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• <br />• <br />• <br />concentration. The adsorption of calcium and magnesium ions and the release of sodium <br />ions by exchange reactions with clay minerals and organic materiels also force a decrease <br />in hydrogen-ion concentration and en increase in bicerbonate-ion concentration. The net <br />result of the above reactions is a moderately alkaline water containing mainly calcium, <br />magnesium, sodium, end bicarbonate ions. <br />If pyrite (FeSZ) is present in the ozidixing near-surface environment, calcium sulfate <br />(Ca504) may be formed. Generalized reactions for the principal processes involved are: <br />4 Fe52 + 15 OZ + 14 Hz0 <br />2+ 2- <br />Ca + S04 CaSO4 <br />(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 />Under anaerobic conditions, sulfate may be reduced to Gulf ide by bacteria: <br />2- <br />504 + 2c (organic) + 2HZ ~ H25 + 2HL03 <br />(l1) <br />Depending on the hydrological, geochemicel, 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-Soecific Ground Yeter chemistry. The results of chemical analyses performed on <br />ground water samples collected within and adjacent to the Yoest area are discussed below. <br />Detailed descriptions of sampling and laboratory analytical procedures can be found in Teb <br />15, "Hydrologic Monitoring Program", of this Permit llpplicetion Package. The locations of <br />Yoast ground water monitoring sites ere shown on Exhibit 7-1. <br />In the following discussion, ground water chemistry will be analyzed by geologic unit. <br />Seasonal veriebi lity in measured water quality ere addressed, end where appropriate, <br />suspect water chemistry date ere noted. As with other hydrologic analyses contained in <br />this PAP, all date collected through the 1993 calendar year are incorporated in the <br />following discussions. Rew water quality date, statistical water quality summaries, <br />2- <br />4 Fe(OH)3 + 8 504 + 16 H (6) <br />39 <br />