Distributed Temperature Sensing for hydrogeological risk mitigation in the San Lorenzo tunnel

Optical fiber technology has emerged as a remarkable tool for groundwater monitoring. It provides high-resolution spatial and temporal data alongside exceptional sensitivity. Increasingly applied to monitor groundwater parameters such as pressure, temperature, flow, and contamination levels, this technology provides a comprehensive framework for real-time hazard assessment and risk management by integrating geotechnical and environmental variables. This study focuses on the San Lorenzo tunnel, a site characterized by significant water inflows that pose substantial hydrogeological challenges. The primary objective is to monitor these inflows closely and develop effective risk mitigation strategies. Preliminary investigations have shown that the rock mass enclosing the tunnel is highly fractured, resulting in substantial water ingress, particularly during heavy rainfall. While water in the tunnel is well-documented, the precise localization and timing of these inflows remain inadequately understood. Distributed Temperature Sensing (DTS) was employed to address these gaps to monitor water inflows and temperature variations along approximately 700 meters of the tunnel. Temperature data were combined with conductivity measurements to infer the origin of the aquifers. Initial findings suggest the presence of at least two independent aquifers, potentially fed by a common upstream source. Three types of optical fibers were installed along the tunnel. The function of these fibers is to detect both the presence and temperature of inflows of water that are intercepted and channeled into conduits. This study aims to detect and assess the location of inflows along the tunnel, temporal and spatial evolution of the water accumulation and discharge associated with external meteorological events. By correlating these observations with environmental variables such as rainfall, snowmelt, groundwater levels, and the discharge of nearby springs, we seek to refine the hydrogeological conceptual model and evaluate the feasibility of extending the draining tunnel as a mitigation measure. This research demonstrates an innovative application of fiber optic technology to monitor subsurface water flow. It contributes to hydrogeological risk mitigation in the San Lorenzo tunnel and advances our understanding of groundwater dynamics in complex fractured rock environments.