Investigation on Confined Aquifer & its Role on Subsidence Occurrence in Hashtgerd Plain

Based on recent research by geological survey of Iran (GSI), an extensive subsidence area was observed in Hashtgerd plain (Tehran province). The geographical position of this area is 35° 47' 45" to 35° 03' 05" N and 50° 29' 05" to 50° 54' 28" E. The maximum and mean rates of this subsidence are 16 and 8.4 cm per year. In general, the main aquifer of plain is composed of Kahrizak and Tehran formations. These formations are alluvium type. The Kahrizak formation consists of gravel, sand and silt. This formation scatters in north of plain. Tehran formation is composed of pebble, gravel, sand and clay in fluvial cone form. Toward the center and south of plain the amount of fine material increases. The subsidence is shown in Tehran formation only. Based on geoelectrical and well-logs data, the confined to semi confined aquifer area is recognized in south and west of Hashtgerd plain. This area is in accordance with subsidence area. In the upper part of confined aquifer we can find an unconfined aquifer. The whole of penetrated water from surface of plain recharges the unconfined aquifer. The extortion of water from confined aquifer and unconfined aquifer in north of plain has caused subsidence phenomena. The unique recharge resources of confined aquifer is unconfined aquifer in north of plain. To take notice of the position and properties of confined and unconfined aquifers in this place is very important in suitable planning and management to prevent and mitigate subsidence occurrence in future.