Integration of geostatistical and hot spot analysis to identify drivers of radionuclide spatial heterogeneity in the high-mountain lake basin topsoil

This study evaluates the drivers of radionuclide spatial heterogeneity in topsoil of the Lake Sevan Basin (Armenia) - the largest high-mountain freshwater body in the South Caucasus - using integrated statistical and spatial analyses. A total of 170 soil samples were analyzed for Ra-226, Th-232, K-40, Cs-137, and gross beta activity, together with in-situ dose was measurement. Statistical analysis revealed strong inter-correlations among natural radionuclides (with Spearman's test) and bimodal distribution patterns for Th-232 and gross beta activity, indicating the presence of distinct source domains. Land-use analysis confirmed significant effects on K-40 and Cs-137 distributions, with higher Cs-137 in low-disturbance soils and lowest in arable land, whereas K-40 was notably elevated in forest soils. The spatial heterogeneity of these parameters was modeled using geostatistical methods including Empirical Bayesian Kriging (EBK) and Getis-Ord Gi∗ (Gi∗) Hot Spot analysis. The analysis identified statistically significant hot spots of Ra-226, Th-232 and radium equivalent activity in southwestern, volcanic-rock-dominated part of the basin, and cold spots in lacustrine-derived soils in the eastern area. K-40 displayed additional enrichment in northwestern agricultural zones, suggesting a potential contribution from long-term agricultural practices. Cs-137 exhibited limited but distinct hot-spot clustering in minimally disturbed soils, reflecting post-depositional fallout processes. Radiological risk assessment revealed Ra-226 and Th-232 as the main risk contributors for the southwestern located settlements. The study demonstrates that integrating statistical inference, spatial modelling, and radionuclide geochemistry enables process-based interpretation of soil radioactivity and provides a transferrable framework for contamination assessment, radioecological targeted monitoring, and management prioritization in complex mountain environments.