FRESHWATER TRACER INJECTIONS IN SHALLOW BRINE WELLS AT THE BONNEVILLE SALT FLATS, UTAH

At the Bonneville Salt Flats (BSF) in western Utah the exchange of solutes between surface salt crust and shallow subsurface brine is critical in understanding observed change of this dynamic desert playa. Both natural and anthropogenic alterations of the local water cycle and ion mass balance drive the dissolution, transport, and precipitation of salt minerals. Investigation of the local and regional groundwater flow and solute transport is needed to understand the importance of seasonally transient environmental processes (e.g. surface ponding, salt crust dissolution) compared with equilibrium exchange of ions between subsurface brine and salt crust. To observe flow rates in the subsurface, freshwater tracer injections were conducted in monitoring wells at BSF. Specific conductivity and temperature measurements were collected intermittently in each well along a depth gradient representing changes in both density and strata. Well observations prior to the introduction of tracer showed a gradient of temperature decreasing with depth and specific conductivity increasing with depth. After an introduction of freshwater tracer, some observable stratification of brine by density occurred in all wells. Observations showed a much quicker return to initial conditions at greater depth than at shallower depth in the brine aquifer. Further, the return rate of specific conductivity to initial conditions correlates across some stratigraphic data available from adjacent core logs. Calculations of specific discharge estimates were made using measured changes to specific conductivity values over time. Further, porosity values were obtained using micro CT scans on individual strata of sediment cores allowing for the estimation of lateral flow velocities. These estimates are complicated by vertical convection driven by density gradients within well casings. Future work will use numerical methods to estimate density induced vertical flow within the well casings and draw comparisons between observations and simulations. Investigations of fluid dynamics within well casings will aid future investigations in subsurface flow across the shallow aquifer and assist in dissecting the effects of changing climate and anthropogenic activities on observed changes to both brine and the salt crust at BSF.