Helene Engler, Dr James Watt, Graham Moore and Professor Margaret C. Graham
Elevated concentrations of manganese (Mn), a trace element linked with human neurological conditions, pose a growing threat to drinking water quality globally. In Scotland, several drinking water reservoirs experience seasonally high concentrations, as sediments release Mn into the overlying waters. The exact controls of these releases remain unknown.
We compare sediments from two reservoirs with differing Mn patterns. One experiences periodic summer–autumn peaks in both dissolved and total Mn concentrations, whereas the other shows episodic elevated total Mn but rarely elevated dissolved Mn.
Sediments were collected by gravity corer, then dried and milled. Total Mn was determined by a HBF4 digest and analysed by ICP-MS. Mn’s spatial distribution within the sediment matrix was explored by SEM-EDX and solid phase speciation by XPS. Porewater Mn fractionation and water–sediment-interface associations were investigated by Size Exclusion Chromatography.
We found elevated Mn concentrations in upper porewaters and sediments of both reservoirs. Truly dissolved Mn was marginally more abundant in one reservoir’s porewaters than the other’s. Sediments were highly organic, and large organic–Mn complexes in the colloidal fraction are dominant in the surface sediment porewaters of one site.
The detailed characterisation of sediments and porewaters improves our understanding of Mn release mechanisms and the chemical forms entering overlying waters. This information is crucial for understanding conditions triggering mobilisation events and the fate of Mn in the water column.