Distribution and migration characteristics of heavy metals around rainfall in soil–water system at closed gold and iron mines in Beijing, China

This study investigated the migration, transformation, and clustering characteristics of trace elements in the abandoned mining areas before and after the rain, using rainfall as a basis. The precipitation-driven factors and the potential migration direction of metals in a highly complex soil-surface water-groundwater system were studied. The local spatial auto-correlation analysis used the Moran index to implement LISA clustering, and to clarify the effect of rainfall on the accumulation and dispersion of metals in different environmental mediums of mining areas. The results show that the spatial clustering of Cd, Pb, Cr, Hg, and As is observed in the topsoil, with mean concentrations of 0.23, 30.07, 236.38, 1.32 and 5.60 mg·kg −1 , respectively. Rainfall significantly increased the concentrations of trace elements in the shallow groundwater, and therefore drove water circulation in the watershed. Moreover, trace element ions are migrated with the watershed water. The rainfall led to the migration of at least 13.37% Pb, 10.44% Cr, 29.55% Hg, and 15.71% As from the topsoil, with some exhibiting spatial clustering or appearing as outliers in surface water and groundwater. Pb tends to migrate laterally into the soil in the study area, whereas Cr and As exhibit the characteristics of vertical migration. Hg shows a propensity for both lateral and vertical migration simultaneously. Consequently, trace elements in the soil and water exhibit strong spatial influences in mining areas, resulting in high spatial heterogeneity. Furthermore, trace elements showed greater susceptibility to vertical migration in gold mining areas, and lateral migration with water recharge in iron mining areas.