title: Nitrogen inputs to a river course in a heavily impacted watershed: a combined hydrochemical and isotopic evaluation (Oglio River Basin, N Italy).
abstract: This study aims at evaluating sources and processes affecting NO₃(-) concentrations in the Oglio River. Five sampling campaigns considered the main watercourse, tributaries, point pollution sources, springs, and groundwater. Physico-chemical parameters, N forms, B, Sr(2+), stable isotopes (δ(2)HH₂O, δ(18)OH₂O, δ(15)NNO₃, δ(18)ONO₃, δ(11)B) 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 mgL(-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 the water 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 equilibrium and 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.
authors: Institute of Geosciences and Earth Resources (IGG), CNR, U.O.S. Pavia, via Ferrata 1, 27100 Pavia, Italy; Department of Earth and Environmental Sciences, University of Pavia, via Ferrata 1, 27100 Pavia, Italy. Electronic address: carloandrea.delconte01@ateneopv.it.
authors: C A
authors: CA
authors: Delconte
authors: E
authors: E
authors: Sacchi
authors: E
authors: E
authors: Racchetti
authors: M
authors: M
authors: Bartoli
authors: J
authors: J
authors: Mas-Pla
authors: V
authors: V
authors: Re
keywords: Hydrological modelling; NO(3)(−) contamination; Po plain; Stable isotopes; Surface–groundwater interaction