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A2 <br /> ( The geochemical processes involved are extremely dynamic. Some <br /> ` processes tend to remove contaminants , while others exchange one <br /> contaminant for the other, or actually may add contaminants to <br /> the system. As an acidic plume advances downgradient, reactions <br /> may take place that will alternately remove and redissolve <br /> contaminants . Understanding of such processes is required to <br /> analyze the observed water-quality data and to plan for remedial <br /> measures. Each of the following described processes appear to be <br /> important in considering the environmental impacts of acidic <br /> plumes moving downgradient from acid uranium mill tailings ponds <br /> or evaporation ponds. <br /> I By far the single most significant geochemical reaction is that <br /> of calcite dissolution, wherein the hydrogen ions react with <br /> calcite or other carbonate material present in the soil or rock. <br /> The mechanism is calcite reacting with hydrogen ion to liberate <br /> calcium ion and form bicarbonate ion . This results in the <br /> gradual downgradient movement of an acidic reaction front, and an <br /> acid-front retardation in the saturated zone , relative to the <br /> rate of water travel . Hydrogen ions in the acidic plume are <br /> consumed by calcite dissolution. The retardation of the advance <br /> of the acid front is a result of the number of pore volumes of <br /> water with specified hydrogen ion concentration that must react <br /> jwith a representative volume of the porous media to completely <br /> remove the total mass of calcite in the volume (Cherry, Shepherd, <br /> t <br /> i <br />