A Conceptual Model for Arsenic Enrichment In Stream Sediments and Ground Waters of the Humboldt River Basin and Adjacent Areas of Northern Nevada

The Humboldt River Basin (HRB) is a closed basin that covers approximately 43,700 km². The Humboldt River originates in the mountains of northeastern Nevada and flows west and southwest into the Humboldt Sink river terminus. Based on public domain data of groundwater and stream sediments, the arsenic (As) concentrations in the stream sediments and groundwater range from 0.5 to 1785 mg/kg and 0.2 to 7500 µg/L, respectively. The HRB water is oxic and predominantly alkaline with pH as high as 9.6. Correlation between As contents in sediments and groundwaters indicates release of As to solution at least partially from the exchangeable phases on solid surfaces. This is partly due to prevailing oxidizing conditions, neutral to alkaline pH, high salinity and high As concentrations, where adsorption of As-species is less favored. Correlations between dissolved As and electrical conductivity, chloride, boron, lithium and sulfate in both ground and surface waters indicate several mechanisms for As enrichment: evaporation, mixing with geothermal waters, and oxidation of As-rich sulfide mineral deposits. Mining wastes possibly contribute local sources of As.

We propose a paleo-hydrogeologic model for the distribution of As in both shallow groundwater and sediments in the HRB. A zone of very high As concentrations (up to >1000 mg/kg) occurs in the Quaternary alluvial deposits in the western HRB, which coincides with the extent of Late Pleistocene lakes. Another zone of high As-concentrations occurs locally in the northeastern part of HRB that coincides with extensive middle Miocene tuffaceous sedimentary rocks. These sediments, derived in part from surrounding Miocene volcanic rocks and Tertiary metal-ore deposits, have been eroded and transported down the Humboldt River since late Miocene and are major sources of As in HRB stream sediments. Dilution by sediment inflow from mountain ranges occurred in downstream sections towards Humboldt and Carson Sinks depocenters.