Abstract

Groundwater is the principal source of drinking water for millions of people worldwide, especially in developing countries. However, naturally occurring radionuclides especially Radon-222, may dissolve into groundwater through water-rock interactions, and pose potential radiological health risks. Radon is a radioactive noble gas produced from the decay of radium-226 in the uranium-238 decay series and contributes significantly to the natural background radiation exposure. Its occurrence in groundwater is influenced by lithology, uranium content, groundwater residence time, hydrogeological conditions, fracture density, and mining activities. This review critically evaluate the occurrence of radon-222 in groundwater and the application of Monte Carlo Simulation (MCS) for probabilistic estimation of Annual Effective Dose (AED) and Excess Lifetime Cancer Risk (ELCR), with emphasis on studies from Nasarawa State, North-Central Nigeria. The review synthesizes evidence from Nigeria and International studies to examine groundwater radon occurrence, geological controls, measurement techniques, uncertainty analysis, and radiological health implications. Core attention is given to the contrasting radon concentrations in groundwater reported in Wamba and Nasarawa Local Government Areas, highlighting the influence of uranium-bearing lithologies, aquifer characteristics, groundwater residence time, fracture systems, and mining-related geological disturbances. Deterministic and probabilistic approaches to radiological risk assessment are compared. While deterministic methods provide single-point estimates, MCS incorporates variability and uncertainty through repeated random sampling, generating probability distributions, confidence intervals, percentile estimates, and sensitivity analyses that provide more realistic assessments of population health risk. Available evidence demonstrated considerable spatial variability in radon concentrations in groundwater across different geological environments. Although some areas of Nasarawa State reported relatively low radiological risk, elevated concentrations in mining-influenced and non-mining-influenced environments (e.g.; Usha) emphasizes the need for continuous monitoring and effective risk management. Integrating MCS into groundwater radiological assessments improves confidence in health risk estimation and supports evidence-based environmental monitoring, regulatory decision-making, and public protection. Future research should prioritize statewide groundwater radon mapping, long-term monitoring programmes, development of national drinking-water radionuclide standards, and integration of advanced probabilistic and machine-learning approaches for groundwater radiological risk assessment.

Key words: Radon-222; Groundwater; Monte Carlo simulation; Excess Lifetime Cancer Risk; Annual Effective Dose; Radiological Risk Assessments; Nigeria.