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Figure 6 shows the predicted pH in the hypothetical Dakota sandstone receptor well. The background pH in the Dakota sandstone is about 8.28. Seepage of C-Pit water (pH 12.5) is predicted to increase the pH in the Dakota sandstone about 0.05, resulting in a predicted pH of approximately 8.33. This predicted pH is less than the pH GWPS of 8.5. 5 Conclusions Results of the transport simulations predict that seepage from the ponded water in the C-Pit will not impact groundwater quality in the Dakota sandstone aquifer at concentrations above the respective GWPS. The constituent transport analysis predicts that quantity of C-Pit seepage that might be released to the underlying Dakota sandstone will be sufficiently dispersed during transport so that the existing groundwater quality is no[ adversely affected. Therefore, the ponded C-Pit water does not require accelerated removal or treatment from a groundwater quality perspective. The predicted increases in constituent concentrations are likely overestimates as the model simulations used parameters and assumptions that were conservative. The conservative nature of these predictions results from: • Using aone-dimensional estimate of the seepage velocity using Darcy's Law. Since the underlying hydrogeologic system is three-dimensional, the rate and amount of seepage from C-Pit is likely overestimated, therefore, the quantity of seepage (about 7%) released to the Dakota sandstone may also be overestimated; • Using atwo-dimensional quasi-analytical contaminant transport model for athree- dimensional flow system likely overestimates the predicted groundwater constituent concentrations in the Dakota sandstone. Use of the highest assumed hydraulic gradient (0.01) results in faster constituent arrival times and higher constituent concentrations. Use of small dispersion coefficients results in less spreading of the constituent plumes and higher constituent concentrations in the Dakota sandstone; • Assuming an instantaneous release of C-Pit seepage to the Dakota sandstone, regardless of the fact that the seepage must flow vertically through low permeability claystones, siltstones, and shales a distance of about 650 ft. If the three-dimensional flow and transport and amount of time for the seepage to flow from C-Pi[ to the Dakota sandstone was considered in the analysis, the potential impact on the hypothetical receptor well would likely be several hundred years in the future, if an impact occurred at all; October 29, 2004 6 Revision 0