عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In recent years, rising water table has been reported in many cities around the world. Therefore, water table fluctuations in these areas can affect structures. Mashhad city is one of the major metropolises in Iran and due to indigenous population and tourists, it needs various resources to supply water. The city's water needs are supplied from the Mashhad aquifer, Dousti dam and other catchment areas. The water table uplift in Mashhad city caused problems for commercial and residential complexes with a depth of more than 20 m of excavation, leading to penetration of groundwater to buildings basement. In addition, the existence of groundwater during the excavation in the second subway tunnel has been created serious problems. The lack of full development of drainage systems and municipal sewage networks have led to an increase in rising groundwater levels in some parts of the city. Also, the urbanization development and the ever-increasing depth of soil excavation in urban areas may have affected the ground water situation. Therefore, the aim of this research is to assess the major factors affecting the groundwater uplift in Mashhad city. In order to determine the relationship between these factors and water table fluctuations, time series analysis was used in both time and frequency domains.
According to the groundwater hydrograph, a decreasing trend is observed during the years 2005-2009. While, from 2009 to 2014, these fluctuations followed the uptrend. The time series analysis indicates that rainfall and evaporation did not have significant impacts on the fluctuations of groundwater level in these areas, due to the existence of impermeable surface in the city, low infiltration of rainfall in the soil, and water table depth more than 5 m. The annual expansion of the sewage network and transfer of collected wastewater to wastewater treatment plants with a lag time of 1 to 2 months have a significant effect on reduction of water table in areas with sewage network; while in areas without sewage network, due to the water penetration from septic wells into groundwater, the water table has been increased. The arrival of more than 800 million m3 of water from Dousti dam and reduction of more than 40% of discharged groundwater in the Mashahd aquifer with lag time of 1 to 2 months, are the most influential factors in groundwater uplift. The lack of sewage networks development in the southern regions and some central parts of Mashhad city has led to an increase in groundwater levels in these areas. Due to the soil extraction from saturated and unsaturated zones during the second subway line drilling and the development of excavation and construction of civil engineering projects in the city's central area, especially around the Imam Reza's holy shrine, the soil porosity has been decreased. This porosity was previously filled with water, while they are now occupied by impenetrable materials. Therefore, the water has been forced to migrate to other places.
The aim of this research is to assess the major factors affecting groundwater uplift in Mashhad city, Iran. In order to determine the relationship between these factors and water table fluctuations, time series analysis was used in both time and frequency domains. The obtained results show lag times between water table and rainfall/evaporation, transferred water from Dousti dam to the town, discharged water from aquifer, and expansion of sewage network in regions with and without network were 9, 2-3, 3-4, 1-2, 3-4, 2-3 and 3-4 months, respectively. In this study an expected drawdown by exploitation from aquifer, an expected rise due to water transfer from Dousti dam, an the expected rise in groundwater surface level in regions without sewage network, and an expected drawdown (if no water was transferred from Dousti dam) equal to 0.5, 0.44, 2.7, and 1 m, respectively, have been occurred. Also, the effects of soil excavation in line 2 of subway and urban foundation digging on the groundwater and soil properties were studied. The results indicate that the water storage capacity in saturated/unsaturated zones have been reduced and water table raised about 1.25 m in the town aquifer.