N-loads from subsurface, drains, and groundwater-fed surface (bypass) flows via two riparian zones (crop field and wetland) to a second order stream were investigated by sampling of shallow and deep groundwater on both sides and monthly measurements of flows from springs, drains, and stream including water quality (nitrate). A push-pull test in the crop field gave estimates of first-order denitrification rate (0.23 day–1). Reactive transport modelling evaluated observations of water chemistry and denitrification processes in the groundwater below the crop field showing that nitrate was completely removed by denitrification with pyrite in the aquifer (model rates of 0.6–2.5 mmol NO3 L−1 yr−1). A drain in the crop field routed approximately 10% (bypass) of the regional groundwater inflow to the stream. Buffer efficiency was high at 90%. The wetland on the other side of the stream hosts several locations of focused nitrate-rich groundwater-fed spring discharge, predominantly through a non-maintained drainage system of drainpipes and ditches with bypass accounting for 59% of the regional flow input. Nitrate was completely removed in groundwater by denitrification with dissolved organic matter in shallow groundwater. The regional inflow and N load to the wetland is amongst the highest recorded and data shows that the N load to the stream is very high. The buffer efficiency ranged from 45–83% depending on if all springs contributed to the stream or only the two with visible outflow. A conceptual model for nitrate removal efficiency as a function of Damköhler number and percent bypass flow is proposed.

Changes in the quantity and quality of groundwater and water in the hydrological cycle in general have important implications for the evolution of water resources, the built environment, and terrestrial and aquatic ecosystems, globally. Exploitation of groundwater and other subsurface resources may lead to e.g. land subsidence, salt water intrusion, loss of important terrestrial and aquatic ecosystems and hence biodiversity. Together with biogeochemical flows of nitrogen and phosphorus and changes in the land-system and climate, these are currently considered the main environmental problems of the planet, which are breaching or close to breaching planetary boundaries. Changes in the hydrological cycle including groundwater is closely related to and affecting these changes. It is the ambition of the four GeoERA groundwater projects studying aspects of groundwater quantity and quality issues related to natural processes and human activities to further develop the European Geological Data Infrastructure as a leading information platform for groundwater data in Europe and one of the leading platforms, globally. Here we briefly present the contents and objectives of the four groundwater projects: HOVER - Hydrogeological processes and geological settings over Europe controlling dissolved geogenic and anthropogenic elements in groundwater of relevance to human health and the status of dependent ecosystems; RESOURCE - Resources of groundwater, harmonized at cross-border and Pan-European Scale; TACTIC – Tools for assessment of climate change impact on groundwater and adaptation strategies and VoGERA - Vulnerability of shallow groundwater resources to deep sub-surface energy-related activities. The four projects will deliver “FAIR” (Findable, Accesssible, Interoperable and Reusable) data and information via the European Geological Data Infrastructure easily accessible for all relevant endusers. This will improve our understanding of the subsurface and support common efforts for developing geoethical uses of the subsurface.