Environmental Geophysical Investigation of a Waste Site Using Radiomagnetotellurics

This study investigates the subsurface structure of an old landfill site located in Cologne-Pesch, Germany, using the Radiomagnetotelluric (RMT) geophysical method. RMT is a passive electromagnetic technique that uses natural and man-made VLF–LF radio signals to assess variations in subsurface conductivity. The site, previously used for sand and gravel extraction and later as a waste dump, is characterized by complex and poorly documented fill materials. A total of four profiles were surveyed using the RMT MK5-SM25 system, measuring electric and magnetic field components across a range of frequencies (10 kHz–1 MHz). Data were processed using EM Processor and transformed into the frequency domain via Fourier Transform. A rigorous frequency selection process was applied based on coherency and azimuth criteria. 2D inversion of the data was conducted using the Mackie2D algorithm with nonlinear conjugate gradient (NLCG) and Tikhonov regularization, optimized through L-curve analysis. Results show a distinct high-conductivity anomaly at the landfill site, which corresponds with zones of waste accumulation and possible leachate formation. The anomaly exhibits lateral and vertical heterogeneity, indicating a non-uniform distribution of waste. Despite the limited depth resolution inherent to the RMT method, the study successfully delineates the extent and internal structure of the landfill, highlighting the method's effectiveness for near-surface environmental investigations.