Are oil spills an important source of heavy metal contamination in the Bohai Sea, China?

Accurate evaluation of the heavy metal contamination caused by oil spills is more urgent than that of organic pollution since heavy metals are toxic and persistent. This study applies bulk geochemical parameters and hydrocarbon compositions to explore heavy metal contamination caused by oil spills. The contents of six heavy metals (Cu, Pb, Zn, Cr, Ni, and Co), total organic carbon, aliphatic hydrocarbons, and grain size were measured in 50 subsurface sediment samples obtained from an area affected by oil spills in the Bohai Sea. The values of the contamination factor (CF), the geo-accumulation index (Igeo), and the enrichment factor (EF) indicate moderate contamination with the six investigated heavy metals at most of the sampling sites. Riverine inputs, oil spills, and airborne dust are the three main sources of the heavy metals, and oil spills are the significant source of heavy metal contamination in the areas. The distributions of the mean grain size (Mz) and the contents of unresolved complex mixture (UCM) and Cr suggest that crude oil migrates from deep hydrocarbon reservoirs through geological faults and seeps through coarse-grained sediments until it reaches fine-grained sediments. The spilled oil contributes appreciable amounts of heavy metals to the sediments at the oil-contaminated sites. At these sites, the heavy metals from oil spills overprinted on those contributed by terrigenous materials, which are the predominant source of metals in the subsurface sediments in the areas affected by oil spills.