Introduction The European Union Water Framework Directive (WFD) (2000/60/EC), further abbreviated as WFD, enforces an integrated approach to achieve at least a ‘good status’ for all European water bodies by 2015. The implementation of the WFD requires mixing of legal requirements with issues of technical feasibility, scientific knowledge, and socio-economic aspects which require intensive multi-stakeholder consultations (Quevauviller et al. 2005). A Common Implementation Strategy (CIS) consisting of the priorities for 2005–2006 is endorsed by the water directors in 2004 (European Communities 2004). In this paper a methodology is presented to support a first step of the implementation of WFD, which is the delineation of groundwater bodies, as the basic units for management and monitoring. Unlike for surface water bodies, the WFD guidance document ‘on the identification of water bodies’ (European Commu- nities 2003) gives only a few recommendations on how to delineate groundwater bodies. The only criterion which must be followed is that the chemical and quan- Jan Cools Yves Meyus Solomon Tuccu Woldeamlak Okke Batelaan Florimond De Smedt Large-scale GIS-based hydrogeological modeling of Flanders: a tool for groundwater management Received: 21 February 2006 Accepted: 6 April 2006 Published online: 11 May 2006 Ó Springer-Verlag 2006 Abstract For the implementation of the European Union Water Frame- work Directive (WFD), technologi- cal and scientific support are required. This paper presents a methodology to support a first step of the implementation of WFD, which is the delineation of ground- water bodies. The methodology consists of (1) the development of a complete and generally-accepted hydrogeological classification system for Flanders, named the HCOV code, (2) the development of a geo- graphic information systems (GIS)- managed borehole database, and (3) the development of aquifer and aquitard models by means of a solid modeling approach. For each unit of the hydrogeological classification code for Flanders unit, GIS maps are generated for the three basic characteristics of hydrogeological layers: extent, base level and thick- ness, such that combined, the vol- ume and extent of a hydrogeological layer is unambiguously defined. This GIS-based hydrogeological database has become a useful tool for groundwater management purposes and to provide the input for groundwater modeling. Keywords Hydrogeological modelling Æ GIS borehole database Æ Aquifer models Æ Aquitard models Æ Belgium Abbreviations WFD: Water framework directive Æ CIS: Common implementation strategy of WFD Æ GIS: Geographic information systems Æ HCOV: Hydrogeological classification code for Flanders Æ IRBD: International river basin district Æ AMINAL: Environmental authority for Flanders Æ EU: European union Æ DEM: Digital elevation model Æ FGM: Flemish groundwater model Environ Geol (2006) 50: 1201–1209 DOI 10.1007/s00254-006-0292-3 ORIGINAL ARTICLE J. Cools (&) Æ Y. Meyus S. T. Woldeamlak Æ O. Batelaan F. De Smedt Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium E-mail: jan.cools@soresma.be Tel.: +32-9-2616338 Fax: +32-9-2616301 J. Cools Soresma, Poortakkerstraat 41, 9051 Ghent, Belgium