Spatial Distribution,Seasonal Variation And Sources Of Polycyclic Aromatic Hydrocarbons In The Seawater And Sediments Of The Bohai Sea

With the gradually improvement of industrialization and urbanization,and the soaring increase of energy consumption,the emissions of polycyclic aromatic hydrocarbons(PAHs)are also increasing.PAHs have long-distance migration potential and show strong lipophilicity,as well as teratogenic,carcinogenic and mutagenic properties.They have been widely detected in the atmosphere,water,soil,sediment,and even organisms and humans.The coastal sea connects the land and the ocean.Huge amounts of pollutants are discharging into coastal sea by extensively industrial and human activities from coastal cities and have caused the deterioration of water quality and have endangering the health of ecosystem and human health.Fifteen priority PAHs were analyzed in the seawater and sediment from three research cruises on the Bohai Sea in June,August and November 2019,respectively.The spatial distribution characteristics and seasonal variations of PAHs in the seawater were investigated for the first time in the Bohai Sea,and the phase distributions were discussed for PAHs between two phases(dissolved and particle phase).This paper also studied the concentration levels,composition profiles and distribution characteristics of PAHs in the surface sediments,and briefly analyzed the main input pathways of PAHs in the Bohai Sea.The conclusions of the study are as follows:PAHs were widely detected in the seawater with a range of 3.16-87.9 ng/L.Horizontally,high levels were found in nearshore and low in the central of Bohai Sea,indicating a dominant land-based source of PAHs.Compared with other studies around the world,the overall PAH levels in the Bohai Sea are at medium levels.The estimated reserves of PAHs in the seawater are much lower than the total PAHs emissions from neighboring cities.It is inferred that offshore waters are important environmental media for the migration and transformation of PAHs in the environment.In the spring and autumn,there are no obvious difference for vertical distribution of PAHs in the Bohai Sea,indicating a well-mixed water mass in the Bohai Sea under the strong influence of the East Asia monsoon.However,relatively high concentrations of PAHs were found in the surface water than in the bottom water in the central Bohai Sea in summer.This phenomenon was caused by the formation of vertically stratifications of water masses in summer and the thermoclines have hindered the vertical exchange of water bodies.The seasonal variationas showed that total concentration of PAHs was higher in June than in November.The low concentrations of PAHs in summer may be due to the fast volatilization of PAHs into the atmosphere under high-temperature.The decrease in the number of fishing boats during the fishing ban period may also lower the PAHs concentration from fishing activities.The increased concentration of PAHs in autumn was related to the discrease of ambient temperature and the emission from coal-fired heating in northern China.Source analysis showed that PAHs in the seawater were mainly from the combustion of biomass and fossil fuels,industrial wastewater and domestic sewage,and ship oil spills and fuel combustion.In surface sediment,concentration of PAHs was ranged from 7.23 to 168 ng/g dw,which showed a slight medium pollution level wordwidely.The estimated reserves of PAHs in the surface sediments reached to 226 tons,which indicated that the surface sediments are important sink of PAHs.A positive correlation was found between the concentrations of PAHs and TOC,indicating TOC content is an important tool for assessing the pollution level and distribution characteristics of organic pollutants such as PAHs.Source analysis showed that fossil fuels(such as petroleum and coal)and biomass combustion are the main sources of PAHs in the surface sediments of the Bohai Sea.In summary,as a kind of persistent toxic pollutants discharged from unintentional production,PAHs were produced inadvertently in daily industrial and domestic activities.Land-based pollution is the main sources of PAHs.Seawater and sediment are the two reservoirs of PAHs in the Bohai Sea.PAHs show different scales of variations vertically and seasonally in the Bohai Sea.PAHs mainly exist in the dissolved phase in water bodies with mainly low-ring congeners;while PAHs in surface sediments they were mainly consisted of high-ring congeners.This shows that the more water-soluble PAHs in water bodies are easily transferred to other sea areas along with high-temperature volatilization and ocean currents,while the more stable high-ring PAHs are easily adsorbed or concentrated on particulate matter,and finally settle into marine sediments.Therefore,the results of this study would help us to better understand the environmental behavior of PAHs in the marine system,and to provide a reference for the monitoring and governance of the marine organic pollutants.