Characterization and evaluation of Fe2+ and Mn2+ in reservoirs and sediments in the Jianghuai Region

A dynamic monitoring study of water quality and sediment was conducted in August 2024 at four drinking water reservoirs in Hefei City (Dafangying, Dongpu, Modun, and Zhongxing Reservoirs). Monitoring results indicate: (1) Compared to the summer of 2018, overall Fe and Mn levels in Hefei's reservoirs showed improvement during the summer of 2024. Excessive Fe and Mn concentrations were only observed at deeper water levels, with no exceedances detected in surface layers. (2) Vertical monitoring analysis revealed that due to high summer temperatures, a thermocline formed at approximately 8 m in all four reservoirs. At this depth, the temperature difference between the surface and bottom layers was about 7 °C, resulting in a positive correlation between T, DO, TP, Fe, and Mn. (3) Monitoring of vertical distribution of Fe and Mn in reservoir sediments indicates overall surface enrichment characteristics, with highest concentrations in the surface layer decreasing gradually with sediment depth. (4) Morphological analysis of Fe and Mn in surface sediments indicates that Fe primarily exists as a residual form, while Mn predominantly occurs in a reducible state.