Biophysical evaluation of natural radioactivity in groundwater of Lokoja, Kogi state, Nigeria, and its health implications.

Natural radionuclides are ubiquitous in groundwater, yet their evaluation and health risks remain inadequately characterized in sub-Saharan Africa. This study assessed natural radioactivity in 28 groundwater sources (14 boreholes, 14 hand-dug wells) across Lokoja, Kogi State, Nigeria, underlain by the migmatite gneiss complex of the Nigerian Basement Complex. Activity concentrations of 226Ra, 228Ra, 232Th, and 40K were measured using HPGe gamma spectrometry, while 222Rn was determined via LSC and verified with a DURRIDGE RAD7 detector using rigorous methodological cross-validation (R2 = 0.97; Bland-Altman analysis showed mean bias of -0.6 Bq/L, 95% limits of agreement: -3.2 to +1.6 Bq/L). This cross-validation represents a methodological advancement, demonstrating that LSC can be reliably verified against electronic detection in field conditions, with important implications for routine radon monitoring in resource-constrained settings. Mean activity concentrations were: 226Ra = 0.078 Bq/L, 228Ra = 0.028 Bq/L, 232Th = 0.015 Bq/L, 40K = 12.8 Bq/L, and 222Rn = 27.6 Bq/L. While gamma-emitting radionuclides remained within WHO and USEPA guidelines, 222Rn concentrations exceeded the USEPA MCL (11.1 Bq/L) in 25 of 28 locations (89% exceedance rate). The mean total annual effective dose (0.238 mSv/y) surpassed the WHO/ICRP reference level of 0.1 mSv/y, with inhalation contributing approximately 2.8 times more than ingestion. Estimated excess lifetime cancer risk (ELCR = 0.83 × 10-3) indicates moderate radiological concern. International comparison reveals Lokoja's radon levels are comparable to crystalline basement terrains in Ghana (12.5 Bq/L) and Cameroon (18.3 Bq/L), but lower than high-risk areas in Iran (68.5 Bq/L) and India's monazite-rich Kerala (32.4 Bq/L). PCA extracted two principal components explaining 64.3% of variance, confirming uranium-thorium series and potassium factors. This study highlights a critical regulatory gap: the absence of numerical radionuclide standards in Nigerian drinking water regulations. Based on these findings, we propose a phased regulatory approach, with an initial 222Rn standard of 100 Bq/L (consistent with WHO guidance) transitioning to 11.1 Bq/L (USEPA MCL) over 5-10 years as monitoring capacity and remediation infrastructure develop. We recommend urgent regulatory action by NESREA, continuous monitoring with seasonal sampling, point-of-use aeration systems, and public awareness campaigns targeting vulnerable populations.