SULFATE REDUCTION COUPLED WITH RECHARGE PROCESSES IN AN ALLUVIAL AQUIFER CONTAMINATED WITH LANDFILL LEACHATE

The aquifer at the USGS Norman Landfill research site, adjacent to the Canadian River in Oklahoma, is contaminated with leachate from an abandoned, unlined municipal landfill. A two-year field study was done to determine whether recharge-supplied electron acceptors facilitated biodegradation of organic compounds in the transition zone between leachate and infiltrated recharge. Water samples were collected monthly from May 1998 to May 2000 at 15-cm depth intervals in the top 2 m of the aquifer at a contaminated and an uncontaminated site. At both sites, temporal concentration-depth profiles showed a seasonal cycle of dissolved sulfate production and loss in the aquifer. Dissolved sulfate present at individual cluster well depths ranged from 0 to 15 mM during the study. Samples for δ³⁴S analysis of dissolved sulfate were taken in May and October 2004, to further evaluate controls on sulfate concentrations. Observed δ³⁴S values of >30‰ at both contaminated and uncontaminated sites indicated sulfate reduction had occurred. δ³⁴S values of -8‰ were observed near the water table at the uncontaminated site, suggesting dissolution of sulfides had occurred. Net sulfate reduction rates were calculated using a chloride mixing-model approach, and ranged from 0.007 - 0.61 mmol L^-1 d^-1 (depth-integrated rates calculated between sampling periods, over depth intervals ranging from 0.45 – 1.75 m). There was no decrease in groundwater non-volatile dissolved organic carbon (NVDOC) in the sulfate-reducing zones; this may be because dissimilatory sulfate reduction does not result in complete mineralization of landfill NVDOC, or sulfate reduction may be coupled to oxidation of other electron donors such as methane. The observations suggest that sulfides are oxidized by dissolved oxygen in the capillary zone or unsaturated zone during the summer growing season, when the water table drops by 1 m or more, then the resulting sulfate is entrained by the rising water table and pushed deeper in the aquifer by recharge during fall and winter. Microbial sulfate reduction redeposits sulfides during this time. These results suggest that sulfate reduction associated with seasonal groundwater fluctuations may contribute to natural attenuation processes in other alluvial plain riparian environments.