Process-Based Source Apportionment and Radiological Baseline of Multi-Radionuclides in Soils of a Tourism-Oriented Island

Islands have high ecological and tourism value; however, owing to their remoteness and limited accessibility, environmental radioactivity is often less systematically evaluated than in mainland regions. This study investigates the distribution, source partitioning, and radiological implications of multi-radionuclides (7Be, 137Cs, 210Pb, 238U, 226Ra, 232Th, and 40K) in surface soils of Zhoushan Island, a representative tourism-oriented island in the East China Sea. Activity concentrations of 7Be, 137Cs, 210Pb, 238U, 226Ra, 232Th, and 40K ranged from 3.4 to 585.5, below detection limit −5.7, 45–1490, 33.3–72.4, 32.3–58.9, 37.8–91.7, and 439.6–872.3 Bq/kg, respectively. Using multivariate statistics and geochemical interpretation, we classified radionuclides into three groups: (i) atmospheric deposition-driven nuclides (7Be, 210Pbex), (ii) lithogenic background-controlled nuclides (238U−226Ra−232Th), and (iii) the alkali-metal-like behavior group (137Cs−40K). This shows that soil radionuclide patterns result from atmospheric inputs, geological inheritance, and land-use disturbance, rather than simple concentration variability. Spatial analysis revealed that agricultural disturbance enhances 137Cs redistribution, low-lying terrains preferentially accumulate atmospheric fallout nuclides, and lithogenic radionuclides are higher in the northern island due to parent material and weathering. No significant 40K enrichment was observed in cultivated soils, indicating limited fertilizer influence. Although radiological indices remain within international safety thresholds, several parameters exceed global background levels, indicating elevated natural radiation driven primarily by thorium-rich lithology. Importantly, we show that radiological risk assessments based solely on bulk activity may overestimate environmental significance without considering process controls. This study provides a process-informed radiological assessment for island systems, offering insights for environmental monitoring and risk evaluation in similar coastal and tourism-dominated regions.