1 Summary
In this book I discuss hydrogeological concepts and analysis approaches to address topics associated with mineral resource development. The focus in this book is principally on physical attributes of the hydrogeological setting that need to be considered in mineral resource developments. This book provides a broad discussion of the role of hydrogeology in mineral resource development but the intent is not to cover all types of mining operations for which the principles of groundwater flow are integral in project development and site closure. For example, one method not discussed is the control of groundwater flow patterns and flow rates in the design of an efficient fluid injection and withdrawal system for in-situ leaching operations. Another topic not discussed is groundwater control measures used for aggregate mining for construction materials, one of the most common mining operations.
In the mining industry the application of principles of hydrogeology is a central component in safe operational practice, water management and environmental stewardship. Characterization of the groundwater regime is required during the planning and development of large open pit mines, in underground and block cave mining, and at in-situ or heap leach facilities. Figure 1 shows many of the physical structures encountered at a large open pit mining operation. Ore is being mined at the active open pit seen in the southeast corner of the photo. Mill tailings produced during processing of the ore are retained in the storage facility seen to the north in the photo. This mine uses an earth-fill dam constructed of waste rock to retain the tailings, with tailings deposited as a slurry discharge from spigots located on the crest of the tailings dam. This facility has an extensive tailings beach, submerged tailings, and a supernatant pond of process-affected water. Pond water is re-used as a water supply for ore processing. A historic open pit seen to the southwest in the photo is slowly filling with water as a consequence of groundwater inflows and precipitation, creating a deep pit lake. A heap leach operation for additional mineral recovery is located to the southeast of the tailings facility and a waste rock stockpile is north of the active pit. A pond is present in a sump on the floor of the active pit, where the local water table has been intersected. This site is a copper mine; but similar structures can be found at coal mines, diamond mines, in oil sand operations, and in many other mineral resource developments. Underground mines typically produce much smaller volumes of waste rock than an open pit operation. However, they too require a management system for disposal of the tailings produced during processing of the ore.
Figure 1 – Key features associated with ore extraction and mine waste management at an open pit mine that require consideration of the hydrogeological regime. For scale, the diameter of the historic open pit seen in the lower left side of the photograph is approximately 1800 m. North is to the top of the photograph.
The primary hydrogeologic issues associated with mineral resource projects are those related to: (i) safe and efficient extraction of ore during mining operations (e.g., dewatering and depressurization controls), and (ii) protection of groundwater and surface water quality. Management of groundwater inflows is required to ensure dry working conditions and groundwater control is often a critical element in ensuring geotechnical stability of pit walls, tunnels and mineral waste containment structures. The presence of elevated porewater pressures can compromise the stability of slopes in an open pit, in structures such as tailings dams and process water storage dams, or the stability of waste rock stockpiles. In block caving mining, the ore zone is undercut by open workings, inducing the rock hosting the ore to fracture and move by gravity toward constructed draw points below for removal. In the presence of groundwater there is a potential for the broken rock to move as a mud rush into work areas below the block cave. A mud rush is a mobile mixture of water and solids equivalent to a debris flow. Generally, the higher the rates of groundwater flow into the subsidence zone around a block cave, the greater the risk of a mud rush occurring. Managing the risk of a mud rush is a key focus of safe mining practice in block caving.
An evaluation of the potential impact of mine operations or post-mining effluent release on the quality of adjacent surface or groundwater is a central component of environmental impact assessments for proposed mining projects. These assessments require an understanding of subsurface fluid pathways to enable quantification of potential impacts in both space and time. The scales at which impacts can occur can range over distances of several hundred meters, to several square kilometers, even to domains hundreds of square kilometers in extent. Evaluation of potential mitigation measures to minimize these impacts requires a robust characterization of groundwater flows within the mine area as well as any adjacent areas that may be impacted. Characterization of the hydrogeological conditions must also be undertaken to manage situations where contaminants of potential concern have been mobilized from mine wastes and a remedial program is to be implemented to ameliorate impacts to the receiving environment.