Elemental geochemistry of the rock-soil system in Wenchuan section of the arid valley, Upper Minjiang River

The geochemical characteristics of elements in surface environments are closely related to their parent bedrock, leading to significant differences in soil compositions derived from various rock types. This study investigated vertical bedrock-soil profiles with three types of parent rocks in Wenchuan section of the arid valley in the upper reaches of Minjiang River. A total of 18 bedrock and soil samples were collected and analyzed for their major and trace element concentrations to characterize their geochemistry and to examine element distribution and migration between bedrock and soil. The results indicate that soils developed from the three bedrock types differ significantly in CaO and Na₂O contents. Elements with higher concentrations in the bedrock generally maintain higher levels in the overlying soil. The average contents of trace elements in soils decrease in the order Mn > Zn > Cu > Se, while the heavy metals follow the sequence Cr > Pb > As > Cd > Hg, consistent with their abundance patterns in the bedrock. Among the heavy metals, only Cd and Pb slightly exceed the risk control standards for agricultural soil pollution. Within the bedrock-soil system, soils derived from granite show depletion of Al₂O₃, K₂O, Na₂O, and SiO₂; soils from phyllite are enriched in CaO and Na₂O but depleted in Zn, Cr, and Pb; and soils from crystalline limestone exhibit enrichment of most elements except for Ca, which undergoes leaching and migration.