THE MERITS OF A ONE-DIMENSIONAL GOUNDWATER TRANSPORT MODEL AND ITS APPLICATION TO THE PRESTO-EPA MODEL FOR RISK ASSESSMENTS

This paper presents the merits of the one-dimensional groundwater transport model used in the PRESTO-EPA model. PRESTO-EPA is a multi-media model designed to assess the maximum individual doses within a critical population group and the genetic and somatic health effects to the general population resulting from the disposal of radioactive wastes in shallow trenches. The model employs a one-dimensional groundwater transport model which can considerably reduce the effort of model development and processing. However, the model has two limitations which are often noted. First, it is unable to calculate the concentration distribution of the plume; second, errors may be induced from neglecting the lateral dispersion effects. These limitations have only a very limited impact on the results of a risk assessment. First, the prediction of the concentration distribution is not essential in conducting a risk assessment, because only the concentration in the well water is required for the dose calculation. The potential error that could be induced from neglecting the lateral dispersion depends on the location of the measurement. Since the application of PRESTO is strictly for the assessment of individual doses, the location of the pumping well is always set no more than 200 meters from the edge of the source. Within this limit, the relative error is expected to be less than 6%, which is considered to be negligible. Therefore, a one dimensional model can be used without incurring excessive error.