Potential risk assessment of cadmium pollution using hyperspectral remote sensing
WANG Dan-yu1, WANG Wen1, ZHAO Yan-yun2
1. Center for Spatial Information, School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China; 2. School of Statistics, Renmin University of China, Beijing 100872, China
Abstract:Cadmium (Cd), one of the major heavy metal pollutants in soil, is known to be hazardous and difficult to manage. An understanding of its quantitative spatial distribution characteristics in soil and the development of rapid monitoring methods are very helpful for ecosystem security and human health. This study utilised the Random Forest model to estimate the spatial distribution of potential Cd pollution risk levels in the Yangtze River basin, using soil Cd field data (2002~2015) collected from 10sites along the river basin and HSI hyperspectral remote sensing satellite data. The results showed that: (1) Using atomic standard spectral curve can greatly simplify the process of characteristic bands selection for Cd in soil; (2) Random Forest modelling is a good method for estimating the Cd content in soil at a high accuracy; (3) Cd pollution is widespread in the Yangtze River basin area. The majority of the study areas were found having more than 8% higher Cd in soil than the Chinese Environmental Quality Standard for Cd pollution, with the upstream area more serious than the rest of the basin. The main conclusions are: (1) It is feasible to use the standard spectral curve of Cd atoms for hyperspectral band selection for Cd risks inversion. A more accurate prediction model can be established through the selected characteristic bands. (2) The Random Forest modelling approach for large scale risk monitoring of Cd pollution in soil showed its credibility in most of the sampling areas in the Yangtze River basin except areas in the Qinghai-Tibet Plateau, especially in the Subtropical Monsoon Climate zone. (3) The Random Forest model prediction showed that Cd pollution exceeded the Environmental Quality Standard in all study areas, where most areas had a large portion of land surfaces at high risk levels. Along the river basin, the overall Cd pollution risk and the proportion of areas exceeding the standard in the upstream area is greater than that in the midstream and downstream areas. This finding agreed with the results of other studies. (4) The study showed that the surface Cd pollution in the upstream area is mainly caused by relatively abundant mineral resources and the backward and active of industrial activities, where in the midstream area is mainly due to the rapid industrial and economic development; and in the downstream area is related to agricultural cultivation, high population density and long history of human activities at a high pollution level.
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