Geochem ical distribution,fractionation,and sources of heavy metals in dammed-river sediments:the Longjiang River,Southern China

Xiaolong Lan·Zengping Ning·Yizhang Liu·Qingxiang Xiao·Haiyan Chen·Enzong Xiao·Tangfu Xiao

Abstract In the present study,six sediment cores were collected from six river-dammed reservoirs to reveal the geochemical distribution of heavymetals(As,Cd,Pb,Sb,and Zn)in the Longjiang River,South China,which is highly impacted by nonferrousmetalm ining and smelting activities.The sediments were geochemically characterized,combining geochem ical analysis,sequential extractions,and 210Pb chronology.The results indicated that the river sedimentswere severely polluted by heavy metals in the order of Cd＞Zn≈Pb≈Sb＞As.These heavy metalsgenerally exhibited relatively low enrichment in the upstream sediments because of the lim ited anthropogenic impact,but their abundances drastically increased in the midstream sediments due to local smelting activities.In downstream sediments,the heavy metal concentrations(except for Cd)decreased,owing to the effect of dam interception and detrital inputs.Cadm ium levels tended to increase in downstream sediments,which were attributed to the intensive discharge of Cd during the pollution event in 2012.The sedimentary records were traced back to 1985,and a signiTcant decrease of heavy metal enrichments could be found after the year 2000,suggesting the enhancementof environmentalmanagement in this period.The statistical results indicated that localmetal smelting and mining activities were the main anthropogenic contributors for the enrichment of heavy metals in the dammed-river sediments.High enrichment factor and nonresidual fractions of heavy metals in local sedimentsmay pose a direct threat to aquatic organisms.Cd presents signiTcant danger because of its extreme enrichment and high labile fractions.

Keywords Heavy metals·Fractionation·210Pb dating·Smelting andm ining activities·Sediment cores·The Longjiang River

1 Introduction

Heavy metals can cause serious environmental problems due to their toxicity,persistence,and bioaccumulation(Du et al.2009;Dogùan et al.2013;Gutie?rrez et al.2016).In recent decades,anthropogenic activities,such as industrial ef?uents,domesticsewage,agricultural runoffs,and gasoline combustion have introduced a great quantity of heavymetals into aqueousenvironments(e.g.,rivers,lakes and reservoirs)throughwaste dischargeand/oratmospheric deposition(Bing etal.2011;Hu and Cheng 2016;Liu etal.2017).

Rivers usually constitute critical resources of drinking water in theworld,but these environments are sensitive to heavymetalpollution.Onceheavymetalsenter rivers,they are liable to be adsorbed by colloids andTne-grained particulates(i.e.,organic matters,FeDMn oxides,clay m inerals)(Du Laing etal.2009),and then precipitated in tothe riverbed or transported further downstream.Many cascade dams have been constructed along rivers worldw ide,which play important roles in trapping sediment particles(and therefore heavy metals),due to the rapid decrease of water?ow(Yang et al.2014a).Therefore,the sediments in cascade reservoirsmay re?ect the spatialand temporaldistribution of heavymetalsata catchmentscale.More importantly,the reservoirwatersare usually used for agricultural irrigation and drinking water supply.Considering thatsediments in reservoirsare the prolonged sources of heavy metals for overlying water(Lo?pez et al.2010;Wei et al.2016),assessment of heavy metal pollutions in reservoir sediments are particularly important.Once the heavy metals exceed safe limits,they pose a long-term health risk to both aquatic organisms and humans(Shinn et al.2009;Tao et al.2012).Therefore,heavy metal pollution in river-dammed reservoirs has become one of the most important environmental topics in recent studies(Bing et al.2016b;Wei et al.2016).A number of researchers have utilized sediment cores or surface sediments from reservoirs to reconstruct historical records of contam inant inputs,constrain their origins,and assess their environmental effects(Grousset et al.1999;van Griethuysen et al.2005;Wang et al.2015;Weiet al.2016).Above all,a fullunderstanding of heavymetal pollution in cascade reservoirsmay help local reservoir water quality conservation and management.

Sim ilarly,many rivers have experienced a dangerous pollution of heavymetal in South China.The heavy metal discharges?mainly originating from sulTde processing,tailing,and/or smelting waste discharges?have produced concern for the environmental impact of metal pollution incidents,for instance Tl pollution in the Beijiang River in Guangdong Province in 2010(Xiao et al.2012),Cd pollution in the Longjiang River and Tl pollution in the Hejiang River in Guangxi Province in 2012 and 2013,respectively(Dou et al.2013;Chen et al.2017).Among these rivers,the Longjiang River,a tributary of the Pearl River in north of GuangxiProvince,is intensively dammed w ith cascade reservoirs.It has been suffered from heavy metal pollution caused by waste discharges from industrialized and urbanized activities in Hechi City and Yizhou City.The nonferrousmetalm ining and smelting activities in these cities,especially for Pb,Zn,and Sb,has been the local industry since 1980(YCCH 2016).As a result,the Pb,Zn,and Sb mining and smelting activities have discharged large amountsof heavymetals such as As,Cd,Pb,Sb,and Zn into local rivers.Moreover,the Longjiang River experienced a serious Cd pollution emergency event on January 13,2012.Thisaccidentdischarged a large amount of wastewater(containing 30D40 t Cd)into the river without any treatment from an illegal plant using Cd-rich ?ue ash to reTne indium.Consequentmonitoring results revealed that themaximum concentrations of Cd and As in local river waters were up to 0.41 and 0.31 mg L-1,respectively(Zhang et al.2013).To prevent diffusion of the metal contam inants,a thousand tons of lime,caustic soda,and poly-alum inum chloride(PAC)were poured into the river to elevate the precipitation efTciency.Finally,it was estimated that about 18 t Cd was precipitated in the riverbed through?occulation-precipitation(Zhang et al.2013).To date,little isknown aboutheavymetal pollution in the dammed Longjiang River,which may affect the drinking water supply form illions of people in the Pearl River basin.

The present study aimed to explore the distribution,fractionation,and sourcesofheavymetalsin dammed-river sediments,and to gain new insight into associated environmental risk.Six sediment coreswere sampled from six reservoirs along the Longjiang River to investigate the contam ination of heavymetals(As,Cd,Pb,Sb,and Zn)at a catchment scale.The evolution of heavy metals was reconstructed in one representative sediment core using210Pb chronology.Sequential extraction experiments were performed to determ ine the geochem ical fractions of heavy metals,allow ing assessment of the geochemical fractionation and mobility of heavy metals.Enrichment factors(EFs)were calculated to assess the contam ination degreeof heavy metals.Correlation analysis(CA)and speciTc elemental ratios were employed to identify the potential sources of heavy metals.This study provides novel data sets that could be used in an alliance of heavy metal-contam inated dammed-river sediments for betterwater quality managementincatchmentsw ithcascadereservoir constructions.

2 M aterials and methods

2.1 Study area

The Longjiang River is 367 km long,w ith a drainage area of 16,878 km2.Itbelongs to the South Asia to Central Asia tropical monsoon area.The annual runoff of the mainstream is 12.2×109m3,and the volume in the?ood season(from April to September)accounts for 85.4%(CERLAC 2013).The Carboniferous and Permian carbonates geologically dominate w ithin the catchment and some metamorphic and magmatic rocks expose in the north.Sixcascadehydropower stations(reservoirs),including Bagong(BG)upstream,Lalang(LL)m idstream,and Yemao(YM),Luodong(LD),Sancha(SC),and Noumitan(NMT)downstream were constructed along the Longjiang River in 1976,1971,1997,1971,1995 and 1979,respectively.The total reservoir capacity of BG,LL,YM,LD,and NMT is 8,102,107,62,and 62×107m3,respectively,butno valueswere available in SC due to its no regulating capacity(Zhang 2002).Three main tributaries of the Longjiang River are the Dahuanjiang River,the Xiaohuanjiang River,and the Dongxiaojiang River,whichannuallycontributerunoffsat2.16×109,2.31×109and 0.76×109m3,respectively.

The local land is mainly covered by forest and grass within the catchment(Fig.S1).The concentrated urbanization is located at HechiCity and Yizhou City(Fig.S1).Along the mainstream course,the PbDSbDZn smelters(Fig.1),are locatedm idstream and downstream.Along the three main tributaries,the urbanization is generally undeveloped(Fig.S1).However,the active m ining activities upstream near the Dongxiaojiang River(Fig.1),have resulted in increase of heavy metals into local river sediments(Lan et al.2018).

2.2 Sam pling and analysis

Six sediment cores were collected using a gravity-type PVC coring pipe(100 cm long and 5 cm in diameter)in July 2015 in the BG,LL,YM,LD,SC,and NMT reservoir,respectively.The length of the sediment cores of BG,LL,YM,LD,SC,and NMTwas34 cm,58 cm,54 cm,54 cm,30 cm,and 30 cm,respectively.Each sediment core was sliced into 2 cm layersand then stored in plastic bags.The sampleswere then transferred to the laboratory and stored at 3D5°C until the samples were oven-dried(～50°C)and ground by agate(＜74μm).

Before being ground,each subsample(200mg)was treated w ith 10%HCland 10%H2O2to remove carbonates and organic matters.A laser size analyzer(MALVERN,APA-2000,UK)was employed to analyze grain size.The grain size pattern of sediment samples was categorized using the system of Shepard(1954).Them ineral compositions of selected subsamples were determined by X-ray diffraction(XRD,Empyrean,Netherlands).

Due to its long impoundment history,the LL core was selectedfor chronological determ ination.210Pbwas obtained by gamma spectrometry(GX6020,DSA-1000,USA)at the State Key Laboratory of Environmental Geochemistry,Institute of Geochem istry,Chinese Academy of Sciences.About 10 g samples wereTlled into plastic containers and sealed for a month allowing226Ra and210Pb to reach radioactive equilibrium.The test duration of each sample was 24 h.The radioactivity level of210Pbwas determ ined by gamma em ission at46.5 keV,and226Rawas determ ined w ith 351.9 keVγ-raysem itted by its daughter nuclide214Pb.The standardmaterials from National Institute of Metrology,China,were used to calibrate the absolute efTciency of the detectors.Supported210Pb in each sample was assumed to be in equilibrium with the in-situ226Ra,and excess210Pb activities were determ ined from the difference between the total210Pb andsupported210Pb activities.The measurement errors of210Pb and226Ra were all better than 15%.

Fig.1 Study area and sampling sites

Theanalysis formajorelements(A l,Ca,Fe,Mn,and Ti)and heavymetals(As,Cd,Pb,Sb,and Zn)was done using ICP-AES(Varian VISTA,U.S.A.)and ICP-MS(Agilent 7700x,U.S.A.),respectively,after digestion by HNO3-HClO4-HF-HCl.The detected limitsof Ca,Al,Fe,Mn,Ti,As,Cd,Pb,Sb,and Zn were 0.01%,0.01%,0.01%,5mg kg-1,0.005%,0.2mg kg-1,0.02mg kg-1,0.5mg kg-1,0.05mg kg-1and 2mg kg-1,respectively.The relative deviation wasbetter than±10%for duplicate samples,andtherecoveriesof standardreferences(GBM 398-4c,GBM 908-10,and MRGeo08)were 95%D 110%,for all the elementswe investigated.

A sequential extraction method was applied to determine the geochemical fractions of heavy metals in two sediment cores(LL and NMT).More details about the extraction method have been reported previously(Ahnstrom and Parker 1999).Brie?y,Tve successive fractions from 2 g samples were summarized as follows:F1(soluble-exchange fraction),15 m L 0.1 M Sr(NO)3(treated tw ice);F2(speciTcally sorbed-carbonate bound),30 m L 1.0M NaOAc at pH 5.0(treated once);F3(oxidizable fraction),5m L 5%NaOCl at pH 8.5(treated thrice);F4(reducible fraction),20m L 0.2 M oxalate+0.2 ascorbate(treated thrice);and F5(residual fraction),in which the residueswere digested by HNO3-HF.A 5m L 0.1 M NaCl rinsewasused between each step and pooledw ith the front extract.The recovery rates ofTve heavy metals were approximately 90%D120%.

2.3 Enrichment factors calculation

Enrichment factors(EFs)is a useful indicator for evaluating anthropogenic impacts(Sutherland 2000;Liu et al.2013;Chen etal.2016).EF is deTned as:EF=（Me/M i）sample/（Me/M i）reference

where(Me/M i)is the concentration ratio of targeted element(Me)to normalized elements(M i,usually from natural sources and exhibiting conservative behavior)in the samples and references(uncontam inated background values).In previous studies,Al,Ti,Zr,and Cs have usually been selected as normalized elements(Sutherland 2000;Loska etal.1997;N?Guessan etal.2009;Liu etal.2013).Considering that large amounts of limes and poly-aluminum chloride had been poured into the river during the emergency control for Cd pollution event,Tiwas selected as the normalized element in this study.The geochemical abundance of the Guangxisoil(Ti,As,Cd,Pb,Sb,and Zn at 0.45%,20.5 mg kg-1,0.27mg kg-1,24.0 mg kg-1,2.93 mg kg-1,and75.6 mg kg-1,respectively)were selected as reference background values(CNEMC 1990).EFs at 1.5 is considered a critical value for distinguishing the anthropogenic input and natural input(Wang et al.2015,Wei et al.2016).EFs can be classiTed as:no enrichment(＜1.5),moderate enrichment(1.5D5),signiTcantenrichment(5D20),very high enrichment(20D40),and extreme enrichment(＞40)(Sutherland 2000).

3 Results and discussion

3.1 Physical-chem ical characteristics of sedim ents

The vertical proTles of grain size compositions in the six sediment cores were shown in Fig.S2.Sediments were mainly composed of silt(74%on average)and clay(23%on average).Sand generally accounted for am inor fraction(2%on average),although a higher fraction was present in the 42-cm(73%)and 56-cm(54%)layer of the LL core.Them ineralogy of the selected subsampleswas dom inated by quartz,followed by illite and kaolinite(Fig.S3).Calcite was only present in the subsamples of the BG core.

The vertical proTles ofmajor element(Al,Ca,Fe,Mn,and Ti)concentrations in the six sediment cores are illustrated in Fig.S4.The concentrations ofmajor elements in sediments were sim ilar to their abundances in the upper continental crust,except for Ca,which was remarkable lower than its crustal abundance,ow ing to fact that it is prone to dissolve in water(Taylor and M clennan 1985).Most major elements in the six sediment cores showed sim ilarconcentrationsandgenerallyexhibiteda stable vertical distribution(Fig.S4),but Ca signiTcantly elevated in upstream core(BG)(3.9D8.84%)with high contents of calcites.Moreover,signiTcantly high concentrations of Fe(7.21%)and Mn(0.61%)were also observed at 26-cm layers of YM core,which were probably attributed to discharge of local Mn-smelters.

3.2 Geochem ical distributions of heavy m etals

The concentrations of heavy metals(As,Cd,Pb,Sb,and Zn)in the six sediment cores were listed in Table 1,and their vertical distribution are shown in Fig.S5.The heavy metal concentrations of the six cores(except As in upstream core BG)were higher than the background values of Guangxi soils(CNEMC 1990)and the upper crustal abundances(Taylor and M clennan 1985)(Table 1).They were also higher than those values in the Yangtze River(Yang etal.2014b),the Yellow River(Hu etal.2015),and the Pearl River(Zhang and Wang 2001;Wang etal.2011)(Table 1),suggesting severe heavy metal pollution occurred in the Longjiang River.

Concentrations of heavy metals in the upstream core(BG)were low(Table 1).This isbecause theupstream areaismainly covered by forest and only slightly disturbed by anthropogenic activities(Fig.S1).Nevertheless,heavy metal concentrations drastically increased in m idstream core(LL),indicating increasing anthropogenic inputs of heavy metals.The concentrated urbanization and industrialization in Hechi City,especially nonferrous smelting activities(Lan et al.2018),could be the main reason(Fig.S1).Large amountsof heavymetals(i.e.,As,Cd,Pb,Sb and Zn)would be released into environmentsduring the sulTde processes(Filella etal.2002;Gutie?rrez etal.2016).

Table 1 Heavy metal(As,Cd,Pb,Sb,and Zn)concentrations in the sediment cores of BG,LL,YM,LD,SC,and NMT

In downstream cores(YM,LD,SC,and NMT),the heavy metals(except for Cd)concentrations tended to decrease.The average concentrations of As,Cd,Pb,Sb,and Zn in YM core apparently decreased 1.3,1.8,2.6,3.2,and 2.5 times,lower than those in LL core,respectively.The decrease of heavy metal concentrations was likely caused by the dam interception and the dilution of detrital inputs,which play important roles in heavy metal distributions in river sediments(Wang et al.2015).However,the concentrations of heavy metals(except for As)in LD core slightly increased,which were probably affected by the urban discharge of Yizhou City,and the con?uence of the Dongxiaojiang River where Pb-Sb mining activities contributed to large Sb discharge(Lan etal.2018).Heavy metal concentrations in the SC core apparently decreased but increased in the NMT core subsequently.Thismay be attributed to the land use variation around NMT reservoir which was dom inated by arable land(Fig.S1).Theagriculturalapplications for fertilizersand pesticides could also be responsible for the elevated heavy metals in river sediments(N?Guessan et al.2009).

The average Cd concentrations increased in downstream cores and the highest average Cd concentrations occurred in the NMT core(Table 1),whichmay indicate additional sources of Cd,rather than other heavy metals.Zn and Cd belong to the IIB group of the periodic table.Theoretically,Zn and Cd should be correlated if they are from the same sources,due to the close chem ical behavior of these two metals(Gutie?rrez etal.2016).This is really the case for the six core sediments collected from the Longjiang River,as shown in Fig.2.However,the downstream sediments showed anomaloushigh Cd/Ti ratios,which supported our hypothesis that additional Cd sources could occur at downstream.During the Cd pollution event in 2012,these four downstream reservoirs have been targeted for emergent cleanup for industrially discharged Cd,and consequently produced large amounts of Cd settlement in local river sediments.In addition,two layers w ith highest Cd concentrations(also the atypical values)in NMT core also contained high concentrations of Ca(3.07%D3.63%)and As(100D122mg kg-1)(Fig.S5),which was also consistentw ith the facts that thousand tons of limewere applied for Cd precipitation and that high concentrations of As were also detected in water during the pollution event,respectively(Zhang et al.2013).Therefore,theseTndings demonstrated the elevated Cd in downstream cores was mainly ascribed to the Cd pollution event in 2012.

The proTle of excessive210Pb(210Pbex)activity in LL core was shown in Fig.S6.The vertical distribution of210Pbexactivity globally exhibited an exponential decay withmass depth,suggesting thathydraulic sorting did not take place in the LL core.The average sediment rate was estimated at 1.93 cm yr-1in the LL core,based on a constant initial210Pb concentration(CIC)model(OldTeld et al.1978).Therefore,the LL core re?ected a record of sedimentaccumulationthatspannedapproximately 30 years(1985D2015).According to previous literatures,the peak of sand volume at42-cm coincided w ith the?ood event in 1994(Wei and Gui 2004),and the peak of sand volume at56-cm coincided with the reconsolidation of the dam during 1985D1987(CERLAC 2013),respectively.These two events may have increased the frequency of hydrological cycles in the reservoir,and thusmore detrital inputs were generated from the riparian zone.Therefore,the two events perfectly matched to dating results,validating the CICmodel.

Two stages could be discerned based on the distribution of heavy metal concentrations in dating cores(Fig.3A).TheTrst stage was from 1985 to 1999(corresponding to 58D32 cm),and heavy metal concentrations were extremely high,w ith average value of 118,21.1,503,81.1,and 1305mg kg-1for As,Cd,Pb,Sb,and Zn,respectively.In this stage,China hasexperienced a rapid econom ic grow th that produced severe environmental problems(Xue and Zeng 2010).Sim ilarly,Guangxi Province has experienced a rapid economic grow th(Wang etal.2015;YCCH 2016).Nonferrous metal smelting released a large amount of heavy metals to the rivers(Xu et al.2016).This can be proven by the low percentage of qualiTed industrial discharged wastewater of Guangxi Province at the same period(Fig.3B).Therefore,the high enrichment of heavy metals in the Longjiang River during this stagemight be due to the inadequacy of environmental supervision and low efTciency ofwastewater treatments.This then resulted in large amounts of unqualiTed wastewater being directly discharged into riverw ithout proper treatment in the early stage of the econom ic boom.Similar resultsalso suggested a signiTcantly high enrichment of heavy metals in this stage from other areas in China based on the recorded sediment cores(Bing etal.2011,2016a;Zhao etal.2013;Zhang etal.2015).Two distinct decreases occurred in the layers of 42-cm(1994)and 56-cm(1986),both due to dilution by high silty sand fractions.

Fig.2 Relationship between the ratios of Zn/Tiand Cd/Ti in the sediments of BG,LL,YM,LD,SC and NMT core,respectively.The empty circles in downstream cores(YM,LD,SC,and NMT)exhibited different trends due to obviously high Cd inputs from pollution events

Fig.3 A Vertical proTles of heavy metal(As,Cd,Pb,Sb,and Zn)concentrations in the LL core;B Annual percentage of qualiTed industrial wastewater discharged in Guangxi(BSEG 1985D2015);Annual percentage of surface water better than class III in Guangxi(BSEG1997D2015).The classes of water quality are divided according to SEPA(2002).Class III refers to water in the secondgrade protection zone of a centralized drinking water source,Tsh and shrimp overw intering ground,m igration route,aquaculture area,sw imm ing area,etc

The second stagewas from 2000 to 2015(corresponding to 30D2 cm).Heavy metal concentrations signiTcantly decreased and remained stable after 2000 w ith average values of 48.6,8.16,227,24.0,and 774mg kg-1for As,Cd,Pb,Sb,and Zn,respectively.At this stage,the Chinese government introduced the??sustainable development?? policy for theTrst time,and strict controls on environmental pollution were performed by local governments across the country(Dai et al.2007;YCCH 2016).The decrease of heavy metal concentrations in the studied core since 2000 suggested that heavy metal pollution has been effectivelycontrolledduetoproperenvironmental management.The annually qualiTed ratios of industrial wastewater and surface water quality in Guangxi province were also signiTcantly improved since 2000(Fig.3B).Sim ilar studies also indicated decreased heavy metal pollution after2000 based on the recorded sedimentcores(Dai et al.2007;Bing et al.2016a;Liu et al.2017),which suggested that the decrease of heavy metals during this period could be attributed to direct government guidance and supervision,as well as improvement of wastewater treatment.

3.3 Geochem ical fractionation of heavy m etals

Previous studies suggest that it is important to constrain their geochem ical fractionation prior to evaluate the pollution status of heavy metals in sediments,because bulk concentrations areinsufTcient(Wanget al.2015).Sequential extraction experiments are thought to be an effective way to assess the geochem ical fractionation of heavy metals in sediments(Najamuddin et al.2016).The geochem ical fractions of heavy metals in the selected sediment cores(LL and NMT)were shown in Fig.4 and exhibited a similar distribution.

The exchangeable fractions(F1)of heavy metals in sediments were relatively low(lesser than 8.5%).Heavy metals associated w ith F1 includeweakly adsorbedmetals retained on a solid surface by relatively weak electrostatic interaction,and they are considered the most easily released fraction in environments(Filgueiras et al.2002).

Heavy metal associated w ith carbonate bound fractions(F2)usually precipitate or coprecipitate w ith carbonates(Filgueirasetal.2002).The average percentage of As,Cd,Pb Sb,and Zn in F2was7.06,60.9,33.3,10.5,and 18.6%,respectively.Among them,Cd and Pb were dominant in F2.Heavymetalsassociatedw ith F2were considered to be easily transported and transformed,and absorbed by aquatic organisms(Clemens 2006;Li et al.2014),which suggests that Cd and Pb may possess high potential bioavailability in the Longjiang River.

Heavy metal associated w ith oxidizable fractions(F3)are generally considered to be associated w ith organic matters by complexations or bioaccumulation processes(Wang etal.2015).The average percentage of As,Cd,Pb Sb,and Zn in F3 was 2.32,25.6,27.3,2.52,and 27.0%,respectively.Heavy metals associated w ith oxidizable phases are assumed to remain in soils or sediments for longer period,but theymay bemobilized by decomposition processes(Xu 2016).Such processes would frequently occur in the Longjiang River w ith a monsoon climate accompanied by hydrological events during?ood season(from April to September),and this may cause large amounts of sediment move into aerobic water,leadingheavymetals,especially Cd,Pb,and Sb,to be released into the Longjiang River.

Fig.4 Geochem ical fractions of heavy metals(As,Cd,Pb,Sb,and Zn)contents in LL and NMT core.F1,F2,F3,F4,and F5 mean exchangeable,carbonate bound,oxidizable,reducible and residual fraction,respectively

Heavymetal associated w ith reducible fraction(F4)are typically scavenged by Mn oxides,and amorphous and crystalline Fe oxides(Li et al.2014).The average percentages of As,Cd,Pb,Sb,and Zn in this fraction were 80.5,5.49,30.1,70.6,and 32.8%,respectively.As and Sb were dominant in this fraction,followed by Pb and Zn.Sim ilar results have also reported the dom inance of Asand Sb in F4 in other studies,which suggested this could be attributed to the speciTc geochem ical behaviors of As and Sb(Beauchem in etal.2012;Wang etal.2016).Generally,heavy metals associated w ith this fraction are barely soluble under natural conditions,but are thermodynam ically unstable under anoxic circumstances and are attacked by benthic organisms(Filgueiras et al.2002;Wang et al.2015).

Fig.5 Enrichment factors of heavymetals(As,Cd,Pb,Sb,and Zn)and classiTcation in the sediment cores from the Longjiang River.EF Class 1.no enrichment,2.moderate enrichment,3.signiTcant enrichment,4.very high enrichment,5.extreme enrichment

Heavy metal associated w ith residual fractions(F5)are likely to be incorporated in alum ino-silicatem inerals,and they are quite stable and unable to be released into water under natural conditions(Wei et al.2016).The average percentagesof As,Cd,Pb,Sb,and Zn in this fraction were 8.86,5.66,7.16,15.1,and 19.1%,respectively.Compared with themobilized fractions,heavymetalsassociated with F5 were relatively low.Previous studies suggested the elevated heavymetal inmobilized fractionscould bedue to more anthropogenic contributions(Wei et al.2016),indicating that very high amounts of heavy metals were derived from anthropogenic activities in the Longjiang River.

3.4 Assessm ent of heavy m etal pollutions

The calculated EFs and pollution degrees of heavy metals in the six sediment coreswere shown in Fig.5.Themean EFsof heavymetals(except As in BG)in the six sediment coreswere allhigher than 1.5,revealing thatheavymetals mainly originated from anthropogenic inputs.In general,the average EFsofTve heavymetals in six sediment cores variedintheorderofCd(12.5D192)＞Zn(4.0D15.7)≈Pb(2.2D17.4)≈Sb(3.6D20.6)＞As(1.4D4.7).Spatially,the EFs of heavy metals were relatively low in the upstream core(BG),and then drastically increased in m idstream core(LL).Subsequently,most heavymetal EFsdecreased in downstream cores(YM,LD,SC,and NMT).And the pollution degrees upstream were under none to signiTcant,and moderate to extreme midstreamwhile they were still in moderate to extreme downstream.The highest EFs of As(9.5,signiTcant),Pb(36,very high),Sb(71,extreme),and Zn(29,very high)were located on the LL core.However,Cd(1235,extreme)was located on the NMT core.Like their concentrations,the variations of EFs could be explained by low anthropogenic impacts upstream and increasing anthropogenic inputs in themidstream and downstream areas.

EFs of As,Cd,Pb,Sb,and Zn before 2000 were relatively high,w ith the average values of 6.8(signiTcant),94.2(extreme),24.5(very high),32.9(very high),and 20.5(very high),respectively.Therewas,however,a signiTcant decrease after 2000,w ith the average values of As,Cd,Pb,Sb,and Zn of 2.6(moderate),37(very high),11(signiTcant),9.7(signiTcant),and 11(signiTcant),respectively.Despite the clear decrease of EFs after 2000,the pollution degrees and the potentialmobility of heavy metals were still high,which suggestsmore strictly controlled policies on the pointsourcesof heavymetals should be carried out.

Since the bioavailability and toxicity of heavymetals in sediments depend on their chem ical forms and total concentrations(Wei et al.2016),it could be inferred that heavymetalsw ith the high EFsand high labile fractions in sediments have high potential ofmobility and bio-toxicity in aquatic ecosystems.In addition,Cd,whichwas themost enriched in the sediments and mainly occurred in easily released fractions,could pose the highestecological risk to the Longjiang River.

3.5 Source identif ication of heavy metals

ThecorrelationcoefTcientsamongthesedimentary parameters(clay and silt fractions),major elements(Al,Ca,Fe,Mn,and Ti),and heavymetals(As,Cd,Pb,Sb,and Zn)in six sediment cores were listed in Table S1.In the BG core,As,Pb and Sb generally displayed signiTcant positive correlation(p＜0.05)w ith Al and Ti.Al and Ti usually come from natural weathering process,indicated that naturalmaterials are one of the important sources of these heavy metals in the BG reservoirs.Though,Cd was only correlated w ith Zn(p＜0.01).The potential sources of Cd and Znmay originate from sulTdeweathering or the agricultural activities,which are dispersedly distributed upstream(Fig.S1).In the LL core,theTve heavy metals were signiTcantly correlated w ith each other(p＜0.01),and also correlated w ith Fe and Mn(p＜0.05),indicated theymay have a common origin.It iswell known that the wastes from m ining and smelting activities notonly enrich theseheavymetalsbutalso contain high amountsof Feand Mn(Wang et al.2015).In downstream cores,As,Cd,Pb,Sb,Zn,Fe,and Mn were also generally correlated with each other(p＜0.05).However,theTve heavy metals were also correlated w ith lithospheric elements(Aland Ti)in SC core,which suggested the natural detrituswere also one of the important sources of heavy metals in the sedimentsof SC reservoirs,and thus decreased the heavymetal concentrations(Table 1).

In river sediments,the sourcesofheavymetals could not be easily identiTed because the hydrodynam ic conditions accelerated thehomogeneity of the sedimentsand obscured heavy metal source signals(Bing et al.2016b).Previous studies considered that speciTc elemental ratios could further constrain the sources of heavy metals(Bi et al.2006;Bing et al.2016b).Elements w ith sim ilar geochem ical behavior may provide important geochem ical signatures of initial sources.Twogroups of coupleelements,Zn and Cd,As and Sb,both have similar geochem ical behaviors(Filella et al.2002;Gutie?rrez et al.2016),respectively,and their ratios of Zn/Cd versus Sb/As in six sediment coresm ight be able to reveal the original signal of heavy metal endmembers,and further constrain their sources in the six sediment cores.

Fig.6 Zn/Cd versus Sb/As in the sediments of six sediment cores(BG,LL,YM,LD,SC,and NMT)from the Longjiang River.Tributaries(DXJ,XHJ,and DHJ)from Lan et al.2018;SS:soils impacted by smelting dusts near the smelters in Hechi City(Xiang et al.2011);SM:smelting wastewater collected from Hechi City(unpublished data);CWW:citywastewater collected from HechiCity(unpublished data).GXS:background valuesof Guangxisoil in China(CNEMC 1990);UCC:average values of upper continental crust(Taylor and Mclennan 1985).The blue dash line is the linear trend of the subsamples of LL core(r=0.654,p＜0.01)

The resultsof Zn/Cd versus Sb/As in six sedimentcores were shown in Fig.6.Source-speciTcmaterials,including sediments of three tributaries(Lan et al.2018),Guangxi soil in China(CNEMC 1990),UCC(Taylorand M clennan 1985),smelting and city wastewater(unpublished data),and soils impacted by smelting dust in Hechi City(Xiang et al.2011)were also shown in Fig.6.ThisTgure gives a broad picture on what themain heavy metal sourceswere presented in the Longjiang River.In the BG core,a large variation of Sb/As ratiosmay be affected by Pb-Sbm ining activities(Fig.1).Nonetheless,the plots of Zn/Cd versus Sb/As for LL core distributed around the linear trend betweennaturalendmembersandsmeltingwastes(p＜0.01)indicates the m ixing model of these two sources.This result further suggested smelting activities could be themain contributors to the increase of the heavymetal enrichmentsm idstream.

Mostof the plotsof Zn/Cd versus Sb/As in downstream coreswere very close to the linear trend displayed by LL core(Fig.6).This further indicated the heavy metal pollutions derived from midstream were still prominent in downstream reservoirs.Conversely,some values of subsamples of LD core were close to the values of PbDSb m ining impacted sediment of the Dongxiaojiang River(Fig.6),indicating that PbDSb m ining from the Dongxiaojiang River was also one of the sources downstream.Additionally,the plots of Zn/Cd versus Sb/As in downstream cores generally exhibited low ratios of Sb/As and Zn/Cd(Fig.6),whichm ightbe due to the effectof the Cd pollution event in 2012.

4 Conclusions

The Longjiang River has suffered different degrees of heavy metal(As,Cd,Pb,Sb,and Zn)pollution,and Cd pollutionwas themostsevere.Themean EF valuesofTve heavy metals were all higher than 1.5(except As in upstream),revealing the anthropogenic inputs.The low concentrations of heavy metals in the upstream core(BG)indicate the lim ited impact of anthropogenic activities.However,heavy metal concentrations in m idstream core(LL)drastically increased and reached the highest values(except Cd)due to the active smelting activities.In downstream cores(YM,LD,SC and NMT),most heavy metal concentrations clearly decreased due to the effectof dam interception and natural attenuation.However,an additional source of Cd was observed in downstream sediments,which originated from the Cd pollution event in 2012.The heavymetals in upstream corewere under no to moderate pollution degreeswhile they were inmoderate to extreme moderate pollution degrees in m idstream and downstreamcores.Oneturningpoint exists w ithin 1985D2015 for the Longjiang River sedimentary series,which can deTne two stages,prior to 2000 and after 2000.TheTrst stage was the time when Hechi City developed rapidly,and the Longjiang River suffered high enrichment of heavy metals(signiTcant to extreme)due to an inefTcient model of econom ic grow th.The second stage was characterized by the enhancement of environmental regulations and wastewater treatments,and the heavy metal enrichments tended to decrease(moderate to very high).The geochem ical fractionation results indicated that Cd was primarily associated w ith carbonate-bound fractions,As and Sb prevailed in the reducible fraction while Pb and Zn were comparatively distributed in carbonate-bound,oxidizable,reducible and residual fractions,respectively.The high enrichments and non-residual fractions of heavy metals,especially Cd,in sediments posed high ecological risk to aquatic organisms.The anthropogenic sources of heavy metals in the sediments were mainly related to smelting andmining activitiesw ithin the catchment,which could be themain contributors for the increaseof theheavy metals.In addition,although theenvironmental controland natural attenuation effects have sustainably decreased the heavy metal concentrations,the pollution degrees and mobilities of heavy metal remained high in the dammedriver sediments.Therefore,more caution should be paid on the environmental management for river-dammed reservoirs in the Longjiang River.

Acknow ledgem entsThis research was funded the National Natural Science Foundation of China(41473124,41673138).The authors thank Yanlong Zhao from Water Resources Protection Bureau of Pearl River for the assistance ofTeld sampling.We also thank Jake Carpenter from UCLA,USA,for language editing.