Polycyclic aromatic hydrocarbon bioavailability in contaminated soils and sediments

The bioavailability and toxicity of polycyclic aromatic hydrocarbons (PAHs) to soil and sediment invertebrates were characterized in contaminated soils and sediments collected from industrial manufactured-gas plant (MGP) sites. At these locations, Town Gas was manufactured from coal and oil feed stocks from the 1860's through 1940's leaving potentially toxic concentrations of PAHs in soils and sediments. However, despite high levels of contamination, the PAHs present in many soils did not bioaccumulate in the earthworm, Eisenia fetida, nor were many sediments toxic to the freshwater aquatic amphipod, Hyalella azteca. Surprisingly, the bioaccumulation and toxicity of PAHs to earthworms and amphipods were generally unrelated to the total concentration of PAHs in soils and sediments measured by exhaustive Soxhlet-extraction methods. The bioaccumulation of PAHs to earthworms was related to the rapidly released fraction of PAHs determined by mild supercritical CO2 extraction (FSFE) and aqueous desorption using XAD-2 resin (FWATER). The toxicity of PAHs to aquatic amphipods was also related to rapidly released concentration of PAHs determined by mild supercritical CO2 extraction and the concentration of dissolved PAHs measured in sediment pore water. Predictions of the bioaccumulation of individual PAHs by earthworms were dramatically improved by modification of the simple equilibrium-partitioning model to include measures of the rapidly released fraction (FSFE or FWATER), organic matter quality (anthropogenic vs. natural C), and physical/chemical properties of each PAH. These models predicted the concentration of individual 2- to 6-ring PAHs in earthworms to within an order-of-magnitude over a 5 order-of-magnitude range in tissue concentration, a 4 order-of-magnitude range in soil PAH concentration, and 2.6 to 90 wt % soil organic C content. These results demonstrate that high concentrations of PAHs present in soils and sediments at MGP sites often have low bioavailability. Measurements of the rapidly released PAH fraction (FSFE and FWATER) concentrations of PAHs using mild supercritical CO2 and aqueous desorption are particularly useful for estimating the bioavailability and toxicity of PAHs to earthworms and amphipods.