Benefits of an Airborne Electromagnetic Survey of Former Opencast Lignite Mining Areas in Lusatia, Germany

A large contiguous former opencast lignite-mining district is located in Lusatia, Germany, about 100 km south of Berlin. Since this region has been heavily polluted by acid mine drainage from opencast lignite mines, time- and cost-efficient investigation methods are required to obtain comprehensive information on aquifers. In summer 2021, BGR carried out a helicopter-borne electromagnetic (HEM) survey with its helicopter and a RESOLVE system covering an area of about 250 km2. While current surface water levels are derived from in-flight altitude measurements, the spatial resistivity distribution shows indications for the water table in the entire survey area. Apparent resistivities agree very well with water resistivities measured in larger lakes, where the distance to the lakeshores outreaches the system’s footprint. While groundwater sampling requires observation wells, we use HEM results to close the gaps between the sparsely distributed wells. The analysis of groundwater samples shows that groundwater EC correlates with dissolved iron (Fe2+) and sulfate (SO42−) content. HEM resistivities help to approximate groundwater EC, and further Fe2+ and SO42− contents at the observation wells, and to estimate these within the mining area where oxidative pyrite weathering processes dominate other contamination processes.