Water systems from the Pădurea Craiului Mountains and the northeastern extremity of the Pannonian Basin (Oradea–Felix) are investigated for deuterium, global salt content, and major solutes. All types of groundwater (springs, wellhead water, domestic wells, karst springs, cave water) display δD values (−76.1‰ to −62.3) similar to the surface running water (−76.5‰ to −50.7), suggesting that they are meteoric in origin. The global salt content ranges from 49 mg/l to 1300 mg/l for the groundwater and from 46 mg/l to 665 mg/l for the surface water. From the co-variation between the δD values, the global salt content, and the major solutes, as well as from the seasonal variation of these parameters, genetic links could be established. The mineralization of the groundwater is due to intense underground circulation, mostly in the karst system developed within the Mesozoic deposits of the Pădurea Craiului Mountains.