Three-dimensional geologic modeling of the Virttaankangas aquifer, southwestern Finland Artimo, A. 1 , R.C. Berg 2 , C.C. Abert 2 , J. Mäkinen 3 and V.-P. Salonen 4 1 Department of Geology, FIN-20014 University of Turku, Finland; 2 Illinois State Geological Survey, 615 E. Peabody, Champaign, IL 61820, USA; 3 Department of Geography, FIN- 20014 University of Turku, Finland; 4 Department of Geology, FIN-00014 University of Helsinki, Finland; E-mail: A. Artimo at aki.artimo@uti.fi Introduction . The Virttaankangas glaciofluvial/glaciolacustrine complex is part of the largest esker system in southwestern Finland (Fig. 1). A plan has been prepared for the area that will involve the production of an artificial groundwater reserve from infiltrated river water for the use by the city of Turku and its surrounding area. The amount of river water infiltrated into the esker aquifer will be about 1.3 m 3 s -1 . This is about 0.5 % of the mean flow of the River Kokemäenjoki, which will be the initial source of the water (Jaakko Pöyry Infra 2001). The artificially infiltrated groundwater will be supplied to 285,000 inhabitants of the Turku area in the beginning of the year 2007. The complex structures within the unconsolidated Quaternary deposits hosting the groundwater reserve needed to be mapped in detail. Therefore, a three-dimensional (3-D) hydrogeological conceptual model of the Virttaankangas area that identified and characterized the main hydrogeological units of the aquifer, was constructed. This model, along with subsequent groundwater flow models, will be the keys in planning and guiding both the infiltration of the river water into the aquifer and pumping of the resulting groundwater from the aquifer to the Turku area. Figure 1. The location of the study area. 5