Predicting Sediment Toxicity at Former Manufactured Gas Plants Using Equilibrium Partitioning Benchmarks for PAH Mixtures

This study was conducted to examine the application of Equilibrium Partitioning Sediment Benchmarks (ESBs) for assessing the toxicity of polycyclic aromatic hydrocarbons (PAHs) in sediments at former manufactured gas plant (MGP) and coke sites. Samples of freshwater sediment from four MGP and coke sites in the U.S. Northeast and Midwest were analyzed for 34 individual PAHs, total organic carbon, black carbon (potentially composed of pitch, soot, and other forms of pyrogenic carbon), and sediment toxicity (28-day Hyalella azteca toxicity test). The sum of the Toxic Units in each sample was calculated from a one-phase model that accounts for sorption of PAHs to total sediment organic carbon, and a two-phase model that accounts for sorption to black carbon as well as to natural organic carbon. Although both the one-phase and two-phase models accurately predicted concentrations of PAHs that were not toxic to aquatic invertebrates, the two-phase model was more often in agreement with results of sediment toxicity tests. While the bioavailability and toxicity of PAHs may vary at other sites, the two-phase model correctly predicted that sediments from these sites with concentrations of total PAHs as high as 52 mg/kg were not toxic to invertebrates.