Predicting Toxicity of Metals in Soil – The Terrestrial Biotic Ligand Model (TBLM).
Herbert Allen1, Sagar Thakali1, Alexander Ponizovsky2, Dominic Di Toro2, Corinne Rooney3, Fangjie Zhao3, and Steve McGrath3. (1) Department of Civil and Environmental Engineering, University Of Delaware, Newark, DE 19716-3120, (2) University of Delaware, Dept of Civil & Env. Eng., Newark, DE 19716, (3) Rothamsted Research, Agriculture and Environment Division, Hertfordshire, United Kingdom
Although it is widely recognized that the total metal content of soils is not a good predictor of ecotoxicological effect, environmental assessments continue to use total metal content as their basis. In this talk we review a new approach, based on the study of copper or nickel added to soils collected in the European Union, to provide prediction of soil-specific ecotoxicological response. The overall approach which we have termed a Terrestrial Biotic Ligand Model (TBLM) is composed of two submodels. The first submodel considers the partitioning of metal from soil to soil solution. We review the factors that control this partitioning and approaches to relate the concentration of copper or nickel and their free metal ions in soil solution to soil properties. Both empirical relationships and chemical speciation models have been used to predict metal concentration in soil solution. For soils in which metal partitioning is dominated by soil organic matter (most non-calcareous soils), we have computed the speciation of the metals in soil solution using the Windermere Humic Aqueous Model (WHAM VI) using easily determined soil chemistry parameters as input. The second submodel of the TBLM considers interaction of metal ion in soil solution with receptor sites on biota to produce a toxicological response. This approach explicitly permits consideration of competition of other ions such as H+ or Ca2+ at the biotic ligand and separation of this effect from that of these ions on the partitioning of the Cu from the soil. Application of the approach is demonstrated with results for barley root elongation measurements for soils that have been spiked with metal. The TBLM provided better correlation to the observed root elongation to the measured values than did either a model based on the total metal concentration in soil or the Free Ion Activity Model (FIAM). Using the TBLM we were able to predict EC50 values for almost all soils considered to within a factor of 2 compared to measured values.