Restoration Assessment Of REE Mining In Southern Jiangxi

Rare earth elements(REE)are an important strategic resource for the development of high-tech applications across various industries,especially,in the national aerospace industry,artificial intelligence and military fields.Although exploitation of rare earth(RE)resources can bring economic benefits,it has a serious negative impact on the ecological environment.Since the 1970 s,RE resources have been over-exploited,particularly,in a disorganized way,and long-term open-pit mining and chemical extraction processing of REE have caused serious land degradation,including vegetation destruction,soil erosion and groundwater pollution.The central government of China has attached great importance to the environmental protection and restoration since 2010.Hence,a variety of restoration measures have been taken in the REE mines in southern Jiangxi,with an obvious effect.But various management techniques and measures implemented in different mines seem to have a spatial variability in effectiveness,including both clear and tiny differences.This can be revealed and timely assessed by remote sensing technology,which has been widely applied in environmental monitoring and assessment on a large scale since decades ago.However,available indicators or indices lack wide enough dynamic range and sensitivity to reveal such tiny changes.It is one of the objectives of this research to develop a new indicator to meet this requirement and to conduct a remote sensing(RS)-based monitoring and assessment using multitemporal Landsat data.In order to illustrate the tiny difference in characteristics of spatio-temporal distribution and changes of mining and recovery of the REE mines with accuracy,a new set of remote sensing indicators namely mining and restoration assessment indicators(MRAIs)were developed based on the contrasted features between mining and restoration of various biophysical indicators to achieve this purpose.Meanwhile,soil was sampled in the managed mines for chemical analysis to reveal the change of REE between managed and unmanaged mines.This innovative research by coupling remote sensing assessment and soil analysis was expected to provide technical support for decision-makers for the rational layout of the restoration in the study area.The research and results are demonstrated as follows:(1)Based on the time trajectory and spatial analysis,the vegetation indices NDVI,GDVI,SARVI,ARVI,SAVI,EVI and other non-vegetation indices,such as albedo(α),land surface temperature(LST),and Tasseled Cap Brightness(TCB),which reflect the biophysical characteristics of the surface,were extracted and analyzed.The result shows that the vegetation index,GDVI,has a wider dynamic range and higher sensitivity for such low vegetal mining areas than other indices.In addition,the vegetation indices such as GDVI and NDVI,are negatively correlated with the non-vegetation indices,e.g.,albedo,LST and TCB,and there is a clear mirror contrast of these two groups of indices in their temporal trajectories from mining,mined and to managed states..(2)In view of such contrast and negative correlation between the GDVI and non-vegetation indices,the new MRAIs were composed in such a way that GDVI is the numerator and the sum of non-vegetation indices as the denominator.These MRAIs are of wider dynamic range and higher sensitivity than any other vegetation indices.In the meantime,they have both characteristics of vegetation and non-vegetation,and are capable of highlighting the subtle change and tiny difference of the land surface.This set of new indicators are hence more suitable for mining monitoring and restoration assessment.(3)A differencing-and-thresholding approach was applied to the multitemporal MRAIs of the period 1988-2019 to highlight the changes in the mining and mined areas and the results show that in general land degradation took places at an increasing rate and then a decreasing one.In the period 1988-2000,the degraded area caused by REE mining increased by about 1288.01 ha while it was about 2993.75 ha in 2000-2010,showing a sharp increase.Nevertheless,land degradation from 2010 to 2019 dropped significantly resulted from the market control and thanks to the restoration measure.This indicates that the restoration of the mined areas has been to certain extent succeeded.Merely,from the analysis of six typical mines we got to know that the restoration in vegetation cover in natural way is slow.Thus,the relevant departments still need to strengthen the implementation of artificial reclamation projects in the later period.(4)The lab analysis of the soil samples from the managed REE mines show that REE concentration in the planted soil declined as a result of intake by green plants over time in comparison with the unmanaged mines.But REE% is higher in both managed and unmanaged mines than the background values in southern Jiangxi.Thus,it is necessary to carry out ecological restoration based on the characteristics of different mining areas in the successive management.