RETRASO simula los procesos de transporte (advección, dispersión y difusión) y las reacciones químicas (reacciones ácido-base, redox, complexación, adsorción, intercambio de cationes, precipitación y disolución de minerales). Se puede asumir que las reacciones químicas siguen un enfoque de equilibrio o enfoque cinético. La solución numérica se lleva a cabo usando el implícito global o el enfoque de substitución directa. Esto significa que todas las ecuaciones se resuelven simultáneamente aplicando el método Newton-Raphson.
The program is able to simulate simultaneously groundwater flow, heat transport and multicomponent reactive solute transport under the following conditions:
- 2-D, confined or unconfined, steady-state or transient groundwater flow. General boundary conditions are allowed.
- Transient heat transport considering conduction and convection processes
- Reactive solute transport equations. Reactions included are:
(a) Aqueous complexation, (b) Redox, (c) Surface reactions,(d) Dissolution-precipitation.
Program RETRASO is based on the direct substitution approach (DSA) to solve the coupled hydrological transport processes and chemical reactions. The chemical equations are substitued as a source/sink term in the transport mass-conservation equations which are solved at the same time. Solution of these non linear partial-diferential equations is carried out using Newton-Raphson iterative procedures. After convergence, the chemical source term is updated.
Triangle finite element techniques are used for spatial discretization. RETRASO can cope with problems having irregular boundaries and composed of nonuniform physical and geochemical properties. The code can handle heterogeneous and anisotropic media in two-dimensional horizontal plane for confined or unconfined aquifers, in two-dimensional vertical planes and in three dimensional regions exhibiting axial symmetry for confined aquifers. Both steady state and transient flow regimes can be simulated. Prescribed head, water flux boundary and mixed boundary conditions are included. Both point and areal fluid sources can be specified. Implicit, explicit and general finite difference schemes are used for time discretization.
Transport processes included in the code are: advection, molecular diffusion and mechanical dispersion. Solute transport boundary conditions include: (1) specified solute mass fluxes, (2) specified solute concentrations and (3) solute sources associated to fluid sources. The code can handle the following types of reactions under the local equilibrium assumption: acid-base, aqueous complexation, redox, mineral precipitation or dissolution (including gas), and sorption or desorption reactions. The code also handles mineral precipitation and dissolution reactions under a large set of experimental kinetics laws. RETRASO can take into account no limit number of aqueous and sorbed species, minerals and gases. Heat transport are solved for updating equilibrium constants and the constants of calculating activity coefficients that are the functions of temperature during each time step. In addition to heat flux carried by input water to the system like salute transport, heat flux (source or sink term) from geological source or other heat sources are considered.
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