TRACE-METAL ACCUMULATION IN BRINES AND SALTS OF FRANKLIN LAKE PLAYA AND THE ASH MEADOWS AREA OF NEVADA AND CALIFORNIA

Trace elements known to form oxyanions are present in large concentrations in the saline ground water beneath Franklin Lake Playa, California. High concentrations of arsenic (16 mg/L), chromium (0.5 mg/L), molybdenum (12 mg/L), selenium (0.7 mg/L), tungsten (0.3 mg/L), uranium (4 mg/L), and vanadium (1.2 mg/L) were measured in the most saline sample (specific conductance = 100 mS/cm). Other trace metals (Cu, Pb, Mn, Co, Ni, Pb) are generally <0.1 mg/L. The enrichments of metal oxyanions are derived partly from evaporative concentration of ground water discharged in the area of Ash Meadows, Nevada, 5-15 km upgradient. Additional metals likely are supplied by reaction of the saline fluid with clastic fragments along the flow path toward the playa. Previous work established that the spring discharge in Ash Meadows yields water with ~1 mS/cm specific conductance, 13 µg/L As, 0.3 mg/L B, and 3 µg/L U. Surface discharge of this water coupled with evaporation in the area of Ash Meadows and along the connecting drainage of Carson Slough toward Franklin Lake Playa precipitates a range of carbonate, chloride, and sulfate salts at the ground surface that collectively accumulate arsenic and other trace elements in approximate proportion to their content in the water. Water extractable salts on the surface in the Ash Meadows and upper Carson Slough areas contain concentrations as high as 600 ppm As, 140 ppm Mo, 4 ppm Se, 11 ppm W, 50 ppm U, and 80 ppm V. In contrast, the trace metals in similar salts on the surface of Franklin Lake Playa are substantially depleted relative to the major ions. Saline water is 1-3 m below the ground surface beneath the playa rather than <0.5 m depth in salt-accumulation areas in Ash Meadows and along Carson Slough. The contrast in metal concentrations may be attributed to preferential retention of metals in the brine beneath Franklin lake playa because capillary transport is the primary mechanism of salt supply to the ground surface. The metal-rich salts are of particular concern as they are entrained in wind-blown dust that disperses the metals over a broad area.