Heterogeneous distributions of heavy metals in the soil-groundwater system empowers the knowledge of the pollution migration at a smelting site

Heavy metal contaminations of soil-groundwater systems from smelting activities is a widespread problem affecting >30% of industrial lands worldwide. Contaminants from the soil migrate into the groundwater and spread further into the surrounding area, causing persistent hazards to human health and the environment. In the present study, migration behaviors and patterns of four persistent pollutants—cadmium (Cd), zinc (Zn), lead (Pb) and arsenic (As)—in a soil-groundwater system were investigated through their heterogeneous distribution characteristics. The results show that these heavy metals (HM) migrate in the vadose zone vertically downwards by gravity while in the saturated aquifer, migration patterns were more probably dominated by solute transport due to groundwater flow. In addition, the migration potential of HMs based on their speciation was ranked as Cd > Zn > Pb > As, which equally reflected their associated environmental risks. Correlation analysis suggested that external pollutant input and hydrogeological characteristics are the main factors controlling the migration and transformation of HMs. The original form of HMs in smelting waste slag determines the release rate. Moreover, partitioning of HMs with the exchangeable phase was closely related to the formation, distribution and extent of groundwater pollution. The migration behaviors and patterns of HMs in the soil-groundwater system of a smelting site were confirmed for the first time through field tests, which could provide valuable insights into contaminant transport and potential barrier control interventions at polluted sites.