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Long-term toxicity to aquatic invertebrates

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Key value for chemical safety assessment

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In a key, semi-static 21 day Daphnia magna chronic toxicity test (OECD 211, KS=1), one daphnid/replicate was exposed to the Water Accommodated Fraction (WAF) of light catalytic cracked gas oil (MRD-10-576) at nominal concentrations of 0.05, 0.10, 0.18, 0.34 and 0.65 mg/L. The 21 day NOELR was 0.05 mg/L based on growth and reproduction and 0.18 mg/L based on mortality (ExxonMobil 2012d).

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater invertebrate NOEL (No Observed Effect Level) value is 0.053 mg/l based on immobility and numbers of live young produced per adult by Day 21.