Radioecological dynamics of primordial radionuclides in rice agroecosystems: A comprehensive assessment of transfer pathways and health risk assessment in the Manavalakurichi NHBRA

This Study provides a comprehensive assessment of radioecological impacts within agricultural systems of the Manavalakurichi Natural High Background Radiation Area in Tamil Nadu, India. The study quantified primordial radionuclides (²³⁸U, ²³²Th, and ⁴⁰K) in soil and rice samples while analyzing their transfer dynamics using gamma-ray spectrometry with NaI(Tl) detection systems across multiple sampling locations. Results demonstrated substantially elevated soil radioactivity concentrations, with ²³⁸U ranging from 136 to 468 Bq kg⁻¹, ²³²Th spanning 399–1059 Bq kg⁻¹, and ⁴⁰K extending from 621 to 1355 Bq kg⁻¹, all significantly exceeding established global reference standards. Rice grains exhibited corresponding contamination patterns with activity concentrations of 7.16–23.61 Bq kg⁻¹ for ²³⁸U, 22.33–35.68 Bq kg⁻¹ for ²³²Th, and 52.78–70.56 Bq kg⁻¹ for ⁴⁰K. Soil-to-plant transfer efficiency followed the hierarchy ⁴⁰K > ²³⁸U > ²³²Th, with respective mean transfer factors of 0.059, 0.051, and 0.040. Radiological hazard assessment revealed concerning exposure parameters substantially exceeding safety thresholds. Radium equivalent activity measurements of 784–2049 Bq kg⁻¹ and absorbed dose rates of 344–904 nGy h⁻¹ exceeded international safety limits. Annual effective doses reached 1.69–4.43 mSv y⁻¹ indoors, surpassing public exposure recommendations, while lifetime cancer risk calculations indicated an increase compared to normal radiation environments. Statistical analysis identified strong uranium‑thorium correlations in soil (r = 0.98) and rice (r = 0.88), indicating coupled transport mechanisms. Cluster analysis revealed two distinct spatial groupings with differing contamination patterns controlled by soil properties. This research establishes the first comprehensive framework for radionuclide transfer characterization in the Manavalakurichi region, providing essential predictive capabilities for risk assessment in comparable geological contexts and emphasizing the critical need for systematic radiological monitoring and evidence-based agricultural management interventions.