Hydrogeology and Hydrogeochemistry of the Shallow Alluvial Aquifer Zone, Las Vegas Valley, Nevada

摘要

In Las Vegas Valley, Nevada, the shallow alluvial aquifer zone is a possible source of contamination to the principal alluvial aquifers that provide 30% of the public drinking water supply for the valley. Development of the principal aquifers has lowered pressure head in the principal aquifers and created the potential for downward seepage from the shallow aquifer zone. This study was undertaken to characterize the hydrogeology and hydrogeochemistry of the shallow alluvial aquifer zone and to compare the hydrogeochemistry of the shallow and principal alluvial aquifer zones.A 37 well shallow ground water monitoring network was established and water-level, water-quality, and isotopic data were collected between June, 1988 and December, 1989. Water levels fluctuate seasonally and are influenced by land use practices. Irrigation influenced water levels are higher in fall and lower in winter while the natural water level pattern has lows in the fall and highs in the winter. Water temperature, pH, and EC appear to be unaffected by local land use practices. Temperatures are high in fall and low in spring. pH remains fairly constant near neutrality throughout the year. EC appears to be controlled by alternating variable length cycles of concentration and dilution of saline water near the water table.Water quality evolves along flow path from a fresh Ca2+-Mg2+-HCO3- type water with TDS around 300 mg/l in the north to a moderately saline Ca2+ - Mg2+-SO42- type water with TDS around 8000 mg/1 in the southeast near Las Vegas Wash. TDS varies temporally but does not follow a seasonal pattern. Ion ratios remain constant throughout the year. Water samples are generally oversaturated with respect to calcite, dolomite, and quartz, but are undersaturated with respect to gysum and amorphous silica. Delta D and o18 0 indicate that the water in the shallow aquifer zone originated as principal aquifer zone water.Comparison of shallow to principal aquifer zone data reveals that Ca2+, Mg2+, Cl-, SO42-, TDS, Si02, TOC, PO43-, B, Mn, Se, and tritium are all suitable for use as natural tracers for tracing the downward leakage of water from the shallow to the principal aquifer zone.Comparison of historical shallow aquifer zone data to the data generated during this investigation reveals that water levels in the shallow aquifer zone rose by an average of about 0.5 meter from 1972 to 1989 and that TDS increased by around 570 mg/1 between 1981 and 1989.