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• A4 <br /> I <br /> 1 <br /> Although not commonly recognized , it is possible for hydrogen <br /> ions in an acidic plume to be removed and pH increased by the <br /> process of ion exchange , wherein hydrogen ions replace other _ <br /> cations on ion exchange sites such as clay mineral surfaces. This <br /> process , while increasing the pH and possibly precipitating other <br /> metals , does not reduce the total dissolved solids but rather <br /> liberates other major cations and produces an alkali earth-rich <br /> solution. However, such major cation liberation may , in some <br /> cases , be countered by the process of gypsum precipitation. <br /> Both the process of calcite dissolution and the ion exchange with <br /> clay can result in the presence of high concentration of calcium <br /> ion in acidic contaminant plumes. The use of sulfuric acid for <br /> leaching results in the presence of extremely high sulfate <br /> concentrations. These concentrations of calcium and sulfate ions <br /> will , in an aqueous solution , result in the precipitation of <br /> gypsum in the subsurface. Note that the precipitated gypsum <br /> occupies more volume than the dissolved calcite thereby resulting <br /> i in a reduction of permeability . Radium has chemical behavior <br /> i similar to calcium, and hence is coprecipitated with the gypsum. <br /> As was previously noted, calcite dissolution can take place by <br /> means of calcite reaction with ferrous iron to precipitate <br /> siderite. Subsequent to the removal of all calcite mineral from <br /> a soil column , this siderite may , in turn , react with acidic <br /> fplumes to consume acid with the side reaction of dissolution of <br />