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This paper describes the differences between the phreatic aquifer and the deep confined aquifers in the territory of Lodi Province (Northern Italy), derived by the reconstruction of the hydrogeological structure of the subsoil and with the aid of hydrochemical and isotopic techniques. Firstly, the presence of a substrate with a marine origin near to the topographic surface influences the chemical characteristics of groundwater in some parts of the territory. Secondly, the distribution of the most important chemical parameters, and of some minor chemical parameters with sampling depth, shows that the phreatic aquifer, with a calcium-bicarbonate facies, has a medium-low mineralization, with a decreasing trend in relation to the increase in depth. The isotopic results obtained from the analysis of nitrogen-15 and oxygen-18 in the nitrate molecule permit us to verify the origin of this compound (essentially agricultural), and the occurrence of denitrification phenomena, due to the establishment of a reducing environment at relatively moderate depths, thus testifying to the selfpurifying capacity of the phreatic aquifer with respect to nitrates. Finally, the isotopic investigation on oxygen-18 in the water molecule allows for the identification of different recharge areas and, therefore, the origin of the different aquifers.