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

摘要

This study aims at evaluating sources and processes affecting NO_3 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~2H_(H_2O), δ~(18)O_(H_2O), δ~(15)N_(NO_3),δ~(18)O_(NO_3),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 NOf-rich groundwater which could be quantified using hydrological modelling. Groundwater inputs are responsible for the sharp, tenfold increase in NO_3~3 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 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 NO_3~- which is subsequently denitrified; when returned to the Oglio River, groundwater modifies the river water composition by dilution, in the case of NO_3~-, or by addition, for other constituents (e.g. Cl~-, B). The results of this study indicate that, in order to reduce the NO_3~- transport towards the Adriatic Sea, groundwater contamination should be addressed first, with expected long recovery times.