| The Yellow River is the second largest river in China and is well known as a sediment-laden river in the world.Every year,a large amounts of freshwater,particulate matter,nutrient,organic matter,and heavy metal were carried into the sea through the Yellow River.Ecological habitat was formed in this condition for organisms to grow up in the estuary and coastal water,meanwhile a large amount of particulate matter and adsorbed pollutants were accumulated.The larger surface area of sediments and a large amount of organic ligands strongly adsorbing heavy metal ions were the important trap for heavy metals.The Flow-sediment Regulation Scheme had been carried out continuously for 19 times since 2002,which not only obviously changed the runnoff and sediment load from the Yellow River to the sea and the sedimentary environment of the Yellow River estuary and its coastal area,but also had a significant impact on spatial and temporal distribution and potential ecological risk of heavy metals in sediments.Therefore,it is necessary to understand the spatial and temporal distribution of heavy metals in sediments and the variation of its fraction,to explore their pollution sources to evaluate their pollution index and its potential ecological risk under the impact of the long-term implementation of Flow-sediment Regulation Scheme.The results can provide an important scientific basis to improve the environmental quality and ecological protection of the Yellow River estuary and and its coastal area for the next step.This paper based on the surface sediments of the Yellow River estuary and offshore in 2013 and 2016,this paper analyzed the spatial and temporal distribution of the contents and fractions of heavy metal(Cr,Ni,Cu,Zn,Pb,Cd)and As in the surface sediments of the Yellow River estuary and offshore,exploring the sources and bioavailability of heavy metals and As,and use some methods which include the the marine sediment quality guidelines in china,index of geoaccumulation,pollution load index,single-element pollution indices,integrated pollution indices and ratios of secondary phase and primary phase to evaluate pollution status contents of fraction and contents of heavy metals and As;use some methods which include potential ecological risk index,the biological effect concentration and toxic unit idicate to protect pollution status contents of fraction and contents of heavy metals and As.The main conclusions are as follows:(?)The spatial distribution of Cr,Ni,Cu,As,and Pb in the sediments from the Yellow River estuary was basically the same as the coastal area while the distribution of Zn and Cd was different in 2013.In 2016,the spatial distribution characteristics of Cr,Ni,Cu,and As in the sediments from the Yellow River estuary was relatively the same as offshore area whereas differed in spatial distribution of Zn,Pb,and Cd.The differences between 2013 and 2016 in the spatial distribution characteristics of heavy metals and As in sediments of the Yellow River estuary and offshore area were mainly due to the heterogeneity of sediment parameters(such as grain size,organic matter and susceptibility etc.)caused by differences in sedimentary environments between two years.The average content of heavy metals and As in surface sediments of the Yellow River estuary and offshore area was in the order of Zn(68.52 mg·kg-1)>Cr(66.79 mg·kg-1)>Ni(26.87 mg·kg-1)>Pb(22,84 mg·kg-1)>Cu(19.4 mg·kg-1)>As(11.28 mg·kg-1)>Cd(0.25 mg·kg-1)in 2013 while was in the order of Zn(93.46 mg·kg-1)>Cr(76.61 mg kg-1)>Ni(28.06 mg kg-1)>Pb(24.77 mg·kg-1)>Cu(18.57 mg·kg-1)>As(12.91 mg·kg-1)>Cd(0.16 mg·kg-1)in 2016.(?)Compared to 2013,the average concentration of Cu and Cd decreased in the surface sediments in 2016,however,the average concentration of Cr,Ni,Zn,As and Pb was significantly increased in 2016.The consistent or similar sources of Cr,Ni,Cu,As and Pb were mainly oil extraction,fossil fuel combustion,ore mining and natural source of compound pollution along the Yellow River and in the delta in 2013.Cd might mainly represent compound pollution of domestic wastewater,agricultural activity,electroplating industry,ship navigation,atmospheric deposition and oil spill while Zn was mainly derived from the discharge of domestic sewage.Compared to 2013,the pollution sources of heavy metals and As might have been partially changed in sediments of the Yellow River estuary and offhore area in 2016.The reason might be related to the industrial transformation along the Yellow River from 2013 to 2016,and the long-term implementation of the Flow-sediment Regulation Scheme which led to change the sedimentary environment in the estuary and offshore water.(?)The average content of Zn,Cr,Cu,Pb and As increased before the implementation of the Flow-sediment RHegulation Scheme(1996?2002).The variation of heavy metals and As in sediments was divided into two periods after the implementation of the Flow-sediment Regulation Scheme(2002?2016).In the first(2002-2010),although the concentrations of Cr,Ni,Cu,Zn?Pb and As fluctuated,the values decreased greatly.Nevertheless,the Cd levels did not show great change in this period;except for Cd,the content of heavy metals slightly increased in the second period(2010-2016).(?)There was no significantly difference in fraction content of heavy metals and As between 2013 and 2016 in surface sediments of the Yellow River estuary and offshore area.Cd was mainly in the acid-soluble fraction while other heavy metals were mainly in residual fraction in the surface sediments in 2013 and 2016.In addition,the distribution of four fractions of heavy metals and As in surface sediments were also different in the two years.The fraction of As was in the order of F4(residual fraction)>F2(reducible fraction)>F3(oxidizable fraction)>F1(acid-soluble fraction)in 2013 and 2016,Cr and Ni was in the order of F4>F3>F2>F1,Cu,Pb and Zn in the order of F4>F2>F1>F3,and the order of Cd fraction was F1>F4>F2>F3.(?)Compared to 2013,the total extractable fraction(F1+F2+F3)of Cr,Ni,Cu,Zn,As,Cd and Pb generally decreased in 2016.The major factors influencing available fraction content of heavy metals and As were silt,organic matter,high frequency susceptibility,low frequency susceptibility,frequency susceptibility and sand in 2013,whereas the major factors were organic matter and clay,low frequency susceptibility,high frequency susceptibility,frequency susceptibility and sand in 2016.(?)The analyses of the index of geoaccumulation,pollution load index,toxic unit and potential ecological risk index all implied that the pollution,ecological risk and toxicity increased in sediments of the Yellow River estuary and offshore area before the implementation of the Flow-sediment Regulation Scheme(1996?2002).However,the variation of the pollution,ecological risk and toxicity changed in different periods with the implementation of the Flow-sediment Regulation Scheme(2002?2016),which could be divided into three periods:stage ?(2002?2016,a fluctuation downward trend);stage ?(2002?2016,an increasing trend);and stage ?(2002?2016,increased firstly and then decreased gradually).It was at a high level of pollution and ecological risk although the pollution or ecological risk showed an fluctuation downward trend of heavy metals and As in the sediments of the Yellow River estuary and offshore area under the influence of long-term implementation of the Flow-sediment Regulation Scheme during this study period(2013?2016).In addition,the pollution,ecological risk and toxicity of the Yellow River estuary and offshore areas increased suddenly because of the Penglai 19-3 Offshore Oil Spill in 2012,indicating that the influence of the sudden pollution incident on the pollution,ecological risk and toxicity was higher than the long-term implementation of the Flow-sediment Regulation Scheme. |
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