RESUMO
We describe here a first attempt to estimate the water seepage rate of an active crater lake using radioactive cesium dispersed into the environment by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 as a hydrological tracer. Kusatsu-Shirane volcano is one of the most active volcanoes in Japan and has an active crater lake named Yugama. There is no outflow such as a river from Yugama crater lake. The content level of stable cesium (133Cs) in Yugama water was almost constant at 15-20 nM during the sampling period of Nov. 2011 to Nov. 2014. In contrast to 133Cs, however, the radioactive cesium (134Cs and 137Cs) concentrations in Yugama water decreased at a more rapid rate than expected by radioactive decay. Based on the decreasing rates of activity concentrations of 134Cs and 137Cs in Yugama water during the three years between 2012 and 2014, it is estimated that 700-800 m3 of the Yugama water, which corresponds approximately to 0.1% of the total volume, leaks through the lakebed per day. In the estimation, balance between leakage of radioactive cesium contaminated water though lake bed and geothermal inflow of radioactive cesium-free water in the volcano was taken into account. Consequently, the water seepage rate of the Yugama crater lake was calculated to be 8.1-9.3 L s-1.
