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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 19 15. Ball, J.W. and D.K. Nordstrom, WATEQ4F for calculating speciation of major, trace, and redox elements in natural waters: US Geol.Surv. Open-File Report 91-183, 1991, Menlo Park, California. 16. The crystal composition is complex, suggesting that growth would be difficult, except in (probably)limited circumstances.Additional references were sought,but not found in a cursory review of literature, suggesting an uncommon or rare occurrence. Computed equilibrium deposition potential exceeds 3 tons per month, but no deposition at all seems plausible also. If large accumulations of sludge do not develop during early operations then this variety of apatite may be disregarded. 17. Amorphous silica saturation is reached at 120 mg/kg at 77°F. Concentrations higher than that, risk deposition of silica on available surfaces. For the injection system, critical surfaces include the rock porosity near the wellbore. 18. For a concentration multiplier of 1.86 and concentration limit of 120 mg/kg, maximum input concentration would be 120/1.86=64.5 mg/kg.Statistics for measured concentrations are; avg = 59.5, std. dev = 11.2. The maximum tolerable input concentration (64.5 mg/kg) corresponds to +0.45 standard deviations above the average. Based on a Gaussian (normal) distribution, such a normally varying fluid would be injectable 67 percent of the "time". Irregularities of the silica concentration changes could cause doubt that the distribution is Gaussian. An alternative method to estimate statistical utility of injection/disposal can be based directly on the data. Using the values on which Figure 2 is based, and limiting the tally to different-day samples, 13 of 18 sampling days (72%) had silica <64.5 mg/kg. These two results are not statistically distinct. Nominally, injectability can be expected to be practical for about 2/3 of the time. 19.The procedure above disregards silica inhibitor (dispersant) that is planned for addition to the RO input fluid, specifically to stabilize silica there. Residuals of inhibitor may plausibly be present in the injectate mixture. Furthermore, it could appear practical to augment the inhibitor concentration for fluids which exceed the injectability index in order that they may be"safely" injected. However, at the present level of documentation, the stabilization actually imparted to the supersaturated silica concentrations is ambiguous for conditions in rocks near the wellbore. Until unambiguous support is developed, injection of supersaturated concentrations of silica into the well could be considered imprudent. 20. If chemically active fluid were in the well after closure, it could be displaced by pumping in two, or more, casing volumes of non-active fluid. Alternatively, plant operating conditions might be susceptible to modification yielding a smaller net concentration factor. This would require in-plant adjustments of rates involved with equation(2). The utility of this option has limits which deserve exploration prior to the need. DON MICHELS ASSOCIATES -Missoula,Montana USA