CLOSED BASIN FRESHWATER-BRINE GROUNDWATER INTERFACE AND INTERACTIONS - PILOT VALLEY, UTAH-NEVADA

The 5 to 10 mile wide and 30 mile long Pilot Valley closed basin playa and salt pan, Utah-Nevada, was occupied by Lake Bonneville. The playa is bounded to the west and east by the 9,000 foot high Pilot Range and the 6,000 foot high Silver Island Mountains, respectively. Numerous fresh to saline water springs issues from the base of the steep alluvial fans at the base of the Pilot Range. No springs issue from the more gently sloping fans of the Silver Island Mountains. Shallow groundwater along the western edge of the playa chemically evolves from fresh water to brine over a distance of approximately 2,000 feet. The fresh water-brine interface is inverted as the brine overlies fresh water. Conventional thinking is that the spring discharges are forced to the surface by playa clay layers, and that the playa groundwater system is largely recharged from the west by underflow from the alluvial fans and to a lesser extent by direct infiltration on the playa surface. It is also thought that the fresh-brine interface is the result of fresh groundwater from the alluvial fan pushing against the brine beneath the playa. Computer modeling results have suggested that the brine groundwater beneath the playa moves convectively under salinity gradients.

However, water level and pressure transducer measurements of more than 200 shallow monitoring wells, over a period of years, demonstrate that the playa groundwater system is largely recharged by underflow from the alluvial fans at the base of the Silver Island Mountains, this groundwater flows westward, and there are no bine convection cells in the shallow subsurface. The Pilot Range is not a significant source of playa groundwater recharge or playa surface flooding. Precision surveying of springs at the base of the Pilot Range alluvial fans indicate that spring discharges are due to a break in slope at approximately the same elevation along a 30 mile front and that this break in slope is a Lake Bonneville shoreline. The fresh water-brine interface at the base of the Pilot Range alluvial fan is controlled by hydrodynamic and osmotic pressure.