بررسی تغییرات شوری رودخانه به روش هیدروشیمیایی در مناطق بیابانی (مطالعه موردی: رودخانه نمکلان، استان ایلام)

آب برخی از رودخانه‌های مناطق خشک و بیابانی، ضمن عبور از برخی سازندهای زمین‌شناسی شور شده و کیفیّت آن کاهش می‌یابد. پژوهش حاضر به منشایابی شوری رودخانه نمکلان در ناحیه‌ای گرمسیری در استان ایلام به روش هیدروشیمیایی می‌پردازد. نمونه‌گیری از آب این رودخانه در چهار فصل سال، از شش نقطه در طول مسیر رودخانه نمکلان و از دو نقطه از رودخانه گدارخوش قبل و بعد از تلاقی با رودخانه نمکلان انجام گرفت. تیپ آب رودخانه نمکلان، کلرور سدیمی بوده و به‌دلیل ماندگاری و عدم تغییر معنی‌دار بیشتر این عناصر در آب، ترکیب شیمیایی آب رودخانه در طول مسیر ثابت است که این موضوع با آزمون آماری t مستقل در سازندهای گچساران و آغاجاری بررسی شد. بر اساس نتایج به‌دست‌آمده از آنالیز نمونه‌ها با استفاده از دستگاه جذب اتمی، میزان املاح و یون‌های مختلف قبل از تلاقی رودخانه نمکلان به گدارخوش، در رودخانه گدارخوش کمتر بوده، امّا پس از اینکه رودخانه نمکلان به گدارخوش می‌پیوندد، میزان این املاح و یون‌ها در رودخانه گدارخوش افزایش معنی‌داری می‌یابد. علّت این تغییر کیفیّت، ورود املاح و یون‌های مختلف از رودخانه نمکلان به رودخانه گدارخوش است. به‌طوری که حداکثر میزان هدایت الکتریکی، نسبت جذب سدیم و کربنات سدیم باقی‌مانده در دی‌ماه در رودخانه نمکلان به ترتیب 336، 75 و 23 برابر مقادیر آنها در رودخانه گدارخوش است. با توجّه به آنالیز نمونه‌ها و تیپ آب رودخانه نمکلان مشخّص می‌شود که منشا شوری رودخانه، افق‌های نمک‌دار سازند گچساران در منطقه است که به‌طور عمده در عمق قرار گرفته و در محدوده مورد بررسی رخنمون سطحی ندارند و توسّط چشمه مولد رودخانه نمکلان به سطح زمین راه می‌یابد.

Evaluation of the River Salinity Changes using Hydrochemical Method in Arid Area(Case Study: Namaklan River, Ilam)

In arid and semiarid regions, the water quality of some rivers decreases by salinization while passing through some geological formations. This study aims to find the source of the hydrochemical salinity of Namaklan River. Sampling was performed at 6 points along the Namaklan River for 4 seasons and also from before and after the confluence of Godarkhosh River with the Namaklan River. Namaklan River water type is sodium chloride,and due to the persistence and insignificant changes of these elements in water, the chemical composition of river water is constant along the path, which was analyzed by independent samples ttest in Gachsaran and Aghajari Formations. The results showed that, the concentration of different ions and salts before the confluence is low in the Godarkhosh River while it increases at the point of conjunction. This is due to the arrival of different salts and ions from Namaklan River to Godarkhosh River. So that the amount of EC, SAR and RSC in Namkalan River in January were 336, 75 and 23 times more than before joining to the Godarkhosh River, respectively. Based on the chemical analysis and the existence of specific compounds in water samples, it was found that the source of salinity was the influence of Gachsaran formation, because the minerals and salts in the water of the river correspond to the chemical composition of Gachsaran formation.Extended Abstract1IntroductionIncreasing the salinity of natural sources of drinking water has been reported as one of the problems affecting lowincome countries, but it has not yet been thoroughly investigated. Since sodium chloride is the main salt solution in water, even at a considerable distance from any evaporate formation, and understanding the chemical properties of water in sedimentary basins is important for several geological processes, such as the exchange of liquids and their transmission routes in rocks and the mechanisms for absorbing hydrocarbons. Salinity, which is one of the primary parameters to assess the quality of river water and its increase, has negative impacts on river consumption and river ecosystems, also leads to decrease the quality of surface water and underground water. The factors that negatively impact the quality and quantity of water resources include the geographic location, topographic features, and the expansion of the impervious geological formations, as well as the impact of these formations on water resources. Among the abovementioned factors, geological formations, especially the Fars group (Gachsaran, Mishan, and Aghajari Formation), have the highest impact on water quality. Therefore, the general objective of this paper is to determine the salinity of the Namaklan River using the hydrochemical method, to study the spatial variation of salinity in the river, and to study the effect of geological formations on the salinity of this river.2Materials and MethodsThe Namaklan River is located in the southwest of Ilam province in a hot region near Iraq border, and the water of the Namaklan river flows into a large river in the south, called Godarkhosh River. Some of the saltwater is evaporated during the river path and the remains salt on the riverbanks, especially in the warm seasons, forms the special and interesting features.Initially, base map of the study area, stream network and geological formations were prepared, and then, the suitable sample points were identified. Formerly, by field survey, data of the source of water and the other 7 points were obtained at random interval method during the four seasons of the year, which the point 7 and 8 were located on the Godarkhosh River. The first six sampling points are located on the Namaklan River, with its first three points in the Gachsaran Formation and the next three points in the Aghajari Formation. The points were selected in areas where the flow of water was stable and without turbulence which can mix sediment to the water sample and at each point, the water samples collected by 1liter bottles. Meanwhile, at the time of sampling, temperature, Electrical Conductivity (EC), pH and Total Dissolved Solids (TDS) were measured by portable electrical conductivity meter (EC meter). The samples were then transferred to the laboratory, and different ions (Ca, Mg, Na, HCO3, SO4, CL, NO3, PO4, Br, B, Zn, Hg, Pb, As, Mn, Fe, and Al) were measured. Finally, by examining the results using independent samples t test, the amount and trend of ion changes in the points and the different seasons were determined, and the river salinity source was determined based on measured parameters.3Results and DiscussionAfter the analysis of water samples, the values of Na, Sodium Adsorption Ratio (SAR), Electrical Conductivity (EC), and Residual Sodium Carbonate (RSC) parameters were measured at different sampling points to determine different water grades based on the Wilcox classification.Based on the SAR and EC at points of 1 to 6, water quality was classified as C4S4 (very salty unsuitable for farming), but for the point 7 as the C4S3 class (very salty unsuitable for farming) and point 8 as the class C3S2 (salty usable for agriculture). Accordingly based on the percentage of sodium, the water quality for the point of 1 to 6 was bad, at point 7 in the susceptible class of and at the point 8 in the class of acceptable. Also, water quality based on RSC is suitable at all points.To determine the ionic frequency, first the concentration of anions or cations calculated, then the anions and cations ordered based on concentrations decreasingly. The water type is, in fact, the dominant anion and water facies are the dominant cationic. The water type in the 1 to 6 points was chlorine type and at the 7th and 8th places was sulfate type. Water facies at all sampling points was sodic. The abundance of ions indicates that from 1 to 6 points, which is related to the Namaklan River, the abundance of chlorine ions is due to the dissolution of salt at the depths of the Gachsaran Formation by intrusive waters. Based on the results obtained from the analysis of the samples, the amount of salts and different ions varies from place to place, which is influenced by the Gachsaran Formation.4ConclusionGachsaran is one of the most important formations of the Zagros zone. The unique features of this formation have been given a special status from the point of view of earthquake, geology, engineering, hydrology and mechanical stratigraphy. Due to the similarity of the composition of the elements in the river water of Namaklan as well as Gachsaran Formation, it is concluded that the source of salinity of the spring is due to the geological formation of the area and also the mechanism of salinity of the spring water, which is penetrated by water and its composition with the bottom layer Gachsaran Formation shows that the salinity of the river is due to the geological formation.