Assessing risk in soils with elevated heavy metal contents: is bioavailability enough?

Abstract

Total concentrations of metals in soils are a poor indicator of metal toxicity because metals exist in different solid phase forms which can vary greatly in terms of their bioavailability. In an attempt to acknowledge this, risk assessment procedures are moving away from totals-based approaches and towards a measure of bioavailability as an indicator of the risk. For example in aquatic systems, environmental risks are increasingly being predicted using the Biotic Ligand Model, and in soils, the use of neutral salt extractants such as 1.0 M NH4NO3 have been adopted as part of the regulatory tool-kit in several European countries. However, reliance on this approach to assess risk requires an assumption that the bioavailable pool of metals is relatively static, i.e. that metals cannot migrate between different solid-phase pools. Our results demonstrate that in fact small environmental perturbations can result in the ready conversion of apparently non-labile pools of metals into labile forms, suggesting that risk assessment procedures which depend solely on bioavailability are likely to significantly underestimate the potential risk. We have developed a new strategy ("Risk Potential Index" (RPI)) for assessing the latent risk of metals in these non-labile solid-phase pools. We discuss how the RPI in combination with a bioavailability assessment can be used to provide a more appropriate measure of risk.