Researchers at the University of Delaware used chlorine isotopes as chemical tracers to determine the age and origin of groundwater in the Eastern Desert of Egypt. The paper, recently published in the Earth and Planetary Science Letters journal, said that in order to measure the age and origin of the groundwater. Twenty-nine samples were collected from different wells during several field expeditions in Egypt.
The researchers used the long-lived radioactive isotope chlorine-36 to estimate the age of the groundwater. This isotope forms in the atmosphere and travels to earth and has a half-life of 300,000 years.
Mahmoud Sherif, doctoral candidate at the University of Delaware and the main author of the paper, told Daily News Egypt that the study focused on using chlorine isotopes to identify sources of chloride, and to estimate the relative age of groundwater from the Nubian aquifer and the shallow alluvial aquifers, in addition to the fractured basement aquifers in the Eastern Desert of Egypt.
Groundwater from the deep wells tapping the Nubian aquifer reported the oldest ages (more than 600,000 years), whereas groundwater from the fractured basement rocks were the youngest.
The shallow alluvial aquifers surprisingly reported old parent ages which reached 200,000 years. Age dating results indicate that old water from the deeper Nubian aquifer is discharged into the shallow alluvial aquifer through deep-seated faults in the Eastern Desert that belong to the Najd fault system associated with the Red Sea rift system.
Responding to our inquiry about the importance of the study, Sherif noted that the Eastern Desert of Egypt is a severely arid region, and thereby the presence of that promising water resource is very crucial to meet stresses in water demands, and to accommodate developmental plans in that region of the country.
"I started working on this project since 2017. The project was partially funded through a fellowship provided by the Egyptian government. The work has been done through several field trips to collect groundwater samples from these aquifers," said Sherif. He added that geochemical analysis of water samples and interpretation of the results has been completed in the United States.
Regarding the methods that the researchers used in the study, the researcher told DNE that they have analysed the chlorine-36 isotope in groundwater as a good tracer to date groundwaters.
"Chlorine-36 isotope has a long half-life (300,000 years) in addition to a conservative behaviour in the aquifer (non-reactive) that allows us to date very old groundwater," he said, adding that "Chlorine-36 is a cosmogenic isotope produced in the atmosphere by spallation of argon-36 during interaction with cosmic ray protons. It enters groundwater systems through precipitation and undergoes radioactive decay over time."
According to the researcher, groundwater from the shallow alluvial aquifers were expected to have young ages since it is mainly recharged by flash floods after heavy rains. "However, our findings revealed that these groundwaters are relatively old (up to 200,000 years). This indicates that older water from the Nubian aquifer comes up along faults in the rocks and mixes in with the shallow water," Sherif said, noting that they also have used the stable isotope ratios of oxygen and hydrogen to quantitatively estimate old and young water mixing fractions.
The study reveals natural discharge of groundwater from the deeper Nubian aquifer into the shallower alluvial aquifer. The deeper Nubian aquifer is considered the biggest groundwater body in the world. It extended beneath four countries in north-eastern Africa, including Egypt, Libya, Chad, and Sudan, covering around 2m sqkm.
It also extends beneath around 80% of the total area of Egypt including the eastern desert. Models have indicated that the amount of groundwater contained in the Nubian aquifer in Egypt alone is tremendously considerable (enough to keep the Nile flowing for 500 years), according to Sherif.
He further explained that the presence of such discharge makes the shallower alluvial aquifer a significant and promising water resource which meets developmental plans in these areas. "However, more studies are still needed to accurately quantify the amount of water contained in that aquifer," he noted.
The researcher stressed that the findings of the study are very important and a significant step toward accommodating water stress in these arid zones of the country. "We are currently applying sophisticated isotopic measurements to identify potential recharge sources for the Nubian aquifer in the eastern desert and the western desert as well. This will also be of great importance toward developmental plans in these regions," he said.
Meanwhile, Ali Akanda, assistant professor at the Civil and Environmental Engineering Department at the University of Rhode Island, said that Egypt's water resources are facing a dilemma of diminishing supplies and increasing demand.
"As impacts of climate change are becoming more prominent (changing precipitation in the highlands, increasing evaporation from Lake Nasser, etc) and upstream nations are asserting their rights for a larger share of the Nile flow–providing water for its growing population and agricultural demand will be a long-term challenge and uncertain at best. To make matters easier, one could see how Egypt becomes more reliant on its groundwater resources in the years to come," he noted.
In his opinion, taking lessons from its neighbours' experiences (such as Saudi Arabia, Libya, Yemen), Egypt should consider its groundwater sources as a strategic reserve and carefully develop a plan to tap this resource.
This should include accurately determining the recharge zones and rates and restrict exploitation in areas that allow a sustainable management of these precious resources. That would require academic research as well as government intervention to identify new areas to develop, (for agricultural expansion, urban development, tourism, etc) and the protection of natural recharge zones such as wetlands and outcrops.
Akanda added that the rapid expansion of Cairo and the Nile Delta region will create a lot of new water demand in the decades to come. Innovative solutions involving the region's food-energy-water connections will be critical to meet this demand and ensure Egypt's long-term water security.
