Biogeochemical behaviour of 137Cs and 90Sr in the artificial reservoirs of Mayak PA, Russia

Abstract The Mayak Production Association (PA) in the southern Urals, Russia was the site of the first weapons-grade plutonium production reactor complex in Russia. The site and surrounding area have been significantly contaminated by direct discharges of radionuclides for over 40 years, the Techa River alone having received more than 100 PBq of waste in the period 1949–1956. The aim of this study was to consider the levels of 90 Sr and 137 Cs in water, sediment and biota samples for two industrial reservoirs in the Mayak PA area, thus allowing a biogeochemical assessment of the behaviour of radionuclides in the system. Four sediment cores were collected and sectioned along with four water samples and seven fish samples (pike, perch and roach). Samples were analysed using (i) standard gamma-spectrometric techniques (HPGe and NaI(Tl) detectors) for 137 Cs determination; and (ii) radiochemical separation and beta-counting (low-background, anti-coincidence and Geiger–Muller counters) for 90 Sr determination. Maximum specific activities (dry weight) of 3350 kBq kg −1 137 Cs and 720 kBq kg −1 90 Sr were measured in sediments from Reservoir 10. Activity levels of sediment-bound radionuclides in Reservoir 11 were 403 kBq kg −1 137 Cs and 670 kBq kg −1 90 Sr. Water concentrations in Reservoir 10 were as high as 100 Bq l −1 137 Cs and 8.4–14 kBq l −1 90 Sr. A dramatic decrease in 137 Cs concentrations was observed in Reservoir 11, i.e. 1.1–1.5 Bq l −1 , but 90 Sr levels fell to a lesser extent, i.e. 1.9–2.4 kBq l −1 . Sediment and water activity data allowed the calculation of distribution coefficients ( K d values). This parameter fluctuated for both radionuclides reflecting the heterogeneous nature of the sediment deposits in the reservoirs. Caesium-137 Concentration Factors (CFs) as high as 1400 l kg −1 were calculated for pike from Reservoir 10. A pronounced ‘trophic level’ effect was evident in Reservoir 11 (pike CF=1000, roach CF=240).