Understanding groundwater flow in a buried valley and bedrock aquifer system in Kurikka, Finland, using hydrochemical indicators and residence time tracers 3H and 14C

Growing demand for groundwater in western Finland has drawn attention to a regional network of buried valleys with high potential for domestic and municipal water supplies. This study aims at understanding the groundwater flow in the buried valley and bedrock aquifer system using hydrochemical data and residence time indicators ( 3 H, 14 C). A multivariate statistical analysis (hierarchical clustering on principal components) revealed six water groups reflecting the origins and different stages of groundwater evolution. Overall, a modern groundwater component (< 60 years old) and active flow system are present in all parts of the system. Longer residence times are found deeper within the buried valleys ( 3 H-free waters) and at greater depth in a few locations in the bedrock ( 14 C ages > 5000 years). The occurrence of geochemically evolved waters in the deeper, downgradient areas of buried valleys indicates suitable (in terms of quantity and quality) and safe (low vulnerability) water supply relative to demand. More broadly, the multi-tracer approach provides insights into the geochemical evolution of groundwater, revealing groundwater flow conditions, geological history and aquifer unit composition. Residence time indicators also illustrate the relative hydraulic isolation of buried valleys, which has implications on vulnerability and sustainable exploitation. To fully clarify the exchanges between bedrock and granular aquifers, a sampling scheme targeting deep flow paths beneath the Paloluoma and Kyrönjoki valleys would be necessary. This study will support the development of a numerical model of the aquifer system and contribute to informing future management decisions.