Migration Simulation And Control Of Rare Earth Elements In Red Soil Erosion Watershed Of Southern China

China has the largest reserves of rare earth elements（REEs）resources and is the largest producer and exporter of REEs in the world.The massive exploitation of REEs has seriously damaged the surface vegetation and soil,weakened the anti-erosion ability of the soil,and caused serious soil erosion.Under heavy rain,REEs transported with runoff and sediment may occur in the soil environment.Consequently,a considerable amount of REEs moved downstream with soil erosion,potentially causing harm to the surrounding ecological environment,which has become a new environmental problem after heavy metals and organic pollutants.Changting County of Fujian Province is enriched in REEs reserves with the most abundant and the earliest industrial development in the province.However,it was also one of the most serious soil erosion counties.Based on the foundation theories of soil and water conservation,soil science,botany,and geochemical correlation,the transport and plant resistance control effects of REEs in a small watershed developed on typical granite red soil in southern China（the Zhuxi watershed）were analyzed using model simulation and field experiment.In this paper,a Soil and Water Assessment Tool（SWAT）model and an artificial rainfall experiment were conducted to reveal the REEs transported with runoff and sediment in the watershed.Moreover,the spatial and temporal distribution of key sources of REEs in the watershed were analyzed.At last,based on the conclusions of REEs migration,the control effects on REEs migration were analyzed under different soil and water conservation measures,different soil erosion intensity,and different ecological restoration years.The key findings are included:（1）REEs transported with runoff and sediment in the Zhuxi watershed based on SWAT modelFirstly,build the SWAT model of Youwuzhen,one of the Zhuxi sub-watershed.The simulation results can reach the precision of the model requires.Then the hydrological similarity analysis between the Zhuxi watershed and Youwuzhen small watershed was taken out,which showed the two watersheds were completely similar.Consequently,it can be transplanted the calculated watershed model parameters of the Youwuzhen small watershed into the Zhuxi SWAT model to simulate the average annual runoff and sediment amount of the Zhuxi watershed from 2013 to 2017,which can reach 4.79×10¹⁰L and 1.04×10⁷kg,respectively.Secondly,according to the artificial rainfall experimental,the unit REEs content with runoff and sediment migration were 10.91μg·L^-1and 2.28×10⁵μg·kg^-1,respectively.Most of the REEs are transported by sediment,just only a few are lost by runoff.Hence,soil and water loss significantly affects the horizontal migration of REEs.Finally,it can be concluded that the average annual horizontal migration of REEs with runoff and sediment in the Zhuxi watershed from 2013 to 2017 were 1553.56kg·a^-1and 2367.09kg·a^-1,respectively.（2）Identification of the key source areas and critical periods of REEs distribution in the Zhuxi watershedIt can be known that the maximum spatial distribution areas of runoff and sediment yield were the key source areas of the horizontal migration of REEs according to the SWAT model simulation.Furthermore,according to the REEs spatial distribution by ordinary Kriging interpolation,the key source areas of the heavy REEs（HREE）distribution migrated with runoff were low-lying western lowland,and the key source areas of the light REEs（LREE）distribution were in the middle of the watershed.In terms of time,the critical period of REEs migrated with runoff was mainly from March to August,while migrated with sediment was concentrated in March to June.In a word,the most critical period of REEs migration was mainly from March to June in the Zhuxi watershed which is similar to the annual distribution of precipitation.（3）The sediment yield and control of REEs migration under different measures based on SWAT simulation in the Zhuxi watershedTaking the variation of USLE＿C and USLE＿P parameters into account,the effects of different soil and water conservation measures on sediment reduction capacity were as follows:the whole slope grass>mixture of trees,shrubs and grass>orchard reconstruction>mixture of trees and grass>pure arbor.It can be concluded that the measures of the whole slope grass and the mixture of trees,shrubs,and grass can more effectively improve the vegetation coverage,reduce soil erosion to the full extent,and have a stronger ability to control the horizontal migration of REEs on the surface.In the other hand,considering the impact of LUCC,the average annual sediment reduction rate per unit area reached 3.49 percent and the average annual sediment reduction amount per unit area was 3.54t·ha^-1·a^-1between 2007 and 2017.Consequently,the average annual reduction of REEs migration per unit area was 0.81kg·ha^-1·a^-1in the Zhuxi watershed.We can conclude that reasonable land-use adjustment is a major measure to control the water and soil erosion in the watershed,to prevent REEs migration,and reduce pollution to the downstream environment.（4）REEs migration and control effects in Benggang system of the Zhuxi watershed under different soil erosion intensityThe maximum REEs content of the worst erosion Benggang was in the part of the channel,while the slope without erosion was located at the top of the slope.It was indicated that the migration capacity of REEs to the downstream was enhanced with the erosion intensity of the Benggang.On the other hand,with the restoration of vegetation,the Benggang system became more stable to prevent and control REEs migration.Nevertheless,the average REEs content of Benggang with different erosion intensity was still enriched in REEs.Therefore,further investigations should be conducted on the ecological restoration of the extremely degraded ecosystem to prevent and control the REEs migration downstream.It was also indicated that the exchangeable REEs content in Benggang systems was significantly higher than the non-collapsed slope.In a word,the ecological restoration measure about finding hyper-accumulated plants will be beneficial to prevent and control the horizontal migration of REEs in the degraded ecosystem of Benggang.Consequently,due to the distribution of Dicranopteris dichotoma,one of the REEs accumulation plants,available state REEs content in the rhizosphere soil was higher.It showed that the available REEs migrate from the original soil to the rhizosphere soil,and then to the Dicranopteris dichotoma,to control REEs migration.（5）REEs migration and control effects in sample polts of the Zhuxi watershed under different years of ecological restorationWith the increasing years of ecological restoration,the TREE contents in surface soil gradually increased.It is demonstrated that by increasing vegetation coverage,the condition of soil and water loss has been improved to reduce the surface REEs loss.On the other hand,the REEs enrichment coefficient A_Xhad the trend of firstly increasing and then decreasing,indicating that the increase of REEs in the soil can promote the REEs migrating to the leaves of the Dicranopteris dichotoma in a short time.Moreover,the leaves of the Dicranopteris dichotoma were a priority to the enrichment of LREE in the soil,which indicated that Dicranopteris dichotoma can control REEs migration to a certain extent and may prevent REEs pollution by biological measures.Above all,the characteristics of REEs migrated by runoff and sediment were revealed by the SWAT model and an artificial rainfall experiment.The REEs migration control by different ecological measures,different soil erosion intensity,and different ecological restoration years were analyzed from point to point.Consequently,the results can expand the ecological restoration fields of REEs control and guide the watershed about the ecological management.