Relationship between Sinkholes of Famenin - Kabudrahang- Ghahavand Plain and the Bed Rock of the Area

Famenin–Ghahavand- Kaboudrahang plain is located in the north of Hamedan province in Iran. The thickness of aquifer in the area is about 70-100 meters and its exploitation has been more than aquifer capacity. Annual falling of water table is about 2.5-3m. At least 19 small and large sinkholes have been formed in the plain during the past 10 years and this is a problem for inhabitants of the area. In this study, field investigation, analysis of water samples and calculation of free carbon dioxide were carried out and the characteristic features of the sinkholes recorded. Results show that the presence of specific conditions of limy bedrock and high amount of free carbon dioxide (more than 1500 mg/l) in groundwater are the main causes for the development of karst in the area. Furthermore, the presence of large holes, water channels, joints and fracture systems in the bedrock and water pumping are the main sources for the creation of sinkholes, whereas other factors such as sand-washing of layers, gas release from wells and fast falling of water table are minor effective causes. Therefore, to stop this phenomenon, drilling in bed rock should be prevented, suitable gravel pack should be used and all wells with high water yield form bedrock should be filled.