Influence of flooding on exchangeability and release of stable and radioactive cesium in contaminated paddy soil

An incubation experiment was conducted to clarify how soil flooding influences the mobility of radioactive cesium (RCs: ¹³⁴Cs and ¹³⁷Cs) in paddy soil after aging, focusing on the effects of ammonium increase and soil reduction. We used two contaminated paddy soils (A and B, both Gleyic Fluvisols) aged for 26 months after the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant accidents, and analyzed both the RCs and stable cesium (¹³³Cs). The soils were incubated in a flooded condition, with or without the addition of ammonium fertilizer, for 1, 15 and 30 d, and in an unflooded condition for 30 d. After the incubation periods, we quantified ¹³³Cs concentration in the soil solution, ¹³³Cs and RCs released from soil in 0.01 mol L^–1 calcium chloride solution as parameters of release intensity, and ¹³³Cs and RCs extracted with 1 mol L⁻¹ ammonium acetate solution as exchangeable quantities, and analyzed their relationship with ammonium content and redox condition in soil. The increase of ammonium by both ammonification and fertilizer application promoted release of exchangeable ¹³³Cs and RCs to the soil solution and calcium solution. When ammonium content became low during unflooded incubation, exchangeable ¹³³Cs and RCs themselves decreased. When soil reduction progressed with flooded incubation, however, exchangeable ¹³³Cs and RCs decreased, despite high ammonium content. To estimate the influence of soil reduction on the exchangeability of RCs, soil A was sequentially extracted with sodium hydrosulfite (a reducing agent) and ammonium acetate solutions. Compared with a control treatment using sodium sulfate instead of sodium hydrosulfite, the total RCs extracted by the reducing treatment was 42% lower, indicating that soil reduction decreases RCs exchangeability. Through these analyses of ¹³³Cs and RCs, we conclude that soil flooding influences the mobility of aged RCs through two opposite effects: the release of exchangeable RCs into soil solution is enhanced with increased ammonium, while the exchangeable RCs itself decreases due to soil reduction.