Subsoil geostatistical modeling as tool for hydrogeological modeling: Transitional Probability approach applied upon a heterogeneous site

Nowadays, a detailed hydrogeological characterization is the base of an effective hydrogeological modeling aimed to plan and manage both the groundwater and the environment systems. Every single groundwater numerical simulation needs a hydrogeological conceptual model of the subsoil in order to define parameters and boundary conditions as well. The better the subsoil is reconstructed, the more accurate the results of the modelling will be. Several methods are available to perform a 3D simulation of the subsoil; however, their application depends on the heterogeneity of the investigated area. The stratigraphic setting can be simulated using the transition probability that models heterogeneities such as fining or coarsening-upward trend. This study aims to perform a 3D model of a heterogeneous site of 3.8 km2 close to the Venice lagoon (NE Italy). This area is relevant from the environmental perspective because the shallower aquifer is affected by arsenic contamination. Approximately 119 stratigraphic logs are available in this site, confirming the presence of alluvial deposits (clay, silt, sand and peat) with high heterogeneity. The flood plan deposits are dominant, and the frequently avulsions of the rivers contributed to increase the heterogeneity of the system. Considering that, we decided to use a transition probability approach to perform the 3D subsoil model. Firstly, we analyzed borehole data to calculate material proportions, mean lens lengths and transition probabilities. Secondly, the transition probabilities were used to generate multiple realizations of the subsoil heterogeneity. Every realization was conditioned by borehole data, preserving the geologic tendencies. The procedure shaped a detailed 3D structure of lithological architecture, which allows for the spatial estimation of the likely arsenic source materials. This information is fundamental to perform a further reactive transport model for arsenic contamination, improving the geochemical learning on arsenic mobilization. Therefore, the 3D subsurface model assumes importance both as starting point to perform further environmental analysis and aid for local stakeholders’ decision process.