Editorial for the Special Issue “Tailings Dams: Design, Characterization, Monitoring, and Risk Assessment, 2nd Edition”

1. Introduction

Tailings storage facilities (TSFs) are among the largest engineered structures in the mining industry and, at the same time, among its most persistent sources of safety and environmental risk. As ore grades decline and the demand for metals rises, the volume of tailings produced continues to grow; thus, the number and size of active TSFs increase worldwide. The worldwide failure rate of tailings dams has been estimated at approximately 1.2%, two orders of magnitude higher than the 0.01% reported for conventional water-retention dams (Contribution 1). High-profile failures over the past three decades, including those at Aznalcóllar (Los Frailes, Spain, 1998), Ajka (Hungary, 2010), Mount Polley (Canada, 2014), Fundão (Samarco, Brazil, 2015), Brumadinho (Córrego do Feijão, Brazil, 2019), Jagersfontein (South Africa, 2022), and Williamson (Tanzania, 2022), have repeatedly demonstrated the human, environmental, and economic consequences of inadequate characterization, design, monitoring, and risk management (Contribution 2).

The first edition of this Special Issue assembled ten articles on the design, characterization, monitoring, and risk assessment of tailings dams, which were subsequently reprinted by MDPI as an edited volume [1]. Building on that foundation, this second edition collects seven peer-reviewed papers that extend the discussion across the life cycle of TSFs, from monitoring and surveillance, through the physical, geotechnical, and geochemical characterization of tailings, to the forensic analysis of failure and the treatment of tailings at the source. Together, the contributions reflect both the consolidation of established practice and the emergence of new tools, and they reinforce the central message that the safety of tailings dams depends on integrated, evidence-based management throughout design, operation, and closure.

2. An Overview of the Published Articles

The seven contributions to this Special Issue fall into four interrelated themes: monitoring, surveillance, and safety guidelines (Contributions 1 and 2); the characterization of tailings and tailings storage facilities (Contributions 3–5); the forensic analysis of failure (Contribution 6); and the treatment of tailings (Contribution 7).

Zare et al. (Contribution 1) reviewed the major international guidelines and systems for tailings dam safety monitoring and surveillance. Their review compiles the visual parameters that should be monitored across the life cycle of a TSF, including the post-closure period, and synthesizes good practice for the design of monitoring programs. The authors emphasize the growing role of sensors, unmanned aerial vehicles (UAVs), and satellite imagery as supplementary safeguards against failure, situating these tools within the surveillance frameworks recommended by leading guidelines.

Complementing this overview, Cacciuttolo et al. (Contribution 2) presented a systematic review, conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and supported by bibliometric mapping, of sensor technologies for the safety monitoring of TSFs in the Industry 4.0 era. They examined the state of the art in wireless real-time monitoring, remote sensors, UAVs, unmanned survey vessels (USVs), artificial intelligence, cloud computing, and the Internet of Things, and identified the principal advances, gaps, and future trends in the digital monitoring of structural health and safety management. The review frames these technologies as scalable and increasingly reliable solutions whose importance is heightened by climate-change-related hazards.

Rodríguez-Pacheco et al. (Contribution 3) proposed a conceptual model for the hydromechanical behavior and internal zonation of TSFs constructed by the hydraulic backfill method, which accounts for more than 98% of TSFs worldwide. The model distinguishes four zones, namely the dike, discharge, transition, and distal zones, each with characteristic physical, geotechnical, and hydraulic properties. The authors show that seven parameters (grain size, friction angle, shear strength, dry density, permeability, shear wave velocity, and liquefaction capacity) decrease systematically from the dike toward the settling pond, whereas thirteen parameters, including fine content, porosity, cohesion, plasticity, degree of saturation, water content, capillary height, and suction, increase along the same direction. By integrating geological, geotechnical, hydrogeological, and geophysical information, the study provides a holistic framework spanning design, construction, operation, and closure.

Farhadian et al. (Contribution 4) examined the role of granular soils, namely clay, gravel, sand, and silt, in the behavior of tailings dams and in the transport of contaminants. Using numerical solute-transport modeling of a tailings dam incorporating different soil types, they showed that finer particles such as clay and silt exhibit higher adsorption capacities that retard contaminant migration, whereas coarser materials such as sand and gravel permit faster transport and a greater potential for rapid dispersion. The study underscores the coupling between particle-size distribution, contaminant properties, and the environmental performance of tailings dams.

Reyes et al. (Contribution 5) characterized the geochemistry of Chilean mine tailings using the SERNAGEOMIN database of 653 deposits. Applying a multivariate framework that combined completeness assessment, descriptive statistics, and hierarchical clustering of 56 elements, they identified four distinct geochemical profiles: silicate copper tailings; zinc–lead–cadmium–arsenic polymetallic tailings, which carried the highest concentrations of heavy metals and rare earth elements; carbonate-matrix tailings, with buffering potential against acid mine drainage; and clay-rich (kaolin) tailings, marked by anomalous cobalt enrichment. These profiles provide source signatures for receptor models and a basis for classifying deposits that lack geochemical information, linking environmental risk assessment with the prospect of secondary resource recovery.

Oliva-González et al. (Contribution 6) reported a forensic investigation of the 1960 failure of the La Luciana TSF in Reocín, Spain, in which the release of approximately 300,000 m³ of tailings caused 18 fatalities, extensive flooding, and contamination of the Besaya River. Integrating historical documentation, in situ testing, laboratory analysis, and numerical modeling, the authors reconstructed a seepage-induced, retrogressive flow failure. Their analyses indicate that the factor of safety dropped to unity when the decant pond approached to within approximately 20 m of the dam, and they distill lessons for tailings governance, including the need to maintain unsaturated tailings, to ensure effective drainage and decant systems, and to monitor the proximity of the pond to the dam. The work also demonstrates a replicable forensic methodology of relevance to contemporary regulatory and industry frameworks for tailings management.

Boshrouyeh et al. (Contribution 7) investigated the treatment of saline tailings by inline flocculation with an anionic polyacrylamide. Laboratory characterization showed that polymer amendment reduced the water content from 107% to 53% and increased the dry density from 0.711 to 1.069 t/m³ relative to the untreated material, while consolidometer testing revealed improved consolidation behavior, including approximately 60% less settlement within the first 48 h, about 50% more free-water drainage, a lower compression index (Cc of 0.74 versus 1.05), and a higher coefficient of consolidation. The authors conclude that inline flocculation can enhance dewatering and long-term stability, and thereby improve tailings management under highly saline conditions.

3. Conclusions and Future Directions

Taken together, the seven contributions to this second edition trace a coherent arc across the tailings dam life cycle. The two reviews (Contributions 1 and 2) consolidate current monitoring and surveillance practice and chart the rapid adoption of sensor-based and digital technologies. The characterization studies (Contributions 3–5) deepen the understanding of the physical, geotechnical, and geochemical heterogeneity of tailings and TSFs, from internal zonation and contaminant transport to basin-scale geochemical classification. The forensic analysis of the La Luciana failure (Contribution 6) connects this knowledge to the mechanics of catastrophic failure and to the governance measures that could have prevented it, while the treatment study (Contribution 7) points toward engineering interventions that improve dewatering and stability at the source.

Several priorities for future research emerge. First, the integration of dense, real-time sensor networks with physically based and data-driven models is needed to convert monitoring data into reliable early indicators of instability. Second, characterization frameworks should be linked more explicitly to risk assessment and to closure design, so that spatial heterogeneity in geotechnical and geochemical properties is carried through to long-term safety and environmental performance. Third, the lessons of historical failures should be embedded in standardized, transferable forensic methods and in the continued strengthening of international frameworks for tailings management. Finally, treatment and valorization strategies, including dewatering and the recovery of critical metals from tailings, deserve further study as routes to reduce both the volume and the hazard of stored tailings. We hope that this Special Issue contributes to these goals and supports the continued advancement of safe and sustainable tailings management.