استفاده از تکنیک‌های ژئوشیمیایی در شناسایی منشأ یون‌ها و فرآیندهای کنترل کننده کیفیت آب آبخوان شاهرود

IntroductionGroundwater is in contact with different materials and undergoes several reactions, in its flow way, from the recharge point to the discharge point. In this path, some of the materials are dissolved in water and some others are deposited in it, and the water quality changes in the aquifer. Determining and identifying the factors affecting water quality, can be an effective step towards the quality management of aquifers. In the north of Shahrood aquifer, the groundwater type is bicarbonate with a good quality and its EC is about 671 μs/cm, while water quality decreases gradually in the southward direction and eventually it changes to saline water which is from sodium chloride type and the amount of EC (μs/cm) is 11210. The present study was carried out to identify the origin of ions and important processes controlling water quality in Shahrood aquifer. Materials and MethodsIn order to investigate the geochemical processes of this aquifer, the results of analysis of 120 groundwater samples taken in June 2018 were used. The maps of EC dispersion, water type, ion concentrations, two-variable diagrams such as Ca2+Mg2 versus HCO3+SO4, Ca+Mg versus TDS, Na versus Cl and Ca versus HCO3, ion exchange diagrams and saturation indices of halite, gypsum, calcite, dolomite and aragonite were used. In the present study, ions with a common origin, or twin ions, such as sodium and chlorine, were studied together and the imbalance between them was considered as significant factors and processes affecting the quality of Shahrood aquifer.Results and DiscussionAccording 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’ dispersion in the eastern and southern parts of the aquifer. This is consistent with the high salinity areas of the aquifer. However, there are evaporative formations in the southwest parts of the aquifer too, even though the salinity of samples taken from these parts is low. Based on the results of this study, halite and gypsum dissolution processes and precipitation of calcite and dolomite and direct and reverse cation exchanges are the most important processes for controlling water quality in Shahrood aquifer. The creation of the water divide line in the eastern and southeastern parts of the region has led to changes in the general trend of groundwater flow in the aquifer. The general direction of groundwater flow in the Shahroud aquifer is from the northern, eastern and southeast parts to the central, western, and southwest parts. But the operation of groundwater divide line, changed the flow direction to eastward around the village of Royan. This hydrogeological complexity has also affected the aquifer's groundwater quality. In fact, the groundwater divide line has prevented the penetration of saline water from the eastern and southeastern parts to the central and western parts of the aquifer. This has caused these saline water masses to be non-effective on the hydrochemistry of the Shahrood aquifer.ConclusionBased on the results of this study, halite and gypsum dissolution processes and precipitation of calcite and dolomite and direct and reverse cation exchanges are the most important processes for controlling water quality in Shahrood aquifer. The groundwater divide line, has prevented the penetration of saline water from the eastern and southeastern parts to the central and western parts of the aquifer. This has caused these saline water masses to be non-effective on the hydrochemistry of west the Shahrood aquifer.