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 heavy metals in soil of mountain farmland around Mojiang Mining Area in Yunnan of 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个土壤样品及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面临中等生态风险水平。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−1,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: With the increasing exploitation of mineral resources, the issue of heavy metal pollution in agricultural soils surrounding mining areas has become more severe, and this trend poses significant threats to the quality of the soil, the security of the surrounding ecosystems, and the health of local human populations. Therefore, it is becoming more critical to monitor and assess heavy metal contamination in agricultural soils, especially those near mining areas. This is particularly important in complex, multi-pollutant scenarios where multiple contaminants interact and potentially have compounded adverse effects. Efficient inversion techniques are required to be developed for pollution assessment and prevention. In this study, we focus on an agricultural area near a gold mine located in Mojiang County, Yunnan Province, an area that has been severely impacted by mining activities. A total of 121 soil samples were collected from the study area, and laboratory hyperspectral data along with heavy metal content data (arsenic (As), chromium (Cr), copper (Cu), and nickel (Ni)) were obtained. The Nemerow pollution index method and the potential ecological risk index method were employed to assess and analyze heavy metal pollution in the study area. Simultaneously, the CARS-IRIV method was applied to select sensitive bands for soil heavy metals, and 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 in the study area was analyzed. The results indicated that the Nemerow pollution index method revealed severe pollution in the study area's soil, while the potential ecological risk index suggested a moderate level of ecological risk. First-order and second-order derivatives, standard normal variate, and reciprocal transformations were found to significantly enhance spectral responses. By combining the efficient band selection capability of the CARS algorithm with the variable refinement advantages of the IRIV method, sensitive bands were successfully selected, and the spectral response mechanisms of heavy metals were revealed. This method was demonstrated to be more effective in terms of the number of characteristic bands, computational runtime, and model inversion accuracy when compared to the CARS and IRIV methods used separately. Among the various inversion models, the back-propagation neural network (BPNN) was found to achieve 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−1, respectively, and the relative percent differences (RPD) were 3.25, 3.84, 1.74, and 4.17, respectively. The results of spatial analysis indicated that high-value areas of heavy metals in the study area were primarily concentrated in the upper and lower parts, while low-value areas were distributed along the edges, showing an overall trend of gradual decrease from the interior to the exterior. This result reflects the spatial heterogeneity of pollution impacts caused by mining activities on the surrounding soils and reveals specific distribution patterns of soil heavy metal concentrations. This study combines traditional pollution assessment methods with hyperspectral inversion techniques to quantitatively evaluate heavy metal pollution in mountainous agricultural soils and to construct spatial distribution maps. It provides a reference for monitoring complex heavy metal pollution in agricultural soils near mining areas and offers data support for soil pollution assessment and remediation. This approach is expected to contribute to the improvement of soil pollution monitoring accuracy and is anticipated to provide valuable insights for related research and practical applications.

       

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