Stable Isotope Geochemistry of Sulfate in Groundwater of Southern Arizona: Implications for Groundwater Flow, Sulfate sources, and Environmental Significance.

Abstract

The usefulness of stable isotopes of dissolved sulfate (δ₃₄S and δ₁₈O) as well as sulfate concentration was evaluated in four cases of study in southern Arizona. In Tucson basin, they have been used to determine the sources of sulfate in the groundwater, to delineate groundwater flow paths and recharge process, and to identify the groundwater domain. In Sonoita Creek near Patagonia, in combination with hydrochemical analysis and other environmental isotopes they were used to identify the sources of sulfate in the groundwater and surface water, the groundwater residence time, the interaction between groundwater and surface water, and the sources of base flow in Sonoita Creek. In the Patagonia Mountains, they were used to investigate the oxidation mechanisms of sulfide minerals. In evaporite deposits, δ₃₄S and δ₁₈O were used to determine the sources of sulfur, and to estimate the contribution of different S sources. This work shows that δ₃₄S and δ₁₈O of sulfate are excellent tracers of sulfate sources in groundwater, surface water, acid mine drainage and evaporite deposits in southern Arizona. The distinctive isotope compositions of the sulfur sources permit a clear identification of sources in aqueous environment, and quantification of the contribution from respective sources in one case. The results demonstrate the lithospheric origins of sulfur in the aqueous environment and evaporite deposits, and with Permian evaporite the most important source of sulfate.