Geochemical and Ecotoxicological Assessment of Contaminated Bottom Sediments in the Shagan River (Kazakhstan)

Radioactive and chemical contamination of aquatic systems remains a key environmental concern in areas affected by historical nuclear testing. The objective of this study was a comprehensive geochemical and ecotoxicological screening assessment of bottom sediments in the Shagan River, which partially flows through the Balapan test site within the Semipalatinsk test site. This 2 km stretch is potentially affected by groundwater discharge hydraulically connected to the Atomnoye Lake retaining zone. A total of 21 bottom sediment samples were collected over a 2 km stretch, and their elemental composition was determined using inductively coupled plasma mass spectrometry (ICP-MS). Bottom sediment quality was assessed using the geoaccumulation index (Igeo) and ecotoxicological criteria, including the threshold effect concentration (TEC), probable effect concentration (PEC), least impact level (LEL), and high impact level (SEL). The results revealed significant spatial heterogeneity in the element distribution and local geochemical anomalies. Manganese (Mn) and molybdenum (Mo) corresponded to the Igeo index category of severe contamination, while zinc (Zn) demonstrated an extremely high contamination level (Igeo = 6), exceeding the PEC threshold by 16 times. Mn concentrations also exceeded the SEL by 15 times. These results provide a baseline geochemical profile for a previously understudied and ecologically sensitive section of the Shagan River, identifying zones of local geochemical enrichment in bottom sediments and laying the foundation for further hydrogeochemical, mineralogical, granulometric, and ecotoxicological studies.