. Field and experimental results show that biodegradation can occur under a wide range of biogeochemical conditions in the wetland.