Evaluation of groundwater discharge in Lake Ezu, Kumamoto, based on radon in water

Abstract

Approximately 400,000-500,000 m³ day^-1 of groundwater are discharged at Lake Ezu from two aquifers with different depths. These two aquifers are separated by an aquitard composed of lacustrine deposits. Since this aquitard is partially absent, groundwater in both the deep and shallow aquifer can discharge into the lake. However, the amount of groundwater discharged from each aquifer into the lake is unclear. Our previous study showed that the radon concentration in groundwater could be used as an ideal tracer to separate shallow and deep groundwater components in the study area. In the present study, we collected water samples from around the study area once a month since June 2009 and then calculated the contribution ratio of groundwater discharge from both the shallow and deep aquifer into the lake using a radon mixing model. The results revealed that the groundwater discharge from the deep aquifer was estimated to be the dominant source of groundwater discharged into the lake (58% on average), with seasonal fluctuation. The results of this study could be used to more precisely estimate the mass balance in Lake Ezu.