Processes controlling manganese distributions and associations in organic-rich freshwater aquatic systems: The example of Loch Bradan, Scotland

Recent increases in manganese (Mn) concentrations in surface waters, including drinking water supplies, have triggered renewed interest in its geochemical behaviour in freshwater systems. This study, involving analysis of bottom sediments and ultrafiltered water (stream, loch and sediment porewater) samples, identified changes in aqueous phase speciation of Mn upon entering the loch waters and during its transit from the inflow to the outflow of Loch Bradan, a drinking water reservoir in SW Scotland. Diffusion out of the bottom sediments during calm periods or mixing of porewaters with loch water during resuspension events also released Mn into the overlying waters. Although 65% Mn was in colloidal form (3kDa–0.2μm) in the main streamwater inflow at the western end, 57–66% was present in the <3kDa fraction in the proximal loch waters, at least partly as a result of the release of Mn²⁺ from the bottom sediments. Towards the outflow at the eastern end, however, the increase in the amount of Mn associated with large organic colloids (100kDa–0.2μm) correlated with the speciation of Mn in the bottom water and the bottom sediment porewaters. While the inflow waters do have some impact upon Mn speciation at the western end of the loch, it appears that within-loch processes have a greater impact on Mn speciation near the outflow. These findings emphasise the importance of understanding the geochemical controls on Mn behaviour in aquatic systems: it is clear that although Mn may be present as truly dissolved Mn²⁺ in some parts of the loch, it can also be associated to a significant extent (35–47%) with large organic colloids. These findings are important not only with respect to water treatment but also in terms of understanding the likely consequences of climatic change which may exacerbate losses of Mn from the bottom sediments.