Arsenic in the waters and sediments of the Humboldt River, North-Central Nevada, USA: hydrological and mineralogical investigation

Arsenic (As) concentration in the Humboldt River (HR) water ranges from 0.012 to 0.06 mg/L, with an average of 0.032 mg/L. The river water is alkaline with pH 8.4–9.3 and oxic with average 12 mg/L of dissolved O₂ and + 139 mV of oxidation–reduction potential (ORP). The river water contains an average specific electrical conductivity of 1000 µS/cm with elevated B (average 0.77 mg/L), Li (average 0.15 mg/L), Cl (average 158 mg/L), and SO₄ (average 178 mg/L). Factor analysis of river water chemistry and sediment chemistry suggests several physical and geochemical processes operating in three sub-regions: upper, middle and lower reaches of the river. Oxidative dissolution of As-bearing sulfide minerals in the upstream is the primary source of As to the water. Groundwater–surface water interactions affect As concentrations in localized area between the middle and lower HR. Further enrichment of As occurs by evaporation in the lower reach and the terminal sink of the river. Results of sequential extraction analysis and scanning electron microscopy (SEM) of the riverbed sediments suggest that As distribution in the river sediments is controlled by partitioning to clay minerals and Fe-oxy-hydroxides.