Hydrological setting controls Cs and Sr concentrations in a headwater catchment in the Chornobyl Exclusion Zone.

Concentration-discharge relationships are widely used to understand the hydrological processes controlling river water chemistry. This study investigates how hydrological processes affect radionuclide (Cs and Sr) concentrations in surface water in headwater catchments within the Chornobyl Exclusion Zone (ChEZ) in Ukraine. In the flat wetland catchments, the depth of the saturated soil layer changes little throughout the year, but changes in the saturated soil surface area during snowmelt and immediately after rainfall affect water chemistry by changing the opportunities for contact between the surface water and the soil surface. On the other hand, in the slope catchments where there are few wetlands, the water chemistry of river water is governed by changes in the relative contributions of "shallow water" and "deep water" due to changes in the catchment water supply pathways feeding the rivers. In this study, no correlations were observed between dissolved or suspended Cs concentrations and either discharge rates or competitive cations, but the solid-liquid ratio of Cs was found to be significantly and negatively correlated with water temperature. However, Sr concentrations in surface water were found to be strongly related to the water pathways for each of the catchments. Moreover, contact between the surface water and the soil surface and changes in the relative contributions of shallow and deep waters to stream water were correlated with changes in Sr concentrations in surface water in wetland and slope catchments, respectively. The study concludes that Sr in rivers inside the ChEZ are strongly affected by the water pathways in headwater catchments. Additional studies will be necessary to clarify the details of sorption/desorption reactions.