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 has been developed to estimate long-term fluxes of reactive compounds in strong tidal estuaries. The model is applied to a preliminary analysis of the carbon cycle in the Scheldt estuary in Belgium and The Netherlands. The model provides a realistic description of the residual circulation 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 estuaries and, in particular, the effect of these non-steady state conditions 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 also been used to investigate biogeochemical interactions characterized by a large spectrum of time scales, which it does by including simultaneous equilibrium reactions and kinetically-mediated processes. Simulations carried out with the model suggest that a formulation based upon microbially-mediated, kinetically-controlled reactions provides a superior description of solute profiles in the Scheldt estuary than does a global equilibrium redox formulation. The mixed equilibrium-kinetic formulation also makes it possible to track simultaneously two master variables: the redox state of the system and the pH. By providing strong constraints on the system, these two master variables can be used to test the modelʹs self-consistency. The simulations carried out with the model suggest the pH profile in the Scheldt estuary is the result of a balance of biogeochemical reactions which produce H+ and degassing which consumes H+ and not the result of simple mixing between seawater and freshwater.