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1 1 1 1 ^ I SIX 6.1 OBJECT{VES 6roun1 The primary objective of groundwater modeling was to assess whether an ea is feasible. L` so, the model was to be used to estimate exVaction well spacii required to achieve capture of the shallow groundwater migrating through ti Values of the aquifer hydraulic properties (i.e., hydraulic conductivity, transtrus~,..,,, _~ yield) used for modeling were estimated based on analyses of the pumping and slug test data collected as part of this investigation. 6.2 MODEL DEVELOPMENT Groundwater flow within the alluvial aquifer downgradient of the tailing impoundment was modeled using the U.S. Geological Survey's (USGS) modulaz three-dimensional finite:- difference groundwater flow model MODFLOW (McDonald and Hazbaugh, 1988). This model was chosen because it can simulate very complex groundwater flow systems and it is widely accepted by groundwater professionals and regulatory agencies. In addition, the particle tracking model PATH3D (Zeng, 1991) was used to simulate groundwater capture by the extraction well system. The groundwater flow model was constructed specifically to estimate the pumping rates and well spacing necessary to capture groundwater migrating through the alluvial aquifer. Findings of previous hydrogeologic investigations indicate that little vertical exchange of groundwater occurs between the alluvium and the underlying Troublesome Formation. So, a one-la;~er MODFLOW model was developed that simulates groundwater flow solely through the alluvial aquifer. The mode] domain was selected to cover a sufficient area such that its boundaries would not be significantly affected by implementation of the extraction well system. Figure 8a illusn~ates the boundaries of the model domain. The entire model domain covers an area of approximately 1 square mile (3,500 ft x 7,860 ft). Grid spacing varies from 25 to 100 feet with the smalilest cells placed at the proposed extraction well locations. The entire model domain consists of 72 columns and 153 rows, which provides a total of 11,016 cells. Lithologic logs of geotechnical borings installed along the proposed slurry wall alignment (Woodwazd-Clyde, 1997) and wells installed as part of this investigation were used to estimate the bottom elevations of the alluvial channel, which forms the base of the model layer. According to these logs, the bottom elevation of the alluvium is approximately 8,555 feet, with a thickness ranging from 25 to 30 feet. A thickness of 25 to 30 feet was assumed throughout the model area where the alluvium occurs. At the tailing impoundment, the bottom elevation of the model was determined by subtracting the alluvium thickness and the dam height from the groundwater surface elevation. Based on the geotechnical boring logs, the alluvial chamie! was assumed to extend from TH-6 on the west and TH-8 on the east. A large portion of the model domain was designated inactive where bedrock outcrops. These areas were assumed to have no impact on groundwater flow within the alluvial aquifer. Boundary conditions include constant head boundaries placed upgradient at the tailing impoundment and approximately 1,800 feet downgradient from Ute Park Pump Station (see Figure 8a). The upgradient constant head boundary at the tailing impoundment was es[irnated 1~1~+ \156Y1iFILEf\EIMPq~IECISEMU•W,IIpgER$CI1_MILI~iWll16 WI.O ry1W_pFJ.MFEVI$Ep TEG YEM}RI.DXl1(19i Y.,<pM 6-1