2020-08-21_REVISION - C1994082 (36)

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in the Grassy Creek and Sage Creek drainages cause variation in the predicted drawdowns. The radial distance to a one-foot drawdown in the Wadge Overburden was predicted to be 401 feet in the Grassy Creek drainage and 564 feet in the Sage Creek drainage. The one- foot drawdown radius in the Wadge Coal was predicted to be 369 feet in the Grassy Creek drainage and 468 feet in the Sage Creek drainage. Radial distances to the one-foot drawdowns in the Wolf Creek Overburden and Wolf Creek Coal in the Sage Creek drainage were predicted to be 452 and 491 feet, respectively. Because the drawdowns due to pit inflows were to be mostly contained within the leasehold area, and adjacent water users are located beyond the one-foot drawdown contours, no significant impact on groundwater use due to pit inflows was predicted. Impacts of Production Well Seneca Property, LLC predicted that maximum drawdowns in the Trout Creek aquifer resulting from nine years of pumping in the Yoast Production Well would be 186 feet. After 20 years of recovery, the residual drawdown within one mile of the well is predicted to be less than two feet. The radial distance to the one-foot drawdown contour after 20 years of pumping was predicted to be 10 miles. The only known Trout Creek wells within this radius are associated with the Twentymile Coal Mine and are between seven and ten miles from the Yoast Mine well. The drawdown at this distance from the Yoast Mine well should not significantly impact these wells. Streamflow Depletion Predictions Streamflow depletions in the Yampa River, Fish Creek, Middle Creek, and Trout Creek channels due to changes in the groundwater regime were all predicted to be less than 0.55 acre-feet per year. However, streamflow depletions in Sage Creek and Grassy Creek were predicted to be higher: 4.9 acre-feet per year in Sage Creek and 5.8 acre-feet per year in Grassy Creek. Impact of Replaced Spoil on Groundwater Seneca Property, LLC predicted that the replaced spoil would have a 25% higher hydraulic conductivity than the undisturbed material. Other than that, the primary effect on groundwater would be degradation of quality with respect to total dissolved solids (TDS). It was predicted that the increase in TDS concentrations in the bedrock aquifers will range from 70% in the Wadge Overburden to over 300% in the Wadge Coal. These predictions are for groundwater in close proximity to the re-saturated spoil, within the permit area. As groundwater migrates away from the re-saturated spoil, mixing, dilution from unaffected recharge, and geochemical changes will decrease the TDS concentrations. Well yields in these bedrock units, the Wadge Coal and Overburden and the Wolf Creek Coal and Overburden, are insufficient for irrigation purposes. The alluvial aquifers that will be affected by the re-saturated spoil include the Armand Draw, Upper Grassy Creek, Lower Grassy Creek, and Sage Creek alluvium. Within the Grassy Creek basin, above the confluence with Scotchman's Gulch, the magnitude of the TDS increase will be high immediately below the re saturated spoil, but will be undetectable a short distance beyond within the Upper Grassy Creek Alluvium. This diminution of TDS will result from low hydraulic conductivities in the Grassy Creek 17