Nitrogen inputs to a river course in a heavily impacted watershed: A combined hydrochemical and isotopic evaluation (Oglio River Basin, N Italy)

This study aims at evaluating sources and processes affecting NO3− concentrations in the Oglio River. Five sampling campaigns considered the mainwatercourse, tributaries, point pollution sources, springs, and groundwater. Physico-chemical parameters, N forms, B, Sr2+, stable isotopes (δ2HH2O, δ18OH2O, δ15NNO3, δ18ONO3, δ11B) and discharge were measured. Hydrological modelling was performed using mass balance and End Member Mixing Analysis equations. During the irrigation period, in the upstream reach, up to 90% of the natural river flow is diverted for irrigation and industrial purposes; excess water drained from agricultural fields is returned to river in the downstream reach. Results evidenced, in the middle reach, a large input of NO3−-rich groundwater which could be quantified using hydrological modelling. Groundwater inputs are responsible for the sharp, tenfold increase in NO3− in the river water, from 2.2–4.4 up to 33.5 mg L−1, and are more evident in summer, when discharge is lower. Nevertheless, river water preserves its natural B isotopic composition, indicating that the two tracers do not have a common origin and are not co-migrant. In the lower plain, surface–groundwater interconnections and human disturbances in thewater cycle favour the recycling of the compounds in the environment, and lead to a similarity in composition of the different water bodies (Oglio River, tributaries and groundwater). The long lasting agronomical practices have profoundly modified the surface–groundwater equilibriumand chemical characteristics, resulting in a highly buffered system. Infiltrating irrigation water leaches down NO3− which is subsequently denitrified; when returned to the Oglio River, groundwater modifies the river water composition by dilution, in the case of NO3−, or by addition, for other constituents (e.g. Cl−, B). The results of this study indicate that, in order to reduce the NO3− transport towards the Adriatic Sea, groundwater contamination should be addressed first, with expected long recovery times.