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' BMRI--San Luis 1998 Backfip Geochemi.~ry ' rate for sulfate from the gneiss in the backfill would be 274.5 mg SO< per kg of rock, or 0.275 g SO. per kg of pink gneiss. The total mass of backfill in the West Pit is 5,779,207 tons (R. Zumwalt, personal communication, 29 Oct 1998). One US ton is 0.91 E+3 kg. Therefore the total mass ' of solid backfill is 5.26 E+9 kg rock. To be very conservative, it is assumed that all of this rock could produce as much sulfate to solution as the median value for the pink gneiss. Therefore, the total mass of sulfate that could be released from the backfill ' would be 1.45 E+9 g SO.. In fact, not all of the backfill is gneiss, and so tit e calculated total mass o_f sulfate released for the backfill_- assuming all of the material is i ~ - is co e. It also is likely t~Ftai the backfilled rock had not fully ' reacted before being inunda , as the more rigorously oxidized HCT samples surely did, so the conservatism of this analysis more than offsets the use of the medilan value ' rather than the maximum value. Assuming that the density of intact rock is about 2.6 g/cm' (2.6 E+3 kg/m3), the ' volume of intact rock equivalent to the backfill would be 2.02 E+6 m'. BMRI estimate a swell factor for the backfill of 33% (R. Zttrnwalt, personal communication, 29 Oct 1998), so the total volume occupied by the backfill is approximately 2.69 E+6 ' m'. Therefore, the effective porosity of the backfilled mass is 0.33, and the notal volume of water in the backfill is 8.88 E+5 m'. ~,,,,.n. ,o::.nn,~ ~ ¢~-r~H Concentration is the mass of a solute divided by the voltune of the solution. Therefore, the expected concentration of sulfate in the backfilled pit should be approximately: ' 1.45E + 9 g SO4 _ 8.88E + 5 m3 groundwater - 1633 g/m3 SOa. 1 A concentration of one g/m' r~ a concentration of one mg/L, so the maximum ' "expected" concentration would be about 1600 mg/L 504. The observed concentration in BF-2 in early October 1998 was 1370 mg/L. Given that the analytical precision of sulfate analyses at these levels is about ± 10% (ls), the estimated and observed values ' are essentially indistinguishable. ' Therefore, the observed available data on TDS and sulfate in BF-2 can be explained fully by the leachability of sulfate as "wash off" from gneiss in the West Pit backfill. Analysis of the solution chemistry from BF-2 using standard computer speciation codes (e.g., PHREEQCI, using the WATEQ4F database) indicates that the solution is saturated with both calcite and gypsum (Attachment 1). Therefore, the sulfate and ' calcium concentrations will not rise discernibly above those currently observed because the addition of either or both would cause additional gypsum to form, bufferiAg the Ca and SOa concentrations. Because sulfate and calcium constitute 88 percent of the ' Geochimica, Inc. lt~Nov-98