Predicting the partition coefficient of strontium-90 and uranium in an alluvial aquifer: From the laboratory scale to the field scale

This study demonstrates the robustness of an approach called component additivity (CA), which was developed to predict the partition coefficient (K D ) of strontium-90 and uranium in an alluvial aquifer near nuclear facilities across scales, from the laboratory to the field. CA uses two thermodynamic sorption models requiring only the proportion of reactive solid phases (illite, smectite, and ferrihydrite) and pore water chemistry. First, the reliability of the approach was first evaluated at the laboratory scale by comparing the simulated K D values with experimental sorption data acquired from four solid samples in their raw and carbonate-free <2 μm clay fractions. These fractions are representative of alluvial deposits and underlying marls. At the field scale, long-term monitoring of historical radiological events over periods of up to 50 years and distances of up to 500 m enabled the determination of in-situ K D values. The values ranged from 1.7 to 7.4 L kg −1 for strontium-90 and from 3.5 to 7.1 L kg −1 for uranium. Next, the CA approach was applied to the calculated K D values based on the characteristics of the four levels of the alluvial deposit and the chemistry of groundwater sampled from 12 piezometers. There was a very good agreement between the in-situ and simulated strontium and uranium K D values, particularly, in the alluvial level through which groundwater primarily flows. This predictive approach's reliability demonstrates that the same mechanisms govern strontium-90 and uranium sorption in submillimeter powdered solid samples in the laboratory as well as in the field at very large temporal and spatial scales under environmental conditions (i.e., in the absence of acidic, alkaline, or saline plumes). This indicates that there is no scale effect for sorption in this case. • First demonstration of sorption prediction reliability at a large field scale for Sr(II) and U(IV) • Major role of clay minerals in Sr(II) sorption revealed in the lab and in the field • Major role of ferrihydrite in U(IV) sorption revealed in the lab and in the field • The alluvial aquifer level where groundwater flows controls Sr(II) and U(VI) partition • No scale effect for partition coefficients of Sr(II) and U(VI) from the lab to the field