Downstream changes in bed-sediment and streamwater metal concentrations along a watercourse in a rehabilitated post-industrial landscape in South Wales

Over 250 years of metal smelting in Swansea (South Wales) left metal-rich slag across the Nant-y-Fendrod valley floor and aerial metal pollution over the wider landscape. Reclamation since 1965 included: (1) partial removal of spoil, flattening the rest and capping it with topsoil, (2) culverting watercourses and creation of two flood-relief lakes and (3) revegetation of the aerially polluted landscape. This paper assesses downstream changes along the Nant-y-Fendrod stream–lake system in metal levels of (a) fine bed-sediment and (b) streamwater and seepages. In the upper, ‘aerial-pollution zone’ total Zn, Cu, Pb and Cd in surface bed-sediment increase marginally within the first lake to 892, 207, 212 and 7.2 mg/kg, respectively. Farther downstream, Cu and Pb rise sharply, and Zn and Cd progressively, to 12,853, 595, 871 and 155 mg/kg, respectively, as the stream traverses the ‘capped metal-rich waste zone.’ Zn and Cd reach 22,671 and 229 mg/kg, respectively, in the second lake, before falling 35–56% below its outlet. Streamwater metal levels rise (but seepage metal concentrations remain stable) in most storm events, though patterns (including whether levels are reduced downstream of the lake) vary with antecedent conditions. Possible interactions between seepages, bed sediment and streamwater metal dynamics are explored.