Environmental impact assessment of radionuclide and metal contamination at the former U site at Kadji Sai, Kyrgyzstan

Abstract

During 1949–1967, a U mine, a coal-fired thermal power plant and a processing plant for the extraction of U from the produced ash were operated at the Kadji Sai U mining site in Tonsk district, Issyk-Kul County, Kyrgyzstan. The Kadji Sai U legacy site represents a source of contamination of the local environment by naturally occurring radionuclides and associated trace elements. To assess the environmental impact of radionuclides and trace metals at the site, field expeditions were performed in 2007 and 2008 by the Joint collaboration between Norway, Kazakhstan, Kyrgyzstan, Tajikistan (JNKKT) project and the NATO SfP RESCA project. In addition to in situ gamma and Rn dose rate measurements, sampling included at site fractionation of water and sampling of water, fish, sediment, soils and vegetation. The concentrations of radionuclides and trace metals in water from Issyk-Kul Lake were in general low, but surprisingly high for As. Uptake of U and As was also observed in fish from the lake with maximum bioconcentration factors for liver of 1.6 and 75, respectively. The concentrations of U in water within the Kadji Sai area varied from 0.01 to 0.05 mg/L, except for downstream from the mining area where U reached a factor of 10 higher, 0.2 mg/L. Uranium concentrations in the drinking water of Kadji Sai village were about the level recommended by the WHO for drinking water. The 234U/238U activity ratio reflected equilibrium conditions in the mining pond, but far from equilibrium outside this area (reaching 2.3 for an artesian well). Uranium, As and Ni were mainly present as low molecular mass (LMM, less than 10 kDa) species in all samples, indicating that these elemental species are mobile and potentially bioavailable.The soils from the mining sites were enriched in U, As and trace metals. Hot spots with elevated radioactivity levels were easily detected in Kadji Sai and radioactive particles were observed. The presence of particles carrying significant amount of radioactivity and toxic trace elements may represent a hazard during strong wind events (wind erosion). Based on sequential extractions, most of the elements were strongly associated with mineral matter, except for U and As having a relatively high remobilization potential. Low Kd was obtained for U (3.5 × 102 L/kg d.w.), intermediate Kds (∼3 × 103 L/kg d.w.) were obtained for 226Ra, As and Ni, while a high Kd (2.2 × 105 L/kg d.w.) were obtained for Pb. The accumulation of metals in fish gills reflected the LMM species in the Issyk-Kul water, and did not show any bioaccumulation. The muscle Hg concentrations in all fish species were low and did not represent any health risk even for groups at risk.Total gamma and Rn dose rate to man amounted to about 12 mSv/y, while the highest calculated dose rate for non-human species based on the ERICA Assessment Tool were obtained in terrestrial plants (164 μGy/h) due to the Ra exposure. The results obtained showed that radiation doses to resident public at all of the investigated sites in the Kadji Sai area were in general relatively low. Low radiological risk and no detrimental health impact on resident public can be expected at these sites. However, exposure to Rn and Tn in the living environment can be further reduced by implementing simple countermeasures such as ventilation of dwelling cellars. More focus in the Kadji Sai area should probably be put on trace elements, especially the As uptake in fish in Lake Issyk-Kul.