Weathering of hydrocarbons in mangrove sediments: testing the eects of using dispersants to treat oil spills Kathryn A. Burns a, *, Susan Codi a , Catherine Pratt a , Norman C. Duke b a Australian Institute of Marine Science, Box 3, Townsville, Qld, 4810, Australia b Botany Department, University of Queensland, St Lucia, Qld, 4072, Australia Abstract This ®eld study was a combined chemical and biological investigation of the relative eects of using dispersants to treat oil spills impacting mangrove habitats. The aim of the chemistry was to determine whether dispersant aected the short- or long-term composition of a medium range crude oil (Gippsland) stranded in a tropical mangrove environment in Queensland, Australia. Sediment cores from three replicate plots of each treatment (oil only and oil plus dispersant) were analyzed for total hydrocarbons and for individual molecular markers (alkanes, aromatics, triterpanes, and steranes). Sediments were collected at 2 days, then 1, 7, 13 and 22 months post-spill. Over this time, oil in the six treated plots decreased exponentially from 36.6 216.5 to 1.2 20.8 mg/g dry wt. There was no statistical dierence in initial oil concentrations, penetration of oil to depth, or in the rates of oil dissipation between oiled or dispersed oil plots. At 13 months, alkanes were >50% degraded, aromatics were 0 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 processes were evaporation (R27%) and dissolution ( r 56%), with a lag-phase of 1 month before the start of signi®cant microbial degradation (R17%). The most resistant fraction of the oil that remained after 7 months (the higher molecular weight hydrocarbons) correlated with the initial total organic carbon content of the soil. Removal rate in the Queensland mangroves was signi®cantly faster than that observed in the Caribbean and was related to tidal ¯ushing. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Oil spills; Mangroves; Weathering; Dispersants; Biomarker patterns; Degradation 1. Introduction Oil spills are known to cause severe and long-term damage to mangrove ecosystems (e.g. Wardrup, 1987; Burns et al., 1993; Duke et al., 1997). Mangroves are important in coastal estuaries and bays on all sides of the Australian continent. Because shipping terminals, industries and municipalities are also concentrated in the estuaries, these important nursery habitats for many commercially important species of ®sh and prawns are particularly vulnerable to oil spills (Volkman et al., 1994). When mangrove trees die, the very structure and cohesion of the mangrove habitat becomes unstable (Nadeau and Berquist, 1977; Duke and Pinzon, 1993; Garrity et al., 1994). Therefore, the methods used to reduce the impact of oil spills on Organic Geochemistry 30 (1999) 1273±1286 0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(99)00101-1 * Corresponding author. Tel.: +61-747-534211; fax: +61- 747-725852. E-mail address: kburns@aims.gov.au (K.A. Burns)