Changes in 90Sr transport dynamics in groundwater after large-scale groundwater drawdown in the vicinity of the cooling pond at the Chornobyl Nuclear Power Plant

Migration of long-lived radioactive ⁹⁰Sr introduced by nuclear accidents and radioactive waste requires long-term monitoring and protection management due to its half-life of 28.8 years and high mobility in water. Presently, 37 years have passed since the largest worldwide ⁹⁰Sr contamination was released and deposited around the Chornobyl Nuclear Power Plant (ChNPP). In the vicinity of the ChNPP, the water level of the cooling pond (CP) has declined since May 2014 following the decommissioning phase of the Unit 3 reactor. The drawdown of the CP lowered the groundwater level in a massive vicinity (about 70 km²), and the change in the groundwater system due to the drawdown has caused concerns about possible changes in ⁹⁰Sr concentrations in water and transport dynamics to the Pripyat River. Therefore, this study evaluated how ⁹⁰Sr transport dynamics were influenced due to changes in the groundwater flow system from 2011 to 2020 based on observed data and results of the groundwater flow simulation in the CP vicinity.

The numerical simulation was conducted from 2011 to 2020 on monthly time-step using USGS MODFLOW with PM11 GUI and calibrated to groundwater heads measured at monitoring wells. In the location between the CP and the Pripyat River, estimated pore velocities near the river were reduced compared to velocities before the CP drawdown due to the decrease in the hydraulic gradient between the CP and the river. Decrease in groundwater velocity results decrease in groundwater discharge and delay of ⁹⁰Sr transport. Therefore, the amount of ⁹⁰Sr transported from the CP to the river is smaller than the period prior to the CP drawdown. The reduced ⁹⁰Sr transport is expected to have less impact on the radioactivity in the river water even in the Pripyat River floodplain northwest of the CP where ⁹⁰Sr concentrations significantly increased after the CP drawdown. In addition, the measured and simulated changes in groundwater flow direction and velocity suggested the possibility of ⁹⁰Sr accumulation at the floodplain caused by stagnant groundwater from reduced velocity and additional ⁹⁰Sr infiltration from surrounding ponds located at the Pripyat River floodplain. Therefore, enhancing the current monitoring of ⁹⁰Sr concentrations near the floodplain would be needed for long-term monitoring and protection management to prevent the risk.