ARSENIC ENRICHMENT IN WATERS AND SEDIMENTS OF THE HUMBOLDT RIVER BASIN OF NORTHERN NEVADA

The Humboldt River Basin (HRB) is a naturally occurring internally draining river basin that covers approximately 43,700 km², and is the only source of water for municipal uses, irrigation and mining activities in this arid region of northern Nevada. The river system originates in the mountains of eastern Sierra Nevada and drains through some important hydrothermal alteration zones and epithermal deposits, including silver, copper, and arsenic-rich gold veins of well-known Carlin deposits. As a result, the waters and sediments from the HRB are highly enriched in arsenic (As). The concentrations of As in the waters (including groundwater, springs, and thermal waters) range from below detection level to 3.3 mg/L, whereas the concentrations in stream sediments range from below detection level to 1785 mg/kg. This distribution of high As concentrations in waters correlates with Quaternary playa deposits, and the most prominent area of high concentration is located around the Humboldt Sink where the subsurface flows of all tributaries of the HRB System converge. The water in the HRB is very oxic (average 13.35 mg/L of dissolved O₂) and saline with an average of 2665 µS/cm of specific electrical conductivity and highly enriched in B and Cl with up to 120mg/L and 25000 mg/L respectively.

Correlation between As content in sediments and As concentrations in the waters indicate that part of the As in solution is being extracted from the exchangeable phases on solid surfaces. This is partly due to the prevailing oxidizing conditions, neutral to alkaline pH, high salinity and high As concentrations, where adsorption of As-species is less favored. Correlation between As and sulfate in the waters indicates that oxidation of As-rich sulfides may play an important role in As enrichment. Lack of correlation between As and dissolved iron in the waters may indicate removal of iron into solid phases increasing the ratio of As to iron in the waters. Together these findings infer that As distribution in the HRB area is the result of complex coupled physical and geochemical interactions. Mining wastes, as well as As-rich geothermal spring waters, possibly contribute additional local sources of As in the area. Ongoing geochemical modeling work will shed significant light on the processes that are important to mobilization and cycling of arsenic.