Remote sensing of preferential groundwater discharge with high-resolution geophysical measurements of FloaTEM and tTEM compared with lake surface temperature anomalies

At Lake Nørresø, Denmark, a suite of geophysical, traditional hydrogeological and two surface temperature methods were combined to investigate how phosphorus-rich groundwater from a deep confined aquifer enters a lake through clay-dominated glacial deposits. 46 % of the water discharge to the lake is from this groundwater source. The high-resolution resistivity data acquired with the FloaTEM method (a floating transient electromagnetic system) showed structures interpreted as glacio-tectonically stacked layers beneath the lake that connect the deep aquifer with sand layers just beneath the lakebed. Satellite-based temperature data (Landsat) were used to create a lake surface anomaly map showing where sufficient volumes of relatively cold (9 °C) groundwater might seep into the lake with an average water surface temperature of 20–23 °C on the warmest summer days. Drone-borne thermal infrared imaging of the winter ice cover indicates thaw holes (i.e., hotspots) created by upwelling relatively warm groundwater. Traditional methods, including in-lake boreholes sampling of the lakebed sediments, and installation of in-lake piezometers were used to support and constrain the geological interpretation of geophysical data and detect hydraulic gradients. The combined use of field-methods showed a lakebed covered by thick low-permeability clay till only accommodating focused groundwater discharge to subaqueous springs in areas where tilted layers derived from glacial tectonics convey the artesian pressure from a deep aquifer.