Geochemistry of trace metals in the Gironde estuary

Using clean techniques, we measured the dissolved, particulate, and (by cross-flow filtration) colloidal fractions of Cd, Ni, Zn, Cu, Pb, Mn, and Fe in the Gironde, an estuary in southwestern France. The fractions of the particulate riverine metals that are apparently mobilized in the estuary vary from >90% for Cd to less than 2% for Pb. Observed mid-salinity maxima for Cd, Ni, and Zn are well reproduced by a simple steady-state conservation model which accounts for the inorganic complexation of the metals by seawater anions. The concentration profiles of other metals, except Fe, can also be modeled by choosing an appropriate desorbable fraction and maintaining equilibrium between particles and solution. While colloidal iron decreases rapidly at low salinities, the colloidal concentrations of the other metals are quasi-conservative in the estuary. It appears that the colloidal fraction contains both iron oxide particles that separate from the rest by coagulation and organic macromolecules which bind most of the other metals and remain in solution.