User Guide to Thresholds and Classification - Environmental ...

351User Guide for Thresholds and ClassificationsThe mechanisms for uptake and depuration rates of metals are very complex and variable and there is atpresent no general model to describe this. Instead the bioaccumulation of metals according to theclassification criteria should be evaluated on a case-by-case basis using expert judgement.While BCFs are indicative of the potential for bioaccumulation there may be a number of complications ininterpreting measured BCF values for metals and inorganic metal compounds. For some metals andinorganic metal compounds there is an inverse relationship between water concentration and BCF in someaquatic organisms, and bioconcentration data should be used with care. This is particularly relevant formetals that are biologically essential. Metals that are biologically essential are actively regulated inorganisms in which the metal is essential. Since nutritional requirement of the organisms can be higher thanthe environmental concentration, this active regulation can result in high BCFs and an inverse relationshipbetween BCFs and the concentration of the metal in water. When environmental concentrations are low,high BCFs may be expected as a natural consequence of metal uptake to meet nutritional requirements andin these instances can be viewed as a normal phenomenon. Additionally, if internal concentration isregulated by the organism, then measured BCFs may decline as external concentration increases. Whenexternal concentrations are so high that they exceed a threshold level or overwhelm the regulatorymechanism this can cause harm to the organism. Also, while a metal may be essential in a particularorganism, it may not be essential in other organisms. Therefore, where the metal is not essential or when thebioconcentration of an essential metal is above nutritional levels special consideration should be given to thepotential for bioconcentration and environmental concern.Application of classification criteria to metals and metal compoundsIntroduction to the classification strategy for metals and metal compoundsThe schemes for the classification of metals and metal compounds are described below and summariseddiagrammatically in Figure 19D.1. There are several stages in these schemes where data are used fordecision purposes. It is not the intention of the classification schemes to generate new data. In the absenceof valid data, it will be necessary to use all available data and expert judgement. In the following sections, thereference to the L(E)C50 refers to the data point(s) that will be used to assign the classification for the metalor metal compound.When considering L(E)C50 data for metal compounds, it is important to ensure that the data point to be usedas the justification for the classification is expressed in the weight of the molecule of the metal compound tobe classified. This is known as correcting for molecular weight. Thus while most metal data is expressed in,for example, mg/L of the metal, this value will need to be adjusted to the corresponding weight of the metalcompound. Thus:L(E)C50 metal compound =L(E)C50 of metal × Molecular weight of metal compoundAtomic weight of metalChronic NOEC data may also need to be adjusted to the corresponding weight of the metal compounds.January 2012 EPA0109