2010-06-04_PERMIT FILE - C1982056A (10)

Home Browse Search

Raw data compiled from the leach study are in the form of spoil and leach solution quantities, EC, and cumulative leachate weight measurements for each incremental stage of the leaching process. Each of the 218 • incremental stages was measured for EC correlation of the incremental EC with TDS values of the leachate samples, results in a TDS /EC ratio of 0.73. Using this ratio, multiplication of incremental EC values by 0.73 will give an estimated TDS value. Initial concentrations of leachate are in the order of 4000 mg/l, which is very similar to observed concentrations of actual spoil, discharges at the adjacent Eckman Park Mine. Water chemistry characteristics of leachate and spoil discharge are also very similar. This indicates that the leaching tests yield a reasonably good prediction of initial water quality that might be expected to occur in the caved overburden material after flooding of the mine workings. The approximately 50 percent higher TDS values observed in some areas of the mine indicate that the flushing times calculated from these tests may have to be increased by as much as 50 percent. The primary purpose of the leaching study was to determine the time interval required for the solute concentration of the leachate to return to the baseline level. The leaching study indicated that significant decreases in EC levels occur when the ratio of the volume of leachate to the weight of overburden material is about 0.9 1 /kg (Exhibit 38, Figure 3). Using a bulk density of 1.45, this is equivalent to leaching of approximately 0.6 and 1.0 (r) pore volumes of the overburden material. Based on the results of the leach study, when the volume of water which has percolated through the caved overburden material is equivalent to about 1.5 1/kg of overburden material, the ground water TDS value will be at the baseline level, and no additional salt concentrations will occur above baseline at that point. The leaching ratios to reduce TDS levels to a given value can be converted to time by applying factors for a rate of water movement through the caved overburden, and assuming values for the pore volume of the caved areas. This is determined from the equation: t = rV R where: t = time in years to reduce TDS to required level r = water volume /caved material pore volume ratio to reduce TDS to required level (from leach test results) V = Pore Volume of caved material in acre -feet R = Rate of movement of ground water through caved material in acre -feet per year Recharge to the caved material from the undisturbed overburden unit was not included in the calculation since it amounts to a negligible portion of the total. The pore volume of the caved material, which is likely to exist after mining, is estimated to be about 186,000 acre -ft (V). This number is derived in Dewatering and Potentiometric Drawdown in Bedrock Units This is based on a 24 square mile mining area, a thickness for the caved zone of about 100 feet and an effective porosity of 10 percent. After re- establishment of potentiometric levels, the recharge rate to the Wadge overburden should be similar to pre- mining conditions, which has been estimated at 12.3 acre -ft per year (R). These values were assigned for this calculation. Solving for "t" in the above equation for leaching of 0.6 and 1.0 pore volumes, results in time factors of 9,100 and 15,000 years, respectively. When taking into account the higher TDS values measured in the mine workings, this calculation indicates that if ground water flow rates through the caved material are about 12.3 acre -ft per year, it will take at least 14,000 years of ground water TDS levels to start declining significantly and 23,000 years to return to baseline levels. As potentiometric levels in the Wadge overburden recover to premining conditions, lateral ground water flows in the unit from recharge areas in the southern and western margins of the Twentymile Park Basin to the major discharge area along the northeast margin of the basin will also tend to be re- established. This will result in movement of lower quality ground water from areas directly overlying mined zones into the undisturbed overburden units to the east of the proposed mine area. PR09 -08 2.05 -144 04/27/09