Groundwater/surface-water interactions and their effect on nitrate pollution in the Oglio River basin (N Italy)

In the Po Plain of northern Italy, one of the larger alluvial basin in Europe, groundwater quality is threatened by nitrate pollution. The main aim of this work is to assess the origin of this pollution and understand how groundwater/surface-water interactions affect nitrate concentrations in the region. The study area is part of the Oglio River basin and covers around 1,900 km2 between the outflow from Lake Iseo and the confluence with Mella River. The northern part of the area (i.e. higher plain) hosts a sandy mono-layer aquifer that passes southwards into a multi-layer aquifer (lower plain) with increasing silt and clay contents. The transition between higher and lower plain is marked by a series of semi-natural springs, the so called “springs belt”. The Oglio River is losing in its first ~30 km crossing the higher plain, then becomes gaining. The area is crossed by hundreds of irrigation channels fed by Oglio River water. Six field surveys (November 2015, February, June, September 2016, March and July 2017) were carried out and up to 58 groundwaters, 20 Oglio River and tributaries waters, 7 spring waters and 1 Lake Iseo water were sampled in each survey. Analyses were performed for major ions, trace elements and stable isotopes (δ18O/ δ2H in water, δ18O/ δ15N in nitrates and δ11B). Concentrations of NO3 were higher in groundwater from the higher plain and spring water (median of 39.8 and 40.6 mg/L, respectively) than in groundwater from the lower plain, where NO3 was generally below the detection limit. The losing reach of the Oglio River had low concentrations of NO3 that nevertheless increased downstream from a median concentration of 2.5 mg/L at Lake Iseo to 20.6 mg/L in its lower gaining reaches. This is due to the gaining of groundwater from the higher plain that is higher in NO3. Isotope data showed that NO3 originated from different anthropogenic sources, such as animal manure, septic tanks and, to a lesser extent, synthetic fertilizers. Beneath intensively-irrigated areas, the isotopic composition is shifted through the compositional range of synthetic fertilizers. This could be due to an increased dissolution of fertilizers related to the intensive irrigation. Irrigation also plays an important role in controlling groundwater NO3: concentrations are lower (mean 38.4 mg/L) where irrigation water is sourced from the channels and higher (mean 56.4 mg/L) where irrigation is performed using groundwater. The difference arises because irrigation channels carry Oglio River water, that has low NO3 concentrations (mean 8.5 mg/L). Therefore, in this case, irrigation water contributes to diluting the high anthropogenic NO3 contents in groundwater. This finding is supported by water isotopes since the Oglio River is more depleted than local precipitation. This work was supported by Fondazione Cariplo, grant 2014-1282.