To reduce radiation exposure rates at the site and prevent atmospheric resuspension of radionuclides released by the Chernobyl reactor 4 explosion (April 1986), about 800 trenches were dug on site to dispose contaminated material. Since 1999, the Chernobyl Pilot Site (CPS) was set up to study the migration of radionuclides from one of these trenches, the trench T22, in the Environment (biosphere, unsaturated zone, saturated zone). The aim of this study is to investigate migration processes in groundwater.
At first, the maximal extent of the contaminant plume is studied based on the understanding of the conservative tracer 36Cl behavior. High contamination of groundwater by 36Cl is shown, with 36Cl/Cl ratios 1 to 4 orders of magnitude higher than the theoretical natural ratio. This contamination is attributed to migration from the trench. Then, a reactive approach is considered. A conceptual model of the main geochemical processes in groundwater is proposed based on the study of major elements concentrations ([Cl-], [HCO3-], [SO42-], [NO3-], [Na+], [Ca2+], [K+] [Mg2+], [Si]), [Fe2+] concentrations, [Mn2+] concentrations, δ18O and δ2H. Meteoric origin of groundwater is showed. Some element concentrations are mainly governed by migrations from the trench. However, natural geochemical processes are also assessed to have an influence on groundwater geochemistry: thus, weathering of minerals, cation exchanges and leakage are supposed. Next, uranium and strontium migrations are investigated based on measurements of 238U/235U, 86Sr/88Sr, 87Sr/86Sr ratios. Indeed, dissolution fuel particles buried in the trench and the release of associated radionuclides is supposed to have a significant impact on those ratios in groundwater. However, in spite of an increase of [238U] concentrations downgradient of the trench, measured 238U/235U ratios in groundwater are in the natural range. Analytical procedure for 86Sr/88Sr and 87Sr/86Sr ratio measurement does not allow observing some trend downgradient of the trench; however, 87Sr/86Sr ratios clearly decrease with the depth.