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Battle Mountain Resourc~ • <br />• January 31, 2000 <br />Page d <br />Anal}'sis of BF-1 <br />The concentrations of all constituents measured in groundwater samples collected from BF-1 <br />have always been lower than in any other location in the backfill. The general conse~rsus has <br />been that the low concentrations in groundwater measured at BF-1 are most due to the flushing <br />action afforded by the inflow of Rito Seco alluvial groundwater. With the observations of <br />groundwater chemistry as a basis, a hydraulic analysis was conducted of groundwater tlows in <br />the vicinity of the backfill monitoring wells BF-1 to evaluate [he number of pore volumes that <br />may have flushed the portion of backfill material represented by BF-l . <br />First, though, the issue of backfill homogeneity must be addressed. That is, the observed water <br />quality at BF-I may only be due to differences in the eeoehemistry of the backtil] materials that <br />were placed in the area of BF-l . The well completion logs that were presented in Appendix A of <br />TR-26 demonstrate that the aggregate properties of the backfill materials do nog differ among <br />backfill groundwater monitoring well locations. In addition, the mining, stockpiling, and <br />backfilling of waste rock typically result in homogenization of an originally heterogeneous <br />material simply due to the amount of handling and mixing that are necessarily in~~olved in each <br />of mining, stockpiling, and backfilling. Therefore, the assumption that the backfill materials <br />sampled by BF-I are comparable geochemically to the rest of the backfill materiels in the West <br />Pit is reasonable. <br />The crur• of the hydraulic analysis was selection of a reasonable value of specilic yield (S~). <br />Calculations of total, unsaturated, and saturated pore volumes as presented in TR-26 were based <br />on a conservative value of S~ of 30 percent. Field observations made during completion of the <br />backfilling operations indicated that the groundwater level in the West Pit increased rapidly, <br />which could be explained by a low value of SY, perhaps on the order to ~ to 10 percent. More <br />commonly, SY is taken to fall within the range of 20 to 2~ percent. Using a range in S~ tiom ~ to <br />25 percent yields pore volume tushing times for the area around BF-1 from appro>:imately 0.2 to <br />1.1 years. The quality of groundwater at BF-1 has not varied greatly since the first sample was <br />taken in October 1993, which would suggest that the secondary sulfate salts associated with the <br />3~Y01 Automatist W"ay, Suite /00, Fort Collins. Colorado <br />Telephone (970) Z33-9600 / FA,Y (97 0) 323-7171 <br />