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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 14 indicates; TDS=14066, C1=7000, HCO3=62, CO3<1, Ca=761, Mg=86, SiO2=16.5, pH=7.85. Reaction of injectate with this composition may involve formation of apatite when high concentrations of calcium and modest concentrations of bicarbonate mix with significant concentrations of phosphate and fluoride.Limited deposition should be expected because the zone of mixing is relatively narrow and moves away from the wellbore as injectate displaces native fluids. Amounts of deposition would be limited by amounts of orthophosphate and fluoride in the injectate and mixing zone. Accordingly, less deposition may be expected in the rocks than is indicated in Table 2 because some of the limiting fluoride and orthophosphate will have been removed, prior to injection, as sludge or in filters. Heavy metals are expected to be negligible in the Domengine formation water due to the abundant bicarbonate and moderate pH. V. WELL COMPONENTS The well contains casing, tubing, and screens made generally from carbon steel. Corrosion should be avoided and the injectate appears neutral in this regard. Filtration and ultrafiltration likely remove BOD so the waters will not have much capacity, if any, as chemical reducers. Sodium sulfite is added at multiple places in the plant, notably to cooling tower liquid to reduce levels of dissolved oxygen. All injectate mixtures are dominated by streams that have been purposefully depleted in oxygen so all injectates may be expected non-oxidizing, initially. However,small amounts of air,inadvertently allowed into injectate,could convert the liquid from neutral to mildly oxidizing,because liquid carries no buffer to consume minor inputs of oxidizers. Injectate waters contain substantial amounts of important nutrients, especially nitrate, sulfate,and phosphorous. Thus, the injection well may be considered as potentially threatened by bacterial accumulations. Bactericides are among the intended additives to fluid entering the ultrafilter and to the cooling tower. Residuals in the injectate would be helpful for protecting the well. Some attention should be given to augmenting residuals of bactericides in injectate, as appropriate. Low levels of BOD will help suppress bacterial accumulation. VI. SILICA MONITORING AND FORECASTING The variability of silica concentrations in fluid from the WSWPCF complicates the tactic of injection/disposal. For statistically substantial increments of time, the system multiplier, operating on above-average silica concentrations, will yield wastes exceeding 120 mg/kg. That value designates a limit for safe injection/disposal 17. Based on the apparent standard deviation for the silica concentrations, injection will be safe for about two-thirds of the time18,19 Part of the discharge from the waste receiver tank always will be directed back to the WSWPCF because the injection well capacity is smaller than the normal plant discharge rate. At times when silica concentrations are favorable, discharge from the waste receivor tank can be divided, 100 or 200 gpm may directed to the injection well, STIG #1, with the balance, ^-200 to DON MICHELS ASSOCIATES -Missoula,Montana USA