Sinkholes and underground mining activities: the key role of monitoring for the hazard assessment and mitigation

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.