Comprehensive Monitoring and Ecological Risk Assessment of Heavy Metals in Soil and Surface Water of Chishui River Basin in Upper Reaches of the Yangtze River

Chishui River is an important ecological security barrier area in the upper reaches of the Yangtze River in China. Therefore, it is of great significance to conduct research on soil and water ecological risks in the Chishui River basin. In this paper, the risk of heavy metals pollution and its control factors was evaluated systematically by using surface water and soil samples from 16 tributaries in the Yunnan section of the Chishui River basin. The method of soil environmental capacity and ecological risk index were studied. The results showed that the average concentration of heavy metals in the surface water of the main stream was in the order of Fe > Mn > Zn > Cu > Pb > Cd > Hg. Except for Hg, all the concentrations of heavy metals were far lower than the Class I water limits in the Environmental Quality Standards for Surface Water (GB3838-2002) issued by the Ministry of Ecology and Environment, PRC. The average concentration of Hg concentration was 0.056 mu g.L-1, which was slightly higher than the limit value of Class II. Heavy metals in the surface water were distributed in a point-like manner in the main stream of the Chishui River, which was mainly affected by mining drainage, township sewage, and human production activities. Meanwhile, the environmental capacity study showed that the heavy metals in soil were in the order of Zn > Pb > Cr > Ni > As > Hg > Cu > Cd, and the environmental capacity were significant differences among different soils: purple soil > limestone soil > loess > yellow-brown soil. Soil Cd tended to migrate out of the soil under the control of the occurrence form, vegetation coverage, and human production activities, while Cr, Cu, and Ni tended to accumulate in the soil. The average comprehensive ecological risk index (RI) of heavy metals in all tributaries ranged from 44.86 to 154.15, mainly distributed in medium and low ecological risk. Therefore, it is recommended to dynamically monitor and control these pollution points in the Chishui River basin to prevent the risk of heavy metals from escalating.