EPA and Hardrock Mining: A Source Book for Industry in the Northwest and Alaska

January 2003

APPENDIX C

CHARACTERIZATION OF ORE, WASTE ROCK, AND TAILINGS
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TABLE OF CONTENTS

1.0 GOALS AND PURPOSE OF THE APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.0 ANALYSIS OF PHYSICAL CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.1 Extent of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.2 Physical Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2

2.3 Mineralogical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

3.0 ANALYSIS OF CHEMICAL COMPOSITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

3.1 Analysis of Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

3.2 Analysis of Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

4.0 ANALYSIS OF CONTAMINANT MOBILITY FROM SOLIDS . . . . . . . . . . . . . . . . . C-8

4.1 Mineralogical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

4.2 Physical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

4.3 Acid Generation Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

4.3.1 Static Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

4.3.1.1 Acid-Base Accounting Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

4.3.1.1.1 Methods to Determine Acid Generating Potential . C-11

4.3.1.1.2 Methods to Determine Acid Neutralizing Potential C-13

4.3.1.2 Static Tests that Produce a Single Indicator Value . . . . . . . . . . C-15

4.3.1.3 Interpreting Static Test Results . . . . . . . . . . . . . . . . . . . . . . . . C-15

4.3.1.4 State Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-16

4.3.2 Kinetic Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-18

4.3.2.1 Kinetic Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-18

4.3.2.1.1 Conventional and Modified Conventional Humidity

Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-18

4.3.2.1.2 SRK Humidity Cells . . . . . . . . . . . . . . . . . . . . . . . . C-19

4.3.2.1.3 Soxhlet Extractions . . . . . . . . . . . . . . . . . . . . . . . . . C-20

4.3.2.1.4 Column Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-20

4.3.2.1.5 Shake Flask Extractions . . . . . . . . . . . . . . . . . . . . . C-21

4.3.2.1.6 Field Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-21

4.3.2.2 Interpreting Kinetic Test Results . . . . . . . . . . . . . . . . . . . . . . . C-21

4.3.2.3 State Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

4.3.3 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

4.3.4 Mathematical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

4.4 Leaching Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-24

4.4.1 U.S. EPA Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-25

4.4.1.1 EP Toxicity Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-25

4.4.1.2 Toxicity Characteristic Leaching Procedure Test . . . . . . . . . . . C-25

4.4.1.3 Synthetic Precipitation Leaching Procedure Test . . . . . . . . . . . C-25

4.4.1.4 Monofilled Waste Extraction Procedure . . . . . . . . . . . . . . . . . . C-26
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TABLE OF CONTENTS 

(continued)

4.0 ANALYSIS OF CONTAMINANT MOBILITY FROM SOLIDS (continued)

4.4 Leaching Procedures (continued)

4.4.2 State Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-26

4.4.3 Other Leaching Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-27

4.4.3.1 British Columbia Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . C-27

4.4.3.2 U.S. Army Corps of Engineers Procedures . . . . . . . . . . . . . . . C-27

4.4.3.3 ASTM Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-28

4.4.4 State Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-28

4.4.5 Comparison of Leaching Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-28

5.0 ANALYSIS OF FATE AND TRANSPORT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-29

5.1 Developing a Conceptual Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-30

5.2 Mathematical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-30

5.2.1 Categories of Mathematical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-31

5.2.2 Chemical Equilibrium Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-33

5.2.3 Physical Flow and Transport Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-33

6.0 SAMPLING PROGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-34

6.1 Objectives of a Geochemical Sampling Program . . . . . . . . . . . . . . . . . . . . . . . . . C-34

6.2 Sample Representativeness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-35

6.2.1 Proposed Mine Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-35

6.2.2 Existing or Abandoned Mine Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-36

6.3 Quality Control and Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-38

6.3.1 Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-38

6.3.2 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-38

7.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-39

LIST OF TABLES

C-1. Summary of Commonly Used Static Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . C-12

C-2. Suggested Guidelines for Static Test Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . C-17

LIST OF FIGURES

C-1. Conceptual physical model of ground water flow from Anderson and Woessner . . . C-31

C-2. Conceptual physicochemical model of metal transport in a river from Schnoor . . . . C-32

C-3. Minimum number of samples as a function of rock mass recommended 

by the BC AMD Task Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-37
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1.0 GOALS AND PURPOSE OF THE APPENDIX

EPA expects that applicants will conduct a sufficient number and variety of

environmental tests on a representative suite of samples in order to support projections of

wastewater and solid waste management practices and effluent quality. This appendix describes

the methods used to characterize the solid wastes from mining activities and the rationale for

their implementation. The materials in this appendix complement those in Appendix B,

Receiving Waters and Appendix F, Solid Waste Management.

Determining the physical and chemical character of solid waste materials is a prerequisite

to delineating the area that would be affected by waste disposal; recognizing the physical,

chemical, and biological impacts of waste disposal; and developing appropriate mitigation

measures. Environmental test samples should be collected as part of a comprehensive program

designed to examine the range of conditions that occur or could occur. For areas in which

mining has concluded or is on-going, tested materials should be produced by normal mine

operations. For areas in which mining is proposed or production methods are expected to

change, tested materials should include batch and pilot-plant waste products. Physical and

chemical characterization studies should be conducted in a manner that provides conservative

estimates of the potential environmental impacts. 

An environmental sampling program should be related to the mine plan and should be

designed to represent the different lithologic units that have been or will be encountered,

excavated, processed, disposed of, or exposed (for example in pit walls). It should establish the

chemical and physical variability of each geologic unit encountered at the mine site, including

borrow materials. It can have the benefit of reducing or eliminating the potential future costs

associated with mismanagement of disposed materials. For proposed or expanding mining

activities, ore sample testing should be representative of the range of materials that will be mined

and wastes that will be generated. Although simple in concept, developing and implementing a

reliable environmental sampling program may be a complex endeavor.

This appendix presents the methods used to determine the physical and chemical

characteristics of waste materials, describes the environmental tests used to assess contaminant

mobility, outlines the conceptual models used to analyze contaminant fate and transport, and

discusses the elements of quality assurance and quality control engendered in an environmental

testing program.

2.0 ANALYSIS OF PHYSICAL CHARACTERISTICS

2.1 Extent of Analysis

The proposed mine plan should be used to determine the types and volumes of materials 

that will be excavated or otherwise disturbed and the management of those materials. This

information, some of which can be presented in the form of maps and cross-sections, provides

the basis for determining the types of characterization studies that will be needed. For example,

if waste rock materials will be used in road construction, then the potential effects on water
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quality will need to be ascertained. If the gangue rock at the site consists of several lithologic

types that will be mined in sequence, then the resulting waste rock dump could contain vertical

or lateral changes in rock type that might impact water quality models and geotechnical stability. 

Because many material or waste dumps cover significant areas, characterization studies of

substrate materials can determine whether lateral changes in physical properties are present that

could impact dump stabilities and contaminant transport models. Although the physical and

chemical characterization of solid materials can be an intricate process, a well-planned and

executed program can provide the benefits of improved project design and environmental impact

mitigation.

2.2 Physical Parameters

The physical characteristics of waste materials govern their hydrologic properties and

physical stability. Important parameters that affect porosity and permeability include particle

size, particle-size distribution, particle-size grading, stratification, and mineral composition. 

Important parameters that affect stability include stratification, mineral composition, cohesion,

compaction, moisture retention, shrink-swell potential, Atterberg limits, and bulk density. For

existing waste rock dumps and tailings piles, physical characteristics testing should determine

whether the disposed material contains vertical or lateral changes in physical properties

sufficient to affect the flow of leachate or the stability of the pile. Such variations could arise

from changes in mining, processing, and disposal methods; variations in the geology of the ore

or gangue materials as mining progressed; or the effects of subaerial weathering, alteration, and

secondary mineral growth after the materials were emplaced.

Particle-size characteristics (median diameter, sorting, size distribution) are determined

through mechanical analyses (sieve analysis). Those of fine-gained materials (smaller than 50

microns) are determined using methods based on particle settling velocities (e.g., pipette

analysis) or optical techniques (e.g., Coulter counters). The American Society for Testing and

Materials provides methods for determining particle-size characteristics (ASTM, 1996);

additional methodologies can be found in Sobek et al. (1978).

Particle-size grading (i.e., changes in particle size normal to a bedding surface) typifies

many waste rock dumps constructed by end-dumping. Grain-dispersive forces that occur as

materials avalanche down the working face of a waste rock dump can create deposits that

become coarser upward and outward (e.g., Blatt et al., 1980). Changes in particle-size grading

potentially can form preferred pathways for the flow of water through waste rock piles. 

Stratification can be created within waste rock and spent ore dumps and tailings piles by

construction practices. In addition to affecting fluid flow, bedding surfaces can serve as planes

of weakness along which slope failure can occur. The presence of stratification can be noted

from visual observation of existing waste materials or drill cores obtained from these materials.

Methods to measure cohesion, compaction, moisture retention, shrink-swell potential,

Atterberg limits, and bulk density have been developed by the American Society for Testing and

Materials (ASTM, 1996). These parameters are particularly important for assessing the stability
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of waste rock and spent ore dumps, tailings piles, and pit benches. For existing waste materials,

vertical or lateral changes in the amount and type of clay minerals can cause many of these

parameters to change throughout a deposit. Consequently, existing waste deposits should be

sampled in several locations and at several depths to determine the range of values that occur. 

For those tests that cannot be conducted on materials in situ, appropriate ASTM procedures

should be followed to ensure sample integrity. The stability of waste rock dumps and tailings

piles is discussed in more detail in Appendix F.

2.3 Mineralogical Composition

Mineralogical composition and mineral textures can be determined using a petrographic

(polarizing light) microscope equipped with both transmitted and reflected light. Samples can be

viewed in thin-section, as grain mounts, or as discrete grains. Mineral percentages can be

estimated through counts of a statistically significant number of points or grains. Thin-sections

are particularly useful for recognizing mineral reaction (alteration) textures and products that

may influence the interpretation of geochemical test results as described in the next section. 

Moreover, they permit identification of reaction products that may form as a consequence of

mineral processing (by examining samples “before” and “after” processing). Petrographic

techniques, including oil immersion, are well-established and widely accepted (Kerr, 1977;

Sobek et al., 1978; Gribble and Hall, 1993; Craig and Vaughn, 1994). 

X-ray diffraction (XRD) is used to identify minerals that are difficult to resolve with a

petrographic microscope and to characterize crystal structures. The method measures the

diffraction of an incident beam of X-rays during its passage through a crystal structure caused by

atoms or atomic layers in the crystal (e.g., Hutchison, 1974; Bish and Post, 1989). The technique

is a quick and easy means to determine the compositions of clay minerals that are associated

with many ore deposits (e.g., Sobek et al., 1978). Analyzing clay minerals, which have different

sorptive properties, can provide useful data that can be used in the design of waste rock and 

tailings piles, drainage covers, compacted liners, and remediation plans.

Scanning electron microscopy (SEM) can be used to image reaction products and grain

coatings that cannot be resolved with an optical (petrographic) microscope. For example, it can

be used to gather data on secondary mineral growths in the pore spaces of waste materials. This

knowledge can be used to refine models of fate and transport by clarifying the potential for

contaminant sorption onto the surfaces of clays or other minerals. In addition, the technique can

be used to gather quantitative or semi-quantitative chemical data on the major constituents of

minerals at scales that vary from a few microns to a few millimeters. The SEM scans a tightly

focused beam of high-energy electrons across the surface of a prepared sample. The beam

dislodges secondary electrons from the atoms in the sample, which are then collected, counted

and formed into an image of the specimen surface (e.g., Goldstein et al., 1981). Because the

energies with which secondary electrons are emitted are unique to each element, secondary

electrons also provide compositional data through energy dispersive microanalysis.

Electron microprobe (EMP) analysis is used to determine the compositions of mineral

grains in a sample. The EMP focuses a beam of high-energy electrons onto a fixed spot on a
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sample surface (typically 1 to 2 microns in diameter). The beam dislodges secondary electrons

that emit radiation in wavelengths and energies characteristic of particular elements. Similar to

SEM analysis, EMPs can be operated in an energy dispersive analysis mode. However, these

machines typically are operated using wavelength dispersive detectors, which provide lower

detection limits and more accurate analyses. Because it utilizes a tightly focused incident beam

of high energy, EMP microanalysis is poorly suited for determinations of light elements(atomic

number less than 10) and volatile elements. 

3.0 ANALYSIS OF CHEMICAL COMPOSITION

Acceptable techniques for determining the concentrations of inorganic and organic

constituents in solid and liquid wastes are given in 40 CFR, Part 136.3. Analytical methods are

detailed in publications by the U.S. EPA (1983; 1986a), American Public Health Association

(APHA et al., 1992), American Society for Testing and Materials (ASTM, 1996), and the U.S.

Geological Survey (Fishman and Friedman, 1989). Considerations regarding the number and

types of samples that should be tested are described in Section 6.0.

3.1 Analysis of Solids

The chemical composition of solid materials such as waste rock, tailings, or spent ore can

be determined using a variety of techniques. Most analytical techniques require solubilization of

the solid material into a liquid form prior to analysis. An exception is X-ray fluorescence

(XRF), which is a common technique used to determine the major and minor chemical

constituents of rocks and minerals (Norrish and Chappell, 1967; Bertin, 1970; Johnson and

Maxwell, 1981). The technique analyzes sample materials in solid form (either as compacted

powders or powders that have been fused into glass) by bombarding the sample with X-rays of

known wavelength and energy. Excitation by the primary X-rays induces emissions of

secondary photons (fluoresence) with energies and wavelengths characteristic of individual

elements. The number of photons emitted (intensity) at a given wavelength or energy is

proportional to the abundance of a given element. X-ray fluorescence is capable of determining

the abundance of many elements that occur in concentrations of a few parts per million. It is an

inferior technique for light elements, volatile elements, and many elements occurring at

concentrations of less than 10 ppm.

Solid samples commonly are solubilized using strong-acid dissolution. Methods to digest

solid materials in nitric acid are common and widely accepted (ASTM D5198 [ASTM, 1996];

EPA Method 3051 [U.S. EPA,1986a]). The subsequent liquids can be analyzed by several

methods that most commonly include atomic absorption spectrometry, inductively coupled

plasma spectrometry, and colorimetry. 

In atomic absorption (AA) spectrometry, samples are vaporized at high temperatures and

the concentrations of selected elements are determined by measuring the absorption of light at

wavelengths characteristic of that element (Harris, 1987; Patniak, 1997). The technique is highly

sensitive, comparatively simple, and permits determination of a variety of metals to levels of
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parts per million or less. In the direct aspiration method, sample solutions are injected into a

flame, where they are dissociated and made amenable to absorption. The more sensitive graphite

furnace technique uses an electrically heated furnace to vaporize the sample solution. The

graphite furnace technique affords lower detection limits, but is more sensitive to matrix

interference effects; it works best on relatively “clean” samples (U.S. EPA, 1986a). A primary

disadvantage of the AA technique is that it is time-consuming, because each element must be

analyzed separately (i.e., a sample must be analyzed repeatedly). Accepted atomic absorption

techniques using both methods are given in U.S. EPA (1983; EPA 200 series methods) and U.S.

EPA (1986a; EPA 7000 series). Methods for determining trace metal concentrations at levels of

a few tens to hundreds of parts per trillion were recently developed by U.S. EPA (1996d, f). The

absorption of elements that occur at low concentrations can be masked by interference from

elements at higher concentrations. Consequently, chemical separation is used to isolate these

elements and permit their analysis without interference. The cold-vapor technique (EPA

Methods 245.1 and 245.2, U.S. EPA [1983]; EPA Method 7470A, U.S. EPA [1986a]; EPA

Method 1631 for low detection limits, U.S. EPA [1996a]) is used to reduce and isolate mercury

for analysis. The gas hydride method is used to reduce and isolate selenium (EPA Method

7741A; U.S. EPA [1986a]) and arsenic (EPA Method 7061A; U.S. EPA [1986a]; EPA Method

1632 for low detection levels; U.S. EPA [1996b]) for analysis. A co-precipitation method (EPA

Method 218.5, U.S. EPA [1983]; EPA Method 7195, U.S. EPA [1986a]) is used to remove

trivalent chromium from solution, permitting measurement of hexavalent chromium in the

remaining solution by AA. 

In inductively coupled plasma (ICP) spectrometry, aqueous samples are ionized at

extreme temperatures in an argon plasma. The ions are focused into a stream of material that is

accelerated toward detectors that measure either the photon emissions at specific wavelengths

(ICP-AES, atomic emission spectrometry) or the masses of specific isotopes (ICP-MS, mass

spectrometry) (Robinson, 1990). Standard ICP techniques can detect elements in concentrations

of a few parts per billion to parts per million, but recently developed guidelines permit detection

of a few to a few hundred parts per trillion. The primary advantage of ICP analysis is that it

permits rapid, simultaneous or sequential determination of multiple elements in a single

analytical session (i.e., a sample need only be analyzed once). Disadvantages include

interference from the plasma gases, background radiation from other elements, and interferences

from large excesses of single elements (U.S. EPA, 1986a). Accepted standard ICP techniques

using both methods are given in U.S. EPA (1986a; EPA Method 6010A for ICP-AES; EPA

Method 6020 for ICP-MS). 

“Ultraclean” ICP-MS techniques that permit low detection limits are given in U.S. EPA (1996e,

1996g).

Colorimetry is a type of spectrophotometric analysis that uses the absorption of visible

radiation (Harris, 1987; Patniak, 1997) to determine concentration. The technique uses a

spectrophotometer or filter photometer to determine the concentration of a constituent in a

specially prepared aqueous solution by measuring the absorbance at a specific visible light

wavelength. An accepted colorimetric technique for hexavalent chromium (EPA Method

7196A) is given in U.S. EPA (1986a). Colorimetric techniques also have been developed for

nitrate-nitrogen, nitrate plus nitrite-nitrogen, ammonium nitrogen, and total cyanide.
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3.2 Analysis of Liquids

Samples of waters and wastewaters typically are filtered in the field prior to analysis. 

Methods developed by EPA require filtration using a 0.45 :m filter. Care should be taken when

reusing field filters to ensure that they do not become sources of contamination. Importantly,

some colloidal particulates can pass through this filter and will report as dissolved constituents in

water quality analyses. Because some of these constituents (e.g., iron oxyhydroxides) readily

adsorb metals from solution, the presence of colloidal particles smaller than 0.45 :m can

influence measurements of dissolved metals such as cadmium, copper, lead, and zinc. 

Liquid samples may be analyzed as collected, but they typically are treated following

collection to preserve their chemical constituents. In many cases, multiple splits of a given

sample are preserved using a variety of techniques. Electrical conductivity and pH should be

measured on untreated samples at the time of collection. In contrast, samples that must be

delivered to a lab for analysis of their inorganic and organic constituents are preserved to

preclude precipitation of metal compounds or the volatilization of organic compounds between

the time of sample collection and analysis. Samples collected for total metals analysis should be

acidified to pH <2.0 using nitric acid and stored at 4°C to permit dissolution of suspended

constituents (EPA Method 200.0; U.S. EPA [1983]). In contrast, samples collected for cyanide

analysis should be adjusted to pH >12.0 using sodium hydroxide and stored at 4°C to prevent the

formation of hydrogen cyanide (EPA Method 335.3; U.S. EPA [1983]). Samples collected for

analysis of their organic constituents should be preserved at 4°C and left untreated or treated

with sodium thiosulfate (EPA 3500 and 5000 series methods; U.S. EPA [1986a]). 

Many metals in ambient waters occur in concentrations of less than 1 part per billion,

which are below the detection limits of most standard analytical techniques. To permit accurate

determinations of background water quality, the U.S. EPA recently released draft Method 1669

(U.S. EPA, 1996h). This method provides guidance for collecting samples that will be analyzed

by newly developed “ultraclean” ICP-MS, AA, and ion chromatographic techniques (U.S. EPA,

1996a-g). Using these sampling and analytical methods, trace metal constituents in ambient

water can be determined at levels of a few to a few hundred parts per trillion.

Prior to analysis, organic constituents are separated using solvent extraction or purgeand-trap techniques. Nonvolatile and semi-volatile organic compounds are extracted using

solvents such as methylene chloride and techniques that include liquid-liquid extraction, soxhlet

extraction, or ultrasonic extraction (EPA 3500 series methods; U.S. EPA [1986a]). Volatile

organic compounds are extracted by bubbling an inert gas (either N2 or He) through the sample

solution to liberate the volatile components which are trapped in a sorbent column (EPA 5000

series methods; U.S. EPA [1986a]).

The concentrations of metals and other inorganic cationic constituents in samples of

surface water, ground water, waste rock leachate, or mine drainage are analyzed using the AA,

ICP, and colorimetric methods described above. Other techniques used to analyze aqueous

samples include titrimetry, gravimetry, ion-selective electrode analysis, ion chromatography, gas

chromatography, liquid chromatography, and Fourier transform infrared spectroscopy. 
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Titrimetric analysis is used to measure the acidity and alkalinity of aqueous samples

(Patniak, 1997). Acidity is measured by titrating a solution to a predetermined pH endpoint

using sodium hydroxide (EPA Method 305.2; U.S. EPA [1983]). Alkalinity is determined by

titrating a solution to a predetermined pH endpoint using a strong acid (EPA Method 310.1; U.S.

EPA [1983]). In both cases, the amount of titrant is converted to milliequivalents of acidity or

alkalinity per liter of solution.

In gravimetric analysis, the mass of a reaction product is used to determine the quantity

of the original analyte (Harris, 1987). Although these techniques are among the most accurate in

analytical chemistry, they are no longer widely used because they are time consuming. 

However, gravimetric analysis remains the most common method for determining total dissolved

solids (TDS) and total suspended solids (TSS) in a sample. To determine these parameters, a

sample is filtered through a standard glass fiber filter. The filter is dried and weighed, with the

weight increase representing TSS concentration (EPA Method, 160.2; U.S. EPA [1983]). Total

dissolved solids are measured by evaporating the filtrate and weighing the residual solids (EPA

Method 160.1; U.S. EPA [1983]).

Ion-selective electrodes respond to a single ionic species in solution (Harris, 1987;

Patniak, 1997). The electrodes measure the electrical potential difference across a membrane

between a solute at constant chemical activity within the electrode and the activity of the solute

in the solution of interest. Ion-selective electrodes can be used to measure the concentrations of

fluorine, cyanide, and ammonia in water samples (Standard Method 4500 series; APHA et al.

[1992]).

Chromatographic techniques, in which constituents of interest are separated from one

another to permit their identification, include ion chromatography, gas chromatography, and

high-performance liquid chromatography. Ion chromatography is used to measure the

concentrations of common anionic constituents (EPA Method 300.0; U.S. EPA [1983]). The

technique uses a series of columns filled with ion-exchange resins to separate the anions from

solution and combine them with hydrogen to form acids (Harris, 1987; Patniak, 1997). The

electrical conductivities of the different acids, which are variably strong electrolytes, are

measured using a conductivity detector, from which anion concentrations can be determined. A

method for determining low levels of hexavalent chromium by ion chromatography was recently

developed by U.S. EPA (1996c). Gas chromatography is used to measure the concentrations of

a wide variety of organic constituents. In this technique, a liquid sample is vaporized and carried

by an inert gas through a column filled with a partitioning material (Harris, 1987; Patniak, 1997). 

Organic compounds are separated in the column by their variable affinities for the partitioning

material, which causes the different compounds have discrete retention times prior to emerging

from the column and flowing to a detector. Several detector types are employed including

electrolytic conductivity detectors, electron capture detectors, and flame ionization detectors

(EPA 8000 series methods; U.S. EPA [1986a]). More sensitive detection can be accomplished

by using mass spectrometers (EPA 8200 series methods; U.S. EPA [1986a]). Constituents that

cannot be differentiated by mass (i.e., isomers) can be distinguished using Fourier transform

infrared spectroscopy, in which isomers are distinguished by their infrared absorption

frequencies (EPA Method 8410; U.S. EPA [1986a]). High-performance liquid chromatography
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also is used to measure the concentrations of organic constituents. This technique uses columns

filled with adsorbent material (typically microporous silica with a covalently bonded stationary

phase) to separate the compounds of interest, which are then eluted from the column by solvents

(Harris, 1987; Patniak, 1997; EPA 8300 series methods, U.S. EPA [1986a]). Liquid flow is

accomplished under high pressure to increase efficiency of the system. Absorbance, refractive

index, and polarographic monitors are used to detect solutes eluted from the column. Potential

interferences occur in all chromatographic techniques when two or more solutes have similar

retention times in the separation column or, for mass spectrometry, have similar masses.

4.0 ANALYSIS OF CONTAMINANT MOBILITY FROM SOLIDS

Rigorous geochemical testing programs can reveal whether the rocks exposed by the

mining process or the wastes and materials produced by extractive operations are likely to

release metals or other contaminants that could degrade the environment at or surrounding a

mine site. Testing programs are aimed at determining the potential for acid generation and

constituent release through weathering and leaching. Because these laboratory programs are

conducted in a manner intended to speed natural processes, test results must be interpreted with

caution. Particle size and mineralogy play pivotal roles that govern the long-term behavior of

materials in the environment. Consequently, these variables should not be ignored by a testing

program. Considerations regarding the number and types of samples that should be tested are

described in Section 6.0.

4.1 Mineralogical Considerations

It is critical to understand the mineralogy of waste rock, tailings, and spent ore materials

in order to establish a sound geochemical testing program. Because many ore deposits and their

gangue materials are chemically and mineralogically zoned (also true of some waste rock dumps

and tailings piles), selecting appropriate test materials requires knowledge of mineral

composition, abundance and distribution. Recognizing spatial variations in mineral abundance is

especially important for potentially reactive sulfides (e.g., pyrite), nonreactive but leachable

sulfides (e.g., galena), acid- and nonacid-sulfates (e.g., jarosite and gypsum), readily soluble and

comparatively insoluble carbonates (e.g., calcite and siderite), and other minerals that may affect

test results (e.g., clays and feldspars). Smith et al. (1994) showed that alteration zoning can have

a significant impact on the pH and metals content of drainage generated from a quartz-alunite

epithermal deposit. Testing programs need to recognize the mineralogical changes that

secondary alteration may have imparted to a given rock unit and characterize the range of

environmental behavior that could occur as a result.

Mineralogical studies provide a framework for interpreting the results of the geochemical

tests outlined below. For example, hydroxide coatings on calcite or sulfate coatings on pyrite

may preclude these minerals from participating in acid neutralization or generation in existing

waste rock dumps. Samples of this material that are crushed to fine particle sizes prior to acidbase accounting tests may exhibit net neutralization potentials significantly different from that of

the in situ waste material. Having knowledge of mineral coatings would allow one to interpret
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the test results in a more sound scientific manner. Mineralogical studies also can provide

information regarding the sorptive properties of host minerals (e.g., clays) which could allow a

determination of whether they are likely to retard the movement of certain contaminants. Studies

of mineral compositions could permit identification of the mineralogical sources of trace metals

in leachates and provide a basis for designing effective disposal plans.

4.2 Physical Considerations

The ability of a material or solid waste to generate acidity or alkalinity, or to contribute

metals or other constituents to the environment through leaching, depends partly on the particlesize characteristics of the waste material. Interpretation of test results is complicated if the

particle size of the test materials differs significantly from the particle size of a waste material as

it is or will be disposed of in the environment. Particle-size characteristics impact both reaction

rate and reaction duration by affecting the reactive surface area, the distances between

potentially reactive particles, and the porosity and permeability of the waste. 

Test materials that are finely ground can impact the results of acid-base accounting tests

(Robertson and Broughton, 1992; Lapakko et al., 1998). Crushing to small particle sizes

increases the surface area of reactive sulfide and neutralizing minerals. In addition, fine crushing

can increase the acid generating potential of a sample by releasing reactive sulfides that are

enclosed in inert minerals (e.g., pyrite enclosed in quartz) and which would not be exposed to

oxidation in coarser materials (Lapakko et al., 1998). The distance between reactive particles

and neutralizing particles is greatly diminished in fine-grained materials, which may inhibit the

formation of localized zones of low pH that are known to occur in coarse-grained waste rock

piles (Robertson and Broughton, 1992).

The leaching characteristics of waste materials also are affected by changes in particle

size. Smaller particle sizes increase the surface area of materials amenable to leaching. 

Moreover, smaller particle diameters and a smaller range of particle sizes (better grading) affects

pore sizes and permeability, both of which influence the volume of extraction fluid held in the

pore spaces of granular materials and the amount of time that it is retained by the material.

4.3 Acid Generation Potential

Materials that contain iron sulfide minerals such as pyrite, marcasite, or pyrrhotite can

generate acid if exposed to moisture (for example, humid air) and an oxidant (either oxygen from

the atmosphere or a chemical source such as ferric iron). In addition, some sulfate minerals,

such as jarosite, can dissolve to form acidic solutions (e.g., Lapakko, 1991). Bacteria commonly

accelerate the process of acid generation from sulfides by enhancing the rate of ferrous iron

oxidation (e.g., Kleinman and Erickson, 1983) or the rate of reduced-sulfur oxidation (BC AMD

Task Force, 1989). The rate at which acid is generated depends on the composition of the

sulfide mineral (e.g., Lundgren and Silver, 1980), its crystal size and shape (surface area;

Caruccio et al., 1977), the presence of reaction coatings that may form on the surfaces of sulfide

minerals (Goldhaber, 1983; Nicholson et al., 1990; Sherlock et al., 1995), and the environmental

conditions (for example, pH, humidity, oxygen fugacity, temperature) at the site of oxidation
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(BC AMD Task Force, 1989). In general, acid generation involves a rather complex set of

chemical reactions that change through time (BC AMD Task Force, 1989). 

The potential for acid generation is offset by the ability of a material to neutralize acid. 

Acid neutralization is imparted by various minerals including calcium- and magnesium-bearing

carbonates, oxides and hydroxides of calcium, magnesium, and aluminum, some silicate

minerals, and some phosphates (Sherlock et al., 1995). In general, dissolution rates (and hence

neutralization) are considerably faster for carbonate minerals than for other neutralizing

minerals. Factors that influence mineral dissolution rates include pH, dissolved carbon dioxide

content, temperature, mineral composition, crystal size and shape, redox conditions, and the

concentration of “foreign” ions (e.g., trace metals) (Sherlock et al., 1995). 

Static predictive tests are used to define the balance between potentially acid-generating

minerals and potentially acid-neutralizing minerals in a sample (BC AMD Task Force, 1989). 

These tests, which are quick and comparatively inexpensive, cannot be used to predict the

quality of effluent that may drain from waste materials in the future. However, they are useful

for determining which geologic units have the potential to generate acidity and, in essence, serve

as positive/negative indicators of the theoretical potential for acid generation (Robertson and

Broughton, 1992). When coupled with mineralogical and petrological data from the test

samples, certain static test procedures can provide some measure of neutralization rate (Mills,

1998a). Kinetic tests are used to define reaction rates through time under specific environmental

conditions. These tests are significantly more expensive and may take months or years to

complete. 

In general, acid mine drainage testing programs utilize a two-step approach in which

static tests of numerous samples are used to identify potentially acid-generating geologic units

and to characterize the variability that occurs within them. Kinetic tests are then run on samples

deemed representative of the range of compositions within potentially reactive units to determine

whether acid drainage will occur. Although New Mexico (NMED, 1996) and Nevada (NV DEP,

1990; 1996) have specific guidelines mandating static and kinetic testing of mine wastes, the

states of EPA Region 10 have not adopted a similar approach.

4.3.1 Static Tests 

Static test methods, which were developed initially to determine the potential for acid

generation from coal mine wastes, have been adapted for use in the metal mining industry. The

variety of static test methods that are available are collectively referred to as acid-base

accounting (ABA) analyses. Static test methodologies are described and evaluated in reports by

Lapakko (1991; 1992), Lawrence and Wang (1996), and Mills (1998a; 1998b); digestion

methods are compared and evaluated in Skousen et al. (1996). Table C-1 summarizes several of

the more commonly used test methods.
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4.3.1.1 Acid-Base Accounting Tests

Specific procedures for conducting acid-base accounting (ABA) tests are compiled in

Mills (1998a; 1998b). Although a few tests produce a single value that can be used to indicate

the likelihood for acid generation (Section 4.3.1.2), most static tests determine separate values

for the acid generating potential (AP) and acid neutralizing potential (NP) of a sample. These

values, expressed in units of tons of CaCO3 equivalent per kiloton of material, are used together

to indicate whether a sample has a stochiometric balance that favors net acidity or net alkalinity. 

In general, determinations of acid generating potential are relatively straightforward. This is not

true of tests to measure neutralizing potential. The problem stems from the widely variable

solubilities and reaction rates of minerals that have the potential to neutralize acidity (e.g.,

carbonates vs. silicates), the relative differences in aggressiveness of the various methods used to

determine neutralization potential, and the different titration endpoints employed by each test

(e.g., Mills, 1998a). Studies in which the neutralizing potential of a sample was determined

using different methods concluded that the NP value is highly sensitive to test methodology

(e.g., Lapakko, 1994). Consequently, it is important that any program established to test wastes

and materials prior to or during operation use a single test method to ensure that the program

produces data that are internally consistent.

4.3.1.1.1 Methods to Determine Acid Generating Potential

Acid generating potential is determined from the sulfur content of a sample (expressed in

weight percent). This value is converted to acid generating potential (AP) by multiplying by a

factor of 31.25 that is derived from the molar stoichiometry of the oxidation and neutralization

reactions. The conversion factor assumes that all reported sulfur occurs as pyrite, that pyrite is

completely oxidized to sulfate and ferric hydroxide, and that hydrogen ions produced in the

oxidation reaction are neutralized by CaCO3. Acid generating potential is reported in kilograms

of CaCO3 equivalent per metric ton of sample (also expressed in units of metric tons of CaCO3

equivalent per kilotonne of material). 

Samples typically contain sulfur in more than one form, not all of which are capable of

generating acidity. The sulfur speciation tests of Sobek et al. (1978) are the most commonly

used methods to determine sulfur content. Alternative methods include the hydrogen peroxide

method (O’Shay et al., 1990) and reactive sulfur tests. 

Sobek et al. (1978) describe procedures to determine the total sulfur, HCl-extractable

sulfate sulfur, HNO3-extractable sulfide sulfur, and organic sulfur contents of a sample. The

tests require a sample crushed to particle sizes smaller than 60 mesh (0.25 mm), which is split

into three parts that are analyzed for total sulfur using a Leco sulfur analyzer. One split is left

untreated and provides a measure of the total sulfur content of the sample. A second split is

leached with HCl and a third split is leached with HNO3. Acid-extractable sulfate sulfur (e.g.,

gypsum and anhydrite) is computed from the difference between the total sulfur contents of the

untreated and HCl-treated splits. Acid-soluble sulfide sulfur (e.g., pyrite) is computed from the

difference between the total sulfur contents of the HCl-treated and HNO3-treated splits. 

Nonextractable organic sulfur is computed as the total sulfur content of the HNO3-treated split. 
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The test methods have disadvantages that include the potential removal of highly reactive sulfide

by HCl and the potential nondetection of sulfide that is slow to oxidize under experimental

conditions, but which may form acid in the environment (BC AMD Task Force, 1989). 

It is important to recognize that sulfur speciation tests like those described above do not

distinguish acid-insoluble sulfates, such as barite or jarosite, which will report as sulfide sulfur. 

As a result, samples containing significant quantities of these minerals will appear to have more

sulfide sulfur than they actually do. Although acid-insoluble sulfates will not oxidize to produce

acid, some of these minerals (e.g., jarosite, alunite, and melanterite) may dissolve, hydrolyze,

and generate acidity (Carson et al., 1982; Mills; 1998a). Mills (1998a) states that whole-rock

barium concentrations can be used to correct sulfide sulfur determinations when barite is present. 

However, barium also may be present in common alteration phases such as potassium feldspar

and biotite (Deer et al., 1992). Consequently, caution must be used when applying a barium

correction of this type. As pointed out by Mills (1998a), it is rarely acknowledged that each step

in the sulfur speciation tests introduces analytical error; these errors are cumulative.

Table C-1. Summary of Commonly Used Static Test Methods

Static Test Method Reference Comments

Sobek Sobek et al. (1978) AP uses sulfur speciation and Leco analyzer.

NP uses fizz test and heated HCl that dissolves

carbonates and most silicate minerals; NaOH titration

endpoint of 7.0. This is an aggressive test that

provides “best case” values.

Modified Sobek NP Lawrence and Wang

(1997)

NP uses fizz test and HCl at ambient temperature

that dissolves carbonates and reactive silicate

minerals; NaOH titration endpoint of 8.3. Less

aggressive test due to use of ambient temperature

acid. Lapakko (1992) suggested that the alkaline

titration endpoint may lead to overly optimistic

estimates of NP.

Sobek NP Siderite

Correction

Skousen et al. (1997) NP uses fizz test and heated HCl; hydrogen peroxide

added prior to titration to oxidize ferrous iron from

dissolved siderite. Yields less alkaline NP than

standard Sobek method when siderite is abundant.

BCRI Initial Duncan and Bruynesteyn

(1979)

AP uses total sulfur by Leco furnace or wet chemistry.

NP uses H2SO4 added to pH 3.5 at ambient

temperature that dissolves carbonates and possibly

limonite and chlorite; gives “most likely case” values.

Lapakko NP Lapakko (1994) NP uses H2SO4 added to pH 6.0 at ambient

temperature for up to 1 week that dissolves

carbonates; gives “worst case” value.

Net Acid Generation (NAG) Miller et al. (1997) Crushed sample is boiled with hydrogen peroxide

then titrated to pH 4.5 with NaOH. NAG value,

expressed in units of kg H2SO4/tonne, provides

indication of potential for net acidification.
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Table C-1. Summary of Commonly Used Static Test Methods

Static Test Method Reference Comments

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Carbonate Carbon ASTM (1997) Samples are either dissolved in acid or combusted

and the amount of CO2 gas evolved is measured and

converted to CaCO3 equivalent. 

Paste pH Sobek et al. (1978)

Page et al. (1982)

Sample is mixed with water and pH measured by

meter. pH value provides indication of potential for

net acidification.

Summaries include information from Mills (1998a and 1998b).

The hydrogen peroxide method (O’Shay et al., 1990) has been used to determine the

pyrite content of coal mine wastes. In this test, a sample crushed to particle sizes smaller than

150 microns is soaked in HCl for two hours to remove carbonate minerals. The treated sample is

mixed with hydrogen peroxide and pH is monitored at intervals of 1 to 2 minutes. Curves of pH

versus time are compared to curves generated from synthesized standards. Potential acidity is

determined using the conversion factor of 31.25.

Reactive sulfur tests treat sample splits with hydrogen peroxide to oxidize sulfide

minerals to sulfates. The sulfate content of the peroxide leach solution is used to determine the

amount of reactive sulfur, which is converted to potential acidity using the conversion factor of

31.25. Producing accurate results with this test method, which is not widely used, requires strict

temperature control (Hinners and SAIC, 1993), because pyrite decomposition is exothermic.

4.3.1.1.2 Methods to Determine Acid Neutralizing Potential

A variety of procedures are used to determine the neutralizing potential of a sample

(Table C-1). In general these methods involve reacting a sample with a known quantity of acid,

determining the base equivalent amount of acid consumed by the sample, and converting

measured quantities to neutralization potential (NP), which is expressed in units of tonnes of

CaCO3 equivalent per kilotonne of material (Mills, 1998a).

The Sobek and Modified Sobek methods, which are perhaps the most widely used

procedures, both use a “fizz test” to determine the quantity of acid that will be used in the NP

determination. In essence, the test consists of adding a small amount of acid to a small quantity

of test sample and subjectively assigning a fizz rating of “no”, “slight”, “moderate”, or “strong”

to the resulting effervescence. Each of these ratings corresponds to a different quantity and/or

normality of acid that is added to the sample (Sobek et al., 1978). Lawrence and Wang (1996)

and Skousen et al. (1997) conducted studies to examine the effects of assigning different fizz

ratings when determining Sobek NP values for a variety of samples. Their results showed that

NP values could differ by amounts that varied from a few percent to a few hundred percent for

one or two category changes in fizz rating.
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Neutralization potential (NP) by the Sobek and Modified Sobek methods is determined

by treating the sample with an excess of hydrochloric acid and then titrating with sodium

hydroxide to determine the amount of unconsumed acid. In the original test procedure outlined

by Sobek et al. (1978), the sample is reacted with hot acid and titrated to a pH of 7. In the

Modified Sobek procedure outlined by Coastech Research (1989), the sample is agitated with

acid at room temperature for 24 hours and titrated to a pH of 8.3 (cf., Lawrence and Wang,

1997). In both cases, the amount of titrated base is converted to a calcium carbonate equivalent

in units of kilograms per metric ton of sample (also expressed in units of metric tons of CaCO3

equivalent per kiloton of material). 

The Sobek and Modified Sobek tests determine the maximum amount of neutralization

potential available in a sample, but do not predict the rate of neutralization nor indicate the pH to

which a sample can neutralize acidity. Lapakko (1992) showed that both tests provided a fairly

reliable estimate of NP for samples composed of quartz, alkali feldspar, and mica, but

overestimated NP in samples with abundant calcic feldspar, chlorite, clay, pyroxene and olivine. 

Similar conclusions were drawn by Skousen et al. (1996) who showed that NP estimates for a

single sample could vary by an order of magnitude depending on sample mineralogy and

digestion method. Other criticisms of the Sobek and Modified Sobek methods (see Lapakko,

1991; 1992 and Hinners and SAIC, 1993) include: 1) the small particle size used in the tests may

produce unrealistically high values for NP, 2) hot acid which is mixed with water and heated to

boiling in the Sobek method may increase analytical scatter, 3) hot acid may digest siderite (iron

carbonate) and clay minerals that increase NP values but provide little alkalinity, 4) NP may be

overestimated because pH is back-titrated to values of 7.0 or 8.3, not 6.0 which is a typical water

quality standard, and 5) NP may be overestimated if metal hydroxides precipitate during the

addition of the sodium hydroxide base.

The BCRI Initial test (Duncan and Bruynesteyn, 1979; Bruynesteyn and Hackl, 1984)

and Lapakko NP test (Lapakko, 1994) both use sulfuric acid at ambient temperature to determine

neutralizing potential; neither test requires a subjective fizz test rating. In both tests, the sample

is suspended in water and acid is titrated into the suspension until a stable, pre-determined pH

value is achieved. The BCRI Initial test uses a titration endpoint of 3.5, whereas the Lapakko

NP procedure uses a titration endpoint of 6.0. The volume of titrated acid is used to compute a

value for acid consumption, which is expressed in units of kilograms per tonne. Neither test is

particularly aggressive in dissolving minerals in addition to the carbonates. Nevertheless, the

higher titration endpoint of the Lapakko procedure makes it the most conservative (lowest NP

estimate) of the static NP test procedures. Lapakko (1992) showed that the BCRI test

overestimated NP for samples containing significant siderite (iron carbonate).

Carbonate analysis may be used in conjunction with neutralizing potential tests to

determine the amount of neutralizing potential that is likely to react quickly with acid formed

through sulfide oxidation. There are several methods to analyze carbonate carbon. In one

method, a sample is digested in acid in a sealed chamber. Carbon dioxide (CO2) gas evolved by

reaction is absorbed into a solution and measured using coulometric titration (Crock et al., 1999).

Alternatively, the sample can be combusted, with carbon analyzed using a Leco or similar
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furnace (e.g., ASTM E-1915-97). In both cases, the carbonate content of the sample is

determined from the amount of CO2 gas evolved, with the result converted to CaCO3 equivalent. 

The titration test offers the advantage of determining the carbonate content of samples with a

wide range of values but can suffer interference if samples contain significant quantities of

sulfide minerals. Combustion tests with Leco analysis should not be used if samples contain

significant pyrrhotite (Fe1-xS), because this mineral will react to form sulfur dioxide gas that

interferes with the Leco analyzer (BC AMD Task Force, 1989). Combustion tests also provide a

measure of total carbon (including organic carbon) unless pretreatment steps are taken to remove

this component.

The alkaline production potential test was developed for use by the coal mining industry. In this

method, a sample crushed to minus 23 microns is mixed with HCl and allowed to react for two

hours at room temperature. The mixture is then titrated to pH 5.0. Although this method

reduces dissolution of less reactive carbonate minerals (e.g., siderite), it may not permit reaction

of all of the buffering carbonates present in the sample (Coastech Research, 1989). 

4.3.1.2 Static Tests that Produce a Single Indicator Value

Two test procedures have been developed that provide a means for quickly indicating

whether a sample is likely to have a stoichiometric balance that favors acid production. The net

acid generation (NAG) test (Miller et al., 1997) uses a peroxide solution to oxidize sulfide

minerals to sulfates. The oxidation process produces acid which reacts with alkaline minerals in

the sample. Upon complete reaction, the solution is titrated to pH 4.5 using NaOH. The volume

of titrated NaOH is used to compute a NAG value, which is expressed in units of kg of H2SO4

per metric ton of material.

Paste pH is a simple and inexpensive method to indicate the presence of reactive

carbonate or readily available acidity. In this test, powdered rock and water are mixed in a

specific ratio to form a paste. The pH of the paste is determined using a pH meter and pH

reference electrode assembly. The test offers no indication of the relative proportions of

acidifying or neutralizing components in a sample (BC AMD Task Force, 1989).

4.3.1.3 Interpreting Static Test Results 

Static test results provide a preliminary indication of whether a sample is likely to

produce acidic drainage in the environment. These tests do not, however, provide any data

regarding when acidification may occur or the rates at which acid generation and neutralization

reactions will proceed. As such, they are useful only for screening samples for their potential

behavior. It should be kept in mind that most static tests are conducted using crushed or

pulverized samples that may have particle sizes significantly smaller than materials as they will

be disposed of. This can significantly change the chemical availabilities of reactive minerals as

described in Section 4.2. In addition to these factors, interpretations should incorporate

knowledge of sample mineralogy. 
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Static test results are generally interpreted within an empirically developed framework. 

Interpretations are based on the net neutralization potential and the neutralizing potential. The

net neutralizing potential (NNP) is defined as the difference between the acid neutralizing

potential (NP) and acid generating potential (AP) of a sample. It is computed by subtracting the

latter from the former (NP-AP) when both are expressed in units of kilograms of CaCO3

equivalent per metric ton of material (or metric tons per kiloton). The neutralizing potential ratio

(NPR) is the ratio of acid neutralizing potential to acid generating potential (NP/AP) and also is

computed from static test results when both are expressed in units of kilograms of CaCO3

equivalent per metric ton of material (or metric tons per kiloton). 

Many static test interpretations use a value for acid generating potential computed from

the total sulfur content of a sample because it provides the most conservative (highest AP value)

measure of acidification potential. In contrast, sulfide sulfur values (or values of total sulfur

minus sulfate sulfur) provide more realistic estimates of acid generating capability because these

analyses do not report sulfur in forms that are not acid generating (e.g., gypsum). The Canadian

metal mining industry has adopted the use of sulfide sulfur as its standard method to compute

acid generating potential (Mills, 1998a). It should be recognized that the assumptions inherent

in the derivation of the stoichiometric conversion factor lead to additional uncertainty, since the

factor could be significantly greater or less than 31.25 (BC AMD Task Force, 1989; see Section

4.3.1.1.1). In fact, some workers advocate using a value of 62.5 (Brady et al., 1990). 

The values given in Table C-2 provide general guidelines for interpreting static test

results, but they should not be interpreted as definitive values. Instead, the values should be

viewed in light of the sulfur content of the sample, the aggressiveness of the test method used to

determine neutralizing potential, sample mineralogy and expected ambient conditions. Because

exceptions to these guidelines can and do occur, kinetic tests should be conducted to confirm the

static test results. As always, operators are encouraged to communicate with state and federal

regulators regarding their preferred method to interpret these test results.

In both schemes shown in Table C-2, there are “gray” areas where static acid-base

accounting tests point to uncertainty. Under the Robertson and Broughton scheme, the gray area

exists where NNP is between -20 and +20 tonnes/kilotonne and NPR is between 1 and 3. In the

scheme of Price et al., uncertainty is present where NPR is between 1 and 4. Samples falling

into the uncertain areas should be tested kinetically (section 4.3.2) to determine their acid

generating capability. Regardless of their acid generating character, representative samples from

all geochemical groups should be tested for metals mobility using one of the leach tests

described in section 4.4.

4.3.1.4 State Recommendations

The States comprising EPA Region 10 presently have not established formal regulatory

guidelines for conducting static tests of mine wastes and materials. The State of Nevada (NV

DEP, 1990) recommends use of the Sobek et al. (1978) method to determine neutralization

potential and either the Sobek et al. (1978) or the peroxide method (presumably O’Shay et al.,
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Table C-2. Suggested Guidelines for Static Test Interpretation

Guidelines from Robertson and Broughton (1992)

Potentially Acid

Generating

Uncertain Behavior * Potentially Acid

Neutralizing

NNP < -20 tonnes/kilotonne > -20 to < +20

tonnes/kilotonne

> + 20 tonnes/kilotonne

NPR < 1 1 to 3 > 3

* Samples exhibiting uncertain behavior should be tested kinetically.

Guidelines from Price et al. (1997)

Paste pH NPR Potential for

ARD

Comment

Sulfide-S <0.3% >5.5 --- None No further ARD testing required provided

there are no other metal leaching concerns. 

Exceptions: host rock with no basic minerals,

sulfide minerals that are weakly acid soluble.

Sulfide-S >0.3% <5.5 <1 Likely Likely to be ARD generating.

1 - 2 Possibly Possibly ARD generating if NP is insufficiently

reactive or is depleted at a rate faster than

that of sulfides.

2 - 4 Low Not potentially ARD generating unless

significant preferential exposure of sulfides

occurs along fractures or extremely reactive

sulfides are present together with

insufficiently reactive NP.

>4 None No further ARD testing required unless

materials are to be used as a source of

alkalinity.

1990) to determine acid generating potential. Those samples in which NP exceeds AP by 100

percent (NP/AP >2) are considered non-acid generating and do not require additional testing

(NV DEP, 1990). Samples that do not meet this criteria should be tested kinetically. The State

of New Mexico recommends determining the acid potential of representative samples using total

sulfur and the neutralization potential using either the ABA, modified ABA, BCRI, or alkaline

production methods (NMED, 1996). Kinetic tests are suggested for those samples with NP/AP

ratios less than 3. Samples with ratios exceeding 3 are considered non-acid generating. The

states of Nevada and New Mexico illustrate that states may view different test methodologies as

acceptable. Applicants should check with state agencies to determine whether they have

preferences that may not be codified.
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4.3.2 Kinetic Tests

Kinetic test procedures are designed to accelerate the natural weathering process in order

to provide information about the rates of acid consumption and acid production over time. A

variety of kinetic test methods are available, including conventional and modified conventional

humidity cells, SRK humidity cells, soxhlet extractions, column leach tests, shake flask

extractions, modified B.C. Research tests, simulated environment studies, and field lysimeter

tests; humidity cells and columns are most commonly used by the mining industry. According to 

Lapakko (1991), there is no single test that produces all of the chemical information needed to

evaluate all mine wastes under all conditions of disposal. Most of the kinetic testing procedures

are complex, time-consuming, and require considerable operator skill to produce consistent

results. 

4.3.2.1 Kinetic Test Methods 

The various kinetic tests described below are similar to one another in that a sample is

subjected to periodic leaching, the leachate is collected and analyzed, and rates of acid

generation, metals release, and neutralization capacity depletion are computed. The methods

differ in the amount of sample used in the test, the particle size of the tested material, test

conditions (lab vs. field), and test duration. Although not specifically stated in most procedures,

it is typical for splits of the starting sample and final leached product to be tested for static acidbase properties and total metals; mineralogical analyses also should be conducted on these

samples because these data can provide important constraints to assist the interpretation of test

results (Mills, 1998c). 

4.3.2.1.1 Conventional and Modified Conventional Humidity Cells

The conventional humidity cell (Sobek et al., 1978) is a bench-scale test that uses a

comparatively small amount of sample (200 to 300 g) crushed to particle sizes smaller than 2

mm. A split of the sample is analyzed for metals and other constituents to assist in the

evaluation of water quality from the tests. The sample is placed in a sealed plastic box and dry

air is passed over the sample for 3 days, followed by moist air for 3 days. Every seventh day, the

sample is flushed with a specified volume of water. To simulate the composition of regional

acidic rain, the pH of the water may be adjusted to slightly lower pH. The leachant is collected

and analyzed for sulfate, pH, acidity, alkalinity, and electrical conductivity. This 7-day process

is repeated for 10 weeks, although some samples may require a longer reaction period (Coastech

Research, 1989). Test durations of 20 weeks are used commonly in the metal mining industry

(see discussion in Section 4.3.2.2).

The modified conventional humidity cell designed by Lawrence (1990) uses a bigger

sample size and larger volume of water for the flush cycles. The test is conducted in a manner

generally similar to the Sobek method.

ASTM procedure D5744-96 (ASTM, 1998), which was designed specifically for mining

wastes and materials, uses a modified column as a humidity cell. The test is conducted on a
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kilogram of sample crushed to particle sizes smaller than 6.3 mm. The test is run for 20 weeks in

a manner similar to the Sobek method, with 3 days of dry air, 3 days of moist air, and a weekly

flush with 0.5 or 1.0 liter of water. The procedure includes provisions for pre-leach and postleach mineralogical and chemical characterization of the solid sample and directions for

preparation and use of an optional bacterial (T. ferrooxidans) spike.

Few data are available to document the reproducibility of humidity cell data (Mills,

1998c). Experiments designed to test the validity of conventional humidity cell results for

tailings and waste rock samples are summarized in Lapakko (1991; 1992). In general, the

conventional humidity cell is able to indicate many of those samples that become acid producing. 

However, some validation tests noted indefinite pH trends that were difficult to interpret and

some tests failed to predict acid generation, suggesting that these experiments should have

continued for longer durations to permit depletion of the neutralizing capacity. Criticisms of the

conventional humidity cell are given in Broughton and Robertson (1992). These authors argue

that the small particle size used in the tests masks the influence of particle size on acid

generation, making them unsuitable for waste rock samples; however, the particle sizes used in

the tests are similar to tailings. Moreover, they point out that the complete sample flush may

affect the development of local low pH and disrupt the natural storage and flushing of oxidation

products. Other workers, however, feel that the small particle size is not a limiting factor since

the most highly reactive products in waste rock piles typically occur in the smaller size fractions

(Hinners and SAIC, 1993). For existing waste rock dumps, Price (1997) recommends using only

the sub-2 mm size fraction of (i.e., crushing larger clasts should be avoided) in humidity cell

tests. For proposed waste rock dumps, Price (1997) recommends crushing drill core material to

80% less than 6 mm. Clay-rich samples can pose problems for humidity cell testing because the

clay particles can be easily lost during weekly flushing and they can clog filters used to prevent

the loss of fine materials (Mills, 1998c). 

4.3.2.1.2 SRK Humidity Cells

Broughton and Robertson (1992) present a modified humidity cell (termed the SRK

humidity cell) designed to test coarse waste rock samples. This test uses material crushed to

sizes smaller than 10 cm which is placed into a cylindrical column with a diameter of 30 cm and

height of 45 cm. Humid air is cycled constantly through the cell. Flush water is introduced at

several points along the upper surface of the waste rock so that it percolates downward along

discrete pathways. The volume of flush water approximates (per unit area) conditions

encountered in the field. The cells can be stacked to allow leach water from one test cell to be

used as flush water in an underlying cell. 

The SRK design eliminates complete flushing of the oxidation products, permitting local

areas of low pH to develop within the cell (Broughton and Robertson, 1992). The coarse size

fraction more closely approximates the separation distance between acid-producing and acidneutralizing minerals in waste rock samples.
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4.3.2.1.3 Soxhlet Extractions

Soxhlet reactors recirculate water or other fluids through a sample to simulate conditions

of weathering. The method of Sullivan and Sobek (1982) uses distilled water at 25°C to leach a

sample over a period of six weeks, although the test duration can vary. A technique described by

Renton et al. (1988) uses as the leach material a pulverized coal waste sample that has been

oxidized in an oven. The sample is leached in a soxhlet reactor with distilled water at 85°C and

the leachate is analyzed for water quality parameters. The sample is returned to the oven for

additional oxidation prior to the next leach cycle. The oxidation-leaching cycle is repeated 5

times. 

Soxhlet extractions require sophisticated equipment and considerable operator skill,

especially for the Renton et al. procedure. Evaluations of the Sullivan and Sobek (1982) method

by Coastech Research (1989) indicate that it may provide reliable results for tailings samples. 

The aggressive oxidation of samples and elevated leaching temperatures used in the Renton et al.

method tend to overestimate the acid producing capability of a sample by accelerating the

dissolution of carbonate minerals (Bradham and Caruccio, 1990).

4.3.2.1.4 Column Tests

Column test procedures have not been standardized (Mills, 1998c). Consequently, they

are highly flexible tests that permit a range of column designs, test material characteristics, and

flow rates. Column tests can be conducted in a manner similar to conventional humidity cells,

but they can also be run in an “upflow” mode to simulate subaqueous disposal or as subaerial

columns without forced oxygenation (i.e., the top of the column is open but air is not forced

through the sample) (Mills, 1998c). Columns, which typically have diameters of 15 cm and

lengths of up to 2 m, can be constructed with larger diameters and lengths to accommodate larger

sample sizes (10 kg to 3 metric tons; Broughton and Robertson, 1992). Particle sizes up to 2 cm

are commonly used in these tests. Materials can be inoculated with bacteria or stratified with

neutralizing materials (for example, limestone) to test disposal options.

Subaerial columns are used to simulate the effects of precipitation infiltration into and

drainage from materials that are exposed to the atmosphere. A fixed amount of water may be

added to the column on a regular basis or the amount may be varied and added irregularly to

simulate seasonal variability (Mills, 1998c). Moreover, water may be added to specific portions

of the column surface to promote flow along preferred pathways, which allows oxidation

products to accumulate on particle surfaces within the column (Mills, 1998c). 

Subaqueous columns are used to simulate water infiltration into and drainage from

materials that are stored under a water cover. To simulate seepage to ground water, columns can

be constructed to permit downward displacement of pore waters by supernatant water (Mills,

1998c). They also can be constructed to allow slow upward movement of deoxygenated water in

a manner that simulates submarine disposal.
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Experiments designed to determine the validity of column tests for tailings and waste

rock samples are summarized in Lapakko (1991; 1992). Several of these studies (e.g., Doepker,

1989) concluded that pyrite oxidized more rapidly in columns that remained unsaturated between

flushes, producing lower pH leachate than saturated columns. In general, column tests appear to

distinguish potentially reactive materials from benign materials, but the leachant compositions

may not reflect what occurs under natural settings (Doepker and O’Connor, 1990).

4.3.2.1.5 Shake Flask Extractions

Also termed batch reactor tests, shake flask tests utilize a split of powdered sample

immersed in distilled water that may be inoculated with bacteria. The flask is sealed and placed

on a shaker table where it is vibrated for a period of days to weeks. Samples are removed

periodically and analyzed to determine the sulfate content, pH and other water quality

parameters.

The shake flask test is relatively simple and inexpensive. However, for long duration

tests, water may need to be added to maintain volume and submersion of the sample may inhibit

oxidation of reactive sulfides (BC AMD Task Force, 1989). Interpretation of test results is quite

complex if water has been added periodically. 

4.3.2.1.6 Field Tests

Field lysimeter tests are conducted using sample quantities that vary from barrel-scale to

piles. The tests can be conducted for protracted periods (years) under natural climatic

conditions. In cases where samples have a small to moderate amount of neutralization potential,

long test durations are required to overcome the effects of neutralization and the lag period that

precedes bacterial oxidation (Lapakko, 1991). Test piles are typically equipped with lysimeters

or set atop impermeable liners to facilitate collection of drainage samples and are constructed in

a manner similar to actual or proposed waste rock or tailings piles. Drainage volumes and

concentrations can be used to calculate the mass release rates of metals per unit mass of waste.

A major advantage of field tests is their conduct under the environmental conditions at

the disposal site, which provides more realistic estimates of water quality and the rates of acid

generation and neutralization than bench-scale lab tests (Price, 1997). In addition, they allow

control options, such as limestone addition (Humphreys, 1990), to be tested under natural

conditions. However, it is critical that the tests be conducted for durations of sufficient length to

smooth the effects of short-term climatic variations. Consequently, their long duration makes

these tests difficult to use, especially for evaluating proposed actions. 

4.3.2.2 Interpreting Kinetic Test Results 

The interpretation of kinetic test results, for which accepted criteria are generally lacking,

can range from relatively straightforward to extremely difficult (Ferguson and Erickson, 1988;

Price, 1997; Mills, 1998d). All interpretations should be based on knowledge of sample

mineralogy, static test data, particle size characteristics, and water flow (Mills, 1998d). Scaling
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issues are a significant obstacle when using bench-scale kinetic test results to quantitatively

estimate acid generation in waste rock and tailings piles. Included are the effects of grain size

and reactive surface area, infiltration rates, and flushing rates and volumes (see comments in

Hinners and SAIC, 1993). 

Most investigators use temporal trends in leachate quality, including pH, sulfate, acidity,

alkalinity, and trace metals, to identify the progression of the acid mine drainage process (e.g.,

Ferguson and Erickson, 1988; Lapakko et al., 1995; Price, 1997; Mills, 1998d). Because trends

in leachate composition reflect changing sample mineralogy and geochemical equilibrium

conditions, they must be interpreted cautiously. Equilibrium chemical speciation programs,

such as MINTEQA2 (Section 5.2.2), can be used to identify the precipitation/dissolution

reactions that are likely to control leachate composition. It is important to keep in mind that labscale kinetic tests are specifically designed to accelerate the natural weathering process. 

Consequently, these tests cannot be used to determine when materials may begin to generate acid

in the environment (only that they will or will not), and they generally will produce leachates

with higher metal concentrations than would be produced naturally (Mills, 1998c). For most

bench-scale tests, samples are considered strongly acid generating if leachate pH falls below 3;

acid generating with some neutralization occurring if pH is between 3 and 5; and not

significantly acid generating (or generated acid is overwhelmed by excess alkalinity) if solution

pH exceeds 5 (BC AMD Task Force, 1989; Humphreys, 1990).

Sample mineralogy plays a pivotal role in controlling leachate quality (Mills, 1998d). 

For samples lacking sulfate minerals, the production of aqueous sulfate may be used to monitor

the sulfide oxidation process. In contrast, when gypsum or other soluble sulfate minerals are

present, their dissolution will provide aqueous sulfate that can mask sulfate produced by sulfide

oxidation. In some cases, high aqueous sulfate concentrations produced by gypsum dissolution

may delay the onset of sulfide oxidation in kinetic tests (Mills, 1998d). Test samples collected

from existing waste piles may contain previously formed oxidation products that dissolve at

varying rates to contribute metals to kinetic test leachates. Hydrolysis of these metals can lead to

reduced pH. Depending on reaction kinetics, secondary mineral dissolution is likely to overprint

the effects of sulfide oxidation, which complicates calculations of sulfide oxidation rates (Mills,

1998d). Price (1997) provides a list of equations that can be used to interpret laboratory kinetic

tests.

Whether kinetic test samples may eventually begin to produce acidic leachates depends

on the proportions of acid generating and acid neutralizing materials, their relative dissolution

and reaction rates, and the particle size characteristics of the test materials. Kinetic test duration

is a critical issue (Price, 1997). Kinetic tests must be conducted for a period of time that is

sufficient to permit the dissolution of neutralizing minerals and accumulated oxidation products

and to overcome the lag-time that precedes the onset of bacterial oxidation. Although 20-week

test lengths are common in the metal mining industry, there is a growing trend toward longer test

times. For example, Price (1997) recommended minimum test durations of 40 weeks and Mills

(1998c) reported that test lengths commonly exceed 104 weeks in western Canada. In long-term

studies reported by Lapakko et al. (1998), some samples did not begin to produce acidic drainage

until more than two years into the kinetic tests. Particle size also strongly influences kinetic test
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results. The reduced particle sizes used in many bench-scale tests enhance reactivity by

liberating sulfides enclosed in silicate minerals (e.g., pyrite enclosed in quartz; Broughton and

Robertson, 1992; Lapakko et al., 1998; Mills, 1998e). In coarser samples, these sulfides would

not be exposed to oxidation. Moreover, smaller particle diameters increase the total surface area

of acid generating and acid neutralizing minerals exposed to reaction which, in turn, affects

reaction rates and drainage quality (Lapakko et al., 1998; Mills, 1998c). 

Finally, it is important to consider that differences between lab test conditions and the

natural environment are likely to complicate extrapolation of kinetic test results. Differences

between lab and ambient atmospheric temperature, lab wetting cycles and natural precipitation

frequency, and complete flushing flows in the lab vs. incomplete or channelized flow in actual

waste piles are cited by Mills (1998c) as factors that require consideration. 

4.3.2.3 State Recommendations 

The states comprising EPA Region 10 presently have not promulgated formal guidelines

that cite specific kinetic procedures. The State of Nevada accepts kinetic testing methods that

include shake flask extractions, soxhlet extractions, conventional humidity cells, column tests,

and field tests (NV DEP, 1990). Although kinetic tests are required for samples of spent ore,

tailings, and waste rock, the State does not provide guidelines for the interpretation of test

results. The State of New Mexico recommends the use of humidity cells and columns for most

kinetic test applications, but will accept soxhlet extraction test results as appropriate (NMED,

1996). The State recommends shake flask extractions for simulating closure conditions that

require underwater storage (NMED, 1996). The State does not provide criteria by which to

interpret kinetic test results. Applicants should check with state agencies to determine whether

they have preferences that may not be codified.

4.3.3 Other Methods

In addition to laboratory analysis of environmental samples, insight into the potential for

certain geologic materials to become acid generating can be gained through empirical studies of

pre-mining water quality, alteration history (including weathering), mineralogy, and water

quality in analogous mined terranes. These types of studies may help to overcome issues related

to sample representativeness and the applicability of laboratory conditions to the natural

environment (Plumlee et al., 1999). Plumlee and coworkers have shown that geologic features

(e.g., deposit and alteration mineralogy), hydrologic setting, climate (e.g., rainfall and

evaporation), and mining methods affect drainage composition at hard rock mines. Although

empirical field studies can be used to anticipate problems before they occur and to guide

laboratory investigations, they should not be used as a basis for quantitative predictions of

drainage quality from particular mines, dumps or impoundments. 

4.3.4 Mathematical Models

Neither static nor kinetic test results provide the types of data that determine

unequivocally the potential for acid generation from waste rock and tailings piles. Instead, test
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results must be extrapolated to longer time frames and different environmental conditions and

scaled to account for the differences in waste volumes, particle sizes, particle separation

distances, infiltration rates, flushing rates, and flushing volumes between laboratory test samples

and waste deposits. Mathematical models can help to bridge this gap and can help planners

determine the potential effects of waste rock and tailings piles runoff.

Empirical models of acid generation utilize trends observed in test results to extrapolate

future conditions, typically using “best-fit lines” through test data points (BC AMD Task Force,

1989). The accuracy of an empirical model, which is by definition a site-specific model,

depends on the quality of the test data. Major sources of uncertainty include differences in

particle-size distributions between test materials and actual waste materials and lack of model

calibration to conditions as they will exist in the waste disposal setting (BC AMD Task Force,

1989).

Theoretical or deterministic models solve a series of equations that represent different

physical or chemical aspects of the acid generation process in order to predict the temporal

evolution of acid generation (see Perkins et al. [1995] for a review of the application of

geochemical models to predictions of acid generation). Models include the Reactive Acid

Tailings Assessment Program (RATAP) model (SENES and Beak, 1986; 1988); the mine

tailings oxidation (MINTOX) model (MEND, 1997); the sulfide oxidation model of Davis and

Ritchey (1986); and the MINEWALL model (MEND, 1995). RATAP was developed to assess

acid generation and ground water quality in fine-grained pyritic tailings. MINTOX can be used

to predict the kinetic behavior of sulfide oxidation within mine tailings impoundments and

simulate the speciation and transport of oxidation products through tailings and into downstream

aquifers. The Davis and Ritchey model determines an approximate analytical solution that

allows a user to evaluate the amount of time required for oxidation of all material in a mass of

waste and estimate the amount of time that materials can pose a threat in the environment. The

MINEWALL model can estimate water chemistry continuously through operational and closure

phases of a mining operation.

Uncertainty is introduced into theoretical models by an incomplete understanding of the

system which is being modeled or through use of simplifying assumptions (BC AMD Task

Force, 1989). In general, theoretical models may fail to properly describe fluid transport through

constructed waste piles, accurately predict thermal gradients that may arise due to the oxidation

process, and correctly determine the transport of oxygen and reaction products in

compositionally and physically heterogeneous wastes (BC AMD Task Force, 1989; Nicholson,

1992).

4.4 Leaching Procedures

Spent ore, waste rock, or tailings materials that are exposed to the environment can

potentially contribute metals or other contaminants to the environment. Metals can be leached

from geological materials even under neutral conditions, but it is accelerated by materials that

generate acid as a consequence of sulfide oxidation. Consequently, a variety of leaching tests are
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used to determine which constituents in waste materials are potentially mobile under the

expected environmental conditions. 

4.4.1 U.S. EPA Procedures

EPA has developed three leach test procedures. Of these, the Synthetic Precipitation

Leaching Procedure (SPLP) test and Toxicity Characteristic Leaching Procedure (TCLP) test are

the most widely applied by the mining industry. The SPLP test is most applicable to metals

removal from mining wastes and materials.

4.4.1.1 EP Toxicity Test

The Extraction Procedure (EP) Toxicity Test (EPA Method 1310A) was developed to

determine whether a particular waste material exhibits the characteristics of a hazardous waste. 

The method, which has been replaced by the TCLP test for regulatory purposes, is outlined in

U.S. EPA (1986a), with the most recent version of the experimental procedure dated July 1992,

revision 1. The method uses an extraction fluid composed of acetic acid diluted to pH 5.0 ± 0.2. 

Solid samples of approximately 100 g are crushed to sizes smaller than 9.5 mm and placed into

an extraction bottle; special procedures are used for mixed solid/liquid waste. A 16:1 weight

ratio of extraction fluid:sample solid is added to the bottle, which is agitated for 24 hours. 

Following extraction, the leachate is filtered and analyzed for metals.

4.4.1.2 Toxicity Characteristic Leaching Procedure Test

The Toxicity Characteristic Leaching Procedure (TCLP) Test (EPA Method 1311;

ASTM Method D5233) was designed to evaluate the mobility of inorganic and organic

constituents in liquids, solids, and mixed wastes in a sanitary landfill. The method is outlined in

U.S. EPA (1986a), with the most recent version of the experimental procedure dated July 1992,

revision 0. For non-alkaline materials, the method uses an extraction fluid composed of acetic

acid diluted to pH 4.93 ± 0.05. For alkaline materials, the method uses an extraction fluid

composed of acetic acid diluted to pH 2.88 ± 0.05. Samples containing volatile organic

components are leached using a zero head space tumbler and the pH 4.93 extract fluid. For nonvolatile materials, samples of approximately 100 g are crushed to sizes smaller than 9.5 mm and

placed into an extraction bottle. A 20:1 weight ratio of extraction fluid:sample solid is added to

the bottle, which is agitated for 18 ± 2 hours. Following extraction, the leachate is filtered,

preserved with nitric acid, and analyzed for metals.

4.4.1.3 Synthetic Precipitation Leaching Procedure Test

The Synthetic Precipitation Leaching Procedure (SPLP) test (EPA Method 1312) was

designed to determine the mobility of organic and inorganic analytes in liquids, solids, and

mixed wastes using a batch leach technique. The method is outlined in U.S. EPA (1986a), with

the most recent version of the experimental procedure dated September 1994, revision 0. For

areas west of the Mississippi River, the method uses an extraction fluid composed of a 60/40

weight percent mix of sulfuric/nitric acid diluted to pH 5.00 ± 0.05 to simulate regional acidic
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precipitation. Samples containing cyanide or volatile organic components are leached using

special procedures and distilled water as the extraction fluid. For non-volatile materials, samples

of approximately 100 g are crushed to sizes smaller than 9.5 mm and placed into an extraction

bottle. A 20:1 weight ratio of extraction fluid:sample solid is added to the bottle, which is

agitated for 18 ± 2 hours. Following extraction, the leachate is filtered, preserved with nitric

acid, and analyzed for metals. 

4.4.1.4 Monofilled Waste Extraction Procedure

The Monofilled Waste Extraction Procedure (MWEP) is a sequential batch extraction test

developed to predict the composition of leachate produced from solid waste under field

conditions. The procedure is outlined in U.S. EPA (1986b). Solid materials are crushed to pass

a 9.5 mm sieve and are combined with extraction fluid in a 10:1 liquid:solid ratio. The mixture

is tumbled at room temperature for 24 hours. The procedure uses reagent grade water as the

extraction fluid, however, the test can be conducted using process waters, ground waters, or

other fluids that occur at a site. Following extraction, the leachate is filtered and analyzed. The

solid residue is returned to the extraction vessel and the leach process is conducted using fresh

extraction fluid. Four leachings per sample are recommended. Not only does this procedure

allow single samples to be leached repetitively, but it permits more than one sample to be

leached by the same extraction fluid.

4.4.2 State Procedures

The State of Nevada recently developed a leach test specifically for mining wastes. The

procedure has been broadly accepted by the mining industry and is being used to test wastes that

would be disposed of in other regions.

The State of Nevada uses a single-pass column leach test termed the Meteoric Water

Mobility Procedure (MWMP) to determine the potential for waste rock, spent ore, and tailings to

release certain constituents to the environment. The test is required by guidance documents

issued by the Division of Environmental Protection (NV DEP, 1990; 1996). The procedure is

provided in NV DEP (1996) and available (as of February 1999) on the internet

(www.enviromine.com/ard/Acid-Base%20Accounting/metal_leaching.htm).

The MWMP test uses 5 kg of material crushed to particle sizes smaller than 5 cm which

is loaded into an extraction column. A volume of extraction fluid equal to the dry weight of the

sample (milliliters of fluid equal to grams of sample) is passed through the sample in a 24 hour

period. Although the procedure states that the pH of the extraction fluid should “reflect the pH

of precipitation in the geographic region in which the mine rock is being evaluated,” the

procedure uses Type II reagent grade water (distilled or deionized as produced by Method 1080

in APHA et al., 1992) as the extraction fluid. The pH values of the initial leachate and

homogenized leachate at the end of testing are recorded. The homogenized leachate is filtered

and analyzed for dissolved constituents.
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4.4.3 Other Leaching Procedures

Leach test procedures also have been developed by the Province of British Columbia, the

U.S. Army Corps of Engineers, and the American Society for Testing and Materials (ASTM). 

These tests are not widely used by the American mining industry. 

4.4.3.1 British Columbia Procedures

The British Columbia Special Waste Extraction Procedure (SWEP) is a single batch

extraction that uses an acetic acid lixiviant, a 16:1 liquid:solid mass ratio, and an extraction time

of 24 hours. According to Mills (1998f), for mine wastes in British Columbia, it is standard

practice to used distilled water or 0.1 N hydrochloric acid as the extract fluid, a liquid:solid mass

ratio of 3:1, and an extraction time of 24 hours.

4.4.3.2 U.S. Army Corps of Engineers Procedures 

The U.S. Army Corps of Engineers developed a procedure to conduct sequential batch

leaching tests (SBLT) of dredged materials (Brannon et al., 1994). This procedure, which

determines changes in the equilibrium distribution of a contaminant between solid material and

an aqueous phase, can be used to investigate the quality of water that might be expected to occur

during episodic flushing of mining wastes (for example, during wet winters and dry summers). 

The SBLT procedure uses a liquid:solid weight ratio of 4:1 and a 24-hour leaching time for each

step. Samples are placed into a tumbler and tumbled using deoxygenated water as the leaching

medium. The leachate is separated by centrifuge, filtered, preserved with nitric acid, and

analyzed for electrical conductivity and metals. A minimum of four sequential cycles are

recommended. The procedure provides a conservative estimate of leachate concentrations under

conditions of anaerobic leaching of freshwater sediments (Brannon et al., 1994). The SBLT

procedure could be applied to analysis of tailings and other fine-grained materials, such as

borrow soils used for growth media and covers. However, the procedure does not define the size

fractions that should be tested and its applicability to tests of coarse waste rock has not been

demonstrated. 

Myers and Brannon (1988) and Myers et al. (1991) describe a procedure developed by

the U.S. Army Corps of Engineers for column leach testing of dredged freshwater sediments. 

These tests are recommended to confirm the results of sequential batch leaching tests and can be

used if the potential for contamination is high. The Myers et al. (1991) procedure uses an

improved column design that increases the number of pore volumes that can be eluted in a given

period of time by using a decreased column length and increased column diameter (producing

pore water velocities of approximately 10-5 cm/sec). The test, which uses kilogram samples, is

conducted using deoxygenated water as the leaching medium.

Graded serial batch tests are described by Houle and Long (1978; 1980). In these tests,

solid waste is mixed with an extraction fluid in a liquid:solid ratio of 2:1 and shaken

intermittently for 24 hours. The sample is filtered and the leachate analyzed, with residual solid

material returned the extraction vessel for subsequent leaching. The liquid:solid ratio is doubled
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for each succeeding extraction (i.e., 4:1, 8:1, 16:1, etc.), with a total of seven leach cycles

recommended for each sample. The extraction fluid can be reagent water or any site-specific

fluid, thus permitting a determination of the constituents that can be removed from or adsorbed

by the solid waste.

4.4.3.3 ASTM Procedures

The American Society for Testing and Materials provides methodologies for conducting

shake flask extractions (ASTM Method D3987) and sequential batch extractions of solid wastes

(ASTM Methods D4793 and D5284) (ASTM, 1996). The tests use liquid:solid mass ratios of

20:1 and extraction times of 18 hours. In the sequential batch tests, 10 leachate samples are

produced from a single solid waste sample. Methods D3987 and D4793 use water for the

extraction fluid whereas method D5284 uses an acidic extraction fluid with a pH similar to that

of the average regional precipitation in the disposal area.

4.4.4 State Recommendations

The states comprising EPA Region 10 presently have not promulgated formal guidelines

that specify use of a particular leaching procedure. The State of Nevada recommends use of the

Nevada Meteoric Water Mobility Procedure to test representative samples of waste rock, spent

ore and tailings for their potential to release contaminants (NV DEP, 1996). The State of New

Mexico (NMED, 1996) recommends use of EPA method 1312 (SPLP test) to test samples for the

potential to release contaminants. Applicants should check with state agencies to determine

whether they have preferences that may not be codified.

4.4.5 Comparison of Leaching Procedures

Batch leach tests vary significantly in their ability to extract metals from solid materials

depending on the type of extraction fluid employed. The determination of which leach test

method should be applied to mining wastes is the subject of continuing regulatory discussions

and there may be differences between state and federal requirements. As such, operators should

maintain open lines of communication with all regulatory agencies on this topic.

In 1995, EPA stated its position that EPA Method 1311 (TCLP) tests were applicable to

evaluations of mineral processing wastes. In general, Method 1311 is applicable to any miningrelated material that is not Bevill-exempt. However, where the materials are Bevill-exempt (e.g.,

waste rock), particularly when they will be managed in a monofill, EPA Method 1312 (SPLP)

may be the preferred method because it utilizes strong acids similar to those that would be

generated under oxidizing conditions. However, the SPLP test uses a combination of sulfuric

and nitric acids as the extraction fluid, which precludes determination of sulfate and nitrate

concentrations in test leachates. Because these constituents may be of interest (sulfates as

oxidation products of sulfides or hydrolysis products of acid-sulfate minerals; nitrates as blasting

residue), it may bedesirable to modify the procedure to substitute a strong acid such as

hydrochloric acid, which has similar, albeit less oxidizing, qualities, as the extraction fluid. The

SPLP test also can be modified to be more aggressive by decreasing the pH of the extraction
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fluid. The SPLP test is run under conditions of high fluid to solid ratio (20:1) and short duration

(18 hours), which limits the extent to which biological oxidation will breakdown reactive sulfide

minerals. States may have their own requirements or preferences, and operators are advised to

consult with their state regulatory authorities.

Sequential leach tests provide data regarding the rate at which constituents could be

released to the environment. In particular, these tests can show whether the concentrations of

metals in a leachate exhibit temporal trends. However, extrapolating the results of sequential

leach tests to the expected conditions of waste disposal may not be straightforward since most

tests are conducted on material that may have significantly different reaction kinetics than the

actual waste (due to particle size) and because extraction durations and the amount of time

between extractions do not replicate either natural wet-dry cycles or conditions of atmospheric

oxidation. 

Many leaching tests use reagent-grade water as the extraction fluid (e.g., Nevada

MWMP), which may not simulate the expected natural conditions, for example, where

acidification occurs at depth in a waste pile. To more closely approximate leaching in regions

where rainfall is acidic or where percolating water contacts oxidation products, reagent water

can be acidified using strong acids to pH values typical of the regional precipitation. A more

acidic extraction fluid makes leaching tests chemically more aggressive; consequently, their

results provide a more conservative estimate of the potential impacts of mining materials on

water quality.

A recent study by Doyle et al. (1998) leached samples of mining wastes using batch

(SPLP) and continuous column procedures. They found that batch tests frequently, but not

always predicted higher metals leachability than the column tests, suggesting that they typically

provide a more conservative estimate of environmental behavior. However, the study did not

indicate which test methods better represented actual field conditions.

5.0 ANALYSIS OF FATE AND TRANSPORT

Analyzing chemical fate and transport at mine sites is a complex task due to the

interactions between the hydrologic cycle, pollutant cycle, and sedimentation (watershed) cycle

(Bonazountas, 1983). Consequently, fate modeling includes processes that occur on the land

surface (soil, atmosphere and water), the unsaturated zone, and the saturated zone (Bonazountas,

1983). Anderson and Woessner (1992) describe a modeling protocol for ground water systems

that can be extended and applied to mine sites. It includes establishing the purpose of the model,

developing a conceptual model, selecting governing equations and an appropriate computer

code, and designing, verifying and calibrating a numerical model.

5.1 Developing a Conceptual Model

A conceptual model is a pictoral representation of a complex system, frequently in the

form of a block diagram or cross-section (Anderson and Woessner, 1992). The conceptual
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model simplifies a complex field problem and makes it more amenable to modeling. In

particular, it helps to determine the dimensions of the numerical model and the design of an

appropriate grid. An example of a conceptual physical ground water model taken from

Anderson and Woessner (1992) is shown in Figure C-1. A conceptual physicochemical model of

metal transport in a river, taken from Schnoor (1996), is shown in Figure C-2.

Four information components are needed to develop a conceptual site model (Bedient et

al., 1994). Geology provides the physical framework within which subsurface fluids collect and

flow and an understanding of the characteristics of the materials and solid wastes that must be

handled. Hydrology describes the movement of fluids across the surface and through the

physical framework (subsurface). Chemistry defines the nature of the chemical constituents

transported by the surface and subsurface flow systems, including aspects of biochemistry as

they apply to fluid chemistry. Climate provides data to describe interactions between

precipitation, evaporation, surface flow, subsurface flow, and infiltration. 

The amount of data required to develop a mine-site conceptual model of fate and

transport are considerable (Schnoor, 1996; Hemond and Fechner, 1994; U.S. EPA, 1989). The

mine plan provides information about the locations, character, and volumes of materials and

wastes, surface and subsurface disturbances, ground water withdrawals, surface water diversions,

and outfall locations and discharges. The solids balance describes the amount and character of

material that will be excavated, processed, and disposed. The water balance characterizes the

effects of climatic variations, drawdown, surface water diversion, and waste water discharge. 

Surface water 

hydrology provides information regarding discharges and their seasonal variation, surface water

chemistry, and storm runoff. Ground water hydrology describes flow rates (flux), hydrologic

gradients, ground water volumes, ground water chemistry, and flow paths. Geology provides

data on vertical stratigraphy (including aquitards), lateral changes in stratigraphic relations, the 

locations and density of faults and fractures, and mineralogy. Aquifer characteristics include

physical aspects such as hydraulic conductivity, porosity, and fracture and matrix flow and 

chemical aspects including adsorptive or neutralizing components and biogeochemical

processes. Contaminant characteristics describe the chemistry, density, discharge, volume, and

chemical and physical stability of solid and liquid wastes and materials. 

5.2 Mathematical Models

Mathematical models that couple physical flow and chemical mass balance equations are

used to simulate the flow and transport of contaminants through the environment. Because

models used for predictive purposes are only as good as the data input to them, high quality, sitespecific data are required to produce confident and realistic model predictions. 

5.2.1 Categories of Mathematical Models

Mathematical models can be grouped into three general categories (Knox et al., 1993). 

Analytical models solve governing equations using simplifying assumptions. They are generally

one- or two-dimensional models that assume steady-state flow. Stochastic models incorporate
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Figure C-1. Conceptual physical model of ground water flow from Anderson and Woessner

(1992).

uncertainty by using mean values coupled with a measure of variance. Numerical models, which

are the most commonly used model form, are computed solutions to coupled partial differential

equations of flow and mass balance equations of contaminant fate. Numerical models are solved

in one-, two-, or three-dimensions using either finite element, finite difference, or method of

characteristics techniques. Detailed discussions of each of these methods can be found in Knox

et al. (1993) and Bedient et al. (1994).
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Figure C-2. Conceptual physicochemical model of metal transport in a river 

from Schnoor (1996).
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5.2.2 Chemical Equilibrium Models

Numerous physical, chemical, and biological processes occurring in surface and

subsurface environments can affect the transport and fate of contaminants. These can be divided

into abiotic and biotic processes (Keely, 1989a). Abiotic processes are physical and chemical

interactions that cause contaminants to move at a rate different from than that of surface or

ground water. They include hydrolysis, sorption, cosolvation, immiscibility, ionization,

radionuclide decay, complexation, volatilization, photodegradation, precipitation, dissolution,

and reduction-oxidation (Johnson et al., 1989; Schnoor, 1996). Biotic processes are microbially

mediated transformations or adsorbtion of contaminants. They include biodegradation and

bioaccumulation. Other physical processes that may affect contaminant concentrations include

hydrodynamic dispersion, molecular diffusion, and density stratification (Knox et al., 1993).

Chemical equilibrium models calculate changes in chemical concentrations assuming

equilibrium. Aqueous models of trace metal concentrations compute chemical species by

accounting for aqueous-phase complexation (e.g., by naturally occurring humic acids), surface

complexation (e.g., by ion-exchange on the surfaces of clays), adsorption and sedimentation by

particles (e.g., lead adsorbed on the surface of ferric hydroxide), mineral precipitation (e.g.,

ferric hydroxide), mineral dissolution (e.g., calcite dissolution by acid), aggregation/flocculation

(e.g., the formation of colloidal suspensions by electrostatic processes), redox reactions that

affect solubility (e.g., Cr+3 and Cr+6), and adsorption by soil particles (Johnson et al., 1989;

Schnoor, 1996). Summary descriptions of three chemical equilibrium models, Mac:QL,

MINEQL+, and MINTEQA2, are given in Schnoor (1996).

5.2.3 Physical Flow and Transport Models

Flow and solute (mass) transport models are available for surface water, ground water

(saturated zone), and the vadose zone (unsaturated zone). They typically are used in conjunction

with one of the equilibrium chemical models described above. The mathematical development

of the governing flow and transport equations used in many of these models is given in Schnoor

(1996).

Models commonly used to compute river water quality include QUAL2EU, NONEQUI,

and WASP (summary descriptions are given in Schnoor, 1996 and are available via the internet n

sites for the U.S. EPA’s Robert S. Kerr Environmental Research Lab and Center for Exposure

Assessment Modeling). QUAL2EU is a steady-state model for pollutants in branching streams

and well-mixed lakes that incorporates uncertainty analysis into the model results. 

Keely (1989b) points out that many ground water models are inappropriate for use in

areas where subsurface flow is controlled by fractures or karst features. Consequently, the

choice of models determines whether realistic model predictions can be computed for these

areas. Bedient et al. (1994) provide summary model descriptions and a listing of modeled

processes for a variety of unsaturated and saturated flow and solute transport models. Included

are 6 vadose-zone flow models, 11 vadose-zone solute transport models, 12 saturated zone flow

models, and 9 saturated zone solute transport models. Additional model descriptions are
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available via the internet from the U.S. EPA’s Robert S. Kerr Environmental Research Lab. 

Among the more widely used saturated zone models are MODFLOW, a three-dimensional finite

difference model, and USGS-MOC, a two-dimensional finite difference and methods of

characteristics model for ground water flow and solute transport. Anderson and Woessner

(1992) describe three conceptual models that can be used to approximate flow through a

fractured system for input to models based on saturated or unsaturated flow. Each of these

conceptual models uses assumptions that oversimplify flow through the fractured system. 

6.0 SAMPLING PROGRAMS

The environmental sampling process should follow a sequence of steps to ensure that

collected samples are representative and adequate (Triegel, 1988). It is important to first identify

the goals of the sampling program and the levels of confidence required. The number of

required samples then can be determined by characterizing the sources of variability (e.g.,

sample heterogeneity). Using these data, the sample program can be designed. The design

should consider the types of analyses that will be conducted on the samples and include the

number and distribution of samples and their manner of collection. The following sections

specifically address geochemical testing programs.

6.1 Objectives of a Geochemical Sampling Program

Establishing a reliable geochemical testing program is a difficult, but critical, aspect of

mine site development. By indicating whether control technologies or alternative disposal

methods should be added to the existing mine plan, a robust program that uses representative

samples can diminish, perhaps eliminate, the costs of contamination mitigation and control that

would be encumbered should environmental problems arise in the future (Robertson and

Broughton, 1992).

The geologic history and nature of mineralization observed at a mine site is unique to that

particular location. As a result, geochemical sampling programs will differ from site to site. 

Nevertheless, all sampling programs should strive to capture the range of variability that occurs,

provide an accurate statistical representation of the materials present, and objectively test the

feasibility of the disposal methods described by the proposed mine plan. A geochemical

sampling program should consider several factors that could affect the chemical or physical

character of samples and, consequently, impact test results. Included are the method of sample

collection, the length of time that a sample will be (or has been) stored prior to analysis, and the

environment in which samples are (or were) stored (U.S. EPA, 1994).

For proposed mines, sampling and testing programs use fresh samples to predict the

potential for acid generating conditions to develop or metals to leach from materials and wastes

(Robertson and Broughton, 1992). A sampling program should be developed within the context

of geochemical rock units and be related directly to a mine plan that outlines the area to be

mined, the locations of pit walls and benches or underground workings, the locations and

amounts of ore and waste rock that will be excavated, and the approximate timing of excavation
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and final placement of the materials (BC AMD Task Force, 1989; Price, 1997). The latter is

especially important for determining the potential for contaminant release from waste rock

dumps and other managed materials because these features can vary in particle size, mineralogy,

and chemical composition over short distances and over the life of the mining operation. The

sampling program also should include materials (e.g., tailings) produced during bench-scale or

pilot-scale processing tests of samples that encompass the range of materials that will be

processed over the life of the operation. Geochemical and mineralogical variability can be

evaluated using three-dimensional geostatistical techniques similar to those used to characterize

the ore body (Robertson and Broughton, 1992). While these methods are well-developed, they

are beyond the scope of this appendix. 

Sampling and testing programs at existing or abandoned mines should address questions

regarding the quantity of acid products stored in the materials and wastes and how contamination

emanating from them is likely to change in the future (Robertson and Broughton, 1992). For

studies of existing waste rock dumps, spent ore heaps, or tailings piles, a sampling program must

establish the physical, mineralogical, and chemical variability of the materials and wastes (see

Nash et al., 1998).

6.2 Sample Representativeness

Samples used in geochemical tests should be representative of the materials that will be

mined and processed. According to Smith et al. (1988), representativeness expresses the degree

to which data accurately and precisely represent a characteristic of a population, parameter

variations at a sampling point, or a process or environmental condition. Indeed, the major source

of uncertainty in a sampling and testing program lies in the samples themselves. In particular,

the question of how accurately a sample represents a larger volume of material can only be

addressed by establishing the variation inherent in the geochemical rock unit by taking multiple

samples and examining their frequency distribution (BC AMD Task Force, 1990). In this regard,

sampling programs should establish criteria for sample size, the appropriateness of compositing

samples, and collection method to meet data quality objectives related to representativeness. 

6.2.1 Proposed Mine Sites

Tests to determine physical and geochemical variability should be conducted initially on

each lithologic unit that will be excavated, exposed or otherwise disrupted in a mine site area. 

They should use as their basis the mineralogical zonation observed within the ore body and, if

possible, the mineralogical distinction that separates ore material from waste rock. The results of

initial tests can be used to define units with similar geochemical and leachate production

attributes (i.e., geochemical rock units; Brodie et al., 1991). In some cases, test results will

require that a heterogeneous lithologic unit be divided into two or more geochemical rock units,

whereas in other cases, two or more homogeneous lithologic units may be grouped together. 

Each geochemical rock unit should be tested further to define the range of its geochemical

characteristics. In essence, a sampling program should use an iterative process to assess

variability and it should be designed to be sufficiently flexible to respond to changes in the

mining plan (Robertson and Broughton, 1992).
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Geochemical test samples should be collected from each geochemical rock unit over the

full vertical and areal extent of the mine site or area of interest. Geographical representativeness

can be depicted using maps and cross-sections. The number of samples that should be tested

depends on the volume and variability of the rock unit in question. In general, sample

requirements increase with chemical and mineralogical heterogeneity, but there are no widely

accepted guidelines. For example, the BC AMD Task Force (1989) recommended a minimum

number of acid-base accounting test samples appropriate for a rock unit with a given mass. As

shown on Figure C-3, this approach can lead to extensive sampling requirements for large

facilities and result in high sampling costs. Price (1997) also provides minimum sample

numbers based on unit tonnage. Alternatively, Runnells et al. (1997) suggested that the number

of required samples should reflect the heterogeneity of the materials within the facility. The

appropriate number of samples is obtained when statistical variability in sample results is within

acceptable limits. Using this approach, the number of samples needed to characterize a facility

will vary from one facility to another because each facility is unique. The Runnells et al. (1997)

method can be applied easily to existing facilities, but may be difficult to apply to materials that

would be disposed of in proposed facilities. Nevertheless, sampling programs that use a fixedfrequency sampling approach should be designed to ensure that sample variability can be

described with statistical validity (e.g., BC AMD Task Force, 1990).

Geologic materials, which are composed of one or more minerals, are by definition

composite materials. For the purposes of geochemical testing, sample sizes should be large

enough to smooth the effects of small-scale heterogeneity, but small enough to reveal the

variations present in the rock unit of interest. The effects of composite sample size on the

distribution of net neutralization potential values obtained from a highly variable rock mass are

described by Robertson and Broughton (1992). For waste rock and overburden materials,

samples are commonly lengths of drill core or drill cuttings. Robertson and Broughton (1992)

suggest restricting drill core lengths to less than 0.5 meters for acid-base accounting tests to

ensure that the chemical behavior of a waste rock pile can be evaluated on small and large scales.

6.2.2 Existing or Abandoned Mine Sites

Existing or abandoned mine sites can pose special problems for geochemical test

sampling because the history of the mine and the detailed composition of materials and wastes

may be unknown or unrecorded. Changes to processing methods and efficiency that may have

occurred during active production or time gaps when mining did not occur can produce chemical

and physical heterogeneity within piles of materials that are not evident from their exposed

surfaces. Consequently, sampling programs designed for existing or abandoned mine sites

should determine the variability of all materials disposed of or exposed on the surface (see

discussion of Runnells et al. 1997 in Section 6.2.1 and Nash et al., 1998) or through a wellplanned composite sampling program (Smith et al., 2000). For pit walls, this will require
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Figure C-3. Minimum number of samples as a function of rock mass recommended by the BC

AMD Task Force (1989)
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collecting samples vertically and laterally across the exposed rock faces. For waste rock dumps,

spent ore heaps, and tailings impoundments, it will require collecting samples laterally and

vertically throughout the deposit (typically by drilling) (Nash et al., 1998). Data gathered from

these samples can be used to construct a three-dimensional image of the volume and chemical

and physical character of the waste materials. As described in the previous section, the number

of samples required by the program depends on the volume and variability of the materials in

question, but generally increases with chemical, mineralogical, and physical heterogeneity.

6.3 Quality Control and Quality Assurance

A recent report by Downing and Mills (1998) describes the application of quality

assurance and quality control procedures as they apply to acid rock drainage studies. QA/QC

guidance and procedures prepared by EPA are available in Adobe format on the EPA Region 10

QA website (www.epa.gov/r10earth/offices/oea/qaindex.htm). New guidance for the preparation

of QAPP documents is in review and is scheduled for issue in early 1999.

6.3.1 Quality Control

Taylor (1988) defines quality control as the application of good lab practices, good

measurement practices, and standard procedures for sampling. The latter should include

specifications for chain-of custody, storage and preservation, stabilization methods, labeling, and

sample containers. 

Physical and geochemical tests conducted using approved methods (EPA or otherwise)

will produce analytical results with accuracy and precision sufficient for all likely applications,

providing that methods are chosen for their ability to meet the data quality objectives described

in the next section. In this regard, it is important for applicants to select analytical methods that

have the necessary detection limits. Applicants should periodically submit replicate samples for

testing and analysis to confirm laboratory assessments of analytical performance. 

6.3.2 Quality Assurance

Quality assurance is the process of monitoring for adherence to quality control protocols

(Taylor, 1988). Smith et al. (1988) list five data quality objectives of a quality assurance project

plan (QAPP): precision, bias, representativeness, completeness, and comparability (cf. U.S.

EPA, 1980; 1998a; 1998b). Precision leads to a measurement of variance (e.g., standard

deviation) and is the mutual agreement among individual measurements under prescribed similar

conditions. Bias refers to the degree to which a measurement reflects an accepted true or

reference value, commonly expressed as a percentage. Representativeness, as described above,

expresses the degree to which data accurately represent a characteristic of a population. 

Completeness is a measure of the amount of valid data compared to the amount expected to be

obtained under normal conditions. Comparability is a measure of confidence that one data set

can be compared to another. 
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A QAPP will ensure that procedures are established prior to the beginning of sample

collection and will help to balance the costs of implementing a quality-assured program against

the liabilities of a poorly designed and executed sampling program.

7.0 REFERENCES

American Public Health Association, American Water Works Association, and Water

Environment Federation (APHA et al.), 1992. Standard Methods for the Examination of

Waters and Wastewaters, 18th edition, American Public Health Association, Washington,

D.C..

Anderson, M.P. and Woessner, W.W., 1992. Applied Groundwater Modeling: Simulation of

Flow and Advective Transport, Academic Press, Inc., San Diego, CA, 381 pp.

ASTM, 1996, Annual Book of ASTM Standards, American Society for Testing and Materials,

Philadelphia, PA.

ASTM, 1998, Annual Book of ASTM Standards, vol. 11.04, American Society for Testing and

Materials, Philadelphia, PA, pp. 259-271.

BC AMD Task Force, 1989. Draft Acid Rock Drainage Technical Guide, Volume 1, British

Columbia Acid Mine Drainage Task Force Report prepared by Steffen Robertson and

Kirsten, Norelco Environmental Consultants, and Gormely Process Engineering, August

1989.

BC AMD Task Force, 1990. Monitoring Acid Mine Drainage, British Columbia Acid Mine

Drainage Task Force Report prepared by E. Robertson in association with Steffen Robertson

and Kirsten, Inc., BiTech Publishers, Ltd., Vancouver, B.C., 66 pp.

Bedient, P.B., Rifai, H.S., and Newell, C.J., 1994. Groundwater Contamination: Transport and

Remediation, PTR Prentice-Hall, Inc., Englewood Cliffs, NJ, 542 pp.

Bertin, E.P., 1970. Principles and Practice of X-Ray Spectrometric Analysis, Plenum Press, NY,

679 pp.

Bish, D.L. and Post, J.E., eds., 1989. Modern Powder Diffraction, Reviews in Mineralogy,

Volume 20, Mineralogical Society of America, Washington, D.C., 369 pp.

Blatt, H., Middleton, G., and Murray, R., 1980. Origin of Sedimentary Rocks, 2nd ed., PrenticeHall, Englewood Cliffs, NJ, 782 pp.

Bonazountas, M., 1983. Soil and Groundwater Fate Modeling. In: Swann, R.L. and

Eschenroeder, A., eds., Fate of Chemicals in the Environment, ACS Symposium Series 225,

American Chemical Society, Washington, D.C., p. 41-65.
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Bradham, W.S. and Caruccio, F.T., 1990. A Comparative Study of Tailings Analysis Using

Acid/Base Accounting, Cells, Columns, and Soxhlets, Proceedings of the 1990 Mining and

Reclamation Conference and Exhibition, Charleston, WV, p. 19-25.

Brady, K.B.C., Smith, M.W., Beam, R.L., and Cravotta III, C.A., 1990. Effectiveness of the

Addition of Alkaline Materials at Surface Coal Mines in Preventing or Abating Acid Mine

Drainage: Part 1 & 2, Proceedings of the 1990 Mining and Reclamation Conference and

Exhibition, Volume 1.

Brannon, J.M., Myers, T.E., and Tardy, B.A., 1994. Leachate Testing and Evaluation for

Freshwater Sediments, U.S. Army Corps of Engineers, Waterways Experiment Station,

Miscellaneous Paper D-94-1, 64 pp.

Brodie, M.J., Broughton, L.M., and Robertson, A.M., 1991. A Conceptual Rock Classification

System for Waste Management and a Laboratory Method for ARD Prediction from Rock

Piles. In: Proceedings of the Second International Conference on the Abatement of Acidic

Drainage, Montreal, Quebec, vol. 3, p. 119-136.

Broughton, L.M. and Robertson, A.M., 1992. Acid Rock Drainage from Mines - Where We Are

Now. In: Science Applications International Corporation, Predicting Acid Generation from

Non-coal Mining Wastes: Notes of July 1992 Workshop, Draft report prepared for the

Environmental Monitoring Systems Laboratory, Office of Research and Development, U.S.

Environmental Protection Agency, Las Vegas, NV.

Bruynesteyn, A. and Hackl, R.P., 1984. Evaluation of Acid Production Potential of Mining

Waste Materials, Minerals and the Environment, vol. 4, p. 5-8.

Carson, C.D., Fanning, D.S., and Dixon, J.B., 1982. Alfisols and Ultisols with Acid Sulfate

Weathering Features in Texas. In: Kittrick, J.A., Fanning, D.S., and Hossner, L.R., eds.,

Acid Sulfate Weathering, SSSA Special Publication No. 10, Soil Science Society of

America, Madison, WI, pp. 127-146.

Caruccio, F.T., Ferm, J.C., Horne, J., Geidel, G., and Baganz, B., 1977. Paleoenvironment of

Coal and its Relation to Drainage Quality, U.S. Environmental Protection Agency Report

EPA 600/7-77-067, 118 p.

Coastech Research, 1989. Investigation of Prediction Techniques for Acid Mine Drainage,

MEND Project 1.16.1.a, Canada Centre for Mineral and Energy Technology, Energy Mines

and Resources Canada, 61 pp. plus appendices.

Craig, J.R. and Vaughn, D.J., 1994. Ore Microscopy and Ore Petrography, 2nd ed., John Wiley

and Sons, NY, 434 pp.

Crock, J.G., Arbogast, B.F., and Lamothe, P.J., 1999. Laboratory Methods for the Analysis of

Environmental Samples, In: G.S. Plumlee and M.J. Logsdon, eds., The Environmental
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