ИСТИНА

A major earthquake and devastating tsunami on 11 March 2011 caused a nuclear accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). As a result of the FDNPP accident, 134Cs and 137Cs deposited on the underlying surface in North-Western direction from the FDNPP. Mobility of radiocesium (r-Cs) of accidental origin in the aquatic environment is governed by the ratio of r-Cs chemical forms in fallout and site-specific environmental characteristics determining the rates of leaching, fixation/remobilization, as well as sorption-desorption of the mobile fraction. R-Cs in the environment is strongly bound to soil and sediment particles. This is due to high selective reversible sorption by micaceous clay minerals (illite, vermiculite etc.) and fixation. The objective of this work is to study current dynamics of r-Cs in ponds/reservoirs within exclusion zone in the proximity to the FDNPP and to characterize r-Cs exchange between sediments and water column. Field observations were carried out on four ponds (Inkyozaka, Suzuuchi, Funasawa and Kashiramori) and their catchments in the close proximity to the FDNPP - Okuma town. Water samples of 2 L volume were collected in the ponds three times during 2015 in duplicate and filtered in the laboratory using membrane filters of 0.45 μm pores. Filters and filtrates were assayed separately for 134Cs and 137Cs. Dissolved K+, NH4+, Na+, Ca2+, Mg2+ in water samples were measured by ion chromatography, and stable 133Cs+ was measured by ICP-MS. Soil samples were collected to a depth of 30 cm using a liner corer. The 137Cs and 134Cs activity concentrations were measured by gamma spectrometry using a high-purity germanium detector (HPGe) CANBERRA GC3018. The measurement time was set to ensure a statistical error not more than 5%. Total r-Cs activity concentration in water of the ponds was found to be relatively high as compared with the rivers or large lakes in the area and was as high as 68 Bq/L for the most contaminated pond Suzuuchi. Activity concentration of particulate r-Cs in pond’s water was found to be higher than that in the top soil of the catchment surrounding the pond and bottom sediments. Kd values in the ponds of Okuma town appeared relatively high (mostly in range 100-400 L/g) and comparable with Kds observed for rivers and large lakes in Fukushima area. Kd values in Inkyozaka, Suzuuchi and Funasawa were quite stable during observation period, especially for Suzuuchi. At the same time, in Kashiramori Kd varies within an order of magnitude. The effective dispersion coefficients in the Fukushima soils were found to be relatively high (in range 2-10 cm2/year) as compared to Chernobyl data. It is shown roles of K+, NH4+ and 133Cs+ in r-Cs remobilization are comparable, but their relative contributions in forming dissolved r-Cs concentration in water can vary with time.