Tesis doctoral de Leonardo David Donado Garzón
Transport of non-conservative species or solutes in porous or fractured media is highly influenced by heterogeneity. Additional complexity is added to the processes due to the presence of different types of chemical reactions that control the fate of species concentrations in the medium. Many of these chemical reactions are governed by mixing of waters with different geochemical signature. Mixing yields instantaneous chemical disequilibrium in the resulting mixed water, and reactions take place to re- equilibrate the system. this dissertation studies transport in heterogeneous media covering different problems (flow, conservative transport and reactive transport) and in different aquifer types. First, we analyze flow and transport in low permeable highly fractured massifs. These are studied using the discrete fracture network (dfn) approach, where a dense network of water-conducting intersecting fractures is considered. The dfn approach traditionally has lacked the possibility of analyzing transport (as well as flow) in an inverse problem framework. The actual tracer test, performed with a conservative solute (deuterium), evidences non-fickian behavior, characterized by tailing in the breakthrough curve. as a consequence, transport of conservative solutes in heterogeneous media can be modeled with an effective equation involving a mass transfer term between the mobile and some immobile zones. In the second part of the thesis we explore the possibility of extending this idea to account for transport of reactive species. We start by considering species where local chemical equilibrium conditions are reached instantaneously. The impact of the medium heterogeneity on effective transport is represented by a multi rate mass transfer approach, which models the medium as a multiple continuum of one mobile and multiple immobile regions, which are related by kinetic mass transfer. Even though all regions (mobile and immobile) are assumed to be well mixed (local equilibrium), globally equilibrium is not preserved. The imposition of local equilibrium at all points implies the need for reactions to take place all through the domain, driven by both local dispersion and mass transfer. We derive explicit expressions for the reaction rates in the mobile and immobile regions and study the impact of mass transfer on reactive transport. The reaction rates can change significantly compared with the ones that would be obtained in a homogeneous media. For a broad distribution of residence times in the immobile zones, the system may take much more time to equilibrate globally than for a homogeneous medium. the last topic addressed in this thesis is the analysis of transport of species undergoing non-instantaneous (kinetic) chemical equilibrium. Reactive transport at the local scale is analyzed under two situations: (i) with a single kinetic reaction and (ii) with two simultaneous reactions: one considered instantaneous and the other one being slow related to the transport characteristic time. In the first problem of these problems, we find that the problem can be rewritten only in terms of the initial state of the system plus a non-linear partial differential equation for the reaction rate. the key result is that the equilibrium reaction rate depends on a mixingrelated term, the kinetic reaction rate, which is actually controlling the availability of reactants in the system, and the distribution of (conservative and kinetic) linear combinations of aqueous species concentrations. From an operational standpoint, our expressions allow direct computation of equilibrium reaction rates without the need to calculate aqueous species concentrations. To illustrate the results, the dissolution of calcite in the presence of precipitating gypsum in a one-dimensional fully saturated system is analyzed. The example highlights the highly nonlinear and non monotonic response of the system to the controlling input parameters.
Datos académicos de la tesis doctoral «On multicomponent reactive transports in porous media: from the natural complexity to analitycal solutions.«
- Título de la tesis: On multicomponent reactive transports in porous media: from the natural complexity to analitycal solutions.
- Autor: Leonardo David Donado Garzón
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 05/10/2009
Dirección y tribunal
- Director de la tesis
- Xavier Sánchez Vila
- Tribunal
- Presidente del tribunal: Jesús Carrera ramírez
- Francisco Javier Elorza tenreiro (vocal)
- philippe Gouze (vocal)
- Andrés Alcolea rodríguez (vocal)