An evaluation of heavy metal pollution within historic cultural strata at a specialized salt production site at Zhongba in the Three Gorges Reservoir region of the Yangtze River, China

The concentrations of Hg, Cu, Pb, Cr, and Ni in soil samples collected from a specialized salt production site at Zhongba in the Three Gorges Reservoir region of the Yangtze River in China were analyzed to reconstruct the heavy metal contamination contexts of different historic periods over the last 4,500 years. The results show that the observed sequence for individual levels of heavy metal pollution was as follows: Hg > Cu > Ni > Pb ≈ Cr. Hg pollution was high during every time period except the Ming Dynasty, with peaks being observed from the Spring and Autumn period. The pollution of Cu and Ni peaked during the Xi Zhou and Xia Dynasties, respectively. The pollution level of Pb has gradually increased since the Qin Dynasty and has coincided with the use of leaded gasoline. Cr contamination was moderate in all soil strata with little indication of change. Comprehensive heavy metal contamination was high during all of these periods, except during the Ming Dynasty, with peaks being observed between the Spring and Autumn Period and throughout the Warring States Period. Enrichment factors (EFs) were used to obtain information on heavy metal sources. The EFs indicate that most of the Hg and Cu originated from human activities, whereas Pb, Cr, and Ni predominantly came from crust weathering. Several preliminary inferences regarding the development of heavy metal utilization in the area were generated. Cu usage had developed well during the Xi Zhou Dynasty and the Autumn Period. Hg usage emerged during the Xia Dynasty and matured between the Spring and Autumn Period and the Warring States Period. Ni usage conceivably started during the Xia Dynasty. Other factors, such as religious activities, technology, environmental awareness and the intensity of salt production, have also affected heavy metal pollution concentrations.