Dissolved 137Cs concentrations in stream water and subsurface water in a forested headwater catchment after the Fukushima Dai-ichi Nuclear Power Plant accident

Numerous studies have been conducted on Cs-137 transport in the environment in Fukushima, Japan, since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Many studies have monitored river and stream water, and a declining trend in dissolved Cs-137 concentrations has been reported. In addition, temporary increases in dissolved Cs-137 concentrations have been observed during rainstorm events. However, the source and process of such temporary increases remain poorly understood. To address these gaps in knowledge, intense multi-point sampling and analysis of dissolved Cs-137 in stream water, soil water, and groundwater were conducted in a forested headwater catchment in Yamakiya district, located 35 km northwest of the FDNPP, from July 2015 to November 2016. The average dissolved Cs-137 concentration in stream water during baseflow conditions was 17 mBq/L, whereas the concentration during rainstorm events was 31 mBq/L (maximum: 170 mBq/L on July 15, 2015). The dissolved Cs-137 concentration in groundwater (average: 0.93 mBq/L) was lower than that in stream water. Conversely, high concentrations of dissolved Cs-137 in soil water were observed (average: 81 mBq/L), particularly at lower parts of the slope near the spring point and in wet conditions throughout summer. Consequently, the dissolved Cs-137 concentrations in stream water were considered to be derived mainly from soil water and diluted by groundwater. The high dissolved Cs-137 concentration in soil water may be derived from litter leachate in the shallow soil layer stored on the slope. During rainstorm events, the saturated area expands upward; therefore, the temporary increase in the dissolved Cs-137 concentration in stream water observed at the beginning of a rainstorm may be caused by the release of high levels of dissolved Cs-137 in soil water stored in the shallow soil layer.