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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 17 and between injectate and formation minerals appear to be of negligible likelihood or significance. The use of water from WSWPCF for the STIG project will yield waste water mixtures that are variably suitable for injection/disposal, but only one component, silica, is critical in that regard. Diversion of the waste stream to the injection well or return to WSWPCF can be based on monitoring silica. NOTES AND REFERENCES 1. Russell Associates, Supplementary Environmental Impact Report, STIG Project, Northern California Power Agency. 2.The earliest plotted points in Figure 2B represent the averages of points plotted in Figure 2A. 3. Testing of polyacrylate materials from two suppliers was reported in March 1994 (Al Benenati,Nalco).Concentrations to twice the natural solubility of amorphous silica(240 vs. 120 mg/kg) were reached in waste streams. However, chemical data provided to this reviewer was ambiguous for showing whether or not silica deposition occurred in the membrane. Three of five tests with two chemicals at two levels of addition showed that some silica was not accounted for.Specifically,mass balance verification was possible from measured silica concentrations in input, concentrate, and permeate. The discrepancy is plausibly explained by deposition in the membrane. However, the amounts indicated were at about the level of resolution of the chemical analyses.If analytical results are to be accepted,it must be concluded that silica deposited in the membrane. Alternatively, if the analyses are not accepted, which may also be reasonable, the tests must be judged as too insensitive for a conclusion to be made. Comments about pressures associated with the test suggest that no deposition occurred. Also,analyses of the membranes were said to not show silica. No pressure records or membrane data were provided.Thus,chemical data are compelling for this review.Accordingly, no credit for chemical stabilization of silica is accepted here, regarding injection, when silica concentrations exceed the nominal solubility limit of 120 mg/kg. 4. The standard deviations for concentrations of liquid components are mostly in the range of 10 to 15 percent of the concentration values. That describes a normal variation of input water composition. Factors causing changes smaller than the range of natural variation may be regarded as negligible for many aspects of this review. 5. Normal input rate for the three main waste streams is about 406 gpm. If discharge from the tank is the same rate, so that a stable liquid level is maintained, then clearance of the tank occurs at(406/V) [working volumes per minute], where V is the liquid(working) volume held in the tank. The tank will be operated at less than its 60,000 gallon capacity in order to accommodate surges and minor upsets in the system. Thus, V could be nominally 30,000 gallons, but an engineeringly practical range could be from 5000 to 40,000 gallons. A 30,000 gallon level yields a clearance rate of 0.0135 volumes/minute. 6. That clearance rate can be recast into the concept of a time to clear half of a parcel of liquid initially in the tank. Numerically, a clearance rate of 0.0135/min is equivalent to a clearance half-time of t1/2=0.853 hours.Lower(winter)rates involve average discharges nearer 314 gpm which have correspondingly longer clearance half-times for the same fill levels of the tank.On the other hand, if the operating level of the tank is less than 30,000 gallons, clearance half- DON MICHELS ASSOCIATES -Missoula,Montana USA