Radiological Risk and Ecological Transfer of Natural Radionuclides From Urban Waste Dumps in Gusau Metropolis, Nigeria: A Quantitative Assessment Using Gamma Spectrometry and the Erica Tool

Uncontrolled urban waste dumps in low- and middle-income countries act as unregulated reservoirs for naturally occurring radioactive materials (NORM), posing potential radiation exposure risks to both humans and non-human biota. This study provides a high-resolution radiological characterization of five major dumpsites in Gusau, Nigeria (G1–G5). Activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in 60 soils, 60 plant, and 25 well water samples were determined using NaI(Tl) gamma spectrometry (relative uncertainty <10% at 95% CI). Mean soil activities were: ²²⁶Ra: 5.8–14.1 Bq/kg (global avg: 35; UNSCEAR, 2020); ²³²Th: 36.7–46.9 Bq/kg (global avg: 30); ⁴⁰K: 243–487 Bq/kg (global avg: 400). Localized extreme ⁴⁰K (1894 Bq/kg at G1S1) and ²³²Th (84.1 Bq/kg at G1S5) indicate anthropogenic hotspots, consistent with findings from similar dumpsites in sub-Saharan Africa (Akinyemi et al., 2023; Popoola et al., 2025). Soil-to-plant transfer factors (TFs) for ²²⁶Ra were unexpectedly high (mean TF = 0.86; range 0.00–3.23), exceeding unity at 28% of sampling points, indicating active biological uptake via Ca²⁺ substitution as described by Vandenhove et al. (2024) and IAEA (2022). In contrast, TFs for ²³²Th (mean = 0.32) and ⁴⁰K (mean = 0.29) were lower and biologically regulated (Isinkaye et al., 2023; Yusuf et al., 2025). All human health hazard indices (Raeq max 186 Bq/kg < 370 limit; Hₑₓ max 0.50 < 1; AEDE max 0.106 mSv/y < 1 mSv/y public limit; ELCR max 0.37×10⁻³ vs. global 0.29×10⁻³) were within safety thresholds, though G2 showed a slight ELCR elevation comparable to values reported for Indian urban dumpsites by Singh et al. (2024). The ERICA Tool Tier 1 screening flagged three sites (G2, G3, G4) with total Risk Quotient (RQ) >1 (max 1.34), necessitating Tier 2 (Brown et al., 2022; Hosseini et al., 2024). Tier 2 refined assessment showed expected RQs for all reference organisms (plants, birds, mammals, reptiles) were <0.40, with only lichen and bryophytes showing a conservative RQ up to 1.10. The overall absorbed dose rate for the most exposed organism (lichen) was 11.0 μGy/h (conservative) vs. 3.6 μGy/h (expected), both at or near the 10 μGy/h screening value recommended by ICRP (2021) and IAEA (2023). Well water showed strong attenuation (mean AEDE = 0.02 mSv/y; mean ELCR = 0.05×10⁻³), posing negligible risk (WHO, 2024). Pearson correlation (r > 0.96, p < 0.01) identified ²³²Th as the dominant external dose driver, while ²²⁶Ra dominated internal food-chain risk. We conclude that while direct external human risk is low, enhanced ²²⁶Ra plant transfer creates a chronic internal exposure pathway requiring biomonitoring. The ERICA Tool successfully identified lichens as the critical ecological receptor, consistent with recent European recommendations.