Polycyclic aromatic hydrocarbons (PAHs) were analyzed for surface sediments and a sediment core from the Yellow River-dominated margin. The concentration of 16 USEPA priority PAHs in surface sediments ranged from 5.6 to 175.4 ng g−1 dry weight sediment (dws) with a mean of 49.1 ng g−1 dws. From 1930 to 2011, the distribution of PAHs (37.2 to 210.6 ng g−1 dws) was consistent with the socioeconomic development of China. The PAHs’ concentration peaked in 1964 and 1986, corresponding to the rapid economic growth in China (1958–1965) and the initiation of the “Reform and Open” policy in 1978, respectively. The applications of molecular diagnostic ratios and principal component analysis suggest that PAHs are predominantly produced by the coal and biomass combustion, whereas the contribution of petroleum combustions slightly increased after the 1970s, synchronous with an increasing usage of oil and gas in China.
Polycyclic aromatic hydrocarbons (PAHs) with two to six rings are a class of organic contaminants mainly derived from incomplete combustion of organic matter, such as coal, fossil fuel, and wood, as well as from forest fires, volcanic activities, and petroleum seeps [
Over the past several decades, the fossil fuel use in China has dramatically increased with the rapid economic development [
Due to the large sediment load of the Yellow River, the Bohai Sea has an exceptionally high sedimentation rate, and its sediment is an excellent recorder of historical PAHs flux. Several studies reported increasing PAH concentrations during the 20th century in the Yellow Sea, the South China Sea, and the East China Sea [
The Bohai Sea, a shallow, semiclosed sea in northeastern China, has a surface area of 77,000 km2 and an average water depth of 18 m [
Map of study area and sampling sites in the Yellow River-dominated continental margin. The arrows show prevailing northwesterly wind in winter and spring. Star denotes the site of sediment core.
The detailed methods about 137Cs and 210Pb dating have been reported [
The sediments were freezing dried at −50°C and grounded into fine powder. After the addition of excess 1 N hydrochloric acid to completely remove inorganic carbon, TOC was measured by an Elementar Vario EL element analyzer. The standard deviation based on the replicate analyses was ±0.02% for TOC.
Freeze-dried sediments (~10 g) were homogenized by a mortar and pestle. After addition of surrogate standards (2-fluorobiphenyl,
PAHs were determined on an Agilent 7890A gas chromatography (GC) coupled to a 5975C mass selective detector (MSD). The separation was achieved on a HP-5 MS capillary column (30 m × 0.25 mm i.d. × 0.25
All samples were analyzed for 16 USEPA priority PAHs, including naphthalene (Nap), acenaphthene (Ace), acenaphthylene (Any), fluorine (Fl), phenanthrene (Phe), anthracene (An), fluoranthene (Fla), pyrene (Pyr), benz[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene(BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcdP), dibenz[a,h]anthracene (dBA), and benzo[g,h,i]perylene (BPe). Additionally, alkyl PAHs such as 1-methyl phenanthrene (1-MP), 2-methyl phenanthrene (2-MP), 3-methyl phenanthrene (3-MP), and 9-methyl phenanthrene (9-MP) were also measured and quantified according to the respective peak areas. The four methyl phenanthrene isomers were summed and reported as total methyl phenanthrenes (MP).
Procedural blanks, spiked blanks, spiked matrixes, and parallel samples were performed to control data quality. As low molecular weight (LMW; 2 + 3 rings) PAHs like Nap and Phe were abundant in the atmosphere, two procedural blanks were added for every ten sample pretreatments. The mean blank value was subtracted from all samples. The detection limits for individual PAH ranged from 0.2 to 2.2 ng g−1 dws (dry weight sediment). The recoveries of PAHs in the matrix spiked samples (
To better assess the spatial differences in the PAHs’ distribution, a principal component analysis (PCA) was performed (SPSS 17.0, Illinois USA). All 39 samples were included in our PCA. However, two PAHs, Nap and An, were excluded in the PCA because Nap is the most volatile PAH with low environmental stability, while the concentration of An is too low to be quantified in several samples. PAHs were processed on basis of their concentrations (ng g−1 dws).
The concentration of 16 USEPA priority PAHs (Σ16 PAHs) ranged from 5.6 to 175.4 ng g−1 dws with a mean of 49.1 ng g−1 dws in our study (Figure
Cross plot of PAHs based on different diagnostic ratios for surface sediments from the Yellow River-dominated continental margin. (a) IcdP/(IcdP + Bpe) versus Fla/(Fla + Pyr); (b) BaA/(BaA + Chr) versus Fla/(Fla + Pyr); (c) MP/P versus Fla/(Fla + Pyr). IcdP: indeno[1,2,3-cd]pyrene; Bpe: benzo[g,h,i]perylene; Fla: fluoranthene; Pyr: pyrene; BaA: benz[a]anthracene; Chr: chrysene; MP: methyl phenanthrenes; P: phenanthrenes. The dash lines represent the thresholds for different sources [
Sedimentary TOC can play an important role in PAH accumulation. This was investigated in the present study. Table
Linear correlations (
PAHs | lower YR | Modern estuary | Old estuary | Coast | Entire areas |
---|---|---|---|---|---|
2 + 3 rings | 0.60 | 0.60 | 0.62 | 0.97 | 0.63 |
4 rings | 0.57 | 0.39 | 0.82 | 0.96 | 0.53 |
5 + 6 rings | 0.59 | 0.55 | 0.75 | 0.95 | 0.54 |
All probability (
In our study, the PAHs with 2-3 rings, 4 rings, and 5-6 rings accounted for 60 ± 11%, 29 ± 7%, and 11 ± 5% of total PAHs, respectively (Supplementary material Table 2). The predominance of low molecular weight (LMW) over high molecular weight (HMW) PAHs may reflect either preferential inputs of LMW PAHs or an important contribution of petrogenic products or biomass and coal burning at low to moderate temperatures [
PAHs with a petrogenic origin are usually abundant in alkyl PAHs relative to their parent compounds, while those with a pyrogenic (combustion) origin contain little or no alkyl PAHs [
Yunker et al. (2002) proposed that the Fla/(Fla + Pyr) ratio of 0.4-0.5 and >0.5 is indicative of liquid fossil fuel combustion and coal/grass/wood combustion, respectively, whereas the BaA/(BaA + Chr) ratio of 0.2, 0.20–0.35, and >0.35 suggests PAHs primarily from petroleum, mixed petroleum/combustion, and coal/wood combustion, respectively. In addition, the IcdP/(IcdP + Bpe) ratio of <0.2, 0.2–0.5, and >0.5 is characteristic of petroleum, petroleum combustion, and coal/wood combustion, respectively [
As semivolatile and relatively refractory compounds, PAHs can be transported from land to sea via atmosphere [
Spatial distributions of PAHs in surface sediments along the transect of lower Yellow River, river mouth, and adjacent sea.
PAHs are easily adsorbed on surface or occluded inside of suspended particles because of their hydrophobic properties. The particle-associated PAHs can be transported to the sea via runoff [
The PCA result shows that the first three principal components (PC) account for 96.5% of total variances (Figure
3D loading plot of principal component analysis (PCA) for PAHs in surface sediments from the Yellow River-dominated margin. PC: principal component.
Figure
Historical distributions in abundance of total 16 PAHs (ng g−1 dws) and percentage of 2 + 3, 4, and 5 + 6 ring PAHs in sediment core from Southern Bohai Sea.
Relationship between sedimentary PAH concentrations (ng g−1 dws) in southern Bohai Sea and energy consumption of China.
From 1930 to 2011, the relative abundance of 2 + 3 ring PAHs generally decreased (84.2% from 1934 to 1949 versus 66% from 1997 to 2011), whereas the relative abundance of 5 + 6 ring PAHs increased from 4.8% to 10.6% (Figure
Historical profiles of diagnostic ratios of PAHs in sediment core from southern Bohai Sea, China. IcdP: indeno[1,2,3-cd]pyrene; Bpe: benzo[g,h,i]perylene; Fla: fluoranthene; Pyr: pyrene; BaA: benz[a]anthracene; Chr: chrysene; MP: methyl phenanthrenes; P: phenanthrenes. The dash lines represent the thresholds for different sources [
We have examined the spatial and temporal distributions of PAHs in surface sediments and a sediment core from the lower Yellow River, estuary, and Laizhou Bay. Based on data about molecular diagnostic ratios, relative number of aromatic rings, and PCA, four conclusions have been drawn.
(1) Compared to the previous studies, the concentration of 16 USEPA priority PAHs in our study is lower, partially attributed to recent implementation of new energy policy in China. (2) The homogenous distribution of PAHs and the positive relationship between TOC and PAHs’ abundance suggest a mixed input by atmospheric and riverine transport. (3) The historical PAHs’ pattern in the Laizhou Bay presents a good correlation with the Chinese socioeconomic conditions during the period of 1930 to 2011, confirming that PAHs are a sensitive tracer for anthropogenic activity. (4) The PAHs in the southern Bohai Sea are primarily derived from the combustion of coal and biomass. The contribution of petroleum combustion is minor, but it increased after 1970s, consistent with the change in the Chinese energy structure.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was financially supported by NSFC (41006042, 41176164), the Ministry of Education of China (SRFDP), and special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control (PKU).