Application of a transport-reaction model to the estimation of biogas fluxes in the Scheldt estuary

In the frame of the BIOGEST project, the fulltransient, one-dimensional, reactive-transportmodel CONTRASTE has been extended for thecomputation of biogases in the Scheldt estuary. The CONTRASTE model (Coupled, Networked, Transport-Reaction Algorithm for Strong T> idal Estuaries) provides a satisfactorydescription of the estuarine residualcirculation (including daily freshwaterdischarge and a complete description of thetide) and a flexible implementation of thevarious physico-chemical and biologicaltransformations, including bothkinetically-controlled and equilibriumreactions. The model allows resolution of thecomplex, nonlinear collective behaviour of thistype of system and investigation of thenon-steady-state phenomena which governestuarine dynamics. Variables currentlyimplemented in the model include salinity,suspended matter, oxygen, inorganic carbonspecies, degradable organic carbon andnitrogen, inorganic nitrogen species,freshwater and marine phytoplankton. Biologicalprocesses described are heterotrophicrespiration, primary production, nitrificationand denitrification. Equilibrium formulationsallow for DIC and NH4+/NH3speciation. Physical processes include gastransfer at the water/air interface, dependingon both wind speed and current velocity. pHprofiles are explicitly computed and constitutea very sensitive check of the overall modelconsistency. Results of the CONTRASTE model arein very good agreement with the measuredlongitudinal distribution of the variablesconsidered, in particular O2, pH,pCO2 and N2O concentrations. However,discrepancies are observed between thecalculated fluxes of CO2 and thoseestimated using an in situ floatingchamber. It is shown that the evaluation of gastransfer can be affected by serious errors ifthe variations due to changes in currentvelocity and water depth during one tidal cycleare not taken into consideration. The modelalso shows that the fluxes of biogases inestuaries are greatly influenced by thequasi-exponential increase of the exchangesurface area with decreasing distance to thesea. Our estimation of the total daily flux ofO2, CO2 and N2O is equal to+28500, −19000 and −17 kmoles.day−1respectively for the Scheldt estuary in July 1996.