Estimation of water seepage rate in the active crater lake system of Kusatsu-Shirane volcano, Japan, using FDNPP-derived radioactive cesium as a hydrological tracer

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.