Distribution, sources and potential ecological risks of soil heavy metals in the main stream of the Yangtze River

This study aims to explore the spatial distribution of soil heavy metals in the Main Stream of the Yangtze River. The contents of As, Cd, Cr, Cu, Pb, Ni and Zn were collected from the 741 topsoil samples. The positive matrix factorization (PMF) model was used to analyze the sources of heavy metals. The geoaccumulation index and potential ecological risk index were selected to assess the pollution and ecological risks of soil heavy metals. The results showed that: 1) The average contents of soil heavy metals As, Cd, Cr, Cu, Ni, Pb, and Zn were 13.50, 0.49, 79.96, 40.85, 34.33, 32.65, and 103.72 mg/kg, respectively. Except for Ni, the maximum content of the rest elements already exceeded the screening value, according to the national standard of soil environmental quality. But the average content of each element was within the screening value. Meanwhile, the accumulation of As, Cd, and Cu was found in the highest exceeding rate and severe pollution. The coefficient of variation of each element ranged from 0.34 to 2.20, where Cd and Cu shared the coefficient of variation higher than 1.0, indicating the uneven distribution and significant influence from local pollution sources. 2) The content of various heavy metals presented a strip-shaped spatial pattern with multiple high-value areas. The soil environmental quality was negatively affected by human activities. There were similar distribution patterns of As and Pb, Cd and Cr, as well as Cu and Zn. The spatial distribution of Cd differed significantly from As element. Specifically, the areas of high-value Cd were in Wuhan and Ezhou, Hubei Province, while the rest were in Anqing and Chizhou, Anhui Province. The content of soil heavy metals ranged from the middle and lower to the higher level in the study area, such as chemical, smelting, printing, and dyeing industries. 3) The natural, industrial emission, transportation, and agricultural production sources contributed the rates of 36.65%, 28.48%, 20.07%, and 14.80%, respectively. Among them, Cd was related to industrial activities, Pb and Zn were from transportation emissions, Cr and Ni were closely related to natural sources, as well as As and Cu were from agricultural production. Overall, 63.35% of soil heavy metals were from human activities. The important sources were chemical, steel, transportation, and agricultural production. 4) The highest degree of Cd pollution was 81.88% of points, indicating a level of mild pollution or above. Furthermore, more than 40% of the sites were contaminated with As, Cu, Ni, and Pb with a moderate level of pollution. More than 75% of the sites were not contaminated with Cr and Zn, where the soil condition was less affected by these two heavy metals. The average of the single ecological risk index was ranked in descending order as follows: Cd>As>Ni>Cu>Pb>Zn>Cr. The average comprehensive ecological risk index was 63.17 at a mild risk level, indicating the spatial distribution pattern. The soil investigation can provide the scientific basis for soil environmental management and heavy metal pollution remediation.