API Publ 4701:2000 pdf download

API Publ 4701:2000 pdf download.BIOACCUMUL ATION: AN EVALUATION OF FEDERAL AND STATE REGUI ATORY INITIATIVES.
22.2 Environmental Variables
Environmental conditions also play a key role in determining a chemical’s potential to bioaccumulate in aquatic organisms. In particular, the amount of organic carbon in sediment and dissolved in the water column is typically the most important factor influencing the amount of chemical available for uptake. Because sediment organic carbon (or sediment organic matter) is a large sink for lipophilic chemicals, the higher the organic carbon content the lower the fraction of chemical available for uptake by aquatic organisms.
In addition to organic carbon content, other factors such as sulfide, pH, salinity, biological activity (environmental degradation processes), and water clarity also play a role in bioaccumulation of chemicals. The level of sulfide in sediment is a particularly important factor influencing the bioaccumulation of certain metals. Changes in pH may also affect chemical speciation, resulting in either increases or decreases in bioavailability and bioaccumulation. For example, naphthenic acids in refining effluent become more water soluble and less bioaccumulative as pH increases. Similarly, biological activity and other environmental degradation processes that reduce concentrations of the parent compound, such as polycyclic aromatic hydrocarbons (PAHs), can lead to reductions in bioaccumulation, although such processes can result in the formation of more toxic breakdown products. In the case of PAHs, photolytic breakdown and metabolism by higher trophic level organisms (i.e., fish) will reduce environmental concentrations; however, some bioaccumulation will occur due to the ongoing releases of these chemicals to aquatic systems (API, 1997).
2.2.3 Organism-Related Variables
Aquatic organisms accumulate chemicals through diet and direct uptake from water. If the rate of intake is greater than the rate of elimination, then bioaccumulation occurs. Organism-related factors that affect bioaccumulation rates include lipid content, species-specific differences in chemical uptake and elimination rates, ability to metabolize certain types of chemicals, and gender, as described below.
Within organisms, hydrophobic organic chemicals tend to partition into lipid stores (i.e., fat). For this reason, organisms that contain higher lipid levels tend to accumulate higher levels of hydrophobic organic chemicals. Differences in lipid content among fish species are one of the factors used in the GLI to estimate bioaccumulation levels for fish of different trophic levels.
Factors that affect chemical uptake rates depend on whether uptake occurs directly from water or via the diet. For many aquatic organisms, direct uptake from water across the gills is the major route of exposure to chemicals (Spacie and Hamelink, 1982). Even very lipophilic materials for which food chain transfer is important are accumulated through water as well. McKim et al. (1985) reported gill uptake efficiencies for 14 organic chemicals from five chemical classes ranging from seven percent to over 60 percent, depending on the chemical’s log K0. Consistent with other studies on log K0 (Opperhuizen et al., 1985), uptake efficiencies were greatest for chemicals with log K0 between one and six. Although log K0 is more influential in predicting uptake, differences in gill structure from species to species may affect uptake to some extent.
Similar to uptake from water, dietary uptake of chemicals is variable and depends both on the chemical and the organism. If ingested materials are mostly comprised of nondigestable materials (e.g., sediments), gut assimilation of chemicals will be limited by the desorption of the chemical from organic matter (API, 1997). Different species also have different gut uptake efficiencies for the same chemicals. For example, the efficiency of PAH uptake by fish, crustaceans, and marine worms ranges from less than ten percent to greater than 70 percent. Part of the reason for this variability may be species differences in the ability to breakdown ingested organic matter (API, 1997). Finally, when chemicals are moved through gastrointestinal membranes, a molecular size limitation (circa 9.5 A) appears to hold true. Above this size limitation, absorption of chemicals through the gastrointestinal tract will be limited.
Bioaccumulation of chemicals only occurs if the rate of chemical uptake exceeds the rate of elimination.