SEDIMENT PHASE ASSOCIATIONS OF TUNGSTEN IN THE ALLUVIAL AQUIFERS, FALLON, NEVADA

Investigations into possible environmental causes of a childhood leukemia cluster in Fallon, Nevada, revealed high concentrations of tungsten (W) in airborne dust, lichens, and in groundwater of the local alluvial aquifers. While the connection between W and cancer remains under investigation, these observations have underscored our lack of knowledge of the W geochemical cycle in the environment. Natural sources of W in the Fallon area include weathering products of scheelite (CaWO₄) from W occurrences in the surrounding mountain ranges and W from geothermal waters. The purpose of this research is to determine the concentration, spatial variation, and phase association of W in the aquifer sediments.

Aquifer sediments, in the surrounding areas of Fallon, NV, were collected using a split spoon drill core, processed under a nitrogen environment, and characterized by total bulk chemistry (ICP-AES/MS), SEM/XRD, and selective extractions. Sediments consisted of coarse sands, silts, and black clays, composed of augite, biotite, K-feldspars, plagioclase, quartz and amorphous Fe coatings. The sands contain hematite and illmenite, whereas the finer grained sediment consistently contained pyrite, illite and kaolinite. Concentration of W in bulk sediments ranged from 1.3 – 7.9 mg/kg, and is a function of grain size. The average concentration of W in the sands (2.7 mg/kg) was significantly different from the concentration in silts and clays (5.3 mg/kg) as determined by a t-test at P=0.009. Approximately 90% of the extracted W partitioned into three sediment phases: non-crystalline iron oxides, crystalline iron oxides, (collectively referred to as FeOx), and organic matter (OM). Extracted Fe and bulk organic C (Org C) concentrations were greater in the silts and clays (FeOx = 8.1%; Org C = 3.4%), than in the sands (FeOx = 5.8%; Org C = 0.19%), with a significant difference in mean values between the two grain sizes (P=<0.001). Concentration of W in sediments sampled at the depth of the well screen correlated (R² = 0.94) with W concentration in groundwater (12.8 – 661 µg/l W) collected adjacent to sediment cores. From this data we conclude the concentration of W in sediments is controlled by the presence of FeOx and OM phases, and correlates with W concentrations in the groundwater.