Dispersion and remobilisation of heavy metals in the River Severn system, UK

Abstract

Metal mining used to be a major source of heavy-metal contamination for river systems and in England and Wales affected areas exceed 4000 km2. Under flooding conditions, heavy metals stored in riverbeds and floodplains can be remobilised and become secondary sources of diffuse pollution. During remobilisation, heavy-metal species and their association with soil/sediment particles have been changed. This paper investigates heavy metals in floodplain sediments of the River Severn, UK. Four floodplains representing the upper catchment (Caersws), the middle reaches (The Burf and Berwick Farm) and the lower catchment (Tewkesbury Ham) were studied for the concentration and distribution of Pb, Zn, Cu, Co and Cd and their associations with sediment particles. In the floodplain sediment from Caersws, heavy metals are associated with sand-sized particles and are in the form of native metals. With increasing distance downstream, these particles are broken into smaller fragments and heavy metals are released into the system where they combine with finer particles and are deposited on floodplains downstream during flooding. The highest concentrations of Pb are in the floodplain sediment at Caersws and the concentration decreases downstream due to "dilution" by local sediment supply and fragmentation. Across the floodplain, heavy metals are deposited adjacent to the present channel in the upper reaches, but are deposited in localities further away from the channel on the other floodplains further downstream, reflecting the fact that heavy-metal association with sediment particles controls their distribution.

Concentration of heavy metals along vertical profiles increases first and then decreases with depth with peak values reached at varying depth between profiles. This pattern of variation along profile reflects the history of heavy metal accumulation, which is determined by the rate of heavy metal release from mining sites upstream. Although the varying depth that peak value reaches at different profiles is affected by translocation and hydroperiod, it still corresponds with the change of historical mining output upstream and can be used to calculate the sedimentation rate of floodplain surfaces.