MESSA A PUNTO DI UNA METODOLOGIA PER LA MODELLAZIONE TRIDIMESIONALE E MULTISCALA DELL'IDROSTRATIGRAFIA, SU BASE GIS
Creators: name: CAVALLI, EMMANUELE
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: Realistic hydrostratigraphic models are necessary to compute the hydrological balance of surface vs. ground waters. In this perspective a key-sector of the Lombardy alluvial plain NE of Milan was selected to study the relations between Adda river and the phreatic nappe. The study focuses on a 10x10 km area close to Cassano d?Adda, that includes the local uppermost alluvial terraces (Middle Pleistocene glacio-fluvial sediments), the intermediate ?Livello Fondamentale della Pianura? (Late Pleistocene sandur) and the lowermost post-glacial terraces within the Adda valley. In the subsurface, the transition between the northern glacio-fluvial alluvial fans and incised valleys, and the southern braided river deposits, occurs. This hinge corresponds to the transition from a single phreatic aquifer (North) to a stack of partly confined aquifer complexes (South), that is accompanied by emergence of the phreatic nappe along a belt of plain springs (?fontanili?). In this area many canals derive up to 95% of the discharge of the Adda river. In this work, the new geological and hydrostratigraphic model of the area is presented. The work is based on surface geology and subsurface stratigraphy, in a GIS environment linked with a 3-D modelling software, that allowed for a ?try and error? procedure of stratigraphic correlation. The results show the architecture of a large T-R sequence, from the marine Early Pleistocene to the present-day fluvial setting, punctuated by low-order cycles. The lowermost Geological Unit (GUO, Early Pleistocene) is formed by gently tilted marine shales with sporadic gravel and sand lenses. Three complex channelized sequences, with erosion relationships (GU1a-b-c) are incised into, and stacked above the marine deposits. These sequences are interpreted as the local effect of Middle Pleistocene eustatic cycles and uplift of the Alpine substratum. In this time the coastline moved at least three times from N to S, temporarily exposing the previous coast and shelf environments. The resulting incised valleys were filled by the three GU1 sub-units. The subsequent regional regressive pulse is witnessed by the glacio-fluvial sandur of GU2-3-4 (Middle Pleistocene), which buries the GU1 sequences, after a large south-eastward shift of the coastline. GU2-3-4 and the overlaying GU5 (Middle p.p. - Upper Pleistocene amalgamated gravels and sands) are separated by a very continuous red paleosoil throughout the entire study area. During latest Pleistocene (after LGM) decametric silt layers cover the ancient fluvio- glacial gravelly sediments. The Postglacial (PG) to recent sediments are confined within the Adda valley. This geological reconstruction leads to a hydrostratigraphic scheme that includes: 1) Basal Aquiclude (GU0), laterally continuous at regional scale with local, small, confined and partitioned Aquifer systems ; 2) Intermediate Aquifer Group (GU1a-b-c) with a complex alternation of fining upwards sequences and juxtaposed gravel and fine-grained bodies due to erosion; 3) Uppermost Aquifer Group (GU2-3-4, GU5), decametric amalgamated gravel and sandy gravel units, locally separated fine-grained layers (preserved palaeosoils); 4) Shallow Aquitard (GU6, GU7) forming a discontinuous sandy silt cover above the underlying aquifers. The geological and the hydrostratigraphic units have been modeled in 3-D, obtaining the maps of their boundaries, the location of their pinch-outs and the distribution of the internal genetic units (fining and coarsening upwards sequences). A descriptive statistic approach was used to characterize the sediments of these units. Multivariate analysis permitted to quantify the vertical and lateral variation trends of lithofacies and poro-perm properties. The quantitative geological model yields the shape of the highest-order stratigraphic units within which the internal architecture and the permeability, porosity and electrical field gradients can be interpolated and/or simulated.