Objective  The grading standards of potential ecological risk index is optimized to be more suitable to assess the potential ecological risk of soil heavy metal contamination.

  Methods  According to Håkanson principle, six heavy metals (Cr, Cu, Cd, Ni, Pb and Zn) in the topsoil samples from Chengkou County were selected as research subjects. This study estimated and verified the suitability of optimized grading standards of potential ecological risk index (RI) of soil contamination, based on the toxic-response factor (T_rⁱ) and Nemerow pollution index (P_N).

  Results  ① The potential ecological risk levels of soils contaminated by the 6 heavy metals were suggested: RI < 60, 60 ≤ RI < 120, 120 ≤ RI < 240, 240 ≤ RI < 480 and 480 ≤ RI represent the low, moderate, considerable, high, and very high potential ecological risk, respectively. ② The mean values (arithmetic mean, similarly hereinafter) of the six heavy metals in the study area were 5.0, 0.39, 0.52, 0.44, 0.23, and 0.76 folds higher than the standard value for soil contamination of agricultural land, respectively. ③ The value of P_N was 3.94, indicating that soil heavy metals contamination was severe pollution level (P_N > 3) in the study area. The Cd contamination in soil was severe pollution level P_i (Cd) = 5.25, while other soil heavy metals were no pollution. ④ Potential ecological risk assessment results showed that RI value ranged from 22.9 to 1582 with a mean of 196. Before optimization, soil heavy metals contamination (RI = 196) was the moderate potential ecological risk (pre-optimization classification, 150 ≤ RI < 300). After optimization, soil contamination, however, was the considerable potential ecological risk.

  Conclusions  Case verification indicated the optimized grading standards of the potential ecological risk index could really show the potential ecological risk level of soil heavy metal contamination.