Research on the sources and remediation target value of soil cobalt contamination at a decommissioned chemical plant site

With the relocation and industrial upgrading of chemical enterprises, a large number of idle chemical sites are facing soil heavy metal pollution issues. Among these heavy metals, cobalt (Co) has attracted significant attention due to its potential ecological and health risks. This study focused on an idle chemical site in Hebei Province and analyzed the spatial distribution of soil cobalt concentrations and sources based on survey data. Using the relative cumulative frequency method, we determined the upper limit of environmental background value of soil cobalt in the site as the remediation target value. The results showed: 1) The cobalt concentrations in 502 soil samples from the site ranged from 1.02 to 85.5 mg/kg. Among them, 160 samples exceeded the screening value for the first category of land use (20 mg/kg) specified in Soil Environmental Quality - Risk Control Standard for Soil Contamination of Development Land (GB 36600-2018), with an exceedance rate of 31.9% and a maximum exceedance multiple of 3.28. 2) The upper limit of the environmental background value of cobalt in the site soil was determined to be 61 mg/kg using statistical methods, which significantly exceeded the cobalt background value (20 mg/kg) determined based on the soil types (Fluvo-aquic soil and Cinnamon soil) in Appendix A of GB 36600-2018, as well as the regional background value of Hebei Province (32.9 mg/kg). 3) The site-scale upper limit of soil cobalt environmental background (61 mg/kg) used as the remediation target value could significantly reduce the remediation volume compared with that of directly using the screening value (20 mg/kg) as the remediation target value for soil cobalt in the site, saving 2.166×10⁷ yuan in remediation funds. This study can provide a scientific basis for reasonably formulating remediation target values, optimizing risk control strategies, and reducing unnecessary remediation costs in high-background sites, which is in line with the strategic needs of green low-carbon development and sustainable development.