Analysis of the recharge process of Alpine Spring through an integrated approach: the case of Perrot spring (Aosta Valley, Italy)

The Perrot spring (1305 m a.s.l.), located on the right side of the Chalamy Stream, inside the Monte Avic Natural Park (Aosta Valley, NW Italy), is an important source of drinking water for Champdepraz municipality.

The spring is placed on a large slope characterized by the presence of debris covers of various origin (glacial, fluvial and landslide) above the bedrock (serpentinised peridotites and metabasites of the Zermatt-Saas Zone, Penninic Domain) which crops out only in the upper part of the basin.

The water source is fed by rainwater infiltrating and flowing into the shallow deposits, with a permeability by porosity, and into the most fractured portion of the substrate. The water emerges at the contact between the topographic surface and impermeable or semi-permeable basal lithologies (unfractured crystalline rocks and glacio-lacustrine deposits).

The aim of this study is to delineate the recharge processes of the spring and the definition of the recharge area extension that is very important for its conservation.

In this view, an analysis of groundwater spring parameters (e.g. daily discharge, temperature and electrical conductivity) were conducted for the years 2018-2020.

The flow rate ranges between 22 and 47 L/s with two maxima, one in spring and one in autumn; the electrolytic conductivity varies between 60 and 75 S/cm. The variation of groundwater temperature is very low, between 4.9°C and 5.5°C.  The low discharge and temperature variations suggest a relatively high average share of the supply area and a sufficiently deep flow circuit.

The analysis of these data shows the presence of two contributions to the spring supply: a spring contribution from snowmelt, characterized by a low INCREASE in flow rate, and an autumn contribution from rainwater infiltration.

Moreover, sampling campaigns were also carried out in the entire Chalamy stream basin in August 2021 and January, July, September 2022. In particular, water from lakes, rivers, spring and rainwater was sampled.

During the field campaigns, pH, electrical conductivity and temperature were measured in situ. Chemical analysis of major ions and stable isotopes (δ²H and δ¹⁸O) were then conducted on water samples.

The chemical analyses show a groundwater chemistry coherent with the regional geology: the hydrochemical facies is bicarbonate-sodium, the main cations are Ca²⁺ and Mg²⁺ and the anions HCO₃^-.  Finally, isotopic analyses of precipitation and spring water suggest a recharge elevation of around 2,500 m a.s.l.