The Piedmont Plain (NW Italy) is characterized by a shallow phreatic aquifer, overlying a deep confined/semiconfined aquifers, the latter are essential for the supply of drinking water.
The aim of this study was to analyze the trends (period 2000–2021) in the main physicochemical parameters (electrolytic conductivity (EC), pH) and main ions (Ca, Mg, HCO3, Na, Cl, NO3 and SO4) in 70 wells in the deep aquifers in order to identify the ongoing processes. The temporal distribution of threshold exceedances for the sum of pesticides was also evaluated. The interaction with shallow aquifers was evaluated comparing the average concentrations for the main ions between shallow and deep aquifers and evaluating the trends of the Saturation Index (SI) of calcite. Additionally, the temporal trends of ion exchange (Ca+Mg/Na index) were evaluated to highlight the contribution from silty-clayey layers, which represent the less permeable portions of the deep aquifers.
Results highlight relevant increasing trends for EC, Ca, Mg and Cl in 57-69% of the monitored wells, and increasing trends for HCO3 and Na in 41-46% of the monitored points. Decreasing trends exist for 3-10% of the monitored points. SO4, NO3 and pH show heterogeneous trends. The sum of pesticides shows greater exceedances of the threshold values in the most recent period compared to the previous one. The temporal trends of ion exchanges reveal the presence of trends in 61% of monitored wells.
Average concentrations in shallow aquifers show higher values than in deep aquifers. The decreasing trends of Calcite SI confirm the interaction between aquifers.
These results suggest an increase in the flows from the shallow aquifers to the deep aquifers and an increased contribution from silty-clayey layers of the deep aquifers. These processes are consistent with excessive withdrawal from deep aquifers.
In conclusion, the existence of impacting and worrying processes at a regional scale were defined.