<p>To explore methods supplementing the insights gained from chemical analysis in understanding landfill conditions, we analyzed the relationship between landfill waste composition, microbial community structure in leachate, and leachate water quality. The principal component analysis of water quality and classification of landfills by waste composition showed that landfills with more plastic had lower total contamination than others. The types of organic and inorganic components in the leachate also varied with the landfill waste composition. The microbial community structure at the phylum level in leachate of inert-waste landfill sites was <i>Proteobacteria</i> emerged as the predominant phylum (55% on average) and <i>Bacteroidetes</i> followed as the second most prevalent (12% on average), while <i>Firmicutes</i> was less dominant (0.3% on average). Additionally, the amount of plastic in the landfill waste was related to cyanobacteria, known for their role in plastic degradation. Microbial taxa involved in organic metabolism and oxidation reactions were related to bicarbonate ion (HCO₃^‒), and those involved in oxygen-deficient environments and methane fermentation were connected to water quality parameters, such as COD, TOC, EC, inorganic ionic components, and Co. In the future, assessing the proportions of characteristic microbial taxa in leachate could help understand landfill conditions.</p>