Multiple geophysical methods are efficient for mapping the geometry of old waste dumps, detecting illegally buried waste, identifying areas enriched in leachate and biogas in landfills, and mapping contaminated sites. This paper describes the results of an electrical resistivity tomography (ERT) and magnetometric (MAG) investigation into the extensive dumping of construction and demolition waste installed in a former quarry in Northeast Brazil. Preliminary investigations using a geotechnical survey in the upper portion of the waste massif indicated the occurrence of methane gas, possible leachate at depth, concrete blocks with iron rebar, and even other metals in smaller proportions. The modeling of resistivity and magnetic data was used to delineate volumes containing metal, corresponding to low electrical resistivity values and high magnetic susceptibility. On the other hand, high resistivity measurements and low magnetic susceptibility are associated with gas or compacted and unsaturated terrain. From deep environmental drilling services and excavations in the region under study for the implementation of a subway, it was confirmed that the geophysical anomalies interpretation identified the presence of portions rich in organic matter, with high humidity, inside the dump and the presence of methane, concrete blocks, and ferrous material derived from the demolition of the construction industry.