Heavy Metal Accumulation and Release Risks in Sediments from Groundwater-River Water Interaction Zones in a Contaminated River under Restoration

This study investigated the accumulation and release risks of heavy metals As, Cd, Co, Cr, Cu, Ni, Pb, and Zn in representative contaminated river-bed and river-side sediments from a groundwater-river water interaction zone (GRIZ) currently under restoration. The results revealed different accumulation and release risk behaviors of the heavy metals in the river-bed and river-side sediments that are a complex function of contamination history, local hydrogeological and chemical properties, and metal-specific chemistry. In the river-bed sediments, the concentrations of all the metals first increased and then decreased with depth, reflecting the histories of metal-containing wastewater discharges and sedimentation of metal-containing particles in river water. In the river-side sediments, the concentrations of As, Cd, Co, and Pb were higher in finer grain materials, while those of Cr, Cu, Ni, and Zn were higher in coarser materials, reflecting the combined effects of stronger sorption capacity in finer materials and preferential water flow in coarser materials that carried metals from contaminated river. Speciation analysis indicated that Cd was mainly controlled by surface complexation, Cu was mainly controlled in complexation with organics, and other metals dominantly associated with iron-oxide and structural fractions with a higher structural fraction in the river-side sediments. Risk analysis indicated that release potentials were metal-specific, following an order of Cd > Cu > Ni > Cr > Co > Zn > As > Pb. The results have a strong implication for understanding heavy-metal distribution and accumulation in GRIZ sediments and for the management of contaminated sediments during and after watershed restoration.