Application of ²²²Rn to the Analysis of the Exchange between River Water and Groundwater

Abstract

As ²²²Rn is an inert gas generated from ²²⁶Ra in soils and rocks, ²²²Rn concentration in groundwater is higher than in river water. It is, therefore, possible to determine a section by measuring ²²²Rn concentration in the stream where groundwater is seeping into the river. Moreover, the simultaneous equations of water balance and ²²²Rn balance make it possible to quantify groundwater seepage and river water infiltration at the same time. However, as the ²²²Rn loss in a stream is estimated directly by measuring the decrease in ²²²Rn concentration in river water, the traditional method of analysis using ²²²Rn is applied only to a river which has a section with no groundwater input.
In this paper, the authors proposed a new method to analyze an exchange between river water and groundwater using ²²²Rn which is applicable to a river in any condition. The ²²²Rn loss in a stream was calculated assuming that a stagnant film existed at the boundary between water and air. Its thickness was estimated as 20μm from field investigations and the previous research. The method using ²²²Rn was applied to the Omoi River Basin, Tochigi, and the groundwater seepage and the river water infiltration were quantified simultaneously. The obtained values were within reasonable ranges and the applicability of the method using ²²²Rn was troved.