Monitoring Groundwater Thermal Treatment Using a Fiber-Optic Distributed Temperature Sensing Network.

Groundwater contaminated with organic chemicals can be treated by raising the subsurface temperature, thereby enhancing the rate of microbial degradation. This process requires careful monitoring through space and time to ensure that heat is being delivered to the most contaminated regions of the groundwater system. Here, we demonstrate the effectiveness of a fiber-optic distributed temperature sensing (DTS) system as a high spatial and temporal resolution monitoring strategy. The DTS sensing system required the installation of fiber optic cable in the subsurface. Boreholes were drilled with hollow casings, a fiber optic cable was inserted into the casings, and then, the casings were withdrawn to allow the formation to collapse around the fiber. The fiber was then fusion-spliced into a single continuous fiber that could be interrogated by a Raman-based DTS unit. Temperature measurements were collected at 30 min intervals over a 575 m span with 0.25 m spatial sampling, resulting in over 110,000 temperature data points per day. With this high resolution monitoring the development of thermal plumes emanating from solar-heated borehole heat exchangers could be closely monitored. The pseudo-3D monitoring network showed the lateral and upward migration of the induced thermal plumes over time. This information was valuable for assuring the heated groundwater was contacting the intended treatment zone.