Objective To analyze the pollution status of heavy metal elements in farmland soil in Lanzhou City from 2018 to 2020, so as to provide scientific evidence for governmental policymaking on farmland soil environment protection and pollution prevention.

Methods Based on the 2018‒2020 rural environmental health monitoring data in Lanzhou City and the farmland around the county under its jurisdiction as the monitoring point, samples were collected through the environmental soil specimen collection method, and the content of five heavy metal elements, namely cadmium, mercury, arsenic, lead and chromium, in soil samples was determined through atomic fluorescence spectrometry (AFS) and/or flame atomic absorption spectrometry (FAAS). In this study, the contamination status of farmland soil was evaluated through the single-factor pollution index (P_i ), Nemero composite index (P_c) and potential ecological risk index (RI) evaluation system. Statistical analyses were performed using Spearman’s correlation analyses and Kruskal-Wallis H-tests.

Results The test results showed that the median mercury content in the 192 soil samples was 0.022 6 mg·kg^-1, which was 1.13 times of the background value of 0.02 mg·kg^-1 in Gansu Province. The contents of the rest four metal elements (cadmium, arsenic, lead and chromium) did not exceed the background value of soil environment in Gansu Province. Over the three years, the P_c of the measured soil metals was 1.53, and the potential ecological RI was 64.71, indicating that the soil was in a light pollution level in general. However, the P_c of elemental mercury in 2020 was 2.23, and the ecological risk coefficient ($E_{\mathrm{r}}^i$) of elemental cadmium in 2018 was 62.82, indicating that mercury pollution reached a medium level and cadmium posed a medium potential ecological risk that year. The statistical test results showed that there was a correlation between the soil heavy metal content and pH in different years (P<0.05), and there was a statistically significant difference for the pollution index of single factor of each heavy metal element in different years (P<0.05).

Conclusion From 2018 to 2020, the soil in the study area in Lanzhou City was low potential risk level as a whole, with mercury and cadmium as the primary potential ecological risk factors for the soil, thus required attention from relevant departments.