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The resulting carbonic acid (H2CO3) dissociates: <br /> H2CO3 11 H+ + HCO3 (2) <br /> HCO3- H+ + CO32- (3) <br /> The effect of reactions 1 , 2, and 3 is to produce a slightly acidic environment conducive <br /> to the dissolution of calcite (CaCO3) and dolomite CaMg(CO3)2 <br /> CaCO3 Ca 2+ <br /> + CO32 (4) <br /> CaMg(CO3)2 Ca 2+ <br /> + Mg 2+ + 2CO32- (5) <br /> Reactions 4 and 5 result in an increase in the concentrations of calcium (Ca 2+) and <br /> magnesium (Mg 2+), and an increase in the concentration of carbonate (CO3 2- <br /> ), which forces <br /> a decrease in hydrogen (H ) concentration and an increase in bicarbonate (HCO3 ) <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 (FeS2) is present in the oxidizing near-surface environment, calcium sulfate <br /> (CaSO4) may be formed. Generalized reactions for the principal processes involved are: <br /> 4 FeS2 + 15 02 + 14 H2O 4 Fe(OH)3 + 8 SO42 + 16 H (6) <br /> 2+ + SO4 2-Ca — CaSO4 (7) <br /> The sulfate ions (SO 42 ) 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 />