Mobilisation of iron and manganese from sediments of a Scottish Upland reservoir

High concentrations of manganese (>50 μg L-1) and iron (>226-467 μg L-1) have caused water quality problems at the Megget Reservoir during an excessive draw down of the reservoir water level in 1997/98. Sediment column studies were carried out to investigate how internal processes, namely sediment resuspension and diffusion from the sediment, contribute to the mobilisation of these elements in the water column. It was found that sediment resuspension is by far the more effective mechanism in mobilising iron and manganese and also causes persistent discolouration of the water. Concentrations of up to 80 mg L-1 iron and 16 mg L-1 manganese were observed in the experimental core water columns which are equivalent to 490 μg L-1 and 97 μg L-1 in a 27m water column (= maximum water depth during reservoir draw down in 1997/98). From sediment particle size data it was calculated that wind speeds over 13 m s-1 can trigger sediment resuspension at water depths of up to 20 m and have the potential to disturb sediments in large parts (>25% at full capacity) of the reservoir. Under current climatic conditions, such wind events are infrequent but increasing wind speeds and rainfall intensities, as predicted by climate change scenarios, may cause resuspension events to become more frequent, reducing the reservoir's water quality over prolonged periods and limiting its use as a drinking water resource. Such resuspension events are also expected to occur in other water bodies in the study region, adversely affecting their water quality and increasing the ecosystem productivity. Diffusion from the sediment mostly affects the cycling of manganese. Maximum concentrations in the experimental columns were equivalent to 70-130 μg L-1 in a 3-5 m water column and were comparable to in-situ concentrations measured in the bottom waters of the reservoir (3-5 m above the sediment) during summer stratification. Sediment release is unlikely to cause serious water quality problems as only a limited amount of available manganese is found within the sediment. However, the accumulation of manganese and iron in the bottom waters may increase with changing climatic condition, in particular when the external inputs of these elements increase as a result of higher catchment loading associated with the predicted rise in rainstorm intensities.