Ni PECs, or dietary Ni concentrations, were identified for each of the food chains in Table 1. As noted in Figure 2, the assumed dietary compositions for birds and mammals were sometimes refined from a diet based on conservative assumptions to a diet based on more realistic assumptions, such as a variable diet rather than feeding exclusively on an organism that tends to bioaccumulate the highest Ni concentrations. In addition, the basis of the data used to derive the PEC varied for some food chains. In most cases the Ni PEC was estimated from water or soil Ni concentrations using a bioaccumulation factor (BAF; see Text Box 2), while in some cases measured Ni concentrations in prey were used. For the marine food chain, it was assumed that C. edule, which accumulates Ni more than other marine organisms, would be a potential food item for the oystercatcher. For the harbor seal, and for the oystercatcher at other locations, it was assumed that C. edule would not be a relevant food item and that they would feed on prey items (e.g., fish and mollusks other than C. edule) that do not bioaccumulate Ni to the same level. Where Ni concentrations in prey items were estimated from seawater, a BAF of 1,631 L/kg was used for scenarios including C. edule and a BAF of 270 L/kg was used for scenarios excluding C. edule (a realistic BAF for fish and other bivalves). For the freshwater food chains, a BAF of 270 L/kg was likewise used to estimate Ni concentrations in freshwater prey. For the terrestrial food chains, PECs were estimated for an earthworm diet, as well as diets that included a mixture of earthworms and isopods (DeForest, 2012). A BAF of 0.30 was used to estimate the Ni concentration in the tissue of the earthworm, while a BAF of 0.06 was used to estimate Ni in the tissue of isopods. In addition, the Ni concentration of the soil in the earthworm’s digestive tract was estimated using recommendations from the TGD. The Ni PECs used to evaluate the marine, freshwater, and terrestrial food chains are summarized in Table 2.
Text Box 2
Bioaccumulation factors, or BAFs, are ratios of the chemical concentration in an organism (Concorg) to its concentration in its abiotic environment (Concenv, i.e., surface water for aquatic organisms and soil for terrestrial organisms):
For example, if it is assumed that Ni concentrations of 50 mg kg-1 and 0.1 mg/L are measured in an aquatic organism and its exposure water, respectively, the resulting BAF would be 500. A water or soil BAF for Ni can then be multiplied by Ni concentrations in water or soil, respectively, to estimate what the Ni concentration would be in an organism at another site or under another exposure scenario. It should be noted that BAFs for Ni and other metals, even within a species, will vary depending on multiple factors, including differences in Ni bioavailability between sites and the magnitude of the Ni exposure concentration (the BAF tends to be inversely related to the exposure concentrations). Accordingly, application of BAFs to estimate Ni concentrations in prey only provides an approximation of the potential Ni concentration at a given site or under a given scenario.
The following basic equation was used to derive PECs from water or soil Ni concentrations (Concenv) using BAFs:
PEC = Concenv × BAF