Combining passive and active distributed temperature sensing measurements to locate and quantify groundwater discharge variability into a headwater stream

Abstract

Abstract. Exchanges between groundwater and surface water play a key role for ecosystem preservation, especially in headwater catchments where groundwater discharge into streams highly contributes to streamflow generation and maintenance. Despite several decades of research, investigating the spatial variability in groundwater discharge into streams still remains challenging mainly because groundwater/surface water interactions are controlled by multi-scale processes. In this context, we evaluated the potential of using FO-DTS (fibre optic distributed temperature sensing) technology to locate and quantify groundwater discharge at a high resolution. To do so, we propose to combine, for the first time, long-term passive DTS measurements and active DTS measurements by deploying FO cables in the streambed sediments of a first- and second-order stream in gaining conditions. The passive DTS experiment provided 8 months of monitoring of streambed temperature fluctuations along more than 530 m of cable, while the active DTS experiment, performed during a few days, allowed a detailed and accurate investigation of groundwater discharge variability over a 60 m length heated section. Long-term passive DTS measurements turn out to be an efficient method to detect and locate groundwater discharge along several hundreds of metres. The continuous 8 months of monitoring allowed the highlighting of changes in the groundwater discharge dynamic in response to the hydrological dynamic of the headwater catchment. However, the quantification of fluxes with this approach remains limited given the high uncertainties on estimates, due to uncertainties on thermal properties and boundary conditions. On the contrary, active DTS measurements, which have seldom been performed in streambed sediments and never applied to quantify water fluxes, allow for the estimation of the spatial distribution of both thermal conductivities and the groundwater fluxes at high resolution all along the 60 m heated section of the FO cable. The method allows for the description of the variability in streambed properties at an unprecedented scale and reveals the variability in groundwater inflows at small scales. In the end, this study shows the potential and the interest of the complementary use of passive and active DTS experiments to quantify groundwater discharge at different spatial and temporal scales. Thus, results show that groundwater discharges are mainly concentrated in the upstream part of the watershed, where steepest slopes are observed, confirming the importance of the topography in the stream generation in headwater catchments. However, through the high spatial resolution of measurements, it was also possible to highlight the presence of local and highly contributive groundwater inflows, probably driven by local heterogeneities. The possibility to quantify groundwater discharge at a high spatial resolution through active DTS offers promising perspectives for the characterization of distributed responses times but also for studying biogeochemical hotspots and hot moments.

Fiber optics passive monitoring of groundwater temperature reveals three-dimensional structures in heterogeneous aquifers

“…Klepikova et al 42 showed that passive temperature tomography, although not performed with fiber optics, was sufficient to identify preferential pathways in a fractured aquifer under the combination of different pumping conditions. Furthermore, passive DTS requires less practical and technological effort, making it more suitable for applications over large spatial scales and for long-term monitoring 43 .…”

“…This is because the ambient temperature field dynamics would become of secondary relevance with respect to the artificial perturbations induced by the tests. Assessing groundwater-stream exchanges, Simon et al 43 found that while long-term passive DTS was able to detect and locate groundwater inflows over a long distance, it required assumptions about thermal properties and boundary conditions that induced high uncertainties on fluxes quantification, for which active DTS over short spatiotemporal scale proved superior. The authors remarked on the importance of the complementarity of the two approaches, which has the potential to provide an imaging of the spatial variability of groundwater inflows.…”

Distributed Brillouin optical fiber temperature sensor for groundwater flow measurement

“…Combining passive and active distributed temperature sensing measurements to locate and quantify groundwater. 10,11 Using this setup, we claim to validate the Brillouin optical fiber sensor technology by measuring the temperature difference by constant and uniform heating of the optical fiber cable and the background temperature of the groundwater system, and horizontal groundwater flow velocity. Figure 1 represents the test site of port Douvot.…”

Estimation of Groundwater Flow Rate by an Actively Heated Fiber Optics Based Thermal Response Test in a Grouted Borehole

“…Using the MILS, flow rates in porous media can be obtained by calculating so‐called inflection feature points of different thermal response stages independent of the jacket effect (Simon et al., 2021). This method was applied in scenarios where the fiber‐optic cable is in direct contact with the porous media, such as for vertical flow rate estimation in a riverbed (Simon et al., 2022). A concern similar to the "skin effect" happen in the measurement based on a borehole; the borehole with different thermal conductivity materials would further change the observed temperature response curve.…”