Mining of PT Freeport Indonesia’s Grasberg open pit required filling the Wanagon Valley and constructing an Overburden Stockpile (OBS) in this major drainage system. OBS material placement began in the late 1990s and incorporates a coarse drain zone constructed at the base of the valley fill and designed to convey flows exceeding 3.2 cubic meter per second. The Wanagon OBS is more than 3,300 m long, varies from 250 to 1,200 m in width, has a slope height of 1,000 m with fill depths that vary from 1 to 400 m, and a volume of approximately 430 million cubic meters. As the open pit operations have ended, closure of the Wanagon OBS is continuing. A primary factor critical to the overall project success is in understanding the pore pressure conditions within the OBS to help ensure designs meet acceptance criteria, to optimize construction requirements, to provide inputs for stability assessments, and to support operational and long-term safety.

Estimating pore pressures distributions within stockpiles is controlled by the complex distribution of hydraulic properties linked to the OBS construction methodology and sequence and poses a significant challenge in industry. A novel approach has been developed by CNI to model the Wanagon OBS hydraulic system to provide accurate and efficient interpretations of pore pressures at an appropriate scale for stability analysis using an unstructured FEFLOW mesh. The new methodology and advanced tools provide the capability to efficiently create an unstructured 3D FEFLOW model and facilitates assigning complex material property distributions to match the OBS construction sequence. A seamless integration between FEFLOW and geotechnical stability models allows for direct transfer of pore pressure distributions into the mechanical stability model eliminating the need for interpolation between the models.

With this new approach the resolution of the pore pressure model was enhanced and an excellent calibration to hydraulic monitoring data within the OBS was achieved. In addition, using this unstructured approach resulted in an increase in the numerical stability of the pore pressure model with more efficient time stepping, convergence properties, and mass balance behavior.

The hydro models have been used to identify areas of hydrogeological/geotechnical concern, provide guidance for monitoring targets, and inform/optimize design geometries and material requirements for stability. Using this new approach improves analysis efficiency and risk assessments, development of appropriate Trigger Action Response Plans (TARP), and implementing monitoring systems in support of the Wanagon OBS closure project.

Neves Corvo is a world-class underground copper-zinc mine located in the south of Portugal. The mine has been operated since 1988 by Somincor. The pyritic tailings produced by the operation have a very high acid generation potential and have been placed, since mine startup, subaqueously in the Barragem Cerro do Lobo tailings storage facility (TSF), a large tailings pond created by a rockfill dam across a natural river valley. Due to the finite capacity of the existing impoundment, alternatives for provision of sufficient storage capacity were studied by Somincor. Surface disposal of thickened tailings was identified as the preferred option, which was considered a novel approach at the time given the high reactivity of the tailings and the arid climate at Neves Corvo. In 2010, the TSF was converted from a sub-aqueous to a thickened tailings deposition facility, without requiring any future raises of the main and perimeter rockfill embankments. The design included disposal of tailings with run-of-mine waste rock, which is potentially acid generating, in a co-disposal system. The waste rock is used for construction of peripheral berms and covers.

Monitoring data collected since late 2010 include records of produced /thickened tailings densities, yield stress, particle size distribution, and specific gravity. Settlement, deposition slopes and piezometric levels in the paste and underlying subaqueous deposit are also routinely monitored. The environmental component of the monitoring program, which is the focus of this paper, includes the determination of ponded water quality as well as compositional profiling of the paste and underlying slurry tailings with depth through acid base accounting (ABA), net acid generation (NAG) testing, chemical composition, paste pH and conductivity, and short-term leach testing.

The concentration profiles demonstrate that oxidation is limited to approximately the upper 2 meters of the paste deposit. The waste rock dikes enhance oxidation, likely through promoting horizontal drainage and depression of the water table in their vicinity. Oxidation when paste is exposed is more prominent than when a cover is present while oxidation is also more pronounced at higher elevations in the facility. Metal leaching trends are consistent with geochemical principles, with lower pH typically resulting in enhanced leachability.

The ongoing monitoring of the tailings mass and operational experience at Neves Corvo indicate that surficial deposition of high-sulfide tailings in an arid climate represents a feasible alternative that enhances operational flexibility, facilitates concurrent reclamation and permits co-disposal.

Since 1979, an active treatment system has been operating at Legacy Mine X, located in the Tohoku area of Japan, to prevent water contamination in the surrounding region. Although effective, this system relies on energy-intensive processes that generate major CO₂ emissions and incur high operational costs. In 2021, a pilot-scale bioreactor passive treatment system was introduced to treat manganese and zinc, achieving a 98% removal efficiency. However, the environmental impacts, particularly CO₂ emissions and operational costs of scaling up the passive treatment system have not yet been fully evaluated.

This study employs life cycle assessment (LCA) to compare the environmental and economic aspects of the current active treatment system with scenarios for a full-scale passive treatment system, based on the existing pilot-scale setup and assuming the treatment of 4046 m³ of mine drainage water per day. The assessment focuses on CO₂ emissions using a midpoint evaluation method, with the functional unit expressed as kilograms of CO₂ equivalent per year during the operational period. Operational costs are integrated into the LCA for economic comparison. The system boundaries encompass all components of the treatment plants, including energy use, water sampling, sample handling, transportation, and labor. Proposals for reducing CO₂ emissions in the passive treatment system by utilizing various energy sources are also included. The cost for the active treatment system was calculated based on annual mine drainage treatment costs and mine drainage volume

The passive treatment system is estimated to emit 154,078 kg of CO₂ equivalent per year, with an operational cost of 20 million JPY (Japanese yen) annually. In contrast, the active treatment system generates approximately 10 times more CO₂ emissions annually, with a total cost of around 147 million JPY (METI, 2010). The unit processing cost per m³-waste water was 99.5 JPY/m³ for active treatment, compared to 13.5 JPY/m³ for passive treatment. However, switching to solar energy could reduce these emissions to 16,210 kg of CO₂ equivalent annually, while lowering energy costs by 9 million JPY per year.

The results suggest that scaling up the passive treatment system to full operational capacity at Legacy Mine X would substantially reduce both operational costs and CO₂ emissions. These findings could serve as a model for implementing more sustainable remediation strategies at other legacy mine sites.

Evapotranspiration (ET) covers are an industry-standard tool for mine waste leachate mitigation in arid and semi-arid climates. A new ET cover was designed, field-tested, and optimized with computer modeling for the tailings storage facility (TSF) at the Zangazeur Copper-Molybdenum Complex (ZCMC) in Kapan, Armenia. Collected field data from ET cover test cells, soil characteristics, and climate data were combined to create a variably saturated groundwater flow model which simulated the effectiveness of the new cover for over 10 years. The model was validated to the field-observed measurements of moisture. A minimum thickness of cover that would prevent breakthrough was recommended.

The Almadén Mining District (AMD) is an area of concern due to the significant contamination generated by mining activities that have impacted regional ecosystems. Inadequate mine closure practices have contributed to the worsening of this issue. In this context, it has been confirmed that the local population consumes crayfish (Procambarus clarkii), a species known for bioaccumulating heavy metals, making it a valuable bioindicator for the aquatic ecosystem. Despite studies conducted on the impact of contaminants on the health of the local population, the potential of this crustacean species as a bioindicator in the area has not been thoroughly explored, nor has its consumption's effect on consumer health been evaluated. Therefore, the objective of this study is to assess the non-carcinogenic human health risk associated with the consumption of Procambarus clarkii in the AMD.

Muscle and hepatopancreas samples from crayfish were analyzed using Energy Dispersive X-ray Fluorescence (ED-XRF) to identify Potentially Toxic Elements (PTEs) such as As, Hg, Cd, Pb, Cu, Zn, and Sb. Total mercury concentration (HgT) was determined through Zeeman-corrected atomic absorption spectrometry. A probabilistic human health risk assessment was conducted using Bayesian models, which allowed for the estimation of non-carcinogenic risk in terms of the Hazard Quotient (HQ), using data on PTEs concentrations and surveys applied to the local population.

Results showed HgT concentrations of up to 3.4 mg kg⁻¹ in abdominal muscle, exceeding the European Union's allowed limit for human consumption of 0.5 mg kg⁻¹. Additionally, high concentrations of other PTEs were found, particularly As and Cd, which showed significant accumulation in the hepatopancreas.

These findings are expected to be relevant in understanding the environmental impact of mining activities on aquatic ecology and their potential repercussions on local populations through the bioaccumulation of PTEs in this species and their subsequent incorporation into the food chain. Additionally, this study may serve as a reference for future research, as the crayfish species used here is considered a valuable bioindicator, and the analysis was carried out in the Almadén Mining District, recognized as one of the most significant mercury mining regions globally.