Biogeochemical Processes in a Contaminant Plume Downgradient from a Landfill, Norman, Oklahoma

By Isabelle M. Cozzarelli, Joseph M. Suflita, Glenn A. Ulrich, Steve H. Harris, Martha A. Scholl, Jamie L. Schlottmann, and Jeanne B. Jaeschke

ABSTRACT

Studies of an alluvial aquifer contaminated with landfill leachate, at Norman, Oklahoma, indicate that the non-uniform distribution of electron acceptors and biogeochemical reactions in anoxic ground water result in steep chemical gradients and the formation of distinct reaction zones. A combined geochemical and microbiological approach was used to delineate different biogeochemical zones along a transect parallel to regional ground-water flow downgradient from the landfill. The important microbially mediated reactions in the anoxic plume are iron reduction, sulfate reduction, and methanogenesis. Dissolved H₂ measurements in ground water at several depth intervals at one location near the edge of the landfill indicate that sulfate reduction is a dominant respiratory process, but near- saturation levels of methane were detected in some intervals. Cycling of sulfur was apparent in a thin interval at the water table where the highest rates of sulfate reduction (13.2 micromoles per day (µM/day)) and sulfate concentrations well above background levels (up to 4.6 millimolar (mM)) were measured. In this zone, sulfur isotope analyses indicate that the sulfate is enriched in ³⁴S (³⁴S of SO₄^2- was 67-69 per mil). Elevated concentrations of sulfate near the water table appear to result from the oxidation of sulfides as oxygenated recharge water mixes with anoxic plume water during recharge events. Sulfate availability in this vertically thin zone near the water table appears to control the high sulfate reduction rates measured in the zone. Iron reduction occurred at the edges of the sulfate-depleted plume, whereas methanogenesis was detected only in the center of the plume. This study underscores the importance of examining the availability of electron acceptors in contaminated environments and using a combined geochemical and microbiological approach to elucidate the spatial distribution of biogeochemical processes.