Radiological dose assessment of waterborne radon and Monte Carlo–based evaluation of lifetime excess cancer risk in uranium mining regions of South Africa

Abstract

Radon ( 222 Rn) is a naturally occurring radioactive noble gas found in groundwater, particularly in regions with uranium-rich geology and extensive mining, such as South Africa. This study presents a multi-site comparative assessment using a unified methodology and represents one of the first South African investigations to explicitly integrate ingestion and water-to-air inhalation pathways within a probabilistic lifetime excess cancer risk (LECR) framework. Using a RAD7 radon monitor, we measured radon concentrations in 80 boreholes across Carletonville, Wedela, Mokopane, Beaufort West, and Sutherland to calculate the total effective dose and lifetime excess cancer risk. Radon concentrations ranged from 18 ± 2 to 369 ± 96 B q L − 1 , with an overall mean of 127 ± 32 B q L − 1 , resulting in total effective doses (ingestion + inhalation of waterborne radon released into indoor air) ranging from 0.09 ± 0.02 to 1.87 ± 0.50 m S v y − 1 . The calculated mean radon concentration exceeds the World Health Organization (WHO) reference level of 100 B q L − 1 . Although the mean total effective dose remains below the International Commission on Radiological Protection (ICRP) public dose limit of 1 m S v y − 1 , the result indicates a non-negligible public health concern that warrants mitigation. Monte Carlo simulation (100,000 iterations, water-only pathway) yielded a mean lifetime excess cancer risk (LECR) of 2.58 × 1 0 − 3 and a median of 2.36 × 1 0 − 3 , both above the U.S. Environmental Protection Agency (USEPA) acceptable risk threshold of 1 × 1 0 − 4 . These translate to approximately 2580 and 2360 additional cancer cases per million people exposed over a 70-year lifetime, respectively, with 100 % of simulated iterations exceeding the USEPA threshold. The reported risk reflects only the waterborne pathway; soil-gas and building-material contributions, which typically dominate total indoor inhalation dose, were not measured. Even so, the waterborne component alone exceeds international risk benchmarks across the entire simulated distribution and at every site sampled, supporting the case for explicit national radon-in-water regulation, systematic monitoring, and complementary indoor-air radon assessment.