Using Fibre Optic Distributed Temperature Sensing for Detecting Groundwater Plume Thermal Anomalies

Groundwater temperature has been used in the past as an environmental tracer for studying changes to groundwater quality, water flow and ecosystem health. Groundwater thermal plumes originating from subsurface geothermal energy use, urban infrastructure, waste disposal sites or mining activities can significantly impact groundwater systems. As such, continuous monitoring of groundwater temperature can be crucial when the aim is rapid detection of changes to the local thermal groundwater regime early on.

Here we demonstrate the suitability of Fibre Optic Distributed Temperature Sensing (FO-DTS) for tracing the arrival and propagation of heat plume-related temperature signals in steel and plastic-lined steel blind tubes. The use of blind tubes is increasingly being considered when monitoring the subsurface of waste processing and waste management areas including landfills and nuclear waste sites. For our laboratory experiments, plastic tanks filled with saturated sand were equipped with different blind tube configurations and a FO-DTS system (Silixa XT-DTS) and subjected to periodic heating and cooling to monitor dispersive heat transport under various thermal conditions along the blind tube. Laboratory experiments were supported by heat transport modelling.

Experimental results showed that the FO-DTS setup was well suited to detect temperature changes along the blind tube wall as low as 0.1^oC at high temporal resolution. Detectability of the thermal signal was not significantly impaired by the plastic-lining where this was present inside the steel tubes. We observed that the lag of the thermal signal through the blind tubes was typically less than two minutes and that only little smearing of the temperature signal along the blind tube walls occurred, which was further confirmed by modelling results. As such, the experimental setup has the potential to be further developed into an effective on-site monitoring system, which will help to support decision making in contaminated site management and ultimately lead to an improved management of groundwater resources.