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• The resulting carbonic acid (HZC03) dissociates: <br />H2 C03 ~ ~ H+ + HC03 (2) <br />HC03 ~ H+ + C032 (3) <br />The effect of reactions 1, 2, and 3 is to produce a slightly acSdic environment conducive <br />to the dissolution of calcite (CaC03) and dolomite CaMg(C03 )2 . <br />CaC03 ~. Ca2+ + C032 (4) <br />CaMg(C03 )2 ~ ~ CaZ+ + Mg2+ + X032 (5) <br />Reactions 4 and 5 result in an increase in the concentrations of calcium (Ca2+) and <br />Z+ p- <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 />(Ca604) may be formed. Generalized reactions for the principal processes involved are: <br />4 Fe62 + 75 DZ + 74 H2O ~ 4 Fe(OH)3 + B 5042 + 16 H (6) <br />CaZ+ + 5042 ~_ CaB04 (7) <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 />In anaerobic conditions, sulfate may be reduced to sulfide by bacteria: <br />7-29 Revised 04/11/88 <br />