Assessment and Monte Carlo Risk Simulation of Radon-222 in River Sediments: A Case Study from the Lower Zab, Kurdistan Region, Iraq

This study assesses the concentration and radiological impact of ²²²Rn in river sediments collected from 18 locations along the Lower Zab River in the Kurdistan Region of Iraq. The ²²²Rn activity concentrations in sediments were measured with a RAD7 radon detector. They range from 25 ± 1.6 to 267 ± 16 Bq m–3 with a mean of 62 ± 9 Bq m–3. The highest level, observed at the Jaly site, exceeded the International Commission on Radiological Protection (ICRP) safety threshold of 200 Bq m–3. Monte Carlo simulation (10000 iterations) was used to model the exposure variability, yielding a regional average AED of 1.31 mSv year–1 (95% CI: 0.86–1.76). Occupancy analysis showed that the annual effective dose (AED) remains below 1 mSv year–1 only if the indoor occupancy is below ~63% for most locations and below ~12% for the high-risk site. Radon exhalation rates reached up to 14.8 ± 0.9 Bq m–2 day–1, and radium concentrations, up to 2.46 ± 0.15 Bq kg–1. The sediment-to-surface-water radon concentration ratios ranged from 7.6 to 533, confirming sediments as a significant source of environmental radon. A strong correlation between ²²⁶Ra and radon exhalation further supports this origin.