Sulfur Biogeochemistry and Isotope Fractionation in Shallow Groundwater of Owens Dry Lake, California

The redox status of hypersaline, strongly alkaline groundwaters at Owens Dry Lake was investigated to help guide mitigation efforts for attenuating dust generated from the dry lakebed. Shallow (<1 m), anoxic groundwaters have been identified as a major limitation to vegetation establishment on the lakebed due to the inability of roots to growth in anoxic conditions. Previous work indicates that sulfate reduction is the dominant reaction regulating the redox status of shallow groundwaters. The purpose of this study was to evaluate sulfur biogeochemistry and formation of solid-phase sulfides in the shallow groundwater/sediments using selective sulfur speciation techniques coupled with isotopic measurements. In addition to groundwater and subsurface sediment samples (1-2 m depth) at sites representative of different groundwater pathways, selected sediment samples at 5 different depths (from oxic to anoxic layers) were collected. Sediment samples were examined for monosulfide, pyrite, sulfate, organic sulfur, and total sulfur. Organic sulfur was less than 0.01% of the total, and pyrite was the predominant sulfur-bearing phase below the groundwater capillary zone ( ∼20cm depth) where anoxic conditions were developed. The concentration of monosulfide and pyrite were less than detection limits above the capillary zone as these unsaturated layers were exposed to oxygen. High concentrations of dissolved sulfide (4.81 to 134.7 mg /L) and low concentrations of dissolved Fe (generally <0.5 mg/L) indicate that the availability of Fe limits pyrite formation. The high values ( ∼50‰ ) of isotopic fractionations between δ ³⁴S_pyrite and δ ³⁴S_sulfate(Δ _{sulfate-pyrite}) in anoxic zones suggest that bioavailability of organic carbon is a limiting factor for the reduction of sulfate. The values of Δ _{sulfate-pyrite} along the hydrologic flowpath indicate that the isotopic fractionations were significantly correlated with dissolved sulfate concentration, which was strongly controlled by evaporation. This indicates that spatial variations in the concentration of dissolved sulfate due to evaporation can be reflected in the pyrite content of sediments in groundwater. The important role of evaporation on the concentration of sulfate in groundwater was confirmed using hydrogen and oxygen isotope values of pore fluids.