Natural and anthropogenic variations in the Po river waters (northern Italy): insights from a multi-isotope approach

Po is the main Italian river and the δ¹⁸O and δ²H of its water reveal a similarity between the current meteoric fingerprint and that of the past represented by groundwater. As concerns the hydrochemisty, the Ca–HCO₃ facies remained constant over the last 50 year, and only nitrate significantly increased from less than 1 mg/L to more than 10 mg/L in the 1980s, and then attenuated to a value of 9 mg/L. Coherently, δ¹³C_DIC and δ³⁴S_SO4 are compatible with the weathering of the lithologies outcropping in the basin, while extremely variable δ¹⁵N_NO3 indicates contribution from pollutants released by urban, agricultural and zootechnical activities. This suggests that although the origin of the main constituents of the Po river water is geogenic, anthropogenic contributions are locally significant. Noteworthy, the associated aquifers have the same nitrogen isotopic signature of the Po river, but are characterized by significantly higher NO^– ₃ concentration. This implies that aquifers’ pollution is not ascribed to inflow of current river water, and that the attenuation of the nitrogen load recorded in the river is not occurring in the aquifers, due to their longer water residence time and delayed recovery from anthropogenic contamination.