Unraveling the nexus: exploring river-groundwater interaction as the primary driver of eutrophication in river ecosystems

•The nitrogen input decreased from the 2000′s but contamination did not.•River-groundwater interaction affects rivers quality and flow.•Eutrophication is triggered by groundwater nitrate.•Hydrological and biogeochemical time lags could delay ecosystems restoring.•Ecohydrogeological factors will gain importance under climate change scenarios. Over recent decades, increased agricultural activities have significantly modified nitrogen (N) and water cycles, leading to a worsening of the environmental quality and widespread eutrophication. The present work investigates the critical issue of N contamination and its impact on eutrophication in three rivers located in the central part of the Po Plain (Northern Italy), one of Europe’s hotspots of N-fertilizers input and loss to aquatic ecosystems. The primary scientific problem addressed is the role of river-groundwater interactions in exacerbating eutrophication, primarily driven by nitrate (NO3-). Historical data from the past ten years on dissolved inorganic N forms in groundwater and rivers were analyzed and interpreted in relation to different watershed managements. This analysis quantified both the volumetric and qualitative contributions of river-groundwater interactions to rivers eutrophication. Results indicate that river-groundwater interactions can be indeed the main cause of eutrophication in intensively cultivated watersheds, with effects surpassing those of typical causes like wastewater. The study highlights how the simultaneous presence of inefficient irrigation practices promotes surface water (and groundwater) overexploitation, reducing dilution and increasing contamination. All the analyzed rivers showed localized increase in NO3- concentration and worsening of their trophic status. Given the foresaw increase in groundwater and surface water use for irrigation under climate change pressures, this research provides a crucial empirical example of future challenges for regions with high N inputs and close relations among soil, groundwater, and surface water. The findings emphasize the urgent need for improved water and agricultural management to mitigate river-groundwater interaction-induced eutrophication.