The Montellina Spring as an Example of Water Circulation in an Alpine DSGSD Context (NW Italy)

Abstract

Alpine areas, with normally fissured bedrock outcrops, do not typically contain important hydrologic reservoirs, except for small aquifers located in Quaternary sediments. By contrast, mountainous areas affected by deep-seated gravitational slope deformations (DSGSD), especially if covered by glacial sediments, contain large aquifers and are consequently promising for water exploitation. This last geological setting is observed, for example, in the lower Dora Baltea Valley (near the confluence with the Renanchio Basin) in which the Montellina Spring is located and exhibits a very high discharge. A multidisciplinary approach (detailed geological survey of the bedrock and Quaternary cover, as well as hydrogeological research based on tracer tests, hydrochemical analyses, and water balance studies) was used, allowing for a reconstruction of the geological and hydrogeological setting of the investigated area, also considering its environmental implications. The consequent hydrogeological model derives from the coexistence of some factors. In detail, the thick glacial cover, widespread in the intermediate sector of the slope, represents an important aquifer with a slow groundwater flow to the spring. The buried glacial valley floor, hosting this cover, can convey the groundwater from the high Renanchio Basin zone towards the low sector. The loosened bedrock of the low sector, consequent to DSGSD phenomena, favors the concentration of groundwater along the contact with the underlying normal fissured bedrock outcropping at the base of the slope. Finally, the flow until the spring essentially takes place through N100° trend open fractures and trenches. Part of the Montellina Spring discharge is also fed by the low Renanchio Stream, as highlighted by fluorescein tests, essentially using NE-SW oriented open fractures on the bedrock. The results of the investigation on the Montellina Spring can provide some insight regarding the hydrological potential of other alpine areas with a similar geological setting.