Laserfiche WebLink
<br />"r- <br />C"~- <br />C"J <br />N. <br />Co <br />C..J <br /> <br />The estimated total porosities calculated from eqn. 2 range from 0,37 to 0.62; these high values <br />probably reflect the large fraction of clay and. in the upper few feet of alluvium. peaL Had cores been <br />obtained in the more highly permeable zones of the aquifer, estimated total porosities would probably be <br />much lower. However. the coring device used to collect the samples was designed for shallow sampling; <br />hence. deeper cores were not available for estimating porosity. These estimated values were used for the <br />shallowesI part of the saturated aquifer, but the porosities of deeper parts had to be estimated on the basis <br />of grain-size analyses (table 1; Freeze and Cherry, 1979, p. 37). <br /> <br />Recharge to the Detention Basin <br /> <br />Estimating the quantity of recharge entering the detention basin is an important SIeJi'in developing <br />a conceptual model that accurately describes the hydrologic conditions of the study area, The magnitude <br />of recharge was estimated on the basis of the hydrogeologic propenies discussed in the previous sections. <br />Recharge enters the detention basin as (1) ground water from upgradient areas, (2) surface water primarily <br />from Monson Road floodway and upgradient sewage effluent, and (3) precipitation from storms, Because <br />storms are infrequent, their effects are difficult to quantify; therefore, only the first two sources are <br />estimated and considered in this study, <br /> <br />Ground water enters the detention basin through the shallow alluvial aquifer. This water originates <br />primarily from treated sewage effluent in the flood plain adjacent to Las Vegas Wash (wasteway channel). <br />Some additional flow may originate from secondary recharge as a result of lawn and golf-course watering <br />and urban runoff in East Las Vegas (fig. IB). The quantity of ground water entering the detention basin <br />was estimated from Darcy's law using averaged horizontal hydraulic conductivities (table 3) along cross <br />section W-Z (parallel to the flow path) shown in figure 4 and hydraulic gradients determined from the water <br />table map in figure 7. An average volumetric inflow of 4.0 ft2/d (0.0000463 fills) was estimated for a 30-ft <br />aquifer thickness with a unit width of 1 ft. If ground-water inflow is assumed to be constant over the area <br />of the detention basin, the estimate of inflow to the basin through a strip of aquifer 4,200-ft wide is 16,000 <br />ft3/d (0.2 f~/s). The 4,200-ft wide strip was determined by measuring the distance between the arms of <br />the proposed slurry wall along a water-level contour (the estimated capture zone of the detention basin), <br /> <br />Althougb the estimate of ground-water inflow is based on available, information, its accuracy is <br />uncertain because of uncertainties in the aquifer thickness at cluster-well site WG035N (where volumetric <br />flux was estimated), errors in estimating hydraulic conductivity, and the assumption that flow is areally <br />constant over the thickness of the aquifer. Evidence that the estimated inflow rate may be low is provided <br />by pumping and water-level data from the Clark County Sanitation District (CCSD) for an area <br />approximately 3,000 ft northwest of well WG025 (figs. 3 and 4). In this area, the shallow aquifer was <br />dewatered to allow for the construction of below-ground clarifiers. Pumping for this project began in mid- <br />1986 and ended in the spring of 1987. Monitoring wells installed by CCSD to depths of 10 to 20 ft in the <br />vicinity of the pumping wells went dry after 2-4 months of continuous pumping at an estimated constant <br />rate of 2.5 ft3/s, The resulting cone of depression grew during the period of pumping to encompass an area <br />larger than the area of influence of the detention basin. Water-level declines in CCSD monitoring wells <br />more than 3.000 ft from the pumping wells were between 1 and 3 ft, Consequently, the overall gradient <br />toward the detention basin was lowered, resulting in a lower volume of water entering the basin (fig, 7), <br />An estimate of ground-water inflow to the detention basin was made for conditions without the influence <br />of pumping from the CCSD facility. This was done by extending the capture zone of the detention basin <br />upgradient to the pumping zone and estimating the percentage of overlap between the zone of influence of <br />the pumping wells and the capture zone of the detention basin, This percentage was then multiplied by the <br />average pumping rate to determine the potential additional inflow to the detention basin. Using this <br />approach. the additional estimated inflow was 0,8 ft3/s and probably represents a maximum. Thus. estimates <br />of ground-water inflow to the detention basin prior to CCSD pumping probably ranged from 0.2 to 1.0 ft3/s. <br />Seasonal variations in inflow are likely to be small relative to the total volumetric inflow rate, but probably <br />are greatest during summer months when water use and urban runoff are at their peale <br /> <br />-17- <br />