2010-06-25_HYDROLOGY - M1977300 - Laserfiche WebLink

Home Browse Search

June 25, 2010 12 of 15 The bedrock surrounding the mine has very low permeability, which prevents any significant movement of groundwater. Shaft Sinking. In the late 1950's, the shaft was being sunk from the Steve adit. No dewatering was required until the shaft reached a depth of 250 feet below the Steve level. The first dewatering pump was installed in September, 1958, when the total shaft depth was 285 feet and groundwater pumping was required for the first time. The mine was dewatered by pumping for only two hours per day at 15 to 20 gallons per minute (gpm), which is the equivalent of a sustained pumping rate of 1.25 to 1.67 gpm. If the mine had been in hydraulic connection with the creek, pumping rates would have been significantly higher. Underground Packer Tests. Packer permeability tests were conducted underground in 1999, before the mine shut down. The results of all 22 tests gave a geometric mean hydraulic conductivity of 4.7 x 10.7 cm/sec. The lowest packer test result was 9.9x10-8 cm/sec and the median value was 2.7x10-7 cm/sec. For comparison, the EPA requires that municipal landfill covers have a hydraulic conductivity less than 1x10'5 cm/sec. Soil barriers in hazardous waste landfills are typically compacted to no greater than 1x10"7 cm/sec. Mine Inflow. At its total maximum depth of 2,200 feet, when the mine was fully dewatered groundwater inflow to the Schwartzwalder mine was only 189 gpm. These very low flow rates indicate that rock mass has very low permeability, and there are no high-permeability faults or conduits that could convey water to (or from) the underground workings. If high-permeability conduits existed, then inflow to the mine would have been significantly greater. WRT Pilot Test. Sustained pumping rates during a pilot test in July and August, 2007 (with the pump set at 27 ft below the collar of the #2 shaft in the Steve Level) were only 1.2 gpm. It should be noted that the entire mine workings were in connection with the shaft, with no resistance to flow in the workings themselves. Calculations that evaluated the mine pool as a 2,200 feet deep "large diameter well" show that an overall bedrock permeability of 2.8x10-7 cm/sec would produce the observed 1.2 gpm pumping rate with 14.6 feet of drawdown (Whetstone, 2007). Saturated Zone Above the Mine. Monitoring wells MW 10 and MW 11 are located on the hillside above the mine (Figure 4) and contain water because the rock is too tight to drain into the unsaturated workings below. The low permeability of the rock mass prevents groundwater from draining from near surface, where the wells are located, downward into the unsaturated upper mine workings. The same process prevents groundwater in the mine pool from draining toward Ralston Creek, or any other point. Creek Flow. Ralston Creek was never drained during mining, although the mine was dewatered to nearly 2,200 feet below the creek level. If a strong hydraulic connection existed between the mine and the creek, then the creek flow would have dried up as a result of mine dewatering during operations. Instead, no effect on flow in the creek was discernable. Mine pool dewatering and treatment at this time will preclude the opportunity to demonstrate that treatment of the alluvial groundwater will address the Ralston Creek issues, and that mine dewatering and treatment are not necessary. Based on technical information compiled by Cotter and its consultants, Cotter believes that treatment of groundwater in the alluvium and fill will be effective in reducing uranium concentrations in Ralston Creek. Cotter is committed to pumping and treating from Sump 1 by July 31, 2010. Further, if pumping from Sump 1 does not capture sufficient alluvial groundwater to meet drinking water standards in the creek, Cotter is committed to pumping and treating groundwater from additional locations in the alluvium and fill. Cotter needs the opportunity to demonstrate that this approach will be successful in lowering uranium