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Page 4-235, insert as new second paragraph <br />Rain and snowmelt that infiltrate the Horse Gulch and Buttress fills may discharge from the toe of each <br />fill and enter surface streams. The discharges would flow to sediment control ponds whose discharges are <br />permitted under the Colorado Discharge Permit System (CDPS). <br />A reasonable assumption is that 25% of annual precipitation will infiltrate a fill. This assumption is based <br />on lysimeter studies of spoil infiltration conducted by the U.S. Geological Survey at the Seneca II Mine in <br />Routt County, approximately 14 miles east of the Trapper Mine (U.S. Geological Survey Water <br />Resources Investigations Report 92-4187). Using this assumption, the average discharge rate of water the <br />to the land surface from the toe of the Horse Gulch Fill can be projected to be 31 gallons per minute for <br />several months each year, calculated as follows: <br />115.1 acres X 43,560 cubic feet per acre X 25% infiltration rate X 16.49 inches of <br />precipitation per year X 12 inches/foot = 1,461,075 cubic feet. <br />If this water discharges over an 8-month period (243 days), the average discharge rate <br />during that period would be 31 gallons per minute, calculated as follows: (1,461,075 <br />cubic feet X 7.5 gallons per cubic foot) / (243 days X 1440 minutes per day) = 1,461,075 <br />gallons / 349,920 minutes = 31 gpm. <br />The Buttress Fill will cover 49 acres, or 43% of the area of the Horse Gulch Fill; therefore, the average <br />discharge rate of water to the land surface from the toe of the Buttress Fill can be projected to be 13 <br />gallons per minute. <br />The quality of the discharges from the fills can be expected to be similar to Trapper spoil water. These <br />discharge rates and quality are/are not sufficient to cause exceedances of the CDPS permit or cause <br />material damage to the hydrologic balance. <br />Page 4-238f, replace existing last sentence of first paragraph <br />Discharge from the North Horse Gulch spring is/is not expected to be affected by the Horse Gulch fill <br />because the spring is/is not located downstream from the fill. <br />Page 4-238h, insert as new second paragraph <br />Water from rain and snowmelt that infiltrates the Horse Gulch and Buttress fills is/is not expected to <br />significantly infiltrate bedrock stratigraphic units underneath the fills because the bedrock units have low <br />permeability and the fills are/are not constructed on sloping land which should cause the water to flow to <br />the toe of each fill where it will discharge to the surface. <br />Page 4-238i, insert as new last paragraph <br />The preceding section titled "Post-mine Surface Water Quality" describes possible discharges of water <br />from the toe areas of the Horse Gulch and Buttress fills. Based on that description, these discharges