Radon-thoron exhalation rates in mineral-enriched beach sand of the Chennai coast, India

The short-lived progeny of radon (²²²Rn) and thoron (²²⁰Rn) are significant global sources of ionizing radiation exposure and require continuous environmental monitoring due to health risks associated with inhalation. This study examines the spatial distribution of radon and thoron in soil, their mass and surface exhalation rates, and the mineralogical factors that influence beach sands along the coastline of the Chennai megacity in southeast India. A total of twenty-four intertidal soil samples were analyzed using a RAD7 radon–thoron monitor (Durridge Co., USA) and an electrostatic solid-state alpha detector, optimized specifically for measuring thoron. The radon mass exhalation rates ranged from 2 to 12 mBq kg⁻¹ h⁻¹ (mean: 2.42 mBq kg⁻¹ h⁻¹), whereas thoron surface exhalation rates varied widely from 162 to 31,623 Bq m⁻² h⁻¹ (mean: 3,688.08 Bq m⁻² h⁻¹). The highest soil-gas radon concentrations, ranging from 4.5 to 12 Bq/m³, were recorded in Kokkilamedu, an area rich in riverine placer deposits. The exhalation rates of radon and thoron are inversely proportional to grain size, with finer sediments exhibiting higher release rates than coarser sediments. However, higher heavy mineral content, increased bulk density, and finer grain fractions can also restrict exhalation, indicating a strong mineralogical and textural influence on radionuclide mobility. The sediments contain up to 12.5% heavy minerals, including ilmenite, zircon, and monazite, contributing to gamma radiation levels of up to 7.5 µR/h. Annual effective dose rates range from 0.12 to 0.45 mSv/y, remaining within global safety limits. Elevated thoron exhalation in monazite-rich areas highlights the need for regular radiological monitoring, particularly for workers in placer deposits. Overall, these findings provide baseline data on the radiological characteristics and sediment provenance of the Chennai coastal zone, supporting environmental risk management and public health planning.