Evolution Of Sediment Transport And Ammonia Nitrogen Discharge Of Rare Earth Mining Area In The Tao River Basin Of South Jiangxi Province

Rare earth is an important raw material for the development of modern industry,national defense and science and technology,and the southern Jiangxi Province is the main producing area of ionic rare earth resources in China.In the past 50 years,three kinds of mining technology,i.e.pond leaching,heap leaching and in-situ leaching,have been adopted for rare earth exploitation,which destroy surface vegetation,aggravates soil erosion,and even bares the original woodland.In addition,ammonia nitrogen pollution is caused by using ammonium salt as leaching agent in mining process.Therefore,it is important and urgent to study the evolution of sediment transport and ammonia nitrogen pollution of rare earth mining area.In this paper,rare earth mining area in the Tao river basin of south Jiangxi Province has been selected as the study area.Based on the meteorological and hydrological observation data,a systematic analysis was made to explore the changing trend of precipitation,temperature,evapotranspiration,runoff,and sediment.Moreover,the change of land use/cover(e.g.for rare earth mining)with time was analyzed on the basis of remote sensing data.Employed scenario simulation method of SWAT distributed hydrological model for Tao River Basin,quantitative separation and assessment of the impact of climate change and human activities(rare earth exploitation,water conservancy project construction)on runoff volume and sediment amount were conducted.Combining with field water quality monitoring in typical small watersheds of rare earth mining area,the hydrological model method was used to analyze the effects of two kinds of rare earth mining technology(pond/heap leaching and in-situ leaching)on runoff volume,sediment amount and ammonia nitrogen discharge.Finally,the influence process of in-situ leaching on ammonia nitrogen discharge in groundwater is analyzed by GMS groundwater model.The main results are as follows:(1)The variations of precipitation,temperature,potential evapotranspiration,runoff and sediment in Tao River Basin with time from 1960 to 2015 turned out to be inconsistent,i.e.the trend of annual precipitation is going lightly downward while that of temperature is upward,where as that of potential evapotranspiration is moving downward at first and then upward with a inflection point around 1991.The annual average value of measured runoff volume and sediment amount in Tao River Basin have decreased significantly,with a sudden change in 2002 and 2003,respectively.(2)The analysis of land use type interpretation maps in 1995,2005 and 2015 shows that forest land,rice paddies and agricultural land are the three main land use types,and the change of land use area is not significant.However,the area of type A land for rare earth mining,for which pond leaching or heap leaching technology was adopted showed a fluctuating downward trend,while that of type B land for rare earth mining,for which in-situ leaching technology is adopted,and urban land shows an upward trend,while that of forest land shows a downward one.(3)Using scenario simulation method based on SWAT distributed hydrological model,the impacts of climate change and human activities on runoff volume and sediment amount at the outlet of Tao River basin are quantitatively separated and evaluated.Land use change has a positive effect on runoff volume and sediment amount,while climate change and water conservancy project construction have a negative effect on runoff volume and sediment amount.The construction of water conservancy project is the main factor to reduce runoff volume and sediment amount.(4)Using land use type replacement method based on SWAT model,the contribution rate of main land use types to runoff volume and its sediment and ammonia nitrogen content is determined.For the whole of Tao River Basin,the contribution ability of each land use type to runoff is in the order of rare earth land type A > urban land > rice paddies >agricultural land > pasture land > rare earth land type B > forest land.The contribution ability to sediment content in runoff is in the order of rare earth land type A > agricultural land > rice paddies > pasture land > rare earth land type B > forest land > urban land,this shows that pond leaching or heap leaching is the main cause of soil erosion in rare earth mining area.For the small watershed of abandoned rare earth mine at about 18 years later on the termination date of mining,the contribution ability to ammonia nitrogen content in runoff is in the order of rice paddies > agricultural land > rare earth land type B > urban land >rare earth land type A > pasture land > forest land,this shows that in-situ leaching can cause long-term ammonia nitrogen pollution in water system of mining area.(5)Based on GMS groundwater numerical model,the migration process of ammonia nitrogen pollution in groundwater caused by in-situ leaching is as follows,at the beginning of in-situ leaching,ammonia nitrogen pollutants migrate and diffuse along the downstream river direction with the flow of groundwater.After termination of leaching,the cleaning process can reduce the peak concentration of ammonia nitrogen pollution to a certain extent without any interception effect on the migration and diffusion of pollutants.It is hopeful that the pollution of groundwater caused by in-situ leaching technology will be thoroughly solved by installing a seepage interception device downstream of in-situ leaching site by filler filtration or hydraulic control means.