2013-01-29_PERMIT FILE - C1981010A

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Where elevated total dissolved solids (TDS) levels exist in the backfill aquifer, elevated TDS levels will be exhibited in base flows derived from the backfill aquifer. To date, Johnson Gulch is the only surface water drainage that definitely shows mining effects on TDS during base flow conditions. The higher TDS base flow concentrations in Johnson Gulch are expected to gradually decline with time as the TDS concentrations in the backfill aquifer decline. This mechanism is described in more detail under the Groundwater Impacts discussion, Section 4.8.3.2. No measurable changes in water quality attributable to mining are anticipated in the Yampa River. The surface water flow rates typically associated with spoil spring discharges are very small during base flow periods and insignificant as compared to the Yampa River flow. The majority of the base flows from the Trapper Mine drainages will not make it to the Yampa River due to infiltration and evaporative losses along the way. Rain and snowmelt that infiltrate the Horse Gulch fill may discharge from the toe of the fill and enter the surface stream. The discharge would flow to a sediment control pond whose discharge is permitted under the Colorado Discharge Permit System (CDPS). A reasonable assumption is that 25% of annual precipitation will infiltrate the fill. This assumption is based on lysimeter studies of spoil infiltration conducted by the U.S. Geological Survey at the Seneca II Mine in Routt County, approximately 14 miles east of the Trapper Mine (U.S. Geological Survey Water Resources Investigations Report 92- 4187). Using this assumption, the average discharge rate of water to the land surface from the toe of the Horse Gulch Fill can be projected to be 31 gallons per minute for several months each year, calculated as follows: 115.1 acres X 43,560 cubic feet per acre X 25% infiltration rate X 16.49 inches of precipitation per year X 12 inchestfoot = 1,461,075 cubic feet. If this water discharges over an 8 -month period (243 days), the average discharge rate during that period would be 31 gallons per minute, calculated as follows: (1,461,075 cubic feet X 7.5 gallons per cubic foot) / (243 days X 1440 minutes per day) = 1,461,075 gallons / 349,920 minutes = 31 gpm. The quality of the discharges from the fill can be expected to be similar to Trapper spoil water. This discharge rate and quality is not sufficient to cause exceedances of the CDPS permit or cause material damage to the hydrologic balance. Although discharge from the fill may increase dissolved solids in surface stream flows, the increase will not be sufficient to impair the water for livestock or wildlife use. Major constituents in the base flow in Johnson Gulch are being affected by discharge from the backfill aquifer in this area. The total dissolved solids (TDS) concentrations have increased roughly 1000 mg/I over baseline conditions. Typical base flow in Johnson Gulch is approximately 20 gpm (0.045 cfs). Combining this water with typical base flow conditions for the Yampa (200 cfs and 280 mg/l TDS; based 4 -235 ?kluo 6-11z/26#)