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.

实验样地设置于澳大利亚格拉德斯通港务局批准填海区域内的三处成熟红海榄（Rhizophora stylosa）林分中，样地编号为FLNS、FLSN、FLIS。三处样地经目视匹配，在潮汐高程、红树林组成与健康状况、沉积物组成以及底栖无脊椎动物种群等方面均保持一致。每个样地内设置3块面积为6㎡的重复样地。在每块样地周边开挖宽约0.5m的步道，切断支柱根以安装塑料挡墙；挡墙埋入泥层20cm深度，支撑高度达1m，并加装闸门，在留存油污的同时允许潮汐水自由进出。

每块样地接受以下三种处理之一：无油污（对照组）、纯油污以及经分散剂处理的油污。另于填海区域外选取三处未受干扰的对照样地（FLNN、FLSS、CISS），仅通过树木间拉伸的标记带划定样地边界。每块样地上方的树冠层均悬挂枯落物收集器。

在施加油污处理前，需开展初始生物学测量，并采集沉积物样本以分析粒度组成、总有机碳含量以及本底烃类含量。本实验所用油污为产自澳大利亚南部巴斯海峡盆地的中质原油，分散剂为Corexit 9527。在施加至样地前，原油以及原油-分散剂混合体系均经过预风化处理，以模拟来自外海的溢油场景。按照行业推荐使用比例，将分散剂与原油按体积比1:20混合。处理液在涨潮时施加至对应样地，施加速率为5L/㎡，并在退潮过程中使用泵体将油污均匀分布于样地内。

处理后40小时，通过采集表层沉积物（0-2cm）样本计算每块样地的平均施药量。每块样地采集4份重复样本，每份样本至少由5份混合的表层沉积物样本组成，用于后续分析。

分别在处理后1、7、13和22个月，从每块样地采集4份直径10cm的沉积物岩心，将岩心分层后，将每份重复样本的0-2cm、10-12cm以及20-22cm层位的沉积物混合均匀。在样本冷冻保存以待后续分析前，需对其开展观测，包括根系、动物、洞穴以及油污的存在情况。

针对沉积物样本开展以下分析：重量法测定总可提取有机质；总油量通过紫外荧光分析法（UV Fluorescence analysis，缩写UVF）测定；总烃类含量通过气相色谱-火焰离子化检测法（GC-FID）测定；未分辨烃类占比；单个类异戊二烯与正构烷烃的浓度（单位：µg/g 干重）；通过选择离子监测-气相色谱/质谱联用法（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共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；通过SIM GC/MS法（m/z 191）测定藿烷系列中三萜烷生物标志物的总含量；通过SIM GC/MS法（m/z 217与m/z 177）测定选定的甾烷与潜在脱甲基藿烷生物标志物；通过特定生物标志物比值表征生物降解程度。

本研究是"Fate and effects of oil and dispersed oil on mangrove ecosystems in Australia"（即《澳大利亚红树林生态系统中石油与分散油污的归趋与影响》）项目的组成部分，旨在评估溢油的即时与短期生态影响。烃类分析的目标为明确溢油处置中使用分散剂是否会影响：1. 沉积物吸附油污的初始浓度；2. 油污在沉积物中的渗透深度；3. 油污随时间的残留持久性；4. 油污的生物降解速率；5. 油污内部三萜烷与甾烷生物标志物的分布模式。

本实验设计模拟了一场从海面逼近红树林的灾难性溢油事件。实验样地位于格拉德斯通港扩建项目拟砍伐移除的数百公顷红海榄林范围内。