Groundwater is often the only water source in semi-arid regions of Turkey. Gunyuzu Basin, located in the Sakarya River basin, SW of EskiAYehir, exhibits semi-arid conditions. The study area is composed of Paleozoic metamorphic rocks, Eocene granitic rocks, Neogene sedimentary rocks, and Quaternary alluvium. In the basin, Paleozoic Marbles are the main reservoir rocks for hot and cold water, bordered by impermeable diabases dykes at the sides and by impermeable granites and schists. Neogene-aged limestones, conglomerates and alluvium represent the other significant aquifers. Water samples chosen to exemplify the aquifer characteristics, were collected from springs and wells in both the dry and the wet seasons. The cation and anion permutation of the samples show that carbonates are the dominant lithology in the formation of chemical composition. delta O-18 (-11.2 to -8.9aEuro degrees) and delta H-2 (-79 to -60aEuro degrees) isotopic values show that all waters (thermal and cold) are meteoric in origin. The hydrological, hydrochemical, and isotopic properties of the waters reveal that there exist two main groups of groundwater systems; one of these is deep circulating, while the other one is shallow. Tritium values, 0-4 TU (Tritium Unit) indicate the presence of old, static water in these aquifer systems.