Quantifying the Efficiency of Fibre-Optic Distributed Temperature Sensing and Thermal Infrared Imaging for Detecting Lacustrine Groundwater Exfiltration: a Mesocosm Experiment

Abstract:

Detecting groundwater inflow into lakes and reservoirs still remains a challenge. The buoyancy of groundwater during winter and early spring can be used for identification of groundwater up-welling related hotspots on surface water by TIR imaging (TIR). TIR has been successfully used to image and fast screen relatively large surface areas of lakes, reservoirs and large rivers for groundwater contributions. Still, quantitative interpretations of groundwater fluxes are hampered by the lack of understanding how the groundwater up-welling signal is propagated from the sediment-water interface through the water column to the water-air interface and what perturbations and signal losses occur along this pathway.

In the present study, groundwater discharge to a surface water body was simulated in a mesocosm experiment. Under winter conditions water of 14° C to 16°C was discharged at the bottom of a 10x2.8 m mesocosm where surface water varied from 4°C –7.4°C. Four layers (20, 40, 60 and 80 cm above the sediment) of the 81 cm deep mesocosm were equipped with fibre-optic distributed temperature sensing (FO-DTS) for tracing thermal patterns in the mesocosm and TIR imaging was deployed to monitor temperature pattern at the water surface in order to: (1) analyze the propagation of the temperature signal through the water column by FO-DTS (2) characterize the spatial distribution of groundwater-borne hot spots on the lake surface by FO-DTS and TIR and, (3) conduct inverse modelling from surface water TIR data to identify the groundwater source at the sediment-water interface. In order to assess the reliability of the model we compare modeled data with FO-DTS observations. Different exfiltration rates were simulated in order to establish the minimum rate of GW upwelling that can be reliably detected at the water surface by TIR imaging. The experiments also allow us to benchmark scale dependencies and adequacy of both methods, FO-DTS and TIR. They also reveal that weather conditions can have important impacts on the detection of groundwater exfiltration at surface water-atmosphere interfaces at larger scales.