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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 16 (2) based on current discharge rates for the sub-systems. (2.91R + 2.17C + 1.41U + 0.010B)/(R + C + U + B) = m* (2) Coefficients differ between eqs. (1) and (2) because the #1 silica monitor tracks concentration after the mild separation effect of the ultrafilter. The logic is otherwise identical for both equations. [Alternatively, a reasonable facsimile of m* might be used to simplify an automated response system.This would require verifying "alert"signals with a genuine value for m*, current for the sampling time.] If the product of (m*)(conc) exceeds 120, or if the rate of change of (m*)(conc) seems aimed past 120, injection should be precluded. Silica monitor#2,sampling the discharge mixture from the waste receiver tank(Figure 1), has two roles. During times when silica concentrations are sub-critical, it serves a verification function.Its output should be consistent with the concentration indicated by monitor#I multiplied by a current value for m*. Mis-match should trigger a system check to find the cause. During times when silica concentrations are greater than critical, a wait ensues for when injection may again become a safe option.Decision to re-start injection should coincide with both silica monitors returning to sub-critical indications. VII. SUMMARY A calculated range of injectate fluid compositions has been derived from chemical data for the source water, effects of plant processes, and blending proportions of the several process streams. An equation was derived for computing the net concentrating effect on input water in terms of the waste discharge rates of sub-systems in the plant. Selected compositions were used as input to a computer program which identifies chemical saturation for more than 100 relevant minerals over a wide range of compositions. About a dozen minerals were identified as near saturation or super-saturated in terms of the liquid compositions. Dissolved silica is found to be a critical component. Solid, amorphous silica is calculated to form at conditions expected in the STIG operation, from waters that are modestly higher than average in silica. Accordingly restrictions on injection/disposal are recommended, based on silica concentrations derived from continuous monitoring. A system for monitoring silica is proposed, including methods for using the data in decisions about whether or not to inject the waste into a well. A variety of phosphate minerals and barite are expected to form in the plant and to be routinely removed as sludge and filtrate. Chemical additives are specifically identified to discourage those minerals from forming as hard crusts (scales). Several iron-and copper-bearing minerals appear supersaturated,but their actual presence is doubtful due to analytical factors. Amounts that might form from liquid components would not be significant in any case. Formation of iron-bearing minerals should be regarded as an indicator of active corrosion, not chemical deposition, and treated on that basis. Reaction of injectate with natural waters in the receiving rocks may yield minor amounts of phosphate minerals. However, expected dispersion would result in their causing no significant impairment to the injection zone.Other potential reactions between injectate and formation water DON MICHELS ASSOCIATES -Missoula,Montana USA