Characterization of urban surfacing ground water in northwest Henderson, Clark County, Nevada

Return flow from urban landscape irrigation is now a major recharge source in northwest Henderson, Nevada, and has created shallow and locally surfaci ng poor quality ground water To evaluate the physical and geochemical evolu tion of the surfacing ground water, a regional evaluation was performed, wh ich included construction of 31 shallow monitoring wells and chemical testi ng of waters from 46 locations, Results show an uppermost stratum, 3 to 6 m thick, geochemically characterized by enriched O-18 and H-2 isotopes, elev ated tritium levels (29 to 42 pCi/L), nitrate concentrations up to 129 mg/L , and TDS concentrations ranging from 3,000 mg/L to in excess of 12,000 mg/ L, Dominant ions are sulfate, chloride and sodium. Near-surface soils in th is arid environment contain abundant soluble salts, which along with evapo- concentration processes have resulted in degraded water quality, Mineral di ssolution appears to be the dominant concentrating process, with PHREEQE pr edicted solubility for gypsum (CaSO4. 2H(2)O), halite (NaCl), sylvite (KCI) , sepiolite (Mg4Si6O15(OH)(2). 6H(2)O), anhydrite (CaSO4), and mirabilite ( Na2SO4. 10H(2)O). Calcium concentrations remain only moderate due to appare nt cation exchange with sodium and magnesium and precipitation of calcite, Factors contributing to the development of near-surface ground water includ e a natural shallow water table and fine-grained soils which are limited in capacity to transmit water, A compaction fault system appears to form a la teral hydraulic barrier to ground-water flow, contributing to up-gradient s urface discharge, Varying degrees of mixing of deeper ground water with the low quality upper stratum are present, especially in the compaction fault zone, The study supports the hypothesis that excessive urban irrigation is probably responsible for evolution of surfacing poor quality ground water, Management of urban irrigation practices may ultimately be necessary to mit igate surfacing of high salinity ground water.