What is the best chemical approach to estimate the bioavailability of pyrethroid insecticides to benthic invertebrates?

The traditional approach for predicting the risk of hydrophobic organic contaminants in sediment is to relate organic carbon normalized sediment concentrations to body residues or toxic effects in organisms. This method is limited, however due to the plethora of variables that can influence bioavailability. Therefore, a matrix independent method of predicting bioavailability needs to be developed in order to be universally applicable. Solid phase microextraction (SPME) and Tenax are two commonly used bioavailability-based methods. While both SPME fiber and Tenax extractable concentrations can be correlated to tissue residues of aquatic species, the majority of this research (with a few exceptions) focuses on compounds that are not acutely toxic or biotransformed. Less is known about the potential applicability of these methods to predict bioaccumulation, and ultimately toxicity, for highly toxic, rapidly biotransformed compounds, such as pyrethroid insecticides. This class of compounds is of particular concern due to frequent environmental detection in sediments at concentrations lethal to benthic species. This research has four specific goals: Determining exposure conditions that may change the concentration on the SPME fibers at equilibrium (Chapter 2); Comparing the ability of SPME fibers and Tenax to predict the bioavailability of two pyrethroids (permethrin and bifenthrin) (Chapter 3); Developing bioavailability-based toxicity endpoints for bifenthrin using two aquatic species (Chapter 4); and, Validating these techniques using sediments from known contaminated field sites (Chapter 5). Overall this research was focused on comparing and contrasting the ability and applicability of SPME fibers and Tenax to adequately predict the exposure of pyrethroids under varying conditions. While comparing these two methods, they were optimized to better provide accurate predictions of bioavailability and toxicity for pyrethroids from sediments. Regardless of the fiber or animal density examined, the SPME fibers exposure did not significantly affect fiber concentrations for permethrin or DDE. Additionally, bioaccumulation of parent permethrin and bifenthrin was predicted using both SPME fibers and Tenax using 6 or 24 h extraction times. Further, a single regression model predicted bioaccumulation across compounds and species using Tenax extractable concentrations. Once demonstrated that these techniques could predict bioaccumulation, median lethal and effect levels were examined for bifenthrin and as expected the bioavailability-based endpoints were more uniform across sediments than use of whole sediment concentrations. Additionally, the relationships among the two methods were compared across multiple sediments. Despite the SPME fiber's ability to determine toxicity in laboratory sediments, the field validation study determined that lethal levels were often too low to detect on the SPME fibers using current methodologies, but Tenax extractable concentrations correlated to toxicity. Overall, while both methods could predict bioavailability, the limitations of SPME fibers including lower sensitivity, inability to function across compounds, and long equilibration time, made Tenax extraction a preferable method.