استفاده از تکنیک‌های ژئوشیمیایی و ایزوتوپی برای شناسایی منشأ شوری آب زیرزمینی در دشت شاهرود

Salinity is the most important factor in reducing the quality of groundwater resources, especially in arid and semi-arid regions. Due to droughts, water shortage and over-exploitation of aquifers, salinity has become a growing problem in Iran. Shahrood aquifer, is one of the aquifers that needs exact monitoring and management to control and prevent the spreading of salinity in its water resources. Groundwater in the northern part of the Shahrood aquifer is a bicarbonate type and has good quality, but in the southeast suddenly, its quality declines and eventually turns into saline water with sodium chloride type. The groundwater salinity in Shahroud plain has caused many problems for farmers, including crop reduction and land degradation. The present study is conducted to investigate and identify the origin of groundwater salinity in the southeastern part of the aquifer.
In order to determine the source of groundwater salinity in Shahroud aquifer, 120 wells were sampled. The electrical conductivity (EC), pH and temperature were measured in the field. Samples were analyzed for determining the major ions and rare elements (Br and I) at the Geochemistry Lab of university of Ottawa. Also, 34 samples were analyzed for determination of ¹⁸O and ²H isotopes in GGHagh lab and 3 samples were analyzed for measurement of tritium isotope in AMS laboratory of university of Ottawa.
The EC in the Shahrood aquifer ranges from 671 to 11210 microseisms per cm. The amount of this parameter is highest in the eastern and southeastern parts of the aquifer that indicates the existence of salinity sources in these parts. Despite the high salinity of samples in the eastern region and the generally flow direction of the groundwater (which is from east to the west of aquifer), western samples have a desirable quality and lower EC. In the central part of the aquifer, there are samples with different amounts of EC that are the result of mixing of the eastern and southeastern saline waters with freshwater. The suddenly changes of EC in the Shahrood aquifer represent a geochemical discontinuity that could have occurred for various reasons, including the performance of an impermeable barrier, such as a fault, presence of an impermeable layer, or a change in the flow direction. According to the geology and lithology situation of Shahrood aquifer, the most probable hypothesis for the groundwater salinity in this aquifer is the dissolution of evaporative formations whose dispersion in the eastern and southern parts of the aquifer is consistent with the high salinity areas in the aquifer. But despite the fact that there are also evaporative formations in the southwest parts of the aquifer, the salinity of samples taken from these parts, is low. Different hydrochemistry diagrams and ion ratios such as Na and Cl ratios, and relationship between Cl and I with Br, as well as the amount of saturation index of different minerals showed that the most important source of groundwater salinity in Shahrood aquifer is dissolution of evaporative minerals such as gypsiferous marl. These results were confirmed by the linear relationship between Cl and ¹⁸O. The location of fresh water samples on the Shahrood meteoric water line indicates that the source of these samples is meteoric water and present precipitation. However, saline water samples, despite the similarity in δ¹⁸O isotopic amounts, show a depletion in δ²H relative to fresh water and present precipitation. The amounts of tritium in the saline water samples of Shahrood aquifer is less than 0.8 TU and in freshwater sample is 2.8 TU. Considering the age distribution of water based on the concentration of tritium and the different results based on tritium in saline and fresh samples, at least two water resources with different ages can be identified in Shahrood aquifer. Since the fresh water has a higher tritium content than saline samples, it can be represented that the freshwater is younger than saline water in this aquifer. The southern saline water with tritium content less than 0.8 TU represents a mixture of relatively old water that fed before 1952 and newer meteoric waters. In order to determine the source of the saline water in the southeast part of the aquifer, the average isotope content of saline waters was plotted with a horizontal straight line on the SMWL. The position of the intersection point on SMWL showed that the source of saline waters is the past meteoric water, which has a lighter isotopic content than the current precipitation. Regarding the lower dexess content of saline samples, the past meteoric conditions are wetter and therefore, the depletion of old precipitation and consequently saline samples is justifiable. The movement of water in the aquifer and water-rock reactions, such as the dissolution of gypsum, cause the change in the water type and enrichment of ¹⁸O (δ¹⁸O-Shift), then the saline samples shifted to right in SMWL. Due to the difference in the age of saline and fresh waters, and the difference in the stable isotopes’ content, as well as the hydrochemical characteristics, it is necessary to study the hydrogeological factors that are effective in the occurrence of these conditions. Based on the hydrogeological map of Shahroud aquifer, in the central part, a water divide line has been created due to the concentration of productive wells and aquifer over exploitation. Due to the operation of this water divide line, a part of the input flows that flow from the northern, eastern and southeastern boundaries are diverted to the east. In other words, this dividing line decreases the flow rate of groundwater, which reduces the hydraulic connection of input flows from the east and south with the western parts of the aquifer. Reducing the flow rate of groundwater, while increasing the age of water by increasing its resident time, provides conditions for more mineral dissolution, thereby increasing the salinity and variations in the groundwater type. The gypsum dissolution, in addition to changing the type of water to sulfate water, results in water enrichment of oxygen-18 (Shift-¹⁸O) and therefore, the shift of saline samples to the right of the SMEL.