The geochemistry of groundwater and sediments governing tungsten concentration in the basin-fill aquifers, Fallon, Nevada

The wide range of tungsten (W) concentrations (&sim0.7-700 ppb) in the groundwater of Fallon, Nevada, initiated an investigation to understand the geochemical cycle of W in the basin-fill aquifer. Tungsten in the groundwater became a potential health concern due to its observed concentrations in the human population of Fallon; the potential sources of W from nearby W ore deposits, mines, processing plants, and geothermal waters; and a possible cluster of childhood leukemia cases that was documented in the area. Spatial variation, concentration, and the mineralogical phase association(s) of W in sediments of the basin-fill alluvial aquifer near Fallon, NV, were identified in this study. These sediment characteristics were then compared to groundwater concentrations of W in an attempt to understand the geochemical controls on W in the aquifer environment. It was hypothesized that Eh levels control the distribution of W between solid and aqueous phases. Key findings of this study are: 1) concentration of W in bulk sediments ranged from 1.3&mdash7.9 mg/kg, and is a function of grain size and sediment chemistry, 2) no W minerals were observed in the aquifer sediments, 3) approximately 90% of the W partitioned into three sediment phases: non-crystalline iron (Fe) oxides, crystalline Fe oxides, (collectively referred to as FeOx) and organic matter (OM), 4) the association of W with FeOx and OM was unique compared to other trace elements (Mo, Cr, As, U, and Co) that predominately associated with the sulfide mineral fraction in the aquifer sediments, and 5) solid phase W concentrations correlate with dissolved W (R2 = 0.94), where groundwater with W concentrations >50 &mug/l had high alkalinity (>200 mg/l), reducing conditions (-0.7e4 mg/kg. Model results suggest dissolved As and P competed with dissolved W for adsorption sites on the Fe oxide surface, resulting in less W adsorption than expected. One conclusion of this work is that dissolved W was greatest in wells with the lowest pe levels. However, Fe oxides were observed in all environments, suggesting that adsorption between W and the Fe oxide surface occurs in all environments, and provides a control on dissolved W concentration