New Mexico Geological Society Annual Spring Meeting — Abstracts

The Gallinas River is the primary source of water for Las Vegas. NM (population 18,000), providing 95% of its domestic water supply. Additional surface water is diverted to the Storrie Lake Water Project and divided among multiple users, including the Las Vegas National Wildlife Refuge, farmers, and Ranchers. Growing demands for water necessitate that the Gallinas River be managed to ensure adequate supply and quality. Water quality studies show that the river contains elevated concentrations of arsenic (0.039 ppm) during periods of elevated flow (1.98m³/s). These results exceed the USEPA drinking water standard of 0.010 ppm. Total recoverable arsenic in water samples is strongly correlated with total suspended solids (R² = 0.98) and this is hypothesized to come from a natural source. We initiated a study to test this hypothesis by collecting twenty-three rock and twenty-six soil samples from major geologic units underlying the Gallinas Watershed and analyzing these samples from arsenic content. The results show that most of the geologic units contain arsenic concentrations less than 1 ppm, except for shales of the lower Pennsylvanian Madera and Cretaceous Niobrara and Carlile formations, which exhibit arsenic concentrations between 7-10 ppm. All soils show arsenic values >20 ppm with soils from the shale outcrop locations having especially high arsenic concentrations of >50 ppm. The Cretaceous shales contain pyrite, which has been correlated with high arsenic values in rock. The weathering of pyrite may release arsenic into soils, which is then introduced into the river during storm events as suspended solids. These results call for further studies to examine the relationship between arsenic source areas and impacted surface and ground water to identify regions where users are potentially at risk.