Delineating the source and mechanism of groundwater salinization in a semi-arid region of southeastern Iran using geophysical and hydrochemical approaches

Introduction
Groundwater salinization is a major environmental problem, especially in arid and semi-arid regions of the world. Different natural processes and anthropogenic activities can cause groundwater salinity. Factors such as rainfall, evaporation, groundwater pumping, agricultural and industrial activities, and artificial recharge of aquifers can affect the salinity of groundwater. One of the important factors that cause the salinity of freshwater aquifers is natural saline waters. Natural salinization of groundwater has occurred in many regions of the world. This phenomenon has been introduced as "dryland salinity". As a result of this phenomenon, salts accumulate in soil and water and affect human life and natural ecosystems. Several factors cause the natural salinity of groundwater in semi-arid regions. These factors include locally derived cyclic salts, salts in wind deposits, salts in marine deposits, salt domes, unsaturated zone salts and, salts resulting from rock weathering. Delineating the origin and mechanism of salinity is an important help in preventing the degradation of groundwater quality and optimal management of groundwater resources. In recent years, the phenomenon of salinization of groundwater has been observed in some areas of the Faryab plain in southeast Iran. This study aims to identify the origin and mechanism of groundwater salinity in Faryab plain aquifer.
 
Materials and Methods
This paper presents an integrated geophysical and hydrochemical investigation of the saline water intrusion into the Faryab plain. Geophysical studies were conducted by the geoelectrical method and include 55 electric soundings. The results of geoelectric studies have been analyzed using iso-resistivity maps and geoelectrical profiles. Twenty-seven water samples were also collected from abstraction wells to assess the quality of groundwater. These samples have been analyzed to determine the concentration of main cations and anions. The results obtained from the chemical analysis of water samples have also been examined and analyzed using spatial distribution maps of qualitative parameters, bivariate diagrams, and time series of water salinity. The mixing rate of saline and fresh groundwater has also been evaluated using ionic ratios of Na/Cl and Cl/HCO₃.
 
Results and Discussion
The lowest electrical resistance was recorded in the central part of the plain. The specific resistance of the saturated zone decreases towards the center of the plain. Therefore, fresh water and saline water are hydraulically connected. According to the spatial variations of specific resistance, the Faryab plain is divided into two regions: one area with a resistance of more than 50 ohm-m and one region with a resistance of less than 10 ohm-m. The area with low specific resistance (less than 10 ohm-m) is observed in the center of the plain. The most important reasons for the existence of this area are the high groundwater level, the surface saline layers, and the salinity of groundwater. The electrical conductivity of groundwater reaches 64000 μmohs/cm in the center of the Faryab plain. The highest amount of sulfate and chloride ions are also observed in the water samples of this area. According to the Gibbs diagrams, the groundwater has been also influenced by the evaporation process.
 
 
 
Conclusion
In this study, the origin and mechanism of groundwater salinity in the Faryab plain in southeast Iran were investigated. Fine-grained sediments have been deposited in the central part of this plain. In the past, the underground water level in this area was a little far from the ground level and caused an evaporation zone of surface and underground water. As a result of water evaporation in the central part, evaporative sediments including chalk and salt sediments were formed and the amount of underground water salts increased.
Based on the results of geoelectrical studies, in the central areas of the plain, the amount of specific apparent resistance decreases, which indicates the presence of fine-grained sediments containing salt water in these areas. Investigating the quality characteristics of underground water also shows the occurrence of processes such as dissolution of halite, dissolution of gypsum and the occurrence of cation exchange process in the aquifer. Evaporation from underground water has also affected the water quality, especially in the middle part of the plain. As a result of high pumping from the production wells, the hydraulic load of the aquifer has decreased and the saline groundwater zone has expanded towards the production wells. Saltwater intrusion has caused salinity and quality degradation of underground water. According to the results of this research, the most important reason for the salinity of underground water in Faryab plain is excessive extraction of underground water and disturbing the natural balance of the aquifer. The most important solution to deal with the development of the salt water front in the Faryab plain is to reduce the exploitation of the aquifer, especially in its middle areas. It is also suggested to modify and improve existing artificial feeding facilities and locate new artificial feeding projects to control saltwater intrusion. Sampling from different depths of the aquifer and measuring minor ions such as iodine and bromine will greatly help to better understand the groundwater salinization process. Measurement of environmental isotopes such as oxygen-18, deuterium and chloride-36 will also help to enrich future studies. By preparing the mathematical model of the aquifer, the effect of different management measures on the salinity control of the aquifer can be investigated. By using the results of the mathematical model, the aquifer balance can be checked and the amount of underground water withdrawal can be determined.