Application of a Sigma polycyclic aromatic hydrocarbon model and a logistic regression model to sediment toxicity data based on a species-specific, water-only LC50 toxic unit for Hyalella azteca

Two models, a Sigma polycyclic aromatic hydrocarbon (FAH) model based on eq uilibrium partitioning theory and a logistic-regression model, were develop ed and evaluated to predict sediment-associated PAH toxicity to Hyalella az teca. A Sigma PAH model was applied to freshwater sediments. This study is the first attempt to use a Sigma PAH model based on water-only, median leth al concentration (LC50) toxic unit (TU) values for sediment-associated PAH mixtures and its application to freshwater sediments. To predict the toxici ty (i.e., mortality) from contaminated sediments to H. azteca, an interstit ial water TU, calculated as the ambient interstitial water concentration di vided by the water-only LC50 in which the interstitial water concentrations were predicted by equilibrium partitioning theory, was used. Assuming addi tive toxicity for PAH, the sum of TUs was calculated to predict the total t oxicity of PAH mixtures in sediments. The Sigma PAH model was developed fro m 10- and 14-d H. azteca water-only LC50 values. To obtain estimates of LC5 0 values for a wide range of PAHs, a quantitative structure-activity relati onship (QSAR) model (log LC50 - log K-ow) with a constant slope was derived using the time-variable LC50 values for four PAH congeners. The logistic-r egression model was derived to assess the concentration-response relationsh ip for field sediments, which showed that 1.3 (0.6-3.9) TU were required fo r a 50% probability that a sediment was toxic. The logistic-regression mode l reflects both the effects of co-occurring contaminants (i.e., nonmeasured PAH and unknown pollutants) and the overestimation of exposure to sediment -associated PAH. An apparent site-specific bioavailability limitation of se diment-associated PAH was found for a site contaminated by creosote. At thi s site, no toxic samples were less than 3.9 TU. Finally, the predictability of the Sigma PAH model can be affected by species-specific responses (Hyal ella vs Rhepoxynius); chemical specific (PAH vs DDT in H. azteca) biases, w hich are not incorporated in the equilibrium partitioning model; and the un certainty from site-specific effects (creosote vs other sources of PAH cont amination) on the bioavailability of sediment-associated PAH mixtures.