Controls on the geochemical composition of surface water in Alfeios River basin in the transition era of lignite mine closure at Megalopolis, Greece

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 (δ²H-H₂O and δ¹⁸O-H₂O, δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻, δ³⁴S-SO₄²⁻), were employed to assess the geochemical status of surface waters and identify the origin of major ions and trace elements. The chemical parameters of NH₄⁺ (maximum 3 mg/L), SO₄²⁻ (range from 3 to 1010 mg/L), Mn (range from 1 to 338 μg/L), and Mo (range from 0.2 to 1027 μg/L) were identified as the most environmental concern, exhibiting significant variability among samples. Samples located near the old mining area showed the highest concentrations of major ions and trace elements. Stable isotope analysis of δ²H-H₂O and δ¹⁸O-H₂O pointed to evaporation as a process affecting surface waters. Stable isotope analysis of δ¹⁵N-NO₃⁻ (range from 15.3 ‰ to 17.6 ‰) and δ¹⁸O-NO₃⁻ indicated that the dominant source of NO₃⁻ was sewage from the town of Megalopolis. The δ³⁴S-SO₄²⁻ isotope analysis (range from −7.8 ‰ to −2.1 ‰) revealed that pyrite oxidation was the primary source of the high SO₄²⁻ concentrations in the water. We conclude that the major ion and trace element composition in the Alfeios river is predominantly influenced by the basin catchment lithology. Finally, the observed differentiation between wet and dry sampling period in water quality suggests that Alfeios River is expected to be more sensitive for major ions to drought conditions due to limited river capacity for dilution.