Distribution of TENORM-contaminated soils in residential areas of Bangka Island, Indonesia.

Radioactive contamination in residential areas of Bangka Island, Indonesia, a major tin- and rare earth-producing region, has rarely been studied, although material stored near mineral-processing workshops may pose a long-term health risk. We collected 46 soil samples around 9 mineral-processing workshops in the Selindung District, Pangkal Pinang City: residential soils from 20 locations at two depths (n = 40) and 6 workshop-related sediments. High-purity germanium detector analysis showed that 13 of the 40 residential soil samples exceeded the screening criteria of the IAEA Safety Standards Series No. RS-G-1.7 for naturally occurring radioactive material. Residential contamination was highly heterogeneous and was not explained by a simple monotonic distance relationship (Spearman's ρ = -0.12, p = 0.61). Two upper outliers occurred at 5 and 10 m from the nearest workshops, including the maximum residential activity of 12,992 Bq/kg detected at 5 m, in areas frequently accessed by local residents. Five of 17 locations with quantifiable paired measurements (including 1 tied pair; n = 16 for the Wilcoxon signed-rank test) showed higher activity at 20 cm than at 10 cm, and the paired depth difference was not statistically significant (Wilcoxon p = 0.079), indicating an irregular rather than consistently surface-dominated vertical distribution. Screening-level outdoor annual effective doses for the quantifiable residential samples ranged from 0.051 to 8.46 mSv/y (median 0.42 mSv/y), with the maximum obtained 5 m from a workshop. Nuclide composition analysis further revealed that the activity concentrations of the 238U and 232Th series increased with total radioactivity, whereas 40K showed only a weak correlation. Maximum activities of 238U and 232Th approached levels reported by the IAEA for high-purity monazite sand (6,000-20,000 Bq/kg). These findings demonstrate spatial and vertical heterogeneity of technologically enhanced naturally occurring radioactive material in residential zones, indicating a need for site-specific assessment and long-term monitoring.