Geological constraints to model complex hydrostratigraphy: case studies from the Quaternary Po Hydrogeological Basin (Northern Italy)

Hydrogeological modeling of porous aquifers at different scales is best performed if the hydrostratigraphic architecture is well constrained to the geological history of aquifer origin and groundwater saturation. Reliable hydrostratigraphic models should account also for the tectono-sedimentary history of aquifer building and disruption. Critical controlling factors on good models are the hierarchy of sedimentary bodies, which depends on the intensity and duration of depositional/erosional processes, the resulting nested multiple scales of subsurface heterogeneities and the syn- to post depositional deformation history. Focusing on the best modelling procedure of the Quaternary hydrostratigraphy of the Po Hydrogeological Basin South of Milan, we chose two key-sectors to apply the method: i) the glacio-fluvial and alluvial terraced landscape at the northern alpine margin of the Basin, directly influenced by the Pleistocene glacial pulses; ii) the Apennine tectonic reliefs which emerge in the southern Po Plain and involve the Quaternary, alpine-sourced alluvial succession in Apennine folding and faulting. The specific aim of the work is to define a consistent 3-D stratigraphic framework of the two sectors at different scales, with the help of static geomodeling techniques, constrained by all the available data. Field-based geological reconstruction provided the entry data for multi-scale GIS management. Original geological mapping, stratigraphic, sedimentological, pedological, paleontological, geomorphological and structural observations were firstly carried out. Hundreds of subsurface borehole and geophysical data were also used, after normalized digitalization based on a specifically built litho-textural Code. Field-based ‘hard data’ were combined to 1-D facies analysis of subsurface logs, then correlated into a fence of 2-D geological sections. In this stages of pre-processing for the 3-D analysis, GIS software was interfaced with GeoModeller® software to rapidly simulate and visualize the stratigraphic/tectonic relations, honoring the geological constraints. Several forward models were computed to compare different and even contrasting architectures and evolutions. The 3-D results permit a first comparison between the contrasting hydro-stratigraphic architectures of the two sectors, in relation to the different geological evolutions. At the Alpine border, uplift related to glacio-isostatic and tectonic rebound determined nesting of entrenched pre-glacial valleys and glacio-fluvial terraces. At the Apennine border, thrusting and wrenching determined the palimpsest of tectonic culminations and depocentres, lately cut by the post-glacial river valleys. The 3-D model of this highest rank framework could be filled with the nested, low-rank hydrostratigraphic units, confining the potential field modelling within each highest rank geological volume, that means constraining the geological geometries to the shape of the top boundary of each unit, which is determined by the predating incremental geo-history. This attempt yielded a satisfactory image of the spatial arrangement of the different rank bodies; hence, it deserves to be taken in consideration to orient further simulations of the internal facies heterogeneities. The tectonically active sector at the Apennine border, permits also to deal with modeling of aquifer bodies whose geometries and thicknesses change dramatically within short distances owing to syn- and post depositional folding and faulting. The attempt to constrain the 3-D realizations with the incremental geological evolution required to model the geological hierarchy. Since ‘Hierarchic stratigraphic piles’ are not encompassed in the GeoModeller suite, we propose some hints to overcome this limit, starting from ad hoc scripts that we are carrying out at the present state of this ongoing research.