Vertical distribution, environmental occurrence, and risk assessment of organic pollutants in lacustrine sediments in southeast China

To clarify the impact of human activities on the natural environment, as well as the current ecological risks to the environment surrounding Zhushan Bay in Taihu Lake, the characteristics of deposited organic materials, including elements and 16 polycyclic aromatic hydrocarbons (∑16PAHs), in a sediment core from Taihu Lake were determined. The nitrogen (N), carbon (C), hydrogen (H), and sulfur (S) contents ranged from 0.08 to 0.3%, 0.83 to 3.6%, 0.63 to 1.12%, and 0.02 to 0.24%, respectively. The most abundant element in the core was C followed by H, S, and N, while elemental C and the C/H ratio displayed a decreasing trend with depth. The ∑16PAH concentration was in the range of 1807.48–4674.83 ng g−1, showing a downward trend with depth, with some fluctuations. Three-ring PAHs dominated in surface sediment, while 5-ring PAHs dominated at a depth of 55–93 cm. Six-ring PAHs appeared in the 1830s and gradually increased over time before slowly decreasing from 2005 onward due to the establishment of environmental protection measures. The ratio of PAH monomers indicated that PAHs in samples from a depth of 0 to 55 cm were mainly derived from the combustion of liquid fossil fuels, while the PAHs in the deeper samples mainly originated from petroleum. The results of a principal component analysis (PCA) showed that the PAHs in the sediment core of Taihu Lake were mainly derived from the combustion of fossil fuels, such as diesel, petroleum, gasoline, and coal. The contributions of biomass combustion, liquid fossil fuel combustion, coal combustion, and unknown source were 8.99%, 52.68%, 1.65%, and 36.68%, respectively. The results of a toxicity analysis indicated that most of the PAH monomers had little impact on the ecology, and the annual increase of a small number of monomers might have toxic effects on the biological community, resulting in a serious ecological risks, that requires the imposition of control measures.


Introduction
Sediments are a significant compartment of the aquatic environments, especially in shallow lakes (Wang and Jiang 2019;Chen et al. 2020;Song et al. 2021). Due to the long-term discharge of industrial wastewater and urban sewage, as well as the accumulation of organic matter, large amounts of pollutants such as organic matter, nitrogen (N), and phosphorus (P) are stored in sediment (Wu et al. 2014;Muller et al. 2021;Patil et al. 2022). As a result, lake sediments generally contain valuable historical records of sediment-related nutrients and pollutant transport across timescales of decades to thousands of years (Hubert-Ferrari et al. 2017;Pongpiachan et al. 2019;Wan et al. 2020). Sediment cores from aquatic environments provide a unique opportunity to study the changes and origins of pollutants from earlier periods (Gong et al. 2019), and therefore, many studies have been conducted to determine pollutant concentrations in Responsible Editor: Hongwen Sun sediment cores (Shumilin et al. 2012;Li et al. 2017;Shen et al. 2018a).
Based on samples collected from sediment cores, the concentrations of organic compounds and elements have been measured to investigate their sources, transport processes, and productivity in lacustrine sediments. Additionally, the potential for redox elements to reveal the impact of anthropogenic loadings or to separate anthropogenic from geogenic inputs has been evaluated (Choudhary et al. 2018;Macdonald et al. 2008;Chaudhary, et al. 2013). Vertical profiles of polycyclic aromatic hydrocarbons (PAHs) were assessed, and possible sources and distributions were identified through the analysis of sediment cores (Chen et al. 2016;Bigus et al. 2014;Terence and Masni 2021). However, the depths of sediment cores in most studies are concentrated around 30-50 cm, with some extending to 70 cm. Taihu Lake is the third largest freshwater lake in China and is located within an economically developed area with one of the highest population densities in China. Sediment cores taken from depths of more than 50 cm are rare in Taihu Lake. Dredging, silting, and other operations are frequently conducted in Taihu Lake, and it is therefore necessary to obtain a deeper sediment core for tracing historical changes in pollutants.
In the present study, a sediment core was taken to a depth of 93 cm in Taihu Lake with the aim of (1) determining the variation of internal and external sources of sediments; (2) conducting a quantitative analysis of the vertical distribution characteristics of 16 PAHs in Taihu Lake Basin; and (3) investigating the occurrence and origin of PAHs in sediments at different depths and further evaluating the potential ecological impact of different PAH monomers in Taihu Lake.

Site description and sampling
Taihu Basin is an open natural ecosystem that extends over a large area and has been extensively disturbed by human activities. Due to the ongoing human activities in the natural area of the basin, the lake sediments of Taihu Basin have become an important sink for the accumulation of pollutants. The sediments are not only a sink of pollutants but also a source of environmental pollution once they are disturbed. It is of great significance to study the variation of pollutant levels in the lake on a centennial scale. A sediment core was therefore collected in April 2021 from an area that had not been dredged (31° 27′ 35″ N, 120° 2′ 13″ E), which was located in Zhushan Bay on the northern edge of Taihu Lake (Fig. 1). This region contains some of the most polluted water in the basin, with a black algal bloom frequently occurring in summer (Yao and Xue 2010;Fan and Xing 2016).
The sediment core was collected with a stainless steel static gravity piston corer (KC-Denmark A/S, Silkeborg, Denmark) and had a total length of 93 cm. The sediment core was immediately cut into segments at 1-cm intervals from the surface to the bottom. Each segment was individually wrapped in plastic, labeled in the field, and then shipped to the laboratory on ice. After each section of sediment was collected, the slicing tool was washed with distilled water. Sediment samples transported to the laboratory were stored in a − 18 °C freezer prior to analysis. After vacuum freeze-drying, the sediment core samples were sieved once with a standard 100-mesh sieve (particle size < 0.15 mm) prior to an analysis of PAHs and the elements commonly occurring in organic compounds.

Chemicals and reagents
All solvents including dichloromethane and n-hexane were purchased from Leybold (Germany) and were of high-performance liquid chromatography (HPLC) grade. The 16 PAHs were purchased from AccuStandard (New Haven, CT, USA) with a purity of over 98%. Tin capsules were purchased from Elementar (Langenselbold, Germany). The PAH solution, which contained all the analytes, was prepared in n-hexane and stored at 4 °C in the dark. Water Fig. 1 The specific location of the sediment core. Note: This map was created by ArcGIS 10.2 software, and the base map was provided by "Yangtze River Delta Science Data Center, National Earth System Science Data Center, China National Science and Technology Infrastructure" (http:// nnu. geoda ta. cn: 8008) was purified in an ultrapure water system (Aquapro, Taipei, Taiwan).

Analytical methods
The concentrations of the 16 priority PAHs (as designated by the US Environmental Protection Agency (USEPA)) were analyzed: naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), phenanthrene ( Approximately 2 ± 0.0001 g of the freeze-dried sample was transferred into a 34-mL extraction cell, which had been pre-covered with a gasket, and the sample was well mixed. The PAHs were extracted in an accelerated solvent system with a hexane/acetone (1:1, v:v) solution (ASE 300) at 1500 psi and 100 °C. The extract was transferred to a round bottom flask and concentrated to 2 mL in a 40 °C water bath using a vacuum rotary evaporator. Then, the concentrate was transferred to a prepared column filled with 1.5 g of anhydrous sodium sulfate, 1 g of silica gel, and 1.5 g of anhydrous sodium sulfate from top to bottom, and washed with 15 mL of n-hexane/dichloromethane mixture (1:1, v:v) and 5 mL of n-hexane. The rinsing fluid was collected in a flask and evaporated to 0.5 mL using the rotary evaporator again. The residual solution in the flask was rinsed with n-hexane and filled to 1 mL (Chen and Liang 2021;Xia et al. 2021;Sun et al. 2021). The samples were measured after passing through a DB-5MS (0.25-µm film thickness, 30 m × 0.25 mm i.d.) silica capillary column in a gas chromatography-mass spectrometry (GC-MS) system (Agilent 8860-5977, Agilent, Santa Clara, CA, USA). The system used 1 µL of unseparated sample and 1 mL min −1 of high purity (99.999%) helium as the carrier gas. The detector temperature was 280 °C and the injector temperature was 250 °C. The system procedure was 50 °C for 1 min, 15 °C for 180 °C, and 5 °C for 280 °C for 5 min. The mass detector was maintained at 70 eV and the ion source temperature was 280 °C. The PAHs were determined in full scanning mode (50 ~ 550 amu).
The nitrogen (N), carbon (C), hydrogen (H), and sulfur (S) contents were determined by an organic elemental analyzer (Elementar). Each 10-12 mg sample was placed in a tin capsule and oxidized to four gases at 950 °C: nitrogen gas (N 2 ), carbon dioxide (CO 2 ), water vapor (H 2 O), and sulfur dioxide (SO 2 ). Helium (He) and oxygen (O) with 99.99% purity were used as carrier gases for oxidation. The combustion products were mechanically homogenized in the gas control zone and separated in the gas chromatography column. Subsequently, the eluted gas was sent to a thermal conductivity detector and the N, C, H, and S contents were obtained.

Assessment of ecological risk
The effects range low (ERL), effects range median (ERM), and ERL average quotient were calculated to evaluate the toxicity and carcinogenicity of PAHs. This is currently the most widely used toxicity evaluation method (Raza et al. 2013;Raeisi et al. 2016;Meng et al. 2018). Based on the evaluation criteria, when PAH concentration was lower than the ERL, there were no adverse impacts of PAHs in this area; but when the PAH concentration was higher than the ERM, there may be toxic effects on the biological community, resulting in serious ecological risks. When the PAH concentration was between the ERM and ERL, there was a potential pollution risk.

Quality control and quality assurance
All glass containers used in the study were baked at 500 ℃ for 4 h in advance of their use in the analysis. Using an external standard method, a 7-point calibration curve (50,100,250,500,1000,2000, and 2500 ng mL −1 ; r 2 > 0.996) was constructed to quantify the PAHs. No target compounds were detected in the method blanks. In all samples, the average recovery rates of PAHs were > 75%, and the concentrations obtained did not exceed the recovery rate correction. The relative standard deviations of PAHs were less than 10% in repeated samples. The samples used in the organic elemental analyzer were weighed with an analytical balance sensitive to 0.001 g, and the tin capsules were handled with tweezers throughout the analysis. The results were expressed based on the dry weight of sediment core samples.

Absolute principal component scores-multiple linear regression receptor model (APCS-MLR)
APCS-MLR quantitative source analysis was carried out based on the normalized factor scores and eigenvectors obtained by principal component analysis (PCA) (George and John 1985). The raw data was standardized, as shown in Formula (1): where C ij meant the concentration of j pollutants in the sample I, Cj and j were the average concentration and standard deviation of j pollutants in all samples.
All manual samples with a concentration of 0 were introduced and standardized, as shown in Formula (2): The factor scores of the standardized pollutant concentration samples could be obtained through PCA. The APCS of each pollutant element could be estimated by subtracting the factor score of Z 0 from the factor score of the real sample. APCS was used as an independent variable, and the measured concentration of PAHs was used as a dependent variable to conduct multiple linear regression analysis. Through the regression coefficient, APCS were converted into the content contribution of each pollution source to each sample, as shown in Formula (3): where Cj was the concentration of pollutant j, b 0j was the multiple regression constant term of pollutant j, b kj was the regression coefficient of source k to pollutant j, APCS k was the absolute principal component factor score, p was the number of factors, and b kj × APCS k meant the contribution rate of source p to Cj.

Statistical analysis
All statistical analyses were conducted using the SPSS 26.0 software (SPSS Inc. Chicago, IL, USA). The potential sources of PAHs were determined by the calculation of the BaA/(BaA + Chry), Ant/(Ant + Phe), IcdP/(IcdP + BghiP), and Flua/(Flua + Pyr) isomeric relationships. A PCA was applied to the raw data for dimensionality reduction processing. The correlation coefficients between variables were combined into fewer factors, and the source of the pollutants was determined through the correlations between factors. (2)

Vertical distribution of elements in the sediment core of Taihu Lake
The elemental composition of the sediment core samples from Taihu Lake is shown in Fig. 2. The N, C, H, and S contents were in the ranges of 0.08-0.3%, 0.83-3.6%, 0.63-1.12%, and 0.02-0.24% and the mean concentrations were 0.17 ± 0.04%, 1.55 ± 0.44%, 0.83 ± 0.09%, and 0.10 ± 0.04%, respectively. Carbon was the most abundant element in the samples, followed by H, which was consistent with previous studies (Anjum et al. 2019). Taihu Lake was a eutrophic lake, but the N content in the sediment core was lower than that of C and H, which was also reported in previous studies (Alcocer et al. 2021). This might be due to the large biomass of the primary producers, which incorporated large amounts of N, preventing its incorporation into lake sediments (Alcocer et al. 2018). The decay of algal biomass in shallow eutrophic lakes consumes O and N from sediments through denitrification (Huang et al. 2019;Zhu et al. 2020). The H content was relatively consistent with depth. The S, N, and C contents and the C/H ratio displayed a decreasing trend with depth, with consistent changing trends for the N and C contents, and the C/H ratio. The C/N ratio suddenly increased at a depth of about 50 cm, with an increase from 7-8 to 10-13, which proved that the source of algae in the sediment below 50 cm decreased sharply (Xue et al. 2007). Through sediment boreholes, it has been reported that the deposition rate of the sediments in West Taihu Lake was about 0.35-0.42 cm a −1 (Wang et al. 2001). Using a 137 Cs and 210 Pb dating method, the average sedimentation rates were calculated to be 0.35-0.41 cm a −1 ). Based on 137 Cs Fig. 2 Elemental composition of the 93-cm sediment core samples in Taihu Lake. Note: C, H, N, and S represent carbon, hydrogen, nitrogen, and sulfur activity, it has also been confirmed that the average deposition rate was 0.3-0.4 cm a −1 (Li et al. 2019a). Considering the above studies, it was speculated that since the 1900s, the source of organic matter in Taihu Lake sediments has changed, with endogenous inputs becoming more significant. In general, larger endogenous algae or bacteria inputs would improve the N stock, and thus reduce the C/N ratio Yu et al. 2022). This might be due to the rapid increase in the concentration of various pollutants in Taihu Lake sediments from this period onward (Sun et al. 2009), which has led to increased eutrophication of the water. Compared with East Taihu Lake, the C/N ratios of surface sediments in Zhushan Bay were significantly lower , which was related to the properties of the lake. East Taihu Lake is located in a typical grass-type area (Wei 2010), while Zhushan Bay is located in a typical algae-type area ).

Occurrence of PAHs and their monomers in the lake sediment core
Sixteen different PAHs were detected in the sediment core of Taihu Lake, with concentrations in the range of 0-1460.08 ng g −1 , as shown in Fig. 3. The Acy, Ace, and Flu concentrations were generally lower than those of the other PAHs, and the vertical concentrations were all lower than 170 ng g −1 . The Phe and BaP concentrations were higher than 1000 ng g −1 at some depths, which were the highest PAH concentrations measured in this study. It was not possible to detect IcdP, DahA, and BghiP below a depth of about 70 cm. The concentrations of most PAH monomers decreased with depth, and there were large fluctuations at the depth of 47-50 cm, which was consistent with the elemental analysis results. Therefore, it was speculated that the concentration of PAH monomers changed around the 1900s, which was driven by a variety of factors including climate change, land reclamation, damming, nutrient loading from agriculture, aquaculture, urbanization, and industrialization Ge et al. 2018).
The ∑ 16 PAH concentration was 1807.48-4674.83 ng g −1 , with an average of 2847.57 ng g −1 . The ∑ 16 PAH concentration displayed a downward trend with depth, with some fluctuations (Fig. 4). There was also a large fluctuation at a depth of about 48 cm. The highest ∑ 16 PAH concentrations were observed in the surface layer and were followed by a decrease in deeper layers, which concurred with previous studies (Notar et al. 2001;Fisner et al. 2013). The overall concentrations of 2-ring PAHs (Nap) and 4-ring PAHs (Flua, Pyr, BaA, and Chry) fluctuated less than those of the other PAHs. The 3-ring PAHs (Acy, Ace, Flu, Phe, and Ant) dominated at the depth of 0-2 cm. The proportion of 5-ring PAHs (BbF, BkF, BaP, and DahA) in all samples was higher than 22%, and they dominated at a depth of 55-93 cm. The proportion of 6-ring PAHs (IcdP and BghiP) gradually decreased and was below the detection limit at a depth of 73 cm, i.e., the 1830s. The 6-ring PAHs are considered to be markers of vehicular traffic (Shen et al. 2018b), and it was therefore speculated that vehicle emissions affected the lake sediments from this point onward. From the 1800s, the Cd concentration increased slightly in the Bohai Sea (Hu et al. 2017), and Taibai Lake began to experience eutrophication (Yang et al. 2008). By the late 1800s, mercury contamination had become serious in Gonghai (Liang et al. 2022), which indirectly indicated that sediment pollution began to gradually increase at this time. The concentration of 6-ring PAHs continued to increase and only began to slowly decrease at a depth of 6 cm, i.e., around 2005. Through the long-term monitoring of Taihu Lake water, it was found that the lake water quality is deteriorating, and it did not improve until 2005 (Ma et al. 2013). The changing pollution situation in the lake was basically consistent with the changes in the concentration of 6-ring PAHs.
The proportion of PAH monomers in sediment cores at different depths was calculated in this study, as shown in Fig. 5. The Ant/(Ant + Phe) ratios in the sediment core samples from Taihu Lake were all > 0.10, indicating that PAHs mainly originated from combustion sources, and the BaA/ (BaA + Chry) ratios also displayed the same phenomenon, with all values being > 0.5. The PAHs at a depth of 16 cm originated from petroleum combustion, but elsewhere in the core the Flua/(Flua + Pyr) ratios were all > 0.5, indicating that biomass and coal combustion were the main sources of PAHs in the sediment of Taihu Lake. Unlike the Flua/ (Flua + Pyr) ratio, the IcdP/(IcdP + BghiP) ratio indicated Fig. 4 Percentages of 16 priority PAHs with different ring numbers and the total concentration of ∑ 16 PAHs (unit: ng g −1 dw). Note: 2-ring included Nap; 3-ring included Acy, Ace, Phe, Ant, and Flu; 4-ring included Flua, Pyr, BaA, and Chry; 5-ring included BaP, BbF, BkF, and DahA; 6-ring included IcdP and BghiP that the PAHs in the samples from a depth of 0-55 cm mainly originated from the combustion of liquid fossil fuels, while the PAHs in the deeper samples were mainly derived from petroleum. The reason for the differences in the results might be that the Flua/(Flua + Pyr) ratio in shallow sediments was very close to 0.5, and it was therefore impossible to accurately determine its origin. However, it was concluded that since the 1900s, the PAHs in the sediment core of Taihu Lake mainly originated from the combustion of fossil fuels, such as coal and petroleum. This was also the conclusion of previous studies Li et al. 2019b;Lang et al. 2022). The main source of PAHs in Taihu Lake sediments changed from petroleum to the combustion of fossil fuels in the 1900s, which could be explained by the onset of the industrial revolution in China (Bao et al. 2015). The increase in fossil fuel sources coincided with the reform and opening up of China, which precipitated a social and economic transformation, and rapid industrial and economic growth (Neupane et al. 2022). Regardless of which molecular ratios were used, it was clear that PAHs in lake sediments have transitioned from petroleum sources to fossil fuel combustion sources.
A PCA was conducted on all the data sets and two principal components (PC1 and PC2) were obtained, which together explained 85.9% of the variance of the PAHs in the sediment core samples (Fig. 6). PC1 explained 55.4% of the variance of the PAHs in all sediment core samples, among which BaP, Phe, BbF, and Ant had relatively high factor loadings, while Flua, BaA, BkF, and BghiP had moderate factor loadings. PC2 could explain 30.5% of the variance of the PAHs in all sediment core samples, with BaP exhibiting a relatively high factor loading and Nap, Acy, Ace, and Flu having a moderate factor loading. According to previous studies, BaP can be used to identify emission sources related to diesel emissions and combustion sources (Callen et al. 2011); Phe, Ant, Flua, Nap, and Ace are the main PAH monomers released from coal combustion (Yang et al. 2002;Larsen and Baker 2003;Agarwal et al. 2009); BbF, BaA, BkF, and Flu are mainly derived from residual fuel (Chen et al. 2016); BghiP is the main marker of vehicle exhaust emissions (Gao et al. 2012); and Acy is mainly derived from biomass combustion. In summary, the PAHs in the sediment core of Taihu Lake were mainly derived from the combustion of fossil fuels such as diesel, petroleum, gasoline, and coal. These results further validated the use of the molecular ratio method.
Based on the PCA results, the functional relationship between PAH concentration and pollution sources was established by using the APCA•MLR model, and the contribution rate of each PAH source was calculated according to the regression coefficient of multiple linear regression equation, as shown in Fig. 7. After adjustment, the R 2 of Nap, Acy, Ace, Flu, Phe, Ant, Flua, Pyr, BaA, Chry, BbF, BkF, BaP, IcdP, DahA, and BghiP were 0. 43, 0.92, 0.90, 0.79, 0.75, 0.59, 0.91, 0.98, 0.96, 0.95, 0.88, 0.83, 0.71, 0.94, 0.93, and 0.86, respectively. The R 2 obtained by the model were all  greater than 0.5, indicating that the predicted concentration of the model and the observed concentration fit well, and the source analysis results were reliable. Source identification results showed that the four potential pollution sources were biomass combustion, liquid fossil fuel combustion, coal, and unknown sources. The average contribution rates were 8.99%, 52.68%, 1.65%, and 36.68%, respectively. The contribution of liquid fossil fuel combustion source was the highest.

Toxicological assessment of PAHs
A toxicological assessment of PAHs in the sediment core from Taihu Lake was conducted, and the results are shown in Table 1. The Flua, Pyr, Chry, and BghiP concentrations were all lower than the ERL, and therefore would not cause adverse effects on the environment. The Nap concentration was only slightly higher than the ERL at the depth of 42-58 cm, and in all other samples was lower than the ERL, which would not present an ecological risk. Although the Acy, Ace, Flu, BaA, BkF, and BbF concentrations in some samples were higher than the ERL, they were still much lower than the ERM, with a low risk of an ecological impact. The Ant concentration was very close to the ERM in the middle section of the sediment core, but it was lower than 300 ng g −1 at the depth of 0-25 cm, and the ecological risk was low. The same phenomenon was also observed for BaP. The Phe and DahA concentrations increased year by year and were very close to the ERM in the surface sediments. Their monitoring and control should therefore be increased in the basin. The IcdP concentration also displayed the same pattern, but because its ERM was not available, the potential toxicity could not be analyzed; however, considering the high carcinogenicity of IcdP, further in-depth research is urgently needed.

Conclusions
The sources of organic matter in Taihu Lake sediments have changed since the 1900s, with endogenous inputs becoming more significant as the region has converted into a typical algae-type area. Because of the variety of influencing factors, including climate change, land reclamation, damming, nutrient loading from agriculture, aquaculture, urbanization, and industrialization, the concentration of PAH monomers also changed around the beginning of the twentieth century. Six-ring PAHs first appeared around 1830 and their concentrations gradually increased, but then decreased from 2005 onward due to the establishment of environmental protection measures. The concentrations of 6-ring PAHs were closely related to the overall pollution status of Zhushan Bay. The PAHs in sediments have transitioned from petroleum sources to fossil fuel combustion sources over time, and the contribution of liquid fossil fuel combustion source was the highest. The concentration of some monomers with a high carcinogenicity has increased annually in the lake sediments. These monomers are very likely to have toxic effects on biological communities, resulting in a serious ecological risk that requires the imposition of control measures.