Natural radioactivity of groundwater in Vendian deposits in St. Petersburg Region.

The study is focused on the hydrogeological conditions and the chemistry of groundwater of the Vendian aquifer in the western part of the Leningrad oblast (Karelian Isthmus and the area near Sosnovy Bor town) and St. Petersburg City, where groundwater features higher radioactivity, but nevertheless it is used for drinking water supply. Data on the radiological characteristics, which have been determined in the estimation of the quality of groundwater used for drinking are generalized and analyzed. These characteristics include the gross alpha and gross beta activity and the specific activity of natural radionuclides 222Rn, 226Ra, 228Ra, 210Pb, 210Po, 238U, and 234U. The data were subjected to statistical and correlation analysis to determine the hygienic criteria for the use of groundwater of this aquifer for drinking water supply and to study the sources and the processes of formation of the natural radiological background. Groundwater quality standards were shown to be exceeded in the majority of the analyzed wells. The brackish water in the southern, deeper, part of the aquifer system was shown to have higher radioactivity and relatively high concentrations of 226Ra, 228Ra, 210Pb, and 210Po, compared with fresh water in the northern part of the territory, of which higher, though nonuniform, 222Rn activity is typical. Relationships between the radiation characteristics of groundwater are considered along with the causes of formation of groundwater radionuclide composition as a result of the higher radioactivity of the host deposits and the chemistry of groundwater; changes in the radiological and hydrochemical background groundwater characteristics from the north to the south are characterized in accordance with the subsidence of the aquifer system and an increase in the stagnation of the hydrochemical regime. The analysis of the well-known relationship between the concentrations of radium isotopes in groundwater, uranium and thorium isotopes in the host rocks, and groundwater residence time in the aquifer, along with the comparison of the available field data with calculation results, suggested the conclusion that the concentration of uranium in the water-bearing rocks in the major portion of the area under consideration is higher than its regional mean values.

The status and trends in radioactive contamination of groundwater at a LLW-ILW storage facility site near Sosnovy Bor (Leningrad region, Russia).

The article presents results of field studies at a site of radioactive waste storage and disposal facilities (the so-called LD RosRAO site, Sosnovy Bor, Leningrad region, Russia). The objective of the study is to overview the history and occurrence of groundwater contamination to answer the question whether the radioactive plume is historical (formed due to accidents, which happened at the beginning of the operations with the radioactive waste) or the release of radioactive solutions to groundwater is still occurring. The main method used to study the evolution of radioactive contamination is to analyze long-term observations of tritium, total alpha, and total beta activities in groundwater samples. The role of the hydrogeological conditions of the site, such as flow parameters, groundwater flow pattern, inter-aquifer downward groundwater leakage, and water-table fluctuations, in the evolution of contamination plumes has also been analyzed. In the field investigations, the integrity of the storage buildings to potential leaks was confirmed by different indirect methods. It was concluded that there is currently no significant release of radioactive components into groundwater at the LD RosRAO site and that the present conditions correspond to the stage of rehabilitation of the groundwater environment.

Sorption of 137Cs, 90Sr, Se, 99Tc, 152(154)Eu, 239(240)Pu on fractured rocks of the Yeniseysky site (Nizhne-Kansky massif, Krasnoyarsk region, Russia).

The study demonstrates the effect of sorption properties of fractured host rocks from the Yeniseysky site (Nizhne-Kansky rock massif, Krasnoyarsk region) on the migration of dissolved radioactive components (137Cs, 90Sr, 79Se, 99Tc, 152(154)Eu, 239(240)Pu) in the deep geological conditions of a high-level radioactive waste repository. Estimates of radionuclide distribution coefficients between the aqueous solution and fractured rocks obtained from sorption experiments were used to study the influence of various petrographic types and fracture-filling substances on the retardation of radioactive components by different fractured rock units of Nizhne-Kansky rock massif. Based on the results of sorption experiments, we concluded that the type and attributes of rock discontinuities, as well as the mineral composition of the material in fractures, are crucial for the immobilization of radionuclides during their migration through a geological environment.

Experimental and modeling study of adsorption-desorption processes with application to a deep-well injection radioactive waste disposal site.

Radionuclide (Sr-90 and Cs-137) behavior in the subsurface environment was evaluated with respect to natural attenuation, sorption and desorption kinetics, and equilibrium. Batch experiments were conducted with synthesized groundwater or acid (NaNO3; pH approximately 3) solutions under different temperature (T=20 and 70 degrees C) and pressure (P=Patm and P=3 MPa) conditions. Samples of sedimentary rock were selected as the solid phase from a radioactively contaminated site associated with deep-well injection of the radioactive waste. Groundwater and a NaNO3 waste-brine solution were used as the liquid phase. All experiments revealed hysteresis in radionuclide adsorption. Moreover, some of the experiments indicated that the adsorption process may be irreversible. A simultaneous temperature and pressure increase leads to anomalous behavior of the adsorption kinetics: a period of a rapid concentration drop of the radionuclides in solution, which is caused by their sorption uptake, is changed by a stage of a gradual increase in the corresponding concentrations. To explain the observed phenomena, several hypotheses were examined. Thus, an analytical model describing the mutual interference of adsorption kinetics and dissolution of carbonate minerals was developed resulting in a nonmonotonic behavior of the concentration curves obtained at the adsorption stage. For the description of the batch experiments with radionuclides at room temperature and atmospheric pressure, a dual-site adsorption model has been used.