Evaluating of dominant flow system and discharge mechanism of karst springs in West Mojan

The studied region is located at 8 kilometer of west Mojen in the south of Mojen waterfall, a part of the Shahrood county. In order to study the hydrogeological and hydrogeochemical characteristics of springs in the area, discharge, electrical conductivity and temperature were measured from October 2017 to November 2018. Also, the concentrations of the main ions including Ca2+, Mg2+, Na+, K+, HCO3-, SO42- and Cl- have been measured in the water and environment laboratory of Shahroud University of Technology. The obtained results showed that the coefficients of variations for the physical and chemical characteristics for all springs are negligible and confirm the dominant diffuse flow system in the study area. Recession curve of the studied springs has solely one coefficient of discharge (α) with a gradient of 0.009 which indicates the dominant diffuse flow system in that karstic aquifer. One of the main reasons for the emergence of the springs in the studied region is contact of groundwater in the karstic catchment of these springs (Limestone rock formations of the Cretaceous and Lar) with impervious units (marls and marly limestone). According to the springs of the area are overflow springs and all these springs are drained from an aquifer. 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+ ion to Mg2+ ion concentration is used to determine the predominant aquifer in karst areas. The average ratio of Ca2+ ion to Mg2+ ion for the studied samples is about 2.60, which can be concluded that the predominant lithology of the studied karstic aquifer is limestone. Also in this research, the primary catchment area of the springs was drawed. 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 percent was obtained. Due to the negligible relative error between annually recharge and discharge volumes and as well as the stratigraphic status in the regionin the region, the drawn catchment area is quite accurate.