The Megalopolis region is undergoing a transition from over 50 years of lignite mining to new land uses, with the hydrogeochemical assessment of surface waters being a key factor in post-mining rehabilitation and environmental monitoring. In March 2024, 24 surface water samples were collected from upstream to downstream locations along the Alfeios River and its tributaries, flowing near the mining areas. A combination of tools including hydrogeochemistry, hierarchical cluster analysis (HCA) and stable isotopes (δ