Role of colloids in the discharge of trace elements and rare earth elements from coastal groundwater to the ocean

The concentrations of both the total dissolved (< 0.45 μm) and colloidal (10 kDa-0.45 μm) phases of trace elements and rare earth elements (REEs) in coastal groundwater were measured to evaluate the role of colloids in transporting trace elements and REEs to the ocean through submarine groundwater discharge (SGD). Samples were collected from two different hydrogeological conditions, Hampyeong Bay in the southwestern coast of Korea and Jeju Island, Korea. The average colloidal proportions of Al, Mn, Zn, Ni, Cu, Cd, Pb, 210Po, and 210Pb were 35-45% of the total dissolved phase, which were relatively higher than those of Cr, Fe, Co, and REEs (10-30%), although hydrogeological conditions were variable. Most of the colloidal trace elements (including 210Pb and REEs), except Mn, correlate most significantly with Fe, indicating that their presence is dependent on Fe-containing oxide minerals in the subterranean estuary (STE). On the other hand, the colloidal Mn and 210Po correlated more significantly with dissolved organic carbon (DOC), indicating that dissolved organic matter (DOM) cycling associated with bacterial activity plays an important role in their presence in the colloidal phase in the STE. Our results suggest that the fluxes of trace elements through SGD are greatly dependent on the presence of Fe (for Al, Zn, Ni, Cu, Cd, Pb, and REEs) and DOM (for 210Po and Mn) in coastal groundwater.