HYDROLOGICAL PROCESSES Hydrol. Process. 23, 3365–3371 (2009) Published online 16 September 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7441 Heterogeneous aquitard properties in sedimentary successions in the Apennine chain: case studies in southern Italy Emma Petrella* and Fulvio Celico Groundwater Research Center, Universit` a degli Studi del Molise, 86090, Pesche (IS), Italy Abstract: The aim of this study was to analyse the hydrogeologic role of siliciclastic deposits and marly-clayey-calcareous successions within the carbonate Apennine chain (southern Italy). The study was carried out along the northern part of the Matese carbonate massif through (1) the hydraulic characterization of siliciclastic rocks in a test site, by means of Lugeon tests, and (2) the identification of the groundwater flow system discharging at an important spring located within a marly-clayey-calcareous succession in a second test site, by means of isotopic investigations. The results showed that the investigated siliciclastic deposits and marly-clayey-calcareous successions may allow significant groundwater discharge from carbonate aquifers. Thus, they do not everywhere behave as aquitard, contrary to the previous model. Instead, groundwater flows through the upper part of these successions, where stress release fracturing enhanced rock permeability in the near-surface bedrock. Thus, these successions may locally be a new groundwater source within the southern Apennine chain. Copyright  2009 John Wiley & Sons, Ltd. KEY WORDS aquitard; carbonate aquifer; marly-clayey-calcareous successionsiliciclastic deposits; southern Italy Received 21 February 2009; Accepted 17 July 2009 INTRODUCTION Carbonate aquifers are a primary source of groundwa- ter in southern Italy supplying an average volume of 4Ð1 ð 10 9 m 3 per year (Celico, 1983). Hydrogeologic sur- veys that were carried out mainly at regional or catchment scale suggested that the main springs of these aquifers often occur at the contact with lower permeability silici- clastic deposits or marly-clayey-calcareous successions. These successions are then considered as aquitards in the present conceptual model (Celico, 1986). Somewhere, the permeability contrast between carbonate massifs and the lower permeability rocks mentioned above is further emphasized by low-permeability tectonic discontinuities (Celico, 1986). Nevertheless, to date there are no studies at site scale that analyse relationships between carbonate aquifers and their relative aquitards, despite the possi- bility that these interactions may have a great signifi- cance in controlling the effectiveness of groundwater flow schemes and water balances in carbonate aquifers and in defining the potential of these lower permeability rocks as a groundwater source. The hydrogeologic role of other successions made up of sandstones, marls and siltstones have been recently investigated in northern Apennine (Italy). The research demonstrated that the latter does not behave as aquitard (Gargini et al., 2008; Vincenzi et al., 2009), as reported in previous studies. In such a case, the inaccurate interpretation of their hydrogeologic behaviour led to construction of new high-speed railway * Correspondence to: Emma Petrella, Groundwater Research Center, Universit` a degli Studi del Molise, Contrada Fonte Lappone, (86090) Pesche (IS), Italy. E-mail: emma.petrella@unimol.it tunnels connecting Bologna and Firenze that drained an enormous volume of groundwater. For example, 2 years after completion of the 15-km-long Firenzuola tunnel, the average drainage outflow was 0Ð35 m 3 s 1 (Vincenzi et al., 2009). A few other studies (Eaton and Bradbury, 2003; Eaton et al., 2007) on the hydrogeology of sedi- mentary rock aquitards demonstrated that flow dynam- ics can be more complex than previously believed. The new findings have important implications for predicting groundwater flow and for planning and protecting water supplies. The main goal of this research was to analyse the hydrogeologic role of siliciclastic deposits and marly- clayey-calcareous successions along the northern part of the Matese carbonate massif (southern Italy) through (1) the hydraulic characterization of siliciclastic rocks at a test site, by means of Lugeon tests and (2) the identification of the groundwater flow system discharging at an important spring located within a marly-clayey- calcareous succession at a second test site, by means of isotopic investigations. STUDY AREA The Matese carbonate massif (Figure 1) is an important drinking water source, and supplies an average volume of about 0Ð5 ð 10 9 m 3 per year (Celico, 1983). From the hydrogeological point of view, it can be defined as a basin-in-series aquifer system (sensu Celico et al., 2006; Figure 2), due to fault zones that act as barriers to groundwater flow and compartmentalize the system (Celico, 1983). Thus, several important springs (mean Copyright  2009 John Wiley & Sons, Ltd.