Formation-scale NORM assessment of the Abu-Tartur phosphate plateau, Egypt: Ore-grade control and occupational radiation implications

The Abu-Tartur phosphate plateau in Egypt's Western Desert is a major sedimentary phosphate resource that requires defensible radiological baseline characterization. This study presents an integrated formation-scale assessment of naturally occurring radioactive material (NORM) in Duwi Formation materials using 50 samples distributed across high-grade ( n = 28), medium-grade ( n = 14), and overburden ( n = 8) classes. Mean activity concentrations were 189.26 Bq/kg for 226 Ra and 192.13 Bq/kg for 238 U, and the 226 Ra/ 238 U activity ratio (0.982 ± 0.083, SD) was consistent with near-secular equilibrium. Mean Ra eq , absorbed dose rate, AEDE, and ELCR were 239.39 Bq/kg, 108.85 nGy/h, 133.48 μSv/y, and 0.467 × 10 −3 , respectively. No sample exceeded Ra eq = 370 Bq/kg or H ex = 1, whereas H in exceeded unity in 35 of 50 samples, including all high-grade samples and half of the medium-grade samples. 226 Ra correlated strongly with P 2 O 5 ( r = 0.912; 95% CI, 0.849-0.949), CaO ( r = 0.898), and 238 U ( r = 0.972; 95% CI, 0.951-0.984), while detrital indicators were inversely related, supporting apatite-controlled uranium-series behavior. Principal component analysis resolved a dominant apatite-detrital mixing axis (PC1 = 86.3%), and one-way ANOVA confirmed clear class separation for 226 Ra ( F = 56.53, p < 0.001; η 2 = 0.706). Ordinary kriging suggested a northwestern zone of elevated 226 Ra, but cross-validation (R 2 = −0.04) indicates that the interpolated surface should be treated as a qualitative trend map rather than a predictive model. Abu-Tartur is therefore best interpreted as a moderate-NORM phosphate system in which higher-grade ore warrants targeted follow-up assessment of radon and inhalation pathways, especially in enclosed handling environments.