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• As is suggested by the above CEC ranges, smec ti tes (i.e., montmorillonite) would have the greatest potential for cation exchange reactions. This is largely due to the fact that their interlattice spacing is greatly affected by absorbed water. The greater the interlattice spacing, the smaller is the overall charge on the interlayer balancing cations. The smaller the charge deficiency, the more weakly held and thus, the more readily exchangeable are the cations. The kaolinites, illites, and chlorites have much tighter lattice spacings and the charge imbalance holding the interlayer balancing cations is considerably higher. Cation exchange reactions with these minerals is lower and typically limited to the edges of the clays. The more common exchangeable cations in clay +2 +2 + + + + minerals (particularly montmorillonite) are Ca , Mg , H , K , NH4 , and Na . Montmo ri llonite was found to be present as mixed layered illite/smectite clays in x-ray diffraction analyses of overburden core samples. The clay fraction of different portions of the overburden cores, particularly the shale zones, can exceed 50 percent. Of the clay fraction, 20 to 56 percent is comprised of illite/smectite mixed layered clays. The overburden material has a high potential for cation exchange reactions and these reactions are significant in determining postmi ning water chemistries. Cation exchange reactions • are described by the following general equations. +2 + Nat (clay) + Ca - Ca (clay) + 2Na +2 + Nat (clay) + Mg --~ Mg (clay) + 2Na Water Chemistry and Flow Inferences Based on Seneca II Monitoring Results. Five overburden wells downgradient from mining are presently being monitored at the Seneca II Mine. The wells are identified as 5-21, 5-42P1, S-44P2, 5-40, and 5-41. Three of the wells, S-44P2, 5-40, and 5-41 have not been influenced by upg radient spoil aquifer flow and represent premining overburden water quality characteristics. The premi ni ng overburden water types are MgCa-HC03 and Ca-HC03. Calcium and magnesium are the dominant cations ranging in mean concentrations from 82-108 mg/1 and 40-52 mg/1, respectively. HC03 is distinctly the dominant anion ranging in mean concentrations from 438-455 mg/1. The other two wells, 5-21 and S-42P1 are downgradient or adjacent to resatura ted spoil. The dominant anion is 504 and the dominant cation is magnesium with sodium and calcium showing milliequi valent concentrations greater than 50 percent of the magnesium concentrations. Table 17-11 summarizes this information. • The resultant spoil water chemistry (Table 17-12) is one dominated by CaMg-504. In comparison to the overburden chemistry, the major cations increased from 400 to 900 31