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TECHNICAL MEMORANDUM CH2MHILL Water Chemistry Evaluation of the Existing STIG-1 and Proposed LEC-1 Injection Wells PREPARED FOR: Toni Pezzetti/SAC PREPARED BY: Brian Schroth,PhD/SAC DATE: August 22,2008 Summary of Water Chemistry and Geochemical Modeling Groundwater from the Domengine aquifer had a total dissolved solids (TDS) concentration of 14,066 mg/L prior to injection at well STIG-1. This is well above the EPA Secondary MCL for drinking water (500 mg/L),and is considered a brackish water. It is dominated by sodium and chloride,and the presence of modest concentrations of nitrate indicate that the aquifer is under chemically oxidizing conditions. Water injected at STIG-1 over the last 5 years has had a TDS range between about 500 and 800 mg/L with a sodium-bicarbonate chemistry. Injection of this water has served to dilute the salinity of the aquifer. The water that is proposed for injection at well LEC-1 and LEC-2,if it is drilled,is a sodium- chloride water with a TDS of 2,187 mg/L,more similar to the aquifer chemistry than the STIG-1 injection water,but still about one-seventh of the aquifer TDS. However, concentrations of sulfate,nitrate, and phosphate are much higher than aquifer concentrations and,in the cases of sulfate and nitrate,significantly higher than those in the current injection water for STIG-1 (there are no data available for phosphate concentrations in the STIG-1 water). The USGS geochemical software PHREEQC (Parkhurst and Appelo 1999) was used to simulate mixing of both injection waters with the ambient Domengine aquifer water. The PHREEQC software evaluates water chemistry of each water to be mixed and simulates the effects of mixing waters at user-selected ratios. The programs thermodynamic database is used to calculate concentrations of aqueous species and saturation indices of possible mineral precipitates,providing an assessment of potential well clogging from these precipitates. A water in equilibrium with a given solid mineral phase is defined as "saturated" with respect to that mineral, and the saturation index (SI) is around zero (plus or minus about 0.5). Undersaturation(negative SI value) indicates that the mineral is not thermodynamically favored to precipitate and oversaturation (positive SI value) indicates precipitation is favored. Available water chemistry data were used for aquifer and injection waters. Temperature assignments were based on downhole temperature survey data and measurements on current injectate. Thermodynamic analysis of Domengine formation water from May 15, 1993 showed that the water is in approximate equilibrium with calcite and dolomite, common carbonate mineral phases in aquifer matrix material. Amorphous silica,the most common precipitate of dissolved silica that forms around well screens,is undersaturated in the formation, meaning it is not favored to precipitate. TECHNICAL MEMORANDUM_V3.DOC 1