How are groundwater-surface water interactions and carbon transport interconnected on a river valley scale?

Observations over recent decades in cold regions, including delayed freeze-up, earlier snowmelt, and rapidly increasing precipitation and runoff, underscore the dynamic nature of groundwater (GW)-surface water (SW) interactions. These changes result in distinct spatiotemporal exchange flow patterns, which in turn enhance the variability of biogeochemical processes, with critical implications for carbon cycle and water-resource budgets. Despite their significance, our current understanding of GW-SW interactions and their biogeochemical implications remains limited. Therefore, it is essential to integrate multidimensional analyses, spatiotemporal scales, and interface hydraulic characterization into modelling frameworks. Our target is to provide a comprehensive representation of the interconnections between GW-SW interactions and diverse river valley scale dissolved carbon transport in the north boreal aquifer, at the Oulanka Research Station, Finland. This target can be further divided into three major categories: i) Development of a conceptual model for the entire river valley. This includes understanding GW-SW processes, particularly changes in aquifer permeability during frozen and thawed periods, and devising tailored methods to investigate the flow paths, exchange rates, and associated hydrogeochemical processes. ii) Linking GW-SW dynamics to carbon cycling through hydrogeochemical analyses. This involves identifying spatiotemporal variability in GW geochemistry via statistical analysis and improving this conceptual framework using supplementary data on various redox conditions and stable isotopes (δ²H and δ¹⁸O) as environmental tracers. iii) Under the Digital Waters (DIWA) flagship initiative, we undertake site-specific digitalization and modelling of GW-SW processes across river valleys. This effort aims to accurately simulate water movement and carbon cycling within diverse environments, with a specific focus on examining the connectivity and distinctions between river-aquifer, hillslope-aquifer, and peatland-aquifer systems. The research hypothesis will be tested through hydrological analyses, supported by high-resolution 3D GW flow modelling. This process encompasses data collection and interpretation, hydrogeological and hydrogeochemical characterization, conceptual model development, and numerical simulations using the GMS software MODFLOW package and the Amanzi-ATS software under varying boundary conditions and disturbances.