USING δ34S AND δ 18O OF SULFATE TO RESOLVE PROCESSES AFFECTING SULFUR IN ALLUVIUM BENEATH A RIPARIAN AREA, OKLAHOMA

The δ³⁴S and δ¹⁸O of sulfate in ground water within alluvium of the Canadian River (CRA) contribute to understanding the spatially variable processes affecting sulfur in a 1 km² riparian area of central Oklahoma. The principal supply of sulfate is from dissolution of bedrock gypsum resulting in 5mM sulfate in Canadian river water (δ³⁴S_SO4 12 ‰ VCDT and δ¹⁸O_SO4 10 ‰ VSMOW). Smaller amounts of isotopically similar sulfate are supplied by surface and ground water flowing from the valley sides toward the river. Natural conditions at much of the study site are modified by a reducing (<0.1mM sulfate) leachate plume from a closed municipal landfill. Processes that modify the concentration and isotopic composition of sulfate within the CRA include bacterial sulfate reduction (BSR), sulfide oxidation, and organic matter degradation. BSR produces residual sulfate with δ³⁴S_SO4 and δ¹⁸O_SO4 as high as 80 and 24 ‰, respectively, and iron sulfide minerals that are depleted in ³⁴S (δ³⁴S_pyrite = -10 to -24 ‰). Low water levels favor oxidation of iron sulfide to produce sulfate with δ³⁴S_SO4 of -10 ‰ and δ¹⁸O_SO4 of -7 ‰. Sulfate is also removed from ground water by plant uptake, which retains the δ³⁴S of the sulfate in proximal ground water. Mineralization of plant litter and DOC within the leachate plume releases sulfate. Sulfate in the leachate plume has a δ³⁴S_SO4 of 8 ‰, which is consistent with sulfur in bedrock from the source of yard wastes in the landfill. δ¹⁸O_SO4 of leachate sulfate and sulfate from modern litter are near 6 ‰, presumably a result of ester sulfate hydrolysis. Progressive, microbial reduction of sulfate from river water, sulfide oxidation and mineralization of organic matter define three distinct subparallel trends of oxygen and sulfur isotopic evolution. The slope of all lines is consistent with the 4:1 ratio (δ³⁴S_SO4:δ¹⁸O_SO4) previously reported for bacterial sulfate reduction. Where most of the dissolved sulfate has been reduced, the oxygen isotopic composition of sulfate diverges from the linear trends and approaches isotopic equilibrium with oxygen in H₂O. The ability to measure the magnitude and distribution of these processes has implications for improved geochemical descriptions of riparian areas.