NUMERICAL VALIDATION OF A EULERIAN HYDROCHEMICAL CODE USING A 1D MULTISOLUTE MASS-TRANSPORT SYSTEM INVOLVING HETEROGENEOUS KINETICALLY CONTROLLED REACTIONS

It is demonstrated that at steady state, the ID thermo-kinetic hydroch emical Eulerian mass balance equations in pure advective mode are inde ed identical to the governing mass balance equations of a single react ion path (or geochemical) code in open system mode. Thus, both calcula ted reaction paths should be theoretically identical whatever the chem ical complexity of the water-rock system (i.e., multicomponent, multir eaction zones kinetically and equilibrium-controlled). We propose to u se this property to numerically test the thermo-kinetic hydrochemical Eulerian codes and we employ it to verify the algorithm of the 1D fini te difference code KIRMAT. Compared to the other methods to perform su ch numerical tests (i.e., comparisons with analytical, semi-analytical solutions, between two Eulerian hydrochemical codes), the advantage o f this new method is the absence of constraints on the chemical comple xity of the modelled water-rock systems. Moreover, the same thermo-kin etic databases and geochemical functions can be easily and mechanicall y used in both calculations, when the numerical reference comes from t he Eulerian code with no transport terms (u and D = 0) and modify to b e consistent with the definition of the open system mode in geochemica l modelling. The ability of KIRMAT to treat multicomponent pure advect ive transport, subjected to several kinetically equilibrium-controlled dissolution and precipitation reactions, and to track their boundarie s has been successfully verified with the property of interest. The re quired numerical validation of the reference calculations is bypassed in developing the Eulerian code from an already checked single reactio n path code. A forward time-upstream weighting scheme (a mixing cell s cheme) is used in this study. An appropriate choice of grid spacing al lows to calculate within the grid size uncertainty the correct mineral reaction zone boundaries, despite the presence of numerical dispersio n. Its correction enables us to improve the convergence and to extend the numerical test to mixed advective-dispersive mass transport. Howev er, the skewness factor involves numerical oscillations that prevent t o compute different grid spacing. The use of a different chemically co ntrolled time step constraint in both calculations induces some incons istencies into the validation tests. This numerical validation method may be applied as well as to check a thermo-kinetic hydrochemical fini te element based code, from a 1D heterogeneous systems, and 2D-3D syst ems provided that they are designed so as to be 1D equivalent. A one-s tep algorithm and the use of a numerical reference coming from the Eul erian code to be tested ensure the potential success (accuracy) of the numerical validation method. (C) 1998 Elsevier Science B.V.