Hydrochemical and isotopic study of Maggiore Valley deep aquifers (NW Italy): evaluation of the interactions with the Po River

The Maggiore Valley well field plays a fundamental role in supplying drinking water to a large territory of the Piedmont Region (north-western Italy) and has been intensively exploited since the early XX century. Due to the lack of other relevant sources of drinking water in this part of Piedmont region, this well field represents a drinking water reserve of regional importance.

This water resource  is host in a deep multi-layered aquifer system. The recharge area of the deep exploited aquifer is located towards the Po Plain, west (Turin Plain) and south (Cuneo Plain) of the study area. Most likely, the deep aquifer is recharged from west by Po river, that in this area is a losing river, due to the highly permeable Quaternary gravelly sand deposits in correspondence with the river.

The main purpose of this study was to confirm the interaction between deep aquifer and the Po River through a hydrochemical and isotopic assessment, and to characterize the different water resource quality in this areas (Po Plain, Poirino plateau, Maggiore Valley area).  

Two sampling campaigns were carried out both in the shallow and deep aquifers (March and June 2021) for a total of 128 samples. Physical-chemical analyzes of the main ions on all samples and isotopic analyzes (δ¹⁸O, δ²H) on 50 samples were conducted.

The processing of chemical data has confirmed the bicarbonate-calcium facies for the majority of the shallow and deep aquifers samples. Moreover, clear hydrochemical differences were observed between the investigated sectors; e.g. the shallow aquifer of the Poirino Plateau shows nitrate concentrations superior than the limits, unlike the deep aquifer of Maggiore Valley is characterized by low concentration of nitrate and other ions.  

The processing of isotopic data, combined with previous data, made it possible to identify a gradual increase in values of the isotopic composition along the flow direction into the Cuneo deep aquifer due to the progressive interaction with the shallow aquifer; moreover, isotopic data confirmed the interaction between the Po River (more negative values) and the shallow aquifer (more positive values) along the watercourse in the Turin Po Plain, resulting with a more negative isotopic composition in the shallow aquifer compared to nearby areas.

In the Maggiore Valley, the isotopic signals of the deep aquifer, flowing from the Turin plain and interpreted as potentially influenced by the Po River showed an isotopic composition highly similar to the watercourse, with to the least enriched waters of the area.

The isotopic signals of the deep aquifers in the Maggiore Valley flowing from the Cuneo plain (more positive) and Turin plain (more negative) were distinguished and the mixing between these converging aquifers in the well field area was verified.

In conclusion, the stable isotopes suggest an interaction between the Po River and the deep aquifer of the Maggiore Valley wells.

The study provides an additional tool for a better groundwater management and protection of a regional importance drinking water reserve.