A comparison of thermal infrared to fiber-optic distributed temperature sensing for evaluation of groundwater discharge to surface water

Abstract:

Groundwater has a predictable thermal signature that can be used to locate discrete zones of discharge to surface water. These inputs are important for numerous reasons, including habitat stability, refuge for thermally stressed-aquatic species, and focused contaminant or nutrient loading from aquifers. However, because detection and quantification at the appropriate scale can be difficult, many discrete areas of groundwater discharge are not identified. This study compares two increasingly common heat tracing methods that rely on either direct-contact measurements or remote sensing: fiber-optic distributed temperature sensing (FO-DTS) and thermal infrared (TIR). FO-DTS is used to make high spatial resolution thermal measurements through time within the water column using temperature-sensitive cables. These cables can range up to several kilometers long, and can be wrapped for higher spatial resolution (cm). The personnel requirements, and time to install and georeference the cables can be burdensome, and the control units need constant calibration. In contrast, TIR data collection, either from handheld, airborne, or satellite platforms, can quickly capture point-in-time evaluations of groundwater seepage zones across large scales. However the remote TIR measurements are adversely influenced by a number of environmental and physical factors, and the measurements are limited to the surface "skin" temperature of water features. We present case studies from a range of lentic to lotic aquatic systems to identify guidelines regarding the effectiveness of FO-DTS and TIR for evaluating groundwater discharge. FO-DTS performs well in all systems across seasons, but data collection was limited spatially by practical considerations. TIR is found to consistently locate groundwater seepage zones above and along the stream bank, but in summer submerged seepage zones are only well identified in <0.5 m depth systems with moderate flow.