Impact of groundwater drawdown on land subsidence and creation of vulnerable areas in Neyshabur plain

Introduction
According to the definition of the American Geological Survey, the land subsidence Phenomenon is the collapsed or down warded settlement of the earth's surface, which can be had small displacement vector (Bates and Jackson, 1980). Land subsidence is a geological phenomenon caused to migrate slowly and horizontally part of the earth's surface layers (Hu et al, 2009). The subsidence phenomenon has the type of factors such as natural and unnatural factors which natural factors are included tectonic movements, limestone dissolution, karst sinkholes, magma discharge, organic soil oxidation, and natural soil compaction, which appear on a long time scale while unnatural factors are mainly caused by human activities. These factors like the severe and irreversible drop affected on groundwater level, oil and gas extraction created in a shorter period of time. The pore water pressure has been decreased by extraction of groundwater which is associated with pore water drainage. In other word, the stress caused to the weight of the upper layers gradually transferred the pore water to the grain structure. On the other hand, the influences of weight of sediments in the unsaturated zone will be increased when the submergence force of pore water is lost. Therefore, to compensate for this increase of pressure, the grain structure may change to accommodate this new stress situation. The porosity in the sediments is reduced to bear this additional pressure. This decrease in porosity is associated with a decrease in the volume of sediments, whose surface appearance is subsidence. The oldest known subsidence, based on information provided by UNESCO, has been occurred in Alabama (United States) in 1900. This phenomenon was along with the creation of cracks in the surface of the earth, has been observed in type of areas such as Italy, Japan, England, China, Thailand, Mexico and other parts of the world occurred with over-harvesting of ground water or oil wells. Due to many droughts in Iran (especially in the east and center), excessive exploitation of groundwater has increased that caused to increase the phenomenon of subsidence in many plains. Rahmanian (1986) prepared the first reports related to subsidence in Iran. The phenomenon of subsidence caused to the drop in the groundwater level has been reported in Rafsanjan plain in 1967 a longed with the phenomenon of tube formation in agricultural wells (Hosseini Milani, 1994). Komak-Panah (1997) considered the land subsidence in the Yazd-Ardakan plain as a result of excessive extraction of ground water, dissolution of salts in the soil, and washing of clay cement particles. Nassery (2005) has evaluated Hamedan Faminin plain that the average of annual drop of groundwater level during the nine-year (1991-1999) was about three meters and the amount of alluvial subsidence was different in parts of the plain which the average amount was 45 cm in the mentioned period.
The most of these sinkholes of Faminin Plain was created where made from limestone bedrock and irrigation of agricultural lands was utilized by flooding method.
Moafi and Rahnama (2006) have estimated the amount of land subsidence in the Rafsanjan plain using geographic information system and remote sensing, after correcting and reconstructing the statistics related to the wells in different months. Salehi et al (2012) have estimated the maximum land subsidence in Mahyar plain of Isfahan using InSAR data in the period of 2003-2006 that the subsidence was reached to 8.6 cm per year. The related results confirmed that the subsidence of this area was belonged to the drop in the groundwater level. Tabatabai and Mohseni Nasab (2014) reported that groundwater withdrawal, tectonic factors were also the main causes of subsidence of the Rafsanjan Plain. Parhizkar et al. (2015) have simulated the water level and have evaluated the subsidence of Damghan aquifer using two GMS and GEP models. The related results were shown that there was a significant relationship between the reduction of the groundwater level and the amount of subsidence, and the amount of recent extraction of water resources. Determining the amount and manner of subsidence is very important in its assessment and management. In order to monitoring and measurement of the subsidence rate in the Neyshabour Plain during the period of 2016, there were various methods such as Spirit Level Global Positioning System (GPS) and Artificial Radar Interferometry(SAR). 
Materials and Methods
In order to determine the relationship between the changes of the groundwater level and the subsidence phenomenon in the Neyshabur Plain, the geological and climatical conditions of the area were evaluated and interpreted. Then, the information related to 54 observation wells during 30-year of water period (1987 to 2017) was collected and their corresponding hydrograph have been prepared. The time period 2014/08 to 2018/12 has been prepared using GMTSAR software in Linux environment (Geological and Mineral Exploration Organization of the country, GMTSAR 2020). With comparing the position of the settlement zones with the map of the groundwater level, we have investigated the relationship between two parameters and have analyzed the evidence and consequences of this phenomenon in the studied area.
 
Results and Discussion
Information related to observation wells
According to the hydrograph of the observation wells, the fluctuations of the groundwater level in the observation wells can be classified into three groups: A, B, and C.
-In the observation wells of group, A, the groundwater level did not fluctuate or its fluctuation was very low. This group of wells are located in the northwest area of the plain and they receive very good surface and ground nutrition, so that despite the significant extraction of ground water in their area, they do not show much drop.
-In the observation wells of group B, due to excessive withdrawal from the aquifer in their area, the level of ground water has been constantly falling and the decline was uniform. The observation well of South Hossein Abad Jangal is representative of this group of wells.
-In the observation wells of group C, the ground water level has a sinusoidal downward fluctuation, which means that their water level has decreased, but during the wet season, it has risen as a result of increasing the amount of feeding and reducing harvesting. During the dry period, with the increase in the amount of harvesting, the water level has fallen again, and the resulting drop is much more than the increase during the wet period. Therefore, the water level has dropped more in each water year than the previous year.
Information related to satellite images
The evaluation of subsidence maps prepared in Neyshabour plain have been shown that the highest rate of subsidence in the significant period (2014/08 to 2016/03) was 15.4 cm per year and it was 14.8 cm of subsidence during 2016/04 to 2018/12. It has been over the years. In order to investigate the phenomenon of subsidence more closely and to determine the area of the areas with high subsidence rate, the maps of the average subsidence rate obtained by the radar interferometry method have been classified. The distance between the ranks is considered to be three centimeters per year, based on which six ranks are defined.
The critical areas of subsidence in the Neyshabur Plain have been marked with black oval marks. According to the evaluation of this map, there were four areas with a high subsidence rate (more than 9 cm per year).
-The critical area in the northeast of the plain: this area is located in the east of Neyshabur city and there was the maximum subsidence rate of the plain (14.8 cm/year) where there are the main road and the national railway line of Mashhad-Tehran and also most of the industries area are located in the Neyshabur Plain.
-Central critical area: This area is placed in the west and southwest of Neyshabur city and there is the maximum subsidence rate of the plain as 12 cm per year. The main road, the national Mashhad-Tehran railway line and the most of the farm lands are located in this area.
-Western critical area: there is the maximum subsidence rate of the plain (12 cm per year) in the west of 
Neyshabur section. The main road and the national Mashhad-Tehran railway line have been shown there and the oldest traces of cracks caused by subsidence were shown in Bazoband village.
-South critical area: located in the south and southeast of Neyshabur Plain and the maximum subsidence amount is 12 cm per year. Agricultural activities have been grown up widely in this part of the plain.
Geomorphic implications of the study area
The subsidence related to exploration of groundwater resources can be caused to many of the economic, social, and environmental damages. The type, extent and severity of these damages depend on the amount of subsidence and the affected zone. As the annual surplus of water withdrawal of the non-renewable sources are more than its replacement by rains, this issue can be caused to the risk of spreading subsidence in other parts of the plain. One of the most important consequences of subsidence can be mentioned continuous longitudinal cracks, interrupted cracks, round holes, wide pits and sinkholes. Longitudinal cracks are observed in many parts of the Plain margin which were formed by the joining of discontinuous cracks with a definite extension. Discontinuous cracks which are continued in the initial steps of creation of longitudinal cracks. After rainfall and flooding create continuous longitudinal cracks (Rokni et al, 2015). Circular holes are the result of subsidence of the lower layers and can be seen in parts of the edge of the plain. The creation of countless cracks has created a very high vulnerability potential for transportation infrastructures, power transmission lines, industrial areas and even residential areas. According to the map of vulnerable areas, there are four areas with a high rate of subsidence and vulnerability potential in the northeastern, central, western and southern parts of Neyshabur Plain, and important infrastructures such as oil and gas transmission lines, railway lines, urban transmission lines and Intercity corresponds to these areas. It is obvious that if not controlled, it will impose life and financial risks in the not too distant future.
Conclusion
Generally, what emerges from the results of the studies of ground water level changes in observation wells and the satellite interferometric method in this research is that in the Neyshabur plain, there are four critical areas in the northeast, central critical, western critical and southern critical areas with high rates of subsidence (more than from 9 cm per year. The highest amount of subsidence has occurred in the east of Neyshabur city with the maximum rate of subsidence of the plain (14.8 cm per year), where the main road, the Mashhad-Tehran national railway line and the major part of the industries of the Neyshabur plain are located in this area. are located. In the other three critical areas, the maximum rate of subsidence is 12 cm per year. The graphs of the rate of groundwater level drop and the rate of subsidence of the plain show that the drop of the groundwater level has a significant effect on the land subsidence. In a large part of the Neyshabur Plain, the rate of drop 30 years has been more than 20 meters and the four subsidence zones correspond to the parts of the plain where the drop in the groundwater level has been more than 30 meters. The results of this research show that water withdrawal exceeds the recovery capacity of the aquifer as one It is one of the main reasons for subsidence in the plain, so it is necessary to pay special attention to the management of ground water resources. The consequences of the subsidence phenomenon in the plain have been the creation of continuous and interrupted longitudinal cracks, circular holes and wide pits. The expansion of these cracks to the main road, national railway line and oil and gas transmission lines may cause various risks.