Week-long mesocosm spill tank experiments reveal distinct behaviour, fate, and ecological risks of bio-derived oils

The increasing marine transport of Hazardous and Noxious Substances (HNS), including bio-derived oils, raises concerns regarding their behaviour, fate, and ecological impacts following accidental releases. Bio-derived oils vary widely in composition depending on feedstocks and production processes, leading to uncertainty in associated spill behaviour and potential environmental risk. Improved weathering studies are therefore needed to support marine spill preparedness and response. Week-long mesocosm tank experiments were conducted to examine the fate and biological effects of three representative bio-derived oils (canola oil, biodiesel, and wood oil) in simulated nearshore marine waters. In each test, 1.0 L of oil was released into 1200 L of artificial seawater (15 °C) containing 1.2 kg of pre-mixed sediment and subjected to continuous high-energy wave mixing for seven days. Volatile organic carbon concentrations in the headspace above the spill were negligible for all oils. Water and sediment samples were collected at intervals for chemical analysis and ecotoxicity testing. Wood oil resulted in the highest dissolved and total organic carbon concentrations (up to 280 mg/L), caused sustained seawater acidification (pH decreased from 8.3 to 7.8-8.0), completely disappeared from the water surface, and accumulated extensively in sediments. Biodiesel and canola oil generated lower dissolved concentrations (less than 21 mg/L), with dispersed oil droplets persisting in the water column. The seawater pH remained stable at 8.3 throughout the week-long tests for both biodiesel and canola oil. Approximately 50% of the canola oil remained recoverable from the water surface after one week of wave action, whereas no recoverable biodiesel or wood oil residues were observed on the water surface. Ecotoxicological assays demonstrated sustained toxic effects in waters contaminated with biodiesel and wood oil. These results highlight the diverse weathering behaviours and ecological risks of bio-derived oils, emphasizing the need for oil-specific approaches to marine HNS spill risk assessment and response.