Field experiment on tracer migration through unsaturated loamy layer. (1). Migration characteristics of nonreactive tracers under natural rainfall condition.

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 (Sr, Co, Cs). The present paper discusses the migration characteristics of reactive tracers under the direct influence of natural rainfall and evapotranspiration.The infiltration rates of reactive tracers such as Sr, Co and Cs were smaller than those of nonreactive tracers. Particularly, most of injected Cs remained near the tracer in jection plane, i.e. the ground surface. The retardation phenomenon was caused by the adsorption of reactive tracers by loamy soil. The temporal and spatial variation of the distribution coeffcient (KdT) of reactive tracers obtained by field experiment ranged from 10-2 to 10102ml/g. This widely dispersed KdT value showed dependency on the concentration of the tracer itself and coexisting cation. The KdT value obtained by the field experiment was consistent with that obtained by the batch experiment considering the concentration dependency. In addition, the selectivity coeffcients of reactive tracers were almost constant with respect to coexisting Ca, and this indicates that the adsorption of reactivet tracers mainly resulted from ion exchange reaction.