A particular threat to surface water and groundwater is the migration of leachate, both from uncontrolled and organised landfills. Leachate is generated by infiltration of rainwater, groundwater and leaching of water from waste. It can contain a wide range of pollutants, including soluble organic compounds, inorganic pollutants, suspended solids, heavy metals and hazardous substances. The use of appropriately selected tools can be helpful in assessing the impact of landfills on environmental elements. It is critical to assess groundwater contamination in the areas around landfills, given that groundwater is the most important source of drinking water and is essential to meet the needs of communities. The aim of this study was to assess the variability in groundwater quality using the Landfill Water Pollution Index (LWPI). The properties of groundwater in the vicinity of two municipal landfills in a Central European country (Poland) were analyzed for a period 11 and 15 years of their operation. The results confirmed that LWPI is an effective method for assessing the quality of groundwater in the vicinity of municipal waste landfills. The obtained results confirm the negative impact of landfills, despite the insulation used. LWPI showed poor quality of groundwater, a visible impact of the landfill (landfill W, average LWPI - 2.34) and moderately polluted waters and a minor impact of the landfill (landfill S, average LWPI - 1.37). In most cases, it was observed that two parameters, EC and TOC, are the main factors contributing to the deterioration in groundwater quality. Analysis conducted on multi-year groundwater data using specific indicators can help managers better understand the impact of municipal solid waste on surrounding areas and help avoid potential operational problems in the future.