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WATER CHEMISTRY EVALUATION OF THE EXISTING STIG-1 AND PROPOSED LEC-1 INJECTION WELLS Analysis of the STIG-1 injection water chemistry for a sample collected February 22,2005 showed saturation with respect to calcite, dolomite,and amorphous silica. This sample was deliberately chosen for its high dissolved silica concentration(92 mg/L) relative to other samples over the past 5 years. It is assumed that on average the injection water has been lower in dissolved silica. The analysis also showed oversaturation with respect to the magnesium silicate mineral talc. Many silicate minerals can show oversaturation,but often are not oberved due to very slow kinetics of formation. Talc is an example of a mineral that could form in a short time frame if the temperature becomes elevated. At the assumed temperatures of injection at STIG-1,however,this is not assumed to be likely. The proposed injection water chemistry shows similar mineral saturation to the STIG-1 water chemistry, discussed above. Silica concentration is lower than the average STIG-1 injection water,and amorphous silica and other silicate minerals are even less likely to precipitate, according to PHREEQC calculations. Mixing of Domengine formation water with STIG-1 injection water was simulated at ratios of 3:1 (formation water to injection water) and 1:1. Although the injected water has higher concentrations of sulfate, silica, and (on average)nitrate,these concentrations have not been high enough to cause mineral precipitation at the well screen,according to model simulations. The STIG-1 injection water was nearly saturated (at equilibrium) with respect to amorphous silica,but mixing with formation water only served to dilute the silica. These simulations reflect agreement with observed conditions over the past several years of injection,which have exhibited no suggestion of mineral incrustation or associated loss of production. PHREEQC output results are provided in Table 1. In a second set of simulations,mixing of the same Domengine formation water from May 15, 1993 with the proposed LEC-1/LEC-2 injection water chemistry was simulated in the same manner as with the STIG-1 water (PHREEQC output file shown in Table 2). Under the assumed conditions of the simulation,no precipitates were favored to form. However,it should be noted that proposed concentration of nitrate (253.7 mg/L) is very high,far above the primary MCL for nitrate (45 mg/L). Such high concentrations may stimulate the growth of nitrate-reducing bacteria in and around the well screen if organic carbon is also increased. It is probable that if no significant increase in organic carbon is made to the aquifer,the extra nitrate will simply remain dissolved in the injected water. There is no predicted estimate of organic carbon in the blowdown water,so there is no way to accurately predict this. It is recommended to maintain dissolved silica concentrations as low as possible, along with temperature, as silicate minerals are more likely to precipitate at elevated temperature. Reference Parkhurst,D.L. and C.A.J. Appelo,1999, "User's Guide to PHREEQC (Version 2)-A Computer Program for Speciation, Batch-Reaction,One-Dimensional Transport, and Inverse Geochemical Calculations," U.S. Geological Survey Water-Resources Investigations Report 99- 4259,312 p. Document and compiled program available at the website: http://wwwbrr.cr.usgs.gov/projects/GWC-coupled/phreeqc/index.html TECHNICAL MEMORANDUM_V3.DOC 2