FIELD AND LABORATORY INVESTIGATIONS OF KINETIC CONTROLS ON SULFATE REDUCTION AT HYDROLOGIC INTERFACES WITHIN A CONTAMINATED WETLAND-AQUIFER SYSTEM

Quantifying the rates of redox reactions initiated at interfaces is of great interest because interfaces have been shown to be zones of increased biogeochemical transformations and thus may play an important role in natural attenuation. In our recent work in an aquifer-wetland system impacted by landfill leachate (Norman, OK), we have observed steep gradients of biogeochemical indicators at hydrologic “mixing” interfaces. In the present study, we used variations of push-pull tests both in situ and in the laboratory to evaluate the kinetic controls on sulfate reduction at these interfaces. In order to mimic the aquifer-wetland interface and evaluate reaction rates over time, we introduced sulfate-rich anaerobic aquifer water (~100 mg/L SO₄^2-) into sulfate-depleted wetland porewater. Mini push-pull tests performed in the field showed that sulfate reduction was stimulated by the mixing event and rates were comparable to those measured in other push-pull studies. However, rates did not follow a simple first-order rate constant slope. Interestingly, we observed a lag phase in sulfate reduction that persisted until the mixing interface between test solution and native water was recovered. This lag phase was not eliminated by addition of electron donor (acetate) to the injected test solution. Laboratory investigations targeting the nature of the mixing interface support the importance of the mixing interface in controlling terminal electron accepting processes. These data suggest redox reactions may occur rapidly at the mixing interface between injected and native waters but not in the injected bulk water mass. Under these circumstances, push-pull test data should be evaluated to ensure the apparent rate is a function of time and that complexities in rate constants be considered.