<p>This study investigates the activity concentration of ²²²Rn in surface and groundwater (hand-dug wells and boreholes) in a Nigerian university campus using a Durridge manufactured RAD 7 radon detector. Human health risks associated with ingestion and inhalation of the water samples were evaluated through the calculations of annual effective dose and excess lifetime cancer risks (ELCR). Monte Carlo simulation method was also used to predict the resulting ELCR. The measured ²²²Rn activity concentrations ranged from 0.5 to 67.8&#xa0;Bq L⁻¹, with borehole water exhibiting the highest mean value of 19.3 ± 21.5&#xa0;Bq L⁻¹, followed by well water (11.0 ± 8.2&#xa0;Bq L⁻¹), and surface water (2.4 ± 1.9&#xa0;Bq L⁻¹). While all the values were below the World Health Organization (WHO) limit of 100&#xa0;Bq L⁻¹, they exceeded the Standards Organization of Nigeria’s limit of 0.1&#xa0;Bq L⁻¹. The elevated concentrations in borehole and well water are attributed to greater depths that penetrate radon-rich bedrock. Annual effective dose estimates revealed that borehole water posed the greatest risk, with combined ingestion and inhalation doses reaching up to 342.7 µSv y⁻¹ and a mean of 98.0 ± 109.1 µSv y⁻¹, exceeding the WHO’s reference level of 100 µSv y⁻¹ in some locations. Surface water, in contrast, posed minimal risk with a total mean dose of 12.3 ± 9.5 µSv y⁻¹. Excess lifetime cancer risk calculations using deterministic and Monte Carlo approaches show values exceeding internationally accepted risk thresholds (10^–6 to 10^–4) in some borehole and well water samples, indicating the need for targeted mitigation efforts such as regular monitoring and aeration before use. The findings emphasize the spatial variability of radon risk and highlight borehole water as the most critical source for public health intervention.</p>