A Lagrangian Reactive Transport Simulator with Successive Paths and Stationary-States: Concepts, Implementation and Veri
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A LAGRANGIAN REACTIVE TRANSPORT SIMULATOR WITH SUCCESSIVE PATHS AND STATIONARY-STATES: CONCEPTS, IMPLEMENTATION AND VERIFICATION
R. B. Knapp
L206, Earth Sciences Department, Lawrence Livermore National Laboratory, Livermore, California, U.S.A. 94550
ABSTRACT
A geochemical software package which models static, single-path kinetic water-rock interactions, EQ3/6 , has been modified to incorporate successive-paths and stationary states under high Peclet number transport conditions in a Lagrangian reference frame. These modifications permit calculation of reactive transport with reasonable computational requirements. Results from the new option in EQ3/6 have been compared with analytical results for the simple HCI - SiO2 system; excellent agreements were achieved. Results have also been compared with published results for a portion of the A12 0 3 - HCI - K20 - SiO2 system. The results are in good qualitative and, in some cases, good quantitative agreement. However, the values of some variables differ substantially; these differences can be attributed to use of a different set of Al and Si aqueous species. INTRODUCTION
Emplacement of high-level radioactive waste into the subsurface is expected to cause a thermal perturbation, convective fluid flow and, possibly, the release of radionuclides into the flowing fluid phase. Consequently, fluids may move across isotherms and lithologic boundaries causing a state of overall chemical disequilibrium between the fluid and rock. This will result in kinetic dissolution of minerals initially present in the rock and precipitation of secondary minerals, some of which may contain radionuclides. Thus, there is an intimate coupling among the fluid flow, solute transport and heterogeneous reaction processes; these coupled processes have been called reactive transport. The purpose of reactive transport simulation, within the context of radioactive waste disposal, is to predict aqueous radionuclide concentrations at various times and locations in a system of interest. Two different reference frames can be chosen in any consideration of reactive transport: Lagrangian and Eulerian. A Lagrangian reference frame is one where the evolution of a single hypothetical fluid packet is followed. This is the reference frame selected for this study where we will examine the chemical evolution of successive fluid packets as they move along the same curvilinear trace in space. An Eulerian reference frame is, in contrast, tied to a particular point in the porous medium. During the chemical evolution of an individual fluid packet, the aqueous phase may become saturated with respect to some mineral phases and precipitation can commence. It appears that the locations of these precipitation points (fronts) move very slowly through time; they can be considered stationary in space for significant amounts of time. It is the purpose of this work to be able to calculate the locations of precipitation fronts, the duration of their stationarity and the steady-state aqueous chemistry associated with them. These c
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