Multidecadal comparison of stable isotope and major ion geochemistry in a shallow brine aquifer

The Bonneville Salt Flats (BSF) in Utah's West Desert is a dynamic halite playa valued by land speed racers, mining interests, and the public. Concerns of a dwindling salt crust have led land managers and researchers to investigate observations of hydrologic and geochemical exchange contributing to halite dissolution, transport, and precipitation in this hydrologically closed basin. Preliminary observations examined seasonal patterns of the shallow brine aquifer in contact with BSF's halite-gypsum strata by sampling wells three times in 2015-2016 and analyzing for δ2H, δ18O, and major ion concentrations. Sampled wells were spatially distributed across 188 km2 and depths span from 1.8 m to over 91 m, including a few nested sites. Spatial patterns, annual averages, and seasonal patterns in brine geochemistry from this period were compared with data reported in past USGS and BLM reports ranging from 1992-2002. These hydrogeochemical data contribute to a multi-decadal evaluation of spatiotemporal change at BSF including analyses of Landsat data showing annual variation in salt crust area and vegetation over the same period as the sampled brine. Comparing patterns of change in surface halite extent with subsurface aqueous geochemistry serves to inform the processes driving exchange between the shallow brine aquifer the salt crust. Further understanding of natural and anthropogenic controls at this managed environment will come with the investigation and modeling of hydrometeorology and human use decisions which include the seasonal piping of brine from a nearby mining facility onto the salt crust surface as an effort to mitigate the impact of decades of extraction. Using baseline observations with known human activities will assist in understanding how historic variability in BSF salt crust has been impacted by human management of shallow brine.