Abstract

The Southern Nevada Water Authority (SNWA) has conducted numerous studies to enhance understanding of the water resources in Snake Valley. A significant amount of chemistry and isotopic data from ground-water, surface-water, and precipitation-sampling locations in Snake Valley and adjacent valleys in Utah have been compiled from a variety of sources. Following data compilation, a field effort was conducted by SNWA to collect water samples from areas with sparse data coverage to obtain the most comprehensive data set presently available for this area. The data set was used to characterize the water chemistry of the study area and to delineate areas that have similar properties.

Several trends in the chemistry of the waters of the study area were observed. Ground water in the western portion of the study area (western Snake, Hamlin, Deep Creek, and Pine Valleys) is primarily dominated by calcium, magnesium, and bicarbonate, which indicate ground-water interaction with carbonate minerals. The concentration of sodium also tended to increase from lower relative concentrations in waters in the recharge areas to greater relative concentrations in ground water on the valley floor. Increasing sodium suggests ion exchange or dissolution reactions with sodium-rich minerals along the ground-water flow path. The waters in the eastern portion of the study area (northeastern Snake Valley, Tule Valley, Fish Springs Flat, and Sevier Desert) are dominated by sodium and chloride, indicating dissolution of evaporites and playa and lakebed deposits. These waters also tended to exhibit higher temperatures, in some areas, and significantly higher total dissolved solids (TDS).

The waters of the study area generally tend to be of high quality with respect to the U.S. Environmental Protection Agency primary drinking water standards. Arsenic concentrations exceeded the maximum contaminant levels (MCLs) in a few cases, but the exceedances were mostly observed in waters from newly drilled wells that may not have been adequately developed. Arsenic exceedances were also observed for warm springs located in Tule Valley and in and near Fish Springs Flat. A much larger percentage of samples exceeded the secondary MCLs for chloride, sulfate, and TDS. These secondary MCLs were exceeded for most samples from Fish Springs Flat and Sevier Desert; no exceedances of the Nevada MCLs were observed for ground water in Nevada, but several exceedances of the Utah MCLs were observed for ground water in Utah.

Delta deuterium (δD) values for ground water in Snake Valley and surrounding valleys range from −124 to −87‰, and delta oxygen-18 (δ¹⁸O) values range from −16.9 to −12.3‰. A comparison of the isotopic composition of modern-day precipitation and ground water in the area suggests that the main source of recharge is from precipitation in the Deep Creek, Snake, and House Ranges. Detectable amounts of tritium were observed in many samples in the area. Tritium activities in the Snake and Deep Creek Ranges in the western part of the study area and in the House Range are either close to or greater than values measured in modern precipitation (8.4 to 12.3 tritium units) and suggest these areas as the recharge source for the ground water in the area. The carbon-14 (¹⁴C) content of the ground water is quite variable but generally decreases down-gradient from the mountain ranges to the valley floor. High ¹⁴C values in the high-elevation areas suggest recharge that has very little dilution due to water-rock reactions.