云南墨江矿区周边山地农田土壤重金属的高光谱反演

    Hyperspectral inversion of soil heavy metals in mountain farmland around Mojiang Mining Areas in Yunnan, China

    • 摘要: 为探究矿区周边山地农田土壤重金属的污染状况,实现在复合污染情境下山地农田土壤中多种重金属含量的高效反演。以云南省墨江县某金矿附近的农田区域为例,获取121个土壤样品实验室高光谱数据和重金属砷(As)、铬(Cr)、铜(Cu)、镍(Ni)的含量数据,构建高精度的高光谱反演模型,实现对不同重金属含量的定量反演。结果表明:1)内梅罗污染指数法显示研究区土壤处于重度污染状态,潜在生态风险指数法显示该区域处于中等生态风险水平。2)一阶微分、二阶微分、标准正态变量以及倒数的对数能有效增强光谱响应,竞争性自适应重加权算法(competitive adaptive reweighted sampling,CARS)的高效波段筛选能力结合迭代保留信息变量(iteratively retains informative variables,IRIV)算法的变量精炼优势,能够实现在土壤重金属反演中的敏感波段选择,该方法在特征波段数量、计算运行时间和模型反演精度方面都比单独的CARS和IRIV方法更有效。3)对比发现反向传播神经网络(back-propagation neural network,BPNN)在As反演中取得最佳反演精度,支持向量机(support vector machine,SVM)为Cr、Cu和Ni的最优反演模型, As、Cr、Cu、Ni最优反演模型的R2分别为0.90、0.93、0.67、0.94,均方根误差(root mean squared error of external validation,RMSE)分别为87.33、142.63、2.63、70.31 mg/kg,相对分析误差(relative percent difference,RPD)分别为3.25、3.84、1.74、4.17。4)重金属的空间分布结果显示,高值区域主要集中在研究区的上下部分,而低值区域则主要分布在边缘,整体呈现从中心向四周逐渐降低的趋势。该研究可为监测矿区附近农田土壤重金属复合污染状况提供参考依据。

       

      Abstract: An accurate and rapid identification of heavy metals (HMs) can be required to prevent environmental pollution and health risks near coal mining sites. However, the multiple contaminants have posed a great challenge to monitoring the heavy metals in agricultural soils. In this study, the hyperspectral inversion technique was developed for the soil heavy metals during pollution assessment and prevention. Spatial distribution maps were then constructed to quantitatively evaluate the heavy metal pollution in mountainous agricultural soils. An agricultural area was taken from the mountain farmland that was severely impacted by mining activities near a gold mine in Mojiang County, Yunnan Province, China. A total of 121 soil samples were collected from the study area. The laboratory hyperspectral data was obtained with the contents of heavy metals (arsenic (As), chromium (Cr), copper (Cu), and nickel (Ni)). The Nemerow pollution index and the potential ecological risk index were employed to assess the heavy metal pollution in the study area. Simultaneously, the CARS-IRIV (competitive adaptive reweighted sampling- iteratively retains informative variables) algorithm was applied to select the sensitive bands for the soil heavy metals. The hyperspectral inversion models were then constructed to estimate the contents of the various heavy metals in the soil. Additionally, the spatial distribution of soil heavy metals was obtained in the study area. The results indicated that: 1) The Nemerow pollution index revealed severe pollution in soil, while the potential ecological risk index suggested a moderate level of ecological risk. The first- and second-order derivatives, standard normal variate, and reciprocal transformations were found to significantly enhance the spectral responses. The efficient band selection of the CARS was combined with the variable refinement of the IRIV. The sensitive bands were successfully selected to explore the spectral response of heavy metals. The better performance of CARS-IRIV was achieved in the number of characteristic bands, computational runtime, and inversion accuracy, compared with the CARS and IRIV only. 3) The back-propagation neural network (BPNN) achieved the highest inversion accuracy for As, while the support vector machine (SVM) was identified as the optimal inversion model for Cr, Cu, and Ni. The R2 values for the optimal inversion models of As, Cr, Cu, and Ni were 0.90, 0.93, 0.67, and 0.94, respectively. The root mean squared errors of external validation (RMSE) were 87.33, 142.63, 2.63, and 70.31 mg/kg, respectively, and the relative percent differences (RPD) were 3.25, 3.84, 1.74, and 4.17, respectively. 4) The spatial analysis indicated that the high-value areas of heavy metals in the study area were primarily concentrated in the upper and lower areas, while the low-value areas were along the edges. There was an overall trend of gradual decrease from the interior to the exterior. The distribution patterns of heavy metal concentration were attributed to the spatial heterogeneity of pollution under mining activities. The finding can provide a strong reference to monitor the heavy metal pollution in agricultural soils near mining areas. The data support can be offered for the monitoring accuracy of soil pollution for the subsequent assessment and remediation.

       

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