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Engineered capping systems are efficient tools for the safety concept for landfills and contaminated land, consisting of multilayer barriers installed at the top of the deposited waste, even years after its closure. The efficiency of such systems is closely associated with avoiding rainwater infiltration into the waste body, a process that can be assessed and monitored with geophysical methods, such as Electrical Resistivity Tomography (ERT). A surface geomembrane capping of approximately 900 m2 was installed over an abandoned unlined landfill in southeastern Brazil and ERT surveys were adopted to assay the effectiveness of the intervention measure. It was possible to verify significantly more resistive regions below the geomembrane and an increase in conductivity values only at approximately 6 m, at the groundwater level of the area. Thus, the reduction of rainwater input possibly, even in the short term, reduces the moisture content and prevents the formation of leachate sheets in unlined landfills, leading the deposit to be more isolated from the surroundings. Lateral flow affected distances as far as 2.5 m from the capping borders, showing that about 70% of the capped area was effectively isolated from the rainfall infiltration. Therefore, capping contributed to reducing leachate production and can be considered an effective method for mitigating impacts from old unlined landfills. [ABSTRACT FROM AUTHOR]