Uncertainty means a lack of knowledge. No matter how much information is available, data are always limited since available information sets are constrained in space and time. In groundwater-related studies, it is often assumed that uncertainty can be compensated by simple parameter uncertainty analysis (sensitivity analysis, stochastic simulations, fuzzy arithmetic). However, in the real world, the total uncertainty of a system is controlled by additional sources, like data, conceptual models, design uncertainty, uncertainty related to the selection of modelling tools, and human behaviour. This presentation intends to address these additional sources of uncertainty by using the theory of system analysis.

Beyond the parametrisation of a groundwater system which is inherently non-unique, it is even more critical to fully understand the system`s key characteristics. A false conceptualisation cannot be compensated with the assessment of parameter uncertainty. However, conceptualisation is sometimes subjective, heavily reliant on previous experience and usually non-unique just like system parametrisation. Therefore, a method is required to manage uncertainty associated with the “lack of knowledge” in conceptualisation and with any other potential sources of uncertainty. The nuclear industry has developed the method of features-events-processes method (FEP catalogues) to manage this kind of conceptual uncertainties, and it is a robust, transparent, comprehensive and clear way to develop base case and alternative scenarios which may form the basis of quantitative predictions. However, this method is not used in mining projects to its fullest potential.

In this presentation, the theory of system analysis will be presented with the theoretical and pragmatic limitations of hydrogeological knowledge. The method of system analysis is applied at several mine sites, especially to assess potential risks in post-closure phases. This approach can be used to list all features, events and process affecting a mine groundwater system and their interaction to assess all potential evolution pathway of a mining activity, and the way to develop most likely (base-case) and a set of alternative scenarios. Also, it will be demonstrated how this approach compares with the traditional approach where conceptualisation relies on one single, deterministic model.

It is shown that system analysis may result in several benefits in mine water management:

• Consideration of all sources of uncertainty with potential effect on the groundwater system.
• Finding optimal project design and ranking alternative scenarios.
• Reduced project risks and preparation for hydrogeological “surprises” and for alternative system evolution pathways.
• Cost benefits.
• Meeting regulatory requirements.

Dissolved organic matter (DOM) has been confirmed to be one of the important factors affecting the migration and transformation of antimony (Sb) in the aquatic environment. Studies have shown that terrestrial organic matter with strong aromaticity in natural water in mining areas may be an important carrier of antimony migration. However, the characteristics of DOM and its impact on the migration and enrichment behavior of Sb are still unclear at the molecular level.

In this study, we combined hydrogeochemistry with optical (Excitation-Emission-Matrix Spectra, EEMs) and molecular characteristics (Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR MS) of DOM to characterize the source and molecular composition of DOM in water environment in the antimony mining area and its influence mechanism on antimony migration and enrichment.

We identified three DOM fluorescent components, a tryptophan protein component (C2) and two terrestrial humus-like components (C1 and C3). The humus-like components in groundwater had a higher degree of unsaturation, while there might be a degradation process of C3 to C1 and C2 in surface water. Antimony in groundwater was positively correlated with non-bioactive compounds that were difficult to degrade, such as unsaturated compounds, polyphenols, and polycyclic aromatic, corresponding to C3 humus-like components. They were mainly composed of compounds containing N and S. In addition, CHON and CHO compounds were more conducive to the enrichment of Sb in groundwater than CHOS. Antimony in surface water was positively correlated with easily photodegradable compounds such as lignin and lipid structures, which was consistent with the changes in the C1 components of photolysis products along the stream. Accordingly, complexation was considered to be an important mechanism for the enrichment of antimony in dissolved organic matter in water environments. Non-biologically active compounds and organic matter photodegradation products were important complexing species. At the same time, there was fluorescence quenching after the protein-like components bind to antimony. In addition, organic matters containing O and N functional groups were easier to combine with antimony than organic matters containing S functional groups.

The results of this study provided new insights into the mechanism by which DOM affects the migration and transformation of Sb in aquatic environments at the molecular level. That will improve the biogeochemical process of antimony, and lay a theoretical foundation for the prevention and control of antimony pollution in mining areas.

As the ' antimony capital of the world ', the continuous mining activities have caused the increase of antimony content in the groundwater environment, which is widely distributed in the environment and can carry out long-distance migration. The toxicity and migration process of antimony are closely related to its valence state in the environment. Sulfate is an important factor affecting the valence state transformation of antimony. It is of great significance to study the influence mechanism of sulfate on the migration and transformation of antimony.

In this study, Xikuangshan in Hunan Province was taken as the research area. The interaction process between atmospheric precipitation and surface water and groundwater was analyzed by sulfur and oxygen isotopes, and the sulfate composition in groundwater system was identified. Combined with the valence state analysis of antimony, the influence of sulfate on the migration and transformation process of antimony was further explored.

Isotope analysis of groundwater in different lithology shows that the contribution rate of rock salt dissolution in limestone aquifer is 37.6%, while that in argillaceous limestone aquifer is 65.53%. When the concentration of sulfate increased, the Sb(III)/Sb(total) increased from 2.39% to 88.63 %, indicating that sulfate could inhibit the conversion of Sb(III) to Sb(V), or in the process of sulfate reduction, sulfide reduced Sb(V) to Sb(III). Compared with surface water in winter and summer, it was found that the fluctuation of antimony valence state in summer was more intense in response to sulfate concentration, indicating that sulfate may have microbial effects on the influence of antimony valence state. When the sulfate concentration is low, the antimony concentration is 206.55μg/L, and the antimony concentration is 161.69μg/L when the sulfate concentration is high, indicating that Sb(V) is easier to migrate than Sb(III). Further study of the effect of sulfate sources on antimony found that Sb(III)/Sb(V) was 91.91 % and 59.28 %, respectively, at high endogenous sulfate concentration and high exogenous sulfate concentration, indicating that endogenous sulfate has a more thorough effect on valence state.

The sulfate reduction process can change the valence state composition of antimony in the water and soil environment. By controlling the exogenous sulfate input of groundwater in the contaminated area, it can drive the conversion of pentavalent antimony to trivalent antimony. This discovery provides new ideas and possibilities for the prevention and control of antimony pollution in groundwater.

The development of waste rock dumps closure projects in Peru is substantially based on physical and chemical criteria, which prioritise the stability of the component, leaving aside other important aspects such as biological, vegetative and landscape adaptation.

Since 2021, Compañía de Minas Buenaventura, owner of the Santa Bárbara mining environmental liability located in the department of Huancavelica - Peru (which was one of the first gold and silver mines exploited by the Spanish conquistadors upon their arrival in America), commissioned Amphos 21 to develop basic and engineering studies for the design of a closure project for three waste rock dumps and an open pit. Thus, the unification of these deposits was proposed under a new geomorphological restoration approach, which allowed a complete change from the regulatory closure approach to one that was more compatible with the natural environment.

The most outstanding feature of this pioneering and innovative project is that it is not necessary to use concrete channels for surface water management; an organic soil cover layer was designed that varies in thickness depending on the slopes generated, accompanied by native high Andean species. This will result in lower post-closure maintenance costs and lower socio-environmental risks associated with these types of components.

The construction of this project demonstrated that we still have many challenges in Peru to introduce innovative models and new criteria to approach the closure of mining environmental liabilities and existing operations. The successful implementation of this project is feeding a new way of approaching closure from the perspective of sustainability, ensuring that the quality of the designs and their execution are durable, sustainable and perfectly integrated into the environmental and social environment. In this case, the articulation with the Buenaventura tourism project and the regional government, make this geomorphological restoration an important point in the company's closure strategy.