Study on the Distribution and Source of Heavy Metal Pollution in Surface Sediments of Huanghua Habor

. In order to study the pollution characteristics of heavy metals in surface sediments of Huanghua Sea area, the concentrations of organic carbon, sulfide, petroleum, Hg, Cu, Pb, Zn, Cr, Cd and As in surface sediments of 15 stations in Huanghua Sea area were analyzed, and their sources and ecological risks were evaluated. The results show that all the elements in the sediments can meet the quality standard of a class of Marine sediments. The coefficient of variation of heavy metals ranged from 18.71% to 37.69%, indicating that the distribution of heavy metals among all stations was relatively uniform and the dispersion was small. The correlation of Pb, Cr and As in heavy metals is good, which indicates that these elements have homology. The results of ground accumulation index Igeo calculated by the average value of sediments in shallow sea of China show that Hg and Cd of surface sediments in Huanghua Sea are pollution-free to light-moderate pollution, and the pollution of Cd and Hg is more serious. The evaluation of potential ecological risk coefficient shows that the potential risk index RI of Huanghua Port area ranges from 112.78 to 449.89, all of which are greater than 110, belonging to moderate to strong ecological risk, in which the contribution values of Cd and Hg are greater than 90%, which also indicates that Cd and Hg are the main ecological risk factors in Huanghua Sea area.


INTRODUCTION
Huanghua Port is located in the dome of Bohai Bay, with a wide hinterland and unique location advantages, is the most convenient outlet to the sea in the central and western regions [1] . With the development of Huanghua Port, metallurgical industry, mariculture and other industries, the Marine pollution in Huanghua Port area is becoming more and more serious [2] . The morphological characteristics of the bay lead to a relatively closed hydrodynamic environment, so that after the land-based pollutants enter the bay, the heavy metal pollutants carried by them are adsorbed on the surface of the sediments, and can be released into the seawater again through continuous complexation and adsorption and desorption process, resulting in secondary pollution. Meanwhile, the pollutants will accumulate and precipitate continuously in the seabed sediments, and when the content exceeds a certain threshold, the water will be polluted. It will have a toxic effect on the benthic organisms nearby, affect the quality of seawater and the community structure and composition of benthic organisms, and then affect human beings through the food chain [3~5] . Huanghua Port is located in the southeast of Bohai Bay, and the ecological risk of Marine environment is higher. Many studies have been carried out on the characteristics of heavy metals in sediments in Bohai Bay. Among them, the study by Xu Yan et al. based on the sediment data in 2018 showed that the phenomenon of Hg and Cd exceeding the standard in Huanghua Port and the ecological risk was higher [6] . Based on the sediment data of 2013 by Zhou Xiaobai et al. [7] believed that arsenic and lead were the main factors that caused the heavy metal content in surface sediments of Bohai Bay to exceed the standard point level, which belonged to the minor ecological risk level. At present, there are relatively few researches on the Marine environment of Huanghua Port.
In general, the sediment pollution in Huanghua Sea area is relatively serious. Therefore, based on the Marine sediment monitoring data of Huanghua Port in 2020, this paper analyzes and evaluates its pollution status, which can provide scientific reference for effectively carrying out environmental quality monitoring in Huanghua Sea area, comprehensively controlling environmental pollution in Bohai Sea and protecting Marine ecological environment.

Sample collection and processing
The survey time of this study is April 2020, and a total of 15 monitoring stations are arranged, as shown in Table 1. The collection, pretreatment, storage, transportation, analysis and quality control of sediment samples are carried out in accordance with the Code for Marine Monitoring Part 5: Sediment Analysis (GB17378.5-1998).

Research method
The Geo-accumulation Index (Igeo) proposed by Muller in 1969 can not only evaluate the degree of heavy metal pollution in sediments, but also reflect the influence of natural geological background, different geochemical background and human activities on the pollution degree of heavy metals in sediments. At present, it has been widely used to evaluate sediment heavy metal pollution [8] . The formula is as follows: Where, Ci is the measured value (mg/kg); Bi is the geochemical background value (mg/kg), and the average value of shallow sea sediments in China was selected for Bi in this study.

Ecological risk assessment
In 1980, Swedish scholar Hakanson proposed the potential ecological risk index (RI) [9] to evaluate the ecological risk of heavy metals in surface sediments. This method is based on sedimentological principles. A variety of influencing factors (heavy metal content, toxicity level, sensitivity of water to metal pollution and multi-element synergism, etc.) have been comprehensively considered, and have been widely used by domestic and foreign scholars to evaluate the ecological harm degree of heavy metals in sediments [10~11] . According to this method, the calculation formula is formula (1) [10] .
Where, RI is the potential ecological harm index; r i E is the potential ecological harm coefficient of a single heavy metal; r i T is the toxicity coefficient of heavy metal i; The toxicity coefficients r i T of As, Cd, Hg, Cu, Pb, Cr and Zn were 10, 30, 40, 5, 5, 2 and 1, respectively. [11] f i C is the enrichment coefficient of ; s i C is the measured content of heavy metal i; n i C is the reference ratio of this element, and the Bi background value of shallow sea in China is used as the reference ratio in this study.

Data processing and analysis
SPSS and Excel2003 software were used for data analysis in this experiment. Coefficient of variation (CV) is the ratio of standard deviation to mean.

Distribution characteristics of heavy metal mass concentration
The concentration of heavy metals in surface sediments in Huanghua Sea area is shown in Table 2, and the results show that The average concentrations of organic carbon, sulfide, petroleum, Cu, Pb, Zn, Cd, Cr, Hg and As in the study area were 0.52%, 51.2mg/kg, 147.1mg/kg, 75.0mg/kg, 17.1mg/kg, 21.1mg/kg, 0.084 mg/kg, 51 mg/kg, 0.238 mg/kg, 5.4 mg/kg. The maximum concentrations of organic carbon, sulfide, petroleum, Cu, Pb, Zn, Cd, Cr, Hg and As in the study area were 0.69%, 153mg/kg, 217mg/kg, 28.1mg/kg, 34.2mg/kg, 100mg/kg, 0.128 mg/kg, 73.1 mg/kg, 0.465mg/kg, 6.61mg/kg. All the factors in the sediment can meet the quality standard of a class of Marine sediments. The coefficient of variation of heavy metals ranged from 18.71% to 37.69%, indicating that the distribution of heavy metals among all stations was relatively uniform and the dispersion was small.
The results show that the average concentrations of Zn, Cr, Cd and Hg in this study are higher than those in the previous study, while the average concentrations of Cu, Pb and As in this study are lower than those in the previous study.

Analysis of sources of heavy metals in sediments
Correlation analysis is a method to study the degree of correlation between variables, which can reflect their sources through the correlation between variables [12] .
The correlation analysis results of heavy metals in Huanghua Sea area are shown in Table 3. The research results show that the correlation of heavy metals in sediments is not significant, while the correlation of Pb, Cr and As is good, with correlation coefficients ranging from 0.42 to 0.60, indicating that they have common sources and control factors.
There is no significant correlation between heavy metals and petroleum, organic carbon and sulfide in this study area, indicating that heavy metals in sediments have no homology with petroleum, organic carbon and sulfide, and that degradation of organic matter in sediments has little effect on heavy metals in sediments in Huanghua Sea area.
Pb, Cr and As have a good correlation. Existing studies show that Cu and Zn mainly come from domestic or industrial wastewater such As anti-corrosion coatings, waste batteries and printing and dyeing industries; AS pollution mainly comes from industrial and agricultural wastewater such as mining, metallurgy, chemical industry, pesticide production and fertilizer; Cr pollution mainly comes from machinery manufacturing and chemical industry. Cd pollution mainly comes from fossil fuel combustion (power generation), semiconductor and superconducting industries and agricultural activities using pesticides and fertilizers; Pb mainly comes from exhaust gas pollution of motor vehicles and ships; Hg sources are greatly affected by human activities and are mainly man-made pollution sources, such as industrial and agricultural wastes and domestic sewage discharge [12~13] . Lu Duan et al. showed that the heavy metals discharged from mariculture were mainly Cu, Cr, Pb, As and Cd [14] .
Combined with the high average concentrations of Zn, Cr, Cd and Hg and the good correlation between Pb, Cr and As in Huanghua Sea area, it is suggested that mariculture, pesticide production and fertilizer use may be an important source of heavy metals in Huanghua Sea area.

Heavy metal pollution assessment
The ground accumulation index Igeo calculated by using the average value of sediments in shallow seas of China is shown in Table 4. The results show that As and Cr are in a pollution-free state, Cu, Pb and Zn are in a pollution-light pollution state, Hg is in a pollution-light to moderate pollution state, Cd is in a pollution-moderate pollution state, and Cd is the most serious pollution state. The potential risk coefficient and potential risk index of 7 heavy metals were evaluated, and the results were shown in Table 4, indicating that the average potential ecological risk coefficient of 7 heavy metals from high to low was: Hg>Cd>Cu>As>Pb>Cr>Zn, the potential risk coefficient of Hg ranges from 76.8 to 204.8, and there is a strong ecological risk of Hg in 93.33% of the stations. The potential risk coefficient of Cd ranged from 20.77 to 214.62, and Cd had a strong ecological risk in 66.7% stations, and a moderate ecological risk in 72.9% stations. The potential risk coefficients of Cu, Pb, Zn, As and Cr were all less than 40, which belonged to low ecological risk. This is consistent with the results of Xu Yan et al. [3] . The potential risk index RI in the study area ranged from 112.78 to 449.89, all of which were greater than 110, belonging to medium to strong ecological risk. The ecological risk varied greatly among all sampling points, 60% of the stations had strong ecological risk, and 40% of the stations had medium ecological risk. The average contribution of Cd and Hg to Marine ecological risk is 41.36% and 50.60% respectively, and the contribution values of Cd and Hg are more than 90%, indicating that both Cd and Hg are major ecological risk factors in Huanghua Sea area. Combined with the industrial development of Huanghua, this study believes that due to strict control of aquaculture drainage, Cd and Hg emissions from man-made sources can be reduced, and Cd and Hg pollution in the sea area can be reduced as much as possible.

Conclusion
(1) The monitoring results of surface sediments in Huanghua Sea area show that organic carbon, sulfide, petroleum, Hg, Pb, Zn, Cr, Cd, As and Cu can meet the quality standards of a class of Marine sediments. The concentration of heavy metals in sediments of Bohai Bay shows that Zn, Cr, Cd and Hg are seriously polluted.
(2) The coefficient of variation of heavy metals in the study ranged from 18.71% to 37.69%, indicating that the distribution of heavy metals among all stations was relatively uniform and the dispersion was small, and the correlation between Pb, Cr and As in heavy metals was good.
(3) The results of Igeo, a ground accumulation index calculated by using the average value of sediments in shallow seas of China, show that As and Cr are in a pollution-free state, Cu, Pb and Zn are in a pollutionlight pollution state, Hg is in a pollution-light to moderate pollution state, Cd is in a pollution-moderate pollution state, and Cd is the most serious pollution. The evaluation of potential ecological risk coefficient shows that the potential risk index RI of Huanghua Port area ranges from 112.78 to 449.89, all of which are greater than 110, belonging to moderate to strong ecological risk, in which the contribution values of Cd and Hg are greater than 90%, which also indicates that Cd and Hg are the main ecological risk factors in Huanghua Sea area.