Proposed monitoring network for ground-water quality, Las Vegas Valley, Nevada

Among the 255 hydrographic areas in Nevada, 1,564-square-mile Las Vegas Valley has the greatest need for a network to monitor ground-water quality. Virtually the entire ground-water supply for municipal, industrial, agricultural, and domestic use in the valley (72,000 acre-feet during 1979) is obtained from the upper 1,100 feet of valley-fill sedimentary deposits. In total, 185 wells and one spring have been selected as preliminary candidates for the network. The wells tap water-yielding material in three arbitrary depth intervals: (1) The shallow zone, less than about 30 feet below the water table, which would be the first to feel the impact of landand water-use practices; (2) an intermediate zone, about 30 to 200 feet below the water table, which is commonly tapped for domestic supplies, and (3) a deep zone, more than 200 feet below the water table, which currently yields most of the ground water for public supplies in the valley. Forty-five water-quality characteristics were chosen for monitoring, largely on the basis of drinking-water standards. For each site, the specific array of characteristics and their frequencies of determination (which range from once quarterly to once in 5 years) are based on geographic location and water-yielding zone(s) tapped. Specific procedures will be necessary to ensure the collection of samples that are truly representative of the ground water. The U.S. Geological Survey's automated "WATSTORE" data-management system is the recommended mechanism for storage, retrieval, and analysis of the water-quality information generated as a result of the monitoring network.