ANALYSIS OF SPATIAL DISTRIBUTION AND INVENTORY OF RADIOACTIVITY WITHIN THE URANIUM MILL TAILINGS IMPOUNDMENTResults are presented of the characterization of radioactivity inventory of Zapadnoe uranium mill tailings impoundment situated at Pridneprovsky Chemical Plant (PChP; Dneprodzerginsk, Ukraine). Analyses of radioactivity data set based on analytical studies of core material from 15 characterization boreholes allowed significantly refining waste volume and radioactivity inventory estimates. Geostatistical analyses using variogram function have established that radioactivity distribution in Zapadnoe tailings is characterized by regular spatial correlation patterns. Ordinary kriging method was applied to assess distribution of radioactivity in 3D. Results of statistical analyses suggest significant redistribution of uranium in the dissolved form in the residues (presumably due to water infiltration process). The developed structural model for radioactivity distribution is used for further risk assessment analyses. Derived radioactivity correlation scales can be used for optimization of sample collection when characterizing the PChP Site and similar contaminated sites elsewhere.
The results of the study on speciation and mobility of uranium in the ore processing residues in the Centralny Yar tailings (CY, former uranium processing site -Pridneprovsky Chemical Plant in Ukraine) are presented. Due to poor neutralization sludge material was dumped into the tailings body in acidic state. Several incidents with breakage in the pipeline transporting complex radiochemical solutions caused radioactive material spillover onto the tailings surface. Two features of radiological concern were identifiedsecondary contamination of the tailings surface amid elevated gamma dose rates, and excessive migration of radionuclides of U/Th decay series in strong acidic conditions within the tailings body. The monitoring data collected during 2005-2017 showed fast migration of uranium from the tailings body into the groundwater with specific activity varied in the range from 1 to 20 Bq/L. To support this finding the experimental studies aimed to obtain physical and chemical speciation of uranium in the tailings materials in existing and simulated conditions were undertaken. This was conducted by application of modified BCR sequential extraction methods followed by assessment of uranium speciation in equilibrium conditions, using the geochemical modeling tool MEDUSA coupled with the HYDRA database.
The results of works on reconstruction and development of the hydrogeological monitoring system at the Prydniprovsky Chemical Plant site, Kamyanske (PChP) and on the groundwater survey using the improved observation wells network are presented (first such survey since 2016). During the works, geology structure of the site was précised, hydraulic testing was carried out, and groundwater was sampled at a number of uranium production legacy objects that have not been previously covered by observations. Automated monitoring of groundwater levels (GWL) has been started. As a result, new information on the seasonal dynamics of GWL was obtained. New sources of serious chemical and radioactive contamination of the geological environment are identified at the Southern PChP site, in particular in the area of settling basins № 220 and 230. Radioactive contamination of groundwater with excess of background levels is also observed also in the zone of “historic” settling pond situated below the “Central Yar” uranium tailings. In addition to previously known chemical toxicants (Mn, Ni, Pb), the monitoring study revealed groundwater contamination by arsenic and mercury in the areas affected by the PChP facilities. Thus, groundwater contamination at the PChP industrial site is formed under the influence of more man-made legacy sources than previously thought. The identified new sources of pollution deserve additional characterization and consideration when predicting the long-term impacts of the PChP site on the surface water system of the Konoplyanka River—Dnieper River.
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