عنوان مقاله [English]
Investigating the hydrogeochemical characteristics of springs in Zagros karstic landforms can be a useful guide to determine the extent of Karst development and management of water resources in these areas. The purpose of this research was to compare the development of karst with the help of hydro geochemical characteristics of Shahoo mass karstic springs having an area of 649 km2 in the thrust Zagros zone of the Kurdistan and Kermanshah Provinces, and Karstic aquifer of Islamabad with an area of 818 km2 in the simply folded zone of Zagros in Kermanshah Province. The most important carbonate formation in the Shahoo aquifer is Bisotun Limestone and in Islamabad aquifer is dolomitic limestone of Asmari and Telezang Formation. Therefore, due to the different dissolution capacities of various Zagros formations, this study investigates the dissolution of cations and anions, and in particular the rate of karstification of the two regions, by assessing the hydro geochemical characteristics of the karstic springs in the study areas.
In order to evaluate the hydro geochemical characteristics of the karstic springs in the Shahoo and Islamabad aquifers, after field visits , the karst features and formations, faults and fractures were identified and their location was determined using GPS .Then, 17 water samples were taken from karst springs of the Shahoo and Islamabad aquifers in wet season (May 2017). Finally, using ionic ratios (calcium to magnesium ratio), combinational charts and Calcite and Dolomite saturation indices using R-software and PhreeQC software, different water characteristics of karstic aquifer and dissolution rate of anions, cations and karstification of the two regions were investigated.
Results and discussion
The results of quantitative water analysis helps to explain the karstic features of the areas. Generally speaking, the hydro chemical properties of water, along with geological formations, reflect the degree of dissolution of carbonate compounds and, in turn, the extent of karst development in the regions. Therefore, for the evaluation of hydro geochemical properties of karst springs, 17 water samples were collected from permanent springs in two Shahoo and Islamabad aquifers and the results were investigated by hydro geochemical methods. The results of the high calcium/magnesium ratio in Shahoo springs compared to the Islamabad aquifer clearly indicate the high purity of limestone and the presence of developed karst aquifers in the Shahoo area (Bisotun limestone) compared to the dolomite Karst of Islamabad aquifer (Asmari and Shahbazan Formations).This is confirmed by the high values of the calcite saturation indices and the lower degree of dolomite saturation indices in the Shahoo compare to Islamabad aquifer. This indicates a large connection between groundwater and the lithology of the aquifer, impurities in the lime of the region, diffusion nature of Karst and a long residence time, all of which are indicative of lower Karst development in Islamabad area. The low discharge of the springs of this area along with low fluctuations in the discharge in dry and wet seasons as well as the high dissolved ions (water electrical conductivity and hardness) as well as the amount of bicarbonate and sulfate ions in the water confirm the lower karst development compare to Shahoo aquifer. Therefore, it can be concluded that the type of flow regime in the Shahoo aquifer is mainly conduit flow which the conduits are well developed, while the karst system in Islamabad aquifer is mainly diffuse.
The results of the Ahmad Synthetic Chart (MRD) indicate that the type of water in most of the Shahoo massif is of bicarbonate calcium type (calcite bicarbonate) and water of most springs of the of the Islamabad aquifer is of the bicarbonate magnesium type. This diagram also confirms the lithology of Shahoo aquifer and Islamabad aquifers as limestone and dolomite respectively, and consequently indicating the greater dissolution potential and karst developmental within the Shahoo area (as calcite dissolves faster than dolomite) in contrast to the Islamabad aquifer.
The most important difference between the two Shahoo and the Islamabad aquifers are the difference in rainfall and temperature, the different effects of tectonics and different lithology, causing a difference in the solubility of the region which created springs with different discharges in the regions. High rainfall, snowiness, low temperature, which dissolves more carbon dioxide in water, and lithology and geomorphological features of Shahoo aquifer have led to the higher karst development in this area. Although Asmari lime in the Islamabad area is also susceptible to karstification, but it has youthfulness, purity and less thickness compared to the Bismuth lime, lower precipitation mainly in the rain form, higher temperature and the effect of geomorphologic factors such as aquifer shape, altitude and different effects of tectonic performance, have led to a lower degree of Karst development in Islamabad than Shahoo, which was examined and confirmed by hydro geochemical studies in this study. Considering the importance of the karstic territories of western Iran, and in particular the Kermanshah province and the vulnerability of karst aquifers, their vulnerability assessment is one of the most important management approaches to encounter potential risks, such as pollution, water supplies and the crisis of water resources. The study of karst development can provide a realistic view to investigate the karst system in such a way that with the help of these results, in addition to the accurate knowledge of the chemical and physical properties of these aquifers, we can better manage the amount and time of using of these aquifers that has to be taken into consideration by the relevant authorities.