Issues in ecological risk assessment of inorganic metals and metalloids

Ecological risk assessment (ERA) is a process that evaluates the potential for adverse ecological effects occurring as a result of exposure to contami nants or other stressors. ERA begins with hazard identification/problem for mulation, progresses to effects and exposure assessment, and finishes with risk characterization (an estimate of the incidence and severity of any adv erse effects likely to occur). Risk management initially sets die boundarie s of the ERA. and then uses its results for decision-making. Key informatio n required for an ERA includes: the emissions, pathways and rates of moveme nt of contaminants in the environment; and, information on the relationship between contaminant concentrations and the incidence and (or) severity of adverse effects. Because of specific properties and characteristics of meta ls in general and of certain metals in particular, a generalized ERA proces s applicable to organic substances is inappropriate for metals. First, meta ls are naturally occurring and can arise, sometimes in very high concentrat ions, from non-anthropogenic sources; organisms can and do adapt to a wide range of metal concentrations. Second, certain metals (e.g., copper, zinc) are essential for biotic health, which means there is an effect threshold f or both deficiency and excess, and dial standard body burden indices such a s bioaccumulation factors (BCFs) can be misleading. Third, metals can occur in the environment in a variety of forms that are more or less available t o biota but adverse biological effects can only occur if metals are or may become bioavailable. Fourth, whereas the bioavailability and hence the poss ibility of toxicity of persistent organic substances are mainly dependent o n their intrinsic properties (i.e., lipophilicity), those of metals are gen erally controlled by external environmental conditions. Examples include pH and ligands, which affect the metal speciation and coexisting cations (e.g ., H+, Ca2+) which compete with the metal ions. ERAs involving metals must include the above four major considerations; other considerations vary depe nding on whether the ERA is for a site, a region, or is global in scope.