Quaternary Journal of Iran

Abstract With the development of the drilling industry in the country since the early 1930 and the use of deep well drilling technology to exploit groundwater resources, the balance of renewable water usage has been disrupted, leading to numerous social conflicts. Since the damages caused by such exploitation become apparent later, its irreversible effects may persist for decades or even centuries. This can result in the migration of residents, abandonment of investments, and ultimately, desertification. Unfortunately, in densely populated provinces of the country, reliance on groundwater resources for water supply has led to land subsidence. Despite favorable climatic, hydrological, and geological conditions, and the abundance of surface water resources, Alborz Province has not been spared this hazard. According to studies conducted by the Geological Survey and Mineral Exploration Organization of Iran since 2005, the plains of this province have been classified as critical. Examination of land subsidence evidence in Alborz plains indicates that the most significant impact of groundwater extraction has been observed in the Hashtgerd plain, primarily due to fine-grained sediments in the central and southern parts of the plain and agricultural land use. The geological conditions of the Karaj plain appear to control this phenomenon, minimizing its damages.
According to the results of remote sensing, only 8% of the area of ​​the Eshtehard plain is involved in subsidence at a rate of 3 cm per year. 92% of the plain does not have this risk. The subsidence center is in the west of Eshtehard plain with an area of ​​22.5 square kilometers. The maximum subsidence rate in the Eshtehard industrial town is estimated to be 8 cm per year as a point, which seems to have occurred due to the exploitation of deep wells and under the influence of the subsidence cone.
The Fissures Observed in the Eshtehard plain since 2005 have been Attributed to land subsidence. Due to the expansion of cracks in the area, urban infrastructures including power transmission towers and electric poles and buildings have been damaged. Over time, these collapses have been affected by water erosion, and the bottom layer, which is made of sand, has been emptied, resulting in the collapse of the roofs of the tunnels, and thus, with the development of these channels, a large area of ​​the plain has been involved in these cracks. .
However, fissures in the Eshtehard plain are attributed to fine-grained and dispersive soils that lack load-bearing capacity. Inappropriate land use, disruption of the region’s hydrological cycle, and the alternation of wet and dry conditions have caused these fissures to manifest as various cracks in the clay soils.
From a sedimentological perspective, the presence of a 3-4 meter layer of fine grain silt and clay deposits is evident in all areas involved in the gap. This clay layer is resistant to the passage of water and has no hydraulic conductivity. At a depth of more than 3 meters, there is a permeable layer of sand, which has created deep tunnels with the passage of subsurface flow and the washing of sand over time. Over time, these fissures, under the influence of water erosion, have evolved into large gullies. Analyzing the subsidence phenomenon in any plain requires a thorough understanding of aquifer geometry, geophysical studies, geology, and hydrogeology. Comprehensive studies and the tectonic conditions of the Eshtehard plain indicate the presence of two distinct sedimentary environments in the plain. In the eastern part of Eshtehard, a shallow surface aquifer exists, and due to low water quality and minimal exploitation, subsidence is not an issue. However, the western part of Eshtehard forms a deep sedimentary basin, containing fine-grained clay and silt interlayers up to a depth of approximately 70 meters. In this aquifer, excessive exploitation since 1986 has led to a 40-meter decline in groundwater levels. Near the Eshtehard industrial park, this change in water levels has reversed the hydraulic gradient and disrupted the balance of the groundwater table. Consequently, subsidence has occurred in the deep western aquifer, necessitating measures to control exploitation. However, surface fissures observed 7 kilometers west of the Eshtehard aquifer are not the result of horizontal ground movements due to subsidence. Soil mechanics studies confirm that these fissures are due to the soil and sedimentary characteristics of the region, exacerbated by climatic events and intensified over time by improper land use and loading.

Abstract 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.

Abstract Abstract
The population growth, the development of cities, industry, agriculture, and improper use of resources especially non-renewable resources have led human beings to face the danger of running out of resources. In some cases, in addition to the above, irreversible environmental and geological hazards have occurred due to the overdrawn of resources. Over extraction of groundwater resources is one of the problems that, in addition to human exposure to the risk of water scarcity, also induces risks due to this over-harvesting. The phenomenon of land-subsidence is called subsidence for natural and human reasons. One of the reasons for this phenomenon is human activities, including the overextraction of groundwater resources and the water table drop. Subsidence itself results in some problems such as deep cracks in the ground, well pipes growth and collapse of buildings. One of the common subsidence damages in many plains of Iran is the creation of sinkholes. Sinkholes are deep pits that are mainly involved in the development of karst dissolution and subsequently many risks to urban and environmental areas. The areas affected by the subsidence phenomenon have been identified using the Synthetic Aperture Radar Interferometry (InSAR) method. The interferometry technique uses electromagnetic wave interference to extract information. Radar interferometry uses the phase difference of radar images with a high spatial resolution seeking to produce a digital elevation model of the region and estimate the amount of deformation and displacement of the earth's crust. In the radar interferometry technique, complex radar images containing the phase values ​​and amplitude of the wave returning from the complication to the sensor are combined and an image called an interferogram is produced. An interferogram is an image that results from the subtraction of two images taken at two different times that are aligned on each other. An interferogram contains information of the phase difference between the two images, which indicates the difference in distance between the terrain features and the sensor in the two captured images. By having the phase difference value, various parameters such as the amount of ground displacement can be obtained. Persistent Scatterer Interferometry (PSI) time series analysis method uses stable scattering pixels (pixels that have scattering patterns with a constant scattering pattern over time) to extract information and overcome the limitations of radar interferometry. In the present research, an attempt has been made to monitor the subsidence of Abarkooh using the Synthetic Aperture Radar Interferometry method.
Abarkooh region is located in Yazd province which is an agricultural region in recent decades and the extraction of groundwater resources has been increased in it. Abarkooh city has two urban points, Abarkooh and Mehrdasht. Excess extraction of groundwater caused numerous subsidence and sinkholes, subsequently. Hydrological studies have determined the rate of groundwater level drop equal to 0.6 m/yr. In this paper, by using 55 Sentinel-1A satellite images in the period of 2017 to 2019, 54 interferograms have been used to process time series analysis based on persistent scatterer after deducting the participatory phases from the interferogram phase.




* h.ansari@yazd.ac.ir




Finally, the average subsidence rate map for the Abarkooh region is estimated. According to the analysis, the maximum subsidence rate of the Abarkooh region is 3 to 4 cm /yr and the maximum subsidence rate of the Mehrdasht region is 6 to 7 cm/yr. Accordingly, by considering the location of the subsidence area in the agricultural lands of the studied region and the decrease of the water table level in that region, one may conclude that the subsidence has happened due to the over-extraction of groundwater resources. In order to investigate the impact of subsidence on the environment, the soil salinity map has been calculated. In general, the salinity of the region is increasing as a factor impacting the degradation of surface soils and desertification. Well pipes growth, sanding of water wells, and reduction of well’s discharge, all are some examples of the consequences of subsidence in the destruction of wells. Other consequences of subsidence in this area include turning the plain into a desert, damaging aquifers, changing the topography of the land, and environmental pollution. Due to the un-uniform distribution of the subsidence areas in ​​the region, the damages to roads, railways, urban and industrial buildings, and energy transmission lines have been predicted. One of the most important subsidence consequences in this area is the creation of deep holes and longitudinal cracks along with the subsidence area. To evaluate the results of the interferometry technique, GNSS station data have been used. Since the data related to this area are available from 2011 until now, it is possible to obtain an accurate evaluation of the InSAR results. The results show that the displacement rate is 1 cm based on the permanent station data (GNSS) and the interferometry measurement results show a displacement rate of 0.7 cm. These values indicate the accuracy of the results of radar interferometry.