Comparison of physical, chemical and hydrogeological characteristics of karst springs in the western and eastern regions of Ilam province, Iran

Karst springs in the western and eastern regions of Ilam province have different physical and chemical characteristics. The springs of the western regions, are sulfuric and thermal with high salinity, while the springs in the eastern regions are fresh and cold. Geological, hydrogeological, hydrogeochemical and isotopic studies were performed to compare and determine the factors affecting the physical and chemical properties of these springs. Monitoring of the springs, including measurement of discharge, temperature, acidity, amount of hydrogen sulfide gas, dissolved oxygen in the field, and analysis of major ion chemistry, was conducted monthly for two years. Deuterium and oxygen-18 stable isotopes were measured during two dry and two wet periods to determine the source of the water. Field studies have shown that the geology of the eastern and western regions of Ilam Province is different. Deep faults have affected the western regions, and some calcareous formations have changed facies so that they have anhydrite members. The deep faults of the western regions of Ilam Province have caused the hydraulic connection of karst formations with the old and lower evaporite formations. The ratios of Na/Cl, SO4/Cl and TDS/Br showed that the salinity of the sulfur springs originates from the dissolution of evaporite formations and is not related to the oil field brines. The results of stable isotope analysis of deuterium (2H) and oxygen-18 (18O) in thermal and sulfur springs showed that the origin of these springs is the local meteoric precipitation. The origin of hydrogen sulfide (H2S) of sulfur and thermal springs was attributed to the activity of sulfate-reducing bacterias. According to the hydrograph of the springs, rainfall affects the discharge increase of fresh and cold springs less than one month; while, the discharge of sulfur and thermal springs increases about two months after rainfalls. In other words, the infiltrating waters that recharge the sulfur springs have a deep circulation. Therefore, the high temperature of the springs in the western regions of Ilam Province is attributed to the deep circulation of infiltrating waters from local precipitation along the faults.