Observations of Decadal-Scale Brine Geochemical Change at the Bonneville Salt Flats

Abstract

Over the past century, the Bonneville Salt Flats, which lies on the western edge of the Great Salt Lake watershed, has experienced changing environmental conditions and a unique history of land use, including resource extraction and recreation. The perennial halite salt crust has decreased in thickness since at least 1960. An experimental restoration project to return mined solutes began in 1997, but it has not resulted in anticipated salt crust growth. Here, primary observations of the Bonneville Salt Flats surface and subsurface brine chemistry and water levels collected from 2013 to 2023 are reported. Spatial and temporal patterns in chemistry, focused on density and water stable isotopes, are evaluated and compared with observations across seven periods of research spanning from 1925 to 2023. Declining salinity in the areas to the east of extraction ditches and south of Interstate 80 were observed. Brine extracted for potash production decreased in salinity as extraction rates increased. Between the years 1964 and 1997, the salinity of the shallow aquifer brine located beneath and to the east of the crust experienced a decrease. However, following this period, the salinity stabilized and subsequently increased. Salinity recovery was concurrent with declines in brine extraction and the salt restoration project, with the largest decrease in brine extraction being concurrent with the largest recovery in salinity. The specific impact of the restoration project on the brine salinity increase remains unclear. To the west, the shallow aquifer in the area between the Silver Island Mountains and the salt crust has increased in salinity. This increase is accompanied by a decline in groundwater levels, which enables the underground movement of solutes from east to west, following a salinity gradient away from the saline pan. Over the past 25 years, the alluvial-fan aquifer along the Silver Island Mountains has markedly declined, leading to increasingly more saline and isotopically heavier basinal waters to be extracted for industrial use. This change is concurrent with the onset of the salt restoration project, which relies on alluvial-fan aquifer waters. This compilation of changes in groundwater chemistry provides an important resource for stakeholders working to understand and manage this dynamic and ephemeral evaporite system. It also offers an example of decadal-scale change in a highly managed Great Salt Lake watershed saline system.