Bioaccumulation of a chemical is often reported by bioaccumulation factors (BAFs, lkg-1 lipid; eqn ), to describe the increase of contaminants such as persistent organic pollutants (POPs) from water to biota due to uptake from all exposure routes:
Baf = [popbiota]lipid corrected h
where [POPBIoTA] is the concentration of the contaminant in the organism, corrected for the animal or plant's lipid content, and [POPwater] is the dissolved concentration of the contaminant in water. Whereas BAFs include uptake from all exposure routes, the bioconcentration factor (BcF) describes only the exposure from the abiotic environment, and uptake due to equilibrium partitioning of contaminants between the surrounding environment and the organic phase in the biota. Various chemical management programs categorize contaminants with BCF or BAF higher than 5000 (wet weight basis) as bioaccumulative. BAFs and BcFs are corrected for lipid content, as this is the organic phase into which the organic contaminants dissolve. Therefore, lipid-normalized BAF and BCF values (eqn ) are more useful when comparing across animals, as the variation due to variable lipid content is eliminated.
BAFs can be estimated by empirical or mechanistic models when empirical data are not available. Several studies show a relationship between a chemical's relative solubility in lipids compared to that in water, as measured by the octanol-water partitioning coefficient (Kow), where octanol and lipids are assumed to have similar properties. BCF on a wet weight basis can be predicted empirically from the chemical's Kow. Although there is a theoretic 1:1 relationship between BAF or BCF and Kow on a logarithmic scale, empirical studies show a high degree of variability in BAFs, often 1-2 orders of magnitude.
Bioaccumulation of organic contaminants in organisms can also be predicted by kinetic, mechanistic models that are based on hypotheses about the exposure and elimination processes involved in bioaccumulation:
where A[POPbiota]/Aî is the change in contaminant concentrations in the animal over time, k1 is the respiration and passive diffusion uptake rate, [POPWATER] is the dissolved concentration of the contaminant in water, kDIET is the uptake rate from diet, k2 is the elimination rate due to respiration and passive diffusion, and kM, kE, and ko are the elimination rates due to metabolism, eges-tion, and growth, respectively. An advantage of mechanistic models is that they quantify different processes of varying importance for bioaccumulation, such as respiration and feeding rates, growth dilution, and biotransformation. This is important not only to assess bioaccumulation, but also to consider which major ecological and chemical factors influence the accumulation of various substances. In addition, the source of contaminants can be identified for animals that are living both in the free water masses and that are occasionally exposed to contaminants through the sediment-related benthic food web.
Even if a contaminant is being taken up by an organism, it does not automatically result in bioaccumulation. An organism can modify the absorbed mixture of contaminant; some chemicals are retained, whereas others that are more water soluble or degradable are eliminated from the body, resulting in no net accumulation. In the food web, animals thereby show very different bioaccumulation of various chemicals, both in levels and in the relative composition.
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