RECHARGE OF A FRESH WATER-BRINE GROUNDWATER SYSTEM, PILOT VALLEY, UTAH-NEVADA

The 5 to 10 mile wide and 30 mile long Pilot Valley playa and salt pan, Utah-Nevada, is a remnant of Lake Bonneville. The playa is located in a closed basin bounded to the west by the 9,000 foot high Pilot Range and to the east by the 6,000 foot high Silver Island Mountains. The nearly flat playa has an elevation of about 3,800 feet. 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 at the base of the dryer 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 unusual in that it slopes toward the brine beneath the playa (i.e. saline water 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 on the flanks of the Pilot Range 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 also suggested that the brine groundwater systems moves convectively under salinity gradients.

However, monthly water level measurements of more than 100 shallow monitoring wells, over a 12 month period, suggests that the playa groundwater system is largely recharged by underflow from the alluvial fans at the base of the lower elevation Silver Island Mountains and from localized flash flooding events originating in the Silver Island Range. Vertical gradients in the shallow subsurface are usually upward but may be either upward or downward, depending upon the extent of surface flooding. The playa groundwater does not move convectively. The slope of playa water table toward the Pilot Range, suggests that the Pilot Range is not a significant source of playa recharge. Precision surveying of springs at the base of the Pilot Range alluvial fans indicate that they discharge at a break in slope, along a 30 mile front,that is a Lake Bonneville shoreline. The attitude and chemical evolution of the fresh water-brine interface is controlled by hydrodynamic pressure of the shallow brine.