Evaluation of Application of One-dimensional Groundwater Transport Model to PRESTO-EPA Model

This work evaluates the merits and the magnitude of potential errors expected from Hung’s one-dimensional groundwater transport model as integrated into EPA’s Prediction of Radiological Effects due to Shallow Trench Operation (PRESTO) model. PRESTO is a multimedia model designed to assess both the maximum individual doses for a critical population group and the cumulative genetic and fatal health effects for a general population that result from soil contamination and near-surface waste disposal. Hung’s groundwater transport model was incorporated into PRESTO to balance efficiency with accuracy for the purpose of regulatory analysis. This 1D advective model accounts for dispersion through a spatially-dependent correction factor while maintaining the proper overall time-integrated concentration. Generally, 1D models suffer from two important limitations. They are incapable of calculating the transverse concentration distribution of a contaminant plume, and they may introduce excessive errors caused by their inherent simplifications and assumptions. This paper reevaluates the consequence of these limitations for generic regulatory analyses currently practiced in the United States; for this purpose, the calculation of contaminant distributions in a plume is not necessary because only nearby well-water concentrations are required for dose calculations. For typical releases, errors in Hung’s model relative to calculations from a 3D finite-difference groundwater code, MODFLOW/SURFACT, can be maintained below 3%. In light of the limited impact of its simplifying assumptions and its merits in terms of computational efficiency, this work concludes that the Hung model is a useful 1D groundwater transport algorithm for near-field regulatory and compliance-driven human health risk assessments. In these applications, the receptor well is assumed to be located no more than 100 m away from the edge of the contaminated site.