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Where elevated total dissolved solids (TDS) levels exist in the backfill aquifer, elevated TDS levels will be <br />exhibited in base flows derived from the backfill aquifer. To date, Johnson Gulch is the only surface water <br />drainage that definitely shows mining effects on TDS during base flow conditions. The higher TDS base <br />flow concentrations in Johnson Gulch are expected to gradually decline with time as the TDS <br />concentrations in the backfill aquifer decline. This mechanism is described in more detail under the <br />Groundwater Impacts discussion, Section 4.8.3.2. <br />No measurable changes in water quality attributable to mining are anticipated in the Yampa River. The <br />surface water flow rates typically associated with spoil spring discharges are very small during base flow <br />periods and insignificant as compared to the Yampa River flow. The majority of the base flows from the <br />Trapper Mine drainages will not make it to the Yampa River due to infiltration and evaporative losses <br />along the way. <br />Rain and snowmelt that infiltrate the Horse Gulch and Buttress fills may discharge from the toe of each fill <br />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 Resources <br />Investigations Report 92-4187). Using this assumption, the average discharge rate of water to the land <br />surface from the toe of the Horse Gulch Fill can be projected to be 31 gallons per minute for several <br />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 not sufficient to cause exceedances of the CDPS permit or cause material <br />damage to the hydrologic balance. Although discharges from the fills may increase dissolved solids in <br />surface stream flows, the increase will not be sufficient to impair the water for livestock or wildlife use. <br />Major constituents in the base flow in Johnson Gulch are being affected by discharge from the backfill <br />aquifer in this area. The total dissolved solids (TDS) concentrations have increased roughly 1000 mg/I <br />over baseline conditions. Typical base flow in Johnson Gulch is approximately 20 gpm (0.045 cfs). ? <br />Combining this water with typical base flow conditions for the Yampa (200 cfs and 280 mg/I TDS; based <br />4-235