303 1. INTRODUCTION Tailings storage facilities constitute an essential component of modern mining operations, functioning as engineered systems for the containment of by-products generated during ore extraction and mineral processing. Effective surveillance and assessment of these facilities are fundamental to maintaining environmental integrity, safeguarding public safety, and meeting increasingly stringent regulatory frameworks. In recent decades, advances in Earth observation technologies have significantly expanded the methodological toolbox available for tailings monitoring, positioning multispectral remote sensing as a highly effective and scalable solution. 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 open-access 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
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