Cover Systems... Are We There Yet?

Mike O'Kane

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Geoenvironmental Engineering

ABSTRACT: Calgary, AB, Canada ABSTRACT Cover systems are an accepted prevention/mitigation alternative for managing mine waste seepage affected by acid and metalliferous drainage. The mining industry has advanced theory, practice, construction, and performance monitoring of cover systems to achieve this level of acceptance. However, uncertainties and misunderstandings remain as to benefits -closure strategy, often leading to insufficient characterization and management of reactive waste prior to/during operations. The mining industry is also challenged at times to demonstrate benefits, or rather the magnitude of benefits, provided by cover systems to prevent/mitigate adverse impacts to receiving environments including the implications for water management (quantity and quality) and water collection/treatment systems. There are numerous benefits of cover systems in regards to meeting closure objectives; for example, meeting land use objective, wildlife habitat, etc. However, this paper focuses on demonstrating cover system benefits in regards to developing a better understanding of the relationship between contaminant loading from the waste storage facility and the control of net percolation rates offered by the cover system. 1 INTRODUCTION AND BACKGROUND The mining industry is faced with increasing environmental requirements for the reclamation of mine wastes including waste rock dumps, heap leach piles, and tailing storage facilities. The increased environmental requirements may arise due to more stringent regulatory requirements tied to the operational license and/or general stakeholder perspectives tied to the company's social license. Metal leaching and acid rock drainage (ML/ARD) is produced in almost all mining activities involving the disturbance of sulphide-bearing overburden. Management and control of this drainage is usually a critical aspect of the mine closure plan which will nearly always include the design of some form of cover system. Cover systems are an accepted prevention/mitigation strategy for managing ML/ARD. The mining industry have advanced theory, practice, construction, and performance monitoring of cover systems to achieve this level of acceptance. Cover systems can have numerous design functions, including but not limited to isolation of waste from the natural environment, limiting the influx of atmospheric water and/or oxygen, controlling the upward movement of process-water constituents / oxidation products, and providing a medium for establishing sustainable vegetation (MEND 2004; 2012; INAP 2011). According to the cover system design functions, cover system design alternatives may include store-and-release or ET (evapotranspiration) cover systems, enhanced store-and-release cover systems, barrier-type cover systems, and cover systems with engineered layers (Ayres and O'Kane 2011). Selection of an appropriate cover system design requires completion of a receiving environment impacts analysis. The mining industry is also challenged at times to demonstrate benefits, or rather the magnitude of benefits, provided by cover systems to prevent/mitigate adverse impacts to receiving environments including the implications for water management (quantity and quality) and water collection/treatment systems. The challenges are mainly attributed to the following factors: dynamic variations of acidity loading with/without cover systems; difficulty with predicting the time period in which post-closure costs continue; a discount rate for net present cost (NPC) calculations agreed to by all stakeholders; and uncertainties associated with all processes (values) used for pre-closure, closure, and post-closure costs estimation. In addition, there are differing opinions, evidence, and research with respect to benefits a cover system provides in reducing net percolation to waste rock, which increases the complexity with demonstrating benefits that a cover system provides compared to a direct ML/ARD collection/treatment system. The same challenges also exist when demonstrating relative benefits of various cover system design alternatives. Even given these challenges, cover systems are still constructed for any number of ancillary benefits (driven by social and ecological needs). This paper highlights the important relationship between net percolation in the evaluation of cover system performance and acidity load generation (and solute concentration). This relationship needs to be understood, at least conceptually, on facility-specific basis and for each waste storage facility (WSF) and cover system design alternative. In order to highlight this relationships, the mitigation of ML/ARD is discussed in light of the primary design objectives of limiting oxygen ingress and net percolation. A technology transfer program and NPC evaluation method are considered useful tools for selection of an optimum cover system design alternative

RÉSUMÉ: , Calgary, AB, Canada ABSTRACT Cover systems are an accepted prevention/mitigation alternative for managing mine waste seepage affected by acid and metalliferous drainage. The mining industry has advanced theory, practice, construction, and performance monitoring of cover systems to achieve this level of acceptance. However, uncertainties and misunderstandings remain as to benefits

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Cite this article:
Mike O'Kane (2014) Cover Systems... Are We There Yet? in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper268,author = Mike O'Kane,title = Cover Systems... Are We There Yet?,year = 2014}