Tritium contamination of surface and ground waters at the “Dnepr” peaceful underground nuclear explosions site

Abstract

The article presents results of a study on the tritium content in surface and ground waters in the area of peaceful underground nuclear explosions of the “Dnepr” series. Low-yield thermonuclear explosions (1.8–2.1 kt of TNT equivalent) were carried out inside Mount Kuel’por (Khibiny Massif, Kola Peninsula, Murmansk Oblast, the Russian Federation) in 1972 and 1984. The purpose of the explosions was to crush the ore body (apatite minerals), followed by the extraction of the crushed rock to the ground surface. The main long-term problem generated by these explosions was the flow of tritium-contaminated groundwater onto the ground surface. The area where the explosions took place is actively visited by tourists. Water from local reservoirs, in particular from the Kuniyok River, is used by people for drinking. The purpose of this study was to assess the drinkability of the local waters in terms of activity concentration of tritium. To achieve this goal, 35 water samples were taken in 2019 from wells, boreholes, rivers, streams, lakes and other accessible environmental waterbodies. Activity concentration of tritium in the samples was determined using the Quantulus 1220 low-background liquid scintillation spectrometer. The activity concentration of tritium in the water samples was in a rather wide range: from less than 2 Bq/kg up to 1510 Bq/kg. The maximum value was up to three orders of magnitude higher than the regional background level of approximately 2 Bq/kg. At the same time, the maximum activity concentration was significantly lower compared to the intervention level for drinking water (7600 Bq/kg, according to Sanitary Norms and Rules of the Russian Federation). Based on the results of this study and data obtained by other researchers earlier, it became possible to assess the half-time for decrease of activity concentration of tritium in surface and ground waters in the period 2008–2019. The mean value (± standard error of the mean) of the effective half-time of tritium in water from the mine, the boreholes, and the Kuniyok River was estimated as 4.4 ± 0.2 year. The decrease in activity concentration of tritium in water depended more on ecological processes (dilution with “pure” water) than on physical decay of the radionuclide. In 2019, the estimated value of the effective dose due to ingestion of tritium in drinking water from the Kuniyok River was 0.17 μSv; this was negligible compared to the dose limit of 1 mSv per year for the public.