Modelización de transporte reactivo: aplicación a la dedolomitizacion

Abstract

The replacement of dolomite with calcite (dedolomitization) has been analyzed by means of two numerical models of reactive transports The results of successive calculations under different scenarios have been compared with the observations made on the dedolomites developed on the Triass¡c 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 disolution 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 equilibrium 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 geometric 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 several orders of magnitude lower than that of dolomíte, im agreement with.what is required for reactive transport calculations to generate porosity. The dedolomltization and associated porosity takes place in the first meter of aquifers, whereas downstream the replacement vanishes and does not create porosity.