Spatial Distribution Characteristics, Source Tracing with Cadmium Isotopes, and a Risk Assessment of Heavy Metals in Soils in the Metal Mining Area of the Qinling Mountains, Central China

Abstract

The Qinling Mountains in central China are rich in polymetallic mineralizations (e.g., Pb-Zn-Au deposits), and historic mining has resulted in large amounts of heavy metals being released into the environment. Thus, it is important to evaluate the ecological risk triggered by historic mining at this location. In this work, a gold mine was investigated as a case study to better understand the health risks present in the Qinling Mountains. To address this issue, a total of 84 soil samples and 23 surface water samples were collected from the gold mining district. Major and trace elements in soils, coupled with the pH and SO4−2 contents in water samples, were analyzed. The multivariate statistical analysis shows that human activities were found to have a significant impact on the levels of heavy metals (e.g., As, Hg, and Cd), pH, and SO4−2 in the soil within the study area, and the coefficient of variation for all indicators falls within a medium to high range (0.20–2.73). The maximum concentration of Cd can reach as high as 3005 mg·kg−1. The spatial distribution of Cd and its isotope signatures indicate that sedimentation ponds and slag piles are the primary sources of high pollution within the study area, with heavy metal concentrations decreasing gradually as they disperse into the surrounding cultivated areas. The present study confirms that Cd and its isotopes can be directly used to track heavy metal sources over a large area (0.8 km2), and the conclusions drawn could be valuable for future restoration and management efforts. Simultaneously, this approach could offer a novel framework for the management of heavy metal contamination in soils at other metal mining sites.