Chemical Elements in the Bottom Sediments of the Polluted Section of the Shagan River

The issue of radioactive and chemical pollution of natural water bodies remains relevant in regions affected by nuclear weapons testing. One such object is the Shagan River, located within the Semipalatinsk Test Site. Of particular concern is the «old channel», where contaminated groundwater and fracture water discharge into the surface flow. The aim of this study is to identify the patterns of chemical element accumulation in the bottom sediments of the «old channel» of the Shagan River. The scientific significance lies in clarifying migration and accumulation processes of chemical elements under conditions of localized radioactive contamination. The results provide insights into the transformation of the geochemical background in rivers exposed to long-term technogenic impact, supporting the development of monitoring and forecasting systems. The methodology included field sampling, laboratory analyses (atomic absorption spectrometry, inductively coupled plasma mass spectrometry), statistical data processing, and GIS-based mapping. Contamination of bottom sediments was assessed using the geoaccumulation index (Igeo) and comparison with Clarke values. The results showed geochemical series indicating elevated element concentrations in the upper continental crust (Sr > U > Ca) and clay shales (Sr > Ca > U > Na > Mg). According to Igeo, bottom sediments of the Shagan River ranged from unpolluted to heavily polluted: Li, Mg, K, and Fe corresponded to class 0, Na and U to classes 1–2 (unpolluted to moderately polluted), while Ca and Sr fell into class 3 (moderately to heavily polluted). This study contributes new empirical data on the geochemical characteristics of radioactively contaminated river systems and establishes a basis for further research on ecological and human health risks. The findings also have practical value for environmental protection and the development of long-term monitoring programs.