Investigating the soil surface properties behind elevated natural radiation in Mamuju, Indonesia

Mamuju Regency, Indonesia, is among the world's notable high natural background radiation areas (HBNRAs). This study examines the environmental and geochemical processes responsible for the accumulation of uranium (U), thorium (Th), and potassium (K) in surface soils. Through systematic field sampling, geochemical characterization, and radiological measurements, we found that the distribution of radionuclides is primarily governed by weathering intensity and lateritization. High U and Th concentrations occur in clay-rich, acidic soils, whereas K is enriched in less weathered profiles. High Purity Germanium (HPGe) gamma spectrometry confirmed elevated of ²²⁶Ra, ²³²Th, and ⁴⁰K activity, while survey meter measurements of ambient gamma dose rates exceeded global averages. Multivariate analyses (PCA and HCA) revealed strong correlations between radiological parameters and geochemical indicators, confirming that weathering and lateritization are the dominant factors. These findings advance the understanding of Naturally Occurring Radioactive Material (NORM) behavior in tropical soils and provide essential data for radiological risk assessment and environmental monitoring in HBNRAs.