Hazard assessment of metals and metal compounds in terrestrial systems

Abstract

Metal accumulation in soil can result in adverse effects on soil biota, and may concentrate metals in food chains to levels detrimental to humans and wildlife. A SETAC Pellston Workshop entitled "Hazard Identification Approach For Metals And Inorganic Metal Substances" examined methods for classification, ranking and screening level assessments. The Terrestrial Workgroup debated issues surrounding persistence, bioaccumulation, and toxicity metrics for soils and terrestrial organisms. Because of the long time required for metals to achieve steady-state in soils, the proposed critical load concept (the "unit world model") cannot be calculated at a steady-state but must be phrased in terms of a predetermined time-frame (e.g., 100 yrs). Even so, toxicity data must be generated in a manner that accounts for ageing of metals in soil. Dissolution or transformation of sparingly soluble compounds in soils is very different from that in water, as soils provide sinks for reaction products and have large buffering capacities. Uptake of metals by plants is not well predicted by free ion concentrations of metals in solution, suggesting that other soluble forms may contribute to overcoming diffusional limitations to uptake. Thus, either prediction of complete speciation or measurement of soluble forms of metals in soil solution may be required to predict bioaccumulation. Prediction of fine root accumulation (a dietary source of metals for soil-dwelling invertebrates) from measurement of shoot tissue metal is difficult. In the case of insufficient reliable toxicity data, at least three tests should be performed for the soil ecosystem (microbial, invertebrate, and plant tests), following methods proposed during a former Pellston workshop. Suggested approaches for ranking hazard of metals include a "back-calculation" from the toxicity thresholds for terrestrial organisms to acceptable soil levels, thus incorporating bioaccumulation factors, or using existing soil quality criteria from various countries although most do not incorporate food chain concerns.