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.