GEOCHEMICAL CONTROLS ON ARSENIC CONCENTRATIONS IN RYE PATCH RESERVOIR, PERSHING COUNTY, NV

Rye Patch Reservoir is located in Pershing County along Interstate 80 approximately 200 km northeast of Reno, NV. Based on U.S. Geological Survey (USGS) and Nevada Division of Environmental Protection (NDEP) data, a two to three fold increase in arsenic concentration occurs between the upstream gauging station at the reservoir intake and the reservoir outlet at Rye Patch Dam. The purpose of this study is to identify the geochemical and hydrological processes that may contribute to this observed increase in arsenic concentration in Rye Patch Reservoir.

Methods used to determine the processes influencing arsenic concentration will include soil and water samples, sequential extractions, sediment coring and geochemical modeling. Water and soil samples will be taken at selected points along the flow path. Water, soil and sediment samples will be analyzed for major cations and anions as well as trace elements. Cation concentrations will be determined using inductively-coupled plasma – atomic emission spectroscopy (ICP-AES). Anion concentrations will be determined using ion chromatography. Trace elements will be determined using inductively-coupled plasma – mass spectrometry (ICP-MS). Soil and sediment samples will also be analyzed for mineral phases using x-ray diffraction and scanning electron microscopy. Sequential extractions will be conducted on soil and sediment samples to determine the availability of arsenic to solution. Following conceptual model development, geochemical modeling using EQ3/6 and PHREEQC will be conducted to evaluate the geochemical and hydrologic processes controlling arsenic.

Preliminary results suggest that evaporation and desorption from oxide mineral phases may be the processes that cause an increase in arsenic in Rye Patch Reservoir. Using the described methodology, continuing work will evaluate the importance of these two processes in the control of the arsenic concentrations in the reservoir.