Geostatistical characterization of a regional-scale sedimentary aquifer

We analyze the impact of the uncertainty associated with (a) the spatial distribution of hydraulic parameters, e.g., hydraulic conductivity, and (b) the conceptual model adopted to describe the system on the characterization of a regional-scale aquifer in the context of inverse modeling of the groundwater flow system. The study aquifer lies within the provinces of Bergamo and Cremona (Italy) and covers a planar extent of approximately 785 km2. Analysis of available sedimentological information allows identifying a set of main geo-materials (facies / phases) which constitute the geological makeup of the system. Hydraulic conductivity (K) distributions are modeled on the basis of the following conceptual schemes: (1) Composite Medium approach, implemented by zonation of the system, and, (2) Overlapping Continua model. The local volumetric fraction of each facies can also be interpreted as the probability of occurrence of that facies in the point (volume) considered and can be assessed by means of indicator-based methodologies. For each of the adopted conceptual models the groundwater flow is simulated with the numerical code MODFLOW-2000. Best estimates of hydraulic conductivities are obtained by a Maximum Likelihood fit of computed versus measured hydraulic heads. Information on hydraulic heads is provided in a set of piezometric level measurements recorded in monitoring wells. The results are examined using formal model quality criteria