Groundwater is often the only water source in semi-arid regions of Turkey. Günyüzü Basin, located in the Sakarya River basin, SW of Eskişehir, exhibits semiarid 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. d 18 O (-11.2 to -8.9%) and d 2 H (-79 to -60%) 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.

Abstract. Flash floods are caused by heavy rainfall that has become more frequent. They are more prominent in lowstorage karst regions, although karst terrain often acts as a natural flood control particularly when it is bare and dominated by conduits. A study using a hydrogeochemical approach and assessing data from several springs in different carbonate rock in western Turkey has made it possible to classify karst aquifers based on their response to heavy rainfall events. According to this aim, physico-chemical measurements in wet and dry seasons and discharge rates in springs are compared in order to explain aquifer characteristics. Groundwater samples have a pH ranging from 6.3 to 8.9, temperature (T ) varying from 7 to 35 • C and electrical conductivity (EC) ranging from 140 to 998 µs cm −1 . Groundwater samples with high EC, high T and low dissolved oxygen (DO) represent the deep circulating water, while low EC, low T and high DO are linked to the shallow circulating water. Lower variability between wet and dry seasons reveals that fracture permeability is predominantly controlled by diffuse groundwater flow with low or high storage, and conduit permeability with high storage. However, variability of the physico-chemical characteristics is higher in a conduit permeability with low storage. These types of aquifers with high transfer capability, predominantly controlled by turbulent groundwater flow, affect flash floods.

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Groundwater, which is renewable under current climatic conditions separately from other natural sources, in fact is a finite resource in terms of quality and fossil groundwater. Researchers have long emphasized the necessity of exploiting, operating, conserving and managing groundwater in an efficient and sustainable manner with an integrated water management approach. The management of groundwater needs reliable information about changes on groundwater quantity and quality.

The Tavşanlı Zone (TZ) is a high-pressure/low-temperature metamorphic belt representing subduction and exhumation between the Sakarya Zone and the Afyon-Bolkardağ Zone in western Anatolia. This paper provides new and precise geological data including whole-rock and mineral chemistry, phengite 40 Ar/ 39 Ar ages, zircon laser ablation ICP-MS U-Pb, and apatite (U-Th)/He thermochronological information for a region of the Sivrihisar metamorphic complex that has been less studied than other regions of the TZ. This region comprises Permo-Carboniferous metamorphic units, Eocene granodiorite and microgranodiorite, terrestrial clastics, and Holocene alluvium. The mineral assemblage of the granite-gneiss contains quartz, plagioclase, K-feldspar, microcline, muscovite, and rare biotite and garnet, while the blueschist comprises plagioclase, white mica, biotite, Na-amphibole, and garnet that are considered to represent greenschist and blueschist facies metamorphism, respectively. We estimate the peak metamorphic conditions for schists of the Permo-Carboniferous metamorphics and associated rocks as T = 303-484 °C and P >10 kbar. This high-pressure event occurred at ca. 83 Ma, indicated by a well-defined 40 Ar/ 39 Ar plateau age for phengite. Laser ablation ICP MS U-Pb analyses of euhedral or subeuhedral magmatic zircons from previously unknown microgranodiorite, intruding the metamorphic rocks, yield an Eocene (50 Ma) age. The new results are interpreted to indicate that lithospheric collision and northward subduction beneath the Sakarya Zone occurred between 83 and 50 Ma. Apatite crystals separated from microgranodiorite yield (U-Th)/He ages consistent with cooling of the Tavşanlı Zone to ~65 °C by ~38 Ma.