Mesocosm Study on Weathering Characteristics of Iranian Heavy Crude Oil with and without Dispersants

Abstract

The environmental fate of Iranian Heavy [ME1] crude oil (IHC) with and without an added oil spill dispersant (OSD) has been studied using a 1,000 kL capacity in situ mesocosm. Physical weathering and chemical composition changes of the oil were monitored for 77 days. The OSD significantly changed the chemical fingerprint of the oil. Compound-specific effects of the OSD could be observed as changes over time in the content of the TPH, unresolved complex mixture (UCM), alkanes, PAHs, hopanes and steranes in the oil. When oil weathers, most hydrocarbons show a rapidly decreasing phase followed by a slowdown and stabilization. Recalcitrant biomarkers,however, show an opposite trend. An additional ca. 10% increase in hydrocarbon formation in the form of UCM occurred after OSD treatment. This change, in effect, increased the half-life of the TPH in the oil with the dispersant (OD), while the enhanced solubility of the low molecular weight PAHs by the OSD decreased the half-life of the alkylated PAHs in the OD. After 77 days of exposure at the sea surface, both the oils with and without the OSD exhibited moderate weathering. Most of the source diagnostic indices maintained their source information, and the weathering indices indicated that evaporation, dissolution, and dispersion were the major weathering processes. The mass balance of the weathered oil was calculated using laboratory and mesocosmoil were monitored for 77 days. The OSD significantly changed the chemical fingerprint of the oil. Compound-specific effects of the OSD could be observed as changes over time in the content of the TPH, unresolved complex mixture (UCM), alkanes, PAHs, hopanes and steranes in the oil. When oil weathers, most hydrocarbons show a rapidly decreasing phase followed by a slowdown and stabilization. Recalcitrant biomarkers, however, show an opposite trend. An additional ca. 10% increase in hydrocarbon formation in the form of UCM occurred after OSD treatment. This change, in effect, i