Toxicity and risk management of oil-spiked sediments by diluted bitumen for two freshwater benthic invertebrates.

Diluted bitumen (dilbit) is an unconventional oil produced by the oil sands industry in Canada. Despite the knowledge available on hydrocarbon toxicity, the effects of diluted bitumen on benthic organisms are still largely unknown. Moreover, in Quebec there are only provisional threshold values of 164 mg/kg C-C for chronic effects and 832 mg/kg for acute effects. The protectiveness of these values for benthic invertebrates has not been tested for heavy unconventional oils such as dilbit. Two benthic organisms, the larvae of Chironomus riparius and Hyalella azteca, were exposed to these two concentrations and to an intermediate concentration (416 mg/kg) of two dilbits (DB1 and DB2) and a heavy conventional oil (CO). The aim of the study was to assess the sublethal and lethal effects of spiked sediment by dilbit. The oil was rapidly degraded in the sediment, especially in the presence of C. riparius. Amphipods were much more sensitive to oil than chironomids. LC values for H. azteca were 199 mg/kg C-C for DB1, 299 mg/kg for DB2 and 8.42 mg/kg for CO compared to LC values for C. riparius of 492 mg/kg for DB1, 563 mg/kg for DB2 and 514 mg/kg for CO. The size of the organisms was reduced compared to controls for both species. The defense enzymes (GST, GPx, SOD and CAT) were not good biomarkers in these two organisms for this type of contamination. The current provisional sediment quality criteria seem too permissive for heavy oils and should be lowered.