Paper No. 9
Presentation Time: 3:55 PM
Dust from playas in the Mojave Desert (USA): Controls on metal contents and emission
A framework for assessing potential health hazards associated with dust generated at dry-lake (playa) settings is built on understanding relations among geologic setting, hydrology, ground-water composition, climate, and surface-sediment characteristics. Evaporite-rich sediment occurs on the surfaces of some Mojave Desert playas and related drainages (Franklin Lake playa, Carson Slough, Ash Meadows, and Soda (dry) Lake), where the depth to water below the surface is <4m. Variations in the texture and composition of surface sediments and their potential for wind erosion are controlled by the timing and intensity of rainfall events and depth to the saturated zone. Evaporation of dilute water in the Ash Meadows and Carson Slough areas produces carbonate, chloride, and sulfate salts that accumulate trace elements in proportion to their content in the water. Water-soluble salts on the ground surface in Ash Meadows and Carson Slough contain metal concentrations as high as 600 ppm arsenic, 140 ppm molybdenum, 4 ppm selenium, 11 ppm tungsten, and 50 ppm uranium. Select trace elements are concentrated in the saline ground water and surface salts of the Franklin Lake Playa area. Maximum dissolved concentrations of arsenic (16 mg/L), molybdenum (12 mg/L), selenium (0.7 mg/L), tungsten (0.3 mg/L), and uranium (4 mg/L) are explained by evaporative concentration of dilute ground water along a shallow flow path. However, salts on the Franklin Lake playa surface have concentrations markedly lower than those from Carson Slough and Ash Meadows because of the greater depth to the saturated zone that favors deeper accumulation of the trace elements. Ground-based observations, satellite imaging, and portable wind tunnels that measure potential dust production at varying wind speeds show that these and other evaporite-rich areas are significant sources of atmospheric dust. Dust collected from wind-tunnel tests is being used for simulated biofluid leaching to evaluate bioaccessibility of potentially harmful trace elements.
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