Introduction

Groundwater resources is the largest global reservoir of liquid freshwater, which is under increasing stress due to inappropriate usage and over exploitation (Wang et al., 2019). his water resource plays a essential role in sustaining ecosystems in a vast area of semi-arid land globally (Minaei and Irannezhad., 2016). The quantitative assessment of groundwater resources and study groundwater table changes is principal for manage water resources and sustainable management of this resource, particularly as population growth, socioeconomic development, and climate changes further press the water resources. Groundwater resources is the most important water resource in Iran and other regions with similar types of semi-arid and arid climates. this water resource have decreased due to overdraft and unfortunately, this water resource is endangered. Over-drafting is causing groundwater levels to drop continuously and dramatically in arid and semi-arid regions such as Iran, leading to a global groundwater crisis. In recent years, agricultural development has caused a decrease in Iran’s groundwater resources. Most agricultural lands in Iran are irrigated by groundwater. In this study, an attempt was made to analyze changes in the groundwater table using Modeling of groundwater table fluctuations in Shush plain.



Methodology

Shush plain is located in the southwest of the Iran, Khuzestan province and in the middle to the end of the Dez river basin with an area of 1996 square kilometers and includes Sabili plains, Lor (Andimeshk) plain, Dimcheh plain, Western Dez plain and Eastern Dez plain. be The study area leads from the north to the Molab and Sarab Jaldan basins, from the west to the East Abbas, Avan and Chenane-Sorkheh basins, from the south to the Shushtar and Ahodasht basins, and from the east to the Gotvand and Lali basins (Soltani et al., 2018). The most important rivers in Shush plain include Dez, Kohank, Golal, Balaroud and Shavor rivers, and the water of these drains is widely used, especially in irrigation and agriculture. The average annual rainfall in the area of the plain is calculated to be about 328 mm. Also, the amount of surface runoff drained from the area is equal to 126 mm. In this area, 14 exploratory wells with a diameter of 16 inches have been drilled, and their depth continues up to the extent of determining the bedrock of the groundwater table. The distribution of these wells in the Sush plain is relatively. Also, in order to investigate the layers of the earth, for each meter of depth, a soil sample was taken to investigate the type of sediments. In the following, the information related to excavations has been used in a more detailed investigation and understanding of the condition of the groundwater aquifer. During the research, using the field method, well logs and GIS, the changes made in the study area were periodically compared. To achieve this goal, ArcGIS software was used to extract the required data and zone groundwater sources. After collecting and analyzing the data used for the water level of piezometric wells of Shush plain in the ArcGIS software environment, interpolation zoning maps were prepared using the IDW method. Also, using WinLog software, geological sections of wells were drawn in different directions (east-west and north-south). Wells with more complete data were used to evaluate and analyze the data of water table changes and water table drop. Finally, the fluctuations of the groundwater table were compared and evaluated based on the logs of the wells and the data of the groundwater table of the wells in the Shush plain.



Results and Discussion

Examining the maps of the depth and level of the reservoir (Fig. 2 and 3) showed that the direction of the groundwater flow in the Shush plain is from the north and northeast to the south and west of the region. The highest level of groundwater is mainly in the northeast, which decreases towards the southwest. In the east of the plain, the general direction of the flow is from the northeast to the southwest. The groundwater table controls the direction of flow from northeast to southwest, and it has a north-south flow direction around the city of Dezful (Fig 4). In some intervals, there is no hydraulic connection between the aquifer of the western Dez plain and the river. In general, it can be said that the conglomerate highlands, with the exception of the conglomerate located in the west of Balaroud, are the sources of groundwater in the region and affect the direction of the groundwater flow. Another feeding and draining source is the Dez River, which feeds the right bank (Andimeshk Plain) in the first 2 kilometers of the route, but does not play a role in feeding the left bank. According to the log of piezometric wells and geological sections of wells, in the northern margin of Shush Plain (around the city of Dezful-Andimeshk), the texture is mainly of coarse-grained sediments and it is the result of erosion of Bakhtiari conglomerate. In the logs obtained from these wells, coarse-grained sediments in the sizes of sand are observed. The sediments of Bakhtiari Formation are the most important formation of Dasht Shush due to their permeability, intergranular adhesion and appropriate porosity in terms of groundwater resources and aquifer feeding. Also, the presence of fine-grained sediments in the range of silt and sand in some well logs.



Conclusion

According to the surveys conducted in the Shush plain, due to arid and semi-arid climatic conditions, irregular rainfall distribution in terms of time and place, increasing demand for water resources and limited surface water resources, there is a great dependence on the groundwater of the region. Also, indiscriminate harvesting for agricultural purposes, considering that most of the study area belongs to the agricultural sector, has caused a drop in the water level in the region. Although the Dez River helps to feed the reservoir to some extent, due to over-harvesting and lack of rainfall, this source of power is also in risk, and if pumping from wells is not managed, it faces the risk of water shortage.