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

Schematic diagram of the groundwater and soil water mixing by means of dissolved 137Cs. • Dissolved 137Cs in stream water derived mainly from soil water. • Low impact of dissolved 137Cs in groundwater was confirmed. • 137Cs stored in the shallow unsaturated area was the source of temporary increase. Numerous studies have been conducted on 137Cs 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 137Cs concentrations has been reported. In addition, temporary increases in dissolved 137Cs 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 137Cs 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 137Cs 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 137Cs concentration in groundwater (average: 0.93 mBq/L) was lower than that in stream water. Conversely, high concentrations of dissolved 137Cs 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 137Cs concentrations in stream water were considered to be derived mainly from soil water and diluted by groundwater. The high dissolved 137Cs 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 137Cs concentration in stream water observed at the beginning of a rainstorm may be caused by the release of high levels of dissolved 137Cs in soil water stored in the shallow soil layer. [ABSTRACT FROM AUTHOR]