Radiological signatures and health risks of natural radionuclides in the Nubian Sandstone Aquifer across Egypt's Western Desert oases.
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Groundwater from the Nubian Sandstone Aquifer System (NSAS) represents Egypt's most strategic water reserve; however, comprehensive radiological assessments of this resource remain scarce. This study presents the first systematic evaluation of natural radioactivity in groundwater across four major oases (Al-Farafra, El-Dakhla, El-Kharga, and Paris) in Egypt's Western Desert. Sixty-four groundwater samples were collected and analyzed using high-purity germanium (HPGe) gamma-ray spectrometry to determine activity concentrations of lead-210 (Pb-210), uranium-238 (U-238), radium-226 (Ra-226), radium-228 (Ra-228), thorium-232 (Th-232), and potassium-40 (K-40). The mean activity concentrations followed the order: K-40 (11.86 Bq/L) > Pb-210 (1.89 Bq/L) > U-238 (1.75 Bq/L) > Ra-226 (1.29 Bq/L) ≈ Ra-228 (1.28 Bq/L) ≈ Th-232 (1.22 Bq/L). The mean concentrations of Pb-210, U-238, Ra-226 and Ra-228 exceeded the World Health Organization (WHO) screening levels by factors of 19, 4.7, 1.3, and 13, respectively. Strong positive correlations were observed among uranium and thorium series radionuclides (r = 0.621-0.822, p < 0.01), indicating common geogenic origins, while no significant correlations were detected with total dissolved solids (TDS) or electrical conductivity (EC) (p > 0.05), suggesting that radionuclide mobility is controlled by mineralogical composition and redox conditions rather than bulk salinity. The mean annual effective doses (AEDE) via ingestion were 2119.9, 2054.9, and 1450.1 μSv/y for adults, children, and infants, respectively, with values exceeding the WHO individual dose criterion (IDC) for drinking water of 100 μSv/y (0.1 mSv/y) by factors of 21.2, 20.5, and 14.5, respectively. However, these values remain below the global average annual effective dose from all natural sources (2400 μSv/y) reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Excess lifetime cancer risks (ELCR) ranged from 5.08 to 7.42 × 10-3, exceeding the acceptable threshold of 1 × 10-4 by factors of 50.8-74.2, warranting immediate attention and targeted monitoring programs. These findings establish a critical baseline dataset for radiological quality assessment of the Nubian Aquifer and underscore the necessity of direct radionuclide measurements rather than reliance on conventional water quality parameters for public health protection in arid regions.