Abstract
RESUMEN Se han utilizado dos modelos numericos de transporte reactivo para analizar el proce so de dedolomitizacion: reemplazamiento de dolomita por calcita debido a la infiltracion de agua en un estrato dolomitico. Los resultados de sucesivos calculos en diferentes escenarios se han comparado con observaciones preliminares en las «carniolas» desarro ABSTRACT The replacement of dolomite with calcite (dedolomitization) has been analyzed by means of two numerical models of reactive transporto The results of successive calcula tions under different scenarios have been compared with the observations made on the dedolomites developed on the Trias'ic strata from Prades (Tarragona, Spain). The model based on the local equilibrium assumption for water-rock interaction does not predict the development of the porosity associated to the replacement. The model based on kinetic laws for mineral dssolution and precipitation does predict the observed proportions of calcite, dolomite and porosity. The result of modeling under kinetic laws is sensitive to parameters such as the flow velocity, the chemical composition of the recharge water and the reactive surface of the minerals. The replacement and associated porosity is only formed for infiltration flows higher than 100 mm/year. The water has a neutral to slightly alkaline pH, far from equi librium with carbonates and the atmosphere. The calcium concentrations must be one order of magnitude higher than the average of surficial waters, probably due to sulfate dissolution. The reactive surface 01' dolomite has been estimated from a simple geome tric model of fractures, whereas that of calcite has been inferred from calculations based on nucleation and crystal growth theory. The reactive surface of calcite appears to be
Highlights
The model based on the local equilibrium assumption for water-rock interaction does not predict the development of the porosity associated to the replacement
Con los valores de los parámetros utilizados en los cálculos, y para una concentración de calcio en equilibrio Ceq de 3,6 mol/m-3, el valor de DI para la dolomita es superior a 10, por lo que la disolución de este mineral está controlada por el
El resultado de los cálculos bajo la hipótesis de equilibrio local, muestra que la intrusión de la solución diluida produce la disolución de la dolomita y la precipitación de la calcita en el primer metro de columna
Summary
La ecuación de transporte de un soluto reactivo i (en una dimensión) puede expresarse como: donde U¡, es la concentración de soluto en solución (ML-3); q, es el caudal unitario (el!); D, es un término que engloba la difusión molecular y la dispersión mecánica (UT-!);
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