Interactions between river water and groundwater and their influences on river chemistry: the case study of Oglio River (northern Italy)

Groundwater and surface water can be closely interconnected so much that they can be considered as a single water body, often called blue water. Interactions within its different components (groundwater, rivers, etc.) are a key aspect since they affect either water quality and quantity. The present work focuses on the analysis of interactions between a river and its surrounding aquifers in the Po Plain (N. Italy). More precisely, this study concerns the Oglio River in its stretch between the outflow from Lake Iseo and the confluence with Mella River and covers ~1500 km2 of mostly intensive agricultural areas, with over 10˙000 wells. This work involved a multi-disciplinary approach including: 1) the measurement of river stages, groundwater heads and river flow rates in selected river stations and wells executed in 4 different periods (Oct. 2015, Feb. 2016, Apr. 2016, June 2016) comprising up to 21 river stations and 79 wells; 2) lake, river and spring water, groundwater and rainfall sampling for analysis of major ions, trace elements (As, Fe and Mn) and water stable isotopes (~80 samples for each period); 3) the interpretation of measured data. Measured river stages and groundwater heads suggested that the Oglio River loses water to the aquifer from the outflow from Lake Iseo to approximately 20-30 km downstream, then the Oglio River gains groundwater up to the confluence with Mella River. These findings are consistent with the results of chemical analysis, indeed we found that: 1) groundwater in the higher plain, i.e., the plain upstream the so called “spring belt”, has oxidized hydrofacies with higher concentration of NO3; 2) groundwater in the lower plain, i.e., the plain downstream the spring belt, has reduced hydrofacies with higher concentration of Mn, Fe, As and NH4; 3) the profile of NO3 concentration along the Oglio River has a marked increase from the point where the river starts to gain groundwater (in the higher plain) up to the transition to the lower plain; this can be related to groundwater gaining in the higher plain that has higher concentration of NO3; 4) the profile of Mn, Fe, As and NH4 along the river increases from the transition between higher and lower plain up to the border of the study area (i.e., confluence of Mella into Oglio River); this can be related to groundwater gaining in the lower plain that has higher concentration of these reduced species. This work was supported by Fondazione Cariplo, grant 2014-1282.