BACKGROUND 22.3% of the cultivated land in southwest China has excessive heavy metal content. The widely distributed Emeishan basalt and carbonate rocks in the region are considered to be the main source of heavy metals in the soil. At present, the level of research on the ecological risk of soil heavy metals in southwestern China, especially in the Emeishan basalt areas, still needs to be improved. The content, spatial distribution and ecological risk of heavy metals in soils of different geological backgrounds (earthogenic parent rocks) lack comparison.

OBJECTIVES To understand the content and spatial distribution characteristics of heavy metals in the soils from Emeishan basalt and carbonate rock areas.

METHODS Soil samples from the typical geological background areas of Sichuan were collected. AFS, ICP-MS and ICP-OES were used to determine the content of heavy metal elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) and pH values.The content and spatial distribution of the heavy metals in the soils, as well as the ecological risks were studied using the accumulation index and potential ecological risk index methods.

RESULTS The results showed that: (1) The content of Cd, Cr, Cu, Hg, Ni, and Zn in the soils of the basalt areas were generally higher than those of the non-basalt areas and the background values of Sichuan and China. The content of the above elements were respectively 3.25, 1.08, 5.08, 1.72, 1.55, 1.63 times the Sichuan background values and 2.60, 1.40, 6.87, 1.47, 1.87, 1.91 times background values of China. (2) The high content areas of As, Cr and Pb corresponded well to the carbonate rock areas, whereas the high content areas of Cd, Cu, Hg, Ni and Zn corresponded well to the Emeishan basalt areas. The corresponding relationship indicated that the spatial distribution of heavy metals in the soil was mainly determined by the geological background in the studied area. (3) The Geoaccumulation Index results indicated that the soil pollution degree of Cd, Cu, Ni and Zn in Emeishan basalt areas was obviously higher than those of the non-basalt areas; (4) Ecological hazard index results showed that Cd, Cu and Hg were the highest ecological hazard elements in the studied area. The proportions of "strong ecological hazard", "very strong ecological hazard" and "strong estecological hazard" of Cd, Cu and Hg in the basalt areas were respectively 22.4%, 1.15% and 26.0% higher than those in the non-basalt areas.

CONCLUSIONS The content, distribution and ecological risk of heavy metal elements in the soil in the study area are closely related to the geological background. Reasons include the differences in the element content in the parent rock, the geochemical behavior of the elements during the soil formation, and the secondary enrichment of the elements. The pH of the soil in the study area is low, and it is necessary to prevent the risk of heavy metal activation caused by further acidification of the soil.