Dynamic Model for Calculating Infiltration of Rain Water through an Unsaturated Zone, and Its Application to Environmental Protection Agency’s PRESTO Model

Abstract

The EPA’s multimedia environmental pathway model, Prediction of Radiological Effects due to Shallow Trench Operation (PRESTO), has been applied extensively to scenarios involving both the remediation of soil contaminated with radioactivity and the near-surface disposal of radioactive waste. Designed primarily for regulatory applications, the model can estimate both the individual dose and risk for a critical population group and the collective health effects to a general population. This paper presents the theoretical background of PRESTO’s dynamic infiltration component. The simulation of infiltration involves solving the dynamic equations that govern the transport of liquid and vapor phases of water in the overland flow, subsurface flow, and atmospheric diffusion systems. The basic space-dependent partial differential equations that describe these processes are transformed into space-independent ordinary differential equations; this entails subdividing the soil moisture content into three components: gravity, pellicular, and hygroscopic waters. Modeling validation, as assessed at the Barnwell, S.C. site, indicates that the simulation results agree well with field studies conducted by the USGS and by the US NRC. Tests and validation runs have demonstrated that: (1) the calculation results can simulate infiltration through a saturated/unsaturated subsurface soil layer, (2) instabilities of numerical calculation do not occur, and (3) the run time for model execution is reasonably short. Because of lack of reliable field data, the model can only be verified and not fully validated.