The Gladstone field experiment, Queensland: Weathering of hydrocarbons in mangrove sediment: testing the effects of using dispersants to treat oil spills

Experimental plots were established at three sites within mature stands of Rhizophora stylosa (FLNS, FLSN, FLIS), in an area approved for reclamation by the Gladstone Port Authority. The sites were matched visually for tidal elevation, mangrove tree composition and condition, sediment composition and benthic invertebrate populations. Three replicate 6 m² plots were constructed at each site. Around each plot, prop roots were cut in a path about 0.5 m wide to allow installation of plastic retaining walls. The retaining walls were dug into the mud to a depth of 20 cm and supported to a height of 1 m. A gate was installed to allow tidal waters to move in and out while retaining the oil. One of three treatments was applied to each plot: no oil (control); oil; and dispersed oil. Three undisturbed control sites (FLNN, FLSS, CISS), were chosen outside the reclamation area, and plots were delineated by marking tape stretched between the trees only. A litter trap was also suspended in the trees above each plot. Before treatment with oil, initial biological measurements were taken and each plot was sampled for sediment grain size, total organic carbon and background hydrocarbon content. The oil used in this experiment was a medium range crude oil from the Bass Strait Basin in southern Australia and the dispersant used was Corexit 9527. Prior to application to the plots, the oil and the oil plus dispersant mixture was preweathered to simulate an oil spill arriving from seaward. Dispersant was added to the oil to achieve a 1/20 dispersant to oil (v/v) ratio in accordance with industry recommended usage. Treatments were added to the appropriate plots at high tide at a rate of 5 l/m² and pumps were used to distribute the oil evenly throughout each plot as the tide fell.The average rate of dosing in each plot was calculated from surface sediment (0-2 cm) samples collected 40 hours after treatment. Four replicate samples, each comprised of at least 5 pooled surface samples were collected from each plot for analysis. At 1, 7, 13 and 22 months, 4 replicate 10 cm diameter cores were collected from each plot, sliced and the 0-2, 10-12 and 20-22 cm sections from each replicate were pooled. Observations, including the presence of roots, animals, burrows and oil were made before the samples were frozen for later analysis.The following analyses were conducted on the sediment samples: Total Extractable Organic Matter determined gravimetrically; Total oil determined by UV Fluorescence analysis (UVF); Total hydrocarbons determined by gas chromatography with flame ionisation detection (GC-FID); The percentage of unresolved hydrocarbons; Concentrations of individual isoprenoid and n-alkanes (µg/g dry weight); Sum of 218 aromatic and alkylaromatic isomers in the naphthalene/biphenyl, fluorene, phenanthrene/anthracene, benzanthracene/chrysene, fluoranthene/pyrene and benzopyrene to benzoperylene series as determined by Selected Ion Monitoring-Gas Chromatography/Mass Spectroscopy (SIM GC/MS); Ratios of specific alkyl phenanthrene isomers to illustrate selective biodegradation; Sum of triterpane biomarkers in the hopane series as determined by SIM GC/MS using m/z 191; Selected sterane and potential demethylated hopane biomarkers as determined by SIM-GC/MS using m/z 217 and m/z 177; Ratios of specific biomarkers useful as biodegradation indices. This research was a component of the project "Fate and effects of oil and dispersed oil on mangrove ecosystems in Australia" and was undertaken to provide an assessment of immediate and short term impacts of oil spills.The objectives of the hydrocarbon analyses were to determine whether dispersant usage to control an oil spill affects:1. the initial concentration of oil absorbed by sediments2. the depth of penetration of oil in the sediments3. the persistence of oil over time4. its rate of biodegradation5. the pattern of its internal triterpane and sterane biomarkers The study was designed to mimic a catastrophic oil spill approaching the mangroves from the sea. The study site was within several hundred hectares of Rhizophora stylosa forest designated for destruction in an extension of the Port of Gladstone.

本研究于格拉德斯通港务局批准的围垦区域内的成熟柱果木榄（Rhizophora stylosa）林分中设置3处试验位点（编号FLNS、FLSN、FLIS）。所有试验位点在潮位、红树植物组成与生长状况、沉积物组成以及底栖无脊椎动物种群结构上均经目视匹配一致。每个位点设置3块重复样地，单块样地面积为6平方米。在每块样地周边切割支柱根，形成宽约0.5米的通路，以便安装塑料挡墙；挡墙嵌入泥层深度达20厘米，支撑高度为1米，并加装闸门，可在保留溢油的同时允许潮汐水体自由进出。每块样地接受三类处理中的一种：空白对照（无油添加）、纯原油组、分散乳化原油组。
另在围垦区域外选取3处未受干扰的对照位点（编号FLNN、FLSS、CISS），仅通过在红树间拉伸标记带划定样地范围；每块样地上方的红树枝干处均悬挂凋落物收集器。溢油处理前，先开展初始生物学测定，并采集每块样地的沉积物样本，用于分析沉积物粒度、总有机碳含量以及背景烃类含量。
本试验所用原油为澳大利亚南部巴斯海峡盆地（Bass Strait Basin）产出的中质原油，所用分散剂为科立释9527（Corexit 9527）。在向样地施加处理前，先对原油以及原油-分散剂混合液进行预风化处理，以模拟从海向涌至红树林的溢油场景。按照行业推荐使用规范，将分散剂与原油按体积比1:20（分散剂:原油）混合。在涨潮时向对应样地施加处理药剂，施加速率为5升/平方米，待退潮时使用泵将原油均匀分散至样地各处。
处理后40小时采集表层沉积物（0-2厘米）样本，以此计算每块样地的平均施药速率。每块样地采集4份重复样本，每份样本由至少5份混合的表层沉积物样本组成，用于后续分析。分别于处理后1、7、13和22个月，从每块样地采集4根直径为10厘米的沉积物柱状样，将其分层切割后，每份重复样本的0-2厘米、10-12厘米以及20-22厘米层段混合为一个分析样本。样本冷冻保存以待后续分析前，先完成相关观测，包括根系、动物、洞穴以及溢油的存在情况记录。
针对沉积物样本开展以下分析：采用重量法测定总可提取有机质；采用紫外荧光分析法（UV Fluorescence analysis, UVF）测定总油量；采用气相色谱-火焰离子化检测法（gas chromatography with flame ionisation detection, GC-FID）测定总烃含量；计算未分辨烃类占比；测定单体类异戊二烯烃与正构烷烃的浓度（单位：微克/克干重）；采用选择离子监测-气相色谱/质谱联用法（Selected Ion Monitoring-Gas Chromatography/Mass Spectroscopy, SIM GC/MS）测定萘/联苯、芴、菲/蒽、苯并蒽/䓛、荧蒽/芘以及苯并芘至苯并苝系列中218种芳香族与烷基芳香族异构体的总含量；通过特定烷基菲异构体比值表征选择性生物降解过程；采用选择离子监测-气相色谱/质谱联用法（SIM GC/MS，监测m/z 191）测定藿烷系列三萜类生物标志物的总含量；采用选择离子监测-气相色谱/质谱联用法（SIM GC/MS，监测m/z 217与m/z 177）测定目标甾烷与潜在脱甲基藿烷类生物标志物；通过特定生物标志物比值表征生物降解程度。
本研究为"Fate and effects of oil and dispersed oil on mangrove ecosystems in Australia"（澳大利亚红树生态系统中原油与分散乳化原油的归趋与效应）项目的组成部分，旨在评估溢油的短期与即时生态影响。本次烃类分析的目标为明确溢油防控中使用分散剂是否会影响以下指标：1. 沉积物吸附原油的初始浓度；2. 原油在沉积物中的渗透深度；3. 原油随时间的残留持久性；4. 原油的生物降解速率；5. 原油内部三萜类与甾烷类生物标志物的分布模式。
本试验模拟了从海向侵袭红树林的灾难性溢油场景，试验位点位于格拉德斯通港扩建工程中拟清毁的数百公顷柱果木榄（Rhizophora stylosa）林分内。