Municipal solid waste (MSW) management is recognized as one of the most critical environmental challenges of the twenty-first century. Rapid population growth, increasing consumerism, excessive waste generation, inadequate recycling practices, and the disposal of waste in poorly designed landfills have significantly complicated sustainable waste management. Landfill leachate, in particular, poses a serious threat to soil and groundwater quality due to its complex composition and high contamination potential. In this study, previous research on landfill-related contamination was reviewed, and the effects of leachate generated from the Aradkouh landfill on the shear strength behavior of soil were experimentally investigated. In addition, the applicability of the Contamination Index Parameter (CIP) and the Leachate Pollution Index (LPI) was evaluated to assess the degree of contamination and the potential risk of groundwater pollution.To examine the influence of leachate, uncontaminated soil was used as the control specimen and compared with artificially contaminated samples containing 3%, 10%, and 15% leachate. Direct shear tests were performed on all specimens under different normal stress levels after a curing period of 72 hours.The results demonstrated that increasing the leachate content to 3%, 10%, and 15% reduced the soil shear strength by approximately 22%, 26%, and 28%, respectively. Moreover, leachate contamination adversely affected the soil's internal friction angle. These findings suggest that leachate infiltration weakens the mechanical properties of soil, reduces landfill stability, and increases the potential for environmental and groundwater contamination.