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It was recognized that a large area in Tehran plain, especially in southwest of Tehran (North of Iran), is subject to the land subsidence. Two major zones of subsidence have been identified and one of these zones has been monitored for several years by leveling. Land subsidence can be caused by a variety of processes, but most land subsidence in Iran is associated with aquifer compaction caused pressure decline associated with groundwater withdrawal. This study aims at zoning land subsidence and recognition of geometrical factors in southwest of Tehran. To estimate and predict land subsidence, all the effective subsidence factors were identified. Among the factors, nine most important factors including, downfall of groundwater, thickness of clay, depth of groundwater, annual discharge of water from wells, the distance of well to each other, slop, elevation, land use and geology were evaluated. In this study, we collected and correlated 40 geotechnical boreholes in Eslamshahr city and around it and then prepared soil type distribution maps for several depths. Finally, these maps overlaid with water table and land subsidence maps. On this study also, the Landsat and Spot Satellites data together with the ArcGIS, Surfer and Excel have been applied in order to analyze the data and prepare the maps and diagrams. Furthermore, field surveys and study and number of geomorphic features have been involved in the study. In basin-fill aquifers, like Eslamshahr, most of the aquifer compaction is due to the dewatering and compression of fine-grained sediments. Once the fine-grained units begin to compress and lose porosity, compaction becomes permanent, overall water storage in the basin fill is reduced, and land subsidence occurs. In sedimentary basins, groundwater in unconsolidated aquifers is pumped from the pore spaces between sand and gravel grains, causing a lowering of pore-water pressure. The decrease in pore pressure results in an increase in the effective stress in the high-permeability low-compressibility coarse-grained aquifers and timedependent pore-pressure reduction in the low-permeability high-compressibility fine-grained aquitards, and this increase in effective stress is equal to the decrease in fluid pressure. Because fluid pressure within the pores of a granular matrix helps support overlying aquifer material, a reduction in pore-water pressure causes an increase in overburden stress (weight) to the aquifer matrix, causing the aquifer matrix to change volume (compact). Land subsidence may result in various types of land-surface movements that can potentially cause problems if human development exists within the subsiding area. High subsidence rate in urban areas increases the risk of ground opening and collapses, flooding and damages of their infrastructures due to the induced surface fracturing by

Sedimentology Studies and Environmental Impacts of Land Subsidence in Eslamshahr, SW Tehran, Iran