Investigating the composition of deuterium and oxygen 18 isotopes of precipitation and determining the local meteoric water line in a part of the Middle Zagros region (Zardkoh Bakhtiari)

AbstractThe use of isotopes, especially stable environmental isotopes, is very important in investigating the water cycle and water resources studies. Preparation of the local meteoric water line and its equation in a region is used to determine the origin of the air mass affecting the region and is used as a basis for a more detailed study of water resources, because by maintaining the initial characteristics of precipitation as an input to the groundwater aquifer, the subsequent changes that occur in the groundwater aquifer will be traceable.MethodsThe studied area is located in Chaharmahal and Bakhtiari province and in Chelgerd, Farsan and Ardal cities, and in terms of structure, it is a part of high Zagros (shredded or eroded). The major part of the study area is located in the large Karun basin and part of it is located in the Gavkhouni basin. In this research, in order to investigate the isotopic composition of precipitation, a total of 38 rain samples and 7 snow samples were collected from 9 stations at different altitudes with monthly frequency in the 2017-2018 in the form of cumulative and precipitation event in the rainy season. In order to determine the isotopic values of the collected samples, they were sent to Mesbah Energy laboratory and the isotopic composition of oxygen and hydrogen of all samples was determined by a laser spectroscopy.By examining the composition of oxygen 18 and deuterium isotopes of precipitation samples in hydrological year 2017-2018, the local meteoric water line for the middle Zagros study area in the Zardkoh Bakhtiari area has been drawn and compared with the global meteoric water line and the Meditrranean meteoric water line. In addition, in order to establish a relationship between precipitation isotopes and regional weather conditions, the correlation of altitude, temperature and precipitation parameters on stable isotopes of oxygen 18 and deuterium of event precipitation samples from different altitudes has been investigated. Also, in order to do more detailed studies and determine the origin of moisture masses, the correlation between the amounts of oxygen 18 and deuterium isotopes with deuterium excess and electrical conductivity was also investigated.ResultsBy examining the isotopic results obtained from the collected precipitation samples, for the first time, the atmospheric water isotopic line of the middle Zagros region in the Zardkoh Bakhtiari was obtained with the equation δD =7.1068 δ18O+12.364. The local meteoric water line of the region had differences with global and Meditrranean meteoric water line due to the occurrence of evaporation and transpiration from precipitation. The changes of δ18O value in meteoric precipitation in the region are from -12.92‰ to -1.07‰ (average -6.38‰) and δD value is from -80.16‰ to 7.51‰ (average -33.58‰).Comparing the isotopic content of precipitation in the range with altitude and amount of precipitation indicates that precipitation with richer isotopic composition occurred in areas with lower altitude and amount of precipitation. For every 100 meters change in altitude in this area, about 0.49 and 2.16 permil decrease in δ18O and δD isotopic values, respectively, and for every 20 mm increase in precipitation, about 1.3 permil decreases in δ18O isotopic value and about 10.1 of a permil decrease in the amount of deuterium occurs.Also, the increase in temperature has led to the enrichment of oxygen 18 and deuterium isotopes in the study area. So that the changes of δ18O and δD of the precipitation samples of the middle Zagros range are about 0.9 and 0.4 permil per 5 degrees centigrade of temperature change, respectivelyAlso, the amount of electrical conductivity (EC) of precipitation samples has a direct relationship with the amount of oxygen 18 and deuterium isotopes of these samples, so that with an increase of 20 microsiemens per centimeter of electrical conductivity, the amount of oxygen 18 and deuterium isotopes of precipitation samples increase by about 1.45 and 9.40 permil, respectively. In the conducted studies, the relationship between deuterium excess and isotopic values of precipitation samples was also investigated. deuterium excess of precipitation is usually affected by relative humidity and atmospheric temperature of the source of moisture, which is why it is considered an important indicator in determining the source of moisture. If the cloud humidity drops below 100%, deuterium excess occurs. The increase in the amount of deuterium excess and the decrease in isotopic values of δ18O and δD of precipitation in the study area also indicate the main origin of moisture masses from Mediterranean regions.