Groundwater chemical trends analyses in the deep aquifers of the Piedmont Po Plain (NW Italy): preliminary evaluation of ongoing processes

The Piedmont Plain (NW Italy) is characterized by a shallow phreatic aquifer hosted in fluvial complex (gravel and sand),  overlying a fluvial-lacustrine and marine complex (gravel and sand with silty clayey levels) containing deep confined/semiconfined aquifers.

Deep aquifers are essential for the supply of drinking water in the Piedmont Plain. However, detailed information on deep aquifers is lacking, such as a regional piezometric map, a continuous monitoring of the water table variations over time and a regional characterization of GW quality. Moreover, the deep groundwater chemical values in the Piedmont Po Plain show significant temporal variability and need to be characterized.

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 main ongoing processes. Furthermore, to gain a deeper understanding of specific processes, the temporal distribution of threshold exceedances ​​for the sum of pesticides (period 2009-2021) was evaluated. The potential interaction with shallow aquifers was evaluated making a comparison of the average concentrations for the main ions and parameters between shallow and deep aquifers. In general, shallow aquifers are exploited for agricultural purposes and show higher concentrations compared than  deep aquifers.

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 more than 60% of the monitored wells, and increasing trends for HCO₃ and Na in more than 40% of the monitored points. For these parameters, decreasing trends exist for less than 10% of the monitored points. SO₄, NO₃ and pH show heterogeneous trends. In particular, several monitored wells show significant variation over time, with concentrations doubling from the beginning of the time series. The sum of pesticides shows greater exceedances of the threshold values in the most recent period (2016-2021) compared to the previous one (2009-2015).

The temporal trends of ion exchanges reveal the presence of trends in 61% of the monitored wells, with a prevalence of increasing trends, corresponding to direct ion exchange. For the main ions, the comparison between the average concentrations in the shallow and deep aquifers shows higher values in the shallow aquifers.

These results suggest an increase in the recharge of the deep aquifers by the shallow aquifers and an increased contribution from silty-clayey layers of the deep aquifers. These processes are consistent with excessive withdrawal from deep aquifers. Furthermore, the increasing concentrations represent a significant issue, leading to the progressive deterioration of deep groundwater quality. In conclusion, the main processes responsible for the variation in groundwater chemistry in the deep aquifers were identified, defining the existence of impacting and worrying processes at a regional scale.