Weathering of hydrocarbons in mangrove sediments: testing the effects of using dispersants to treat oil spills

This field study was a combined chemical and biological investigation of th e relative effects of using dispersants to treat oil spills impacting mangr ove habitats. The aim of the chemistry was to determine whether dispersant affected the short- or long-term composition of a medium range crude oil (G ippsland) stranded in a tropical mangrove environment in Queensland, Austra lia. Sediment cores from three replicate plots of each treatment (oil only and oil plus dispersant) were analyzed for total hydrocarbons and for indiv idual molecular markers (alkanes, aromatics, triterpanes, and steranes). Se diments were collected at 2 days, then 1, 7, 13 and 22 months post-spill. O ver this time, oil in the six treated plots decreased exponentially from 36 .6 +/- 16.5 to 1.2 +/- 0.8 mg/g dry wt. There was no statistical difference in initial oil concentrations, penetration of oil to depth, or in the rate s of oil dissipation between oiled or dispersed oil plots. At 13 months, al kanes were >50% degraded, aromatics were similar to 30% degraded based upon ratios of labile to resistant markers. However, there was no change in the triterpane or sterane biomarker signatures of the retained oil. This is of general forensic interest for pollution events. The predominant removal pr ocesses were evaporation (less than or equal to 27%) and dissolution (great er than or equal to 56%), with a lag-phase of 1 month before the start of s ignificant microbial degradation (less than or equal to 7%). The most resis tant fraction of the oil that remained after 7 months (the higher molecular weight hydrocarbons) correlated with the initial total organic carbon cont ent of the soil. Removal rate in the Queensland mangroves was significantly faster than that observed in the Caribbean and was related to tidal flushi ng. (C) 1999 Elsevier Science Ltd. All rights reserved.