REP20177 - Laserfiche WebLink

Home Browse Search

SECTIONSIX Groundwater Modeling 1 from recent water level measurements taken from piezometers installed in the dam. The ' downgradient constant head boundary was estimated fr®m the elevation at that loc:nion and extrapolation of the interpreted potentiometric surface: The constant head along the tailing impoundment was estimated to be 8,730 feet and fke constant head at the downgra~iient ' boundary was estimated to be 8,545 feet. The hydraulic conductivity values used in the groundwater flow model were obtained from analysis of the pumping and slug test data collected as part of this investigation. Analysis of these data indicates a wide range of hydraulic conductivity values for the alluvium, ranging from IS to 238 fdd. A distribution of hydraulic conductivity values used in the calibrated model is ' shown in Figure Sa. The natural aerial groundwater recharge rate used in the model was assumed to be :approximately 10 percent of annual average precipitation. Annual precipitation rates in the vicinity of the Site ' were assumed to be equal to those at Granby, Colorado. According to National Weather Service records, the annual average precipitation at Granby is approximately 22 inches/yeaz (in/yr). Thus, the natural aerial groundwater recharge rate for the.Site was estimated to be 2..2 in/yr. ' The natural groundwater rechazge rate is a highly variable parameter, which depends on a wide range of factors such as weather, altitude, soil condition, topography, canopy cover, and depth to groundwater table. This parameter is difficult to quantify (Sharma, 1986); however, various ' sources have indicated that it can vary from less than 2 percent in the deserts of Ari::ona (University of Arizona, 1980) to more than 40 percent in the mountainous azeas of the ' northwestern United States (Snyder et al, 1994). The natural groundwater recharge rate assumed for this study was estimated without site-specific information, and was based on a general knowledge of the Site. The assumed value is believed to be a reasonable estimate of natural ' groundwater rechazge for afeasibility-level investigation. However, if design of they extraction well system proceeds, it is recommended that site-specific information be collected to verify the assumed natural groundwater recharge rate. 6.3 MODEL RESULTS Figure 8b illustrates the simulated groundwater levels in the alluvial aquifer under e:tiisting conditions. The simulated hydraulic gradient varies from 0.01 to 0.03 ft/ft, which compares reasonably well with the observed hydraulic gradient of between 0.02 to 0.04 ft/fr. however, for ' the purposes of this investigation, the model was not rigorously calibrated because there are insufficient groundwater level measurements to completely define the potentiometri~: surface of groundwater in the alluvial aquifer throughout the model area. Nevertheless, the values assigned ' in the current mode] result in steady-state simulations that reasonably match the existing data for groundwater elevations and hydraulic gradients. Extraction wells were simulated at a variety of locations along the north access road to estimate ' the pumping rate(s) and well spacing needed to capture groundwater flowing through the alluvial aquifer. Figure 8c shows the results of aparticle-tracking simulation for the extraction well layout that best achieves the goal of capture with a minimum number of wells. This figure ' clearly illustrates that the specified extraction well system is capable of capturing all the flow particles placed upstream. The modeling results indicate that 17 wells spaced approximately 50 feet apart with a combined capacity of approximately 82 gpm (i.e., approximately S l;pm each) ' will acwhieve capture. Figure 8d provides details of the pumping rate needed for each well. ~~` MO6YIIFREI~EINWP~IECfS6MA:A1J1E11~ERSON_YII~FFAl151R_WI.O PPOlpE1NAEVI5E0 TEQ1 YEY6HI D[t1,9M ]tBR16-2 ~/-Y1I