Localization of lacustrine groundwater discharge (LGD) by airborne measurement of thermal infrared radiation

Although lacustrine groundwater discharge (LGD) might be important in water and nutrient budgets, it has often been neglected due to the required effort to measure LGD and due to intense spatial heterogeneity of LGD limiting the validity of measurements. Therefore, fast, easy applicable methods for a first snapshot of the LGD pattern are required and might be the basis for choosing relevant and representative sampling sites. In the present case study, which is actually the first application of an airborne measurement of thermal infrared radiation (TIR) to identify LGD pattern for entire deep freshwater lakes, the measurement was substantiated with thermal profiles in sediments and a water-table map. We found that measurement of TIR is a powerful tool to identify LGD pattern in lake-related studies provided that there is a lack of warm surface inflows. A TIR image taken in March 2012 shows that warm groundwater entering the relatively colder lake water in some near-shore areas is visible as a plume floating on top of the lake water. Prerequisites for the application of TIR to detect LGD pattern are the positive buoyancy of the groundwater relative to lake water and weak mixing in the water column. We propose a dimensionless scale for identifying groundwater floating conditions based on weather conditions. Attributing a surface thermal anomaly to LGD depends on careful consideration of other factors that could produce similar patterns and careful consideration of lake physics.