Hydrogen And Oxygen Isotopes And Rare Earth Elements Geochemistry In Typical Cascade Dammed Rivers

The intensive impoundment has dammed the continuous flow of rivers and affected the biogeochemical processes in river water.The Jiulong River,the Mun River,and the Lancang River increase progressively in area,and many dams were constructed in the basin.In this study,river water and suspended particulate matter（SPM）samples were collected from three basins during the wet season.The stable isotopes（H-O isotopes）geochemistry of river water has been studied to investigate water cycling in the catchment,as well as the rare earth elements（REE）of SPM to discuss the controlling factors of its geochemical composition.Especially,the environmental effects of cascade dams on rivers have also been studied by means of H-O isotopes and REE geochemistry.The main conclusions are as follows:Three rivers are different in physical and chemical properties of river water,hydrogeochemical characteristics,and metal element composition of SPM,which also show spatial variation in each river.The chemical weathering degree of the Jiulong River and Mun river is higher than that of the Lancang River.Overall,the cascade damming has made p H decreasing and made total dissolved solids（TDS）homogeneous,but reveals no distinct impact on water temperature.In the Jiulong River,δ²H values are from-52.9‰to-42.8‰,andδ¹⁸O values are from-8.18‰to-6.54‰.In the Mun River,δ²H values are from-93.9‰to-25.4‰,andδ¹⁸O values are from-12.24‰to-2.22‰.In the Lancang River,δ²H values are from-145.2‰to-60.7‰,andδ¹⁸O values are from-18.51‰to-8.49‰.The H-O isotopic composition of three rivers is related to regional precipitation and geographical environment of catchments.The evaporation effect on H-O isotopes is unapparent in the Jiulong River and the Lancang River,but shows significant impact on the Mun River.Precipitation amount is the major controlling factor ofδ²H andδ¹⁸O in Jiulong River water.Spatial distribution of precipitation and land uses mainly controlδ²H andδ¹⁸O in Mun River water.The differences of contribution between glacier melt water and precipitation to river water result in the spatial difference ofδ²H andδ¹⁸O in the Lancang River.Cascade damming results in the homogenization of H-O isotopes in the Jiulong River and the Lancang River during the flood season,while a mixing effect of H-O isotopes is shown in the main stream of Mun River due to extreme rainfall and cascade damming.The order of∑REE content in SPM is as:the Mun River<the Lancang River<the Jiulong River.The∑REE of SPM in Jiulong River are from 110.5 to 1291.5 mg/kg with an average of376.1 mg/kg,those in the Mun River are from 8.5 mg/kg to 192.7 mg/kg with an average of104.9 mg/kg,and those in the Lancang River are from 40.6 to 356.1 mg/kg with an average of171.9 mg/kg.The∑REE of the upper-crust rocks and the SPM in the Yangtze River and World Rivers are lower than the average∑REE value of SPM in the Jiulong River,and higher than that in the Mun River.The average∑REE value of SPM in the Lancang River is similar to the upper-crust rocks,but higher than SPM in World Rivers.The REE contents in SPM are mainly related to the geological background（lithology）in the basin,rather than the weathering degree.The Ce anomaly,（La/Yb）_N,（La/Sm）_N and（Sm/Yb）_N are related to p H,ORP and CIA.Also,the results show that cascade damming has decreased p H of river water that reduces REE contents and/or fractionation in SPM.