1200 Multispectral remote sensing exploits reflectance measurements across multiple wavelengths of the electromagnetic spectrum, allowing detailed discrimination of surface materials based on their spectral behavior. This capability facilitates the detection of subtle variations in surface moisture, material composition, and vegetation condition, thereby providing critical insights into the physical state and temporal evolution of tailings facilities. The use of spectral indices derived from multi-temporal satellite observations further enhances analytical robustness, offering a non-invasive and economically efficient means of generating spatially continuous monitoring indicators. The present study aims to identify and quantify changes associated with operational activities and progressive development of the Benkovski 1 and Benkovski 2 tailings facilities through the use of openaccess satellite data and open-source geospatial software. Freely available imagery from the Landsat and Sentinel-2 missions is processed using platforms such as QGIS and SNAP to evaluate the applicability of multispectral analysis for tailings monitoring without reliance on costly proprietary datasets or software. By integrating archival and contemporary satellite observations within a unified analytical framework, this research establishes a consistent approach for multi-decadal monitoring of tailings facilities. The findings underscore the capacity of open-access remote sensing resources to support robust environmental evaluations across different stages of the tailings facility lifecycle. 2. STUDY AREA This study examines the Benkovski tailings storage complex, operated by Ellatzite-Med AD and located in the Sofia region, Bulgaria, as a representative example of long-term tailings management within an active mining operation. The complex consists of two consecutively developed tailings facilities, reflecting the evolution of production requirements and environmental management strategies over time. Benkovski 1 was constructed during the initial development phase of the mine and operated until its designed storage capacity was reached. Following its phased closure, the facility underwent complete reclamation, including geomorphological reshaping, application of a topsoil layer, and the establishment of stable vegetation cover. These measures resulted in the successful rehabilitation of the site, transforming it into an environmentally stable landform that is currently well integrated into the natural landscape. To sustain ongoing production, the Benkovski 2 tailings storage facility was subsequently commissioned. This facility receives flotation tailings derived from the processing of ores from the Ellatzite porphyry-copper deposit and represents a large-scale engineered structure with a designed area of nearly 490 ha and a total capacity exceeding 312 million m³. The storage facility is subdivided into two operational compartments, Ai Dere and Suludja Dere, which are alternately utilized based on annual operational planning. Tailings are transported via a hydrotransport system as a slurry with a solids-to-water ratio of approximately 1:3. Upon deposition, the slurry undergoes natural segregation, with coarser sand fractions forming the dam embankment prism and finer materials settling upstream toward the decant lakes. Since its inception, Benkovski 2 has been operated in accordance with contemporary environmental management practices, emphasizing continuous monitoring and proactive measures to reduce environmental impacts.
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