Thermal infrared video details multiscale groundwater discharge to surface water through macropores and peat pipes

Shallow groundwater temperatures often strongly differ from surface-water temperatures during summer and winter months (except in equatorial regions). Therefore, groundwater discharge points may be identified as locations of anomalously cold or warm temperatures, depending on the time of year, and can be identified through the remote sensing of heat. Infrared images have been used to evaluate water-surface temperature at large scales (e.g. Torgersen et al., 2001), groundwater discharge dynamics (e.g. Schuetz and Weiler, 2011), mixing of hydrothermal discharge (e.g. Andrews et al., 2011), and aquatic thermal refugia (e.g. Briggs et al., 2013; Dugdale et al., 2015). However, infrared imaging has not been explored for the study of finer-scale groundwater discharge features, such as that from macropores, particularly in real-time video mode. Thermal infrared (TIR) water-surface data were collected on 6 November 2015 at Tidmarsh Farms, a former cranberry bog that is currently undergoing restoration in coastal Massachusetts, USA (41°54′17.6″N 70°34′17.4″W). Data were collected during the late afternoon in an area of known focused groundwater discharge to surface water. This area is located along the western bank of the main drainage channel through the peatland. The channel bank is composed of thin patches of fine-medium sand underlain by peat, with abundant macrophyte growth as shown in the visible light image and by the red leaf-shaped structures along the right side of the TIR images. The TIR imagery was collected in video mode using a georeferenced FLIR T640b× camera (FLIR Systems, Inc.) affixed to a tripod. The camera lens was oriented northeast towards the main channel from a height of approximately 0.5 m, yielding a lateral distance across the bottom of the images of approximately 1 m. The TIR images are composed of 640 × 480 pixels, so the effective 'pixel size' is sub-square millimetre. TIR emissivity was set at 0.98 consistent with the expectation that fresh water approximates a black body (Handcock et al., 2012). Still images have been appended to the beginning of the TIR video to help orient the viewer. First, a visible light photo is shown, with the approximate inset area covered by the TIR data collection. A closer view of the TIR still image is annotated to show important features captured by the video: (1) peat-pipe groundwater discharge, (2) mixing of groundwater and surface water, (3) macropore groundwater discharge, and (4) possible gas ebullition from macropores. Most obvious in the image foreground is a focused groundwater discharge zone located in shallow water (approximately 0.08 m), through a 0.025 m (diameter) hole in the peat known as a 'peat pipe', located just to the right of the TIR imagery. Discharge from the peat pipe was determined to be 3.0 L min−1 using a seepage meter adapted for flowing water (Rosenberry, 2008). Groundwater discharge temperature is 10.5 °C, matching the expectation for regional groundwater, and indicating little conductive influence from the surface. The groundwater temperature contrasts strongly with average channel temperature of approximately 23 °C, creating an effective indicator of discharge locations. A counterclockwise mixing pattern is observed between the slow-flowing side waters and main channel flow. The peat-pipe seepage has a downstream water-surface footprint of approximately 2 m (to the left of the field of view); plunging of the relatively cold, dense groundwater increased the subsurface zone of thermal influence (as evaluated with a digital thermometer), which is not captured by TIR, a limitation that has been observed elsewhere (Hare et al., 2015; Briggs et al., 2013). Several other finer-scale (e.g. millimetre) focused seepage points are located directly offshore and upstream of the main seep mixing zone. The water-surface temperature of these macropore seeps is warmer than regional groundwater and is approximately 20 °C. The underlying groundwater source is likely the same as the large seep; however, the smaller macropore discharge is affected more immediately by heat exchange with the water column or through the porous media. Sporadic apparent gas ebullition from the macropores is indicated by small pulses of colder water, most notably occurring at the 23-s video runtime. The TIR images collected here in video mode are composed of 307 200 quantitative colour pixels; this data-rich mosaic reveals both large-scale and fine-scale groundwater seepage processes that would be difficult to capture in comparable detail with any other current methodology. Funding for these studies was provided by the Tidmarsh Farms Living Observatory and the U.S. Geological Survey Toxic Substance Hydrology Program. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Thermal Infrared Captures Groundwater Seepage Dynamics. Video content can be viewed at https://onlinelibrary.wiley.com/doi/10.1002/hyp.10722