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percent with the largest increases being the sodium and potassium concentrations. The • <br />anions show increases of 22, 2000 and 140 percent for HC03, 504, and C1, respectively. <br />Relating these chemical changes to the possible reactions controlling the spoil chemistry, ~, <br />it is apparent that the dissolution of calcite and dolomite in the presence of carbonic i <br />and sulfuric acid are the principal processes accounting for the increased Ca and Mg <br />concentrations. The oxidation of pyrite, the dissolution of calcite and dolomite in the <br />presence of sulfuric acid, and the dissolution of gypsum are the principal chemical <br />reactions contributing to the marked increase in 504. The increases in sodium and <br />potassium are believed to be principally due to cation exchange reactions, particularly <br />with the finer sized sediments (clays). <br />The water chemistry in the downgradient bedrock aquifers which are receiving spoil aquifer <br />discharge, shows additional changes from that observed in the spoil. The following <br />remarks are based on provisional water quality data analyses provided by the USGS. Nests <br />of downgradient bedrock wells below three different mined areas at Pea body's Seneca Mine <br />were monitored by the USCS from 1987 through 1989. Following mixing of the spoil water <br />with the bedrock aquifers, the resultant water chemistry shows considerably lower <br />concentrations of Ca and Mg, while Na concentrations have increased by a factor of 4 to 5. • <br />504 concentrations are lower but it still remains the dominant anion. In some cases, HC03 <br />concentrations are at least 50 percent of the 504 concentrations, thus making the water a <br />HC03504 type. Table 17-13 summarizes the downgradient bedrock well chemistries. The <br />chemical reactions accounting for these constituent concentration changes are cation <br />exchange between Ca, Mg, and Na, precipitation of calcite, dolomite, and gypsum, and <br />dissolution of sodium rich minerals (halite and nahcolite) from the bedrock formations. <br />The bedrock aquifers were most likely at equilibrium or saturated with respect to Ca, Mg, <br />and 504, thus increasing the potential for precipitation of these species when higher <br />concentrations are introduced to the system. With less oxygen being available further <br />along the flow paths, some sulfate reduction is probably occurring. <br />The downgradient alluvial aquifer and surface water chemistries more closely resemble that <br />of the spoil. Both water types are CaMg-504 and the percentages of the major ions are <br />similar. <br />The above discussion of chemical reactions and resultant chemistries should describe what <br />will happen at the Seneca II-W Mine, because the starting chemistries are similar with the • <br />following exceptions. All concentrations of the major cations in the Seneca II-W <br />34 <br />