Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials

Accurate and timely prediction of acidic drainage and metal leaching are keys to preventing potential environmental impacts and minimizing the high costs of mitigation. Prediction of drainage chemistry is a technically challenging subject, involving a large number of methods, properties and processes. The Manual was developed to provide a comprehensive, in-depth level of understanding for use by technical experts or practitioners to conduct a prediction program and/ or review the results. It also serves as a reference document for members of the public, educators or students studying or reviewing drainage chemistry.

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Summary of Some Important Points in Each Chapter

Chapter 1: Introduction

Drainage waters from sulphidic geologic materials can contain elevated concentrations of metals and other elements at any pH. This may lead to costly environmental management and remediation. Successful, cost-effective, proactive mitigation measures depend on an accurate prediction of future drainage chemistry. The prediction of drainage chemistry from sulphidic geologic materials is therefore important in ensuring that the extraction of Canada’’s mineral resources occurs in a sound fiscal manner and minimizes impacts to adjacent land and watercourses.

Guidance is provided on the strengths and potential limitations of different procedures, analyses, tests and criteria used to predict future drainage chemistry. This Manual recommends site specific prediction of drainage chemistry. Users of the Manual should consider local site conditions, such as the weathering environment, the stage of project development, geologic materials, mine components, environmental goals and project needs when deciding which of the procedures in this Manual to use and how to interpret the results. The document is not intended to limit the properly supported approaches or substitute for individuals with the appropriate technical training and experience.

Chapter 2: Overall Objectives of Prediction

The objective in predicting drainage chemistry is to determine the type, magnitude, location and timing of measures required to prevent significant environmental impacts. These objectives are achieved by: measuring the present drainage chemistry; predicting the potential future drainage chemistry; determining the influential properties and processes and predicting the timing of significant changes in the drainage chemistry and influential properties and processes. Predictions should be made for all excavated, exposed and otherwise disturbed sulphidic geologic materials.

Our understanding of the properties and processes determining drainage chemistry is far from complete. However, the available prediction tools combined with a comprehensive, well- informed approach and cautious interpretation of the results should allow mines with sulphidic geologic materials to meet receiving environment objectives and minimize the liability and risk. The predicted drainage chemistry will often be a range in contaminant concentrations because of the range of properties within each geologic and waste management unit and the limited accuracy and precision of the prediction methods. Uncertainty regarding drainage chemistry should be reduced to the level at which plans that will meet the environmental objectives can be designed and implemented.

Chapter 3: General Principles and Best Practices

The ““best practice”” for drainage chemistry prediction is to take a site specific and proactive approach. Drainage chemistry should be predicted for all geologic materials in the forms that will be excavated, exposed or otherwise disturbed (the resulting project components). Prediction should consider the spatial variability and temporal changes in the contributing properties and processes and use an iterative, phased and scientific approach. Due to the large number of factors involved, proper planning is an essential component of successful prediction. Prediction should occur throughout the life of the project. The objective at each stage is to demonstrate that the project has the necessary understanding, capability, resources and intent to protect the environment. Challenges in prediction include dealing with uncertainty and changes in mine plans. It is important to identify the materials and methods and intended uses for prediction work, use clearly defined terminology and consider the cost-effectiveness prior to initiating each phase of test work. Prediction requires qualified personnel and adequate resources. Maintaining prediction information in an accessible form that facilitates regular review and tracking of changes is also extremely important. Practitioners should be aware of past errors in prediction, act safely and recognize that a proper understanding can only be achieved by reviewing the details regarding site conditions, sampling, sample preparation, analyses, test procedures and the interpretation of data.

Chapter 4: Main Steps and Stages of a Prediction Program

There are three main steps for predicting drainage chemistry. First, the general properties of the project and site should be reviewed. Second, any existing drainage chemistry should be measured and monitored, then potential future drainage chemistry predicted. Third, predictions made from the previous steps should be periodically checked and updated, with any significant information gaps identified and highlighted. The third step should be conducted repeatedly through all stages of a project.

Chapter 5: Parameters and Processes Controlling Drainage Chemistry

There are a large number of parameters and processes that affect site specific drainage chemistry from sulphidic geologic materials. This chapter discusses the more important ones from a geochemical perspective.

Chapter 6: Site Conditions

Because prediction of drainage chemistry requires a great deal of site specific information, this chapter lists and discusses many important aspects of site conditions. Local and regional geography, climate, hydrology, and hydrogeology should be defined. Since drainage chemistry will likely change with mining, detailed investigations of geological issues are also needed, including spatial variations in soils, overburden and rock units. Other important aspects of site conditions are the requirements and expectations of the local community, regulators, company, and other stakeholders.

Chapter 7: The Project and Project Components

Each project and site can be divided into components, such as open pits, underground workings, waste rock dumps, low grade ore stockpiles, tailings impoundments and borrow materials. Drainage predictions can then be developed for each component, based on its unique combination of site conditions and design. These predictions require site specific information on a myriad of combinations of physical, geochemical, biological and engineering properties and processes. This chapter lists and discusses many of these properties and processes, including how they may change through time. For example, initial drainage chemistry from an open pit may reflect weathering of the mine walls, but later chemistry may reflect the ongoing accumulation of finer grained talus with greater reactive surface area. Also, rising or falling water tables can greatly change the rate of sulphide oxidation while inversely affecting the loadings in drainage.

Chapter 8: Selection, Storage and Preparation of Samples

The selection, storage, and preparation of samples are critical steps in the prediction of drainage chemistry from sulphidic geologic materials. If a sample is not selected and stored properly, all the remaining time and cost spent on analyses and interpretations could be wasted. Careful decisions must be made on many issues, such as which material to sample, the method and frequency of sampling, the appropriate volume of the sample, whether to crush or grind the sample, substitution of samples from other sources and separation of coarser less reactive particles from finer more reactive ones. Each sample should be described in detail and preferably geo-referenced to a location and depth at the mine or project. For example, samples of blast hole cuttings are often geo-referenced and placed in site geologic models. Characteristics like colour may provide some indication of weathering, leaching and oxidation to guide sampling, but colour is not always reliable.

Chapter 9: Overview of Static and Kinetic Tests

The analyses and tests for predicting drainage chemistry can be divided into one time ““static”” and repetitive ““kinetic”” tests. There are many types of static tests, such as Acid Base Accounting (ABA) and total elemental analyses; these can be completed relatively fast. Kinetic tests, including laboratory based humidity cells and on-site leach pads, can take years to complete and are more expensive. For these reasons, kinetic testing is often limited to samples identified as important and representative by static tests. Also, some kinetic tests provide primary mineral reaction rates, while others provide direct predictions of drainage chemistry after additional processes. Therefore, the objectives of all testing should be carefully considered and stated. Flow rates should always be measured in all kinetic tests to assist in interpretations. Many static and kinetic tests provide some information that is similar and complementary to others, so any discrepancies should be investigated and resolved. Test results should be carefully tabulated, accompanied by descriptive statistics, and also shown on scatterplots.

Chapter 10: Whole-Rock and Near-Total Solid Phase Elemental Analysis

The analyses discussed in this chapter provides the total or near-total amounts of selected chemical elements in a solid phase sample. This is accomplished in two major steps. First, most or all of a sample is digested in a hot chemical flux or strong acid combination. Second, the digested sample is analyzed by one of several techniques, such as X-ray Fluorescence (XRF) or Inductively Coupled Plasma (ICP). It is important to be aware of the strengths and weaknesses of each method of digestion and analysis because it may affect predictions of drainage chemistry from sulphidic materials. For example, whole-rock analyses may be reported as oxide equivalents, such as CaO and Al2O3, which require mathematical conversions to obtain pure element concentrations. These analyses do not reveal the forms in which an element occurs, such as in one or more minerals, although this can sometimes be estimated using a few assumptions. Also, solid phase levels, whether high or low are not on their own measures of the potential aqueous concentrations in drainage or of the threat to the environment. However, tests in other chapters are combined with these solid phase results for drainage predictions, such as the length of time until elements are fully leached from a sample.

Chapter 11: Analysis of Soluble Constituents

Sulphidic geologic materials are often comprised of suites of minerals whose solubilities range from relatively low to high. The more soluble minerals can often dissolve faster and thus determine immediate drainage chemistry. However, the chemistry of the local water, the contact (residence) time and the water:solid ratio can also affect the dissolution of soluble constituents. The recommended procedure for measuring soluble constituents is to add the sample to shake flasks, with a default ratio of 3 parts solid to 1 part water on a weight basis and gently agitate it for 24 hours. These test conditions can be changed as needed to address site specific predictions questions. Attaining equilibrium, including mineral solubility limits, is important because net dissolution stops and aqueous concentrations do not rise any higher. Therefore, an important aspect in testing for soluble constituents is identifying when equilibrium has been reached. As a check for whether equilibrium limits have been attained, a sample can be leached a second time with fresh leach water or at different water to solids ratios. As a check for whether residence time has affected the results, leaching of the solid residue can be extended or repeated for a longer time. The measurements of surface (rinse), crushed and paste (pulverized) pH also reflect soluble constituents of samples.

Chapter 12: Sulphur Species and Acid Generation Potential (AP)

Sulphur species are the primary source of potentially deleterious acid, acidity and elemental species in the drainage from sulphidic geologic materials. Their effects on drainage chemistry depend on factors like abundance, oxidation state, impurities, physical properties and local environmental conditions. The main sulphur minerals and species are sulphides, sulphosalts, sulphates, organic sulphur and species of intermediate oxidation states. Sulphide primarily occurs combined with iron in minerals such as pyrite, pyrrhotite, marcasite and monosulphides. In contrast, sulphate minerals can be grouped as highly soluble basic or acidic, moderately soluble basic, low solubility acidic and extremely insoluble.

The objective in sulphur analysis is to identify and measure the concentration and composition of different sulphur species with sufficient accuracy and precision. This is important for the calculation of acid generation potential (AP) and the prediction of elemental release under potential weathering conditions. There are several methods for measuring sulphur species discussed in this chapter. For example, Leco is a manufacturer of high temperature induction furnaces, whose name has become synonymous with the most common method for determining total carbon and sulphur. All methods have strengths and weaknesses, which should be understood for proper predictions from analytical data.

Chapter 13: Acid Neutralization Potential (NP)

The term acid neutralization potential (or NP) is presently used for a wide range of different laboratory measurements and field NP predictions. For sulphidic geologic materials, the primary concern is with the neutralization of acid potential from Chapter 12. Acidic drainage pH will result when the exposed acid neutralizing minerals are depleted or the rate of acid neutralization becomes inadequate.

To estimate ““effective NP”” under field conditions from laboratory analyses of NP, several properties and processes are important, including (1) identity, concentration and weathering mechanisms of minerals, (2) their contribution to the measured NP and (3) their cumulative rates of alkalinity production compared to the rate of acid generation under the site specific conditions for each project component. Some carbonate minerals provide a fast neutralization response and thus contribute more to effective NP than ferrous iron and manganese carbonates.

There are several methods for measuring NP, including the Carbonate, Sobek (U.S. EPA 600), several Modified, BC Research and Lapakko procedures. Each method has unique strengths and weaknesses, and thus no one method is the best for estimating effective NP. However, the comparison of Carbonate NP with one of the other ““bulk-NP”” methods assists in estimating the percentage of reactive carbonate contributing to bulk NP.

Chapter 14: Acid Base Accounting and Criteria Used to Predict Potential for Acidic Drainage

Acidic drainage will only result when the rate of acid generation exceeds the rate of acid neutralization. Acid Base Accounting (ABA) is a series of analyses and calculations used to estimate the potential for mineral weathering to produce acidic drainage. ABA includes rinse and paste pH (Chapter11), sulphur species and acid potential (AP, Chapter12), and acid neutralization potential (NP, Chapter 13). Mineralogy (Chapter 17), elemental analyses (Chapter 10) and kinetic testing (Chapter 18) are also important for interpreting ABA results.

The rinse pH is indicative of the present drainage pH of a sample. Material categories for future drainage pH are potentially acidic rock drainage generating (PAG) and not potentially acidic rock drainage generating (Non-PAG). For cases where AP and NP are equally exposed and AP generates acid identical to pyrite and NP neutralizes acid like calcite, samples with an NPR less than 1.0 are PAG and samples with an NPR greater than 2.0 are non-PAG. A sample with an NPR between 1.0 and 2.0 is capable of generating acid rock drainage (ARD).

Site specific factors that may alter the relative magnitude of AP and NP include: AP and NP sources whose generation and neutralization of acid differs from pyrite and calcite, differences in AP and NP exposure and the location and length of flow paths. Other considerations in setting NPR criteria for PAG vs. Non-PAG are external sources of AP and NP and safety factors that account for limitations in the precision and accuracy of sampling, determination of the effective AP and NP and material handling.

The minimum AP, sulphide-S or acidic sulphate-S capable of causing ARD is not a generic number, but depends on the magnitude of the effective NP. A % S cut-off should not be used as the only means of assessing ARD potential unless the minimum NP value is known.

The onset of ARD may occur in a few years or take hundreds of years. The absence of ARD up to the present does not on its own prove that ARD will not occur in the future.

Criteria used to guide decisions regarding the potential for future acidic drainage are a key component of sound environmental and fiscal management. Drainage chemistry prediction should be conducted even for Non-PAG material because environmental impacts can also occur due to near-neutral and alkaline pH drainage.

Chapter 15: NAG Tests

Net Acid Generation or NAG tests use hydrogen peroxide (H2O2), a strong oxidizing agent capable of rapidly oxidizing sulphide minerals, to assess whether a sample is capable of neutralizing the potential acidity. NAG testing may involve a (1) single addition NAG test for low sulphide samples, (2) sequential NAG test for higher levels of sulphide, (3) partial ABA consisting of total sulphur, NP, and paste pH, (4) kinetic NAG test to obtain estimates of mineral reactivity and (5) acid buffering characteristic curve (ABCC). The sequential NAG test should be conducted where the breakdown of the hydrogen peroxide due to reactions with sulphide surfaces, organic matter, sulphide oxidation products or other sources of reactive metals may incorrectly indicate the neutralizing capacity is large enough to maintain pH neutral to alkaline drainage.

Chapter 16: Particle Size Separation and Analysis

Particle size distribution of sulphidic geologic materials can play an important role in drainage chemistry prediction because of its effects on mineral reactivity and the movement of water and gases. These effects result from the relationships among particle size, pore size, grain exposure and exposed surface area. For example, a waste rock boulder may contain a much higher concentration of hard minerals like quartz and K-feldspar, while softer minerals like calcite, gypsum and phyllosilicates are concentrated in the finer size fractions. Geometric surface area, based on particle size distributions, can be calculated from equations in textbooks or by free software.

Chapter 17: Mineralogical Properties

Mineralogical analyses measure properties of individual mineral phases and their contributions to geologic materials as a whole. Mineralogical information is an essential component of drainage chemistry prediction because mineralogical properties determine the physical and geochemical stability and relative weathering rates of geological materials under different weathering conditions. This is important for the selection, design, check of assumptions and interpretation of the results of other static and kinetic tests.

The mineralogical methods discussed in this chapter are: visual descriptions, petrographic analysis, X-ray Diffraction (XRD) preferably by the Rietveld Method, Scanning Electron

Microscopy coupled with energy dispersive X-ray Spectroscopy (SEM/EDS), Electron Microprobe, laser ablation and other microbeam analyses, image analysis and calculated mineralogy from solid phase elemental data. Each method has strengths and weaknesses. As a minimum, prediction programs generally should include visual descriptions, petrographic analysis and X-ray Diffraction analyses.

Chapter 18: Humidity Cell Procedures

For sulphidic geologic materials, the decades old, well-flushed humidity cell with alternating dry and humid air is the recommended kinetic test for predicting primary reaction rates under aerobic weathering conditions. The resulting data provide primary rates of elemental release, acid generation and acid neutralization. This information can provide site specific NPR criteria for interpreting ABA data (Chapter 14) and, when combined with solid phase analyses, also provide depletion times for NP, sulphide and various elements. However, these cells do not usually simulate the precipitation and dissolution of secondary weathering products, which often determine drainage chemistry under field conditions. Cells should continue until rates have stabilized at relatively constant levels for at least five weeks. When a cell is terminated, the closedown procedure should be conducted for better interpretations and for post-test validation of cell results.

Chapter 19: Kinetic Tests that Measure Primary Mineral Weathering and Secondary Mineral Precipitation and Dissolution

Drainage chemistry depends on both the primary mineral reactions (Chapter 18) and the precipitation and dissolution of the resulting secondary minerals. This chapter discusses several kinetic tests that can examine both primary and secondary aspects at the same time and thus provide more direct predictions of drainage chemistry. These tests are: trickle leach columns (both subaerial and subaqueous types), field test cells including leach pads and barrels, MEND wall-washing stations, full-scale monitoring data and previously weathered materials like outcrops or old rock piles. However, large disparities may exist among these tests and full-scale project components due to differences in sample preparation, site climatic conditions, sample size, scale and particle size. Even without these disparities, the equilibrium solubility processes and reaction product retention that play significant roles in determining drainage chemistry cannot always be reliably identified, even after decades of monitoring or testing.

Chapter 20: Modeling Drainage Chemistry

Drainage chemistry modeling can assist with the interpretation of test work and monitoring results and may improve the prediction of drainage chemistry and loadings. However, modeling cannot substitute for good site specific monitoring and understanding. Modeling predictions need to be tested before they can be accepted.

Brief overviews of three basic categories of drainage chemistry modeling are presented in this chapter: empirical modeling, speciation and mineral equilibrium modeling and complex models. If a minimum of hundreds of water analyses are available for a particular site, then these analyses can be compiled into a statistical ““empirical drainage chemistry model”” (EDCM). The second category applies pre-selected chemical reactions, equilibrium constants and mineral solubilities to a particular water analysis to estimate aqueous concentrations of all pertinent chemical species and determine whether minerals are close to saturation. Complex models simulate more than just chemistry and can include water and gas-phase movement across an entire mine site.

Chapter 21: Checklist of Important Information for the Prediction of Drainage Chemistry

This chapter provides a detailed checklist of potentially important information for predicting drainage chemistry from sulphidic materials. This list is intended to make the technical specialist aware of general issues and the generalist practitioner aware of detailed information requirements. Every mine site has unique combinations of environmental, geological and operational conditions. For any particular site, some properties and processes within this list may not be relevant. Similarly, there will be instances where there are additional factors to consider. Minimizing environmental risks and liability includes consideration of near-neutral and alkaline conditions, as well as acidic drainages and the reduction of water and/or oxygen entering a project component. At each stage of prediction, one should consider the purpose of the test work and whether the results will impact site management, liability or the risk to the environment. In some cases, the provision of contingency mitigation measures coupled with operational testing during mining will be more effective than additional pre-mining prediction test work, which could be inconclusive or of limited significance to the overall mine plan. In all cases, there is never complete understanding, so a critical part of any drainage chemistry prediction is identifying and dealing with uncertainty.