In rapidly growing urban areas of developing countries, inappropriate urbanization practices near water bodies lead to significant environmental degradation, including deforestation, soil impermeabilization, and direct waste discharge into rivers and streams. This not only compromises water quality and ecosystem health but also exacerbates flood risks for nearby communities. Dumpsites, common in these regions due to inadequate waste management infrastructure, pose significant environmental threats, including soil and groundwater contamination. Our study employed a frequency-domain electromagnetic survey on an inactive dumpsite adjacent to one of Brazil’s major rivers. This geophysical method provided critical insights for geoenvironmental investigations, facilitating the collection of soil and groundwater samples to understand leachate behavior. We found that the shallower electrical conductivity map aligned with the waste disposal area's boundaries, verified by on-site surveys and excavations. Historical aerial images supported this interpretation and helped explain conductive electrical anomalies in the deeper conductivity map, linked to old irregular housing and waste disposal around the dumpsite. Moreover, the electrical conductivity maps revealed NE-oriented percolation at depth in the study area. Groundwater monitoring wells and soil analyses identified chemical elements exceeding standards set by Brazilian and international environmental agencies in this flow direction. These findings highlight the importance of using investigation techniques with superior spatial resolution compared to traditional environmental monitoring approaches. Electromagnetic methods prove effective in enhancing understanding and management of contamination in dumpsite environments.