REPORT Geological and hydrogeological reconstruction of the main aquifers of the Maltese islands Alessio Barbagli 1,2,3 & Enrico Guastaldi 1,2,4 & Paolo Conti 1 & Miriana Giannuzzi 2 & Iacopo Borsi 5 & Francesca Lotti 5,6 & Paolo Basile 7 & Lorenzo Favaro 7 & Adrian Mallia 8 & Rachel Xuereb 8 & Michael Schembri 9 & Julian Alexander Mamo 9 & Manuel Sapiano 9 Received: 9 November 2020 /Accepted: 2 September 2021 # Springer-Verlag GmbH Germany, part of Springer Nature 2021 Abstract Geological and hydrogeological conceptualizations of the five main aquifers of Malta were performed by means of character- ization of the groundwater bodies’ geometries to assess their hydraulic properties. The starting point was 23 new geological cross-sections, intended to intercept all the main structural features of the Maltese archipelago at once. This conceptual analysis constitutes a fundamental phase in the development of groundwater models, and in turn influences the assessment of transmis- sivity and the numerical models’ structure, calibration and related uncertainties. Besides the construction of the geological sections, a crucial step in building a coherent geological conceptual model involved data homogenization and use of different tuning scales, with respect to both geometric and hydraulic aquifer features. Cross-section quality was spatially assessed by the fit on both a high-resolution digital terrain model and the surface geological data, and the information was subsequently merged with previously determined inconsistencies within borehole stratigraphic data collected from various sources. The geological boundaries data (contact lines and point data) and the dense fault system of the Maltese islands (horst and graben structure) were interpolated by means of the “spline with barriers” algorithm. The resulting surfaces of the main geological formation beds constitute the new detailed geological conceptual model of the whole Maltese archipelago, needed for a more reliable assessment of hydrogeological parameters. The achieved hydrogeological conceptual models of the main Maltese groundwater bodies constitute the basis for groundwater numerical modelling to better understand and quantify the groundwater resources available within the hydrological reservoirs. Keywords Conceptual models . General hydrogeology . Aquifer geometry reconstruction . Malta . Groundwater bodies Introduction Due to the geographic and climatic conditions of the Maltese islands, freshwater is a very scarce resource. Like most of the Mediterranean countries, Malta is facing water stress due to both water scarcity and resource deterioration, moreover it has no surface water that can be efficiently exploited (Mangion et al. 2005). Groundwater abstraction has been estimated to reach around 40 million m 3 , i.e. 61% of the total national water demand (Sapiano 2020). The need for characterizing these hydrogeological reservoirs of the Maltese islands, in order to use these resources more efficiently, is therefore ur- gent. A complete review and update of the existing geological and hydrogeological conceptual models of the islands consti- tutes the preliminary phase for the management of Malta’s groundwater resources (Anderson et al. 2015). Geological conceptualization allows for drafting of the geometry and * Enrico Guastaldi guastaldi@geoexplorersrl.it 1 Center for GeoTechnologies, University of Siena, Via Vetri Vecchi, 34 - 52027 San Giovanni, Valdarno, Italy 2 GeoExplorer Impresa Sociale S.r.l., Via E. Vezzosi, 15 -, 52100 Arezzo, Italy 3 Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy 4 CGT SpinOff S.r.l., Via E. Vezzosi, 15-52100 Arezzo, Italy 5 TEA Sistemi S.p.A, Via Ponte a Piglieri, 8, 56121 Pisa, Italy 6 Kataclima, Strada Statale Cassia, 92, 01019, Vetralla, Italy 7 STEAM S.r.l., Via Ponte a Piglieri, 8, 56121 Pisa, Italy 8 ADI Associated Environmental Consultants Ltd, Kappara Business Centre, 113 Triq Birkirkara, San Gwann SGN 4197, Malta 9 EWA Energy and Water Agency, 9043 Triq Ħal Qormi, Hal Luqa, Malta Hydrogeology Journal https://doi.org/10.1007/s10040-021-02406-z