2013-01-29_PERMIT FILE - C1981010A

Home Browse Search

Third White Sandstone wells which exist within one mile downgradient of the eastern portion of the Trapper Mine permit are permitted wells 80998 and W624-74. These two Third White Sandstone wells exist downstream of proposed mining operations in the Flume Gulch drainage. Mining is not expected to significantly affect the water levels in these wells. The closest Third White Sandstone well (80998) to the eastern mine pits is approximately 6000 feet north of the planned mine cuts in this area. Additional Second and Third White Sandstone wells exist further from the permit boundary. These wells are unlikely to see water levels affected by the Trapper mining activity. Continued monitoring in Third White Sandstone wells GC-2 and GP-9 will be important in defining whether any water quality impacts develop in this aquifer due to mining activities. Second White Sandstone wells in this area are very unlikely to be affected because their outcrop is north of the northern limits of the Trapper pits. Some potential exists for water quality to be affected in Third White Sandstone wells in areas north of mining operations in the Flume Gulch drainage. Elevated TDS levels in groundwater could persist for a prolonged period of time following mining as previously discussed. As this groundwater moves downgradient into the Third White Sandstone, some ambient water quality constituent concentrations could see increases. The percent pyrite concentration in A East pit and the Flume drainage will be reflected in the temporal duration of elevated groundwater quality constituent concentrations observed in this area. TDS and sulfate concentrations from the backfill in this drainage can be expected to fall within ranges previously established in other backfill monitoring wells at Trapper. The overall quality of this water will be dependent upon the degree of dilution that occurs as a result of underburden inflows, however, some degradation in groundwater quality in the Third White Sandstone is possible. Pre-mining sulfate concentrations as defined at wells GC-2 and GP-9 range from approximately 600 to 800 mg/I with TDS values ranging from approximately 1300 to 1400 mg/I. Considering the range of permeability values likely to exist in the Third White Sandstone, calculations characterizing groundwater movement in this area suggest a plume of higher TDS water could migrate at a rate varying from tens to hundreds of feet per year. At a projected groundwater flow rate of approximately 200 feet per year, the plume could be expected to reach the GC-2 monitoring well approximately 10 years after backfilling and resaturaticn of the spoil aquifer in A East pit is complete. Elevated TDS levels could extend to monitoring well GP-9 roughly 40 years after resaturation occurs. The Big Bottom syncline lies north of the Trapper project area in the Flume Gulch drainage and will limit the extent of the migration to the north. The plume can be projected to move generally northward with the structural orientation of the Third White Sandstone acting to change the prevailing direction of groundwater flow to the northwest as one moves further from the mined area. Ultimately, waters in the 4-2381 PRA q JuM