Calibration and Reliability in Groundwater Modelling: A Few Steps Closer to Reality (Proceedings of ModelCARE'2002. Prague, Czech Republic, June 2002). IAHS Publ. no. 277. 2002. 115

Geostatistical modelling of a heterogeneous alluvial aquifer by indicator variables

LAURA GUADAGNINI Dipartimento Ingegneria Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento (DIIAR), Politecnico di Milano, Piazza L. Da Vinci 32, 1-20133 Milano, laura.guadagnini(5),polimi,it

MARCO FARINA & MATTEO SIMONI Municipality of -Unità Ambiante, Via Zamboni 8,1-40127 Bologna, Italy

Abstract We have explored the applicability of indicator geostatistical methodologies for mapping the probabilistic spatial distribution of high and low permeability fades within the alluvial aquifer system of the city of Bologna in . The first step was to analyse stratigraphie and sedimentological well data, identifying the main hydrogeological units of the area. After this, we focused our attention on the detailed reconstruction of the spatial pattern of the aquitard that serves as the separating element between superficially contaminated aquifers and deep aquifers, both exploited for drinking-water purposes. The spatial distribution of materials within the aquifer was then reconstructed on the basis of three data selection criteria, using two indicator geostatistical methodologies: (a) hydrostratigraphic analysis via indicator point kriging and probability cut-off, and (b) Monte Carlo conditional indicator simulations. The reliability of the resulting material distribution images within the aquitard was analysed using the available hydrogeological information.

Key words aquitard; geostatistics; indicator point kriging; Monte Carlo conditional indicator simulations

INTRODUCTION

The study area comprises part of the high-medium alluvial plain close to the city of Bologna in the Regione in northern Italy. The study area of about 50 km2 is located within the alluvial fan. Three major well fields, yielding nearly 80% of all the groundwater used for drinking water and industrial purposes in the Bologna area, are located within the study area. Figure 1 depicts the limits of the area investigated, together with the location of data points of an aquitard called "Alpha". The plain around Bologna is part of the Basin fill, which is a syntectonic sedimentary wedge (Ricci Lucchi, 1984) forming the infill of the Pliocene-Pleistocene fore-deep. The sedimentary evolution of the basin is characterized by an overall regressive trend from Pliocene open marine facies to Quaternary marginal marine (cycle Qm from Ricci Lucchi et ah, 1982) and alluvial deposits (cycle Qc). An array of coalescent fan complexes (Reno, and rivers) characterizes the uppermost portion of the Po Basin at its southern border (Apennines Piedmont area). Alluvial deposits in the Bologna area are more than 300 m thick (Francavilla et al., 1980). According to recent studies (Regione Emilia Romagna, 1998) cycle Qc is 116 Laura Guadagnini et al.

Apennines

Reno River Scale 1:400.000

Fig. 1 Location of the study area with data points and orientation (dashed line) of the cross-section, which is expanded in Fig. 2.

Fig. 2 Cross-section of the aquifer and main geological features. subdivided into two coarse units: Cycle A (corresponding to Unite Alluvionale superiore in Amorosi & Farina, 1994b) and Cycle B (Unità Alluvionale inferiore, ibid.). Such units, about 100-150 m thick, are separated by clayey deposits and located in Cycle C, which in turn is the upper portion of Cycle Qm. The stratigraphy of cycle Qc has been