The variability of heavy metals in floodplain sediments: A case study from mid Wales

A series of vertical alluvial profiles from the upper River Severn floodplain at Welshpool ranging in age from Late Pleistocene to the present have been examined for their Ni, Cu, Zn, Cd, Ba and Pb concentrations. The floodplain deposits have been variably contaminated with heavy metals following extensive metalliferous mining in the upper catchment during the mid-19th century. The data are used to establish whether vertical increases in concentrations can be used to indicate a younging-upwards of the sediments. The data show that grain size and organic content do not influence heavy metal concentrations. Although some of the most recent alluvial profiles contain the highest levels of contamination, a great deal of variation exists, not only between the general vertical distribution of individual metals but also between the absolute values themselves. There was no consistent age-concentration relationship in the vertical profiles. Pb displays the least complex picture with predominantly increasing values towards the top of the alluvial profiles in the historically sedimented sites, indicating a younging-upwards. Rapid sedimentation and post- depositional processes such as leaching, particle translocation or the lateral migration of groundwaters enriched in heavy metals result in different vertical heavy metal distributions at superficially similar sites. It is recommended that sampling strategies for contaminated floodplain sediments utilise a multiple profile approach including the whole range of sediment types, ages and depositional forms that are known to exist at a study site.