SEAWATER INTRUSION EFFECT ON RADIOACTIVE STRONTIUM MOBILITY

Both radioactive and stable strontium, Sr, exist solely as Sr²⁺ ions in the natural environment, showing a geochemical behavior very similar to Ca²⁺. The speciation and mobility of Sr is controlled by the adsorption capacity of minerals as well as the pH, ionic strength, and organic matter of solutions. Seawater intrusion is widely known in groundwater systems near the ocean where most nuclear power plants are located. Particularly, the change of ionic strength in groundwater by seawater intrusion has been considered as the most influential factor for Sr transport because the Sr typically forms outer sphere complex which is relatively weak and the ion exchange process controls Sr removal. In this study, the effect of ionic strength on Sr transport was investigated under varying ionic strengths in solution to simulate seawater intrusion into groundwater. A core rock sample was collected from the Korean Nuclear Power Plant (KNPP) site and < 2-mm fractionized samples were used for batch experiments. Cation exchange capacity (CEC) was measured using both NH₄-acetate and 1 M of NaNO₃ and KNO₃ mixed solution at pH 7. The samples were reacted with additional ion-exchanging solutions (Ca²⁺, K⁺, or Na⁺) of 10^-6, 10^-4, and 10^-1 M concentration, while neutral pH was maintained through experimental reaction. After reaction, the extracted Sr concentration was determined by analyzing Sr concentration in the supernatant using ICP-MS. The solid samples were also reacted with varying concentrations of Sr (10^-6, 10^-4, and 10^-1 M) to determine the exchangeable concentrations of Ca²⁺, K⁺, and Na⁺ ions from the solid.

The measured CEC values were approximately the same, 24 μeq/g from two methods. As ionic strength increased, the Sr concentration in solution also increased, suggesting that accelerated transport of Sr was noticeable after contacting with high ionic strength solution. In particular, the Sr exchange was more relevant with Ca²⁺ ion concentration, but high Na concentration was also enough to decrease Sr sorption (e.g., more Sr release in solution) in groundwater-seawater mixing conditions. The results could be used to predict the amount and the migration pathway of radioactive Sr (⁹⁰Sr) in the subsurface environment of KNPP sites.