Bioaccumulation of natural radionuclides in edible molluscs and associated risk from the Ashtamudi Estuary, situated in a high background natural radiation zone (HBNRA), Kerala, southwest coast of India

The study investigated various natural radionuclides, including ²³⁸U, ²³²Th, ²¹⁰Po, and ²¹⁰Pb, in edible molluscan species collected from different zones of the Ashtamudi estuary. Physicochemical and sedimentological factors played crucial roles in influencing the distribution of radionuclides in water and sediments. Notably, bivalve species, particularly those from the Cyrenidae family, exhibited significant bioaccumulation of ²¹⁰Po (up to 432.8 Bq kg⁻¹) and ²¹⁰Pb (up to 121.3 Bq kg⁻¹), with Zone VI recording the highest biological concentration factor (BCF) and biological sediment accumulation factor (BSAF). The calculated effective dose for humans consuming bivalve meat was moderate, falling above the lower end of the ICRP range of 1 mSv y⁻¹ but well below the upper limit of 20 mSv y⁻¹ for existing elevated radiation zones. The intake of radioactivity from some bivalves corresponded to a cancer risk of approximately 3.2 × 10⁻² (total mortality + morbidity), exceeding the acceptable risk threshold reported by the USEPA. However, this level of exposure indicated that immediate action might not be necessary, especially if further dose reduction proved impractical, in line with the ICRP's approach to optimizing radiation protection.