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Raw data compiled from the leach study are in the form of spoil and leach solution quantities, EC, and <br />cumulative leachate weight measurements for each incremental stage of the leaching process. Each of the 218 <br />incremental stages was measured for EC correlation of the incremental EC with TDS values of the leachate <br />samples, results in a TDS/EC ratio of 0.73. Using this ratio, multiplication of incremental EC values by 0.73 <br />will give an estimated TDS value. initial concentrations of leachate are in the order of 4,000 mg/l, which is very <br />similar to observed concentrations of actual spoil, discharges at the adjacent Eckman Park Mine. Water <br />chemistry characteristics of leachate and spoil discharge are also very similar. This indicates that the leaching <br />tests yield a reasonably good prediction of initial water quality that might be expected to occur in the caved <br />overburden material after flooding of the mine workings. The approximately 50 percent higher TDS values <br />observed in some areas of the trine indicate that the flushing times calculated from these tests may have to be <br />increased by as much as 50 percent. <br />The primary purpose of the leaching study was to determine the time interval required for the solute <br />concentration of the leachate to return to the baseline level. The leaching study indicated that significant <br />decreases in EC levels occur when the ratio of the volume of leachate to the weight of overburden material is <br />about 0.9 llkg (Exhibit 38, Figure 3). Using a bulk density of 1.45, this is equivalent to leaching of <br />approximately 0.6 and 1.0 (r) pore volumes of the overburden material. Based on the results of the leach study, <br />when the volume of water which has percolated through the caved overburden material is equivalent to about <br />1.5 1 /kg of overburden material, the ground water TDS value will be at the baseline level, and no additional salt <br />concentrations will occur above baseline at that point. <br />The leaching ratios to reduce TDS levels to a given value can be converted to time by applying factors for a rate <br />of water movement through the caved overburden, and assuming values for the pore volume of the caved areas. <br />This is determined from the equation: <br />t =rV <br />R <br />where: <br />t = time in years to reduce TDS to required level <br />r = water volume /caved material pore volume ratio to reduce TDS to required level (from leach test <br />results) <br />V = Pore Volume of caved material in acre -feet <br />R = Rate of movement of ground water through caved material in acre -feet per year <br />Recharge to the caved material from the undisturbed overburden units was not included in the calculation since <br />it amounts to a negligible portion of the total. <br />The pore volume of the caved material (Wadge Seam) likely to exist after mining is estimated to be about <br />186,000 acre -ft, and that of the Wolf Creek Seam is estimated to be about 50,000 acre -ft, for a total pore volume <br />of about 236,000 acre -ft (V). These numbers are derived in Dewatering and Potentiometric Drawdown in <br />Bedrock Units. This is based on a 24 square mile mining area for the Wadge Seam and 7 square miles for the <br />Wolf Creek Seam, a thiekness for the caved zone of about 100 feet and an effective porosity of 10 percent. <br />After re- establishment of potentiometne levels, the recharge rates to the Wadge overburden and Wadge /Wolf <br />Creek Interburden should be similar to pre - mining conditions, which have been estimated, respectively, at 12.3 <br />and 0.5 acre -ft per year, for a total recharge rate of 12.8 acre -ft per year (R). <br />These values were assigned for this calculation. Solving for "t" in the above equation for leaching of 0.6 and <br />1.0 pore volumes, results in time factors of 11,100 and 18,500 years, respectively. When taking into account the <br />higher TDS values measured in the mine workings, this calculation indicates that if ground water flow rates <br />through the caved material are about 12.8 acre -ft per year, it will take at least 18,500 years of ground water TDS <br />levels to start declining significantly and 30,800 years to return to baseline levels. <br />As potentiometric levels in the Wadge and Wolf Creek overburden recover to premining conditions, lateral <br />ground water flows in the unit from recharge areas in the southern and western margins of the Twentymile Park <br />TR13 -83 2.05 -144 11103114 <br />