Mesocosm experiments identifying hotspots of groundwater upwelling in a water column by fibre optic distributed temperature sensing

Abstract

Lacustrine groundwater discharge (LGD) can substantially impact ecosystem characteristics and functions. Fibre optic distributed temperature sensing (FO-DTS) has been successfully used to locate groundwater discharge into lakes and rivers at the sediment-water interface, but locating groundwater discharge would be easier if it could be detected from the more accessible water Lacustrine groundwater discharge (LGD), that is, the discharge of groundwater (GW) into lakes, can substantially impact ecosystem characteristics and functions (Baker et al., 2014;Ridgway & Blanchfield, 1998;Warren, Sebestyen, Josephson, Lepak, & Kraft, 2005). Upwards directed GW flow is sometimes called upwelling, especially in the context of hyporheic zones, where commonly both upwelling and downwelling occur along river reaches. In the present manuscript, we use the term upwelling solely for upward transport processes in the water column; this definition is adopted from limnophysics. On the one hand, upwelling of warm water in cold lakes can be caused by natural processes such as GW flow across the lake bed into the cold lake water body during winter conditions (LGD; Lewandowski, Meinikmann, Ruhtz, Pöschke, & Kirillin, 2013) or thermal springs in volcanic lakes (Cardenas et al., 2012). On the other hand, it can be related to thermal pollution ------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Thermal infrared imaging for the detection of relatively warm lacustrine groundwater discharge at the surface of freshwater bodies

“…In such studies, comparison of TIR images with FO-DTS temperature data is problematic because: a) depth-dependent mixing may fundamentally alter the thermal patterns received by the FO-DTS in comparison with TIR and b) the warming effect of incident radiation at the air water interface (eg. Benyahya et al, 2012;Marruedo Arricibita et al, 2018;Ouellet et al, 2014) may not be recorded by the bed-mounted FO-DTS. Our experimental setup provides a partial resolution of this problem, allowing for more direct comparison of the two techniques.…”

“…Finally, an outlet at the other side of the mesocosm was used to maintain a constant water level of 0.82 m. Discharge at this outlet varied from 34 to 48 L min -1 depending on the warm water injection rate (Marruedo Arricibita et al, 2018). <Figure 1 here, please> Temperatures were acquired with a temporal frequency of 10 seconds (mean of the preceding 10 second period; essentially the integration time of the FO-DTS).…”

“…500 m of 2.8 mm diameter Corning ClearCurve simplex fiber (loose tube, 1 multimode fibre, both ends terminated with E2000/APC connectors) were deployed for the experiments. This selected FO cable specification allowed bending and was thus appropriate for installation in the aluminium frame (Marruedo Arricibita et al, 2018).…”

“…The first and last 20 m of fibre optic cable were submerged in this well mixed ice bath, and an external temperature probe connected to the DTS device used to monitor calibration bath temperatures independently. For differential loss corrections, fixed value settings were chosen along the relatively short cable (500 m) without any splices (Marruedo Arricibita et al, 2018). A default value of 0.255 dB/km was used.…”

“…This means that the water surface heated up more than under overcast weather conditions, rendering the signal from the warm water injection (at the base of the mesocosm) difficult to observe with TIR (because of the very small resultant temperature difference between the simulated upwelling and the ambient mesocosm surface temperature). On the contrary, during night-time, a greater amount of net radiation (Rn) was lost during clear sky conditions than under overcast conditions (Marruedo Arricibita et al, 2018). While this increased heat loss explains why the FO-DTS data was not able to properly characterise upwelling (due to rapid dissipation of heat energy at the surface), it does not explain why the TIR images were not readily able to detect the upwelling, as it is this very loss of radiation which is measured by the thermal camera.…”