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h = average height of spoil in feet <br /> <br />= infiltration rate of water from precipitation into spoils in feet per year <br />Recharge to the spoil from the undisturbed overburden aquifer was not included in the calculation since it amounts <br />to a ttegli;ible portion of the total. Since the second dimension remains a constant and would be factored out, the <br />vertical dimension was used instead of an areal dimension. <br />Values of b = 1.45 gm/cc, p = 0.45, h = 60 feet, and i = 3 inches/yr (0.25 ft/yr) were assigned for this <br />calculation. Solving fort in the above equation for leaching ratios of 0.9 and 1.0 I/kg results in time factors of <br />141 and 235 years, respectively. This calculation indicates that if infiltration rates into the reclaimed backfill <br />remain at about 3 incites per year, then i[ will take at lease 141 years for groundwater TDS levels to start declining <br />si~_nificantly and 235 vears to return to baseline levels. If the infiltration rate reduces as a result of backfill <br />compaction and revegetation, then the time periods will be correspondingly longer. <br />Rainwater and snowmelt, which infiltrate into the backfilled mine pits, percolate downward to the mine floor and <br />Flow down-slope to the highwall. There it pools up against the less permeable undisturbed overburden aquifer <br />providing limited recharge to the aquifer. As described above, water that passes through backfill material has an <br />.initial solute concentration of about 4,000 mg/I. The chemical character of the leachate is dominated by calcium, <br />magnesium, and sulfate. Solute concentrations will start to decline after about 140 years and will return to <br />baseline levels after about 235 years. <br />Recharge of water that has passed through disturbed overburden materials causes elevated concentrations of <br />certain cltetnical species, particularly sulfate, calcium, and magnesium, in the groundwater of the undisturbed <br />overburden aquifers downgradien[ from the mining operations. The plume of higher TDS groundwater will move <br />downeradient in the general <br />r l <br />L <br />1996 Permit Ren. Permit Renewal 780-144-18 Revised 3/18/97 <br />