Cs and plutonium isotopes accumulation/retention in bottom sediments and soil in Lithuania: A case study of the activity concentration of anthropogenic radionuclides and their provenance before the start of operation of the Belarusian Nuclear Power Plant (NPP).

Knowledge of the background activity concentrations of anthropogenic radionuclides before the start of operations of the new nuclear facilities in Belarus is of great value worldwide. Inland water bodies in Lithuania (specifically the Neris River, the Nemunas River and the Curonian Lagoon) are near the site of the Belarusian NPP under construction and, for this reason, sediments and flooded soils from these sensitive areas were analysed for radiocesium and plutonium isotopes (macrophytes were analysed only for Cs) in 2011-2012. The Cs and Pu activity concentrations in bottom sediments from the Nemunas River, sampled in 1995-1996 and re-calculated to the year 2016, were compared with those of 2011-2012. The obtained activity of Cs in bottom sediments of the Nemunas River and Curonian Lagoon varied from 1 Bq/kg to 47.0 Bq/kg. The activity of Cs in the tested soils ranged from 5.3 B g/kg to 32.9 Bq/kg. The Pu activity in bottom sediments of the studied sampling sites varied between 0.016 and 0.34 Bq/kg and in flooded soils from 0.064 to 0.55 Bq/kg. The Pu activity values were very low or lower than the detection limit. The activity of Cs in macrophytes varied from values lower than the detection limit to 6 Bq/kg. A strong positive linear correlation for bottom sediments was calculated between: Pu and total organic carbon (TOC), r = 0.86, p-value 0.01; Pu and silt, r = 0.80, p-value 0.029; Cs and silt, r = 0.78, p-value 0.04; and Cs and TOC, r = 0.85, p-value 0.015. The similar peculiarities of Cs and Pu accumulation in bottom sediments and flooded soil allow us to assume that Cs can be used as a tracer for Pu in the initial stage of searching for radionuclide accumulation zones. A remaining impact of the Chernobyl fallout in average comprised: in the Lower Nemunas River and Curonian Lagoon sediments - 51%, in the Middle Nemunas River -90% and in the floodplains of the Nemunas River - 59%, while the provenance of plutonium in studied bottom sediments and flooded soil was the global fallout.