LAUSR

To address knowledge gaps and the lack of benchmarks on the toxicity of dilbit oiled sediments, weathered Cold Lake Blend (CLB) and Western Canadian Select (WCS) were assessed in 10-day sediment tests with the amphipods Hyalella azteca and Leptocheirus plumulosus. Lowest observed effect concentrations (LOECs) and 20% effect levels (EC20s) were determined for wet weight sediment concentrations of TPH and total PAHs normalized to 1% organic carbon. LOECs and EC20s for TPH ranged from 216 to 1165 mg/kg sediment in H. azteca, and from 64 to 75 mg/kg sediment in L. plumulosus. Dilbit LOECs and EC20s for total PAHs ranged from 2.9 to 11.8 mg/kg sediment in H. azteca, and from 0.75 to 0.87 mg/kg in L. plumulosus. Comparison of toxicity-based benchmarks derived from the current study to sediment concentrations from past spills indicate that dilbit spills in aquatic habitats may pose substantial risks to freshwater and estuarine benthic organisms.