Particle aggregation, pH changes and metal behaviour during estuarine mixing: review and integration

Abstract

Estuaries are dynamic mixing zones where river water interacts with seawater, resulting in large and complex geochemical changes. How two key factors, particle aggregation and pH, affect metal behaviour in estuaries is reviewed and integrated in this paper. Riverine particles are coated with organic matter and electrostatic repulsive forces restrict aggregation. In estuaries, increased concentrations of divalent cations reduce the repulsive forces between particles at low salinities, resulting in their rapid coagulation and removal of particulate-associated metals (e.g. Fe and Pb). However, truly dissolved metals may mix conservatively, and metals associated more with colloidal and dissolved organic material (e.g. Cu and Zn) can show variable behaviour. In many field studies and modelling of river inputs with different compositions, pH decreases slightly at low salinity. Geochemical model simulations of dissolved metal speciation indicated that Zn would be desorbed from iron oxide binding surfaces due to these pH and cation concentration changes, with Cu also showing less binding to dissolved organic matter (DOM). DOM, pH and particle surfaces can influence individual metal behaviour at various spatial and temporal scales. Further integrated field and laboratory research in estuaries where key geochemical processes affecting metal concentrations are measured and modelled is needed.