FIELD
EXPERIMENT
HYDROCARBON SAMPLING |
The chief aim of the chemistry work was to determine whether dispersants
affected the rate of penetration, dissipation or long term retention of the
medium range crude oil (Gippsland) stranded in a mangrove environment.
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In the
field study, we measured and evaluated the composition and concentration of
hydrocarbons in sediments to 22 cm depth. Sediment cores from three replicate
plots of each treatment (oil only and oil plus dispersant) were analysed for
total hydrocarbons and for individual molecular markers (alkanes, aromatics,
triterpanes, and steranes).
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There was no statistical difference in initial oil concentrations, penetration
of oil to depth, or in the rates of oil dissipation between oiled or dispersed
oil plots |
Sediments were collected at 2 days, then 1, 7, 13
and 22 months post-spill. Over this time, total oil in the six treated plots
decreased exponentially from 36.6 ±16.5 to 1.2 ±0.8 mg.g-1 dry wt.
There was no statistical difference in initial oil concentrations, penetration
of oil to depth, or in the rates of oil dissipation between oiled or dispersed
oil plots. At thirteen months, alkanes were >50% degraded, aromatics were
-30% degraded based upon ratios of labile to resistant markers.
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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 (< 27%) and dissolution (> 56%), with a
lag-phase of 1 month before the start of significant microbial degradation (<
17%). The most resistant fraction of oil that remained after 7 months (the
higher molecular weight hydrocarbons) correlated with the initial total organic
carbon content of the sediment.
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Removal rates in Queensland mangroves were
significantly faster than those observed in the Caribbean |
Removal rates in Queensland mangroves were
significantly faster than those observed in the Caribbean, and this appears to
be related to differences in tidal flushing. Our chemistry data provided the
context for interpretation of concurrent biological observations.
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Plate 7a, b, c, d, e. During field trials, sediments were sampled for the
presence and quantity of hydrocarbons. Monitoring continued for around two
years. |
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A. Oil initially deposited on the surface, forming waxy layers in
pooled areas and filling crab burrow entrances.
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B. After two weeks,
enclosures were removed since the oil had adhered to sediments and exposed
roots, and was no longer mobile with subsequent flooding tides.
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C.
Sediment cores to 25 cm were taken within oiled plots.
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D. Surface samples
were collected from control plots. All samples were placed immediately into
specially prepared glass sample jars and placed on ice and returned to the
laboratory where they were frozen until analysed.
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E. Sediment cores were
sectioned and samples from 0-2, 10-12 and 20-22 cm were collected for analysis.
Cores were also described, noting holes, the presence of oil, and sediment
character, among other features.
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