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Environmental and Engineering Geoscience; November 1998; v. 4; no. 4; p. 455-477
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Characterization of urban surfacing ground water in Northwest Henderson, Clark County, Nevada

Dwight L. Smith, and Johannes C. Guitjens

Stantec Consulting, Sparks, NV, United States
University of Nevada at Reno, United States

Return flow from urban landscape irrigation is now a major recharge source in northwest Henderson, Nevada, and has created shallow and locally surfacing poor quality ground water. To evaluate the physical and geochemical evolution of the surfacing ground water, a regional evaluation was performed, which included construction of 31 shallow monitoring wells and chemical testing of waters from 46 locations. Results show an upper-most stratum, 3 to 6 m thick, geochemically characterized by enriched 18 O and 2 H isotopes, elevated 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 this arid environment contain abundant soluble salts, which along with evapo-concentration processes have resulted in degraded water quality. Mineral dissolution appears to be the dominant concentrating process, with PHREEQE predicted solubility for gypsum (CaSO 4 .2H 2 O), halite (NaCl), sylvite (KCl), sepiolite (Mg 4 Si 6 O 15 (OH) 2 .6H 2 O), anhydrite (CaSO 4 ), and mirabilite (Na 2 SO 4 .10H 2 O). Calcium concentrations remain only moderate due to apparent cation exchange with sodium and magnesium and precipitation of calcite. Factors contributing to the development of near-surface ground water include 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 lateral hydraulic barrier to ground-water flow, contributing to up-gradient surface 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 mitigate surfacing of high salinity ground water.

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JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
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