| Pearl River Basin(102°14'?115°53' E,21°31'?26°49' N)is a complex basin,which comprises Xijiang,Beijiang,Dongjiang and Pearl River delta rivers.This area covers less than 5%of the total China land area,but its GDP accounts for 13%of whole China.It thus plays an important role in Chinese economics.In recent years,with the development of industry and agriculture,and urbanization,ecological degradation,especially heavy metal pollution,has become an important factor restricting the social sustainable development.At present,studies about heavy metals of the Pearl River Basin mainly focused on the downstream areas characterized with rapid economic development,while relatively few studies were focused on metal speciation and mobility on the upstream basin.With economic development,a comprehensive and systematic investigation on the present status of pollution on the upstream is needed to understand the increasing pollution resulted from mining,agricultural nonpoint-source pollution and industrial point-source pollution.In addition,the landform of the upstream of the Pearl River,especially the Yunnan section is featured with Karst landform.A large amount of Ca could be dissolved into water,which led to a higher pH value in the upstream water than in the downstream.However,the current study on what effect the higher pH value would impose on heavy metal mobility is insufficient.Such questions,as whether the metal pollution of the upstream affects the downstream area and whether the transfer of metal pollutants goes through water-phase or through particle carriage,still remain unsolved.Therefore,a further systematic study is needed to identify the source,concentration and its distribution,speciation of metal elements in the water,and sediment and soil in the upstream of the Pearl River,further to reveal the relationship between its speciation and environmental risks as well as its transfer rules.This line of work will be significant for heavy metal treatment and remediation of the Pearl River.This paper studied the concentration and distribution of heavy metals,alkali metals and alkaline earth metals in water,as well as sediment and soil in upstream of the Pearl River,China,and explored geochemical behaviors of metal elements in multi-media,the main results of this study were listed as follow:(1)Total concentration of sediment and soil heavy metals in upstream of the Pearl River.The concentration characteristics of total Cu?Zn?Cd?Pb,As,Cr,Mn and Fe concentrations in sediment and agricultural and non-agricultural soil were basicly in accordance with the crustal abundance sequence in China,and total concentration of Fe was the highest,followed by Mn,while Cd was the lowest.Total concentrations of Fe,Cu,Zn,Cd,Pb,Cr and As in agricultural soil were higher than those in sediment and non-agricultural soil,and total concentrations of Cu,Zn,Pb,Cr and As in sediment were higher than those in non-agricultural soil.Eight heavy metals in three media were classified into two groups.Group one included Cu,Pb,Cr,Cd,Zn,As and Mn;group two only included Fe.(2)Pollution status quo in the upstream of Pearl River.The water was polluted with N and P to a certain extend,but was not polluted with Cu,Zn,Cd,Pb,As,Cr,Mn and Fe in the study area;Organic pollution indexes of sediment were very high,organic indexes of most samples reached grade ?,which meant organic pollution,while the potential ecological risk indexes of eight heavy metals in the sediment were less than 150,which meant slightly polluted.Total concentrations of Cu,Zn,Cd,Pb,Cr and As in upstream of Pearl River were significantly lower than those in the downstream of Pearl River;in contrast with water environment,the agricultural and non-agricultural soils were polluted by industrial and agricultural activities in the study area,and the pollution in factory district was the worst.The comprehensive pollution indexes of Cd in agricultural and non-agricultural soils were significantly greater than 3,which meant serious pollution,in addition,pollution indexes of Zn,Pb and Cr in agricultural soils were also greater than 3.(3)The pollution sources in the upstream of the Pearl River.N in soil mainly came from residues of crops and fertilizer,while N in the sediment mainly came from zooplankton,phytoplankton and algae in the river,and N in soil could get lost into the river,which led to N pollution in the river.the metal elements in soil and sediment in the study area were mainly from soil parent material,and human activities also contributed a lot to the concentration of metal elements in soil;the concentrations of metal elements in agricultural soil were higher than those in non-agricultural soil,and the concentrations of heavy metals in agricultural soil in the industrial district were the highest.(4)The distribution characteristics and transfer of metal elements along the upstream of Pearl River.The concentrations of metal elements in the sediment dispersedly distributed,there was no progressive increase from upstream to downstream,and the transfer of metal elements along the flow direction was not observed;the metal elements in agricultural soil had not significantly moved to soil in the downstream area,and not moved to the upstream river,it indicated the soil in study area had adsorbed and saved the metal contamination.(5)Speciation of metal elements in the upstream of Pearl River.The percentages of residual fraction for Cr were close to 100%in sediment,and the percentages of residual fraction for Fe were more than 70%,it indicated Fe and Cr were mainly held in residual fraction;Cu and As were mainly held in residual fraction and reducible fraction;Pb and Zn had no obvious dominant fraction,the order of percentages for each fraction of Pb and Zn was as follow:reducible>residual>oxidizable>weak acid soluble;the percentages of weak acid soluble for Mn were the highest.The percentages of each fraction in agricultural and non-agricultural soil were similar,the percentages of residual fractions for Fe,Cr,As,Zn,Cu and Cd were more than 60%,and the percentages of reducible fraction for Mn and Pb were more than 50%.K,Na and Mg were mainly held in residual fraction in sediment,Ca and Sr were mainly held in weak acid soluble fraction;Ca,Sr and Na in agricultural and non-agricultural soil were mainly held in weak acid soluble fraction,K were mainly held in residual fraction,and the percentages of each fraction for Mg was similar.(6)The bioavailability and mobility of metal elements in the upstream of Pearl River.The bioavailability and mobility of metal elements in environment not only depend on their total concentration,but also to a large extent depend on their chemical fraction,the percentages of non-residual fraction can reflect their bioavailability,and the percentages of weak acid soluble fraction can reflect their bioavailability.The order of non-residual fraction in sediment and soil were different on a percentage basis in the study area,the order in sediment was as follow:Mn>Zn>Pb>As>Cu>Fe>Cr;the order in agricultural and non-agricultural soil was as follow:Mn>Pb>Cd>Cu>Zn>As>Cr>Fe,the percentages of non-residual fraction in agricultural soil were higher than those in non-agricultural soil.The bioavailability of Mn was the highest in sediment and soil,which was easy to be released into the environment,leading to pollution again;The bioavailability of Zn,As,Cu and Pb was rather high;The bioavailability of Fe and Cr was lower,which meant low potential impact on environment,The bioavailability of heavy metals in agricultural soil was higher than that in non-agricultural soil.The ratio of weak acid soluble fraction of heavy metals(Mn,Zn,As,Cu,Pb,Cr and Fe)to the total concentrations in sediment and soil was generally similar,the percentages of Mn,Zn and As(Mn>Zn>As)were relatively higher,with rather stronger mobility;the percentages of Cu,Pb,Cr and Fe were less than 1%,with rather low mobility;the mobility of Cd in soil was higher than that in sediment.(7)The effect of total organic carbon(TOC)on heavy metals in sediment and soil.The content of TOC in sediment was significantly correlated with the oxidizable percentages of Cu and Pb;The content of TOC in agricultural and non-agricultural soils was also significantly correlated with the oxidizable percentages of Cu?Zn,Cr,Pb and Cd,Cu and Pb in sediment and soil were strongly adsorbed by and bound with organic matter which had significant effects on their mobility and bioavailability.(8)The effect of metal contamination in the upstream of the Pearl River on the downstream areas.Although the soil in study area was polluted by heavy metals(Cd et al.),due to soil adsorption interception,they were not evidently transferred to downstream areas and rivers;most of the heavy metals remained in sediment in the study area due to the adsorption of particulate matter and sediment,and there were very low concentrations of heavy metals in the aqueous phase,and low posssiblity that the heavy metals would be rapidly transferred with water flow;The heavy metals in the sediment in the weak alkaline water environment were not easy to be released into the aqueous phase and be rapidly transfered,in addition to the interception of the sluices,which reduced the possibility that heavy metals in the sediment would move from upstream to downstream with the movement of bottom mud;furthermore,heavy metals can be absorbed and degraded by organisms from and/or around the river water,which reduced concentration and mobility of heavy metals in the water to a certain extend.Therefore,in the context of low pollution,self purification of rivers can greatly reduce the transfer and hazards of metal pollutants,thus the effects of heavy metals in the study area on the downstream were very low. |
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