Municipal solid waste management is a critical environmental issue, primarily due to the generation of leachate—a highly polluted liquid that can severely impact nearby ecosystems. This study examines the environmental consequences of leachate from solid waste landfills, particularly its effects on groundwater, soil, and the overall health of ecosystems. A review of existing research indicates that the leachate often contains a mix of hazardous substances, including heavy metals and toxic chemicals, which pose substantial risks to public health and the environment. The study highlights the urgent need for effective treatment methods, such as biological and physicochemical techniques, to reduce the harmful impacts of leachate. The impact of leachate infiltration on groundwater quality was evaluated at the Kattumannar Koil open dumpsite, focusing on water samples from three places around the area. Raw leachate and groundwater samples were collected and analyzed using standard laboratory techniques to determine seasonal variations. The analysis revealed that leachate samples had high levels of both organic and inorganic pollutants, indicating severe contamination. The high pH and low BOD₅/COD ratio suggest the dumpsite is old and currently undergoing methanogenic anaerobic decomposition. Groundwater analysis showed that average pH, electrical conductivity (EC), total dissolved solids (TDS), and major ions were higher during the dry season, whereas total suspended solids (TSS), BOD₅, and COD levels were elevated during the rainy season. Most groundwater quality parameters, except for pH and nitrate, exceeded the limits set by the World Health Organization. Based on the pH, EC, and dominant ions, the groundwater near the dumpsite was found to be slightly alkaline, hazardous for consumption, with chloride as the predominant ion. The study highlights the significant influence of seasonal changes and proximity to the dumpsite on groundwater pollution. It is recommended to implement continuous groundwater monitoring and to establish engineered landfills that comply with environmental standards to prevent leachate from contaminating groundwater.