Geological and physicochemical controls of the spatial distribution of partition coefficients for radionuclides (Sr-90, Cs-137, Co-60, Pu-239,240 and Am-241) at a site of nuclear reactors and radioactive waste disposal (St. Petersburg region, Russian Federation)

The paper presents a study of the sorption properties of sediments of different geological ages and lithological types, governing radionuclide retention in the subsurface (up to 160 m beneath the surface) within the area of potential influence of the Northwestern Center of Atomic Energy (NWCAE), St. Petersburg region, RF. The focus of this work is mostly on the sedimentary rocks of two types, i.e., weakly cemented sandstone and lithified clay formations of Cambrian and Vendian series. The first lithological unit is associated with a groundwater reservoir (Lomonosov aquifer), and the second one, with both a relative aquitard in the upper part of the Vendian formation (Kotlin clay) and a regional aquifer (Gdov aquifer) in the lower part of the formation. The main mechanisms responsible for the variability of the sorption distribution coefficient (Kd, defined as the ratio of the concentration of solute on solid phase to its concentration in solution at equilibrium) was identified for radionuclides such as Sr-90, Cs-137, Co-60, Pu-239,240, and Am-241. It was shown that the main factors contributing to the chemical heterogeneity of the Cambrian sandstone were related to the presence of secondary minerals (iron and magnesium oxides and hydroxides produced by the weathering process) in trace amounts, forming correlated layer structures. The statistical analysis of nonlinear isotherms confirmed this conclusion. For the Vendian formation, a determinate trend was established in the Kd change over depth as a result of temporal trends in the sedimentation process and pore-water chemistry. The geostatistical characteristics and the spatial correlation models for describing linear sorption of different radionuclides are not identical, and the exhibition of chemical heterogeneity of sedimentary rock of a particular lithological type depends on radionuclide chemistry. Moreover, variogram analysis for some Kd data sets (both in Cambrian and Vendian formations) demonstrates the total absence of auto-correlation (pure nugget effect). It can be supposed that sampling distances did not allow the evaluation of small-scale variability in sediment sorption properties.