Dynamics of suspended and dissolved radiocaesium in a small irrigation pond based on vertical water quality profiles

Following the Fukushima Daiichi Nuclear Power Plant accident, extensive research has been conducted on the dynamic distribution of radiocaesium in large reservoirs. However, studies on small ponds, particularly those used for rice cultivation, are limited. This study analyzed the vertical water quality profiles, including the bioavailable Caesium-137 (137Cs) in an irrigation small pond classified as discontinuous cold polymictic based on water mixing type, and subjected to a high (2.0 MBq m−2) deposition of 137Cs and evaluated their potential risk associated with agricultural use. During 2014–2015, the suspended and dissolved concentrations of 137Cs in the pond water ranged from 0.10 to 7.7 Bq L−1 and 0.03–0.88 Bq L−1, respectively. The elution of dissolved 137Cs from the bottom sediment occurred only during the summer when the water temperature exceeded 15 °C, NH4+ increased, and dissolved oxygen decreased. Except during flood inflow mixing events, the activity concentration of dissolved 137Cs in the bottom layer did not increase in the surface layer at depths ≤ 1.0 m. In a 3 m shallow pond without a well-defined thermocline, a gradual density stratification primarily occurred in the lower layer during summer, limiting the transport of dissolved 137Cs from the bottom to the surface layer. About 5 % of 137Cs in the suspended solids and bottom sediment are exchangeable, indicating the mica-rich soils and geological conditions with a high fixation capacity of 137Cs reduce the risk of 137Cs elution. Even after three to four years of considerable depositions, the risk of radiocaesium absorption by paddy rice and subsequent increase in radiocaesium concentration in brown rice was negligible as long as the surface water of the pond was used for agriculture.