Presence of aliphatic and polycyclic aromatic hydrocarbons in near-surface sediments of an oil spill area in Bohai Sea
Introduction
Oil spills usually occur in the oceans during oil exploitation, storage, transportation, and maritime accidents (Gong et al., 2014, Mulabagal et al., 2013, Palinkas et al., 1993). It is a major environmental and financial threat to local communities, particularly if large volumes of unrefined hydrocarbons or crude oil are spilled into the sea (Graham et al., 2010, McNutt et al., 2012). In the last 10 years, the number of oil spills has increased with the rapid increase of offshore oil exploitation in China. Although the self-purifying capacity of the marine environment may dilute the spilled oil over time, seafloor sediments can accurately record massive oil spills from oil tankers and drilling platforms (Wei et al., 2015, Xu et al., 2013). Aliphatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) present in the sediment cores have been used for the identification of the source of organic matter (OM) and reconstruction of the historical record of these hydrocarbon inputs (Lima et al., 2012, Yunker et al., 2011, Zhang et al., 2007, Zhang et al., 2013a).
The shallow Bohai Sea, located in North China, is surrounded by Liaodong and Shandong peninsulas, and is connected to the northern Yellow Sea by the Bohai Strait (Fig. 1). It occupies a marine area of 77,000 km2 with an average water depth of 18 m. Several large rivers, including the Yellow River, flow into Bohai, with a total yearly runoff of 890 × 108 m3 (Sündermann and Feng, 2004). It is the second largest crude oil production base in China with more than 50 offshore platforms, which discharged approximately 9.9 million tons of petroleum-contaminated water in 2006, causing 30% of its water to fail quality standards (State Oceanic Administration, 2006). In June 2011, the most serious oil spill to date occurred on an offshore oil field, named Penglai 19-3 operated by ConocoPhillips China Inc., in Bohai Sea. The accident was caused by a geological fault that opened a near-surface reservoir during production, because of pressure from water injection. Approximately 723 barrels of oil and 2620 barrels of mineral oil-based drilling mud seeped into the sea, which led to the pollution of 840 km2 area of its surface (Liu et al., 2015).
Satellite remote sensing was used to model the oil spill trajectory and to assess the damage from Penglai 19-3 (Wei et al., 2015, Xu et al., 2013). However, data on the sedimentary OM involved in this accident were not obtained earlier. Core samples were collected at 0.5 m below the seafloor (hereinafter called the near-surface sediments) in the oil spill area, which is the zone between Penglai 19-3 and other oil and gas fields. Because the sedimentation rate around 0.29 cm/a dated from 210Pb activity in the 30-cm core sample (Hu et al., 2011), these samples were expected to contain seepage hydrocarbon as well as terrigenous and marine matter with little or no anthropogenic pollution. The grain size, total organic carbon (TOC), AHs, and PAHs were quantified to determine the sources of OM in the sediment and to identify the fingerprint of the oil components. Moreover, characteristics of AHs and PAHs may provide a useful tool for detecting oil spills before massive hydrocarbon seepage.
Section snippets
Sample collection
The sampling sites (BS1–BS50) are shown in Fig. 1. In August 2012, 50 sediment cores were collected using a vibro piston corer deployed from R/V Yezhizheng of the Qingdao Institute of Marine Geology, China. This corer can obtain a 2.5- to 3.0-m core sample, via vibration, without any significant disturbance to the substrate. The sediment between 0.50 and 0.55 m of these core samples was used for analysis. All the samples were wrapped in aluminum foils and stored at − 20 °C until analysis.
Analysis of grain size and TOC of sediments
The grain
The grain size and TOC of near-surface sediment
The ranges of TOC and grain size parameters for near-surface sediments of the oil spill area are listed in Table 1. The median grain size (MZ) of near-surface sediments is in the range of 3.74–7.48 Φ, averaging 6.97 Φ. Silt and clay are found to be the predominant constituents of the near-surface sediment samples, with mean compositions of 60.86% and 28.9%, respectively. These fine-grained sediments in the oil spill area are usually observed in the central Bohai, where weaker tidal environments
Conclusions
The grain size, TOC, AHs, and PAHs of near-surface sediments of the oil spill area were analyzed to determine the source of OM. Although the concentrations of AHs were not significantly higher than those of core sediment from a non-oil spill area, the levels of short-chain n-alkanes, UCMs, and EPA 16 priority pollutant PAHs were higher than the non-oil spilled surface sediment developed in a similar semi-closed marginal sea shelf. Sediments of the oil spill area have mixed biogenic and
Acknowledgments
This study was financially supported by the Special Program of Ministry of Land and Resources of the People's Republic of China (Grant No. 201211060), National Natural Science Foundation of China (Grant No. 41503048), and the Key Laboratory Project of Gansu Province (Grant No. 1309RTSA041). The authors extend their special thanks to Chief Editor Dr Charles Sheppard for thorough and constructive reviews that greatly improved the clarity and quality of this article.
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