Ecological risk assessment of heavy metals in farmland soils around karst coal mining areas: A comparison of various methods

In this paper, we use TCLP method and the methods of Nemerow comprehensive index and geoaccumulation index (Igeo) to evaluate heavy metal pollution in the farmland soils around mining areas in Zhijin County of Guizhou province. The results show that the contents of cobalt (Cd), chromium (Cr), copper (Cu), Nickle (Ni), mercury (Hg) and arsenic (As) in the soil are in the range of 0.79～2.08 mg·kg-1, 144.20～464.40 mg·kg-1,91.63～187.50 mg·kg-1, 48.13～104.47 mg·kg-1, 0.14～2.33 mg·kg-1, and 144.35～265.89 mg·kg-1, respectively. And all the total contents of Cd, Cr, Cu, and Ni in the soil is higher than those of the second -class standard of the national codes on soil environmental quality, while parts of the Hg and zinc contents exceed this standard. The TCLP-Nemerow composite index shows that all sampling points are at safe levels and not polluted by heavy metals. The assessment using Igeo method indicates that 65% of the sample points are at a moderate pollution level, 25% of those are at moderate to severe pollution levels.The potential ecological risk index shows that all the sampling points are at a slight ecological risk level. It can be seen that the analytical results by different evaluation methods differ from each other. Practically, it its hence recommended to choose an appropriate evaluation method based on the pollution degree of the soils.