Exploring the use of Fiber Optic Distributed Temperature Sensing for monitoring seawater intrusion
Résumé
As pressure on coastal fresh groundwater resources increases, interest in coastal aquifers monitoring rises. The weakest point of coastal aquifers occurs at the fresh-salt water interface induced by seawater intrusion (SWI), highly sensible to any change in the heads of the fresh and salt water bodies. Therefore, the position, width and dynamics of the interface is one of the main features of a costal aquifer to be monitored and understood, in order to improve coastal groundwater management. Traditionally, changes in electric conductivity have been used to identify and monitor the interface in coastal aquifers. Alternatively, natural differences in temperature between fresh and the saline groundwater bodies are also known to provide useful information. Recently, Fibre Optic Distributed Temperature Sensing (FO-DTS) is increasingly being used in the field of hydrogeology because of its high spatial and temporal resolution. To test the FO-DTS technology for the monitoring and quantification of the SWI dynamics, distributed temperature data were collected every 15 minutes in a Mediterranean granular aquifer, matching with the occurrence of an extreme rainfall event. Thermal response to the recharge event is compared to data collected independently in wells with electrical conductivity and temperature probes. A 2D variable density heat and solutes transport model is performed with CodeBright. The numerical model is used to confirm the conceptual model defined from the interpretation of the FO-DTS data, explore possible future scenarios and prove the usability of this technology for coastal aquifers monitoring. Distributed temperature data reflect thermal responses to the extreme recharge event. However, smaller displacement of the interface, like those produced by tides, were not detected by FO-DTS. Only hydraulic processes with thermal responses larger than 0.15 ºC could be observed with this technology. Therefore, the high spatial and temporal resolution provided by FO-DTS is limited by its temperature resolution, which depends on many factors, such as the DTS sensor, the number of connections, the calibration baths and the calibration process. In any case, it is a promising technology for monitoring fast responses of the interface to large scale processes like recharge events.