Comparison of five methods for measuring sediment toxicity of hydrophobic contaminants

Sediment toxicity from hydrophobic organic compounds (HOCs) is complicated by chemical partitioning among multiple phases and sediment-specific bioavailability. In this study, we used three hydrophobic pyrethroid insecticides as test compounds and derived 10-d median lethal concentrations (LC50s) for Chironomus tentans in three different sediments. The LC50s were expressed using HOC concentrations on a bulk sediment basis (C(S)), organic carbon (OC)-normalized sediment basis (C(S-OC)), porewater basis (C(PW)), dissolved organic carbon (DOC)-normalized porewater basis (C(PW-DOC)), and freely dissolved porewater basis (C(free)). The bulk phase C(S) and C(PW) yielded highly variable LC50s across sediment types, whereas the use of normalized concentrations C(S-OC) and C(PW-DOC) generally reduced variability due to sediment type but not that due to aging. In contrast, LC50s based on C(free) were essentially independent of sediment conditions. The sediment pore water samples contained approximately 20-90 mg L(-1) DOC, and the C(free) expressed as a percentage of the total bulk pore water concentration ranged from 9 to 28% for fenpropathrin (mean = 19%), 8 to 18% for bifenthrin (mean = 13%), and 3 to 8% for cyfluthrin (mean = 6%) across the different sediments. These results indicate thatthe use of C(free) reduces uncertainties caused by sediment variables such as OC properties and aging effects.