Geologic and hydrogeologic factors affecting sinkhole (obruk) development in Central Turkey

Abstract

Konya region is located near Tuzgolu (salt lake) in the center of Turkey. The surface of the study area which is between Cumra-Karapinar and Aksaray-Sultanhani-Obruk plains in the Konya region is 2,100 km2. Obruk landforms are concentrated in a 100 km2 section of the study area which contains approximately 50 obruks. The climate is typically continental and annual average precipitation varies between 314.2 mm/year in Sultanhani, 298.4 mm/year in Karapinar, and 333 mm/year in Besagil. Evaporation value is 1,444 mm/year. In the study area, agricultural activities are carried out on 5,115 ha of land, which is irrigated from the 109 wells drilled and operated by several soil-water cooperatives. The static water levels in the wells vary between 75–91 m. Even though Karapinar and Sultanhani plains are closed basins, there is groundwater connection between them. Since the groundwater flow in this area is canalized between some lithological and tectonic barriers, the underground formations are eroded by groundwater flow moving between the limestone. Over time, dissolution occurs within the limestone formations and they eventually collapse. Generally speaking, circular obruks are formed in horizontal layers and oval ones are observed in layers with inclination. Some obruks reach the groundwater table. The main composition in the study area is, from the oldest to the youngest, Paleozoic schist–fillit–quartzite–crystallized limestone and Mesozoic ophiolites, Cenozoic limestone with pebble, marl, clay, calcite, silica, dolomites and tufa, marl layers with gypsum and salt, lacustrine limestone with tufa layers and andesite near the volcanites, sediments of clay, sand, pebble, and silt of Plioquaternary and basalt flows. Samples collected from all of the sampling stations were analyzed and hydrochemical graphs were prepared. According to hydrochemical evaluations, pH is between 7.1 and 8.2, EC is between 840 and 1,965 μmho/cm, and hardness varies between 40 and 81 FS°, and the water is classified as C3 S1 category according to US Salinity Laboratory interpretations. With respect to irrigation water criteria, the water is of good quality, but not good as drinking water. In the study, the effects of dissolved CO2 in groundwater on karstification were also investigated. Water from wells drilled at Yarimca, Acikkuyu, and Badirik plains contains large amounts of CO2. As is commonly known, those waters which contain CO2 more than 50 mg/l are considered to be corrosive and can cause dissolution of limestone. Through the preparation of topographical elevation maps of the groundwater, water level maps were drawn and the flow direction of groundwater, recharge area of groundwater, groundwater hydraulic balance and level fluctuations of the groundwater, as well as the locations of possible exploitation sites were determined.