Background
SGME was engaged for our expertise in cover system design and maintenance to evaluate the performance of a historic cover on a tailings storage facility (TSF). The mine is a conventional open pit zinc and lead operation located in North West Queensland, approximately 250 kilometres north-west of Mount Isa. The bulk sample tailings dam (BSTD) was constructed in 1994 to manage tailings from the bulk plant mine operation.
Objective
The BSTD was rehabilitated using a store and release cover (the cover) in early 2000; however, this cover failed.
A significant issue that caused cover system failure was production of acid rock drainage (ARD) expressed as low pH (≤ 3) seepage. This issue highlighted the need for investigations into the geochemical processes occurring within the BSTD and effectiveness of the cover in limiting potential contamination of the surrounding environment.
The mine operator wanted to learn from their early rehabilitation attempts and, therefore, required an industry expert to lead an assessment of the cover to identify the cause of failure. Findings from the assessment were crucial for understanding the root causes of cover failure and identifying strategies to mitigate it on the main TSF.
Solution
SGME was engaged to evaluate and assess performance of the BSTD. We conducted an assessment that involved collecting and testing samples for geochemical and physical parameters. Laboratory analysis was then used to build a numerical model for the cover.
The assessment aimed to understand differences between the built cover and its conceptual design, and how the differences lead to its failure. The project consisted of two phases: evaluating cover performance and assessing tailings geochemistry. SGME analysed performance using a numerical model that included environmental inputs (ie rainfall, runoff, infiltration and water storage) and physical characterisation inputs (ie soil water characteristic curves derived from pedo-transfer functions, unsaturated hydraulic conductivity functions and saturated permeabilities). Understanding the sulfide / sulfate balance with depth was crucial for conceptualising and modelling major chemical processes. SGME analysed tailings samples for total sulfur, chromium reducible sulfur, soluble sulfate, net acid generation (NAG) pH, pH, electrical conductivity (EC) and acid neutralising capacity (ANC). Static analysis was enhanced by incorporating kinetic studies.
The goal was to use both geochemical data and historical cover performance to understand their interrelationship. Integrated numerical modelling used the cover and solute transport models to predict seepage quality based on the mixture of infiltrating rainwater, leaching of the unsaturated zone and reactions with the existing saturated tailings. This analysis aimed to provide an understanding of the interactions between the cover and the tailings, enabling the development of more effective strategies for managing tailings and preventing ARD. The BSTD cover failure facilitated a learning opportunity to improve mine stakeholders’ understanding of geochemical processes, cover performance and geochemical processes. Lessons were intended to inform and enhance closure plans for the main TSF to ensure that future covers would be more effective in preventing ARD and other environmental issues. Applying our research, our experience and industry best practices enables SGME to play a significant role in solving geochemistry-based problems in the mining industry.
Our in-depth understanding of the distribution of chemical elements in rocks and minerals, and absorption of these elements into soil and water, enable our clients to mitigate negative impacts on the environment. This underscores our unwavering stewardship and dedication to sustainable practices, and our goal of creating enduring value for the future.