Abstract:
Groundwater resources are very important to guarantee sufficient fresh water supply worldwide. To ensure its quality, which is often endangered by natural and anthropogenic hazards, existing risks and possible protection measures need to be adequately evaluated. Mathematical models describing the transport of reactive solutes in groundwater are the key tool for this evaluation. The models help to understand the complex system of coupled physical and biogeochemical processes in the subsurface at different time and spatial scales. The parameterization and operation of such models may raise difficulties as subsurface properties are typically uncertain and computational costs for three-dimensional simulations might be immense. The travel-time based models simplify the description of reactive transport by replacing the spatial coordinates with the groundwater travel time, posing a quasi-one-dimensional (1-D) problem and potentially rendering the determination of multidimensional parameter fields unnecessary. This alternative approach is based on the assumption that the location of stationary reactive fronts i.e. concentration profiles correspond to certain groundwater isochrones which can be truth for ideal conditions that avoid solute and groundwater mixing processes: stationary flow, constant and uniform penetration of the groundwater and dissolved reactant across the entire inlet boundary, and uniform spatial distribution of the biogeochemical parameters within the domain. Travel time is defined as the time that a particle spent to achieve an observation point from the inlet boundary and it is numerically measurable through the seepage velocity field and conservative transport simulation. The stochastic behaviour of the flow field and the diffusive-dispersive transport mechanisms confer random properties to the travel time, which are expressed by local travel time probability density functions, pdf(τ), at each location. The corresponding mean travel time are commonly used as the independent variable in travel-time based models. The main hypothesis of this thesis is that the chemical-compound concentrations as function of time and travel time at each location can be a good approximation to the spatially-explicit concentrations, even when non-ideal conditions may affect the reactive behaviour. To test the validity of this hypothesis, several representative test cases are considered in this work. These test cases are inspired by real-world observations of surface water-groundwater interactions in the hyporheic zone, where dissolved organic carbon, oxygen and nitrate infiltrate into the groundwater and trigger aerobic and anaerobic degradation of organic matter by aerobic and denitrifying bacteria. In a first study, six scenarios are analysed which are differing in the variance of log-hydraulic aquifer conductivity and in the inflow boundary conditions. The results show that the conceptualization of nonlinear bioreactive transport in complex multidimensional domains by quasi 1-D travel-time models is valid for steady-state flow fields if the reactants are introduced over a wide cross-section, and dispersive mixing is adequately parameterized. Results from a second series of test cases focussing on transient time-periodic flow show that a modified version of travel-time based reactive transport models is valid if only the magnitude of the velocity fluctuates, whereas its spatial orientation remains constant. Finally, in a third study, the model is used to simulate reactive transport in geochemical and geophysical heterogeneous porous media, which request the use of the exposure time, equivalent to the time of reaction between two or more reactants, instead of travel time. The results show that the exposure-time models are able to provide good approximation of nonlinear reactive transport problems when transverse mixing is not the controlling process of the reactive system.
Hidrogeology