The Behavior Analysis And Sourcing Of Heavy Metals During Weathering And Pedogenesis In The Upper Reaches Of The Pearl River

Heavy metal pollution in river basins has always been a major environmental concern.The karst areas in southwest China represented by the Pearl River Basin are studied extensively due to the high ecological ricks of heavy metals in sediment and soil.It was generally believed that human activities such as mining,smelting,and chemical industry contributed to the high background value of heavy metals in this area.Previous studies on the heavy metal pollution in the Pearl River Basin have mainly focused on some river basins that have been severely affected by human activities.However,recent studies have shown that the weathering of parent rocks rich in heavy metals and the special pedogenesis mechanism in karst areas have also greatly contributed to the high background value of heavy metals in the Pearl River Basin.Therefore,in the Pearl River Basin,the effect of heavy metals from geogenic sources on environmental pollution may be underestimated in past studies.In this paper,In order to systematically study the migration and enrichment of heavy metals during natural weathering and human activities,and the effects of different types of bedrock weathering and pedogenesis on heavy metal pollution in the Pearl River Basin.Three types of "mono-lithology" small watersheds(carbonate rock,basalt and mud shale)that are less affected by human activities and the Diaojiang River,which is greatly affected by mining activities,are selected as the research objects in the upper reaches of the Pearl River.We collected samples of water bodies,rocks,riverbed sediments,soils and weathering profiles in the above watersheds systematically,analyzed their main and trace element contents,and Sr / Nd,Pb isotopic compositions.The following conclusions are obtained:(1)Whether it is a small watershed with a natural background or the Diaojiang River that is greatly affected by mining activities,most of the water samples have a low heavy metal content.Therefore,in the upper reaches of the Pearl River Basin,the natural water body is not the main facilitator of metal enrichment.(2)Under the natural background,the type of rocks exposed in the watershed have a significant impact on the migration and enrichment of heavy metals.The transition elements(Cr,Ni,Cu,and Zn)show lost in the soil and sediment of the small basalt watersheds,being similar to the basalt profile.However,Cd,As and Pb which shows lost in the basalt profile soil(? <0)are significantly enriched in the soil and sediment due to the expose of black shales.In the carbonate watershed,most heavy metals are enriched,while Cd,As,and Zn are also significantly enriched in the weathering profile.This enrichment during weathering processes is mainly due to the weathering characteristics of carbonate rocks: the rapid dissolution of carbonate minerals and the formation of clay minerals and Fe-Al oxides.(3)The sediment of Diaojiang River are affected by specific mining activities.Cr and Ni in most sampling sites did not show significant pollution(CF <1),while for Zn,Cd,As,Pb and Cu,most of the samples are highly polluted(CF> 6),and the pollution factors in the reaches flowing through the mine(D2,D3)increased significantly.The Diaojiang River widely covered by carbonate rocks also inherits the enrichment characteristics of heavy metals in the small carbonate watersheds(C1-C4).However,due to the contribution of Nandan black shale,the enrichment of Pb and Cd is much higher than that of the small watershed(C1-C4).(4)According to the element ratio of Zn,Cu,and Cd,the sediment in the Diaojiang River have been greatly affected by mining activities,while the small watersheds in the upper reaches of the Pearl River have not been significantly polluted.Combining the results of Sr / Nd and Pb isotopes,we believe that in the small watershed in the upper reaches of the Pearl River under the natural background,the carbonate rock has the largest proportion of exposed area,and the whole watershed is affected by the heavy metals released by the weathering of the carbonate rock.Due to the secondary enrichment effect of carbonate rock weathering and soil formation,basalt and black shale exposed in the river basin with a high content of heavy metals have also become the potential sources of heavy metals.(5)In small watersheds with a natural background,the content of heavy metals in most soil and sediment are generally higher than the baseline values in China.The \small watersheds as a whole show low-medium ecological risks(150 <RI <600).Among all metal elements,Cd has the highest ecological risk in different small watersheds,and the element As is the second.Due to the impact of mining activities,the Diaojiang river samples have extremely high ecological risk(RI> 600).Among all metal elements,Cd and As have the highest ecological risk factor index due to the high toxicity and the contribution of mining activities.It can be seen that the entire upstream of the Pearl River Basin has been polluted by Cd and As to a certain extent.