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BMRI--San Lais 1998 BackfiU Geochemi.~ry measured TDS (in the O1 Oct 98 sample of BF-2), solubility control on Ca and SO< implies that TDS will not continue to rise much further, either. i f The inal quest on to be evaluated is the length of time it will take for the wash-our effect to move through the system. The batch-leach experiments (Logsdon, 1995; . ' Table 5) showed that the average leachate concentration produced by 1:1 (mass:mass) static leaching of gneiss samples was about 129.3 mg/L, or about 18.5 mg/L above the staffing concentration of 111 mg/L in the "background" water. Based on the test ' protocols described in Appendix A of that report and an extrapolation to releAse rate per kg at 1 pore volume, this amounts to an average sulfate yield of 0.186 g/kg (compared to the 0.275 g/kg from the median kinetic test). ' The change in sulfate concentrations as a function of effective pore volumes in the ' sequential batch tests quite closely follows an exponential decay curve (Figures 2a and 2b) of the form C = Coe-~'p', where p is the number of pore volumes and n is a fitting parameter for the empirical data; the negative sign indicates that the concentration falls as fresh water continues to move past the stationary solids. For ttre sequential batch- ' test data, the value of n would be approximately 0.17, based on alog-linear regression and residuals analysis to estimate a model Co of 331 mg/L SOe at I PV in the batch tests. [f the exponential model for the sequential batch test were applied to a Co of ' 1370 mg/L (the observed concentration at BFZ), then it would require about 10 pore volumes for the concentration to fall to 250 mg/L at source. If one assumes [trot there is no further sulfate production below the water table, then eventually all sulfate that ttas accumulated will be dissolved, and the concentrations in the backfill will return to values near the baseline range of about 100 mg/L. 1 ' RECOMI~~NDATIONS I. It is very unlikely that additional experimental work on leaching of small Samples ' will further refine the understanding of water-quality conditions in the backfilled and there is no large-scale processes controlled b The water uali is bein it , y q ty g p . advantage to conducting small-scale tests and then trying to project those results to ' the full-scale system when one can directly measure the full-scale system. Geochimica does not recommend any additional laboratory leach testing. ' 2. On-going ground-water monitoring is needed in, around and downgradient of the pit. Such monitoring is proposed undo the companion geohydrologic studies. 3. We do recommend that the on-going ground-water monitoring data be evaluated geochemically. We recommend two types of evaluations: ' • Speciation modeling, using a computer model such as PHREEQC, to confirm that the sulfate concentrations remain at or below val~.res for gypsum saturation. ' Ceochimica, Inc. I6-Nov-98