Introduction:
Land subsidence, defined as the downward movement of sediments, is one of the significant geological hazards primarily caused by the decline in groundwater levels and the increase in effective stress in aquifer sediments. This phenomenon leads to considerable damage to surface and subsurface infrastructures, including road networks, urban water and sewage systems, and buildings. However, due to the relatively low immediate human casualties, it has not been considered a serious hazard. Over time, the continued subsidence of land results in irreparable damage to urban infrastructure. In most plains of the country, especially in densely populated provinces, excessive groundwater extraction has led to land subsidence.
Alborz Province is no exception and is considered one of the high-risk areas in the country. Studies on subsidence in Alborz Province began in 2005, conducted by the GSI. In 2017, the organization's Remote Sensing Group updated these studies to monitor subsidence and its development patterns in Alborz Province. Research Findings Using radar interferometry technology, the maximum rate of subsidence in Nazarabad Plain was calculated for the period 2014–2017, revealing a maximum rate of 22 centimeters.
Result:
In addition to the rates obtained from remote sensing studies of the GSI, data prepared by the land subsidence portal under the name COMIT-LICS were also used. The subsidence rates obtained from the processing of COMIT data are consistent with the studies conducted by the GSI. The 9-year subsidence rate for the Hashtgerd plain is more than 90 centimeters cumulatively.
Additionally, the groundwater level decline model for Hashtgerd Plain, developed using data from the Alborz Regional Water Management, along with geological and sedimentological analyses of the area, was used to assess the causes of subsidence in Nazarabad Plain. In the northern parts of the Hashtgerd Plain, due to the coarse-grained nature of the sediments, the aquifer receives adequate recharge, which helps reduce the subsidence rate. In the central parts of Hashtgerd Plain, limited recharge from the north and south, the fine-grained sediments, and the presence of clay interlayers result in the highest subsidence rates. In the northern parts of the Hashtgerd Plain, there is no evidence of land subsidence due to the significant accumulation of coarse-grained sediments from the Karaj and Kordan rivers. Although the greatest decline related to groundwater withdrawal occurs in this Part of the Plain. In fact, the result of high withdrawals from the groundwater table in the north of the plain has been manifested in the form of subsidence due to the drop in water level in the fine-grained sediments in the southern parts.
Discussion: 
Changes in Hashtgerd groundwater level over 25 years using the Surfer model software. The behavior of the level lines is an expression of the aquifer geometry. Therefore, to interpret and analyze the risk of subsidence, a set of different factors must be evaluated. The complexities of the Hashtgerd plain's groundwater table affect water resource management. The geology and hydrogeology of the plain are such that the spread of subsidence can be prevented by aquifer management and balancing. Currently, the central and southern parts of the Hashtgerd Plain are at risk of subsidence. The depletion of groundwater resources and excessive exploitation in the north, on the one hand, will limit the supply to the aquifer in the central parts of the plain, and on the other hand, will lead to the reversal of the hydraulic gradient and the ineffectiveness of controls and balancing.
Conclusion:
The total number of wells drilled in the Hashtgerd Plain aquifer is 2,852, with a withdrawal volume of 241 million cubic meters. The share of wells in the subsidence area is 518, of which 211 are abandoned according to the Ministry of Energy statistics. The volume of water that can be withdrawn in this area is 58 million cubic meters, which 24% of the total water withdrawn from the plain. The results of the study of groundwater level changes in the Hashtgerd Plain show that an area of the plain that is at risk of subsidence, based on field evidence, has fewer wells and fewer discharges compared to the entire plain.
The specific geological and geomorphological conditions of the Hashtgerd Plain and the presence of the permanent and abundant Kordan River have a positive effect on controlling the subsidence rate. Therefore, to control the subsidence rate, monitor and prevent future damage, it is recommended to: manage the aquifer and create underground dams to increase the water level in the north of the plain, control surface water and implement artificial recharge plans; identify the geometry of the aquifer, and identify the type of aquifer in the Hashtgerd Plain using pumping tests.