GEOCHEMISTRY OF TRACE-METALS IN THE GIRONDE ESTUARY

Using clean techniques, we measured the dissolved, particulate, and (b y cross-flow filtration) colloidal fractions of Cd, Ni, Zn, Cu, Pb, Mn , and Fe in the Gironde, an estuary in southwestern France. The fracti ons of the particulate riverine metals that are apparently mobilized i n the estuary vary from >90% for Cd to less than 2% for Pb. Observed m id-salinity maxima for Cd, Ni, and Zn are well reproduced by a simple steady-state conservation model which accounts for the inorganic compl exation of the metals by seawater anions. The concentration profiles o f other metals, except Fe, can also be modeled by choosing an appropri ate 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-conservat ive in the estuary. It appears that the colloidal fraction contains bo th iron oxide particles that separate from the rest by coagulation and organic macromolecules which bind most of the other metals and remain in solution. Copyright (C) 1997 Elsevier Science Ltd.