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Battle Mountain Resources <br />November I, 2000 <br />Page 3 <br />' Equation (1) can be solved for p, the number of pore volumes required to reach a concentration <br />of CP, which, in the original analysis, was taken to be the groundwater quality standard of 250 <br />mg/L sulfate. The initial concentration, Co, and the target concentration, C,,, influence the <br />magnitude of the number of pore volumes, p. <br />In November 1999 when the Response Plan was developed, the only groundwater quality data <br />available in the backfill came from BF-2 and BF-]. At that time, the concentration of sulfate in <br />BF-2 was approximately 1,370 mg/L. In contrast, the concentration of sulfat<~ in BF-I was <br />approximately 200 mg/L. The greater concentration value was chosen to ensure conservatism in <br />the calculation to reflect the small data set used in the calculation. The value of n used in the <br />calculation (0.17) was determined by fitting Equation (1) to the results obta ned from the <br />laboratory. Equation (1) was then solved fore assuming an initial sulfate concentration of 1,370 <br />mg/L (BF-2) and a final target concentration of 250 mg/L (the groundwater quality standard). <br />Based on the analysis using laboratory results and extrapolating to the field, apfroximately 10 <br />pore volumes would be required to pass through the West Pit backfill material to yield a <br />groundwater sulfate concentration of 250 mgJL. <br />The estimated number of pore volumes, as calculated in the Response Plan, was combined with <br />the results of the hydrological evaluation that was conducted in support of TR-26. The residence <br />time of groundwater in the West Pit backfill, under ambient hydrologic conditions, was estimated <br />to be 4 years, based on an outflow rate of approximately 80 gallons per minute (gpm). Ten pore <br />volumes of flushing at a rate of 4 years per pore volume resulted in an estimated tine required for <br />backfill flush of 40 years, as was presented in TR-26 (pp. 38). <br />2.2 January 2000 Update <br />In the January 2000 technical memorandum, revisions were made to the pore volume and flushing <br />requirements as presented in TR-26 using the additional field data and hydrologic analysis. The <br />number of pore volumes required to Flush secondary sulfate salts from the backfill material was re- <br />evaluated assuming that the initial concentration [C~ in Equation (1)] was the average <br />2636 Midpoint Dr., Srrite B, For[ Collins, Colorado 80525 <br />Telephone (970) 484-7704 / I~AX (970) 484-7781 <br />