Numerical and experimental methods for stream and wetland modelling

The general aim of my research was the development of innovative numerical and experimental methods for the study of water bodies, in particular wetlands and streams. The use of constructed wetlands (CWs) for waste water treatment is one technique that has shown potential to remove a variety of contaminants including municipal, agricultural, industrial wastewater and storm water. Also, for terrestrial environments and human activities is of primary importance to ensure quality and health safety of rivers and streams. Water quality degradation is mostly caused by the transport and transformation of solutes (pollutants) in rivers. The study of solute transport in wetlands and in rivers appears scientifically significant within a Doctoral Degree in Industrial Engineering since it is related to anthropogenic impacts mainly of industrial origin on the natural environment and on ecosystem services, in particular on surface water bodies and aquatic ecosystems. For these reasons, the improvements of both numerical and experimental methods used for understanding transport phenomena in transitional environments (rivers and wetlands) has a fundamental role for achieving better knowledge on the pollutants removal processes in such zones and thus better management and design of these water bodies. In Chapter 1 a short literature review is presented about: (i) hydrodynamics and removal performance modelling in constructed wetland systems, (ii) conservative and smart tracer techniques and (iii) solute transport modelling in rivers. Then the specific aims of my doctorate research are described. Chapter 2 presents the numerical modelling developed in COMSOL Multiphysics for the study of suspended sediment transport in vegetated wetlands, with different vegetation densities. The removal efficiencies were estimated and compared for the different vegetation densities and grain sizes. Chapter 3 presents the numerical modelling developed combining Telemac2D and Matlab codes for simulating hydrodynamics and solute transport in wetland with randomly generated bathymetries, but characterized by different statistical parameters determining different configurations of the bed forms. The removal efficiencies were then estimated and compared for the different bathymetries. Chapter 4 introduces first activities carried out on numerical and experimental methods for streams and executed with a classical approach at the retention processes study. The numerical model STIR was applied at several conservative tracer datasets, measured for the same reaches in different flow rate conditions. Classical retention parameters, such as diffusion coefficient, exchange rate, mean residence time, were calibrated and compared for the different flow rates. Chapter 5 focuses on the development and application of an innovative numerical tool for the study of reactive and smart tracers. The theoretical basis of STIR-RST software tool is described, in particular about the introduction of parameters representing decay and transformation of the smart tracer and about the chance of choosing if the 2 storage zones are arranged in-series or in-parallel with the main channel. Finally it is shown its application on a smart tracer field test case where Resazurin was used. Chapter 6 reports the experimental study developed for investigating the mass balance closure of the Resazurin-Resorufin (Raz-Rru) system at the cellular scale. In the designed laboratory experiments, the sorption and photodecay of the tracers were minimized and the use of different microbial communities allowed analysing recovery patterns independent of specific microbial species. For each test, total recovery (Raz + Rru) was monitored in the time for evaluating if tracer mass disappeared during the experiments for uptake by cells. A summary of main results and conclusions obtained in this 3-years research is given in Chapter 7. For an easier search of the bibliographic sources used in the text, references are given separately for each chapter and included at the end of the related chapter.