Remobilization of heavy metals in sediments of the Thames Estuary (UK) from historic coastal landfills

Metals are problematic contaminants in estuarine sediments, where metal remobilization is influenced by salinity, pH and redox. Environmental concentrations of some metals are often higher than the natural background, but show no contiguity to pollution source. Notwithstanding, other metals display a more characteristic contamination plume. This study aims (1) to evaluate mobility of metals under controlled conditions, and (2) to examine the evidence of physicochemical remobilization in a real estuarine environment. Metal-contaminated sediments taken from a saltmarsh adjacent to Newland Landfill (Thames Estuary, UK) were exposed (N=96, 20°C) to water at various environmentally relevant factorial combinations of salinity, pH, and redox potential in the laboratory. The resultant slurry was analysed for pH, redox potential, salinity and organic matter after 24h. Major cations, Fe²⁺, and trace metal concentrations were analysed in the leachate. Metal remaining on sediments were also determined on Aqua Regia extracts. Salinity, pH and redox affected metal mobility, speciation and partitioning (p<0.05), e.g. Cu, Fe, Mn and Pb concentrations in the leachate might be increased up to 1000-fold. These results were also relevant in the field. Measurements in situ of surface (5 cm) and subsurface (up to 4 m) sediment cores revealed that landfill proximity poorly explained metal spatial distribution (N~600, r² <0.15). However, physicochemical parameters explain up to 92% of geochemically normalized metal concentrations in sediments. For instance, Na, Mg and K strongly correlate with Al, Cr, Cu, Fe, Li, and Mn (p<0.01). Organic matter and pH were dominant factors for Cu (p<0.01) and Sr (p<0.01), and for Al and Co (p<0.01), respectively. Chemical mobility altered the spatial distribution, resulting in patchy and varied dispersal of heavy metals. Thus, present results highlight the importance of chemical mobility to determine sediment metal concentrations and estuarine fate of metals even in areas near potential contamination sources.