Lignite mining in the Oropos Neogene basin (North Attica), especially in the areas of Milesi and Markopoulo, operated since last century and ceased in the late 1960’s. Piles of complex waste material consisting of lignite tailings and waste rocks are dispersed in the area between the above mentioned mining sites. Environmental characterization of waste piles is performed by applying leaching tests (EN12457, EN15875) as well as bulk geochemical analysis. Mineralogical study revealed that pyrite is the dominant sulfide phase in local wastes. The high sulfur content and low paste pH and Net Neutralization Potential, i.e. values < -20 CaCO3 kg/t in most analyzed samples, all clearly indicate that the waste is prone to acid generation. The analysis of water leachates showed high concentrations in Ni, Zn and Cd, exceeding the EU regulatory limits for the non-hazardous wastes. The preliminary results suggest that the lignite waste pose a potential threat for ground water contamination close to the waste disposal fields due to acid generation and heavy metal mobilization from the lignite matrix or the waste rock.