Bonneville Salt Flats is a large salt pan deposited by an intermittent playa lake in western Utah. Magnesium and potassium chlorides are recovered from brines collected in a system of ditches and concentrated in large solar evaporation ponds. Recharge to the brine system is by rain and saline well water. Some recharge to the area of brine production may come from overland flow during wet winters.The upper ten feet of the Bonneville sediments constitutes an aquifer of high transmissivity which is attributed to, first, the high hydraulic conductivity of the salt bed at the surface, second, fractures in some of the underlying layers of silty clay, and, third, permeable lenses of sand-size brine-shrimp pellets. Transmissivities in parts of the aquifer exceed 50,000 gallons per day per foot. The occurrence of open fractures in silty clay is explained either by osmotic desiccation of buried clays induced by an increase in salinity of near-surface waters or by syneresis.