Distribution and Radiological Assessment of Natural and Artificial Radioisotopes in Coastal Marine Sediments of Sri Lanka

Monitoring natural and anthropogenic radionuclides in marine sediments is essential for establishing baseline radiation levels and supporting effective radiation protection. Coastal sediments act as long-term sinks for radionuclides and provide valuable information on historical contamination and environmental processes. This study investigated the activity concentrations of natural and artificial gamma-emitting radionuclides in coastal marine sediments along the Sri Lankan coastline, providing reference data for future environmental assessments. A total of 42 surface sediment samples were collected approximately 1 km offshore, covering the entire coastal zone of Sri Lanka. Sample preparation and analytical procedures followed International Atomic Energy Agency (IAEA) guidelines. Activity concentrations were measured using high-purity germanium (HPGe) gamma spectrometry, and quality assurance was confirmed through the analysis of certified reference materials, which showed good agreement with certified values. No ¹³⁴Cs was detected in samples collected during 2012, 2013, and 2019 (detection limit: 0.15 Bq kg⁻¹). Considering its short half-life (~2 years), the absence of ¹³⁴Cs in later years is likely due to radioactive decay below detection limits and does not necessarily indicate the absence of contamination from the Fukushima Daiichi Nuclear Power Plant (FDNPP) incident. Low levels of ¹³⁷Cs (0.17–1.00 Bq kg⁻¹) were detected, reflecting residual global fallout. The activity concentrations of natural radionuclides (²²⁶Ra, ²³²Th, ⁴⁰K, and ²¹⁰Pb) ranged from 2–51, 3–167, 19–658, and 4–182 Bq kg⁻¹, respectively in 2012-2013 period. Radiological indices indicate relatively low radiation levels, however these results should be interpreted with caution, as the indices were developed for terrestrial environments and are used here for comparative purposes only.