Transport of reactive carriers and contaminants in groundwater systems : a dynamic competitive happening

Transport of contaminants constitutes a potential threat for public health and ecosystems. One of the potential pathways for contaminant transport in groundwater systems is transport adsorbed to carriers (colloidal particles, large molecules). Figure 1 shows a detail of a groundwater system with carriers and contaminants. In this thesis adsorption and transport models for carriers and contaminants are developed and applied in order to increase the understanding of the behavior of carriers and the behavior of contaminants in a system with carriers. Emphasis is put on mobile natural organic matter (NOM), which is a major carrier for contaminants in soil and groundwater systems.Fig. 1The first part of this thesis considers the behavior of contaminants in a system with a constant concentration of carriers. Using a model for coupled carrier and contaminants transport (COLTRAP) the effect of reaction rates and non-linearity on contaminant transport is studied. Varying the reaction rates of carrier-solid matrix and contaminant-carrier reactions, it is shown that a range exists in which time dependent reactions determine the transport of contaminants. Transport processes within this range can only be described and understood using models considering dynamic sorption, like COLTRAP. Transport at and beyond the limits of this range can be calculated with equilibrium models. Simulation of a column experiment under natural conditions shows that time dependent contaminant-carrier and carrier-solid matrix interactions are important under these conditions. In this experiment, the mobility of a radioactive contaminant (americium) is dramatically increased due to the presence of mobile NOM. Both desorption of Am from NOM and interaction between mobile and immobile NOM are found to be time dependent (slow).The second part of this thesis considers the behavior of NOM in more detail. As mobile NOM is a mixture of molecules varying from simple small molecules like citric acid to complicated large molecules like humic acid, its adsorption behavior cannot be fully understood from mono-component adsorption models. The NOMADS model (NOM adsorption) is developed in order to describe the dynamic competitive adsorption/desorption of NOM subcomponents on a homogeneous surface. Furthermore, a method to minimize the number of adaptable parameters is developed. The model allows illustrating and quantifying important features of sorption and transport of a mixture of NOM molecules, like apparent adsorption/desorption hysteresis, slow increasing sorption maxima and large tailing of NOM breakthrough curves. It is shown that experimental results over a large range of temporal and spatial scales and surface to volume ratio's can be successfully described using the same set of model parameters. The behavior of NOM is described in laboratory batch experiments with a few grams of soil and some tens of milliliters of water and also in a field experiment with large travel times, distances of some meters and large volumes of soil.The two main mechanisms found in this research are time dependent sorption reactions and dynamic competitive sorption of a carrier mixture. It is shown that these mechanisms are relevant for contaminant and carrier transport under natural conditions. They may determine main features of contaminant and carrier transport in groundwater systems and therefore also determine potential risks for public health and ecosystems.Keywords : adsorption, desorption, adsorption kinetics, transport processes, contaminant transport, facilitated transport, contaminants, radionuclide, competitive adsorption, polydispersity, natural organic matter, NOM, dissolved organic matter, DOM, DOC, humic substances, colloids, colloidal particles, modeling, simulation, groundwater, porous media, soil