Detecting groundwater discharge to a small river channel using ground‐based thermal infrared imagery and radon‐222

Understanding river–groundwater interaction is important for assessing the hydrologic, ecologic and biogeochemical processes in the hyporheic zones. Various field methodologies exist to evaluate their interaction, such as direct seepage-metre measurements (Rosenberry, 2008) or streambed temperature profiling (Anibas et al., 2018). Moreover, a number of different environmental tracers (Cook, 2013) are used in this context, for example radon (Rn), which is generated in the subsurface from uranium-series isotopes and its concentration decreases due to fast decay (half-life of 3.8 days) and gas exchange with the atmosphere. The presence of Rn in river water indicates that groundwater exfiltration to the river takes place at adjacent zones or upstream within short distance (Cook, Favreau, Dighton, & Tickell, 2003). Another field approach is the use of thermal infrared (TIR) imagery and videography. Infrared thermography can remotely sense the surface water temperature (top 100 μm) to assess surface water and groundwater interaction (Ala-aho, Rossi, Isokangas, & Kløve, 2015; Torgersen, Faux, McIntosh, Poage, & Norton, 2001). It has been applied, for example, to capture groundwater upwelling to streams at point or reach scales (Briggs, Hare, Boutt, Davenport, & Lane, 2016; Dilge & Koray, 2018; DoleOlivier, Wawzyniak, Creuzé des Châtelliers, & Marmonier, 2019), to detect submarine groundwater seepage to coastal rivers (Shaban, Khawlie, Abdallah, & Faour, 2005; Tamborski, Rogers, & Bokuniewicz, 2017) and to trace natural spring discharge to surface water in difficult accessible areas (Fitch, Kelleher, Caldwell, & Joyce, 2018). To our knowledge, a combined approach of TIR imaging and Rn has been successfully applied to localize submarine and lacustrine groundwater discharge patterns (Bejannin, van Beek, Stieglitz, Souhaut, & Tamborski, 2017; Wilson & Rocha, 2016). However, few studies have been conducted using these two techniques in riverine environment, particularly at fine scales during winter months. Therefore, we employed this combined and non-invasive approach along a small stream section in our catchment to meet our research objectives, which are (a) to locate potential groundwater discharge zones to the river at small spatial scales; (b) to facilitate further indepth research for quantification of river-aquifer exchange fluxes, such as performing streambed thermal profiling at identified active groundwater upwelling locations. TIR data were collected on 21 January 2019 between 12 p.m. and 2 p.m. Central European Time (CET) in the headwater segment of the Zwarte Beek (51 05039.5600N, 5 21034.1300E), a small tributary of the river Demer in northeast Belgium (0–3 s video runtime). The studied river section is approximately 2 m wide and 20 cm deep, with an daily mean streamflow of 1 m/s at Lummen, Limburg (Belgian hydrological station No. L09_148). The streambed is dominated by fine to medium sand. The vegetation on the stream banks are mostly shrubs and grasses with alder carr in the wettest parts. The weather during data collection was sunny with an average daily air temperature of −3.9 C at Overpelt (Belgian meteorological station No. ME11_002). At this time of the year, a relatively large positive temperature difference between river water and groundwater can be expected (Anibas et al., 2009). TIR data were collected with a handheld infrared camera (T640, FLIR Systems, Inc.). Both still images and videos were taken. The camera covers a spectral range of 7.5–14 μm with a thermal sensitivity of <0.03 C (at +30 C) and image resolution is 640 × 480 pixels. As water bodies are excellent radiators, the emissivity values of water range between 0.97 and 0.99 for observation angles within 30 of nadir (Dugdale, 2016; Handcock et al., 2012; Kay et al., 2005). Consquently, we set this parameter to 0.98. The relative humidity was 93.0% measured at Overpelt (Belgian meteorological station No. ME11_002). The observation distance between the camera lens and the shoot zones was approximately 1–2 m. Received: 30 March 2020 Accepted: 27 May 2020