Characterization of the bioavailability and toxicity of PAHs at MGP sites

The toxicity of polycyclic aromatic hydrocarbons (PAHs) to the aquatic amphipod, Hyalella azteca was measured in 34 sediment samples collected from four manufactured gas plant sites ranging in total PAH (sum of 16 EPA priority pollutant PAHs) concentrations from 4 to 5700 mg/kg. The survival and growth of H. azteca in 28-d bioassays was unrelated to total PAH concentration with 100% survival in one sediment having 1730 mg/kg total PAHs, while no survival was observed in other sediment samples with concentrations as low as 54 mg/kg total PAHs. Twenty-five of the 34 sediment samples exceeded the Probable Effects Concentration (PEC) screening value of 22.8 mg/kg total PAHs, yet 19 (77%) of the 25 samples predicted to be toxic were not toxic to H. azteca. However, the toxicity of PAHs to H. azteca was accurately predicted for all 34 sediment samples when either the rapidly released concentrations as determined by mild supercritical fluid extraction (SFE) or the pore water concentrations were used to establish the bioavailability of PAHs. The unexpectedly low toxicity observed in these sediments indicates that the PEC and ESB screening values on their own do not provide a reliable basis for predicting the toxicity of PAHs in sediments at MGP sites without adjusting for bioavailability. These results demonstrate that the PAHs present in most sediments collected from MGP sites have low bioavailability and that chemical measures for predicting PAH bioavailability can be useful tools for estimating sediment toxicity.