Elemental Geochemistry Of Bed Sediments From Yuanjiang River Inlet To Dingting Lake,Hunan Province Of China

The famous Dongting Lake is one of the most important inland freshwater lakes in China.The Yuanjiang River is the second longest water system of Dongting Lake watershed.In this study,the sediment samples were obtained by drilling in inlet areas from the Yuanjiang river to the Dongting Lake.Multi-elemental concentrations in the sediments were analyzed by using X-ray fluorescence?XRF?,and inductively coupled plasma mass spectrometry?ICP-MS?techniques.Also,age of a sediment core was dated by measuring the sedimentation rate using ²¹⁰Pb method to sediment samples of the core.The results show that,the sediments are relatively enriched in element TiO₂,but depleted in MgO?CaO?K₂O.Major elemental compositions of the sediments show less spatial variations both from the upper to the lower river and from deep to the surface of a core.It is seen that concentrations of elements Al₂O₃,Fe₂O₃,MnO,K₂O,P₂O₅,and LOI are significantly negatively correlated to those of SiO₂ with r²>0.40.Such correlation becomes more significant for sediments from upper to the lower river,as shown by sediment cores from YN through YJ to YW,suggesting a distince grain size effect for concentrations of these elements which tend to be more enriched in fine-grained sediments.The molar ratios of Fe₂O₃/K₂O,Na₂O/K₂O and SiO₂/Al₂O₃ in the sediments are relatively also less variable,which again suggests that the chemical composition of the sediments is relatively homogeneous.In the plot of A-CN-K,the sediments are mostly concentrated in a region of minor variation that is very closely along the A-CN line,suggesting K-bearing minerals in sediments such as microcline were more stable than the Ca-and Na-bearing minerals such as plagioclase,and albite.The ratios of Na₂O/K₂O in the all sediment core was inversely correlated with the CIA values,and the average values of Na₂O/K₂O at the three sites were significantly higher than that in the upper crust,the average soil and the Yangtze River sediments.All these feature suggest that the sediment maturity is low.The sediment were of mostly detrital origin.The results of heavy metal enrichment coefficient?EF value?showed that the sediments are enriched significantly in Cd,and moderately in Sc,V,Mn,Pb,Bi and other heavy metals.Three distinct layers enriched with different heavy metals were found in the three sediment cores,that is,?1?Pb?Tl?Bi enrichment layer at depth of 192 cm;?2?Pb?Cr?Ni?Cu enrichment layer in upper part of the cores from located bewteen 64 cm to 96 cm;?3?V?Cr?Mn?Ni?Cu?Zn?Pb?Cd?Tl?Bi enrichment layer in upper 64 cm.The index of geoaccumulation?Igeo?and the synthetic enrichment index?EI?shown that,from the sediment bottom to surface,the pollution level of heavy metals tends to increase,and the composite of enriched metals changed from V-Cd to V-Cr-Mn-Ni-Cu-Zn-Pb-Cd-Bi.The accumulation and loss characteristics of other trace elements in sediments were significantly different from those of heavy metals,and the accumulation and loss were not obvious.The EF value of the high field strength element don,t appear obvious enrichment layer,and the EF value of rare earth element has three enrichment layers that is different from heavy metals.These characteristics can be used to demonstrate the influence of human activities on the accumulation of heavy metals.The chondrite normalized REE is characterized by a relative enrichment of light REEs and the relative loss of heavy REEs and curves for all sediments are overall uniform,high LREE/HREE ratios,and had an significant negative Eu anomaly.The sediments of heavy metal with pollution are relatively rich in Al₂O₃,Fe₂O₃,MnO,P₂O₅,LOI,the content of heavy metals was obviously linear correlationand with these the major elements.The sediments of heavy metal without pollution are not rich in Al₂O₃,Fe₂O₃,MnO,P₂O₅,LOI,the content of heavy metals was not obviously linear correlationand with these the major elements.The Pearson correlation analysis and principal component analysis of major elements and heavy metals indicated that Al₂O₃?Fe₂O₃?MnO?P₂O₅?LOI,which represent fine-grained minerals?e.g.,clay minerals?Fe-Mn oxides?apatite?organic matter,etc?,is closely related to heavy metal pollution.This suggests that heavy metals are largely transported and deposited together with the fine-grained minerals.The result of ²¹⁰Pb dating on the YW sediment core by showed that the three heavy metal enrichment/pollution layers,from the sediment bottom to surface,corresponds to?i?1905-1922,before the founding of new China and national industrial development early period;?ii?1959-1979 during which the industrial development period of new China;and?iii?1979 to up to now,the period for China's rapid economic growth.The history of heavy metal accumulation and pollution shows that,with increasing human activities,the sediments in the Yuanjiang River Basin tend to be increasingly contaminated by a complex composite of heavy metals.