Abstract
This research article is dedicated to the analysis of the spatio-temporal dynamics of Lake Chagan. The primary scientific goal is the quantitative assessment of the spatial changes in the water surface area over the period 1995–2025 and the identification of the climatic determinants, as well as the anthropogenic stress factors linked to the lake's nuclear heritage, that influenced these changes. The research methodology involved processing Landsat 5, 8 and Sentinel-2 satellite data on the Google Earth Engine (GEE) platform to calculate the Normalized Difference Water Index (NDWI). Comparative analysis was carried out using the Union Overlay geospatial analysis tool in the ArcGIS Pro environment. The main results of the study reveal that the lake's water surface area decreased by 17.3% (from 1.82 km² to 1.50 km²) over three decades. This shrinkage is directly linked to regional climatic warming (with an average warming trend in Kazakhstan of 0.31 C per decade) and increased trans-evaporation. The scientific value of the work lies in demonstrating the ecosystem instability of this unique hydrogeological feature within the Semipalatinsk Test Site and contributing to the assessment of its ecological safety in the context of radionuclide (tritium, plutonium) contamination. The practical value of the results provides a crucial scientific and methodological basis for developing sustainable water resource management strategies