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<br />' alluvium and the Dakota Sandstone. Table 7.4 shows the predicted <br /> concentrations for each constituent for the range of hydraulic gradients <br />' simulated. The solute transport analyses predict that a hypothetical leachate <br />release will not impact groundwater quality, under the scenarios evaluated, in <br /> either the St. Vrain Creek alluvium or the Dakota Sandstone at a hypothetical <br /> receptor well located about 3,000 feet east of the C-Pit. The background <br />' concentration for these constituents in the alluvium and the Dakota Sandstone <br /> groundwater are shown on the table for comparison. The background values <br /> were obtained from water samples collected in the A-Pit and Dakota Sandstone <br />' (Table 6.1). <br />~ Solute attenuation or decay was not simulated in these analyses so that a <br />, conservative simulation was maintained. However, some of the constituents <br /> present in the CKD leachate may undergo attenuation or decay during transport. <br /> Metal cations, such as thallium or selenium, are reactive constituents that may be <br /> readily attenuated during transport if sorptive solid phases such as clay minerals <br /> or iron and manganese hydroxides are present in the aquifer matrix. Because of <br /> the low concentrations of metals anticipated in a potential leachate release, the <br /> presence of sorptive materials in the aquifer matrix would rapidly attenuate most <br /> cations released to groundwater. <br />' <br /> 7.3.5 Mixing Simulation of pH in the Alluvium and Dakota Sandstone <br />' Evaluation of the impact of a hypothetical CKD leachate release on alluvium and <br /> Dakota Sandstone groundwater pH was pertormed using PHREEQC {Parkhurst, <br /> 1995).~PHREEQC is a computer program that performs speciation, mixing, <br /> reaction-path, advective-transport, and inverse geochemical calculations. <br /> PHREEQC uses an ion-association model and Debye Huckel expressions to <br /> account for the non-ideality of aqueous solutions. The speciation and mixing <br />~ , modules of PHREEQC were used in this analysis. In the mixing module, each <br /> solution is multiplied by its mixing fraction and a new solution is calculated by <br /> summing over all of the fractional solutions. <br />' <br /> The mixing ratio of the groundwaters and the CKD leachate were determined <br /> using the annual leachate volumes calculated by the HELP model and the <br />' volume of each aquifer immediately beneath the CKD disposal site, an area of <br /> approximately 20 acres. The volume of water in the alluvium and Dakota <br />' Sandstone aquifers is estimated at 28,575,360 cubic feet and 85,726,080 cubic <br />feet, respectively. The annual CKD leachate volumes estimated using HELP for <br /> the alluvium and Dakota Sandstone simulations are 53,898 cubic feet and 366 <br />' cubic feet, respectively. The initial PHREEQC calculations were pertormed using <br />a mixing ratio of one part aquifer water volume to one part of the annual CKD <br /> leachate volume. <br />The results of the groundwater-leachate mixing simulations indicate that at <br />reasonably expected volumes of CKD leachate, the alluvium and Dakota <br />' Sandstone aquifer pH will not be adversely impacted by a hypothetical CKD <br />leachate release. At higher CKD leachate volumes (which are not reasonably <br />' is <br />