Oxyanion concentrations in eastern Sierra Nevada rivers - 3. Boron, molybdenum, vanadium, and tungsten

Water samples were collected from 10 locations along the Truckee River syst em, 14 locations along the Walker River system, and 12 locations along the Carson River, and analyzed for B, Mo, V, W, Na, Cl, and pH. Boron concentra tions ranged from approximately 2 mu mol/kg in the upper reaches of the Tru ckee River to almost 1,200 mu mol/kg in Pyramid Lake. Molybdenum, V, and W had concentrations in the nanomolal range; Mo varied from a low of about 12 nmol/kg to a high of 3,200 nmol/kg (Walker Lake); V ranged from 9 nmol/kg to approximately 470 nmol/kg; and W varied from a low value around 0.8 nmol /kg (West Walker River) to 1,030 nmol/kg. The high concentrations of these oxyanion-forming trace elements in the rivers reflects (1) the relative sta bility of these oxyanions (e.g., MoO42-, HVO42-, WO42-, B(OH)(3), and/or B( OH)(4)(-)) in the alkaline, well oxygenated river and lake waters, (2) cont ributions of hydrothermal waters (especially for B), and (3) weathering of rocks/regolith with high concentrations of these elements. In the case of M o, V, and W, each exhibited relatively conservative behavior in the upper, oxygenated reaches of all three rivers. During the study period the region experienced a prolonged drought such that the lower reaches of each river w ere typified by no flow or stagnant waters and probably low oxygen and/or a noxic conditions (although not measured). Reductive processes occurring in the low flow to stagnant reaches of each river could have led to removal of Mo, V, and W from solution as coprecipitates with Fe monosulfides, or via sorption to Fe oxides/oxyhydroxides and/or organic matter. Boron, however, exhibited essentially no or minor removal from these rivers, and instead wa s added to each river via B-rich hydrothermal waters (e.g., Steamboat Creek from Steamboat Hot Springs), or by B-rich groundwaters via base-flow durin g the extensive drought.