In 2005, the exploitation activities in the Moncalvo underground gypsum quarry in the Monferrato area (Moncalvo, Piedmont Region, NW Italy), intercepted a karst cave filled with pressurized water (0.3 MPa). After this event, a hydrodynamic and hydrogeochemical methodology was developed and coupled to direct observations to monitor the water flows into the quarry. In particular, all the existing water flows and the new ones generated by the mining activities were mapped. Redox potential, electrical conductivity and water temperature were recurrently measured and integrated with chemical analyses. Piezometric levels were also measured in many piezometers located around the quarry site. The proposed methodology was successfully applied to the Calliano gypsum quarry, which is located in the same geological framework but it is representative of a different karst system. The paper highlights the key role of the hydrogeochemical monitoring as a potential technique for the sinkhole risk related to mining activities.
Karst rocks, such as evaporites and carbonates, cover the 10-15% of the continental surface of Earth and they can be considered one of the most problematic media from a rock mechanics point of view due to their marked anisotropy and heterogeneity (Stevanovic & Milanovic 2015). Moreover, these rock masses are characterized by an intense water circulation (mainly in the carbonates), which leads to dissolution phenomena that generate karst-related morphologies, such as caves and long conduits, often completely saturated by pressurized water. When there is an interaction with human activities (mainly underground mining), severe issues with serious consequences can occur both underground, with huge inrush in the artificial voids, and at the surface where sinkholes take place (Clay & Takacs 1997; Day 2004; Bonetto et al. 2008; Hou et al. 2016; Coli et al. 2020; Golian et al. 2021).
The 2005 Moncalvo sinkhole event (Piedmont region, NW Italy) is a clear example of the possible consequences of the interaction between mining activities and karst systems (Bonetto et al. 2008, Vigna et al. 2010a, Vigna et al. 2010b). During a drift excavation in the Moncalvo gypsum quarry, a karst cave was intercepted, triggering a 60000 m3 inrush of water and fine sediments. Fortunately, no injuries were recorded but a 20 m wide and 10 m deep cover-collapse sinkhole was generated close to the NE sector of the quarry.
