Ministry of Science and Technology, Saudi Arabia.

Determining Groundwater age in the wells to study sustainable water utilization (2020-2022)

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Groundwater age is determined from the measurement of age “tracers”, chemical or isotopic constituents dissolved in the groundwater. These tracers include naturally occurring isotopes, which decay at a known rate; isotopes that were introduced into the atmosphere at known times relating to nuclear tests; and manufactured gases whose concentration in the atmosphere over time is known.

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Young groundwater is commonly defined as water that entered the aquifer since about 1950 because several chemical and isotopic substances related to human activities were released into the atmosphere since that time.  The presence of these substances in groundwater tell us that the water is young. These substances include tritium (3H), which was released into the atmosphere by detonation of nuclear bombs in the 1950s and early 1960s, chlorofluorocarbons (CFCs), which were released into the atmosphere from refrigeration and other uses from the 1930s through the 1980s, and sulfur hexafluoride (SF6), which is used primarily in electrical equipment and manufacturing semiconductors and whose use has been increasing steadily since about 1965. These age-dating tracers can help water-resource managers to develop management strategies for shallow groundwater systems that contain mostly young groundwater.

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Old groundwater is defined as water that entered the aquifer before 1950 and more commonly refers to water older than 1,000 years. Many common and rare isotopes are produced naturally in the Earth’s atmosphere from the bombardment of cosmic rays or solar radiation, and their presence in groundwater can help determine the groundwater age. These isotopes are adsorbed by rainfall and can enter the aquifer with recharge. Argon-39 can be used to identify water that recharged between 50 and 1,000 years ago. Carbon-14 or radiocarbon is the most common method used to determine groundwater ages between 1,000 and 30,000 years. Groundwater older than 30,000 years can be determined using isotopes like helium-4, which is produced from the decay of uranium and thorium in aquifer solids, or by chlorine-36 and krypton-81, which decay over extremely long timescales and thus are useful for determining the age of ancient groundwater—hundreds of thousands of years old or more.

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Determining ground water age has become an integral part of study for many countries for being a useful initial task in an effort of supplying uncontaminated water. Monitoring of water age fluctuations help to know about adverse or beneficial extraction. Historically, H-3 (tritium), a radioisotope of hydrogen, has been the most widely used tracer of young groundwater. As tritium has a half-life of 12.34 years and as because it generally behaves predictably, tritium content of water is a reliable indicator of groundwater age.

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Many common isotopes are produced naturally in the atmosphere from the bombardment of cosmic rays or solar radiation and their presence in groundwater can help determining groundwater age. These isotopes are absorbed by rainfall and can enter the aquifer during recharge. Radioactive tracers such as tritium, radiocarbon, chlorine-36 etc. allow to estimate the groundwater age. Historically, H-3 (tritium), a radioisotope of hydrogen, has been the most widely used tracer of young groundwater. As tritium has a half-life of 12.34 years and as because it generally behaves predictably, tritium content of water is a reliable indicator of groundwater age. In the earth, little quantities of natural tritium are produced by alpha decay of lithium 7. Natural atmospheric tritium, produced by secondary neutron cosmic ray bombardment of nitrogen, decays as carbon-12 and tritium. Tritium atoms then combine with oxygen, forming water that subsequently falls as precipitation. Because most tritium is distributed in the environment as water, it enters the hydrological cycle as precipitation and eventually gets accumulated in groundwater which could be assessed as a tracer to estimate groundwater age.

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The project will be focused to study groundwater resources in Hafr al Batin and surrounding areas and could be further extended to other areas of Eastern Province in Saudi Arabia. After collecting water samples from different wells, the task will be to compute the concentration of tritium in those samples. This will be accomplished by a high technology, advanced Liquid scintillation counter machine – Hidex 300 SL Liquid scintillation counter. Age of groundwaters will be calculated mathematically from the concentrations of tritium. The groundwater which will not have considerable amount of tritium concentration to detect the ages, will be separated from those with certain amount. Those will be considered and classified as old groundwaters in where tritium has decayed with several half-lives. Whereas groundwater with detectable amount of tritium will be considered and classified as relatively young ground water. The differentiations of age in the relatively young groundwaters will be plotted in terms of their ages and as per the locations. Results will be assessed to screen the groundwater resources for planned, sustainable and quality water management referring public health and safety for drinking and irrigation purposes.

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