بررسی سیستم غالب جریان و سازوکار تخلیه چشمه‏‎های کارستی غرب مجن

Karst aquifers are one of the most important water resources in the world. The importance of karstic aquifers comes from the fact that they provide about 25%of the world's drinking water (Ford and Williams 2007). In addition, the in karstic aquifers contain high quality water resources and they are suitable for drinking water. Springs are one of the most important hydrogeological phenomena of the hydrological cycle (Fiorrilo 2014). In karst systems, the dissolution channels are mainly connected to the karst springs. Therefore, evaluation of discharge, its variability and values of electrical conductivity and major ions can reflect the properties of the total system (Rezaei 2000). To study the hydrochemical characteristics of groundwater flow systems, comprehensive information on groundwater chemistry is used which depends on various factors such as location geology, rock weathering and its type, water quality of feeding into the aquifer, and hydrogeochemical reactions (Coetsiers and Walraevens 2006).
Hydrogeological characteristics of karstic aquifers are very difficult to study due to the high heterogeneity of aquifers (Mohammadi and Raeisi 2007, Mohammadi and Field 2009). Therefore, one of the common and low-cost methods for studying karst aquifers in the last four decades is to study the temporal variations in the physical and chemical characteristics of karst springs(Raeisi et al. 2006) .Due to the physicochemical properties of the springs, some characteristics of the of water movement pathway and the type of dominant flow system can be found. The spring discharge variability is the most important criterion in characterizing the dominant flow system type (Karami et al. 2008). Most of the researchers have mentioned that the widespread use of springs in the karst studies is due to their reliable interpretations for the characteristics of karstic aquifers (Mohammadi and Field 2009).
In the west of the Mojen region, there are extensive outcrops of calcareous formations in which karstic aquifers are formed. There are several large and small karstic springs emerging from this karstic aquifer that has not been studied so far. Therefore, the main purpose of this research is to investigate the hydrogeological and hydrogeochemical characteristics of the karstic springs of west Mojen and determine the dominant flow system in karstic aquifer feeding these springs.
In order to evaluate the dominant flow system type and discharge mechanism of karst springs in west Mojen, the physical and chemical properties including discharge, electrical conductivity and temperature, major anions and cations were measured. The geological map of the studied region was prepared using the geological map of Shahrood with the scale of 1:100000 as well as field observations. The precipitation and temperature data-sets for the Mojen region were collected from the Shahrood Regional Water Company and Meteorological Organization.
According to the results obtained in this research, the temporal variations in the physical (discharge) and chemical (electrical conductivity) characteristics of the discharging water from karstic springs in west Mojen are relatively low. Therefore, the coefficient of variations of all parameters indicates the dominant diffuse flow system in the karstic aquifer of these springs. In addition, the recession curve of the studied springs has a coefficient of discharge with a gradient of 0.009, which indicates the dominant diffuse flow system in that karstic aquifer. Such small discharge coefficients are consistent with the geomorphological characteristics of the studied region. The karst springs in the studied region are overflow springs, which the main reason for the emergence of these karst springs in this location is the contact massive limestone (i.e. karst aquifer) and impervious marls and marly limestone. The hydrochemical diagrams related to samples prepared from the studied springs in June 2017 show that all samples are high in Ca²⁺ and HCO₃^-, and are located in a specific range of karst waters. The ratio of Ca²⁺ to Mg²⁺ ion is used to determine the predominant aquifer in karst areas. The average ratio of Ca²⁺ to Mg²⁺ ions for the studied samples is about 2.60, which can be concluded that the predominant lithology of the studied karstic aquifer is limestone. In this research, the primary catchment area of the springs was drew. Then, precision of this primary catchment area was evaluated by the water budget method and comparing its values of annual discharge and recharge volumes. According to the mentioned comparison, a relative error of 0.8%was obtained. Due to the insignificant relative error between annually recharge and discharge volumes and as well as the stratigraphic status in the region, the drawn catchment area is quite accurate.