Ground-water quality and geochemistry of Las Vegas Valley, Clark County, Nevada, 1981-83: Implementation of a monitoring network

Abstract

As a result of rapid urban growth in Las Vegas Valley, rates of water use and wastewater disposal have grown rapidly during the last 25 years. Concern has developed over the potential water-quality effects of this growth. The deep percolation of wastewater and irrigation return flow (much of which originates as imported water from Lake Mead), along with severe overdraft conditions in the principal aquifers of the valley, could combine to pose a long-term threat to groundwater quality. The quantitative investigations of groundwater quality and geocheraical conditions in the valley necessary to address these concerns would include the establishment of data-collection networks on a valley-wide scale that differ substantially from existing networks. The valley-wide networks would have a uniform areal distribution of sampling sites, would sample from all major depth zones, and would entail repeated sampling from each site. With these criteria in mind, 40 wells were chosen for inclusion in a demonstration monitoring network. In developing this network, water samples were collected from 56 wells for determination of as many as 34 water-quality characteristics. » Ground water in the northern half of the valley generally contains 200 to 400 milligrams per liter of dissolved solids, and is dominated by calcium, magnesium, and bicarbonate ions, reflecting a chemical equilibrium between the ground water and the dominantly carbonate rocks in the aquifers of this area. The intermediate to deep ground water in the southern half of the valley is of poorer quality (containing 700 to 1,500 milligrams per liter of dissolved solids) and is dominated by calcium, magnesium, sulfate, and bicarbonate ions, reflecting the occurrence of other rock types including evaporite minerals among the still-dominant carbonate rocks in the aquifers of this part of the valley. The poorest quality ground water in the valley is generally in the lowland parts of the valley in the first few feet beneath the water table, where dissolved-solids concentrations range from 2,000 to more than 7,000 milligrams per liter, and probably reflects the effects of evaporite dissolution, secondary recharge, and evapotranspiration. The most common water-quality constraint on potential groundwater use is the high salinity. No evidence of large-scale contamination of deep ground water was found in this study.