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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 10 minerals, but in each review, only those minerals with all components occurring in a given fluid composition are tested for saturation. With the WSWPCF fluids, 39 mineral species were involved with the major and minor dissolved components. Another 62 mineral forms were considered in regard to trace concentrations of iron, copper, and zinc. A great majority of these mineral forms are not apparently supersaturated and may be disregarded. Among those that are identified as supersaturated, some, like silica varieties quartz, chalcedony, etc., do not deposit for reasons related to their atomic mechanisms of growth. Mineral species identified by WATEQ4F to be super-saturated and plausibly capable of forming under injection conditions are shown in Table 2. That may be regarded as a nominal maximum listing, given the available chemical analyses. Identified minerals involve a fairly wide range of dissolved components and the several forms deserve separate consideration. Accordingly, a "Level of Concern" has been attached to each mineral listed in Table 2. A. Silica Forms Several crystalline forms of silica(Si02) are known and all are super-saturated according to the chemical model,which is based solely on thermodynamics.They include quartz,chalcedony, and cristobalite. However, experience shows that even when greatly supersaturated, these species do not form in environments such as at STIG. Accordingly, only chalcedony is shown in Table 2 as an intermediate representative of the group. An amorphous (non-crystalline) form of silica does deposit readily when super-saturated in industrial contexts. Si02 concentrations are very close to the saturation concentration in the "nominal' case and exceed saturation, due to statistical variations, to seasonally high plant concentration factors, and to excursion conditions in the plant. Silica concentrations in the RO membrane will normally reach saturation conditions and chemicals will be added to prevent problems there. Amorphous silica is a critical component in regard to injection/disposal. Silica concentrations are forecast to range on both sides of the solubility limit, but chemical additions have ambiguous (not reliable) effects on stabilizing concentrations that exceed the solubility. Consequently there will be times when injection/disposal should be precluded. Silica can also be a component of other minerals with more complicated compositions and those require separate evaluations. Such minerals include silicates of sodium and potassium and alumino-silicates, some of which contain magnesium. Alumina was not analyzed, but concentrations in natural waters normally are very low(<O.1 mg/kg). Alumino-siliceous minerals were disregarded. Silicate minerals containing magnesium were not indicated as being near saturation in the nominal compositions. B. Heavy Metals Concentrations of iron, copper, and zinc are low and variable in the unprocessed slough water.For two reasons, their measured concentrations should not be taken at face value. Indicated concentrations are near the analytical detection limits, so resolution of concentrations are likely no better than a factor of two, possibly poorer. Apparent concentrations exceed solubility by relatively large multipliers, especially for iron, suggesting that detected material is present as DON MICHELS ASSOCIATES -Missoula,Montana USA