Radioactive contamination of surface waters from a fly-ash depository at Velenje (Solvenia)

The coal-fired Šoštanj thermal power plant (750 MW el) has been in operation since 1972 and currently produces almost a million tonnes (Mg) of coal ash per year. Fly ash with a U content of at least 25 mg kg −1 is transported as a slurry and was disposed at first into a lake and later into wet ponds on a depository of an area of 0.50 km 2. The deposited ash has direct contact with the lake water. Leaching of radionuclides from fly ash into lake and rain water and pile seepage water are the main sources of radioactive contamination of the lake and its outflowing waters. Results showed enhanced levels of radionuclides in the affected surface waters and sediments. The high alkalinity of lake water (the pH is about 11) strongly influences the solubility and therefore the radionuclide concentrations: a very low concentration of uranium (natural uranium: 0.02 mg m −3) was measured while radium is present in concentrations an order of magnitude higher ( 226Ra: 70 Bq m −3) than in other uncontaminated waters. The impact of the lake water could be detected in the Paka River—in water and sediments—far from its confluence with the lake outflow.