Ricostruzione tridimensionale delle caratteristiche idrogeologiche della pianura lombarda, finalizzata all'applicazione di modelli di flusso e di trasporto

The present PhD project concerns the research field of the applied hydrogeology and the study of complex hydrogeological systems, through the integrated use of computer tools, like database, G.I.S. and 3D models. It fits the last decades developed consciousness of protecting and safeguarding the water resource. According to that, Public Authorities promote the sustainable water use on a long-term protection of available water resources and the detailed investigation of the geological and hydrogeological structure of the subsoil, that influence the resource hydrodynamic behaviour. The main goal of the present project is the 3D detailed quantitative reconstruction of the heterogeneous distribution of the quaternary deposits of the Lombardy plain subsoil and the variation of their hydraulic parameters, beginning with the geostatistical interpolation of georeferenced stratigraphic logs. The study area is the Lombardy plain between the Ticino river to the East, the Oglio river to the West and the Po river to the South. The achieved results were used for developing a ground water flow model in the whole study area, realized by the PhD project of Paola Canepa, and for estimating the amount of fresh water, stocked in the shallow aquifer of the Milano and Monza and Brianza provinces. More than 21000 well data, located in the study area, were collected and stored in a specific database (TANGRAM, Bonomi et al., 1995), where about 8400 georeferenced stratigraphic logs were organized, codified and translated into quantitative percentage values. The database is an useful tool, both in the scientific and the management field. The “acquifero tradizionale” (Martinis & Mazzarella, 1971) was investigated and modelled using 2D (Surfer and Arcgis) and 3D (Gocad) modeler softwares: a 3D grid (more than 520000 active cells) was built according to the size and the shape of the topography surface (DTM, Cnr-Irpi) and of three depositional surfaces (A, B and C, Regione Lombardia & Eni-Divisione Agip, 2002), in order to simulate the whole volume of the aquifer and its geological structure. The interpolation of more than 8400 georeferenced stratigraphic logs inside the grid resulted in a detailed and continuous reconstruction of the heterogeneous distribution of textural and hydrogeological characteristics (hydraulic conductivity and effective porosity), that is consistent with the literature data and previous studies: the sediments size decreases from North to South, South-East, according to the decrement of the transport energy of the main rivers in the study area. It also decreases from the top to the bottom of the hydrogeological system, caused by changes in the depositional environment. The high plain is constituted by permeable sediments, that allow the water exchange among different aquifers; whereas the rise in the thickness of the clay and silt layers obstructs the water exchange in the mid and the low plain. The geological structure and the distribution of the hydraulic parameters (hydraulic conductivity and effective porosity) were directly used for the conceptual groundwater flow model. The 3D interpolation of georeferenced stratigraphic logs was also used to rebuild the distribution of the peat in the study area: this element can be a marker for the geomorphologic study of the Lombardy plain development. The present PhD project also suggests a method to estimate how much water is stocked in the subsoil and the size of the volume of potentially available water, in relation to effective aquifer porosity (the measurement of the amount and the size of subsoil pores), and the spatial and temporal water table changes. This study describes the case of the shallow aquifer of the provinces of Milano and Monza and Brianza, from 1979 to 2005. The applied method relates to the detailed reconstruction of the aquifer geological structure and of the heterogeneous distribution of textural and hydrogeological characteristics (hydraulic conductivity and effective porosity), beginning with a geostatistical correlation among a little less than 3500 stratigraphic logs, stored in the hydrological database, Tangram. It concerns the temporal and spatial reconstruction of the level (m a.s.l.) of the water table too. The 3D reconstruction of effective porosity was combined with piezometric surfaces, resulting in the variability of the zone of saturation (the volume of interstitial water). Then, the final product of this processing provided the volume of potentially available water (about 32 billion m3), according to the variability of percentage classes of effective porosity. However, only the water stocked in coarse and medium size sediments (medium and medium-to-high effective porosity, ≥17%) is available to be pumped out of the aquifer: for the two provinces area this represents about 17 billion m3. The amount of available water in different geographic areas of the two provinces was also been estimated, according to their geomorphologic characteristics and their recharge behaviour: outwash, mid and fluvial terraces, morainic walls, piedmont plain, hydromorphic plain as well as the area of the city of Milan. In conclusion in the present thesis appears two main innovative aspects. The first concerns the 3D reconstruction of the heterogeneous distribution of the Lombardy plain sediments, beginning from codifying and elaborating georeferenced stratigraphy logs: a different hydraulic conductivity value was assigned at each cells of the 3D grid. In this way the strict subdivision into aquifers and aquitards with medium hydraulic conductivity value for each of them is avoid, resulting in an improvement in the achievement of groundwater flow and transport models: the more detailed the geological structure is built and the hydraulic parameters are defined, the better the flow pathlines and the polluted plume are simulated. The second innovative aspect concerns the applied method itself: thanks to computer techniques and tools a large amount of data could be simultaneously managed in a time period shorter than using classical methods.