Groundwater residence time downgradient of Trench No. 22 at the Chernobyl Pilot Site: Constraints on hydrogeological aquifer functioning

International audienceFollowing the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contaminated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer 3H/3He method (t1/2 = 12.3 a) and anthropogenic tracers including CFC and SF6 are investigated along with the water mass natural tracers Na, Cl, 18O and 2H. The groundwater is stratified, as evidenced by Na and Cl concentrations and stable isotopes (18O, 2H). In the upper aeolian layer, the Na-Cl relationship corresponds to evapotranspiration of precipitation, while in the underlying alluvial layer, an increase in Na and Cl with depth suggests both water-rock interactions and mixing processes. The 3H/3He and CFC apparent groundwater ages increase with depth, ranging from 'recent' (1-3 a) at a 2 m depth below the groundwater table to much higher apparent ages of 50-60 a at 27 m depth below the groundwater table. Discrepancies in 3H/3He and CFC apparent ages (20-25 a and 3-10 a, respectively) were observed during the 2008 campaign at an intermediate depth immediately below the aeolian/alluvial sand limit, which were attributed to the complex water transfer processes. Extremely high SF6 concentrations, well above equilibrium with the atmosphere and up to 1112 pptv, are attributed to significant contamination of the soils following the nuclear reactor explosion in 1986. The SF6 concentration vs. the apparent groundwater ages agrees with this interpretation, as the high SF6 concentrations are all more recent than 1985. The persistence of the SF6 concentration suggests that SF6 was introduced in the soil atmosphere and slowly integrated in the groundwater moving along the hydraulic gradient. The apparent age distribution in the lumped parameter models suggests an exponential or piston flow model in the upper geological section, followed by more pronounced mixing processes in the lower section