KINETIC CONTROLS ON SULFATE REDUCTION AT AN EXPERIMENTALLY INDUCED MIXING INTERFACE IN A CONTAMINATED WETLAND, NORMAN LANDFILL

In situ push-pull tests were used to evaluate the rates of sulfate reduction at an interface between wetland sediment porewater and groundwater from underlying anaerobic aquifer sediments at the Norman Landfill research site, Norman, OK. Recent studies have indicated that small-scale mixing interfaces are zones of increased microbial activity and geochemical cycling and are therefore often the most dynamic portions of the system. Unfortunately, the complex interactions controlling the biogeochemistry of these zones are poorly studied due to the small transient nature of mixing interfaces. One such control requiring further study is the role of kinetics on biogeochemical cycling of electron acceptors and donors within these zones. This study was designed to evaluate kinetic controls on sulfate reduction through experimentally inducing small-scale mixing interfaces. This was accomplished using “mini” push-pull tests designed to simulate the exposure of a reduced zone, limited with respect to electron acceptor, to anaerobic groundwater containing abundant sulfate (~100 mg/L), thus simulating the aquifer-wetland interface. A uniform, porous sand lens within the wetland sediments was targeted using small-diameter (2.54 cm, O.D.) “drive-point” wells with a discrete, internally packed 4.5 cm well screen. Mini push-pull tests were then performed by using the wells to inject 10L of aquifer water into the targeted zone. Sulfate-rich water used for the “push” phase of the tests was pumped from the anaerobic aquifer at the site and amended with 100 mg/L bromide (as NaBr) which served as a conservative tracer to track dilution from mixing, advection, and dispersion. The role of electron donor on sulfate reduction rates was also evaluated using push-pull tests by augmenting groundwater with additional electron donor (e.g. acetate). Mini push-pull tests appear to be an effective tool to quantitatively examine biogeochemical cycling in interface zones.