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<br />'t- <br /> <br />c."-2'" <br /> <br />(\1 <br />C\L <br /> <br />r" <br />--' <br /> <br />'::;J <br /> <br />" <br /> <br />, ' <br /> <br />end of the dike. two outlet structures allow surface water to leave the detention basin. The Tropicana <br />f100dway also enters the detention basin, but that inflow leaves the basin through an outlet structure farther <br />to the east (fig. 3), The planned slurry wall would not be constructed unless simulation results suggest that <br />the detention-basin strategy would work as proposed. If constructed, the slurry wall would extend vertically <br />beneath the dike to the base of the aquifer except along the leg adjacent to the Monson Road f100dway <br />(shown with hachures on fig. 3), The detention basin would then act as a catchment for ground water that <br />flows in a southeastward direction (the ground-water level nearly parallels the land surface). <br /> <br />The first phase of the study is described by Emme and Prudic (1991). Its purpose was to evaluate <br />the areal, vertical, and seasonal variations in ground-water quality, and to describe the proc~ses and factqrs <br />controlling this quality. The effort involved (I) drilling numerous test holes and installing 'observation wens, <br />which was done principally by the U.S. Bureau of Reclamation, (2) collecting lithologic samples for <br />mineral-composition analysis, (3) making water-level measurements at the test wells, and (4) collecting <br />water-quality samples for chemical analysis. <br /> <br />Purpose and Scope <br /> <br />This study. done by the U.S. Geological Survey in cooperation with the U.S. Bureau of Reclamation, <br />was made to evaluate'the effectiveness of the proposed detention-basin strategy in reducing the tonnage of <br />salts entering Las Vegas Wash from the shallow ground-water system in the Whitney area. The study,' <br />which took place from spring 1986 to summer 1987, was divided into two phases--a chemical-analysis and <br />synthesis phase and a solute-transport modeling phase. <br /> <br />The second phase of the study is described in this report Its purpose is to develop both planimetric <br />and cross-sectional solute-transport models to examine the effectiveness of the proposed detention basin. <br />These models use both hypotheti,cal and estimated aquifer and solute properties and boundary conditions-- <br />detennined during both phases of this study--to evaluate whether the dike and proposed slurry wall could <br />reduce solute loads entering Las Vegas Wash. Various aquifer properties were tested to detennine the <br />optimum conditions under which the proposed strategy would be most effective,; The scope of this phase <br />of the study involved (I) making aquifer tests at selected test wells to determine the hydraulic conductivity <br />of the shallow alluvial aquifer, (2) collecting soil cores for analysis of total porosity and particle size. <br />(3) making measurements of flow in floodways and from nearby pumping wells to estimate ground-water <br />and surface-water inflow to the area, and (4) mapping phreatophyte and hydrophyte areas to delineate the , <br />areal distribution of evapotranspiration, <br /> <br />Acknowledgments <br /> <br />Appreciation is expressed to D. Art Tuma of the U.S. Bureau of Reclamation, who provided maps, <br />coordinated U.S. Bureau of Reclamation activities. and provided considerable insight regarding the <br />detention-basin strategy. The author also would like to thank Clifford L Voss of the U.S. Geological Survey <br />for his insightful comments, suggestions, and efforts that have led to realistic and accurate conceptual <br />simulations of the study area. <br /> <br />-7- <br />