The role of vegetation in the retention of fine sediment and associated metal contaminants in London's rivers

摘要

Many urban rivers receive significant inputs of metal-contaminated sediments from their catchments. Restoration of urban rivers often creates increased slack water areas and in-channel vegetation growth where these metal-contaminated sediments may accumulate. Quantifying the accumulation and retention of these sediments by in-channel vegetation in urban rivers is of importance in terms of the planning and management of urban river restoration schemes and compliance with the Water Framework Directive. This paper investigates sediment properties at four sites across three rivers within Greater London to assess the degree to which contaminated sediments are being retained. Within paired restored and unrestored reaches at each site, four different bed sediment patch types (exposed unvegetated gravel, sand, and silt/clay (termed 'fine') sediments, and in-channel vegetated sediments) were sampled and analysed for a range of metals and sediment characteristics. Many samples were found to exceed Environment Agency guidelines for copper (Cu), lead (Pb) and zinc (Zn) and Dutch Intervention Values for Cu and Zn. At all sites, sediments accumulating around in-channel vegetation were similar in calibre and composition to exposed unvegetated fine sediments. Both bed sediment types contained high concentrations of pseudo-total and acetic acid extractable metal concentrations, potentially due to elevated organic matter and silt/clay content, as these are important sorbtion phases for metals. This implies that the changed sediment supply and hydraulic conditions associated with river restoration may lead to enhanced retention of contaminated fine sediments, particularly around emergent plants, frequently leading to the development of submerged and emergent landforms and potential river channel adjustments. High pseudo-total metal concentrations were also found in gravel bed sediments, probably associated with iron (Fe) and manganese (Mn) oxyhydroxides and discrete anthropogenic metal-rich particles. These results highlight the importance of understanding the potential effects of urban river restoration upon sediment availability and channel hydraulics and consequent impacts upon sediment contaminant dynamics and storage.