Quantifying infiltration rates within a Managed Aquifer Recharge pilot site using Distributed Temperature Sensing technology

Managed aquifer recharge (MAR) became a fundamental approach in sustainable groundwater management. In this study, we investigate the potential of the use of the DTS (Distributed Temperature Sensing) technology to assess the efficiency of MAR. An innovative approach that couples passive and active-DTS measurements along a Fiber Optic (FO) cable has been developed to quantify and evaluate the spatial variability of infiltration rates at the field scale. An infiltration test was performed in a small infiltration basin in the Geer watershed (Belgium) where a FO cable was buried within the basin sediments. Passive-DTS measurements allow for the monitoring of natural temperature variations at 10 cm depth along the FO cable. Since the injected water through the infiltration test was cooler than the ambient sediment temperature, a decrease of the temperature was measured over time along the FO cable at the beginning of the infiltration test. A heat transport numerical model was developed to interpret the temperature decrease induced by the water injection and estimate the infiltration rates at early infiltration stage (1.2510-5 m.s-1). Then, an active-DTS test were was applied by heating a section of the FO cable. The analysis of the temperature increase, resulting from the heat injection in the sediments, provided estimates of the infiltration rate along this specific section of the FO cable (3.79  10-6 m.s-1). This discrepancy is most likely attributed to the dynamics of infiltration rather than parameter sensitivity as higher recharge rates are observed at the start of the infiltration. The analysis of DTS measurements provides maps of the spatial variability of the infiltration rates along the FO cable at two different stages of the infiltration test. Findings highlight the potential of DTS based methods in accurately evaluating MAR rates throughout space and monitoring the temporal dynamic of infiltration rates.