In-situ depth-resolved gamma spectrometry of natural radionuclides in central Nepal: Radiological assessment across...

Potassium-dominated elevated natural radiation environments are exceptionally rare globally, with documented high background radiation areas typically being thorium-dominated (Kerala, India; Guarapari, Brazil) or uranium-dominated (Ramsar, Iran). This study characterizes such an environment in the Himalayan region within the Markhu Formation of central Nepal and establishes the first depth-resolved radionuclide baseline across eight geological formations from Kathmandu to Hetauda. In-situ gamma ray spectrometry at 10 cm intervals to 50 cm depth was used to quantify ⁴⁰K, ²³⁸U, and ²³²Th distributions and examine depth-dependent behavior. ²³²Th exhibited strong depth dependence reflecting bedrock-controlled immobility, ²³⁸U showed variable correlations reflecting lithological heterogeneity, and ⁴⁰K displayed intermediate patterns controlled by weathering. Dose rates exceeded world averages at all sites (84.4 ± 2.0 to 246.0 ± 6.2 nGy/h vs. 59 nGy/h), yet annual effective doses remained below the 1 mSv public limit. The Markhu Formation exhibited annual effective doses 3.6 times the world average, external hazard index exceeding unity (H_ex = 1.28 ± 0.31), and radium equivalent activity of 477 ± 118 Bq/kg driven by ⁴⁰K of 6.70 ± 0.82 to 9.2 ± 1.0% (four to six times global average) from biotite-rich metamorphic parent rocks. Constant activity ratios and U-Th correlation are consistent with natural geogenic origin rather than anthropogenic contamination. These findings expand the global typology of elevated natural radiation environments and demonstrate that formation-specific baselines are essential for radiological assessment in geologically diverse Himalayan terrain.