The use of fractal theory to study the sources of soil Cd and Pb pollution in Youjiang River Basin

Abstract

Fractal theory can be applied to study the complexity of spatial structures in nature, and operates by extracting the deterministic parameters driving complex processes. However, to date there has been little reported application of fractal theory to the analysis of the sources of soil heavy metal pollution. This study aimed to determine the sources of heavy metal accumulation in the shallow soil of the Youjiang River Basin. A total of 71 shallow soil samples were collected across an area of 1,000 km2 for the analysis of Cd and Pb to determine the spatial distribution of heavy metal pollution in the basin. The fractal characteristics of Cd and Pb for the eastern, central and western parts of the study area were analyzed using a content-frequency fractal model. The results showed that the content-frequency fractal distributions of Cd and Pb in the area were similar and multifractal. Linear models described the three different groupings of the sample points well with a scattering of several high points, indicating a chaotic distribution of high-content Cd and Pb areas. Two linear models similarly could describe the fractal distributions of these two elements in the eastern part of the study area, with a scattering of several high-value points, whereas although the fractal distributions of the central and western parts of the study area also showed linear distributions, the linear fit for the western part of the study area was slightly worse. The fractal distributions indicated that the spatial distributions of soil Cd and Pb show several trends, which indicates the influence of multiple geochemical processes. When combined with regional industrial and mining enterprises and high density population, the fractal Cd and Pb distributions infer that soil Cd and Pb in the area originate from the same sources, and four major geochemical process are responsible for the distribution of Pb and Cd: 1) the soil formation process; 2) modern agricultural production; 3) industrial sources and dense population; 4) aluminum resources development and utilization. Among these, exploitation and utilization of aluminum resources is the most complex, resulting in a chaotic distribution of Cd and Pb.