Interconnected groundwater–surface water interactions and carbon transport across a subarctic river valley

Groundwater–surface water (GW-SW) interactions play an important role in subsurface carbon transport and transformation but remain understudied. This study investigates the aquifer-river system of the Oulankajoki River Valley in northeastern Finland to evaluate how hydrostratigraphy and recharge pathways regulate groundwater hydrogeochemistry and carbon transformations during 2022–2025, using a combination of a 3D geological model, multivariate analyses (PCA, HCA), and stable isotope analysis. Results reveal three major findings. First, carbon dynamics within the aquifer are primarily governed by hydrostratigraphy, recharge connectivity, and groundwater residence time, while redox evolution regulates transformations between organic and inorganic carbon species. Second, groundwater is DOC-poor (1.2 ± 0.6 mg L −1 ) and DIC-rich (20.3 ± 6.7 mg L −1 ) relative to surface water (DOC = 8.3 ± 1.9 mg L −1 ; DIC = 4.5 ± 0.2 mg L −1 ), indicating efficient organic carbon processing and carbonate buffering. Third, multivariate analyses identify a clear hydrogeochemical separation between shallow, recharge-dominated groundwater characterized by transient DOC transport and deeper, carbonate-buffered groundwater associated with progressive DIC accumulation under longer residence times. The conceptual model developed in this study integrates recharge pathways, carbonate weathering, and redox evolution, and carbon transformation within a unified framework. These findings underscore the importance of hydrogeological heterogeneity and groundwater evolution in regulating carbon coupling within boreal aquifer systems.