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This study aims to investigate the physical and chemical effects of interactions between groundwater and surface water (GW-SW) -particularly in streams -on nitrate contamination. The effects of GW-SW interactions are briefly reviewed, with a particular emphasis on processes and environments that influence increases or decreases in nitrate concentration. Then, this paper analyses nitrate concentrations in groundwater and surface water in the western Po plain (Northwestern Italy); this analysis includes the nitrate concentration profiles across the shallow aquifer and intersecting the main streams on the plain. The investigation highlights how the concentration trends are similar, even when nitrate levels in rivers and groundwater are not comparable. The maximum nitrate concentrations in the surface water were generally measured in areas with high nitrate levels in groundwater. An analysis of the nitrate concentration profiles highlighted the mutual influences of GW-SW. The most important streams on the plain (the Po River and Stura di Demonte River), both of them gaining streams, seem to reduce the nitrate concentrations of groundwater at a study scale.The proposed conceptual model indicates how the near-stream environment (the riparian zone, wetlands, hyporheic zone and shallow organic-rich soils in the near-stream environment) and the groundwater flow systems in shallow and deep aquifers, from the recharge zone to the streams, could dramatically affects the nitrate concentrations.

Large volumes of precious water resources are negatively affected by nitrate contamination, and the problem of the world population's exposure to this is becoming an even more pressing issue. To tackle this problem, the application of environmental isotopes has proven to be an effective method to identify the N origins and major transformations in different environments. In this work, nitrate ( 15 N NO3 and  18 O NO3) and boron ( 11 B) isotope analyses performed in the last twenty years in groundwater from shallow aquifers of the Po plain area, a complex hydrogeological system of European relevance, have been compiled in a comprehensive database together with major ionic contents; these data were integrated with additional original results, targeting areas not previously examined or complementing the available information. Such data, previously interpreted on the local scale, are examined at the Po plain scale, providing an understanding of the N sources and dynamics in the shallow aquifers, and defining the most important processes governing nitrate contamination in Northern Italy. The most impacted groundwater is that hosted in the alluvial fans of the Alpine and Apennine foothills, due to a combination of high soil permeability and presence of intensive agricultural activities. Here, aquifers are characterized by fast circulation and by great water table depths. On the contrary, nitrate contamination is absent in most low plain areas, with shallow water table depths but lower soil permeability, due to the presence of denitrification processes. The  15 N median values, calculated for each province, are significantly correlated with pig density. Hence, manure represents one of the main nitrate sources in groundwater from agriculture, the other being synthetic fertilizers. Isotopic compositions enriched due to denitrification are present in 22% of the data, being responsible for nitrate abatement in groundwater affecting up to 70-80% of the original content. The B systematics, in such a low geogenic-B context, proved the presence in the investigated area of another anthropogenic nitrate source of civil origin (i.e. sewage). While new results on the local B sources are reported, the garnering of all groundwater data allowed us to define the range of the expected geogenic B signature ( 11 B =+13 ±2.5‰). This contribution is a significant step forward for the use of the coupled  15 N- 11 B toolbox in the study area, previously limited by a poor definition of the compositional end-members. This georeferenced set of hydrochemical and isotopic data will lay the foundations for future monitoring activities and advanced data treatment or modelling. In addition, since the hydrogeological setting of the investigated area shows common features to alluvial basins located near mountain ranges, the approach and the results presented in this study serve as a reference for other study areas worldwide.

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This paper describes the hydrogeological map of the western Po Plain, located in Piedmont (north-western Italy). Po plain represents a hydrogeological system of European relevance, and the Piedmont Plain is the most important groundwater reservoir of the Region. The 1:300,000 scale map was realised using previous and new data to update the knowledge of this area. The map provides information about the hydrogeological complexes and their type and degree of permeability, water table levels and depth, piezometric level fluctuation, lithostratigraphic cross-sections, thickness, and percentage of the permeable deposits between 0 and 50 m from the ground surface. All this information is essential to public administrations, stakeholders, researchers, and professionals for defining possible tools for groundwater protection and management and for planning new groundwater exploitation (i.e. municipal drinking water supplies).