WSP04273 - Laserfiche WebLink

Home Browse Search

'. ~ C"-l N (;J o o ,', "~. Shallow Ground Water in the Whitney Area, Southeastern Las Vegas Valley, Clark County, Nevada Part n. Assessment of a Proposed Strategy to Reduce the Contribution of Salts to Las Vegas Wash By Thomas J. Burbey .......- ABSTRACT A continuing increase in the contribution of dissolved solids to Las Vegas Wash from various sources, including ground-water seepage, has resulted in the proposal of several salt-reduction strategies. One such strategy is the construction of an impoundment, or detention basin, consisting of a dike structure above ground and a slurry wall beneath the dike that would extend,to the base of the shallow alluvial aquifer, The dike-and-wall system was expected to function by damming deeper, more saline flow in the shallow aquifer, while allowing only shallow, less saline flow to leave the detention basin. Theoretically, the shallow flow would become increasingly fresh over time, because it would no longer come in contact with the Muddy Creek Formation at the base of the shallow aquifer. which heretofore has been considered the principal source of salts in the aquifer. 1bis report describes the results of a study made to detennine whether the proposed slurry wall could be effective in inhibiting saline ground water from seeping to the wash under various possible geohydrologic conditions. Field data were collected and analyzed and fluid- and solute-transport models were developed to test the effectiveness of the proposed strategy. X-ray analyses indicate that salt-bearing minerals, panicularly gypsum, are relatively abundant in the alluvial deposits that make up the aquifer, and are largely responsible for the high dissolved-solids concentrations. 1l1ree conceptual and mathematical models were developed to analyze the effects of the proposed slurry wall. Simulations of ground-water flow and solute transport were made using a finite-element model. Simulation indicates that most ground-water flow is through the center part of the 3D-foot-thick aquifer (between depths of 10 and 25 feet) and therefore does not come in contact with the underlying Muddy Creek Formation; yet, the ground water is high in dissolved-solids concentration, In areas dominated by phreatophytes, simulations have shown that evapotranspiration is the major process influencing the distribution of dissolved solids. In areas dominated by hydrophytes and affected by effluent from upgradient sewage-treatment plants. mixing of the more dilute effluent with the more saline shallow ground water is the major influencing process. Simulation results also indicate that the slurry wall would not induce upward flow in the shallow aquifer farther upgradient than under present conditions. Furthermore, the quantity of flow at the base of the aquifer behind the slurry wall would not be reduced; rather, the flow would be deflected upward immediately adjacent to the waiL Results indicate that the aquifer is too thin for the proposed slurry wall to function as planned. Simulation indicates that much better results would be obtained with the slurry wall if the aquifer was one order of magnitude thicker. Thus, the study indicates that the proposed slurry wall would not decrease the dissolved-solids load entering Las Vegas Wash from the existing shallow aquifer. INTRODUCTION The study area, or Whitney area as it is referred to herein. is situated in the southeast part of Las Vegas VaHey in Clark County, southern Nevada (fig, lA), The Whitney area received its name from the U,S. Bureau of Reclamation's Whitney Verification Program, which in turn is named after the small community of Whitney on the outskirts of southeastern Las Vegas. The l_mi2 study area (fig. lB) represents only a small part of the overall valley, which encompasses about 1,540 mi2 (fig, lA). -1-