Unraveling the interactions between shallow and deep aquifers through analysis of groundwater chemical trends: insights from the Piedmont Po Plain (NW Italy)

Deep aquifers are essential groundwater resources for human activities, and their exploitation has intensified in recent years, as shallow aquifers generally exhibit a higher occurrence of contaminants compared to deep aquifers. The aim of the study is to examine the interactions between shallow and deep aquifers using hydrochemical approaches, considering the spatiotemporal distribution and trends of physicochemical parameters, major ions, heavy metals, pesticides and specific hydrochemical indices in deep aquifers over the 2000–2021 period in Piedmont (NW Italy). In the Piedmont Po Plain, a shallow phreatic aquifer hosted in fluvial deposits overlies deep confined and semiconfined aquifers, hosted in transitional and marine complexes. Results highlight (1) an increase in physicochemical parameter values and major ion concentrations; (2) enhanced introduction of pollutants, such as pesticides; (3) increased CO 2 levels, leading to increased concentrations of some major ions and decreased concentrations of Fe 2+ and the calcite saturation index (SI calcite ); and (4) intensified ion exchange processes. In conclusion, the increasing trends in physicochemical parameter values and major ion concentrations confirm an increase in the recharge of deep aquifers from overlying shallow aquifers, likely driven by the intensified exploitation of deep groundwater resources. This work represents one of the first regional studies of deep aquifers in the Piedmont Po Plain to analyze both physicochemical parameter and major ion trends and the significant role of interactions with shallow aquifers, providing valuable insights for local authorities, particularly as deep aquifers are expected to be increasingly exploited in the future for agricultural and drinking-water purposes.