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• z+ <br />Reactions 4 and 5 result in an increase in the concentrations of calcium (Ca ) and <br />2+ 2- <br />magnesium (Mg ), and an increase in the concentration of carbonate (C03 ), which forces <br />a decrease in hydrogen (H ) concentration and an increase in bicarbonate (HC03 ) <br />concentration. The adsorption of calcium and magnesium ions and the release of sodium <br />ions by exchange reactions with clay minerals and organic materials also force a decrease <br />in hydrogen ion concentration and an increase in bicarbonate-ion concentration. The net <br />result of the above reactions is a moderately alkaline water containing mainly calcium, <br />magnesium, sodium, and bicarbonate ions. <br />If pyrite (Fe52) is present in the oxidizing near-surface environment, calcium sulfate <br />(Ca504) may be formed. Generalized reactions for the principal processes involved are: <br />4 Fe52 + 75 02 + 14 H2O 4 Fe(OH)3 + g 5042 + 76 H (6) <br />Ca2+ + 5042 ~_ Ca504 <br />(~) <br />The sulfate ions (5042 ) produced in reaction 6 either are transported in solution to the <br />• aquifer or are precipitated as calcium sulfate near the surface as in reaction 7. The <br />precipitated calcium sulfate may be redissolved later and transported to the aquifer by <br />deeply percolating recharge water. <br />Under anaerobic conditions, sulfate may be reduced to sulfide by bacteria: <br />5042 + 2C (organic) + 2H2 H25 + 2HC03 (g) <br />• <br />Depending on the hydrological, geochemical, and biological processes that predominate in <br />an area, the ground water can be of a calcium magnesium bicarbonate, sodium bicarbonate, <br />calcium magnesium sulfate, or sodium sulfate type. <br />Site-Specific Ground Water Chemistry. The results of chemical analyses performed on <br />ground water samples collected within and adjacent to the Seneca II-W area are discussed <br />below. Detailed descriptions of sampling and laboratory analytical procedures can be <br />found in Tab 15, "Hydrologic Monitoring Program", of this Permit Application Package. The <br />locations of Seneca II-W ground water monitoring sites are shown on Exhibit 7-1, <br />35 <br />