Water quality functioning of an upland reservoir and its catchment area : with particular emphasis on iron and manganese

Abstract

Water quality problems associated with high iron and manganese concentrations in the water column are a common feature in many upland water supplies and have been experienced at the Megget Reservoir during a period of major draw down in 1997/98. The peaty soils covering such upland catchments favour metal mobilisation processes resulting in high iron and manganese loading in the streams draining into lakes and reservoirs. When removed from the water column by sedimentation, the metals accumulate in the bottom sediments from where they can be remobilised when conditions in the water body change. In this thesis, extensive investigations at the reservoir body and its surrounding catchment area have been carried out to study the hydrological processes in the catchment, the reservoir and processes operating at the sediment-water interface in the reservoir. The temporal and spatial variability in iron and manganese concentrations in the water column of the reservoir and in the feeder streams is a particular focus of the study. A combination of analytical techniques and process-based laboratory experiments has been employed to investigate the relative importance of physical and chemical processes in determining iron and manganese mobilisation within, and release from, the bottom sediments of the Megget Reservoir. Evaluation of the catchment data reveals that stream chemistry is influenced by flow pathways through the catchments and by catchment characteristics, in particular soil type distribution, which control the iron and manganese fluxes into the reservoir, especially during high rainfall episodes. Evidence is presented that sediment release and re-suspension processes can introduce large amounts of iron and manganese into the reservoir water column and appear to have contributed significantly to the experienced water quality problems. The results of this study have significant implications for the management of the water supply itself and also highlight the importance of catchment-based management strategies for water supplies in other upland regions.