Quantifying infiltration rates and their spatial variability within a MAR pilot site using Distributed Temperature Sensing technology

A novel approach integrating Distributed Temperature Sensing (DTS) technology has been developed and tested to assess the efficiency of Managed Aquifer Recharge (MAR) at the field scale. This method involves the collection of passive and active-DTS measurements along a buried Fiber Optic (FO) cable during an infiltration test, allowing for the accurate estimation of recharge rates over space. The study was conducted in a small infiltration basin located within the Geer watershed (Belgium). The basin was filled with cold groundwater and a FO cable was buried within loess sediments at the bottom. Passive-DTS measurements were employed to monitor natural temperature variations at a 10 cm depth along the FO cable throughout the whole infiltration test. Given the cooler recharge water, a soil temperature change was noticeable at the beginning of the infiltration as the cold water reaches the FO cable. A heat transport numerical model was used to interpret this temperature reduction and a mean early infiltration rate of 1.25 × 10⁻⁵ m.s⁻¹ is obtained. Subsequently, active-DTS measurement were collected by heating a section of the FO cable. The analysis and reproduction of the resultant temperature increase using the ADTS toolbox yields to a mean infiltration rate estimate of 3.79 × 10⁻⁶ m.s⁻¹ along the heated section of the FO cable. The observed discrepancy between the infiltration rates obtained from passive and active-DTS data analysis is attributed to the dynamics of infiltration, with higher recharge rates occurring at the onset. The analysis of DTS measurements provided spatial maps of infiltration rates at different stages of the infiltration test. It demonstrates the potential of this approach in accurately evaluating infiltration rates across space and time within MAR sites and in monitoring the infiltration dynamics of the system.