The general aim of this project is to integrate the role of groundwater as sink or source of nutrients (N and P) in river basin mass balances. The complex dynamics of groundwater makes this compartment an understudied regulator of nutrient cycling. Most studies are unable to integrate groundwater N contamination data in the global N models at the watershed scale, and the same applies to P dynamics, with an even higher degree of uncertainty. Groundwater features must be understood as they provide information on the timing of nutrient transfer from and to the surface and on the main sources and transformations in different geographical areas. Long term projection of the evolution of N and P concentrations in surface water cannot be developed unless groundwater is included in the model.

In the Lombardy plain (Northern Italy), the uneven distribution of nitrates in groundwater (whose concentration often exceeds the regulatory limits in the higher plain and is below detection in the lower plain) points at different roles and ecosystem functions of this compartment. Based on what evidenced in the Oglio river basin, we hypothesize that, in the higher plain, the N excess is leached to the subsoil and is therefore subtracted from the shallow environment. Nevertheless this sink is only temporary, since groundwater naturally outflows in correspondence of the transition between the higher and lower plain, in the so-called ―fascia dei fontanili‖ (springs belt). Here, numerous outflows return the deep groundwater, partially mixed with recently infiltrated irrigation water, and the associated nutrients back to the surface water compartment. Evidence of this input is provided by the 10-fold increase of nitrate concentration in the Oglio river water when flowing across the springs belt. By contrast, in the lower plain groundwater is deprived of nitrates because of strong denitrification phenomena and, when contributing to river water, dilutes its nitrate content.

Nutrient budgets at watershed scale, inventory of sources, N and P dynamics in aquifers (water residence time, storage zones, paths) and nutrient upwelling timing and quantity will be studied in two key sub- basins of the Po river, the Adda and the Ticino basins, in their sub-lacual sector. As in the case of the Oglio, these basins are characterized by groundwater upwelling in the ―springs belt‖ and by a strong river hydrological alteration due to multiple water use. However, the three basins strongly differ for crops, fertilizer applications and livestock density, likely resulting in a gradient of nutrient surplus and a consequent different impact on surface and groundwater quality. Nutrient cycling will be investigated also in relation to the type and amount of irrigation, with the aim to understand how this affects groundwater upwelling, N and P mobilization and retention, river water quality and ecosystem functioning.

The comparison between the calculated nutrient surplus and the load exported by rivers at the closing section will allow to quantify the amount retained within the basin that could be denitrified (in the case of N) or leached to groundwater. Hydrochemical data, coupled to aquifer characteristics, will allow to evaluate the present status of the nutrient contamination and to estimate the amounts stored in the reservoirs. In the higher plain, groundwater dating will allow to infer the residence time of nutrients in groundwater and to estimate the timing for groundwater recovery following the remedial actions undertaken at watershed level. Finally, in the lower plain, the factors promoting denitrification and its capability to permanently remove the N excess, and those enhancing the P mobilization or retention will be investigated. This will allow to fully exploit the ecosystem services to the purpose of groundwater decontamination, while reducing the negative environmental side effects (i.e. N2O emissions) of the process involved.

Dissemination activities will play a key role in the project. Through a Social Network Analysis, the main groups of stakeholders (as the actors/groups likely to be affected or that can influence the project) will be identified and involved at an early stage. Using innovative communication instruments such as the social media, results will be made available for the scientific and the social communities to comment in real time. The main outcomes of the project will be summarized in a series of guidance documents addressing 1) the reduction of nutrient excess at watershed level; 2) the sustainable management of water resources in order to match multiple water use and ecological functions of the groundwater-surface water ecotone; 3) the integration of water and land use plans. These and other educational material, such as flyers or posters, will be made available for download in the project website. These guidelines, discussed and shared with stakeholders, will open the way to a sustainable agriculture and water use.

The project addresses the need to restore and preserve good water quality status for future generations, one of the key challenges faced by water resources managers, having to cope also with the changes in water availability due to climatic change and the increasing demand from multiple users. The main originality of the project is the integrated approach, which considers both surface and groundwater and addresses both N and P, two key nutrients whose dynamics and mutual relationships are rarely examined together at the catchment scale. By integrating the role of groundwater as a sink or source for nutrients, the project will investigate the ecosystem services provided by this compartment, identify the main present and future criticalities, and evaluate the potential and time frame for a natural attenuation of the contamination.