Abstract: Polycyclic aromatic hydrocarbons (PAHs) consist of two or more benzenoid rings,which are highly lipophilic and hydrophobic.PAHs can be transferred through food chain,and can accumulate in biological tissues,including human body.Because of their toxicity and persistence as well as their carcinogenic,teratogenic and mutagenic properties,PAHs are a topic of growing concern.PAHs have been detected in many water bodies,including the Yellow River.While there have been many studies of PAH concentrations in water,and PAH concentrations in water during freezing and thawing periods have rarely been reported.The study area is Baotou section of the Yellow River.A global positioning system was used to record the locations of the 8 sampling sites (S1-S8) along the main channel of the Baotou section.Baotou is an industrial city on the banks of the Yellow River.As the city has developed,large quantities of industrial wastewater containing organic pollutants have been discharged into the river.In particular,huge amounts of industrial wastewater have been channeled into the Yellow River from the Kundulun,Sidaoshai,East and West rivers.The Baotou section of the Yellow River is located in a zone of 40.248 9°-42.730 3°N and 109.253 3°-111.440 3°E.Winter lasts for more than three months in a year,and the Yellow River at Baotou is frozen in winter,which means that PAHs from the atmosphere are deposited onto the ice.In light of this unique situation,the distribution of PAHs in the Baotou section of the Yellow River should be investigated.In this study,water samples were collected from eight sites along the Baotou section of the Yellow River in freezing-thawing process during the period from 2012 to 2014.By establishing the pre-processing method used in water from the study area,the information on the PAH concentrations in the Baotou section of the Yellow River in freezing-thawing process was obtained,and the environmental risk of PAHs in this area was analyzed.In this paper,the main results are as follows: There were 11 kinds of PAHs detected in water samples from Baotou section of the Yellow River.The total PAH concentrations in water samples ranged from 6.58 ng·L^-1 to 222.37 ng·L^-1,the mean value was 61.48 ng·L^-1,and PAH concentrations were dominated by fluoranthene.Some of them were higher than the standard of EPA882-Z-99-001,and benzo〔a〕pyrene exceeded the standard of GB3838-2002 “Surface Water Environment Quality Standard”.According to the ecological risk assessment of individual PAHs,the risk probability of anthracene in water was 83.92%,when the percent of aquatic organisms affected more than 5.00%.Water phase Σ₆PAH joint toxicity was higher than that of the ecological risk of anthracene largest single role toxicity,even higher than the monomer ΣPAH overlap area.According to the joint risk assessment of PAHs,the joint risk was much higher than that of individual PAHs in water phase,and there was a significant joint toxicity.

Key words: freezing and thawing periods, PAH, concentration level, ecological risk assessment, Baotou section of the Yellow River