Fixed Point Observations and Characterization of Radioactive Caesium in Tama River

Behavior of radioactive caesium, derived from Fukushima Dai-ichi Nuclear Power Plant Accident, in river water and sediment investigated during 2012–2016. Concentrations of radioactive caesium in river water and sediment were decreased with time exponentially. The residence half-life of radioactive caesium in the river water and sediment were 0.788–1.50 year for 134Cs, 1.22–2.17 year for 137Cs. Any decrease in radioactive caesium concentration in the Tama river is because of weathering effect than radioactive decay. Concentrations of suspended radioactive caesium temporarily increased when sediments were resuspended due to rain. On the other hand, dissolved radioactive caesium is not easily impacted by this factor. Radioactive caesium concentration in sediments was considerably higher than that in river water. It indicated that much of the radioactive caesium in the Tama river existed in the sediments. Sequential extraction, elemental and crystal phase analysis were performed on the sediments and examined the chemical state of radioactive caesium as well as the adsorption mechanism. Radioactive caesium in sediment was present in a stable chemical form, and there is possibility that radioactive caesium was incorporated in biotite.