Verification and sensitivity of the calculational methods used in the PATHRAE code to predict subsurface contaminant transport for risk assessments of SRP waste sites

Presented in this report are an independent verification of the subsurface contaminant transport calculations contained in the code and an assessment of the sensitivity of predicted contaminant concentrations to uncertainties in transport parameters. The subsurface transport approximation incorporated in the PATHRAE risk assessment code was compared with alternate two-dimensional and three-dimensional approximations and with the EPA VHS model. Agreement between the PATHRAE approximation and the alternate two-dimensional approximation was good. Due to its neglect of vertical dispersion, the PATHRAE model predicted higher groundwater (undiluted) concentrations than the three-dimensional approximation and, for EPA parameters, the VHS model. The use of a value of zero for horizontal dispersivity, as specified for 1 m and 100 m wells in SPR waste site analyses, was found to add an additional degree of conservatism to PATHRAE estimates of groundwater concentration, yielding levels that were more than three orders of magnitude higher than those of the three-dimensional model for a 100 m well. Implementation of the transport approximation in the PATHRAE code was verified by comparing code generated concentrations with those of an independent calculation for wide ranges of the input parameters. Agreement between PATHRAE and the independent calculations was excellent.