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Feasibility of Estimating Cu Contamination in Floodplain Soils using VNIR Spectroscopy—A Case Study in the Le’an River Floodplain, China

Heavy metal accumulation can influence the physical, chemical, and ecological processes in the soil ecosystem, and the accumulation of heavy metals has become a serious environmental issue in China, especially in the floodplains downstream from mining and smelting sites. A novel method of estimating the heavy metal contamination of soil is proposed using visible and near-infrared (VNIR) spectroscopy and partial least squares regression (PLSR). Our study focuses on the Le’an river floodplain, Jiangxi Province, China, which houses the largest copper mining operation in China and has suffered a series of environmental setbacks from the extraction of copper. Our study employs PLSR to summarize the relationship between VNIR reflectance spectra and the copper content of collected soil samples, and then estimates copper contamination of the soil using VNIR spectroscopy and the calibrated model. More specifically, with 71 soil samples collected from the Le’an River floodplain, our study aims at (1) exploring the correlation between VNIR and soil constituents, including soil organic matter, total copper, and iron content; (2) assessing the relationship between VNIR determination of copper and the pre-processing of soil samples; and (3) evaluating the performance of data transformation methods in PLSR. The correlation analysis revealed that the mechanism of estimating Cu content lay in its correlation with Fe content. The PLSR model with logarithmic scale transformed copper content and the standard normal variate spectra was chosen for estimating copper contamination from untreated soil samples; the model with logarithmic scale transformed copper content and reflectance spectra was selected for pretreated soil samples. The correlation analyses and regression results in the PLSR models both suggest that the main mechanism for estimating Cu content in this case study lies in its correlation with Fe content. Therefore, the coupling of VNIR spectroscopy and PLSR could serve as an alternative method of monitoring heavy metal contamination of soil.