SULFATE LOADS OF GEOLOGIC AND ANTHROPOGENIC ORIGIN IN THE RIO GRANDE VALLEY – SULFUR ISOTOPE MASS BALANCE

The Rio Grande is the fourth longest river system in United States and is the primary source of irrigation water for the Rio Grande Valley. Evaporation, groundwater recharge associated with salt-rich sedimentary rocks, geothermal heating, and flood irrigation are believed to increase the solute content of the river. To assess the salt loads from various geologic and anthropogenic sources in the Rio Grande Valley, we have been working to determine the chemistry and S isotope composition (δ³⁴S of sulfate) for a 1,400 km long stretch of the Rio Grande, its shallow aquifers and drains in New Mexico and West Texas. The δ³⁴S of sulfate greatly differs for pyrite-, evaporite- and fertilizer-derived sulfate (-25 to 0 ‰, +9 to +17 ‰, and -2 to +4 ‰, respectively). Therefore, the contribution and fluxes of sulfate can be estimated using S isotope mass balance constraints. Our initial results suggest that sulfate fluxes in the recharge areas of the Rio Grande are mainly controlled by sulfide weathering in upstream locations (~50 to 100 %), which appear to be the highest during snow melt events, and dissolution of Paleozoic and Mesozoic evaporites (< 50 %). With downstream flow, the Rio Grande likely gets additional sulfate fluxes from return irrigation flows, controlled by fertilizers (~20 %) and inputs from sedimentary sources (~20 %).