Critical evaluation of the sediment effect concentrations for polychlorinated biphenyls. 
In 2000, a set of sediment effect concentrations (SECs) was published for evaluating the toxicity of polychlorinated biphenyls (PCBs) in freshwater, estuarine, and marine sediments.  According to the developers, these consensus-based SECs reconcile existing sediment quality guidelines (SQGs) that have been developed using various approaches, reflect causal rather than correlative effects, and can be used to determine the spatial extent of injury to sediment-dwelling organisms.  In the present study, a critical evaluation of the SECs was conducted based on the original documents and databases used to develop the underlying SQGs for the SECs, as well as the original documents and data sets used to determine the predictive ability of the SECs.  Results of the critical evaluation indicated that the SECs are simple mathematical constructs that share the same limitations as their underlying SQGs.  The SECs are questionable "consensus" values, because many of their underlying SQGs are dissimilar, misclassified, or redundant with other SQGs.  Because nearly all of the data sets included in the databases used to calculate the underlying SQGs, or to validate the SECs, were affected by elevated concentrations of multiple co-occurring chemicals, it was not possible to-conclusively identify PCBs as the cause of any of the observed sediment toxicity.  The SECs, and most of their underlying SQGs, are likely biased by the fact that their underlying databases are composed primarily of PCB concentrations less than 0.5 mg/kg dry weight.  Comparisons between the SECs and bioaccumulation-based SQGs calculated using the equilibrium partitioning approach provide no information on whether the SECs are causally related to sediment toxicity.  The primary available median lethal concentration (LC50) value for PCBs, determined using spiked-sediment toxicity tests, has limited applicability to most contaminated aquatic environments, because it was determined using an unusually low total organic carbon content.  Finally, site-specific application of the SECs indicated that their predictive ability was very low, that concentration-response relationships were not found for a variety of test species and toxicity endpoints at PCB concentrations greater than the SECs, and that some of the highest survival and growth values in the toxicity tests were found at PCB concentrations considerably greater than the SECs.  Based on the results of this study, we conclude that the SECs for PCBs should be used only in the screening-level evaluations that typically precede more direct assessments of sediment toxicity at individual study sites, and should not be used to predict the presence of sediment toxicity.  Contrary to the conclusions of the SEC developers, the SECs do not reconcile existing SQGs, do not reflect causal effects, and should not be used to determine the spatial extent of injury to sediment-dwelling organisms.