GEOLOGICAL AND HYDROGEOCHEMICAL CONTROLS ON RADIUM AND URANIUM ISOTOPES IN GROUNDWATER OF THE EASTERN DESERT OF EGYPT

Radium isotopes (²²⁶Ra and ²²⁸Ra) and U concentration were analyzed in 39 groundwater samples from the Nubian Sandstone Aquifer System (NSAS) and shallow alluvial aquifers in the Eastern Desert of Egypt. The shallow Nubian aquifer has ²²⁶Ra and ²²⁸Ra activities ranging from 0.016 to 0.750 and 0.018 to 1.421 Bq/L, respectively. The deep Nubian aquifer has ²²⁶Ra and ²²⁸Ra activities ranging from 0.019 to 0.051 and 0.014 to 0.248 Bq/L, respectively. The shallow alluvial aquifers have ²²⁶Ra and ²²⁸Ra activities ranging from 0.008 to 0.086 and 0.006 to 0.081 Bq/L, respectively. Groundwater in the Nubian aquifer may have Ra activities substantially in excess of the drinking water MCL values of the US Environmental Protection Agency (EPA), the European Union (EU), and the World Health Organization (WHO).

The concentration of U varies from 0.06 to 33.06 µg/L, 0.06 to 15.46 µg/L, and 0.01 to 13.19 µg/L for the shallow Nubian aquifer, the deep Nubian aquifer, and the shallow alluvial aquifers, respectively. U concentrations exceed the EPA and WHO MCL values in some wells, mostly from the shallow Nubian aquifer.

The occurrence of elevated U in some may be linked to higher nitrate and alkalinity in these wells. Nitrate can alter U solubility by oxidative dissolution of reduced U(IV) minerals present within the soil, and thereby U could be released into the groundwater. The major sources of nitrate are commonly surface-applied chemicals and organic manure, while alkalinity is originated from dissolution of carbonate minerals as implied from saturation of these minerals in groundwater.