<p>The Surat Thani geothermal springs located along Bandon Bay in southern Thailand have the highest levels of natural background radiation in Southeast Asia. This study aimed to investigate the sources and migration of Ra-226. Gamma-ray spectrometry using a high-purity germanium detector was used to identify the presence of radioactive elements as a proxy for uranium-series (Pb-214, Bi-214, and Ra-226), thorium (Ra-228, and Th-228), and potassium (K-40), in granitic outcrops and alluvial soils. X-ray fluorescence spectroscopy was used to analyze the major element oxide compositions of the granitic rocks. The results showed that the average concentrations of radioactive elements in all granitic samples were higher than their average content in the Earth’s crust. However, the concentrations of natural radioactive material in the alluvial soils were approximately 42 times higher than in the granitic outcrops for Ra-226. Hot springs have developed in the depositional area, where the hot water mixes with brackish marine water at depth. The chloride-rich hot water mobilized Ra-226 as a proxy for uranium-series activity in the sedimentary layers while flowing upward. Then, Ra-226 accumulated near the surface, resulting in an increased concentration. Due to physicochemical conditions thorium (Ra-228) and potassium-40 (K-40) did not experience this mobilization and accumulation. Further understanding of the underlying processes and conditions is necessary and is currently being studied. This study shows that a combination of various geological elements and processes can significantly increase one radionuclide, but not another. Therefore, a careful understanding of all elements involved is necessary.</p> Graphical Abstract <p></p> <p>The graphical abstract highlights how the integration of various geological, radiogeological, and geochemical methodologies will be applied to investigate the sources and migrations of natural radionuclide in the Surat Thani geothermal springs of Southern Thailand’s east coast. The representative granitic (GSR) and alluvial soil (SSR) samples were subjected to determine the radioactive elements (a high-purity germanium detector) and geochemical dispersion patterns (an X-ray fluorescence spectroscopy). The results indicate that the higher dispersion of Ra-226 (as a proxy for uranium-series activity), especially for the alluvial soils taken from the geothermal spring areas were significantly higher than those of the granitic outcrops collected from mountains in western Surat Thani Province by approximately 42 times. The increase in Ra-226, in the alluvial soil of the hot spring areas can be linked to the weathering of the granitic mountains in western Surat Thani Province. Although, detrital sediments are particles derived from weathered rock in the mountains west of Surat Thani Province (representative of natural radionuclides) into the wetlands of the basin depositional settings along Bandon Bay (hot spring areas). Ra-228 largely remains in a solid, refractory form and is transported in the form of colloids. On the other hand, the saline hot water is flowing up along permeable faults through the various sand and clay layers from the wetland deposition and by this mobilizing the Ra-226 from all layers and transporting them to the near surface and accumulating them there.</p>