Land and Water Use Impacts on Ground‐Water Quality in Las Vegas Valleya

Abstract

ABSTRACTMarked changes in the occurrence and quality of near‐surface ground water in Las Vegas Valley, Nevada result from urban and industrial land and water use practices. In‐valley recharge has increased tenfold in the period 1943 to 1973 and now amounts to about 40,000 acre‐feet/year (49.3 million m3). Ground‐water flows leaving the Valley have increased from 250 acre‐feet/year to about 12,000 acre‐feet/year.Twenty to 400 tritium units (T.U.) in shallow ground water confirm widespread addition of recent recharge. Trend‐surface analysis of recent water‐quality data for depth intervals or “slices” of 0 to 50, 51 to 100, and 101 to 300 feet (0 to 15.2 m, 15.5 to 39.5 m, 30.8 to 91.4 m) revealed that natural trends below a depth of 50 feet are explainable in terms of broad hydrogeologic conditions. From 0 to 50 feet quality is highly irregular and markedly more influenced by land and water use practices and waste disposal in particular. Chloride, TDS, and nitrate are particularly diagnostic of return flows as is spring development and (or) a rising water table resulting from increased recharge and low vertical permeability. Statistical tests on water‐quality data for the period 1912 to 1968 yielded generally insignificant change with time. However, the extreme paucity of the data base makes any conclusion questionable.More efficient irrigation practices could reduce the present irrigation water demand by 15,000 acre‐feet/year and reduce return flows by 11,000 acre‐feet/year. Return flows by the year 2000 could easily amount to 75,000 acre‐feet/year or about three times the total water budget of the Valley prior to urbanization. Therefore, groundwater problems are likely to worsen and, if present monitoring practices prevail, go unnoticed.