Stationary and non-stationary geostatistics to model 3-D hydraulic conductivity distribution: a case study in the southern Po river plain

Groundwater numerical modeling has become one of the most important up-to-date water resource management tools because it allows to reproduce the aquifer behavior, that is vital in sustainable water resource exploitation, contamination fate assessment, and remediation scheme design and efficiency evaluation. During numerical model implementation, besides the hydrogeological boundary conditions, the hydraulic conductivity (K) of the considered aquifer is a key parameter to be defined. In general, this parameter is assigned to each hydrogeological complex, relying on values obtained by point permeability tests (e.g. pumping tests, slug tests, etc.). Although the obtained experimental K values could be representative of a portion of the whole aquifer volume, its intrinsic heterogeneities cannot be detected, especially when the considered aquifer is very complex (Bianchi & Pedretti, 2017). For this reason, the main objective of the research is to draw a physically based 3-D hydraulic conductivity model by both stationary (i.e. Ordinary Kriging, OK) and non-stationary (Intrinsic Random Function theory, IRF-k) geostatistical methods, applying these techniques to 182 Cone Penetration Test (CPT) profiles of the tip (qc) and shaft (fs) resistances, collected by the Emilia Romagna Regional Geological Survey. The obtained 3-D qc and fs models were combined by equations present in literature (Robertson, 1990; Lunne et al., 1997) to obtain the lithology index (Ic) and the K models. The selected study area is in the southern part of the Po plain and is characterized by mainly alluvial deposits made up of undifferentiated fine silty-sandy deposits, with coarser (i.e. sandy gravelly alluvial fans and sandy paleo-channels) and finer (i.e. silty clayey lacustrine lenses) geological inclusions. As a result, the OK and IRF-k method allowed to estimate the K values in a 3-D model, starting from CPT data. The obtained models reproduce as closely as possible the actual geological and hydrogeological features, not neglecting the multi-scale heterogeneity. The proposed methodological approach provides a detailed physically based 3-D hydraulic conductivity model. The obtained 3-D K model can represent a useful starting point for hydrogeological numerical modeling and/or a constraint for model calibration, especially when the intrinsic deposit heterogeneity could affect substantially the groundwater flow and contaminant transport in the aquifer.