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

AbstractIn 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.

[1]  Donald J. O'Connor,et al.  Mechanism of Reaeration in Natural Streams , 1958 .

[2]  J. Edmond,et al.  On the calculation of the degree of saturation of sea water with respect to calcium carbonate under in situ conditions , 1970 .

[3]  C. Culberson,et al.  MEASUREMENT OF THE APPARENT DISSOCIATION CONSTANTS OF CARBONIC ACID IN SEAWATER AT ATMOSPHERIC PRESSURE1 , 1973 .

[4]  W. Mook,et al.  Chemistry of dissolved inorganic carbon in estuarine and coastal brackish waters , 1975 .

[5]  R. Weiss,et al.  Nitrous oxide solubility in water and seawater , 1980 .

[6]  Trevor Platt,et al.  Photoinhibition of photosynthesis in natural assemblages of marine phytoplankton , 1980 .

[7]  M. Somville.  Use of nitrifying activity measurements for describing the effect of salinity on nitrification in the Scheldt estuary , 1984, Applied and environmental microbiology.

[8]  Bruce B. Benson,et al.  The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere1 , 1984 .

[9]  M. Frankignoulle.  Field measurements of air-sea CO2 exchange , 1988 .

[10]  T. Platt,et al.  Primary production by phytoplankton : analytic solutions for daily rates per unit area of water surface , 1990, Proceedings of the Royal Society of London. Series B: Biological Sciences.

[11]  R. Wanninkhof.  Relationship between wind speed and gas exchange over the ocean , 1992 .

[12]  P. Herman,et al.  MOSES: model of the Scheldt estuary: ecosystem model development under SENECA , 1992 .

[13]  R. Wollast.  The Scheldt Estuary , 1993 .

[14]  S. V. Damme,et al.  De waterkwaliteit van de Zeeschelde: evolutie in de voorbije dertig jaar , 1995 .

[15]  Michael Whitfield,et al.  A chemical model of seawater including dissolved ammonia and the stoichiometric dissociation constant of ammonia in estuarine water and seawater from −2 to 40°C , 1995 .

[16]  R. Wollast,et al.  Atmospheric CO2 fluxes in a highly polluted estuary (the Scheldt) , 1996 .

[17]  C. Steefel,et al.  Long-term fluxes of reactive species in macrotidal estuaries: Estimates from a fully transient, multicomponent reaction-transport model , 1997 .

[18]  P. Régnier,et al.  A discussion of methods for estimating residual fluxes in strong tidal estuaries , 1998 .

[19]  B. Delille,et al.  Carbon dioxide emission from european estuaries , 1998, Science.

[20]  W. Cai,et al.  The chemistry, fluxes, and sources of carbon dioxide in the estuarine waters of the Satilla and Altamaha Rivers, Georgia , 1998 .

[21]  C. Steefel,et al.  A high resolution estimate of the inorganic nitrogen flux from the Scheldt estuary to the coastal North Sea during a nitrogen-limited algal bloom, spring 1995 , 1999 .

[22]  H. Wilde,et al.  Nitrous oxide in the Schelde estuary: production by nitrification and emission to the atmosphere , 2000 .