Chemical and isotopic analyses of groundwater from piezometers localized around a reclaimed landfill were performed in order to identify the boundaries of groundwater contamination zone. Spatial distribution of dissolved inorganic carbon (DIC) concentration and stable carbon isotopes in the groundwater was used to distinguish the piezometers localized within the contaminated aquifer. Background groundwater was characterized by low DIC concentration (from 1.8 to 5.0 mmol/L) and negative values of δ13CDIC (from -20.6‰ to -12.4‰). Higher DIC concentrations (from 6.0 to 12.5 mmol/L) and higher values of δ13CDIC (from -10.9 to +3.6‰) were determined in groundwater contaminated by landfill leachate. The study confirmed that δ13CDIC value in the groundwater was a useful tracer in determining the extent of the contamination zone around the landfill. In general, upgradient from the landfill, carbon isotopic composition of groundwater depended on natural sources of carbon and δ13CDIC values were negative. Downgradient from the landfill, where groundwater was contaminated by the landfill leachate, δ13CDIC values were higher, sometimes even positive.