LONG-TERM FLUXES OF REACTIVE SPECIES IN MACROTIDAL ESTUARIES - ESTIMATES FROM A FULLY TRANSIENT, MULTICOMPONENT REACTION-TRANSPORT MODEL

A coupled, fully transient, multicomponent reaction-transport model ha s been developed to estimate long-term fluxes of reactive compounds in strong tidal estuaries. The model is applied to a preliminary analysi s of the carbon cycle in the Scheldt estuary in Belgium and The Nether lands. The model provides a realistic description of the residual circ ulation in a strong tidal estuary and includes the essential feedback mechanisms between interdependent chemical species. The model has been used to analyze the fundamentally transient nature of strong tidal es tuaries and, in particular, the effect of these non-steady state condi tions on the long-term fluxes of chemical species out of the estuary. The results indicate that flux estimation techniques based upon steady -state assumptions may result in significant errors. The model has als o been used to investigate biogeochemical interactions characterized b y a large spectrum of time scales, which it does by including simultan eous equilibrium reactions and kinetically-mediated processes. Simulat ions carried out with the model suggest that a formulation based upon microbially-mediated, kinetically-controlled reactions provides a supe rior description of solute profiles in the Scheldt estuary than does a global equilibrium redox formulation. The mixed equilibrium-kinetic f ormulation also makes it possible to track simultaneously two master v ariables: the redox state of the system and the pH. By providing stron g constraints on the system, these two master Variables can be used to test the model's self-consistency. The simulations carried out with t he model suggest the pH profile in the Scheldt estuary is the result o f a balance of biogeochemical reactions which produce H+ and degassing which consumes H+ and not the result of simple mixing between seawate r and freshwater. (C) 1997 Elsevier Science B.V.