TRACE ELEMENTS IN A DRYLAND SETTING OF GROUNDWATER DISCHARGE: LINKAGE BETWEEN GROUNDWATER AND DUST

Potentially toxic trace elements in dust can pose risks to human and ecosystem health. In dryland settings of spring or evaporative discharge where salts accumulate, the composition of wind-erodible material is related to the subsurface sediment grain size as well as depth and composition of groundwater. Studies of the composition of subsurface sediment and salt crusts with respect to their hydrologic setting in the Franklin Lake playa – Ash Meadows area, California and Nevada (110 km NW of Las Vegas, NV) reveal the controls on trace element distribution in salt crusts and ultimately dust. Groundwater that evaporates to form salts ranges from dilute to briny with specific conductance ranging from 0.6 to 100 mS/cm and TDS ranging from 300 to 100,000 mg/L. Relatively high dissolved concentrations (several mg/L) of As, Mo, and U are present in the more saline, alkaline, oxidizing groundwater. In subsurface sediment and salt crusts, maximum soluble-salt contents are 7.99 mS/cm to 18.9 mS/cm, respectively. Soluble metal concentration in surface crusts are as high as 570 ppm As, 174 ppm Mo, and 96 ppm U. The trace element concentration of salt crusts depends more on the proximity of groundwater to ground surface than on the dissolved concentration. Areas where salts and trace elements accumulate in the vadose zone have a relatively deep saturated zone (>3m), coarse-grained subsurface sediment, or both of these characteristics. Areas where trace elements accumulate at the ground surface have a shallower saturated zone, finer particle size of the sediment matrix, or both. Evaporative processes result in high concentrations of soluble trace elements at the ground surface and within the vadose zone. Dust emitted from salt-crusted surfaces contains relatively high average concentrations of water-soluble arsenic (691 ppb) and uranium (64 ppb). Dust from areas where the salt accumulates in the vadose zone tend to have relatively low soluble contents of toxic elements compared to dust from areas covered by surface salts. Overall, local hydrogeologic attributes profoundly affect the distribution of trace elements and the availability of potentially toxic dust.