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Technical Adequacy Review <br /> Two Rivers Application <br /> August 30,2022 <br /> Page 2 <br /> and were not used in aquifer characterization other than measured water levels. The soils <br /> strength and grain size data were used in slope stability analyses and K estimation. <br /> 3. Comment 8— How many vertical layers are in the model?What are the thicknesses?Two <br /> layers were used to simulate unconsolidated alluvial deposits and bedrock. Two hydraulic <br /> conductivity zones were identified for the coarse alluvial deposits and finer grained upper <br /> terrace deposits located near the northwest model boundary. The thickness of the <br /> unconsolidated deposits varied between 12 and 44 feet in the vicinity of the mine. A plate <br /> depicting vertical layers and assigned conductivities is provided in the updated report. <br /> 4. Comment 9 — Is recharge from precipitation accounted for in the model, or is its impact <br /> assumed to be negligible? Lindsey et al., 1982, provides an average annual evaporation <br /> from shallow lake evaporation map, which indicates the study area to have between 48 and <br /> 52 inches of evaporation. Given the high potential evaporation a recharge of 10% of the <br /> annual precipitation total of 15 inches per year(1.5 inches)was used. <br /> 5. Comment 10— Please discuss the validity of the model boundary conditions in the light of <br /> the final pit configuration (which is assumed to be that shown on the maps in the PAP). <br /> The central pit configuration was changed after my analysis without my knowledge. I have <br /> modified the constant head boundary in the central pit with little change to the predicted <br /> drawdown. An updated analysis is provided. <br /> 6. Comment 11 — Please discuss the characterization of the pre-mining water table. How <br /> reliable is the data from MW-1? How do you account for the steeper gradient? Are there <br /> any other data points in the north study area to improve the characterization?AWES was <br /> provided roughly four years of monthly water level data collected from the 12 piezometers. <br /> The levels were collected by using an electric water level meter marked to 1/100 of a foot. <br /> The top of casing and ground surface were surveyed by a professional land surveyor and <br /> water levels typically collected by a registered professional engineer. The quality of the data <br /> is considered excellent. The approximate well locations within the property boundary are <br /> depicted on Figure 1. As can be seen the spatial distribution of the wells provide excellent <br /> gradient control. The boring log of MW-1 shows that water was not encountered during <br /> drilling (indicating low permeability) and that the water table surface is located in silty, <br /> clayey sand. These soils will have orders of magnitude less permeability than clean sands or <br /> gravels which account for steeper gradients. Well records from the Division of Water <br /> Resources were researched and only one well record in close proximity to the northern <br /> model boundary was available. The well log is provided as Attachment B and the well <br /> location is depicted on Figure 1. The well log shows a sustained yield of 5 gallons per minute <br /> over a two hour interval with a drawdown of 7 feet, with the water table located primarily in <br /> shale. Using a form of Darcy's Law a permeability (K) of 13.76 feet per day is calculated <br /> based on reported data. The topographic gradient is dramatically steeper north of the <br /> alluvial valley and dips sharply to the south, southeast — groundwater flow direction and <br /> gradient typically mimic topography, which accounts for the steeper gradients. <br /> �w�s 4SC9 Four Star Court, Fort Collins,CC SCJ24—91C-b9C-3SC1 <br />