Radionuclide geochemistry of Egyptian groundwater
Date
2019
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The Middle East and North Africa region (MENA) is the most water-stressed region of the world. This region has a mostly hyper-arid climate with little or no potable surface water outside a few river valleys. Groundwater is a promising supply to accommodate increased fresh water demand and to achieve development goals in the region, without resorting to the great expense and infrastructure required for desalinization of seawater. Nevertheless, natural radioactivity has been recognized as a limiting factor for groundwater quality that could pose a substantial radiological health risk to millions of people throughout the MENA region. This dissertation provides new data on the occurrence of radium isotopes and uranium in groundwater from several types of aquifers throughout three geographic regions of Egypt - the Sinai Peninsula, the Eastern Desert (east of the Nile River), and the Western Desert (west of the Nile River). Multiple isotopic tracers (e.g., H, O, Cl and Sr), along with geochemical data for various cation and anion concentrations are used to evaluate the mean residence times and sources of solutes in groundwaters. ☐ Long-lived radionuclides (226Ra, 228Ra and 238U) were analyzed in groundwaters from different aquifer systems; including the Nubian Sandstone Aquifer System (NSAS), the alluvial aquifers, and the fractured basement aquifers. The Nubian aquifer groundwater had 226Ra and 228Ra activities ranging from 0.01 to 1.55 and 0.03 to 2.19 Bq/L, respectively. The shallow alluvial aquifers had 226Ra and 228Ra activities ranging from 0.008 to 0.086 and 0.006 to 0.081 Bq/L, respectively. The fractured basement aquifers had 226Ra and 228Ra activities ranging from 0.014 to 0.038 and 0.007 to 0.051 Bq/L, respectively. More than 59% of the investigated samples had total Ra activities (226Ra + 228Ra) in excess of the maximum contaminant level (MCL) for Ra in drinking water. Activities as high as 2500% of the MCL for total Ra were measured in some locations (e.g., Bahariya Oasis). Groundwaters from the shallow Nubian and the shallow alluvial aquifers had U concentration from 0.01 to 34.0 μg/L and 0.01–15.5 μg/L, respectively. The concentration of U was below detection limit (0.01 μg/L) in all sampled groundwaters from the deep Nubian aquifer. U levels in excess of the MCL were measured in a few wells tapping the shallower aquifers in the Eastern Desert (e.g., W. Dara). ☐ The dissertation documents various factors controlling radionuclide mobilization in the investigated aquifers. Ra isotopes in groundwater are produced through decay of parent Th isotopes contained in the aquifer solids. The measured 87Sr/86Sr and 228Ra/226Ra isotope ratios indicated that the aquifer clasts were mainly derived from the underlying Proterozoic basement rocks. These ratios also revealed some extent of interaction with carbonate rocks (e.g., at Kharga). Ra activity in groundwater is largely a function of redox potential, aquifer mineralogy, and residence time of groundwater. Activities exceeding the MCL are measured in anoxic, SO4–poor groundwaters having long residence times, and Ra activities are generally lower in younger groundwaters. Long residence times are optimal for mobilization of Ra to the solution by alpha-recoil processes and the development of anoxic conditions that promote reductive dissolution of sulfate minerals (i.e., barite). The low Ra activities in SO4–rich groundwaters imply partitioning of Ra into sulfate mineral phases including barite. The occurrence of lower levels of Ra isotopes in anoxic, low-SO4 groundwater may indicate lower abundances of U and Th, the parent nuclides of Ra, in the aquifer solids. In some groundwater wells, Ra activities may be limited by sorption mechanisms and cation exchange reactions. However, the presence of high concentrations of total dissolved solids (TDS) counteracts the sorption capacity of Ra in other groundwater wells. Higher U concentrations were measured in only a few shallower wells in the Eastern Desert. U is released to groundwater through oxidative dissolution of reduced U(IV) minerals present within the vadose zone and aquifer solids and is maintained in solution in the form of uranyl-carbonate complexes. In contrast, reducing conditions inhibit mobility of U in deeper groundwaters. ☐ Radiation dose data was evaluated for different life ages for people whose water intake depends primarily on the NSAS waters. Estimates indicated that doses from the majority of groundwater samples would exceed the individual dose criterion (IDC) of 0.1 mSv/yr regulated by the World Health Organization (WHO). Radiation doses are significantly higher for infants and tend to decrease with increasing life ages. ☐ Data for the origins and residence times of groundwaters in the Eastern Desert is also presented. Apparent groundwater 36Cl model ages ranged from 0 (+/−40) to 1300 (+300/−180) kyr. Chlorine-36 model ages as well as stable isotope data for H2O (isotopic compositions of H and O) are consistent with two sources of groundwater: 1) modern meteoric water with short residence time in the fractured basement aquifers, and 2) isotopically depleted paleowater with long residence time in the Nubian aquifer. The Nubian groundwater was mostly recharged during wet climate periods of the Pleistocene. Groundwater in the shallow alluvial deposits exhibits anomalously old 36Cl model ages, likely indicating hydraulic connectivity and mixing of older water from the Nubian aquifer with young meteoric water in the shallow alluvial deposits. ☐ The results of this study indicate that groundwater from the Nubian aquifer must be used with caution, especially in areas where radiation doses exceed the WHO Individual Dose Criterion. Further investigation of radionuclides in groundwaters from the Nubian aquifer is recommended, with a primary focus on mitigation of the broader environmental and ecological impacts of using water having elevated Ra activities.
Description
Keywords
Egypt, Groundwater, Nubian aquifer, Radionuclides, Radium, Uranium