SALT CRUST STABILITY AND BRINE EVOLUTION OF GREAT SALT LAKE’S NORTH ARM

The railroad causeway (ca 1959) separating the north and south arms of Great Salt Lake, Utah, created a perennial halite (NaCl) bottom crust in the north arm, but lake conditions controlling evolution and stability of the crust are less known, including chemodynamic and hydrodynamic factors that influence the empirical degree of the halite saturation (EHS). Prior to the opening of a new bridge in December 2016 the north arm lake elevation was at a historical low (4189 ft) with lake brine at halite supersaturation (~27% salinity) and significant halite precipitation occurred during warm months. After the opening, inflow of less saline south arm water (~14% salinity) raised the north arm elevation and diluted it to halite undersaturation. The following five years have resulted in annual and seasonal fluctuations of halite saturation states related to south arm inflow and temperature.

Beginning in mid-2019, the Utah Geological Survey investigated the north arm brine system to document halite saturation state transitions by field data collection and laboratory experiments. Groundwater contribution and influence on salt crust dissolution was also studied. Within the new hydrologic regime following the 2016 bridge opening, the north arm did not reach halite supersaturation with substantive halite precipitation until August 2020 and again in late May 2021, as lake level dropped. We determined the EHS density of the north arm brine to be ~1.223 g/cm³ at 20°C. The entire north arm water column is halite supersaturated during summer months, with air-water interface, fine-grained halite raft precipitates that sink to the bottom and nucleate coarse, clear halite bottom growths. Observations and geochemical modeling indicate groundwater discharge near the lake margins and that nearshore environments influence halite crust dissolution and contribute to north arm brine evolution. Dissolution holes within lake bottom crust (~20 ft depth) indicate groundwater upwelling, shifting the paradigm for interpreting paleoclimate records from lacustrine salt cores. The north arm saturation state is also seasonally affected by mirabilite/hydrohalite precipitation that occurs during winter months, lowering brine density and salinity, and promoting halite dissolution by sequestering Na⁺, Cl^-, and SO₄^2-.