Iron and manganese cycling in the storm runoff of a Scottish upland catchment

Factor analysis and (two-component) end member mixing analysis (EMMA) were applied to high resolution stream chemistry data from three catchments to infer sources of iron and manganese-rich runoff in the catchment area and to evaluate their temporal and spatial influence on the stream water quality. The results demonstrate that flow related changes in soilwater inputs (associated with changes in flow pathways) exert a major control on stream chemistry during storm events and depend on the soil distribution in the catchments as well as on antecedent conditions and storm intensity. Two Fe and Mn sources were identified: (1) an organic soilwater source, associated with Fe and Mn accumulation in the organic-rich upper soil horizons; and (2) a deep soilwater/groundwater source arising from reduced metal mobilisation in the deeper soils. While (1) provides the dominant inputs during storm events, (2) becomes important just before peak flow when riparian groundwater is displaced into the stream giving rise to total Fe and Mn concentrations of up to 1160 and 121 μg l−1, respectively. The timing of such high Fe and Mn loadings in the stream runoff has important implications for the water supply management of the catchment. Subsequent effects on lakes/reservoirs, for example, must be considered when assigning load limits or load reduction goals for the purpose of improving/ maintaining water quality but also when planning maintenance work on the reservoir. However, a better understanding of the nature of these Fe and Mn sources is required to better (quantitatively) predict when and where undesirable Fe and Mn concentrations will occur.