Spatial distribution, ecological risk and origin of soil heavy metals in Laoguanhe watershed of China's South-to-North Water Diversion Middle Route Project

The central section of China's South-to-North Water Diversion Project has been designated as a national water conservation area, and the soil ecological security in its associated watersheds is of great importance. A total of 204 soil samples (0-20 cm depth) were obtained from the Laoguanhe River Basin. The concentrations of seven elements (Cd, Cr, Cu, Ni, Zn, Pb and Hg) were determined using inductively coupled plasma mass spectrometry and atomic fluorescence spectrometry following a near-total acid dissolution. Data analyses (including potential ecological risk, principal component analysis, geostatistical analysis and the positive matrix factorization model) were applied to evaluate the contamination of soil heavy metals and to identify their sources. The research results demonstrated that the mean contents of these seven elements exceeded background values for Henan Province, China, indicating human disturbance. Ecological risk evaluation revealed that Cd was the most frequently detected and the heavy metal causing the most pollution. Principal component analysis indicated that Cr, Ni and Cu stem from natural sources, while Zn and Cd are predominantly influenced by agricultural activities. Additionally, industrial activities and atmospheric deposition were responsible for the excess presence of Pb and Hg. The study suggests taking measures to control Cd sources in agricultural areas, reducing input of heavy metals into the river and providing scientific support for managing water quality.