- 4.1 Introduction
- 4.2 Site Characterization Approach
- 4.3 Components of Site Characterization
- 4.4 Summary
- 4.5 References
- List of Tables
- List of Figures
4.0 Defining the Problem – Characterization
The generation, release, mobility, and attenuation of acid rock drainage (ARD) is a complex process governed by a combination of physical, chemical, and biological factors (see Chapter 2). Neutral mine drainage (NMD) and saline drainage (SD) are governed by similar factors but may or may not involve the oxidation of sulphides. Whether ARD, NMD, or SD enters the environment depends largely on the characteristics of the sources and pathways. Characterization of these features is therefore key to the prediction, prevention, and management of drainage impacted by the products of sulphide oxidation at mine sites.
In this chapter, the term “ARD” refers to drainage types that are affected by the products of sulphide oxidation, including acid, neutral and saline drainage.
Environmental characterization programs are designed to collect sufficient data to answer the following questions:
- Is ARD likely to occur and what are the potential sources?
- What type of chemistry is expected?
- When is likely to start and how much will be generated?
- What are the significant pathways that transport contaminants to the receiving environment and can those contaminants be attenuated along those pathways?
- What are the anticipated environmental impacts?
- What can be done to prevent or mitigate/manage ARD?
Figure 4-1 shows how the information presented in this chapter is integrated with other chapters of the GARD Guide in the development and execution of a site characterization program to address these questions.
To address these key questions, expertise from numerous disciplines is required, including geology, mineralogy, hydrology, hydrogeology, geochemistry, biology, meteorology, and engineering (Figure 4-2). Fundamentally, the geologic and mineralogic characteristics of the ore body and host rock (or the coal seam and overburden) define the type of drainage generated as a result of mining. The site climatic and hydrologic/hydrogeologic characteristics define whether and how constituents present in mine drainage are transported through the receiving environment to receptors.
Because the geologic and mineralogic characteristics of mineral deposits exert important and predictable controls on the environmental signature of mineralized areas both before mining and during mining (Plumlee, 1999), a preliminary assessment of the potential for ARD is typically made based on review of geologic data collected during exploration. Baseline environmental characterization of elemental concentrations in various media (i.e., water, soils, vegetation, and biota) may also provide an indication of ARD potential and documents potentially naturally-elevated concentrations in the surrounding environment.
The initial assessment of ARD potential is refined during mine development and operation as detailed characterization data of the waste and ore materials are obtained. During mine development, the magnitude and location of sources of mine and process discharges to the environment are identified. The boundaries of the receiving watersheds are delineated based on topography, defining the site characterization boundary. Meteorologic, hydrological, and hydrogeological investigations are conducted to characterize the amount and direction of water movement within the watershed (i.e., the hydrologic cycle) to evaluate contaminant transport pathways. Potential biological receptors within the watershed boundary are identified. Over the mine life, the focus of the watershed characterization program evolves from establishment of baseline conditions, to prediction of drainage release and transport, to monitoring of the environmental conditions and potential impacts.
This chapter presents the approach and methods typically applied to characterize the release, transport, and fate of constituents present in ARD (i.e., major cations and anions, metals, metalloids, acidity/alkalinity) at a mine site. Despite inherent differences at mine sites (e.g., commodity type, climate, regulatory framework); the general approach to site characterization is similar, as presented in the following sections of this chapter.
4.2 Site Characterization Approach
Table 4-1 describes the phases of mine development from exploration through to post-closure. During the early stages of mine development, site-specific information may be limited and therefore a high level of uncertainty is present in site characterization. This uncertainty is reduced as information is obtained during exploration, planning, feasibility and design, and is further reduced during operation and decommissioning.
Table 4-2 and Table 4-3 present the chronology of a characterization program and identify the data collection activities typically executed during each mine phase. The bulk of the characterization effort occurs before mining during the mine planning, assessment, and design phase (referred to as the development phase throughout this chapter). Identification of potential environmental impacts during this phase and incorporation of appropriate prevention and mitigation measures is intended to minimize environmental impacts and serves as a foundation for the environmental and social impact assessment. During the commissioning/construction and operation phases, a transition from site characterization to monitoring of waste materials and geochemical processes occurs, which continues throughout the decommissioning and post-closure phases. Ongoing monitoring refines the knowledge of the site, allowing adjustments for new technologies that may evolve during the mine life and whose incorporation will reduce closure costs and better manage associated risks.
4.2.1 Mine Life Cycle Phases
126.96.36.199 Exploration Phase
The primary objective of exploration is to locate a potentially economic ore body or energy resource. The techniques employed in mineral exploration include literature review, geologic mapping, geochemical sampling (rock, soil, and water sampling), geophysical testing, remote sensing surveys (surface, subsurface, airborne, and satellite), aerial photography, and drilling (SME, 2008). Exploration data are compiled to characterize the ore deposit, including the deposit’s size, grade, mineralization style, and the alteration assemblages present. The exploration geologist maps the lateral and vertical distribution of material types across the deposit. Material types may include distinct lithologies or rock units, ore units, alteration assemblages, coal seam overburden and interburden or soil types that have relatively homogeneous characteristics of importance (e.g., mineralogy, grain size, and porosity) to mineral extraction and processing, waste management, and other uses such as for construction. Three-dimensional digital representations of material types, called block models, are generated from borehole data to develop the economic ore estimation models (see Chapter 5).
If sufficient data are available, geologic block models with an ore evaluation focus can be adapted using ARD potential indicator parameters (e.g., total sulphur and carbonate carbon and ABA results) to provide an early indication of the quantities of potentially acid generating and non-acid generating wastes and the overall ARD potential of the deposit. Determination of parameters such as sulphur and carbon during exploration, therefore, provides significant value at later stages of the project cycle by augmenting the ABA database from the geochemical characterization program.
Exploration data collected to determine the economic value of a mineral deposit also provide information useful in the assessment of environmental impacts from mining. For example, the presence of acid generating and acid neutralizing minerals associated with the ore and host rock can be determined from mineralogical data. Exploration drilling logs are often a source of useful hydrogeologic data. The depth to water and quantity of water encountered during drilling are information that may be used to characterize groundwater occurrence and flow conditions. Soil, water, and sediment sampling provide information on the occurrence and mobility of trace metals in the watershed. Remote sensing data may provide information on the distribution of secondary minerals formed from weathering of mineral deposits.
Plumlee’s (1999) summary of how the geologic characteristics of a mine deposit affect its environmental signature is reproduced as Table 4-4. Much of this information is obtained during exploration. Seal and Hammarstrom (2003) discuss the application of geoenvironmental models for massive sulphide and gold deposits. Samples collected during the exploration phase (core, rejects and pulps) should be cataloged and saved for potential future use.
188.8.131.52 Mine Planning Phase (Pre-Feasibility, Feasibility and Approval)
To decide whether a project will progress, the economic viability is assessed during the mine planning phase. This phase includes completion of a number of trade-off, design and prefeasibility studies, including the environmental and social impact assessment (ESIA) and the feasibility study (FS). The ESIA includes an assessment of the potential environmental impacts associated with development of the deposit. Mine design, development of the mine waste management plan and closure planning are incorporated into the ESIA while detailed design and mine permitting follow and are also considered part of the mine planning phase.
During this phase, the intensity of site and source material characterization is usually high. Laboratory testing programs aim to systematically evaluate the environmental stability of waste and ore materials, including the occurrence and nature of sulphide minerals and minerals with neutralization potential and their respective quantities. Intensive baseline monitoring is conducted to characterize existing environmental conditions. This may include establishment of surface water, groundwater, sediment, and climate monitoring stations. The biological receptors within the watershed are identified and their habitats characterized.
Environmental characterization programs need to be integrated with activities of other mine departments to optimize both data collection efforts and the design of the mine facilities. For example, often ARD characterization activities can be combined with geotechnical investigations (e.g., collection of samples for ARD characterization of ultimate pit walls during drilling to evaluate pit wall stability). Optimal siting of waste facilities should integrate information from multiple disciplines, including mining engineering, metallurgy, geochemistry, hydrogeology, hydrology, geology and geotechnical.
When a positive water balance exists in the beneficiation process and depending upon the expected effluent quality, evaluation of water management (e.g., treatment and discharge) may be required to manage excess water if it cannot be contained on the mine property during and after operation.
Characterization during the mine planning phase must be sufficient to allow estimation of any significant long-term costs of ARD management for inclusion in the economic evaluation of the project. It may not be possible to profitably mine some marginally economic ore bodies or coal deposits that have high ARD management and closure costs.
Initial design considerations for mine closure should begin as soon as possible during the mine planning phase. This ensures a “design for closure” approach. Issues such as reduced mine footprint, ARD prevention in waste rock and tailings (and coal refuse), and the avoidance of long-term water treatment should be integrated into the mine design to ensure that the economic model for mine development fully considers the true costs of the whole lifecycle, including post-closure.
If ARD or any other geochemical issue has been identified as a potential concern, then development of an ARD/geochemical block model is usually required during this phase. Integrated with the mine plan, the block model is used to estimate the quantity and production schedule of the identified geochemical waste types. Incorporation of ARD and other geochemical indicator parameters into the block model facilitates consideration of environmental risk in ore development plans, waste production scheduling, and potential segregation of waste as part of an ARD management plan.
184.108.40.206 Construction and Commissioning Phase
Detailed design typically occurs coincidently with and subsequent to the ESIA review by regulators, communities and other stakeholders, and the securing of mine permits. Modifications to the ARD management plan may be made as a result of this consultation to reflect stakeholder input and permit conditions. Detailed design transitions into construction of all mine facilities (e.g., mine camp, water treatment plant, and waste storage facilities) and infrastructure (e.g., water, power, and roads) required for operation of the mine and associated mineral processing.
The characterization programs initiated during the mine planning phase are usually continued during this construction and commissioning phase. Ongoing laboratory testing of ore and waste materials may now include testing of samples generated during site excavation activities (e.g., pit pre-stripping and road/tunnel construction). Field-scale testing programs may also be initiated at this time, including large test plots of mine waste and pilot testing of water treatment options. Routine surface water, groundwater, sediment, climate, and biological receptor monitoring programs begin during baseline data collection and continue to be implemented during the construction and commissioning phase. Environmental characterization activities to evaluate the construction of engineering controls intended to prevent ARD migration may also occur during this phase (e.g., field testing of liners placed at the base of waste facilities to ensure design permeability criteria are met).
220.127.116.11 Operational Phase
The operational phase is usually defined as the start of milling. Note that early mining operations, including pre-stripping of overburden and waste rock, and ore stockpiling at surface mines or during the development of declines, adits or shafts for underground deposits, often begin in the construction phase. The majority of ARD sources resulting from ore extraction and establishment of mine waste facilities are created during the operational phase.
Laboratory and field testing of source materials generally continue during operations to calibrate/validate predictions and validate/modify design of control measures. Testing also supports tracking of waste segregation by ARD potential, if required. Additional testing must be conducted if materials not included and/or fully assessed in the initial characterization programs are encountered during excavation. The waste management plan commonly stipulates specific testing of waste materials to document the composition of waste facilities or, as part of the waste management protocol, to document operational monitoring (e.g., for segregation of waste materials). Source characterization during operations often includes collection of runoff and seepage samples from potential ARD sources. In some cases, instrumentation of waste facilities is required to fulfill specific data needs (e.g., lysimeters or gas monitors within waste rock piles). In general, monitoring of drainage as close to the source as possible is preferred to minimize dilution effects.
Watershed monitoring (i.e., groundwater, surface water, sediment, climate, and biological receptor) continues during the operational phase with a focus on the identification of environmental impacts and compliance with applicable regulations. The quality of water removed in association with dewatering activities should be characterized, even if recycled, to understand the effects on site water quality, and if discharged, the need for treatment.
During the operational phase, source and watershed characterization data collected during operations are reviewed on an ongoing basis and compared to expected conditions and compliance requirements. Trigger levels should be established during this phase that indicate the need to implement contingency measures (e.g., a change or modification to a mitigation measure or engineered source control measure). Trigger levels may be based on predictive models or permit conditions. As needed, the predictive models are updated to reflect measured conditions.
18.104.22.168 Decommissioning Phase
The decommissioning phase involves activities aimed to reestablish premining conditions (to the extent possible) or conditions suitable for beneficial post-mining land use. Active mining and the activities associated with mining and processing cease (e.g., cessation of dewatering activities, cessation of heap leaching). Site reclamation and rehabilitation activities are often conducted during mine operations (progressive reclamation) but their breadth and intensity increases substantially during the decommissioning phase. The closure measures outlined in the closure plan are revised as needed based on operational experience and are conducted during this phase. If necessary, water treatment facilities continue to operate or passive measures are instituted.
Monitoring continues during the decommissioning phase to assess environmental and geochemical conditions with the view to assessing the performance of the closure plan in meeting targets and/or regulatory requirements, and to identify any need for additional or modified activities. Models are commonly used to predict the performance of closure measures, and data from laboratory and field scale kinetic test work and operational monitoring can be used to calibrate and verify these models for longer term predictions. Additional test work may be necessary or the predictive models themselves may need to be modified.
Water table rebound and surface water inflow during the decommissioning phase present an opportunity to assess actual pit lake or flooded underground mine water quality. Additional data collection and evaluation, including modeling, may be required to refine long-term water quality predictions and assess the need for mitigation measures, including water treatment. The rate of water table rebound is measured to confirm predictions of the time for mine flooding or establishment of a pit lake and its potential consequences (e.g., discharges), including acceptability for post-closure beneficial uses.
22.214.171.124 Post-Closure Phase
With the advance of mine waste management techniques, the post-closure phase is usually characterized by the absence of a continuous presence of personnel on the mine site, though some operations might require ongoing water management and treatment. During the post-closure phase, land use is commensurate with requirements of permits and expectations from adjacent land users and regulatory agencies. The requirements for post-closure monitoring vary with the post-closure objectives and the remaining facilities. Typically, there is a period of at least five to ten years of performance monitoring over which there is a decreasing frequency of activity, based on achieving the predicted performance for chemical and physical stability. Following performance monitoring, some sites may require longer term monitoring and maintenance of, for example, physical structures such as tailings dams, water retaining structures, or covers. There are a number of management models by which the post-closure monitoring can be achieved (e.g., the company, contractors, regulatory agencies, communities, or research institutions).
4.2.2 Sources of Acid Rock Drainage
The disturbances and wastes resulting from mining and processing are the primary sources of ARD. The methods of extraction include surface mining (e.g., open pit mining), underground mining, and solution mining. The depth of the deposit, the nature of the mineralization (e.g., disseminated vs. veins), the concentration and amenability to processing of valuable metals, and the cost of waste removal generally dictate whether a resource is extracted using underground or open pit techniques. The solubility of the ore material determines whether heap leaching or in situ solution mining is an option (SME, 2008).
The selection of mining and mineral processing methods defines which sources of ARD are present (Figure 4-2). Both open pit mining and underground mining create features that are potential sources of ARD (e.g., low-grade ore stockpiles, heap leach piles, waste rock facilities, and tailings storage facilities [TSF]). The mine plan and process waste management plan describe the size and location of these facilities. Because waste facility design must consider the potential for these facilities to produce environmental impacts, material characterization and facility design is an iterative process involving multiple disciplines as explained previously. Solution mining changes the geochemistry of the subsurface and has the potential to result in direct ARD impacts to groundwater and impacts to surface water through waste solutions
This section describes how the geometry, physical properties, and structure of common mine and process facilities influence or control the production and pathways of ARD. Conceptual models illustrate the chemical reactions occurring within each facility, as well as the movement of oxygen, heat, and water that govern sulphide oxidation and transport of oxidation products. Additional technical detail on sulphide oxidation and transport of oxidation products is provided in Chapter 2.
126.96.36.199 Surface Mines
Mining of relatively shallow deposits or very large low-grade deposits often employs surface open-pit mining techniques. This typically involves blasting, excavating, loading, hauling of waste rock and ore, with the construction of permanent waste rock storage facilities and temporary (low-grade) ore stockpiles. An open pit is formed by the successive removal of rock from benches as the mine deepens. The ultimate shape of the pit is usually a function of the shape of the ore body, the economics of ore and waste removal and the mechanical characteristics (or strength) of the rock in the pit walls. The ARD potential of the host rock may also be considered in determination of the pit shape to avoid areas with high ARD potential (SME, 2008) or to include areas of high acid neutralizing potential such as limestone.
Open-pit mining may alter surface water and groundwater conditions. Diversion of surface water or dewatering activities to lower the groundwater table may be required to access the ore body. Fractures generated during blasting alter the hydraulic properties of the host rock and may change groundwater flow patterns. Surface water and groundwater quality may also be affected. When sulphides are present within the surrounding rock, dewatering and blasting activities may expose them to atmospheric oxygen, initiating oxidation and acid generation.
Figure 4-3 schematically illustrates the primary water pathways and geochemical reactions that occur within an open-pit mine during operations. When necessary, dewatering wells are sited around the perimeter of the pit to lower the groundwater table to beneath low stability rock units and mine working areas. Alternatively, sumps within the pit may be used for dewatering. Precipitation falling within the pit capture zone becomes pit wall runoff, or infiltrates into the unsaturated zone. Infiltration flows downward to the groundwater table or horizontally toward the pit wall, where it discharges as seepage. High-porosity blast-generated fractures within the adjacent rock zone and historical mine tunnels intersecting the open pit provide preferential pathways for groundwater flow and can create a zone of groundwater depression around the pit.
The quality of pit wall runoff and groundwater inflow to the pit is a function of the composition and reactivity of the rocks these waters encounter and the contact time. Sulphides exposed to atmospheric oxygen on the pit walls or blast fractures oxidize, causing generation of acid that may result in ARD. Therefore, if the pit is to be filled and reclaimed after mining is completed, and if mining is proceeding laterally as for a surface coal mine, the backfilling process should be coordinated with excavation to minimize the amount of time that the mineral sulphides are exposed. ARD neutralization may occur due to dissolution of buffering materials, when they are present. Sulphide oxidation products, which accumulate on pit walls and fracture surfaces, are flushed by groundwater or surface runoff. Pit wall runoff may collect in pools where the haul road meets the pit wall. During dry periods, evaporation results in the accumulation of secondary minerals. These soluble mineral phases are flushed during storm events and may release metals, sulphate, and acidity, depending on their characteristics. Ultimately, runoff collects in a shallow pool at the bottom of the pit. Water on the floor of the pit may infiltrate into the groundwater system, evaporate, or be actively removed by pumping from sumps. Infiltrated water mixes with underlying groundwater and may be captured by dewatering wells.
At the cessation of mining, a pit lake will form if total water inflow to the pit is greater than water outflow, including evaporation (Figure 4-3).
During the decommissioning phase, dewatering activities typically cease. The groundwater table may rebound to near the premining level and, if the pit has not been backfilled, produce a pit lake that can act as a source to groundwater. In other cases, the pit may develop as a groundwater sink due to low inflow and high evaporative loss.
Filling of the pit may be accelerated by diversion of surface water into the pit. For example, the Island Copper Pit in British Columbia, Canada was flooded with sea water in less than half a year and a similar approach is planned for the Lihir Gold Mine in Papua New Guinea. River water was diverted to accelerate filling of the Kelian Gold mine pit in Kalimantan Indonesia; and the Martha Mine Pit in New Zealand is expected to fill from river water and groundwater over a 5-year period following completion of mining.
Open-pit mines in arid regions with limited surface water resources, low groundwater discharge rates, and high evaporation rates may take tens to hundreds of years to achieve a steady-state lake level, extending lake filling conditions well into the post-closure phase. Alternatively, open-pit mines in such conditions may not contain water at all if the natural groundwater table is below the pit bottom or the evaporation exceeds inflows.
Similar to natural lakes, pit lake water quality may vary seasonally and with depth. Pit lake water quality will be a function of the geochemistry of the wall rock, climatic conditions (i.e., the amount of precipitation and evaporation), the rate and quality of groundwater and surface water inflows, the type and extent of biological activity, and pit limnology (Geller et al., 1998).
Pit lake water quality can present a long-term environmental concern, especially considering the volume of water that some pit lakes contain. Acidic conditions may develop within a pit lake because of the oxidation of sulphides in wall rock, flushing of reactive waste materials backfilled in the pit, the addition of ARD in runoff or groundwater, and the precipitation of iron hydroxide minerals within lake water. An example of a highly acidic pit lake is the Berkeley Pit Lake in Butte, Montana, which has a pH of approximately 2.5 and copper and zinc concentrations of approximate 150 mg/L and 600 mg/L, respectively (Gammons and Duaime, 2006). Fortunately, pit lakes with these extreme characteristics are rare on a global scale and appear to be limited to some porphyry copper deposits with high sulphur contents and minimal carbonate or other neutralizing lithologies. Abandoned coal mine pits in eastern Germany and Pennsylvania, USA, and uranium mines in central Canada have also developed acidic waters. Neutral to basic waters have developed in pit lakes hosted in limestone deposits, where the dissolution of calcite buffers pit lake pH. Saline waters also occur in pit lakes, particularly in extremely arid environments where the evaporative loss raises the concentration of dissolved solids. In some mine pits in central Australia, hypersaline water can accumulate due to the evapoconcentration of naturally highly saline groundwater inflows. Stratification within pit lakes and the presence of thermoclines and chemoclines may result in waters of very different composition with depth.
Because pit lakes may potentially represent a long-term source of ARD that persists after mine closure, prediction of the quality and environmental impacts of these lakes is a key part of the ARD management plan. If impacts are predicted, mitigative options should be considered, including accelerated flooding, raising the flooded water level, and batch treatment such as nutrient addition to facilitate bioremediation. Selective mining of problematic material from pit walls above the final lake level can also be considered, and as a final option, pumping/treatment may be required.
A pit lake may be incorporated into the site water management plans for post-closure (e.g., Island Copper Mine, Selbaie Mine, and Mt. Milligan Project in Canada). Wastes can be stored in pit lakes where very low oxygen or anoxic conditions essentially prevent sulphide oxidation. Physical (e.g., sedimentation), chemical (e.g., mineral precipitation and sorption), and biological (e.g., sulphate reduction) processes that occur in pit lakes can be used to ameliorate water quality.
Additional references on pit lake characteristics, predictive modeling, remediation and post-closure utilization include Geller and Salomons (1998), Castendyk and Eary (2009), and Bowell (2003).
188.8.131.52 Underground Mining
Underground mining typically involves blasting, stoping, mucking (excavating), hauling, and where a shaft is used, skipping (vertical haulage) of waste and ore to surface. Where employed, block caving mining methods may create a large mass of fractured rock above the main underground workings. The rubblized zone in some cases may extend all the way to the ground surface. The physical and chemical properties of this large mass of fractured or rubblized rock can be similar to the properties of waste rock.
Similar to open-pit mining, dewatering activities are typically required to remove groundwater from the underground workings, commonly through use of dewatering wells and sump pumps. In some cases, a drainage tunnel can be constructed below the mining level that permits gravity drainage of groundwater to land surface. Mining exposes sulphides present on mine walls or blast fractures to atmospheric oxygen that enters the underground workings through shafts and other openings that intersect the land surface. The underground workings, as well as the ore and waste piles generated by mining, are a potential source of ARD.
Figure 4-4 shows the water pathways and geochemical reactions associated with underground mines. Dewatering activities during operations alter groundwater flow paths near the underground workings. Depending on the depth of the workings, the primary source of groundwater inflow into the mine may be from the regional groundwater system (deep mines) or the local groundwater system (shallow mines). The shallow groundwater flow system is recharged by precipitation that falls within the underground working capture zone and infiltrates into the ground. The quality of groundwater inflow is a function of the composition and reactivity of the rock it encounters and the contact time. Oxidation of exposed sulphides in the underground workings (mine walls or blast fractures) results in the accumulation of sulphide oxidation products. During mining, a constant supply of oxygen is maintained through the ventilation system and mine shafts and adits that intersect the land surface. Sulphide oxidation products are flushed by inflowing groundwater. Underground mine water quality may also be affected by chemicals introduced during mining activities (e.g., diesel, nitrogen from blasting residuals, grout, lime dusting) or by materials backfilled into the mine during operations or at closure (e.g., paste tailings, waste rock or borrow material). Groundwater removed as part of dewatering activities may require recycle to the mill process or treatment before discharge to the environment. Although the underground mine is typically a groundwater sink during operations, release of impacted mine water to the environment may occur by infiltration to groundwater or surface discharges at mine openings.
Workings during Operation and Closure
At the cessation of mining, dewatering ceases and groundwater inflow into the underground workings may begin to accumulate and flood the workings. The mine workings, due to their elevated permeability and porosity relative to the host rock (even if backfilled during mining), become preferential pathways for groundwater flow. Collapsed or backfilled voids and block cave rubble zones exhibit greater permeability than the surrounding fractured rock and therefore strongly influence groundwater movement. As the water table rebounds, sulphide oxidation products present on mine walls or backfilled waste are flushed, resulting in release of sulphate, metals, and acidity to the mine water while alkalinity from carbonates and other minerals is also introduced to the water. Inundation of sulphide minerals prevents further sulphide oxidation. However, sulphides that remain above the water table represent a long-term source of ARD. As is the case for pit lakes, underground mine pools are frequently stratified (Wolkersdorfer, 2008). In Pennsylvania, USA, water levels in flooded underground coal mines show considerable fluctuation in composition responding to variability in meteoric conditions and local groundwater pumping.
As the water table rebounds, the underground workings may transition from a groundwater sink to a groundwater source. Discharge of mine water to the environment may occur by groundwater or mine openings. In some cases, these openings are plugged to prevent discharge to surface and to further raise the water level (ERMITE-Consortium, 2004).
184.108.40.206 Waste Rock Storage Facilities
Smith and Beckie (2003) provide a comprehensive summary of the hydrologic and geochemical processes occurring in a waste rock pile constructed by the traditional end-tipping from lift heights greater than about 10 m. Figure 4-5 summarizes the water pathways and geochemical reactions associated with this style of waste rock pile. The primary factors controlling the hydrologic characteristics of a waste rock pile are (a) material grain size distribution, and (b) the proportion and spatial arrangement of matrix-supported and matrix-free zones created during construction (Smith and Beckie, 2003).
Typically, waste rock is hauled in dump trucks to the disposal site. The loading process and truck vibrations during hauling result in segregation of the rock into layers of similar grain size, a process called sorting. Waste rock dumping from the crest of a steep slope further enhances sorting because large grained material travels further down slope than fine grained material. Consequently, waste rock piles are generally composed of inter-layered beds of coarse grained material and fine grained material inclined at the angle of repose (33 to 37 degree angle). This method of construction creates the following structures that influence water and gas movement within the pile:
- Discrete zones within the pile containing little to no granular material between larger rock fragments
- Sloping surfaces between coarse and fine grained layers
- Internal low permeability pavement surfaces formed by truck movement during pile construction
This method of construction has been standard practice for more than a century. It has been used all over the world and is entirely appropriate if the materials being placed are geochemically inert. However, when rocks are geochemically reactive, this method can promote oxidation and ARD generation, and may result in the need for long-term management.
Many of these facilities, especially large-volume ones, are a legacy of the mining industry due to problems associated with vegetation failure and poor water quality, and require significant effort and cost to mitigate. To address these problems, waste rock piles need to be designed and constructed based on geochemical performance through the application of appropriate geochemical and geotechnical criteria.
The basis for design should be to construct waste piles to control the generation and leaching of sulphide oxidation products through segregation and controlled placement within a waste rock storage facility. Because oxygen is essential for the oxidation process, preventing advective and convective transfer of atmospheric oxygen into the facility and managing the diffusive oxygen flux such that geochemical reaction rates meet design targets that will not adversely effect the environment, are key design considerations. Coupled with the segregation and selective placement of material types with distinct geochemical properties, this may require use of construction and placement methods that include small lifts and prevent particle segregation. Examples of such methods of construction that provide oxidation control are the Martha Mine in New Zealand and the Phu Kham mine in Lao PDR (papers on both are planned for the 2012 ICARD in Ottawa). Further information is presented in Chapter 2, which contains detail on oxygen transport into and within waste rock piles.
Precipitation falling on a waste rock pile will evaporate, flow over the surface of the pile as runoff, or infiltrate into the pile. Saturated conditions may exist within the waste rock pile. Development of a perched water table at the base of the pile may occur because of the presence of a low permeability layer beneath the pile. Perched water may also be present at higher elevations in a pile because of areas of low permeability created by haul roads. Waste rock seepage may exit along slope faces, at the toe of the pile or may infiltrate into the subsurface underneath the pile.
To prevent infiltration of waste rock seepage into the underlying groundwater system, waste rock piles may be sited in areas where surficial soils have low permeability. Siting such facilities is best accomplished by an assessment of the entire development site to map the levels of permeability for the purposes of determining the optimum location of all facilities. In potentially sensitive areas, drilling, test pitting and geophysical investigation to fully understand the subsurface conditions may be required. In cold climates, permafrost may act as a barrier to the migration of seepage. However, because of the heat generated during sulphide oxidation, the permanence of the permafrost below waste facilities requires evaluation. In some cases, engineered liners may be used as a barrier to seepage migration and drains may be installed to collect drainage.
220.127.116.11 Ore Stockpiles
Ore stockpiles are essentially temporary storage facilities that are established to provide feed to a processing plant or for direct transfer to markets, such as iron ore and some coal. There can be various grades of ore (e.g., high, medium and low), with the higher grades typically stored for short periods (e.g., 1 month) prior to processing in the mill. In some cases, depending on the economics at that time, lower grade ores may never be processed or may be stockpiled for several years and, hence, should be managed as waste rock. The principles that apply to waste rock dumps (Section 18.104.22.168), therefore may also apply to lower grade ore stockpiles.
High grade or run of mine (ROM) ore may only be stockpiled for a short period and ARD management generally relies on early processing before ARD conditions develop and, in some cases of high reactivity, the capture and treatment of drainage.
Lower grade stockpiles, that may or may not be processed, require careful consideration to ensure allowance is made for closure. Frequently, low grade ore can have a higher ARD risk than waste rock. Therefore, careful planning is necessary to avoid a potential long term liability associated with low grade ore stockpiles.
22.214.171.124 Tailings and Process Refuse Storage Facilities
Tailings are discharged to surface storage facilities by several methods, including subaerial slurry, subaqueous slurry, paste and dry deposition. The methods of transport and disposal are a function of the water content of the tailings (e.g., thickened and paste tailings may be disposed sub aerially using pipeline transport, whereas fully dewatered tailings are disposed by dry deposition methods, including use of conveyors and trucks) and the topography and placement of the tailings dam. Tailings may be segregated by grain size (e.g., use of cycloning to separate the sand fraction from the slimes) before discharge. Compared to waste rock, tailings are homogenous with a more consistent distribution of acid generating and acid neutralizing minerals. However, if these minerals are associated with a particular size fraction, segregation may occur during deposition, with a higher sulphur content typically occurring near the deposition pipe discharge on the tailings beach. The fine particle size of tailings results in lower permeability to oxygen and water compared to waste rock.
The tailings grain size, disposal method, and deposition history govern the hydrogeological characteristics of a TSF (Blowes et al., 2003). The sulphide content of the tailings and the availability of oxygen will govern the reactivity of sulphidic tailings. Water covers, in the case of subaqueous disposal, act as a barrier to significant oxygen ingress into the tailings. Other tailings design features intended to limit oxygen or water ingress include use of engineered covers and maintenance of a high degree of saturation (e.g., in tailings paste).
In addition to surface disposal, tailings may be disposed in underground workings. Underground disposal may simply fill void spaces or may be designed to provide structural support for ongoing mining. Reagents may be added to increase strength or improve environmental stability before backfill. Addition of a binder (i.e., cement) to acid generating tailings may provide some, although generally limited, neutralization potential and typically reduces water and oxygen permeability. Tailings segregation (e.g., removal of slimes) or dewatering (e.g., for disposal as a paste) may be performed before backfill. Pyrite recovery from tailings in the mill prior to tailings deposition to reduce or eliminate the ARD risk from the bulk of tailings is current practiced at some sites (e.g., Ok Tedi copper and gold mine in Papua New Guinea) and proposed at others (e.g., Paracatu gold mine in Brazil). Pyrite may also be removed to create a non-acid generating cover near the time of mine closure (e.g., Detour Lake Mine in Canada). The small volume of pyrite concentrate that is then produced can be managed and isolated permanently from atmospheric oxygen. In some cases, the pyrite concentrate may have a market value.
Discharges associated with tailings facilities include runoff and seepage for all disposal methods. Runoff and seepage quality are a function of tailings composition, reactivity, and contact time. Facilities may be sited in areas with low permeability surface soils or an engineered liner may be constructed to prevent migration of tailings seepage. In cold climates, migration of the permafrost into the base of the facility may prevent generation and movement of ARD.
Coal processing typically includes crushing, grinding, and sizing followed by physical separation of pyrite and shale (waste materials) by gravity or floatation. The waste material (locally known as coal refuse, gob, slurry, etc.) from coal beneficiation (also known as coal cleaning, washing, or preparation) generally has a higher acid generation potential than the coal itself because sulphide minerals contained in the coal and waste rock are concentrated in the waste during the coal cleaning process. Coal reject piles, largely consisting of fine-grained shale and pyrite, are typically located near the coal processing plant. Because of the mineral composition, grain size, and high-surface area of these wastes, coal reject piles may be strongly acid generating depending upon their sulphur content (SME, 2008). This waste material is often deposited as a slurry behind a dam, just like tailings. Where it is deposited dry, it should be compacted frequently to minimize the ingress of oxygen. Additional measures may be taken to minimize ARD formation, as discussed in Chapter 6.
Figure 4-6 shows the flow paths and geochemical reactions occurring in a subaqueous TSF as well as the three methods typically employed for dam construction. A dam is first constructed to impound the tailings and supernatant. For stability reasons, tailings dam embankments are commonly designed to be unsaturated and well drained so if they are constructed with sulphide-bearing waste rock or tailings, the tailings dam embankments may be particularly prone to ARD generation. Precipitation onto the surface of the facility contacts the tailings beaches (tailings exposed to atmospheric oxygen), the dam, or falls directly on the tailings pond. During large storm events, discharge through an overflow drain or discharge down the face of the dam may occur. This water may be captured for treatment. Infiltration through the tailings enters into the subsurface or is captured in a seepage collection system. The seepage rate is a function of the permeability of the underlying natural or engineered materials and the infiltration rate through the tailings. During operations, ARD is not normally a concern (except with extremely reactive tailings) because most mill circuits add lime to the tailings. Also, during subaqueous disposal, fresh tailings added to the beaches maintain a relatively high water content for a time. Active management of the tailings pond supernatant (e.g., addition of lime) can be conducted to prevent low pH conditions and mobilization of metals. During post closure, remedial measures that have been designed from the outset are implemented to prevent ARD and improve seepage quality (see Chapter 6).
126.96.36.199 Heap Leach Piles
Sulphuric acid/ferric ion leaching and sodium cyanide heap leaching are two metallurgical processes used to extract metals (copper and gold, respectively) from ore material under different conditions. In both leaching processes, cobble-sized or finer grains of ore materials are placed onto lined pads, either directly from the mine or after size reduction. The leaching fluid, applied to the top of the pile, infiltrates through the material, dissolving the ore bearing mineral. The metal-bearing solution (pregnant solution) is collected from the bottom of the pile and processed to recover metals.
Improper handling of both the leach solutions and the pregnant solution can result in the release of acidic or alkaline process solutions to the environment. Leakage through the lined or unlined base of the leach pad could impact groundwater quality, while seepage flowing from the toe of the facilities and direct runoff may impact surface water. When leaching is concluded, the drain down water or rinse water must also be handled properly. Acidic leaching solutions and drainage may precipitate very fine-grained acid-generating secondary minerals such as jarosite and melanterite, onto grain surfaces particularly under dry conditions. During rain events, these secondary minerals will readily dissolve, releasing the stored acidity and metals. Whether these solutions exit the mine waste depends on the climatic and physical characteristics of the mine wastes and the presence or absence of engineering controls (e.g., liner).
Sulphide minerals remaining in the pile after conclusion of leaching can also contribute to acid formation, depending on the residual sulphide mineral content and climatic conditions. If an alkaline cyanide solution is used for ore leaching, some cyanide compounds may remain sorbed to grain surfaces or dissolved in pore waters after leaching concludes. At the cessation of mining, cyanide present in gold heap leach piles can be removed by rinsing or can be allowed to degrade naturally (SME, 2008). Maintenance of the residual alkalinity (pH 10 to 11) from cyanide leaching through the addition of a cover may also provide environmental benefits. Best operating practices from closed heap leach sites in Nevada have demonstrated that preserving the alkaline conditions, which were essential during the cyanide leaching operations, is best achieved by not rinsing the heaps. However, an engineered cover may be needed to limit infiltration of precipitation and prevent natural leaching of alkalinity to a level that could allow ARD, NMD or SD processes to begin depending on the nature of the material.
188.8.131.52 In Situ Solution Mines
Solution mining makes use of a series of injection and recovery wells to circulate a leach solution through an ore zone. Solution mining is applied to extract solids that are easily leached or dissolved. Leach solutions may be acidic, neutral or basic, heated, or unheated. Blasting may be conducted in boreholes before injection to increase the permeability of the ore zone. Solution mining has been used in uranium, copper, manganese, halite, potash, nahcolite, and sulphur mining (SME, 2008).
Solution mining alters groundwater geochemistry and flow paths near the ore zone. In some applications, leach solutions are oxidants and may promote ARD. If not contained, leaching solutions, particularly oxidizing solutions and acids, may degrade local or regional groundwater quality. To avoid groundwater impacts, solution mining requires rigorous evaluation of the groundwater flow system (e.g., flow paths, capture zone analysis) before injection of leach solutions. Characterization of the groundwater system must include hydrogeologic units in hydraulic connection with the ore body and aquifers above and below the ore zone. Disposal of waste solutions may pose a concern for surface water resources.
4.2.3 Conceptual Site Model Development
A conceptual site model (CSM) describes what is known about the release, transport, and fate of contaminants at a mine site. As such, the model includes the following components:
- Biological receptors
The CSM describes the sources of potential contaminants, the mechanisms of their release, the pathways for transport, and the potential for human and ecological exposure to these parameters. The most important potential sources of ARD at mine sites are the mine pit (walls, benches and floor), exposures in underground workings, waste rock, ore (including any low grade stockpiles) and process residues (tailings and rejects).
Atmospheric oxygen is essential for oxidation of sulphides and to begin the ARD evolution process. Water is the primary environmental pathway for ARD released from these sources. Transport occurs by groundwater, surface water, or infiltration through the vadose zone. Because water is a primary pathway, aquatic resources generally are the receptor of most interest. In addition to water, human and ecological exposure to contaminants may occur by other pathways, including air (e.g., exposure to wind-blown tailings) or from direct contact with a solid phase mine or process waste (e.g., vegetation in contact with metal-laden soils). Indirect exposure may occur along the food chain (for example exposure of bioavailable metals to animals that graze on vegetation in contact with contaminated soils).
A conceptual site model can be developed at any stage of a mine’s life; however, development typically begins in the early phases of a project and is continually validated, revised and updated, as necessary, over the life of the mine as site characterization data and operational monitoring data are collected.
Where regulatory approval for new mine development is required, early involvement by these agencies and other stakeholders, including the local communities, in the development and validation of the model is key, and should be encouraged. Early involvement is needed to establish the hazard considerations for risk assessment (see Section 3.6), as differences in risk tolerance can often be best dealt with through consensus using the evolving site based model.
The principal data requirements of a CSM are shown in Figure 4-7. The ability of the model to accurately describe the release, transport, and fate of ARD is a function of the amount of available characterization data, which generally is proportional to the mine phase. An example schematic of a CSM is shown in Figure 4-7.
4.3 Components of Site Characterization
This section presents a summary of the components and methods commonly used to characterize ARD sources, pathways, and receptors. A comprehensive listing of the tools used in environmental assessments of mining projects, including references and a description of their use, is presented in Plumlee and Logsdon (1999). Price (1997, 2009) presents guidelines for the development of a characterization program, including laboratory testing and interpretation of test results.
An approach for characterization, classification and prediction adopted by Earth Systems is documented in the Characterization Case Study.
4.3.1 Geo-environmental Models
A geo-environmental model of a mine deposit is defined as “a compilation of geologic, geophysical, hydrologic, and engineering information pertaining to the environmental behavior of geologically similar mineral deposits prior to mining, and resulting from mining and mineral processing” (Seal et al., 2002). The key elements of the model include deposit type, deposit size, host rock, wall-rock alteration, mining and ore processing method, deposit trace element geochemistry, primary and secondary mineralogy, topography and physiography, hydrology, and climatic effects. Geo-environmental models are empirical data compilations that are best used as guidelines for the potential range of environmental impacts at a site (Seal et al., 2002) and should not be used to predict pH or element concentrations that will develop at a site or in lieu of site characterization (Plumlee, 1999).
Geo-environmental models provide a starting basis for the level of characterization that will be required at a mine site. Additional discussion on geo-environmental models and their use in water quality definition and prediction is presented in Chapters 2 and 5, respectively.
4.3.2 Source Material Geochemical Characterization
The primary purpose of geochemical characterization of mine materials is to guide management decisions. Therefore, it is critical that a phased assessment program is carried out to ensure sufficient data are available at all stages of the project cycle (exploration, pre-feasibility, feasibility, construction, operation, closure and post closure). Best practice environmental management can only be achieved through the early recognition of the potential for acid drainage and metal leaching.
Geochemical characterization aims to identify the distribution and variability of key geochemical parameters (such as sulphur content, acid neutralizing capacity and elemental composition) and acid generating and element leaching characteristics. A basic screening level investigation is essential and should commence at the earliest possible stage. The need and scope for detailed investigations will depend on the findings of initial screening. Since some tests, such as leach tests or oxidation rate measurements, require a long time frame to provide the necessary data, it is important to initiate this work well ahead of key project milestones.
Reference to other mining operations in the region, particularly those situated in the same stratigraphic or geological units may provide empirical information on the likely geochemical nature of similar ore types and host and country rocks. Early indications are also provided from exploration drill core, and it is best practice to log key indicators such as sulphide and carbonate type, abundance and mode of occurrence, to analyze samples for total sulphur content as a minimum, and to include key environmental elements (such as carbon, calcium and magnesium as possible analogs for ABA parameters) in drill core assays. Mineralogical investigations should examine the type and mode of occurrence of sulfide and carbonate minerals.
This section outlines sample selection and number of samples required for a geochemical characterization program, and provides an overview of the testing programs and classification procedures. More detail and specific procedures and methods are presented in Chapter 5.
184.108.40.206 Sample Selection
Sample selection is a critical task and must be given careful consideration at all stages of a project. Samples should represent each geological material that will be mined or exposed and each waste type. The number of samples should be based on the project phase but ultimately must be sufficient to adequately represent the variability within each geological unit and waste type.
Although drilling and sampling will focus on ore zones in the exploration and pre-feasibility stages, samples of host and country rock should be increasingly represented as the project develops, so that adequate data are available to produce block models and production schedules by geochemical waste types, where required. The available sources of material for testing are typically related to the phase of mine development. Drill core is the most common material source for geochemical testing during the early stages of mine development. Because exploration drilling programs target discovery and delineation of the ore zone, the selection of samples to characterize waste material must include careful examination of the spatial coverage of the drill core relative to the anticipated extents of the pit or underground workings (Downing and Mills, 2007). Other sources of material for testing that are frequently available include rock chips from borehole drilling, hand samples from outcrops, samples from existing waste facilities, development rock, ore composites, and residues from metallurgical testing.
The project geologist is a valuable resource and should be consulted in the selection of representative samples for testing. Determination of the appropriate disposal methods for wastes generated during exploration necessitates initiation of testing early in the mine life. As mentioned previously, any material with the potential to generate ARD or release contaminants should be characterized. Construction materials for roads and site infrastructure are often quarried from the area around a mining development. The geochemical characteristics of these materials should also be evaluated before construction. Due to the spatial extent of placement, use of construction materials with ARD potential may result in a widespread source of ARD. The potential for land disturbances associated with the construction of mine facilities to expose rock with ARD potential should also be considered.
Selection of representative samples should consider the following:
- Material Type – Individual samples selected for testing should be representative of a single material type (e.g., lithology, alteration type). The exploration geologist should be consulted regarding the initial definition of mine units and material types. Based on the results of the geochemical characterization program, material type classifications may require further refinement. For instance, a classification suitable for mineral extraction may not be sufficient to identify the environmental characteristics and corresponding ore and waste management requirements of the various material types. Construction materials should be included in the characterization program. Ore materials should also be included for prediction of ore stockpile discharges.
- Spatial Representation (x, y, z) – Sample selection should ensure good spatial representation (vertical and horizontal) of the area to be mined. In practice, sample locations may be restricted to a one-dimensional line defined by a borehole or mine tunnel, or a two-dimensional plane, such as the wall of an open pit or cross section through the deposit. Additional boreholes increase the distribution of sample points and improve the definition of mine units. Because mine plans change, the spatial representativeness of samples should be reassessed throughout operations. For example, if the location of the pit wall changes, additional testing may be required to characterize pit wall runoff. Mining may also extend into areas that were not characterized during feasibility testing and such mining may encounter new materials.
- Compositional Representation – Sample selection should include all major material types and cover the range of pertinent characteristics for each material type (e.g., pH, carbonate, sulphur, and neutralizing potential content). Personnel tasked with sample selection must be familiar with the geological characteristics of the deposit, including rock types, fracture patterns, weathering, alteration, and mineralization.
- Focused (Biased) versus Random Sampling – Use of focused or random sampling depends on the objective of the characterization program. For example, sample selection may target areas with visual sulphides to provide an indication of worst-case drainage quality. Similarly, focused sampling may be more effective in ensuring a sample set with a complete range of compositional characteristics than random sampling. Random sampling may be appropriate during operations in determining the appropriate location for waste disposal (e.g., waste segregation based on total sulphur content). In this context, “random” still implies a rationale-based program, which may be part of a phased program but lacks adequate samples to be geostatistically complete. Ultimately, if a major ARD problem is predicted from earlier phase test work, the waste should be characterized by a geostatistical model, which includes an adequate number of samples as well as a geological interpretation. From this model, one or more key indicators can then be selected for the operations to use in separating materials.
Appropriate sample storage and handling procedures should be defined. Preference regarding the use of weathered or fresh materials for testing must be determined. In either case, the type of material used for testing must be documented. A photographic log of geochemical test samples is recommended.
Standard operating procedures (SOPs) for geological logging and the collection and documentation of sample selection should be developed and followed. SOPs should include quality assurance/quality control (QA/QC) protocols (e.g., collection and analysis of duplicate samples, sample chain of custody procedures, and inclusion of standard samples with known values). Sample size should be large enough to provide material for all potential geochemical tests and sample for archiving purposes. Archived samples should be easily retrievable. If compositing of samples is required, protocols should be defined. Compositing may be useful for identification of the characteristics of a sample representing a larger core interval or rock volume, such as an open-pit mine bench depth or waste zone. However, information on the smaller-scale characteristics may be lost due to the “smearing” of geochemical properties and analytical results. This “smearing” may lead to samples with anomalous qualities not being recognized, even though it may be those materials that govern the composition of a mine or process effluent. In general, it is recommended to collect discrete samples with clearly-defined characteristics. These can be assayed as individual samples or combined to represent mine bench intervals or other mining or waste units (including rocks immediately surrounding underground workings), with selected individual samples used to evaluate variability.
The mine geologic model and the block model may be used in the selection of representative samples. If geochemical testing indicates that special handling of waste materials will be required, the block model may be populated with diagnostic ARD indicator parameters (e.g., total sulphur). In this case, a comprehensive set of samples would be needed to build the geostatistical model.
220.127.116.11 Number of Samples
The number of samples required for source characterization of each material type depends on the following: (a) the amount of disturbance (i.e., the volume/mass of material extracted or the amount exposed on pit/mine walls or production tonnage as determined by the block model); (b) the compositional variability within a material type; and (c) the statistical degree of confidence that is required for the assessment.
Initial estimates of sample numbers are typically based on professional judgment and experience. The number of samples required commonly increases during each of the early phases of mine development as the knowledge base and project needs develop. Ultimately, for sites characterized as having an ARD potential, a full geostatistical model often provides the basis for control plans where material segregation is part of the mine plan.
Few guidelines are available regarding sample requirements. Table 4-5 provides an example of Australian guidelines for the number of samples during the early phases of the mine life. Although characterization testing is likely to occur during all phases of mine development, the peak of the laboratory testing programs often occurs during the feasibility phase.
Tourism and Resources, 2007).
|Mine Phase||Number of Samples||Description|
(1) Prospect Testing – Include sulphur in list of elements being analyzed for all samples tested; include the full range of pathfinder elements as defined by ore deposit/exploration model; collect and record mineralogical data as per exploration/ore deposit model; where the geology of the deposit is known include static testing of at least 3 to 5 representative samples of each key material type (i.e., lithology, alteration type); analysis of ground water and surface water for acidity and representative pathfinder elements.
(2) Resource Definition – All samples tested for sulphur and representative samples tested for mineralogy as per ore deposit model. Static testing of at least 5 to 10 representative samples of each key material type.
All testing to include QA/QC samples.
|By the end of the resource definition phase, there should be adequate information to accurately characterize the ARD potential of the ore body (high and low grade), although further test work will normally be required to characterize the ARD potential of waste rock and ore and hence tailings.|
Static testing of several hundred representative samples of high and low grade ore, waste rock and tailings, the number dependent on the complexity of the deposit geology and its host rocks.
All drillhole samples analyzed must include sulphur analysis and identified representative metal ions. Sampling density is dependent on complexity of ore deposit and host rock geology interval of representative drill holes but should be restricted to single rock units or lithologies - include minimums.
Kinetic testing of at least 1 to 2 representative samples of each material type.
Surface water and groundwater analysis to include acidity as well as pH, EC and representative metal ions, including Al, Fe, Mn.All testing to include QA/QC samples.
Where required, the number of samples must be sufficient to populate a “resource” block model of the ore and host rocks that will be affected by mining with a reliable distribution of NAPP data (e.g., acid producing potential (APP), sulphur and acid neutralizing capacity (ANC) (or NPR data) on ore, waste rock and wall rock.
|Mine Planning, Feasibility and Design||
Where required, additional static testing as required for block waste resource model refinement – increase density of NAPP (or NPR) characterization.
Inclusion of confirmatory testing (e.g., NAG testing for comparison to NAPP (i.e. APP, or sulphur, and ANC) for metalliferous deposits; and mineralogy or NPR values).
Continuation of kinetic testing.
Upgrade drillhole database and waste resource model for new ore positions.
All testing to include QA/QC sample(s). Apply QA/QC to all analyses, not only ore. Include wall rock.
|Data set must be sufficient to assess ARD potential to support a management plan. If data are insufficient, additional testing will be required.|
Statistical analysis of test results is advisable to confirm that a representative data set has been obtained. For example, histograms may be used to ensure that the entire distribution has been captured in sample selection (Runnells et al., 1997) and samples with “extreme” characteristics have not been overlooked. The number of samples will increase as the heterogeneity (e.g., particle size and composition) of a material type increases. For this reason, characterization of process tailings typically requires fewer samples than characterization of waste rock. Sample representativeness must continually be assessed during the mine life. For example, a change in ore type over the mine life may produce process tailings with different characteristics. Operational monitoring (see Chapter 8) should include a program of systematic ongoing tailings testing to identify changes and implement alternative waste management practices, if required.
The goal of material management is to prevent or minimize ARD. Characterization programs must be designed to provide adequate information to make cost-effective, sustainable, and environmentally protective decisions regarding the management and disposal of waste materials. For materials with an uncertain ARD potential, resolution of this uncertainty by additional characterization efforts may not be necessary if a decision is made to manage the waste with the assumption that the material has ARD potential. For example, detailed characterization of the sulphide content of tailings over time may not be necessary if the tailings will be placed in a contained impoundment with a water cover.
18.104.22.168 Testing Program Overview
Laboratory and field testing is conducted to characterize the acid generation and metal leaching potential of mine materials. Geochemical characterization programs typically follow a phased approach, beginning with laboratory testing followed by field testing. The design of most testing programs is dynamic, with each successive phase building on the results of previous phase or phases. A brief summary of the testing approach is provided below, with significantly more detail presented in Chapter 5.
The laboratory phase of a geochemical characterization program will typically include the following analyses:
- Static Tests
- chemical composition (whole rock and elemental analysis)
- mineralogical analysis
- acid base accounting (ABA)
- net acid generation (NAG)
- water extraction (batch extraction) tests - with solution assay
- Kinetic Tests
- humidity cell leach testing
- column leach testing
Static testing is the first phase of geochemical characterization, and is a precursor to kinetic testing. The objective of static testing is to describe the bulk chemical characteristics of a material. These tests are designed to evaluate the potential of a particular rock type to generate acid, neutralize acid, or leach metals. Static tests provide an indication of the presence of minerals that may generate acid as well as minerals that may act to neutralize any acid formed. In some cases, testing may indicate that a surrogate parameter can be used as an indication of ARD potential (e.g., iron as an indicator of the amount of sulphide, calcium or carbon as an indicator of the amount of neutralization potential) (see Chapter 5 for additional information on the use and interpretation of static tests).
Elemental analysis results are commonly compared to average crustal or mean world soil abundance values as a multiple or geochemical enrichment factor to provide a screening level assessment of elements that are enriched in the sample. A high concentration of a particular element does not necessary imply that this element will be mobilized in concentrations that may lead to environmental or health impacts, but it does highlight an issue that should be further investigated. An essential component of static testing is mineralogical analysis that, at a minimum, includes identification of all sulphur and carbonate minerals. If possible, mineralogical analysis should be quantitative. A description of how minerals with acid generation and acid neutralization potential occur (e.g., grain size, grain morphology, disseminated, fracture coatings, as inclusions) is also relevant in the assessment of reactivity (i.e., rate of oxidation or dissolution).
ABA analysis typically includes analysis of paste pH, sulphur speciation, neutralizing potential (NP) or acid neutralizing capacity (ANC) and total inorganic carbon (TIC). Paste pH is used as an indicator of the presence of stored acidity. Sulphur speciation data, which includes information on the presence of non-acid generating sulphur minerals, are used to calculate the acid generation potential of the material. NP and ANC provide estimates of the acid neutralizing potential of a material (NP in the units of tCaCO3 /kt and ANC in the units of kgH2SO4/t). TIC is used to calculate the carbonate neutralizing value (CNV) or carbonate NP (Ca-NP), and allows assessment of the fraction of NP or ANC attributed to carbonate mineral phases. In some cases the CNV based on TIC (or even total carbon) can be used as a surrogate to estimate the NP or ANC at a particular site. In combination, results from ABA, NAG, and elemental and mineralogical analysis are used to assess the relative proportions of acid generating and acid consuming materials.
Short-term extraction tests (such as 24-hour batch extraction tests using deionised water) provide information on the short term metal leaching potential. The nature of the sample (e.g., unoxidized vs. oxidized; oxidation products absent vs. oxidation products present), test solution to solid ratio, lixiviant, reaction time, and sample particle size should all be considered in the evaluation and comparison of leach test results.
Although the results of static testing may indicate a potential for acid rock drainage or metal leaching, kinetic testing is commonly required to assess the relative rates of the various ARD and metal leaching reactions occurring, and to provide information on the evolution of ARD over time. Field scale leach tests may be initiated before or during the construction or operational phases of mine development to provide a better representation of material reactivity under ambient site conditions.
Physical properties of the testing materials (e.g., surface area, particle size distribution) are also determined because these properties affect material reactivity and are needed in the scale-up of laboratory and field testing results to represent field scale and operational conditions.
Figure 4-8 shows the typical components and evolution of a geochemical characterization program for selected potential source materials. Any waste, construction, or process stream residues that have the potential to generate ARD must be included in the mine characterization program so that appropriate disposal practices and mitigation measures can be employed. This includes waste rock, ore, process residues, treatment sludges, quarried materials for construction, heap leach residues, hydromet residues, slag etc. Chapter 5 presents detailed descriptions of the laboratory and field scale testing methods and their interpretation for ARD prediction.
4.3.3 Watershed Characterization
Because water is the primary pathway for transport of ARD, the quantity, quality, and movement within the mine’s watershed must be characterized. Delineation of the watershed boundary is the first task in watershed characterization. Topographic maps and site reconnaissance are used to determine the surface water boundaries, or divides, that separate the watershed containing the ore deposit from surrounding watersheds. Geographic information systems (GIS) and digital elevation models (DEM) may be used for this task. Groundwater watershed divides are initially assumed to coincide with surface-water divides, with refinements added based on the results of subsequent hydrogeological investigations. The watershed boundary generally defines the site characterization boundary.
Although groundwater watershed divides are typically initially assumed to coincide with surface water divides, groundwater regimes and their boundaries can be complex. When mining in an area with karst, investigations should be conducted very early in the site characterization program to identify karstic limestone features within the watershed boundary. Karst features can be major preferential flow paths which can govern local groundwater regimes and the transport pathways of any seepage from tailings and waste rock containment areas. Siting of mine infrastructure should also consider the presence of major karst features.
22.214.171.124 The Hydrologic Cycle
The quantity of water within a watershed is a function of climate. The key components of climatic characterization are precipitation and temperature. Information on the amount, temporal distribution, and form of precipitation (rain or snow) is used in association with temperature data to characterize the quantity and seasonal distribution of recharge to a watershed. These data are used in development of a site wide water balance (see Section 126.96.36.199).
Characterization of the climatic conditions at a site typically begins with identification and review of available regional data. Site-specific climatic data are obtained by installing a meteorological station to record daily values of temperature, precipitation, wind speed, wind direction, and relative humidity. In cold climates, snowpack should be measured. Evaporation pans or empirical equations are used to estimate site evapotranspiration rates.
Site precipitation data are typically compared to regional data collected concurrently to assess the representativeness of the regional data set. Because the period of record for regional data sets is typically longer, these data are often used to estimate the occurrence, frequency, and magnitude of extreme weather events (e.g., floods and droughts). Characterization of these events is needed to assess ARD release, fate, and transport. During dry periods, sulphide oxidation products will accumulate. ARD loading is often greatest during a rain event that follows an extended dry period.
Hydrologic characterization begins with identification of all surface water features within the watershed (i.e., lakes, streams, and rivers) and points of discharge (i.e., lakes and ocean). Surface water quality, quantity, and direction of flow within the watershed boundaries are characterized. Baseline conditions are characterized before exploration or, more commonly, during the development phase. Monitoring is conducted during the construction and operations phases, and possibly during decommissioning and post-closure phases to assess impacts. Stream flow measurements are required to characterize the amount and rate of flow to evaluate constituent fate and transport and to characterize aquatic habitat. Stream flow is measured by developing a stage (water height) versus discharge relationship and then measuring flow by water elevation (e.g., pressure transducer). The stage-discharge relationship is developed by using a current meter or weirs to measure water flow at various water heights (see Chapter 8). The degree of seasonal variation will dictate the required monitoring frequency. Continuous monitoring systems can be established using data loggers with solar or battery power. These systems characterize changes in flow in response to climatic events. Water quality sampling is conducted to characterize baseline water quality conditions (see Chapter 8). If possible, water quality sampling should precede any land disturbances such as exploration drilling. Multiple sampling events may be required to capture baseline conditions and seasonal variation in water quality related to seasonal variation in flow. The initial water quality survey should be spatially comprehensive, with samples collected throughout the watershed, both upstream and downstream of the ore deposit and future land disturbances. Typically, samples are collected above and below the confluences of each relevant tributary in the watershed, as well as above and below any historical mine features and natural exposures of ARD. This approach allows anomalous high values to be systematically traced to their source. Some of the sampling sites in the initial survey will become part of a long-term monitoring program if and when a mine is developed. With this in mind, siting of sampling locations should consider the locations of future mining features. Sample sites should be surveyed with a satellite based navigations system such as Global Positioning System (GPS), GALILEO (European Global Satellite Navigation System), or GLONASS (Global’naya Navigatsionnaya Sputnikovaya Sistema [global navigation satellite system]). Because metal concentrations may be naturally elevated in mineralized areas, characterization of baseline conditions is critical in later assessments of water quality impacts related to mining. Baseline data may be used to support establishment of site-specific water quality guidelines based on premining conditions. In the absence of adequate and defensible baseline data, water quality impacts may be erroneously attributed to mining operations or post-closure water quality criteria may be set to unachievable levels. For these reasons, special emphasis is placed on historical mine features and natural sources of drainage. These data may be provided to regulatory agencies in advance of mine development to ensure documentation of premining conditions.
Water quality sampling and flow monitoring continues during the operation phase to evaluate environmental impacts. If lakes are present in the watershed or the watershed discharges to an ocean, characterization and monitoring of these systems may be necessary. Chapter 8 discusses lake and marine water quality monitoring and determination of a lake water balance.
Hydrogeologic characterization includes determination of groundwater occurrence, groundwater quality, current and potential future groundwater usage, and groundwater flow direction and velocity. Characterization of groundwater conditions is required to evaluate constituent fate and transport, to design dewatering operations, assess compliance with regulatory criteria for designated uses (e.g., drinking water), and to site mine and process facilities (e.g., preference for siting waste facilities in groundwater discharge zones over groundwater recharge zones and preference for siting of waste facilities over aquitards rather than aquifers).
Topographic maps, site reconnaissance, and aerial photographs are used to identify areas of groundwater recharge (i.e., hill tops) and groundwater discharge areas (i.e., springs, streams, rivers, ponds, lakes, and wetlands). Information on existing groundwater wells and their use is compiled. Existing geologic information for the watershed is reviewed to evaluate the nature and distribution of aquifers and aquitards. Aquifers are saturated geologic units that readily transmit groundwater (e.g., fractured bedrock, unconsolidated sand, and gravel), whereas aquitards are geologic units that do not transmit significant quantities of groundwater (e.g., unfractured crystalline bedrock, most shales, and clay). In many cases, collection of information on the lithology, stratigraphy, and structural features (e.g., fractures, folds, and faults) of the subsurface will result in an understanding of the distribution of aquifers and aquitards. The geologic data collected during exploration and regional geologic survey data should be included in the assessment of geologic watershed information
Groundwater occurrence and the depth of the water table are determined by drilling and sometimes by geophysics. Shallow exploration boreholes can provide locations to measure the depth to the water table. Exploration drilling logs may also include information on depth to water and volume of water encountered during drilling that can be used in the development of the subsequent field investigations. During the mine development phase, a monitoring well network is established. Groundwater levels are measured to create a potentiometric map for the study area from which groundwater flow directions are determined. Groundwater flows from regions of high hydraulic head (e.g., hill tops) to regions of low hydraulic head (e.g., stream valleys). Hydrostratigraphic cross sections for the site are created showing depth to groundwater, aquifer and aquitard thicknesses, and extents. The location of seeps and springs and their flow rates should be documented.
Laboratory or field testing is conducted to characterize the pertinent hydraulic properties of aquifer units (i.e., porosity and hydraulic conductivity). Hydraulic conductivity is estimated from laboratory testing of drill core samples or from hydraulic testing in the field, including piezometer tests (slug test) or larger scale pumping tests. Because pumping tests provide in situ measurements of hydraulic conductivity averaged over a larger aquifer volume than piezometer tests, pumping tests are often the preferred testing method. Pumping tests also allow for determination of the specific storage and transmissivity of the aquifer. Porosity is determined by laboratory testing or estimated from literature values (Freeze and Cherry, 1979). Groundwater flow velocity is calculated from the hydraulic gradient (determined from water-level data), hydraulic conductivity, and porosity. Groundwater flow velocities are required for evaluation of constituent fate and transport. Characterization of the physical flow system is also required to select an appropriate dewatering system and for dewatering system design. When dewatering wells are employed, the dewatering time is a function of the pumping rate, which is dictated by the number of pumps and pump capacity. Dewatering rates dictate the required capacity of the water treatment plant (if treatment is deemed necessary) and this information is also needed for surface water discharge permits. Numerical modeling software is often used to create a two- or three-dimensional representation of the groundwater flow system, which may be used as a tool in constituent fate and transport and dewatering evaluations. The groundwater model may also be used to define inputs to the water balance (see Section 188.8.131.52 and Chapter 5).
Groundwater quality sampling is conducted at all monitoring wells, seeps, and springs to establish baseline conditions. Monitoring wells are sited upgradient and downgradient of sources of mine drainage. Groundwater quality monitoring continues throughout the operation phase, and as required during the decommissioning and post-closure phases to evaluate environmental impacts. For underground mines, characterization of the hydrogeologic conditions is essential to assess dewatering during operations and flooding at closure. Geologic maps are reviewed to identify structural controls on groundwater flows. Exploration drill holes may be converted to piezometers or for measurement of groundwater levels or hydrogeologic testing.
184.108.40.206 Water Balance
Climatic, hydrological, and hydrogeologic data are combined to develop a watershed water balance. The water balance is a fundamental component of the environmental impact assessment as it defines the amount of water transporting chemical components and the water available to “dilute” a constituent load released from a source, thereby defining the concentration of a constituent in a water resource. An accurate water balance is, therefore, key to the accurate prediction of constituent concentrations. The water balance is also used to manage site water consumption, predict discharge from water treatment plants, determine design criteria for storm water collection systems, and predict post-mining pit lake filling (if applicable). The water balance describes the hydrologic regime of the watershed. The water balance is an accounting of all water inputs and outputs and changes in storage. For a watershed in which the surface water and groundwater divides coincide and for which there are no external inflows or outflows of groundwater, the water balance is described as follows (Freeze and Cherry, 1979):
where P is precipitation, Q is runoff, ET is evapotranspiration, and ΔS is the change in storage of the groundwater and surface-water reservoir. A simplified box and arrow representation of the components of a watershed water balance is shown in Figure 4-9. This figure illustrates the interaction between surface water and groundwater resulting in additional components (i.e., overland flow [OF], infiltration to soil and groundwater [IS and IG], and groundwater base flow [B]).
To develop a site wide water balance, each of the water inputs and outputs must be defined using site characterization data. When site data are unavailable, inputs are derived using regional data or established relationships. For example, the precipitation and evapotranspiration inputs may initially be based on regional data and then updated with data from the site meteorological station. The Thornthwaite Method provides a means to estimate monthly ET rates as well as IS and IG values based on average monthly air temperature, latitude, and soil characteristics (Dingman, 2002). The Rational Equation can be used to estimate monthly OF values (Fetter, 2001). Site requirements will dictate the temporal resolution of the water balance. Typically, a daily or monthly time step is applied. Spreadsheet programs, databases, and decision analyses software (e.g., MS Excel, and GoldSim) are well designed for water balance calculations. To evaluate conditions under a range of rainfall events, a multiyear precipitation record is generated. This record typically includes extreme climatic conditions (e.g., droughts and storms) to evaluate the effects of such extreme events. Statistical analysis of historical precipitation records is conducted to determine the frequency and magnitude of extreme events.
220.127.116.11 Assimilative Capacity of the Receiving Environment
The sensitivity of the downstream aquatic life and the ability of the receiving environment to attenuate constituents of potential environmental concern must be characterized to predict the fate and transport of constituents in the environment. The buffering capacity of the receiving environment will affect the fate and transport of acidity and metals present in ARD. For historical mining or natural ARD releases, neutralization of acidity may occur following mixing with alkaline waters or interactions with solid mineral phases. Movement of an acidification front will be slower in a well-buffered system than a poorly-buffered system. The transport of chemical constituents will also be affected by geochemical conditions within the receiving environment, and as such, key geochemical parameters should be measured (i.e., pH and redox). Characterization programs should include collection of solid phase data for stream and lake sediments and aquifer materials that may affect metal transport (e.g., presence of clay and total organic carbon).
18.104.22.168 Biological Receptors
The first step in biological characterization is to identify the receptors within the watershed that may be affected by release of ARD. Biological receptors may include vegetation, aquatic life, terrestrial wildlife, livestock, and humans. Consideration should be given both to current and future use of water resources by humans. During the mine development phase, receptor baseline conditions are characterized, including receptor habitat, when applicable. These studies are completed by ecologists or biologists familiar with the local habitats and biota. During the construction and operation phases, receptor monitoring is conducted to assess impacts. In some cases, potential impacts to receptors are determined indirectly (e.g., monitoring of groundwater quality to ensure drinking water obtained from wells for human consumption and use is not affected). If operational monitoring identifies impacts to biological receptors, the objective of monitoring during the decommissioning phase is to measure recovery in impacted areas. Chapter 8 provides additional detail on receptor characterization and monitoring.
Development of an ARD characterization program at a mine site is critical to the prediction, prevention, and management of ARD. The development of a site characterization program begins with development of a conceptual site model to identify sources of ARD, pathways for transport, and the receptors within the watershed. This chapter identifies and discusses the common components and data collection activities associated with a mine site characterization program. Because the distinctions between characterization, prediction, prevention and mitigation and and monitoring are loosely defined, the contents of this chapter should be reviewed in association with Chapters 5, 6 and 8.
Identification and characterization of source materials is fundamental to the accurate assessment of whether ARD is likely to occur at a particular mine site. Characterization of materials to assess their ARD and metal leaching potential should begin during the early phases of a mine life and continue through to the end of operation, and possibly into closure. At a minimum, the characterization program should include testing of the mineral resource (i.e., ore) and waste materials (waste rock, coal overburden and process residues). Inclusion of other materials (e.g., construction material) may also be appropriate.
The scope and development of an ARD characterization program is ultimately site specific. In some cases, the precautionary principle may be selected as the preferred or most economical method to address uncertainty in material characterization (e.g., placement of tailings in a lined facility). In all cases, the scope and intensity of the characterization program are determined in an iterative fashion.
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List of Tables
- Table 4-1: Mine Phase Objectives and Activities
- Table 4-2: Characterization Activities by Mine Phase
- Table 4-3: Source Material Characterization Activities by Mine Phase
- Table 4-4: Geologic Characteristics of Mineral Deposits that Affect Their Environmental Signatures (Plumlee, 1999)
- Table 4-5: Australian Guidance on Sample Numbers (Australian Government Department of Industry, Tourism and Resources, 2007)
List of Figures
- Figure 4-1: Characterization Chapter Road Map
- Figure 4-2: Major Steps Involved in Extraction Metallurgy of Metals
- Figure 4-3: Sources and Pathways of ARD, NMD, and SD in a Pit during Operation and Closure
- Figure 4-4: Sources and Pathways of ARD, NMD, and SD in Underground Workings during Operation and Closure
- Figure 4-5: Sources and Pathways of ARD, NMD, and SD in a Waste Rock Pile
- Figure 4-6: Sources and Pathways of ARD, NMD, and SD in a Subaqueous Tailings Storage Facility (modified from Blowes et al., 2003)
- Figure 4-7: Typical Data Requirements of a Conceptual Site Model (CSM)
- Figure 4-8: Source Material Geochemical Testing Program Components
- Figure 4-9: Water Balance Box and Arrow Diagram
- NAPP – Net Acid Producing Potential expressed as kg H2SO4 per tonne. Calculated by subtracting acid neutralizing capacity (ANC) from acid producing potential (APP). The Australian Guidelines use NAPP as a measure of the ARD potential. NPR may be used in place of NAPP