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

Abstract

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.

The Impacts of Natural Flood Management Approaches on In‐Channel Sediment Quality

“…Similarly, Gibbs et al . () observed no significant difference in Cr, Cu, Ni, Pb, or Zn concentration between restored and un‐restored reaches because of the variety of restoration approaches used.…”

“…Heavy metals adsorb preferentially to the fine‐grained fraction of the sediment (<63 µm), and significant correlations (using non‐parametric analysis) were observed between percentage of sediment <63 µm and pollutant content (for various pollutants). Therefore granulometric correction was performed by calculation of a dilution factor for each sample (similarly to Horowitz, and Gibbs et al ., ): …”

Recognizing different impacts of human and natural sources on the spatial distribution and temporal trends of PAHs and PCBs (including PCB-11) in sediments of the Nador Lagoon (Morocco)

“…1) were selected on the basis of water depths (Table S1 in the Supplementary material) and the surficial grain size distribution reported by El-Alami et al (1998). In this way the sampling of fine sediment was favoured, as silt and clay fractions are acknowledged to interact preferentially with contaminants (Frignani et al, 2001;Ünlü and Alpar, 2006;Bellucci et al, 2012;Gibbs et al, 2014). In addition, fine-grained materials deposited under low-energy conditions do not favour mixing and resuspension and are more suited for retaining chronological information (Cochran et al, 2005;Mendes et al, 2010;Bellucci et al, 2012).…”

Controls on anastomosis in lowland river systems: Towards process-based solutions to habitat conservation

“…Most riverbanks in the NNP are now lined with reed (75%; Próchnicki, 2005) which can extend laterally from the floodplain into the channel and root into sediment deposits (Best et al, 1981). The aquatic vegetation blocks the channel physically and increases flow resistance (Nepf et al, 2007;Gibbs et al, 2014;Gurnell, 2014). This in turn results in turbulent energy dissipation, creating zones of low velocity and low bed shear stress that encourages deposition of fine organic and inorganic particles.…”