Study of recharging dynamics of a spring in an alpine valley through isotopic and hydrogeochemical approaches: the Montellina Spring case study (NW Italy)

Stable water isotopes are valuable tools to determine the flow pathways, and, when combined with other analyses, can provide insights into the hydrogeology in complex settings.  

This study aims at determining the recharge and flow pathways processes of groundwater in an alpine stream basin, focusing on a spring characterized by exceptionally high discharge.

Montellina Spring is one of the drinkable water springs with the highest discharge in the Turin Province (Piedmont, NW Italy); its discharge varies between 50 and 180 L/s. It feeds the local water supply system and it is therefore necessary to identify and safeguard the recharge area. The spring is located at 380 m above sea level at the base of the Renanchio Stream Basin, on the low alpine Dora Baltea Valley. The peaks forming the watershed are at an altitude of 2000 m a.s.l., approximately. The aquifer feeding the spring consists of an eclogitic bedrock with limited layers of dolomitic marble, fissured due to deep-seated gravitational slope deformations (DSGSD), and is covered by thick layers of glacial sediments.

Surface water, groundwater and precipitation were sampled at several sites along the Renanchio Stream Basin (altitude of the sites: 380 to 1460 m a. s.l.), in different seasons during three sampling campaigns (autumn 2017, winter 2017-2018 and spring 2018). Chemical analyses of major ions and water stable isotopes (δ¹⁸O and δ²H) were evaluated, showing a bicarbonate alkaline-earth facies. Furthermore, waters referred to Montellina Springs are mostly enriched in major ions and in term of isotopic contents, similars to the Renanchio Stream, latter sampled at altitudes of up to at 1460 m a.s.l.

The similar isotopic content and the higher major ions content (especially Mg⁺⁺, Ca⁺⁺ and HCO₃^- due to the dissolution of bicarbonate minerals) indicate an important aquifer and a significant circulation in the bedrock interested by DSGSD and glacial sediments. Lastly, chemical and isotopic data suggested that the spring’s recharge area is located at elevations above 1500 m a.s.l., that the spring is partly feed by precipitation and inflow from the Renanchio Stream and that the DSGSD and glacial sediments play a fundamental role in the recharge of Montellina Spring.

The present work seeks to better highlight the hydrogeological context of the Renanchio Stream Basin and provide a new perspective on water resource research and safeguard in an alpine environment.