Downhole passive fiber optics temperature monitoring for improved characterization of aquifer heterogeneities

Unconfined shallow aquifers are particularly exposed to the risk of contamination. Especially when exploited for drinking water production, for which water quality is of particular concern, careful monitoring of the physical processes and detailed characterization of the subsurface properties are crucial. Furthermore, the possible presence of heterogeneities, such as intricate networks of hydraulically conductive paleo-channels that are often inherent in alluvial aquifers, can establish preferential pathways. Consequently, monitoring activities in these complex environments pose serious challenges and raise the demand for advanced techniques and innovative approaches. In this context, recent advances have been made possible by employing Fiber Optics Distributed Temperature Sensing (FO-DTS). This technology combines the use of heat as a natural tracer with a detailed spatiotemporal resolution and has proven informative in a wide variety of applications. In this study, we applied downhole passive FO-DTS to a cluster of piezometers in a highly heterogeneous phreatic gravelly aquifer. The aquifer is exploited for irrigation and drinking water supply, and exhibits both natural and pumping-induced groundwater temperature fluctuations. Vertical transient water temperature profiles were acquired over a 1-month experiment. Borehole-dependent and depth-related features of the temperature measurements were ascribed to possible spatial structures having different hydraulic conductivity. The collected data were used to invert the three-dimensional saturated hydraulic conductivity field of a physics-based numerical model that couples flow and heat transport. Even without active heating, FO-DTS has demonstrated its ability to provide valuable insights at an unprecedentedly high resolution.