To evaluate the characteristics of groundwater flow and nuclide migration through unsaturated loamy soil, various field tracer experiments have been carried out using both nonreactive tracers (D2O, Br-) and reactive tracers (Co, Sr, Cs). This paper discusses the migration characteristics of nonreactive tracers under the direct influence of natural rainfall and evapotranspiration.
D2O was a more conservative tracer than Br- ion to evaluate an unsaturated flow because the infiltration rate of Br- ion was slower than that of D2O owing to anion adsorption of allophane, a major mineral in loamy soil. The longitudinal and transverse dispersion coefficients were calculated from the spatial distributions of D2O concentration. The linear relation between the maximum concentration depth of D2O and the cumulative rainfall indicates that the rainfall gives a significant influence on the infiltration rate while the evapotranspiration gives little influence.
