An aquifer where receives steady lateral recharge from an AMD-polluted river, was chosen to understand the profound influence of the "solid-liquid phase" evolution on the mobility of trace metals. The results showed that the accumulation of trace metals in groundwater was observed near the riparian zone. Zn and Cd almost remained in the "truly dissolved" phase (<3 kDa), whereas Cu and Pb were mainly in the coarse colloid (0.22 μm-30 kDa) and shifted from the coarse colloid to the "truly dissolved" phase along the flow path. The evolution of relatively high dispersive colloids to low dispersive colloids indicated that the Al-rich mineral colloids as the dominance migrated with the groundwater flow path, and flocculated gradually so that the porous matrix was coated by gibbsite. The coating of gibbsite dissociates OH- in acidic environment, resulting in the decreasing negative charge in porous matrix. Thus, the mobility of Cu and Pb was shifted from the colloid-facilitated transport to the co-precipitation with colloids, whereas the mobility of Zn and Cd was changed from the high electrostatic binding to the low electrostatic binding. Numerical simulation also confirmed the varying retardation factor and the total penetration time followed an order of Pb > Cu > Zn > Cd. Therefore, the changes in "two phases" in the aquifer-not only in water phase but also in colloid phase and porous matrix-have a profound influence on metal migration that take place in shallow groundwater where there are close hydraulic connection between river and groundwater.
Keywords: Colloidal redistribution; Porous matrix; River-groundwater interaction; Trace metal mobility.
Copyright © 2019 Elsevier Ltd. All rights reserved.