Factors affecting on the groundwater uplift in the Mashhad city, Iran

In recent years, rising water table has been reported in many cities of the world. Groundwater and engineering structures are interconnected; 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 water needs are supplied from the Mashhad aquifer, Dousti dam and other catchment areas. The water table uplift in Mashhad city created a problem when commercial and residential complexes with a depth of more than 20 meters of excavation with the arrival of significant amounts of water to the lower classes were encountered. In addition, due to the lack of necessary measures due to the declining groundwater level in the past years; the encroachment on water during the drill of the second metro tunnel has caused 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 development of urbanization and the ever-increasing depth of excavation in urban areas may sometimes have impacts on the ground water situation. Therefore, approach of this research is to assess the major factors effecting on the groundwater uplift in Mashhad city. In order to determine the relationship of these factors with water table fluctuations, time series analysis was used in both time and frequency domains. One way is to move up. According to the groundwater hydrograph, a decreasing trend are observed during the years 2005-2009. While, from 2009 to 2014, these fluctuations followed the uptrend. Time series analysis shown, due to existence of impermeable surface in the city, low infiltration of rainfall in the soil and water table depth more than 5m, rainfall and evaporation did not have a tangible impact on the fluctuations in groundwater levels in these areas. The annual expansion of the sewage network and the transfer of wastewater collected to wastewater treatment plants with a lag tome 1 to 2 months have a significant effect on reducing the water table in areas with sewage network; While in areas without sewage network, due to the penetration of water from septic wells into ground water, it increases the level of water in these areas. The arrival of more than 800 million cubic meters of water from the Dousti dam and reduction of more than 40 percent of discharged groundwater in Mashahd aquifer with lag time 1 to 2 months, are the most influential factors in groundwater uplift. The lack of development of sewage networks in the southern regions and some parts of central Mashhad city has led to an increase in groundwater levels in these areas. Due to the extraction of soil from saturated and unsaturated zones during the drilling of the 2nd line of metro 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, soil porosity has been decreased. This porosity was previously filled with water. But 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 on the groundwater uplift in Mashhad city, Iran. In order to determine the relationship of these factors with water table fluctuations, time series analysis was used in both time and frequency domains. Results show lag times between water table with rainfall/evaporation, transferred water (Dousti Dam) to the town, discharged water from aquifer, expansion of sewage network in region with network and region without network were 9, 2-3, 3-4, 1-2, 3-4, 2-3 and 3-4 months respectively. In this study a calculated drawdown by exploitation from aquifer equals 0.5 m, the calculated rising equals 0.44 m due to transferred water from Dousti Dam, the expected groundwater rising equals 2.7 m in region without sewage network and an expected drawdown equals 1 m (if no water was transferred from Dousti dam). Also, effects of excavation in line 2 of subway and urban foundation digging on the groundwater and soil properties were studied. Results show that the water storage capacity in saturated/unsaturated zones have been reduced and water table raised about 1.25 m in the town aquifer.