Document Type : Research Article
Authors
Ferdowsi University of Mashhad
Abstract
Introduction: Tritium is the only radioactive isotope of hydrogen, with a half-life of about 12.3 years, in water molecule which can be used to determine the age of water in a hydrological cycle. Although hydrogen bomb tests entered a lot of tritium into the atmosphere and then into the hydrological cycle, but at the moment the average amount of tritium concentration in global precipitations is reaching to about 5 TU. The purpose of this paper is to investigate the tritium concentrations in precipitations of the Middle East countries and to determine the tritium concentration in Iran precipitation (especially in precipitations of the west of Kermanshah province) and to determine the relative age of groundwater resources in Paveh, Javanrood, Ravansar and Sarpule Zahab areas using tritium radioisotope.
Materials and Methods: The required tritium data for the Iran and neighbors and for the global precipitations were retrieved from the Global Networks of Isotopes in Precipitation (GNIP) site of the International Atomic Energy Agency (IAEA). To measure the amount of tritium in Kermanshah precipitations, samples were collected from three rain stations, three wells and from nine springs in Paveh, Javanrood, Ravansar and Sarpule zahab areas during fall 2015 and 600 ml in 600 ml water polyethylene containers, all water samples were analysed at Waterloo University Isotope Laboratory.
Results and Discussion: The amount of tritium concentration in precipitations depends on latitude, longitude, temperature, altitude and the vapor mass. The higher amount of vapor and the lower temperature or the higher altitude, decreases the concentration of tritium. In areas such as Karachi, Bahrain and Adena, due to its proximity to the sea and the higher amount of vapor in the atmosphere, the tritium concentration in precipitation is low. In this paper, the tritium concentration in precipitation and groundwater resources of the west of Kermanshah province was measured at the University of Waterloo-Environmental Isotope Laboratory (UW-EIL). Then the average relative age of groundwater was determined. Results indicate that the tritium concentration in precipitation of the west of Kermanshah is about 6.0 TU and it is much lower in groundwater resources. Based on water age division using tritium concentration, the water of precipitations in the west of Kermanshah is modern and the water of groundwater resources are mixture of modern (recently recharge water) and sub modern water (the waters fed before 1950). By determining the amount of electrical conductivity (EC) and the concentration of tritium in the waters of the region, it is concluded that in the direction of flow, with increasing EC and decreasing the amount of tritium, the water age increases. By examining the EC and the relative age of the waters, it can be concluded that in the Sarpule Zahab area, in Ghaleh Shahin plain, groundwater recharge to the alluvial aquifer in Qaleh Shaheen spring area and then it flow in the direction toward Sarabgarm spring. However, in Boshive plain, the groundwater flow from Marab spring towards the Gandab spring. Tritium has a correlation with the air temperature. The higher the temperature, the more the concentration of tritium in the abundant water resources, and the older age for the water sample. In the study area, the average annual air temperature in the Paveh, Javanrood and Ravansar areas are about 15.1, 15.0 and 14.9 degrees Celsius, respectively, and it is about 19.9 degrees Celsius for Sarpule Zahab area. The average concentration of tritium in Pave and Javrroud is about 3.4 TU, however, in Ravansar and Sarpule Zahab areas are about 1.4 TU and 1.1 TU, respectively Therefore, it is evident that the relative age of groundwater is younger in the Paveh region and it is the oldest in Sarpule Zahab region.
Conclusions: The concentration of tritium is associated with the age of water. The lower the amount of tritium is the oldness of the water. The geology and rocks are affected by the movement of water, which is why the age of groundwater resources in the Paveh region due to the development of karst and the rapid transfer of groundwater is less than the Sarpule Zahab and Ravansar areas. On a global scale, the concentration of tritium in the northern hemisphere’s precipitations is much higher than that of in the southern hemisphere, and in the polar regions’ precipitations it is approximately 4 times of the tropical region’s precipitations. By investigating the concentration of tritium in the rain of neighboring countries of Iran it is concluded that the proximity to the sea and the increase of water vapor in the atmosphere have reduced the amount of tritium concentration.
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