Studio sperimentale del trasporto di inquinanti in vasca di filtrazione mediante tracciante fluorescente

The methods and technologies of investigation for determining the state of contamination of an aquifer are largely established and trusted, but we can not say the same about the recovery of the source pollution, a complex inverse problem which, from the theoretical point of view, has found many answers in the literature. All the proposed procedures however, because of their theoretical nature, require validation on experimental data characterized by reliability and completeness. For the matter under consideration, these data are difficult to find; moreover often, these data aren’t accompanied by the necessary information such as hydraulic boundary conditions or dispersion parameters of the aquifer. Hence the need to overcome the lack of such data, trying to build a laboratory device capable to produce them, with the desired characteristics. For this goal, a preliminary physical model was set up, the installation was built at the Laboratory of Hydraulics, Department of Civil and Environmental Engineering (DICATeA) of the University of Parma. It allows to adjust the water flow and to realize basic transport experiments by the injection of tracer. The concentration is detected at any point of the domain by a photographic method. The water velocity was chosen high enough (about 0.1 cm / s), in order to perform the experiments in a reasonable time, highlighting advective-dispersive than diffusive phenomena. The central part of the device consists of a sand-box with transparent Plexiglas walls (inside dimensions of 95x70x10 cm3). It contains a porous matrix and an injector, which allows the input and uniform distribution of the pollutant. As contaminant a solution of sodium fluorescein was adopted. This substance is non-toxic, readily available and easily disposable. A hydraulic circuit guarantees the filtration of water in the porous medium, made by glass spheres with diameters in the range between 0.75 and 1 mm, such material has been adopted because of the inertial proprieties in order to ensure full clean system in reasonable time, at the end of each experiment. The background flow transports the tracer towards downstream and shows the plume at the trasparent wall. The detection of the tracer occurs via image analysis, a fully non-invasive method of investigation. The experimental unit has been gradually improved and integrated with additional devices to control the solute injection, for example the Mariotte vessel to ensure a constant hydraulic head at the injector, also equipped with an inductive-resistive probe level, for a continuous recording of the volume of the transferred tracer. With this instrumentation different experiments have been realized , even in non-uniform field by entering drains or diversion screen in order to modify significantly the path of the tracer. These activities were accompanied by an appropriate numerical modeling; so it is obtained an estimate of the hydraulic and dispersion parameters and, finally, the inverse problem of searching the source has been addressed. At this aim, the methodology of the adjoint state has been applied and, in addition, also a geostatistical methodology has been used to obtain the recovering of the pollutant release history.