The relevance of speciation in the remediation of soils and sediments contaminated by metallic elements--an overview and examples from Central Scotland, UK

The environmental impact of metallic contaminants in soils and sediments is dependent both on the chemical speciation of the metal and the response of the matrix to biological and physicochemical conditions. These factors are responsible for the mobilisation of the metal from the solid into the aquatic phase and hence transport within the immediate vicinity, impacting on the rate of dispersal, dilution, uptake and transfer into living systems. The impact of changing environmental conditions on the contaminant inventory can be to enhance or moderate these phenomena, with subsequent consequences for the broader risk assessment of the contaminants. Remediation of metallic contaminants can only be brought about by their removal from the site or by establishing conditions which favour their retention in the solid phase. A wide range of in situ and ex situ approaches are available and a summary overview is presented. The examples show assessment at both the field and laboratory scale and demonstrate an equally wide range of success in achieving remediation targets. This can be attributed to limitations in ensuring that the desired conditions for the initial removal or immobilisation process are met and maintained over a suitable period of time. Three areas are reviewed which include: the transport and release of metallic contaminants in estuarine sediments and the assessment of their potential to impact on biota; terrestrial contamination systems involving the release of chromium from waste ore contaminating urban environments; the response of metal-contaminated wastes to changing environmental conditions and the impact of natural bioremediation. The focus of the discussion is to highlight the generation of reliable speciation information and the problems associated with impact and risk assessment. Particular issues of concern are the laboratory to field scale evaluation of contaminant behaviour and the approach used to assess the reliability of remediation options. In conclusion, part of a recent initiative in risk assessment and the development of pilot scale experimental systems to study long-term behaviour are addressed as future goals to fill gaps in current research.