Ground-water quality assessment of the Carson River basin, Nevada and California — Results of investigations, 1987–91

Abstract

The Carson River Basin is an area of dramatic contrasts. The Carson River drains pristine wilderness of the forested Sierra Nevada, which provides much of the basin's water. The chemical composition of the Carson River changes from that of a fresh, untamed white-water river in the Headwaters Area to that of stagnant saline sloughs and alkali lakes in the Carson Desert. The ground-water quality, particularly in shallow aquifers, broadly mirrors the chemical changes in the river-a major source of recharge to basin-fill aquifers. Contrasts in ground-water quality within the Carson River Basin are evident across the basin, among the different aquifers, and, to a lesser extent, between shallow ground water beneath urban and agricultural land. Using current drinking-water standards as a measure of overall water quality, ground-water quality in principal aquifers in the upper basin generally is good. Principal aquifers in the upper basin are a major source of supply for municipal systems that provide water to the communities of Minden, Gardnerville, and Carson City. Precipitation falling on the Sierra Nevada, along with recharge from the Carson River in areas of heavy ground-water pumping, is the major source of recharge to principal aquifers. Except for locally high concentrations of nitrate and presence of synthetic organic compounds, water quality in principal aquifers generally results from chemical reactions with aquifer materials. Some ground water in and adjacent to the Sierra Nevada contains uranium concentrations greater than the proposed drinking-water standard. Radon activities in the Sierra Nevada locally exceed 10,000 pCi/L and are highest in the Carson River Basin. Shallow aquifers in Carson Valley contain higher concentrations of most major constituents and, compared to water in principal aquifers, more commonly contain concentrations of some minor constituents that exceed drinking-water standards. Manganese exceeds the secondary maximum contaminant level at more than 25 percent of the sampled sites. Minor constituents that exceed drinking-water standards at less than 10 percent of sampled sites are arsenic, fluoride, nitrate, and iron. Water from shallow aquifers more commonly contains concentrations of arsenic, fluoride, iron, and manganese in excess of the drinking-water standards than does water from the principal aquifers. Shallow aquifers beneath the upper basin locally contain herbicides, pesticides, and volatile organic compounds. Beneath the urban part of Carson City, prometone, trichloroethylene, and tetrachloroethylene were found at concentrations well above the laboratory minimum reporting level. Trichloroethylene was found at concentrations above the drinking-water standard. With a few exceptions, ground water beneath agricultural land in Carson Valley contained, at most, low concentrations of synthetic organic compounds. Principal aquifers beneath the sparsely populated middle Carson River Basin are recharged by precipitation falling on the uplands and, locally, by the Carson River. Concentrations of major constituents in water from principal aquifers in the lower basin generally are higher than in water from the principal aquifers of the upper basin. Concentrations of dissolved solids, iron, manganese, and sulfate more commonly exceed drinking-water standards in principal aquifers of the middle than the upper basin.