Improper landfills (waste dumps) are common waste disposal systems in developing countries and represent sources of environmental pollution. These sites defy researchers and managers because they lack structures to collect liquid, solid and gaseous samples, which make it challenging to monitor local environmental quality. In this work, we show one device for sample collection to monitor leachate quality in a closed waste dump in Brazil. During the installation of this device (Leachate Monitoring Station, LMS), interesting facts about the structural, physical, and chemical composition of an old dump could be visualized. Two different kinds of leachate were found: the accumulated leachate (AL), a thick dark fluid entrapped above non-degraded material, and the mobile leachate (ML), a lighter liquid which flowed into the LMS, and thus was not stagnant like AL. In the AL, the chemical oxygen demand and total ammoniacal nitrogen average concentrations were about 21,500 mg/L and 1000 mg/L, respectively, which were considerably higher than the ML concentrations, of about 1100 mg/L and 200 mg/L, respectively, for the same parameters. Thus, despite the lower concentrations of hazardous substances in the ML, the waste body stores pockets of leachate (AL) with significant concentrations of hazardous compounds, even after 15 years of the dumpsite closure. Moreover, waste solubilization assays showed that the solid material could not be considered inert according to the Brazilian Standard Norm NBR 10004/2004. The installation of the LMS enabled a new understanding about pollutant accumulation inside waste deposits and provided an effective, low-cost tool to monitor leachate production in non-sanitary landfills. The results warn about the risks that old dumpsites still pose to the environment and reinforce the need for a post-care action plan for managing uncontrolled waste deposits.