A one-dimensional dynamical soil–atmosphere tritiated water transport model

A one-dimensional numerical model for simulating transport of heat, water and tritiated water (hereafter referred to as HTO) in unsaturated surface soil and their soil–atmosphere exchange has been developed. This model consists of five prognostic equations for soil temperature, soil water content, soil air humidity and HTO concentrations in liquid and gas phases. The model is applied to a hypothetical and simplified scenario of HTO contamination of an unsaturated soil layer with actually observed meteorological conditions in Japan to analyze the dependencies of the HTO transport and evaporation on the meteorological conditions, hydrological conditions and soil properties. Results obtained indicate that the infiltration of water and the soil moisture conditions largely affect the transport and evaporation. Therefore, precipitation and soil moisture characteristics seem to be the most essential factors. Other meteorological factors are found to have substantial effects through two pathways, one being direct and fast by changing the atmospheric conditions and availability of vapor at the ground surface, and the other being indirect and slow by affecting the infiltration and water content. It can also be demonstrated that the usage of time-averaged meteorological and hydrological conditions reduce the estimated amounts of HTO transport and evaporation considerably, and that high-concentration episodes virtually determine the annual mean HTO concentration and evaporation.