Equation 2 was used to estimate the biolipid concentrations shown in Figure 3b from estimated free aqueous concentrations shown in Figure 3a. Table 2 illustrates median ratios of estimated to measured aqueous and biolipid concentrations of 1 and slopes close to unity for data in Figure 3a and b. As demonstrated by Jonker and Koelmans (8), BC is a heterogeneous material with distinct sorption properties and therefore the use of a single KBC value could be challenged. Yet, for the data in Figure 3 spanning free aqueous concentrations of 7 orders of magnitude and biolipid concentrations of 4 orders of magnitude, normalization to fAOC and fBC appears to minimize systematic differences among the five sediments that were apparent when estimates were based on sediment TOC alone. It is logical that the BC sorption isotherm should eventually become more linear in the very low free aqueous PCB concentration range, since there must be a finite limit to the affinity of PCBs for any type of adsorption site, and the BC sorbent strength for PCBs cannot increase indefinitely with decreasing free aqueous PCB concentrations.