Fate of total petroleum hydrocarbon from Iranian Heavy crude oil in the water column using an enclosed ecosystem

Abstract

The contamination of a marine environment with crude oil and/or its products through accidental oil spill from tankers or dumping causes serious problems to organisms and natural environment. When oil is spilled on the sea, some fraction spreads over the surface of the water to form a thin film, an oil slick. Other fractions disperse or dissolve into seawater and form water-soluble emulsions, although hydrocarbons have low solubility in seawater. We studied the environmental fate of a simulated oil spill and the water-soluble fraction of Iranian Heavy crude oil using experimental 980 liter in situ mesocosm bags (0.5 m in diameter and 5 m in depth) that mimic natural ecosystems. The replicated experimental treatments consist of unoiled control (×3), oil only (×3), and pre-mixed oil+dispersant (×3). The mesocosm experiment was conducted for up to 116 days (Apr. 17 to Aug. 10, 2009). The fate of artificially spilled crude oil in the water column and the effect of dispersant application were monitored by measuring total petroleum hydrocarbons (TPH) and their compositional changes with time. A much faster increase of oil concentration (TPH) in the water column was observed in the dispersant-treated oiled mesocosm (313 ppb/day) than in the untreated oiled one (3.47 ppb/day) by eight days after oil addition. Oil concentrations in the water column remained relatively stable thereafter, and the TPH levels in the ‘oil+dispersant’ mesocosms were about two orders of magnitude higher than those in the ‘oil only’ mesocosm. TPH levels were relatively high according to water depth in order of surface (0.5 m) > middle (2.5 m) > bottom (4.5 m) within several folds. The difference decreased according to time. Depuration rates of TPH were also six times higher in the ‘oil+dispersant’ mesocosm compared to those in the ‘oil only’ group. Quantitative and compositional changes of oil components during the simulated weathering of crude oil were also recognized and compared