Geochemical Study On Heavy-metal Contamination In Sediments From The Four River Inlets Of Dongting Lake,China

Dongting Lake is one of the most important inland freshwater lakes in China,which collects water drainages from the Xiangjiang,Zijiang,Yuanjiang,and Lishui rivers,which are the so-called“Four Rivers”,and it also collects from the Yangtze River and then drainages water into it.The"Four-water"watersheds of the Dongting Lake are very famous for their abunances of non-ferrous metal mineral resources in the country.The exploitation and utilization of these mineral resources throughout these watersheds have contributed a lot to the economic develoment and social construction of the country,and such mining for mineral ores in the areas also has caused a series of ecoenvironmental issues for the Dongting watershed,among which the heavy metal contamination is one of the most serious environmental problems for the area.Thus the heavy metal contamination in sediment of the watershed has attracted many researchers to investigate from many widespread aspects.As the heavy metal contamination in sediments is actually a geochemical processes that heavy metals are transported,concentrated,and enriched during the surface“source to sink”processes,it is necessary and of greatsignificance to take a geochemical study on it,and this thesis study for a PhD degree aims at make a contribution to this.In this study,the sediment samples were obtained by drilling in inlet areas from the"Four-water"to the Dongting Lake.Mineral concentrations in the sediments were analyzed by X-ray diffraction analyzer?XRD?,grain size were analyzed by Mastersizer-3000,X-ray fluorescence?XRF?,inductively coupled plasma mass spectrometry?ICP-MS?techniques,and Multi-receiving plasma mass spectrometer?MC-ICP-MS?and other technical means to systematically analyze the multi-elemental and Pb isotopic geochemical analysis of the sediment.To analysing the element geochemical composition characteristics of the sediment,and determining the temporal and spatial distribution and evolution characteristics of heavy metal contamination in the sediment,tracing sources of heavy metal comtamination,predictiving future trends in heavy metal contamination.And on the basis of determining the element geochemical characteristics of heavy metal contamination sediments,construct the element geochemical indicators to indicate heavy metal contamination sediments.The grain-size results suggest that composition is similar in the sediment of Xiangjiang,Zijiang,and Yuanjiang Rivers,silt fraction?0.002?0.02mm?have the most with weight percentages of the sediments ranged from 56%to 72.8%,followed by the sand fraction?>0.02mm?with weight percentages of the sediments ranged from 12.6%to31.8%,and clay fraction?<0.002 mm?have the least with weight percentages of the sediments ranged from 13.6%to 14.1%.However,the silt fraction and sand fraction have higher contents,which are 49.05%and 42.19%,respectively,and the proportion of clay fraction are lower?8.77%?in the sediment of Lishui River.It is suggested that the provenance and hydrodynamic environment of the"Four Rivers"sediment into the Dongting Lake may be significantly different.XRD analysis results of 12 sediment samples show that the mineralogical composition of the sediments is very complicated,which generally include the detrital minerals,the clay minerals and the carbonate minerals.The proportion of the detrital mineral is between 33.81?91.37%,which mainly includes:quartz?33.81?67.31%?,potassium feldspar?6.25?21.19%?,plagioclase?1.74?10.22%?,sodium feldspar?2.24?13.83%?,feldspar?0.31?15.22%?.The proportion of the clay mineral is between7.83?66.11%,including:illite?5.29?17.99%?,kaolinite?1.34?6.18%?,oblique green Debris?0.31?18.68%?,muscovite?4.76?54.45%?,hematite?0.09?1.40%?.The proportion of the carbonate mineral is between 0?7.93%,mainly dolomite?1.11?4.99?%)and calcite?0.44?3.37%?.Among them,only the Lishui River contains carbonate minerals,which is significantly different from the other three rivers into the Dongting Lake.The composition of the major elements of the sediment is the basis for understanding the mineral composition,analyzing the source of the sediment and the weathering degree of the rocks in the source area,determining the occurrence characteristics of heavy metals,and exploring the characteristics of sediment heavy metal contamination?such as human-made and natural sources?.The results of major element show that Si O₂,TiO₂,Al₂O₃?K₂O are significantly less variable and Fe₂O₃,MnO,MgO,Ca O,Na₂O,P₂O₅,LOI are significantly variable in the sediments of"Four Rivers"into the lake.The proportion of Al₂O₃,Fe₂O₃ in the sediments of Xiangjiang and Zijiang River are higher than those in the Yuanjiang and Lishui River,and MgO,CaO,Na₂O have a reverse tendency.It is shown that the sediments of Xiangjiang and Zijiang River have similar major elemental composition,and that of Yuanjiang and Lishui sediments have similar major elemental composition.The molar ratios of Fe₂O₃/K₂O,Na₂O/K₂O,SiO₂/Al₂O₃,the chemical weathering index CIA,and WIP show that the mineral stability,chemical maturity,and grain size in the sediments of the Xiangjiang and Zijiang River into the lake which are different from those of theYuanjiang and Lishui Rivers.It is suggest that the source of the sediments,the degree of chemical weathering in the source area,and the hydrodynamic environment formed by the sediments are significantly different.This feature corresponds to the significantly different geological background and the physical geography environment of the“Four Rivers”watershed.The major elements in sediments?except Yuanjiang River?are mostly negatively linearly correlated with SiO₂%?r²>0.45?reflecting that major elements has a significant grain size effect in the sediments of“Four Rivers”.Compared with the upper crust and other reference bodies,the sediments are relatively enriched in element Mn O,LOI,but depleted in Mg O,CaO,Na₂O,but Ti O₂,SiO₂,Al₂O₃,K₂O,P₂O₅ do not show an obvious enrichment and/or a depletion characteristics.The results show that the concentrations of heavy metals Bi,Cd,Cr,Mn,Ni,Cu,Zn,Pb,Tl in the sediments of the Xiangjiang River are significantly higher than the corresponding heavy metals in the sediments of Zijiang,Yuanjiang and Lishui River.There is no significant difference in the concentrations and change trends of trace elements such as Ba,Sc,Th,U,Cs,Ga,Ge,Rb,Zr,Hf,Nb,and rare earth elements in the sediments of"Four River".The enrichment factor calculations?EF_C,EF_D value?of heavy metals and others trace elements show that heavy metals Bi,Cd,Cr,Mn,Ni,Cu,Zn,Pb and Tl are significantly enriched in the sediments of the four river inlets?EF>2.0?,while trace elements Ba,Cs,Sc,Th,U,Ga,Ge,Rb,Nb,Ta,Mo,Sr,Zr,Hf,Y,and REEs are less significantly enriched in the sediments?1.5<EF<2.0?,among them,Sr has a significant loss?EF<0.7?.Judging from the results of principal component analysis?PCA?,enriched heavy metals are contributed from both the natural processes?e.g.,rock weathering?and the anthropogenic activities?e.g.,mining,metal refining,etc.?.Less significantly enriched trace elements are mostly contributed from the natural processes developed in the watershed.The assessment by geo-accumulation index(I_geo)calculation shows that the inlet sediments of Xiangjiang River are contaminated by heavy metals as assemblages of Cd-Bi-Pb-Zn-Mn-Cu-Tl,and the pollution degree reached extreme or moderate contamination level,the inlet sediments of Zijiang River are contaminated by heavy metals as assemblages of Cd-Bi,which has reached a significant contamination level,the inlet sediments of Yuanjiang River are contaminated by heavy metals as assemblages of Cd,which has reached a significant contamination level,the inlet sediments of Lishui River are contaminated by heavy metals as assemblages of Cd-Mn-Cu-Ni-Cr,which has reached significant or moderate contamination level.In general,the inlet sediments of the Xiangjiang and Lishui rivers are most seriously contaminated by many kinds of heavy metals,while those of the Zijiang and Yuanjiang rivers are simple.The strategy for protection from heavy metal contamination for the lake watershed should pay a greatest attention to the contamination of heavy metal Cd in the inlet areas of the four rivers.A comparative analysis of ²¹⁰Pb isotopic dating of sediments shows that there may have been three heavy metal contamination incidents in the basin,namely the early sporadic heavy metal contamination before the liberation?1913?,the medium heavy metal contamination with frequent but low pollution intensity before the reform and opening-up?1979-1963?,the recent heavy metal contamination incident with high intensity and high frequency after the reform and opening-up?1979-present?.Heavy metal contamination in the sediment from the“Four River”inlets of lake may have a direct impact on Dongting Lake sediments.The BCR analysis of the occurrence form of heavy metals in the sediments and the linear correlation analysis of the heavy metal concentrations show that:?i?The heavy metals in the sediments of"Four Rivers"have similar occurrence characteristics,Cd,Mn,Co,Cu,Zn,Pb,are mainly in the extractable form?water-soluble state,exchangeable state,carbonate-bound state+iron-manganese oxide-bound state+organic matter and sulfide-bound state?,and the ratio of their concentration is between 34.74 and 86.47%.Heavy metals Cr,Ni and Tl are mainly in the residue form in the sediment,and the ratio of their concentration is between 52.87 and 92.19%;?ii?The extractable form heavy metals mainly exist in clay minerals?the linear correlation coefficient of concentration and Al₂O₃ is r²>0.305?,Iron and manganese oxides?correlation coefficient is r²>0.494 with Fe₂O₃?,organic matter?linear correlation coefficient is r²>0.242 with LOI?,these heavy metals have strong chemical activity and are easily released into the water due to changes in environmental conditions,which has an impact on the environment,so they are the main targets of heavy metal pollution prevention;?iii?The heavy metals mainly in the form of residue are mainly deposited in feldspar and other aluminosilicate detrital minerals,and their chemical properties in the sediment are stable,not easily activated and released.The Pb isotopic composition of sediments is radiogenic rich thorium Pb.Vertically,the lead isotope composition of the Xiangjiang sediment changes relatively stably,with the Pb concentration is the highest?average 163.8 mg/kg?;the lead isotope composition of the Yuanjiang sediment changes greatly,while the Pb concentration is the lowest?average 22.84 mg/kg?;Zijiang and Lishui sediments are between the corresponding ratio and content of Xiangjiang and Yuanjiang sediments.At the same time,Zijiang,Yuanjiang,and Lishui sediments have similar lead isotope compositions,and their ²⁰⁶Pb/²⁰⁷Pb values vary from 1.1311to 1.2092.Moreover,the lead isotope ratio of sediments in the three rivers has obvious stratification at a depth of about 64cm,that is,the ²⁰⁶Pb/²⁰⁷Pb values of the upper sediments of each river are between 1.131?1.193,and the ²⁰⁶Pb/²⁰⁷Pb values of the lower sediments are 1.141?1.209.The isotopic composition and variation characteristics of the sediments of the above-mentioned Zijiang,Yuanjiang,and Lishui River have the same characteristics in terms of source,process,and influence of human activities.The ²⁰⁶Pb/²⁰⁷Pb values of the Xiangjiang sediments ranged from 1.1789 to 1.1887,indicating a significantly different lead isotope composition from the aforementioned three river sediments.The diagram of lead isotope ratio and lead content(²⁰⁶Pb/²⁰⁷Pb-1/[Pb])shows that the lead in the Xiangjiang sediments contains lead from anthropogenic sources such as lead-zinc ores,coal combustion,and lead from natural processes such as granite weathering.The anthropogenic lead in the sediments of the Zijiang,Yuanjiang,and Lishui Rivers is mainly lead by coal combustion,and its natural source is lead brought by the weathering of sedimentary rocks such as black shale.This corresponds to the geological background and human activities of the"Four Rivers"watershed.Pearson correlation analysis results show heavy metals Bi,Cd,Mn,Co,Cu,and Zn,have the same occurrence characteristics as lead in the sediment and have a significant linear correlation,have similar source as Pb.In this study,based on the screening of heavy metal contamination sediments,and determining the element geochemical characteristics of heavy metal contamination sediments to establish the elemental geochemical indicators for identifying heavy metal polluted sediments.Chemical indices AF?=Al₂O₃+Fe₂O₃+Mn O+P₂O₅+LOI?,SI?=SiO₂+Na₂O?,GI?=SI/AF?,and the Al₂O₃/SiO₂,Fe₂O₃/Si O₂ and Al₂O₃/Na₂O ratios can used identify the heavy-metal contaminated sediments in the study area,and sediments that have values of AF>25.6,SI<67.5,Al₂O₃/SiO₂>0.29,Fe₂O₃/SiO₂>0.12,and Al₂O₃/Na₂O>43.0 can be identified to be the heavy-metal contaminated sediments.