Dissolved-solids concentration in the shallow brine ranges from less than 100,000 to more than 300,000 milligrams per liter on both playas. The concentration of dissolved solids in brine in the shallow-brine aquifer varies, but it generally increases from the edges of the playas toward areas of salt crust. Evaporation rates through the surface of thick perennial salt crust were much higher. ![]() Evaporation rates through sediment-covered salt crust and the gypsum surface were estimated at between 3x10 -4 and 4x10 -3 inches per day during the summer and fall of 1976. Salt-scraping studies indicate that the amount of halite on the Salt Flats is directly related to the amount of recharge through the surface (which causes re-solution of halite) and the amount of evaporation at the surface (which causes crystallization of halite). Some water was transpired by phreatophytes, and some leaked into the alluvial fan along the western edge of the playa. Discharge was mainly by evaporation at the playa surface and withdrawals from brine-collection ditches. Recharge to that part of the shallow-brine aquifer north of Interstate Highway 80 on the Salt Flats is mainly by infiltration of precipitation and wind-driven floods of surface brine. The shallow-brine aquifer is the main source of brine used for the production of potash on the Salt Flats. Ground water occurs in three distinct aquifers in much of the western Great Salt Lake Desert: (1) the basin-fill aquifer, which yields water from conglomerate in the lower part of the basin fill, (2) the alluvial-fan aquifer, which yields water from sand and gravel along the western margins of both playas, and (3) the shallow-brine aquifer, which yields water from near-surface carbonate muds and crystalline halite and gypsum. The salt crusts are dynamic features that are subject to change because of climatic factors and man's activities. Geomorphic differences in the salt crust are caused by differences in their hydrologic environments. Five major types of salt crust were recognized on the Salt Flats, but only one type was observed in Pilot Valley. The Pilot Valley salt crust and surrounding playa have been largely unused.Įvaporite minerals on the Salt Flats and the Pilot Valley playa are concentrated in three zones: (1) a carbonate zone composed mainly of authigenic clay-size carbonate minerals, (2) a sulfate zone composed mainly of authigenic gypsum, and (3) a chloride zone composed of crystalline halite (the salt crust). Much of the Bonneville Racetrack has become rougher, and there has also been an increase in the amount of sediment on the south end of the racetrack. In recent years, there has been an apparent conflict between these two major uses of the area as the salt crust has diminished in both thickness and extent. The salt crust on the Salt Flats has been used for many years for automobile racing, and brines from shallow lacustrine deposits have been used for the production of potash. Both areas lack any noticeable surface relief (in 1976, 1.3 feet on the Bonneville salt crust and 0.3 foot on the Pilot Valley salt crust). The Bonneville salt crust covered about 40 square miles in the fall of 1976, and the salt crust in Pilot Valley covered 7 square miles. ![]() The areas are separate, though similar, hydrologic basins, and both contain a salt crust. The Bonneville Salt Flats and Pilot Valley are in the western part of the Great Salt Lake Desert in northwest Utah.
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