EPA530-R-9MG6
NTIS PB94-201829
TECHNICAL DOCUMENT
ACID MINE DRAINAGE
PREDICTION
December 1994
U.S. Environmental Protection Agency
Office of Solid Waste
Special Waste Branch
401 M Street, SW
Washington, DC 20460
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Acid Mine Drainage Prediction
DISCLAIMER AND ACKNOWLEDGEMENTS
This document was prepared by the U.S. Environmental Protection
Agency (EPA). The mention of company or product names in this
document is not to be considered an endorsement by the U.S.
Government or by the EPA.
This technical document consists of a brief review of acid forming
processes at mine sites, followed by a summary of the current
methods used to predict acid formation, selected state regulatory
requirements, and case histories. This report was distributed for
review to'the U.S. Department of the Interior's Bureau of Mines and
Bureau of Land Management, the U.S. Department of Agriculture's
Forest Service, the Interstate Mining Compact Commission, the
American Mining Congress, the Mineral Policy Center,
representatives of state agencies, and public interest groups. EPA is
grateful to all individuals who took the time to review this technical
document.
The use of the terms "extraction," "beneficiation," and "mineral
processing ".in this document is not intended to classify any waste
stream for the purposes of regulatory interpretation or application.
Rather, these terms are used in the context of common industry
terminology.
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Acid Mine Drainage Prediction
.4.3.4 Environmental Risks 41
4.3.5 Treatment , 41
4.4 California Gulch 41
4.5 Clear Creek/Central City 42
4.6 Iron Mountain Mine , 43
4.7 Silver Bow Creek/Butte Area Site 43
5. REFERENCES 45
LIST OF TABLES
Table 1. Comparison of Acid Rock Drainage Factors In Waste Rock Piles and
Tailings Impoundments 3
Table 2. Partial List of Sulfide Minerals 4
Table 3. Sulfide Ore Bacteria and Their Growth Conditions 5
Table 4. Sources of Information on Acid Generation Potential for New and
Operating Mines . 10
Table 5. Summary of Static Test Methods, Costs, Advantages, and Disadvantages 15
Table 6. Summary of Some Kinetic Test Methods, Costs, Advantages, and
Disadvantages 20
Table 7. Summary of State Regulations for Acid Generation Prediction Testing
(August 1994) . 30
m
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Acid Mine Drainage Prediction
TABLE OF CONTENTS
Page
1. INTRODUCTION 1
I.I Oxidation of Metal Sulfides 2
1.2 Source of Acid and Contributing Factors ' 5
2. ACID GENERATION PREDICTION 9
2.1 Sampling ...... 12
2.2 Static Tests 14
2.2.1 Acid-Base Accounting (ABA) 16
2.2.2 Modified Acid Base Accounting 16
2.2.3 British Columbia Research Initial Test (BC) 17
2.2.4 Alkaline Production Potential : Sulfur Ratio (APP:S) 17
2.2.5 Net Acid Production Test 18
2.3 Kinetic Tests 18
2.3.1 Humidity Cell Tests 19
2.3.2 Soxhelet Extraction Tests ! 19
2.3.3 Column Tests 21
2.3.4 British Columbia Research Confirmation Test 21
2.3.5 Batch Reactor (Shake Flask) Tests 22
2.3.6 Field Scale Test 22
2.4 Application of Test Results in Prediction Analysis 23
2.4.1 Some Experience With Static and Kinetic Tests . 24
- 2.5 Mathematical Modeling of Acid Generation Potential 25
2.5.1 Overview of Existing Models 26
2.5.2 Empirical Models 26
2.5.3 Deterministic Models 27
2.6 Conclusions 28
3. CURRENT REGULATORY REQUIREMENTS 29
4. CASE HISTORIES 33
4.1 Newmont Rain Facility, Elko County, NV 33
4.1.1 Introduction '. : 33
4.1.2 Waste Rock 34
4.1-.3. Acid Generation Prediction 35
4.1.4 Treatment . . 35
4.2 Cyprus Thompson Creek, Challis, ID 36
4.2.1 Introduction 36
4.2.2 Waste Rock 37
4.2.3 Acid Generation Prediction 37
4.2.4 Tailings 38
4.2.5 Acid Generation Prediction : 38
4.2.6 Treatment 38
4.3 LTV Steel Mining Company, The Dunka Site, Minnesota 39
4.3.1 Introduction 39
4.3.2 The Acid-Generating Duluth Complex 40
4.3.3 Acid Rock Drainage Prediction Methods 40
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Acid Mine Drainage Prediction
Acid mine drainage prediction tests are increasingly relied upon to assess the long-term potential of
acid generation. This concern has developed because of the lag time at existing mines between waste
emplacement and observation of an acid drainage problem (Univ. of California, Berkley 1988). The
issue of long-term,' or perpetual care of acid drainage at historic mines and some active mines has
focussed attention on the need for improving prediction methods and for early assessment of the
potential during the exploratory phase of mine development. In addition to many other mines,
examples of three mine sites where the potential to generate acid was either not considered or not
expected but later developed include: Cyprus Thompson Creek in Idaho; the Newmont Gold
Company's Rain Mine hi Nevada; and the LTV Dunka Mine in Minnesota. Case studies for these
mines are presented in Section 4.0 of this report. Also included are short case studies of four sites on
the NPL due, in part to acid drainage (U.S. EPA 1991).
Wastes that have the potential to generate acid as a result of metal mining activity include mined
material such as spent ore from heap leach operations, tailings, and waste rock units, including
overburden material. While not wastes or waste management units, pit walls in the case of surface
mining operations, and the underground workings associated with underground mines and subgrade
ore piles, also have the potential to generate ARD.
As mineralogy and size variables change, the ability to accurately predict the acid potential becomes
quite difficult (Brodie, et al. 1991). Waste rock piles and subgrade ore piles, when left onsite, are
both characterized by wide variation hi mineralogy and particle size. Changes in these variables
appear to influence drainage water quality (Doepker 1993). Coarse grain material allows air
circulation; however, fine gram material exposes more surface area to oxidation (Ferguson and
Erickson 1988). Drainage water quality from waste rock piles at several mines hi British Columbia
have demonstrated wide variability. Research at these sites focussed on variables affecting the
frequency of acid effluent observed hi permit-related monitoring (British Columbia AMD Task Force,
1990). The results reflect the diurnal and seasonal fluctuations in drainage quality as well as variation
in mineralogy and particle size common to waste rock piles. In contrast, drainage from tailing
impoundments are more likely to carry a more uniform contaminant load due to their more uniform
mineralogy and texture. Table 1 compares acid rock drainage factors of waste rock piles and tailings
impoundments. In examining this table, it is important to note that diffusion of oxygen into water is
slow and, therefore, oxidation of iron sulfide is inhibited until the water level drops, which can occur
periodically or seasonally hi some cases.
1.1 Oxidation of Metal Sulfides
Acid is generated at mine sites when metal sulfide minerals are oxidized. Metal sulfide minerals are
present in the host rock associated with most types of metal mining activity. Prior to mining,
oxidation of these minerals arid the formation of sulfuric acid is a function of natural weathering
processes. The oxidation of undisturbed ore bodies followed by release of acid and mobilization of
metals is slow. Discharge from such deposits poses little threat to receiving aquatic ecosystems.
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Acid Mine Drainage Prediction
Aero MINE DRAINAGE PREDICTION
1. INTRODUCTION
The U.S. Environmental Protection Agency (EPA), the states, and the Federal Land Management
Agencies all need better tools to manage acid mine drainage at mine sites. This report examines acid
generation prediction methods as they apply to non-coal mining sites. Following a brief review of
acid forming processes at mine sites, the report summarizes the current methods used to predict acid
formation including sampling, testing, and modeling. Selected State requirements for testing the
potential of mining wastes to generate acid are summarized. Case histories from active mining sites
and sites on the Superfund National Priorities List (NPL) are also presented. It is hoped that this
report will assist states and the mining industry in their use of predictive methods. The Agency has
not yet determined whether any one method is more accurate than another. This report also does not
incorporate material presented at the Third International Conference on the Abatement of Acidic
Drainage, held in Pittsburgh, Pennsylvania, in April 1994. The Agency is preparing additional
reports to update this information, including a document containing extended summaries of selected
papers presented at the conference.
The formation of mine acid drainage and the contaminants associated with it has been described by
some as the largest environmental problem facing the U.S. mining industry (U.S. Forest Service
1993, Ferguson and Erickson 1988, Lapakko 1993b). Commonly referred to as acid rock drainage
(ARD) or acid mine drainage (AMD), acid drainage from mine waste rock, tailings, and mine
structures such as pits and underground workings is primarily a function of the mineralogy of the
rock material and the availability of water and oxygen. Because mineralogy and other factors
affecting the potential for AMD formation are highly variable from site to site, predicting the
potential for AMD is currently difficult, costly, and of questionable reliability. The U.S. Forest
Service sees the absence of^acid prediction technology, especially in the context of new mining
ventures, as a major.problem facing the future of metal mining in the western United States (U.S.
Forest Service 1993).
Acid mine drainage from coal and mineral mining operations is a difficult and costly problem. In the
eastern U.S., more than 7,000 kilometers of streams are affected by acid drainage from coal mines
(Kim et al. 1982). In the western U.S., the Forest Service estimates that between 20,000 and 50,000
mines are currently generating acid on Forest Service lands, and that drainage from these mines is
impacting between 8,000 and 16,000 kilometers of streams (U.S. Forest Service 1993). In addition
to the acid contribution to surface waters, AMD may cause metals such as arsenic, cadmium, copper,
silver, and zinc to leach from mine wastes. According to the Forest Service, the metal load causes
environmental damage, and is of greater concern than the acidity in environmental terms.
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Acid Mine Drainage Prediction
Table 1. Comparison of Add Rock Drainage Factors In Waste Rock Piles and
Tailings Impoundments
Acid Generation
Factors
Affecting
Waste Rode Piles
Tailings Impoundment
Sulphide Source
Variable in concentration and
location.
Conditions may vary from sulphide
rich to basic over short distances.
Conditions uniform, often with
very high sulphide content.
Particle Size
Average rock size typically greater
than 20 cm (but highly variable).
Tailings may be 100% less
than 0.2mm.
pH Variation
Highly variable conditions over
short distances.
Fairly uniform conditions with
a few major horizontal zones.
Initiation Of
Rapid Oxidation
Usually starts immediately after first
rock is placed (in "trigger" spots).
Usually starts after tailings
placement ceases at end of
mine life.
Oxygen Entry
Enters freely along highly
conductive flow paths at base of pile
and large open void spaces;
"chimney and lung" effects.
Restricted by water in void
spaces and die lower
conductivity of partially
saturated void spaces.
Temperature
Temperature in pile affected by
convective flow of air from outside
pile.
High oxidation rates results in high
temperatures distributed
convectively through pile.
Temperature transmission
mainly by conductance,
generally slow.
Low oxidation rates result in
low localized temperature
effects.
Storage
Large storage of acid products due
to evaporative concentration.
Small storage of acid
products.
Seepage
Seepage rapid along preferential
flow paths.
Seasonal variations in flow path
"flushes" out stored products
resulting in concentration peaks.
Seepage slow and uniform.
Reduced flow path variation
and stored product "flushing.
ARD Releases
Large infiltration resulting in large
seepage from toe and to
groundwater.
Rapid release following generation,
sometimes with both neutralized and
acid ARD seeps.
Large early top surface ARD
runoff.
Lower infiltration.
Gradual transition in seeps
from process water to
neutralized ARD to low pH
ARD.
(Source: Brodfe et al.. 1991)
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Acid Mine Drainage Prediction
Extraction and beneficiation operations associated with mining activity increase the rate of these same
chemical reactions by exposing large volumes of sulfide rock material with increased surface area to
air and water.
The oxidation of sulfide.minerals consists of several reactions. Each sulfide mineral has a different
oxidation rate. For example, marcasite and framboidal pyrite will oxidize quickly while crystalline
pyrite will oxidize slowly. For discussion purposes, the oxidation of pyrite (FeSj) will be examined
(Manahan 1991). Other sulfide minerals are identified in Table 2.
Table!. Partial List of Sulfide Minerals
Mineral
Pyrite
Marcasite
Chalcopyrite
Chalcocite
Sphalerite
Galena
Millerite
Pyrrhotite
Arsenopyrite
Cinnabar
ComposftioB
FeSj
FeSj
CuFeSj
CujS
ZnS
PbS
NiS
Fe,.xS (where 0 4H*
2FC2
In this step, Sf is oxidized to form hydrogen ions and sulfate, the dissociation products of sulfuric
acid in solution. Soluble Fe2* is also free to react further. Oxidation of the ferrous ion to ferric ion
occurs more slowly at lower pH values:
4H+ -
2H2O
At pH levels between 3.5 and 4.5, iron oxidation is catalyzed by a variety of Metallogenium, .a
filamentous bacterium. Below a pH of 3.5 the same reaction is catalyzed by the iron bacterium
Thiobatillusferrooxidans. Other bacteria capable of catalyzing the reaction are presented in Table 3.
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Add Mine Drainage Prediction
If the ferric ion is formed in contact with pyrite the following reaction can occur, dissolving the
pyrite:
2FeS2(s) + 14Fe3+ + 8H2O -
16H+
This reaction generates more acid. The dissolution of pyrite by ferric iron (Fe3*), in conjunction with
the oxidation of the ferrous ion constitutes a cycle of dissolution of pyrite. Ferric iron precipitates as
hydrated iron oxide as indicated hi the following reaction:
3H2O <-> Fe(OH)3(s) + 3FT
Fe(OH)3 precipitates and is identifiable as the deposit of amorphous, yellow, orange, or red deposit
on stream bottoms ("yellow boy").
Table 3. Sulfide Ore Bacteria and Their Growth Conditions
v Microorganism
Thiobatillus thioparus
T. ferrooxidans
T. thiooxidans
T. neapolitanus
T. denitrificans
T. novellus
T. intermedius
T. perometabolis
Sulfohbus acidocalderius
Desulfovibrio desulfuricans
pH
4.5-10
0.5-6.0
0.5-6.0
3.0-8.5
4.0-9.5
5.0-9.2
1.9-7.0
2.8-6.8
2.0-5.0
5.0-9.0
Temp., *C
10-37
15-25
10-37
8-37
10-37
25-35
25-35
25-35
55-85
10-45
Aerobic
+
+
+
+
+/-
+
+
+
+
-
Nutrition
autotrophic
tf
n
ft
n
n
n
ti
n
heterotrophic
(Source: Thompson "1988)
1.2 Source of Acid and Contributing Factors
The potential for a mine to generate acid and release contaminants is dependent on many factors and
is site specific. Ferguson and Erickson identified primary, secondary, and tertiary factors that control
acid drainage. These factors provide a convenient structure for organizing the discussion of acid
formation in the mining environment. Primary factors involve production of the acid, such as the
oxidation reactions. Secondary factors act to control the products of the oxidation reaction, such as
reactions with other minerals that consume acid. Secondary factors may either neutralize acid or
react with other minerals. Tertiary factors refer to the physical aspects of the waste management unit
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Acid Mine Drainage Prediction
(e.g., pit walls, waste rock piles, or tailings impoundments) that influence the oxidation reaction,
migration of the acid, and consumption. Other downstream factors change the character of the
drainage by chemical reaction or dilution (Ferguson and Erickson 1988). These downstream factors
are beyond the scope of this paper and are not discussed herein.
I
Primary factors of acid generation include sulfide minerals, water, oxygen, ferric iron, bacteria to
catalyze the oxidation reaction, and generated heat. Some sulfide minerals are more easily oxidized
(e.g., framboidal pyrite, marcasite, and pyrrhotite) and hence, may have a greater impact on tuning
and magnitude during acid prediction analysis compared to other metal sulfides. Also important is the
physical occurrence of the sulfide mineral. Well crystallized (euhedral) minerals will have smaller
exposed surface areas than those that are disseminated.
Both water and oxygen are necessary to generate acid drainage. Water serves as both a reactant and a
medium for bacteria hi the oxidation process. Water also transports the oxidation products. A ready
supply of atmospheric oxygen is required to drive the oxidation reaction. Oxygen is particularly
important to maintain the rapid bacterially catalyzed oxidation at pH values below 3.5. Oxidation of
sulfides is significantly reduced when the concentration of oxygen hi the pore spaces of mining waste
units is less than 1 or 2 percent. Different bacteria are better suited to different pH levels and other
edaphic factors (edaphic factors pertain to the chemical and physical characteristics of the soil and
water environments). The type of bacteria and their population sizes change as their growth
conditions are optimized (Ferguson and Erickson 1988). Table 3 identifies some of the bacteria
involved in catalyzing the oxidation reactions and their growth conditions.
The oxidation reaction is exothermic, with the potential to generate a large amount of heat, and
therefore thermal gradients within the unit. Heat from the reaction is dissipated by thermal
conduction or convection. Research by Lu and Zhang (undated) on waste rock using stability analysis
indicates that convective flow can occur because of the high porosity of the material. Convection
cells formed hi waste rock would draw in atmospheric air and continue to drive the oxidation
reaction. Convection gas flow due to oxidation of sulfide minerals depends on the maximum
temperature hi the waste rock. The maximum temperature depends on ambient atmospheric
temperature, strength of the heat source, and the nature of the upper boundary. If the sulfide waste is
concentrated in one area, as is the case with encapsulation, the heat source may be very strong.
Secondary factors act to either neutralize the acid produced by oxidation of sulfides or may change
the effluent character by adding metals ions mobilized by residual acid. Neutralization of acid by the
alkalinity released when acid reacts with carbonate minerals is an important means of moderating acid
production. The most common neutralizing minerals are calcite and dolomite. Products from the
oxidation reaction (hydrogen ions, metal ions, etc!) may also react with other non-neutralizing
constituents. Possible reactions include ion exchange on clay particles, gypsum precipitation, and
dissolution of other minerals. Dissolution of other minerals contributes to the contaminant load in the
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Acid Mine Drainage Prediction
acid drainage. Examples of metals occurring in the dissolved load include aluminum, manganese,
copper, lead, zinc, and others (Ferguson and Erickson 1988).
Some of the tertiary factors affecting acid drainage are the physical characteristics of the material,
how acid generating and acid neutralizing materials are placed, waste, and the hydrologic regime in
the vicinity. The physical nature of the material, such as particle size, permeability, and physical
weathering characteristics, is important to the acid generation potential. Particle size is a fundamental
concern since it affects the surface area exposed to weathering and oxidation. Surface area is
inversely proportional to particle size. Very coarse grain material, as is found in waste dumps,
exposes less surface area but may allow air and water to penetrate deeper into the unit, exposing more
material to oxidation and ultimately producing more acid. Air circulation in coarse material is aided
by wind, changes in barometric pressure, and possibly convective gas flow caused by heat generated
by the oxidation reaction. In contrast, fine-grain material may retard air and very fine material may
limit water flow; however, finer grains expose more surface area to oxidation. The relationships
between particle size, surface area, and oxidation play a prominent role in acid prediction methods.
As materials weather with time, particle size is reduced, exposing more surface area and changing
physical characteristics of the unit. Though difficult to weigh, each of these factors influences the
potential for acid generation and are therefore important considerations for the long-term (Ferguson
and Erickson 1988, Lu and Zhang undated).
The hydrology of the area surrounding mine workings and waste units is also important in the
analysis of acid generation potential. When acid generating material occurs below the water table, the
slow diffusion of oxygen in water retards acid production. This is reflected in the portion of pits or
underground workings located below the water table. Where mine walls and underground workings
extend above the water table, the flow of water and oxygen in joints may be a source of acid. A
similar relationship is evident with tailings, which are typically fine grained and disposed of
subaqueously; the slow diffusion of oxygen inhibits formation of acid. However, since tailings are
placed in either raised or valley impoundments, they are likely to remain saturated for only a limited
period oCtime during mine operation. Following mine closure, the free water surface hi the
impoundment may be drawn down substantially, favoring AMD conditions.
The spatial distribution of mining wastes in units, or waste placement, affects acid generation
potential. For example, the distribution of acid generating wastes with neutralizing wastes may be
controlled by the stacking sequence. Calcareous material may be mixed with or placed above sulfidic
wastes to buffer acid production or provide alkalinity to infiltrating solution prior to contact with acid
generating wastes. An alternative to layering or mixing is encapsulation. This technique attempts to
isolate acid generating wastes from oxygen and water, thereby reducing its potential to produce acid.
It is unclear if encapsulation is feasible over the long-term.
r
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Acid Mine Drainage Prediction
Wetting and drying cycles in any of the mine workings or'other waste management units will affect
the character of any acid drainage produced. Frequent wetting will tend to generate a more constant
volume of acid and other contaminants as water moves through and flushes oxidation products out of
the system. The build-up of contaminants in the system is proportional to the length of time between
wetting cycles (Ferguson and Erickson 1988, Doepker 1993). As the length of the dry cycle
increases, oxidation products will tend to accumulate in the system. A high magnitude wetting event
will flush accumulated contaminants out of the system. This relationship is typical of the increase in
contaminant load observed following heavy precipitation for those areas having a wet season.
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Acid Mine Drainage Prediction
2. ACID GENERATION PREDICTION
The objectives of predictive testing are to: (1) determine if a discrete volume of mining waste will
generate acid and (2) predict the quality of the drainage based on the rate of acid formation measured
(California Mining Association 1991). There are two important points that must be considered when
evaluating the acid generation potential of a rock material, the first is how to collect samples from
the field for use in analytical testing. The second is which analytic test method should be used. Both
points have a profound impact on the reliability of analytical tests. Results from any analytical test
are only as reliable as the samples used for the test. Once the sampling strategy is selected, an
appropriate analytical method (or methods) can be selected. Methods used to predict the acid
generation potential are classified as either static or kinetic. Factors affecting the selection of the
sampling regime and analytical method include an existing knowledge of the geology, costs, and
length of time available to conduct the test. This section will examine sample methodology and
analytic tests used to predict acid generation potential.
The following list of components describes the solid phase composition and reaction environment of
sulfide minerals. Potential contaminants are included to indicate their importance in the scope of acid
generation. These components should be kept in mind while evaluating information on acid
generation potential.
Components affecting the total capacity to generate acid are characterized by:
Amount of acid generating (sulfide) minerals present [Note: assumes total
reaction of sulfide minerals]
-X-
Amount of arid neutralizing minerals present
Amount and type of potential contaminants present.
Components affecting-the rate of acid generation include:
Type of sulfide mineral present (including crystal form)
Type of carbonate mineral present (and other neutralizing minerals, as appropriate)
Mineral surface area available for reaction
Occurrence of the mineral grains in the waste (i.e., included, liberated)
Particle size of the waste
Available water and oxygen
Bacteria.
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Acid Mine Drainage Prediction
Analytical tests used to assess a material's acid generation potential are either static or kinetic in
nature. A static test determines both the total acid generating and total acid neutralizing potential of a
sample. The capacity of the sample to generate acidic drainage is calculated as either the difference
of the values or as a ratio of the values. These tests are not intended to predict the rate of acid
generation, only the potential to produce acid. Static tests can be conducted quickly and are
inexpensive compared to kinetic tests. Kinetic tests are intended to mimic the processes found at
mining sites, usually at an accelerated rate. These tests require more time and are considerably more
expensive than static tests. Data from the tests are used to classify wastes or materials according to
their acid generating potential. This information can be collected and evaluated during the economic
analysis of mines in their exploratory phases. Based on this information, decisions can be made with
respect to specific mitigation practices for existing mines.
/
In this discussion, it will be useful to keep in mind sources of information needed to assess acid
potential. Some of the primary and secondary factors that affect the drainage character from waste
management units are presented in Table 4. The variables identified may be appropriate when
considering other than waste units, such as mine pits and underground workings.
Table 4. Sources of Information on Acid Generation Potential for New and Operating Mines
Information Type
Mine Rock Classification
Mine Rock Distribution
Acid Generation, Leaching
Potential
Drainage Water Quality
New Mine
Outcrop exposures
Exploration drill samples, logs
Geological sections
Core assays
Mine planning
Static testing
Short term leach extractions
Mineralogy
Site comparisons
Kinetic testing
Background water quality
Operating Mine
Outcrop and excavation exposures
Drill core
Production sampling
Core assays
Specific sampling from working
areas and piles
Mine planning
Mine rock placement records
Pit and underground plans and
exposures
Pile surveys
Pile drilling and sampling
Site personnel
Observation of old cores
Field sampling
Static testing of distinct sub-units
from working areas
Regular monitoring
Seep surveys
Kinetic testing
Leach extraction
**.
(Source: Modified from Robertson and Broughton, undated)
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Acid Mine Drainage Prediction
Efforts by both the mining industry and state regulatory agencies to develop the best protocols for
sampling and/or analytical methods to predict acid generation potential have demonstrated that site
specific conditions (e.g., climate and geology) dictate a case-by-case approach when evaluating acid
potential. This is complicated by the fact that a variety of research efforts on different methods by
the Bureau of Mines, EPA, and the Canadian. Mine Environment Neutral Drainage (MEND), as well
as those used by mining companies and their consultants, make comparison of data difficult. Several
authors have conducted comparative evaluations of predictive tests (Lapakko 1992, Bradham and
Caruccio 1990, Coastech 1989). Kim Lapakko of the Minnesota Department of Natural Resources
has conducted comparative evaluations of static and kinetic test methods using a range of rock types.
Bradham and Caruccio Conducted a comparative study on tailings.
When evaluating the acid generation potential, a phased testing plan selects samples appropriate for
the detail needed (California Mining Association 1991). This approach allows investment in acid
prediction testing to be commensurate with a deposit's economic potential and saves time and expense
associated with unnecessary tests. Sampling and testing should be an iterative process, collecting,
testing, and evaluating a small amount of information to establish the acid generation potential. Based
on the preliminary findings, subsequent sampling and testing can be selected to refine the information
as needed.
(T
The typical steps in predicting the acid forming potential, as described in summary documents on the
subject, are listed below (California Mining Association 1991, British Columbia AMD Task Force
1989):
1. Define the geologic (or lithologic) units that will be encountered during mining. Describe
the geology and mineralogy of these units in detail.
2. Develop a sampling plan based on understanding of geology (rock mass, etc.). Collect
samples to represent ranges of compositional variation within a rock unit (see Lapakko
1988, 1990a).
<. 3. Select static or kinetic tests and evaluate potential for acid formation.
4. Evaluate sampling criteria and conduct additional kinetic tests as required.
5. Develop a model as appropriate.
6. Based on findings, classify geologic (lithologic) units as acid, non-acid forming, or
uncertain. (Note: the potential to produce acid may vary within a given geologic unit.)
11
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Acid Mine Drainage Prediction
2.1 Sampling
Selection of samples has important implications for subsequent acid prediction testing. The purpose
of testing rock material is to allow classification and planning for waste disposal based on the
predicted drainage quality from that material. Samples must be selected to characterize both the type
and volume of rock materials and also account for the variability of materials that will be exposed
during mining. When to collect samples for testing is an equally important consideration.
Researchers agree that sampling and testing should be concurrent with resource evaluation and mine
planning (Lapakko 1990a, British Columbia AMD Task Force 1989). Sampling techniques used to
evaluate recoverable mineral resources (assay samples) are similar to those required for prediction of
acid generation potential. Active mining operations for which predictive tests were not conducted hi
advance of mining lack the advantage of front end planning; however, these mines can still use these
samples and other information collected to establish the acid generating potential.
The pressure is increasing for new operations or those hi the exploratory phase to accurately predict
future drainage water quality. By comparison, the acid drainage potential at old mines may be well
established. Examples of information needed from existing operations are the. quantity of existing
acid products, the potential and stage of acid generation in each of the waste units, and the acid
forming potential of future wastes to be generated (see Table 4). Broughton and Robertson
recommend that the first two stages of an acid prediction analysis for either new or existing mines are
(1) to review the geology and mineralogy and (2) classify the rock and collect samples (Robertson and
Broughton, undated; Broughton and Robertson, 1992).
Sample collection for prediction tests for both old and new mines should consider both geologic and
environmental factors.- Geologic factors for sample selection are primarily a good understanding of
the local geology. If available, this may include information from mines, core logs, or other sources
in the immediate area. The exploration geologist or mine geologist is probably the best resource for
understanding and describing the mine's geology in detail. This information is important to both the
sampling program and application of test results. Environmental factors include consideration of the
potential environmental contaminants hi the rock and climatic variables. A quality assurance/quality
control program should be developed and coordinated with the mine plan for sample collection and
acid generationtesting.
There are many opinions concerning the number of samples to be collected in a fixed-frequency
sampling program. One mining consulting firm recommends about 8-12 samples of each significant
rock type or 1 sample for each 1 million tons, at a minimum (Schafer 1993). In this case a
significant rock type represents one or two percent of the total mine rock volume. Gene Farmer of
the U.S. Forest Service suggests that one sample (about 1,500 grams) be collected per 20,000 tons of
waste rock, or about 50 samples for each 1 million tons (USDA Forest Service 1992). These samples
would be collected by compositing from individual drill hole cuttings prior to blasting. The British
Columbia AMD Task Force recommends a minimum number of samples based on the mass of the
12
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Acid Mine Drainage Prediction
geologic unit. Their recommended minimum sample number is 25 for a 1 million ton geologic unit,
or one sample for every 40,000 tons. Using the British Columbia method, as waste volume
increases, the number of samples decreases. For example, for a unit of 10 million tons, the minimum
sample number is 80, or one sample for every 125,000 tons (British Columbia AMD Task Force
1989).
There are reservations to prescribing a fixed number of samples for collection per volume of material.
This is particularly true for existing mines when collecting samples from waste rock dumps for acid
generation potential tests. Waste rock dumps are usually constructed by end-dumping of rock from
trucks, creating heterogeneous deposits that are very difficult to sample with confidence. Tailings are
comparatively more uniform due to milling and depositional methods used, and it is easier to
characterize their variability. Fixed-frequency sampling does not encourage the use of best judgement
on the part of the sample collector (typically a mining company). It also does not provide the
statistical basis to account for variability among samples. The determination of how many samples
should be taken at any one time appears to be dependent on variability of the site's geology and how
the mine will be developed. Due to general uncertainty regarding AMD predictive methods, it may
be prudent to sample wastes or material throughout the life of the mine.
f
Factors to consider in a sampling program for existing or planned mines include the method of
sample collection, length of time samples are to be (or have been) stored, and the sample storage
environment. Each of these can affect the physical and chemical characteristics of a sample. Samples
collected from cores exposed to the environment may be physically and/or chemically altered. If
samples are collected from drill core, contamination may be a problem if a lubricant was used. At
existing mines, tailings samples should be taken over a variety of depths to determine if oxidation of
sulfide minerals is occurring. The influence of lime addition during milling may maintain alkaline
conditions. Collecting samples of waste rock is difficult because of the variability inherent hi these
waste units. Drilling is considered to be the preferred method for collecting samples from waste rock
piles (Ferguson and Morin 1991).
*
Since individual samples will be used to test and classify larger volumes of waste, it is important to
consider how representative samples are to be collected. Compositing is a common practice used to
sample large volumes of material. Typically, composite samples are collected from drill hole cuttings
on benches prior to blasting. However, compositing merges information about the variation of
sample that would be identified if more samples were collected and analyzed. Therefore, information
about sample variability is lost (British Columbia AMD Task Force 1990, Robertson and Broughton
undated). Composite sampling of tailings may be useful as a "first look" for characterizing tailings;
compositing with stratification by lithology and alteration can help to avoid the problems of simple
composite samples (Schafer 1993).
13
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Acid Mine Drainage Prediction
2.2 Static Tests
Static tests predict drainage quality by comparing the sample's maximum acid production potential
(AP) with its maximum neutralization potential (NP). The AP is determined by multiplying the
percent of total sulfur or sulfide sulfur (depending on the test) in the sample by a conversion factor
(AP = 31.25 * %S). NP is a measure of the carbonate material available to neutralize acid. The
value for NP is determined either by adding acid to a sample and back titrating to determine the
amount of acid consumed or by direct acid titration of the sample; the endpoint pH is usually 3.5
(Ferguson and Morin 1991, Lapakko 1993a). Lapakko (1992) reported that using an endpoint pH of
3.5 measures a sample's acid neutralizing potential below 6.0, but noted that a drainage pH in the
range of 3.5 may not be environmentally acceptable. The net neutralization potential (NNP), or
acid/base account (ABA) is determined by subtracting the AP from the NP (NNP = NP - AP). A
ratio of NP to AP is also used. An NNP of 0 is equivalent to an NP/AP ratio of 1 (Ferguson and
Morin 1991). Units for static test results (AP, NP, and NNP) are typically expressed in mass (kg,
metric ton, etc.) of calcium carbonate (CaCO3) per 1000 metric tons of rock, parts per thousand.
<
If the difference between NP and AP is negative then the potential exists for the waste to form acid.
If it is positive then there may be lower risk. Prediction of the acid potential when the NNP is
between -20 and 20 is more difficult. If ratios are used, when the ratio of a sample's neutralization
}potential and acid production potential is greater than 3:1, experience indicates that there is lower risk
for acid drainage to develop (Brodie et al. 1991). For ratios between 3:1 and 1:1, referred to as the
zone of uncertainty, additional kinetic testing is usually recommended. Those samples with a ratio of
1:1 or less are more likely to generate acid. Prediction of drainage quality for a sample based on
these values requires assumptions that reaction rates are similar and that the acid consuming minerals
will dissolve (Lapakko 1992). When reviewing data on static tests, an important consideration is the
particle size of the sample material and how it is different from the waste or unit being characterized.
Information on these and other static acid prediction tests, including summaries of test results, is
available (Coastech 1989, Lapakko 1993b). The following descriptions are excerpted from Lapakko
(1993b). Lapakko (1992) has also conducted comparison tests of static methods using mine waste .
e
samples from different mines. Additional summaries of static tests have been completed by Coastech
(1989) as part of the MEND Project, and the California Mining Association (1991). Five static tests
will be summarized here and in Table 5.
14
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Acid Mine Drainage Prediction
Table 5. Summary of Static Test Methods, Costs, Advantages, and Disadvantages
Add Base Accounting
(Sobek et al, 1978)
MODIFIED Add Base
Accounting
(Coastech, 1989)
BC RESEARCH
INITIAL
(Duncan and Bruynesteyn,
1979)
Alkaline Production
Potential: Sulfur
(Caruccio et al, 1981)
Net Add Production
(Coastech 1989)
ACID PRODUCTION DETERMINATION
Acid Producing Potential
- 31.25
*TotalS
Acid Producing Potential
-31.25
TotalS
Total Acid Production =
31.25
TotalS
Total S used as indicator
300 mL HA added to
5 g rock to directly
oxidize sulfides present
NEUTRALIZATION POTENTIAL DETERMINATION
-60 mesh (0.24mm)
sample
add HC1 as indicated by
fizz test, boil one minute
than cool
titration endpt pH 7.0
cost: 34-110
-60 mesh (0.24 mm)
sample
add HC1 as indicated by
fizz test agitate for 23
hours at room
temperature
pH 1.4 -2.0 required
after six hours agitation
titration endpt pH 8.3
cost: 34-110
-300 mesh (0.038 mm)
sample
titrate sample to pH 3.4
with 1.0 NH2SO,
titration endpt not
applicable
cost: 65-170
-0.023 mm sample
^
20mL0.1NHClto0.4g
solid for 2 hours at
room temperature
titration endpt pH 4.0
cost: 34-110
particle size not
presented
acid produced by iron
sulfide oxidization
dissolves buffering
minerals
titration endpt pH 7.0
cost: 25-68
ADVANTAGES AND DISADVANTAGES
simple and short time'-3
no special equipment
and easy interpretation1
many samples can be
tested9
]
does not relate to kinetic2
assumes parallel acid/
alkaline release2-'
if APP and NP are close.
hard to interpret and
different particle size not
reflected3
simple,
short time,
no special equipment,
and easy interpretation1
does not relate to kinetic2
assumes parallel acid/
alkaline release2-3
if AP and NP are close,
hard to interpret and
different particle size not
reflected3
simple and
fairly short time'-3
no special equipment and
easy interpretation1
many samples can 'be
tested3
assumes parallel acid/
alkaline release,
different particle size not
reflected, and
if APP and NP are close,
hard to interpret9
simple,
short time, and
no special equipment1
moderate interpretation1
simple,
short time,
no special equipment,
and
easy interpretation1
limited reproducibiliry4
uncertain if extent of
sulfide oxidation
simulates mat in field
1 = Coastech 1989, as referenced in Lapakko 1993
2 = Bradham and Caruccio 1990, as referenced in Lapakko 1993
3 = Ferguson 1984, as referenced in Lapakko 1993
4 = Lawrence 1991, as referenced in Lapakko 1993
(Source: Lapakko 1993b)
15
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Acid Mine Drainage Prediction
2.2.1 ' Acid-Base Accounting (ABA)
The acid-base accounting test, a form of static testing, was developed in 1974 to evaluate coal mine
waste and was modified by Sobek et al. in 1978.. The acid production potential (APP) is determined
from the total sulfur content as follows:
1 . »
31.25 x percent S = APP
and assumes that two moles of acid will be produced for each mole of sulfur. Units for APP are tons
of acidity per ton of rock. Neutralization Potential (NP) is determined first by a simple fizz test to
select the acid strength to use in the next step. Based on this information, hydrochloric acid is added
to the sample and the sample is boiled until the reaction stops. The resulting solution is back titrated
to pH 7 with sodium hydroxide to determine the amount of acid consumed in the reaction between
HO and the sample.
The net neutralizing potential (NNP) is determined by subtracting the APP from the NP and is a
t
measure of the difference between the neutralizing and acid forming potentials. The value for NNP
may be either positive or negative. Tests conducted by Ferguson (reported by Lapakko 1993b)
indicate that NNP values less than 20 (kg CaCO3/ton) are likely to form acid. Those with NNP
.values greater than 20 were not likely to form acid. For NNP values between -20 and 20 it was
difficult to determine the acid potential.
*
Assumptions of the test are that all the sulfur in the sample is reactive. This assumption does not take
into account the presence of gypsum and other non-reactive sulfur minerals. A shortcoming of the
technique is the potential to overestimate NP in one or more of the following ways: (1) use of strong
acid may dissolve minerals that would not otherwise react to maintain drainage pH within an
environmentally acceptable range; (2) use of boiling acid may cause an overestimation of NP by
reacting with iron and manganese carbonates, which would not otherwise factor in the natural NP
(this observation js problematic with samples that contain large quantities of these carbonates; (3) the
NP may be underestimated by contribution from metal hydroxides that precipitate during the titration
with sodium hydroxide.
2.2.2 Modified Acid Base Accounting
The Modified Acid Base Accounting method is similar to the previous method with some exceptions.
ft calculates APP on the sulfide sulfur content (Lawrence 1990). This is different from the total
sulfur calculation used in the ABA test in that the sulfur contribution from non-sulfide sources is not
included. Determination of NP uses a longer (24-hour) acid digestion at ambient temperature, rather
man boiling hydrochloric acid as used in the ABA method. When back titrating with sodium
hydroxide to determine the acid consumed in the digestion, an endpoint of 8.3 is used instead of 7.
16
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Acid Mine Drainage Prediction
This modified method assumes that sulfur present as sulfate is not acid producing, and therefore may
underestimate available APP if jarosite or other acid producing sulfate minerals are present.
Conducting the acid digestion at standard temperature may reduce the contribution of iron carbonate
minerals when determining the NP.
2.2.3 British Columbia Research Initial Test (BC)
The B.C. Research Initial Test, as developed by Duncan and Bruynesteyn (1979), is similar to the
ABA test in that it calculates APP based on total sulfur. Consequently, similar concerns should be
kept in mind for the APP values. NP (or acid consuming capability) is determined by titrating the
sample with 1.0 normal sulfuric acid to pH 3.5. Coastech (1989) notes that this requires more
sophisticated equipment (i.e., automatic titrator) than the ABA procedure and is more time
consuming. Samples are crushed to minus 400 mesh. Data for APP and NP are compared by
difference or ratio, as described above. If a sample is determined to be potentially acid generating,
the B.C. Confirmation kinetic test may be conducted. This test is presented in the next section.
2.2.4 Alkaline Production Potential: Sulfur Ratio (APP:S)
The Alkaline Production Potential: Sulfur Ratio test was developed by Caruccio et al. (1981) and
modified by Coastech (1989) to measure the acid forming potential of coal waste. Like the ABA and
B.C. initial tests, the APP:S test uses total sulfur to determine the total acid potential. Again, similar
problems exist for the APP:S test as were experienced with these other tests. A change in
nomenclature should be noted here. The acid consuming potential (NP in the previous tests) is
referred to as the Alkaline Production Potential. The value is determined by grinding a 500 mg
sample to minus 23 micron "and adding 20 mL of 0.1N HC1 and allowing it to react for 2 hours at
ambient temperature. The sample and solution are then titrated to pH 5 to determine the alkaline
production potential.
Samples representative of the geologic variation at the site are collected as in other tests and the
Alkaline Production Potential is determined. Results from the alkaline production potential test are
plotted with the results for total sulfur content of the same samples. Samples of several APP:S ratios
are selected for kinetic testing to determine which will be acid producing. With this calibration, the
acid producing potential of future samples from the various geologic units can be projected based on
the APP:S ratio, rather than depending on kinetic tests, which require more time.
Because this test uses total sulfur, similar to the ABA, to determine acid production, it also tends to
overestimate potential acid production for samples containing sulfate minerals. Coastech (1989) noted
the shorter exposure to less concentrated acid used in the digestion reaction would tend to
underestimate Alkaline Production Potential (NP), and preclude the complete reaction of all buffering
carbonates present.
17
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Acid Mine Drainage Prediction
2.2.5 Net Acid Production Test
In the Net Acid Production Test, hydrogen peroxide is used to accelerate the oxidation of sulfide
(Lawrence et al. 1988). For the test, five grams of material are oxidized by 100 mL of 15 percent
hydrogen peroxide to oxidize the metal sulfide minerals. The reaction generates acid which in turn
reacts with the buffering minerals in the sample. The reaction is allowed to continue for one hour
after all visible signs of reaction have ended. The pH of the solution is determined and then titrated
to pH 7. This gives a value for the. Net acid or neutralizing potential of the sample. This test is
different from static tests described above in that it mimics the reaction of APP and NP and
determines a single value, NNP. One potential limitation of the test was noted. If the extent of
oxidation in the field setting is greater than in the test, the potential exists for the test to underestimate
acid production, creating the possibility that some acid producing waste may be incorrectly classified
as non-acid-producing.
2.3 Kinetic Tests
Kinetic tests are distinguished from static tests in that they attempt to mimic natural oxidation
reactions of the field setting. The tests typically use a larger sample volume and require a much
longer time for completion than for static tests. These tests provide information on the rate of sulfide
mineral oxidation and therefore acid production, as well as an indication of drainage water quality.
Of the different kinetic tests used, there is no one test that is preferred. The preference for tests
changes with time as experience and understanding increase. In a 1988 summary article by Ferguson
and Erickson, the B.C. Research Confirmation Test was considered to be the most widely used. A
similar 1991 article by Ferguson and Morin stated that the use of modified humidity cells was
becoming more common. From information reviewed for this report, there does seem to be a trend
toward the preference for modified humidity cell and column type tests.
Kinetic tests can be used to assess the impact of different variables on the potential to generate acid.
For example, samples may be inoculated with bacteria (a requirement for some tests); temperature of
the sample environment may also be controlled during the test. Most tests require the sample particle
size to be less than a specified sieve size (e.g., minus 200 mesh). Larger sample volumes and test
equipment may examine acid potential from coarse particles. Acid drainage control mechanisms,
such as increasing alkalinity by adding lime, may also be examined using kinetic tests.
It is helpful to supplement kinetic tests with an understanding of empirical data characterizing the
sample. Examples include analysis of specific surface area, mineralogy, and metals. Such
information may affect the interpretation of test data and are important when making spatial and
temporal comparisons between samples based on the test data. As with static tests, it is important to
consider the particle size of the test sample, particularly when comparing test results with field scale
applications.
18
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Acid Mine Drainage Prediction
Seven kinetic tests are summarized primarily from Lapakko's (1993b) review and the BC AMD Task
Force, Draft Technical Guideline, Volume I (1989). Other sources are noted in the text. Brief
descriptions of the kinetic tests discussed are also presented in Table 6.
2.3.1 Humidity Cell Tests
Both Standard and Modified Humidity Cell Tests are used to determine the rate of acid generation.
Tests are conducted in a chamber resembling a box with ports for air input and output. The modified
humidity cell uses crushed samples and resembles a column. There is no standard for either humidity
cell test. ,
The Humidity Cell Test, as conducted by Sobek (1978), leaches a 200 g sample crushed to minus
2.38 mm in an enclosed plastic container. The test is typically run for ten weeks and follows a seven
day cycle. The sample may be inoculated with bacteria. During the seven day cycle, dry air is
passed through the sample container for the first three days and humidified air for the next three days.
On the seventh day the sample is rinsed with 200 mL of distilled water. The solution may be
analyzed for pH, acidity, alkalinity, and specific conductance; redox potential (the oxidation-reduction
potential of an environment),* sulfate, and dissolved metals may also be tested. The humidity cell test
method is very similar to the column test described below.
Depending on the sample, the test duration may need to be extended. Monitoring sulfate and
dissolved metal loads is important to track both the oxidation reaction and metal mobility. Two
points are important when using this and other kinetic tests: (1) if the sample was allowed to react
before testing began (e.g., in storage) there may be a build up of oxidation products in the sample
this would be flushed out in the early water rinses, and (2) neutral drainage may lead to an incorrect
prediction of acid potential if the test period is not long enough.
2.3.2 Soxhelet Extraction Tests
This test-simulates geochemical weathering using a soxhelet extraction apparatus to recirculate
solution through the sample. The sample is placed in a thimble in the unit and solution is circulated
from a reservoir. Two procedures are usedone is the standard test described by Singleton and
Lavkulich (1978); the other is the modified test described by Sobek et al. (1978). In the standard test
the sample is leached using a 70°C solution of acetic acid or distilled water over a period of six
weeks (duration of the procedure may vary). The modified test uses only distilled water at 25 °C.
Research by Coastech (1989) determined that use of acetic acid yielded unrealistic results. Soxhelet
extraction test conditions are more extreme than other kinetic tests. However, it is a shorter test and
may be useful in simulating long weathering trends in a relatively short test time. Drawbacks include
the complex equipment required and the more complex nature of the test in general.
19
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Acid Mine Drainage Prediction
Table 6. Summary of Some Kinetic Test Methods, Costs, Advantages, and Disadvantages
HUMIDITY CELLS
(Sobek et al.. 1978)
SOXHELET EXTRACTION
(Singleton and Lavkulich, 1978;
Sullivan and Sobek, 1982)
COLUMN TESTS
(Bruynesteyn and Hackl, 1982;
Hood and Oertel, 1984)
SUMMARY OF TEST METHOD
-2.38 mm particle size
200g of rock exposed to three days dry air,
three days humidified air, and rinsed with
200 mL on day seven
cost: 425-850
particle size not presented
T=70"C (Singleton and Lavkulich, 1978)
T=25"C (Sullivan and Sobek, 1982)
water passed through sample is distilled
and recycled through sample
cost: 212-425
variable particle size
columns containing mine waste are leached
with discrete volumes or reciiculating
solutions
cost: dependent upon scale
ADVANTAGES AND DISADVANTAGES
models AP and NP well and
models wet/dry3
approximates field conditions and
rate of acidity per unit of sample
moderate to use,
results take long time, and
some special equipment1
moderate ease of interpretation1-3
large data set generated2
simple,
results in short time, and
assessment of interaction between AP and
NP3
moderate to use and
need special equipment1
moderate interpretation1-3
in developmental stage and
relationship to natural processes not clear*
models AP and NP,
models effect of different rock types,
models wet/dry, and
models different grain sizes3
difficult interpretation,
not practical for large number of samples3
large volume of sample1
lots of data generated,
long time, and
potential problems: uneven leachate
application, channelization"
(Source: Lapakko 1993b)
BC RESEARCH CONFIRMATION
(Duncan and Walden, 1975)
BATCH REACTOR
(Halbert et al., 1983)
FIELD TESTS
(Edger and Lapakko, 1985)
METHOD
-400 mesh particle size
15-30g added to bacterially active solution
at pH 2.2 to 2.5, T=35°C
if pH increases, sample is non acid
producer
if pH decreases. 1/2 original sample mass
is added in each of two increments
cost: 170-340
-200 mesh particle size1
sample/water slurry is agitated
200g/500 mL1
cost: 425-850
field scale particles
800 to 1300 metric ton test piles
constructed on liners flow and water
quality data collected
tests began in 1977 and are ongoing
cost: initial construction is expensive,
subsequent costs are comparable
' ADVANTAGES AND DISADVANTAGES
simple to use,
low cost,
assesses potential for biological leaching3
moderate to use,
longer time needed, and
some special equipment needed1
difficult interpretation if pH change is
small,
does not model initial AP step, and
long time for pH to stabilize3
able to examine many samples
simultaneously and
relatively simple equipment4
subject to large sampling errors and
lack of precision4
uses actual mine waste under
environmental conditions
can be used to determine drainage volume
mitigation methods can be tested
expensive initial construction
long time
(Source: Lapakko 1993)
1 = Coastech 1989, as referenced in Lapakko 1993
2 = Bradham and Caruccio 1990, as referenced in Lapakko 1993
3 = Ferguson 1985, as referenced in Lapakko 1993
4 - Babij et al. 1980, as referenced in Lapakko 1993
20
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Acid Mine Drainage Prediction
2.3.3 Column Tests
Column Tests are conducted by stacking the waste or material in a cylinder or similar device.
Wetting and drying cycles are created by adding water and then allowing the column to dry. Each of
the cycles may occur over a period from several days to a week or more, though they typically last
for three days each. Care must be taken to avoid piping along the sample-wall interface when
packing the column. Water added to the column is collected and analyzed to determine the current
oxidation rate, sulfate production, metal release, and other parameters.
Column test equipment, like humidity cells, is a relatively simple apparatus compared to a soxhelet
extraction device. It is easily modified to test control options, such as the addition of limestone, the
influence of bacteria, and water saturation (Water Resources Control Board 1990). Results from
research indicate that column tests of well sorted tailings material greater than 0.5 cm in diameter
accurately represents field test conditions (Bradham and Caruccio 1990). Tests of waste rock material
were not reported. Some of the disadvantages of column type tests are that the long time required,
the associated high costs, and as mentioned above, the potential for channeling.
2.3.4 British Columbia Research Confirmation Test
Originally developed by Duncan and Bruynesteyn (1979), this test is intended to confirm results of the
B.C. Initial (static) Tests; specifically, it is intended to determine if bacteria can catalyze enough
reactions to satisfy their acid demands. As described in the Draft Technical Guide, Volume I (1989),
sulfuric acid is added to a sample volume to a pH of 2.5. Although not identified in the Draft
Technical Guide, other researchers use sample volumes in the range of 15 to 30 g of material passing
a 400 mesh screen (Lapakko 1993b). The sample is shaken for four hours and acid is added to
maintain a solution pH "between 2.5 and 2.8. The sample is then inoculated with Thiobacillus
ferrooxidans and the flask weighed. The flask is plugged with cotton, incubated at 35°C, and shaken
continuously. The pH and metals in solution are monitored for the first three days and the pH
maintained below 2.8.^ Distilled water is added to maintain constant weight. When the pH is
established below 2.8, monitoring for pH and the metal is performed every second day until
microbiological activity stops. This occurs when pH and metal values remain constant. Additional
sample material is then added to the flask and this is shaken for 24 hours. When tested, if the pH is
3.5 or higher, the test is terminated. If the pH of the solution is less than 3.5, more of the sample is
added and is shaken for 24 hours. The pH is tested; if it is greater than 4 or less than 3.5, the test is
terminated. If the pH is less than or equal to 4, or greater than or equal to 3.5, the sample is shaken
for 48 more hours and a final pH reading is taken (British Columbia AMD Task Force 1989).
If the bacteria are sustained in the sample, there is a strong possibility that acid drainage will be
generated hi the waste unit being characterized (British Columbia AMD Task Force 1989). If
insufficient acid is produced, the solution pH will approach the natural pH (above 3.5), and the
21
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Acid Mine Drainage Prediction
sample is determined to be non-acid producing. If the solution remains below 3.5 then there is a
strong possibility that the sample will be an acid producer.
The initial acidification of the sample in this test presents conditions significantly different than in a
typical waste unit. The test does not examine mineral/bacterial reactions above a pH of 2.5 (2.8 as
described above). Reactions above these levels may be a major influence hi determining if acid
drainage is generated (Lapakko 1993b). Other disadvantages are that the test ignores neutralization
potential and sulfide oxidation rates (British Columbia AMD Task Force 1989).
2.3.5 . Batch Reactor (Shake Flask) Tests
In the Batch Reactor test, like the British Columbia Confirmation test, a mine sample and water are
shirried together hi a flask. The solution is usually distilled water, however, nutrients may be added.
Sample size and solution volume are determined by the user. Coastech (1989) conducted tests using
250 g of waste and 500 mL of distilled water. Flasks are shaken continuously during the test. Water
samples are taken at regular intervals to determine water quality parameters such as pH, sulfate, and
metals hi solution. Sampling for water quality analysis during longer tests may require addition of
water to maintain volume. This would complicate interpretation of test data. Data from the tests are
used to estimate the rate of sulfide mineral oxidation and release of contaminants, such as metals.
The batch reactor is relatively simple and allows examination of multiple factors, such as pH and
temperature, which can be tested simultaneously.' The influence of bacteria and control measures may
be used as test parameters. The primary difficulty with the method is that the duration of the test
may not exceed the lag time prior to acid formation (Lapakko 1993b). Other concerns are that the
water volume hi the flask may inhibit acid formation and bacteria may not acclimate hi the test
. conditions (British Columbia AMD Task Force 1989).
2.3.6 Field Scale Test
Fidd Scale Testing, similar to On-site Rock Piles described by B.C. AMD Task Force, use large
volumes"of material to construct test cells hi ambient environmental conditions, typically at the mine
site in question. These tests are very different from laboratory tests where the experiment is
conducted under controlled conditions. Sample size varies and may be as much as 1000 metric tons
or more, depending on space availability. Particle size of the test material is not usually reduced for
the test to better approximate field conditions. The sample is loaded on to an impervious liner to
catch solutions and a vessel is used to collect the leachate. The volume of solution is determined and
an aliquot is analyzed for pH, sulfate, dissolved metals, and other parameters.
Consideration of climatic conditions is important when evaluating results from field scale tests.
Climatic effects must be distinguished from the rate of sulfide oxidation, acid generation,
neutralization, and metal dissolution as determined by analysis of the leach solution. This is
22
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Acid Mine Drainage Prediction
necessary because climatic effects, especially precipitation, determine the flushing rate but do not
influence either reaction rate or the subsequent chemical composition of the leachate (British
Columbia AMD Task Force 1989).
Lapakko (1988) demonstrated that carefully constructed kinetic tests hi the laboratory could be
extrapolated to field scale tests. That research is summarized in Section 5.3 of the report.
Field scale tests have the advantage of being conducted under the same environmental conditions as
the waste or other units they are simulating. They also allow monitoring of the influence of bacteria
and control measures. (Drawbacks to field tests are that they require long test durations. Unlike other
kinetic tests, field test do not accelerate environmental conditions, which tend to assess the potential
to generate acid more quickly. Consequently, field tests will provide information on acid generation
potential for a mine waste unit for that amount of time that they are started before waste emplacement
begins. For some operations this may be 10 years or more and test results may be used to optimize
reclamation design (Lapakko 1993b).
2.4 Application of Test Results in Prediction Analysis
Results from static and kinetic tests are used to classify mine wastes on the basis of their potential to
generate acid. Static tests yield information about a sample's ability to neutralize and generate acid.
The difference or ratio of these values becomes the basis of the classification. As discussed, for
samples with NNP values greater than 20 tons CaCO3/1000 tons of waste (ratio of 3:1), the potential
to generate acid is low (Smith and Barton-Bridges 1991). For NNP values between -20 and 20 (ratios
between 1:1 and 3:1), the potential for acid generation remains, and uncertainty will exist. It is
important to note that each of these values are generalities and can be affected by the relative
availability of surface areas of iron sulfides and calcium-magnesium carbonates.
The determination of AP based on estimated or reactive sulfur content in the sample has some
inherent limitations. When total sulfur is used as the basis to estimate sulfide content, this uncertainty.,
may*e attributable to possible errors in: (1) assessment of true acidity and neutralization in the
sample; (2) calculated acidity based on total sulfur conversion value; and (3) analytical error. Similar
errors exist for static tests that determine reactive sulfide mineral concentrations. Estimating long-
term reactive sulfide based on short-term tests may result in uncertainty due to difficulties hi making
oxidation rate predictions (British Columbia AMD Task Force 1989).
Acid base accounting tests conducted on an iterative basis, where the initial sample set is small, are
helpful when establishing boundaries between lithologic units. As data from static tests is collected
and evaluated, the sampling selection can be refined. The goal of sampling is to collect representative
samples that define the variability of the lithologies present. If significant variability hi the acid
generation or neutralization potential is identified hi the initial sample test results, additional sampling
to refine lithologic boundaries is necessary (California Mining Association 1991).
23
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Acid Mine Drainage Prediction
Kinetic tests are often conducted to confirm results of static tests and estimate when and how fast acid
generation will occur. The test provides insight on the rate of acid production and the water quality
potentially produced and is used to evaluate treatment and control measures. Unlike static tests, there
is no standardized method for evaluating test results. Data are examined for changes through time
and water quality characteristics. Kinetic tests tend to accelerate the natural oxidation rate over those
observed in the field. This may have the advantage of condensing time, and providing earlier insight
into the potential for acid generation.
Generally, kinetic tests are evaluated for changes in pH, sulfate, acidity and a host of potential metals.
According to the B.C. AMD Task Force (1989), samples with pH values less than 3 are considered
strongly acid; between 3 and 5 the sample is acid generating and there may be some neutralization
occurring; at pH values >5, the sample is not significantly acid, or an alkaline source is neutralizing
the acid. Sulfate is a by-product of sulfide oxidation and can be used as a measure of the rate of
oxidation and acid production. When evaluating test data it is important to examine the cumulative
sulfate production curve as an indicator of sulfide oxidation, in addition to other parameters. An
analysis of metals in the sample solution serves as an indicator of contaminant load but is not a good
indicator of acid generation.
«i&
N*-
Based on test data, decisions with respect to the mine plan are made. Similar to static tests, kinetic
tests are refined to address variability of the geology. Information collected from kinetic tests, such
as oxidation rates and water quality, are more commonly being used as inputs to models, which are
discussed in the following section.
2.4.1 Some Experience With Static and Kinetic Tests
«
Ferguson .estimated that for about SO percent of the mines it is easy to determine whether acid
generation is a problem, and noted that predicting the potential for the other 50 percent is more
difficult (U.S. FJ*A 1992a). When data collected from static and kinetic tests is inconclusive it may
be necessary to extrapolate from existing data using oxidation rates and other factors and project how
a sample may react. The soundness of the extrapolation is dependent on the representativeness of the
sample, accuracy of the tests data, and the interpretation of the data.
Ferguson and Morin (1991) found that samples with an NP/AP ratio of less than 0.1 tended to
produce acid during typical laboratory timeframes. They expected that if laboratory tests were
conducted for longer time periods the NP/AP ratio would shift closer to 1 and did not speculate on
what the values for NNP and NP/AP would be in the future. Extrapolating a sample's ability to
generate acid was divided into short (less than one year), medium (a few years), and long-term (many
years) time frames. Short term projections are based on laboratory data. Medium term projections
require knowledge of the neutralization process, primarily consumption of carbonate. Long-term
extrapolations of acid generation potential will require an understanding of weathering rinds and
24
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Acid Mine Drainage Prediction
diffusion of oxygen into and reaction products out of that rind. Long-term projections were identified
as being extremely problematic.
Researchers in British Columbia, Canada, have examined results of static and kinetic tests conducted
on tailings and waste rock (Ferguson and Morin 1991). The results are based on a study of 20 active
or abandon mines in British Columbia. Their findings indicate that for tailings, only those samples
having a negative NNP produced acid. The test method was not identified and the limitations are
therefore not discussed here. According to this report, waste rock data from static tests is very
limited and demonstrates the variability expected with these waste units. They observed that samples
of waste rock that had weathered for one month (prior to sample collection) needed to be flushed
initially to remove existing oxidation products.
Lapakko (1990b) used solid phase characterization of the sample in conjunction with acid base
accounting data and the rates of acid production and consumption to extrapolate information beyond
the timeframe of kinetic tests. The rates of acid production and consumption were based on kinetic
test results over a 20 week period. The time required to deplete sulfide and carbonate minerals was
determined using rates established from kinetic tests. Based on these observations the tune required
to deplete the iron sulfide content was 950 weeks and the tune to deplete the carbonate content was 40
weeks. This prediction agreed with an observed drop hi pH between week 36 and week 56 from 8.7
to 6; after another 20 weeks the pH dropped below 5. This research appears to indicate that kinetic
tests should be run for periods of at least 20 weeks in length.
2.5 Mathematical Modeling of Acid Generation Potential
As the preceding discussion indicates, static and kinetic testing provide only a partial picture of the
potential of mine wastes to produce ARD. Static testing estimates the ultimate APP and NP of waste
material but is generally silent with regard to the rates of generation of acidic and alkaline flows hi
actual waste matrices. Kinetic testing is more helpful with regard to estimating the rates of oxidation
and neutralization^ As discussed above, actual waste units can be very non-homogenous and
anisotropic with respect to the distributions of mineral types, particle size, hydrologic conditions and
so forth. Thus, while a given kinetic test may well approximate the potential for ARD in a portion of
a waste unit, the result may not be representative of the "global" potential for ARD. Equally
important is the practical limitation on the duration of kinetic tests: because kinetic tests are generally
short-lived with respect to the potential period of persistence of AMD, they inadequately mimic the
evolutionary nature of the process of acid generation.
To overcome the uncertainties inherent in short-term testing, as well as avoid the prohibitive costs of
very long-term testing, some researchers have developed mathematical models to aid in predicting the
long-term effects on water quality of acid generating wastes. Predictive modelling offers the hope of
providing tools for estimating the potential extent of acid generation prior to its occurrence. Ideally,
such information may be compared for scenarios entailing alternative management options to identify
25
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Add Mine Drainage Prediction
the design, operating, and closure methods that best meet economic and environmental objectives. As
a practical matter, existing AMD models fall short of the ideal. Nevertheless, these models may
provide valuable information for planning purposes, and may have an important role hi understanding
and predicting AMD.
2.5.1 Overview of Existing Models
A number of distinct approaches to modelling ARD have emerged to date. In general, all the models
attempt to describe the time-dependant behavior of one or more variables of a mine waste
geochemical system in terms of observed behavior trends (empirical models) or chemical and/or
physical processes that are believed to control ARD (deterministic models). Empirical models
extrapolate values for the desired output variables (e.g., acid generation) from laboratory or field data
(British Columbia AMD Task Force 1989). Deterministic models simulate the changes in system
values according to the causal mechanisms relating each element of the system to the others.
It is important to remember that all ARD models are simplifications of reality. Simplification is
required by incomplete understanding of all factors influencing ARD. Simplification can substantially.
reduce the cost and tima required to model the system under study. However, simplifying
assumptions can lead to incorrect conclusions if they result in the omission of important causal
mechanisms. For instance, failure to consider the presence of neutralizing materials in a waste pile
could result hi an overestimation of the rate of acid generation. Similarly, failure to consider
hydrogeochemical conditions within a waste pile may preclude consideration of
adsorption/precipitation reactions involving metals, thereby miscalculating the potential for metals
loading hi effluent streams. Because the importance of any given controlling factor may vary from
site to site, the significance of a simplifying assumption for any particular modelling effort must be
weighed carefully.
2.5.2 Empirical Models
- As stated above, empirical models extrapolate values of sulfide oxidation from existing laboratory
field test data. The methed of extrapolation typically involves determination of the "best-fit lines"
through test data points (British Columbia AMD Task Force 1989). The equations so derived may
then be solved to provide, for instance, the acid generation rate of a particular waste unit at some
time hi the future. Using the projected acid generation rate as an input to a separate
hydrogeochemical model that accounts for attenuation of seepage constituents hi soils and dilution in
receiving waters, the estimated constituent loading rates and consequent receiving water quality at
tune T may be estimated (Broughton and Robertson 1991).
Empirical models generally do not explicitly consider the causal mechanisms driving oxidation of
sulfides and neutralization of seepage. Rather, such models assume that the operation of such
controls is accurately represented hi the test data. Therefore, the accuracy of empirical models hi
26
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Acid Mine Drainage Prediction
predicting AMD depends heavily on the quality of the test data used in the models. Principle sources
of uncertainty may be expected to include variations in the spatial and particle size distribution of
sulfide and alkaline minerals not captured by the data due to insufficient spatial distribution of
samples; changes in the distribution of particle sizes throughout the waste unit (due to weathering) not
captured by the data; and failure to accurately calibrate the model to reflect the actual quantity and
type of materials disposed of (British Columbia AMD Task Force 1989).
It is important to note that empirical models, by their nature, are site-specific. Because the models
i
rely on actual trends observed at a specific site, rather than generic causal mechanisms, the best fit
lines for one site can not be assumed to be representative for another site. Further, significant
changes hi waste unit composition, geometry, or controls over time may invalidate previous
representativeness of empirical models. Nevertheless, empirical models may provide cost-effective
and reasonably reliable estimations of short-term future AMD conditions for sites with sufficient
spatial and temporal data.
2.5.3 Deterministic Models
Deterministic models simulate AMD by solving systems of equations that represent the various
controlling factors in the waste reaction process (Broughton and Robertson 1991). The simulation
approach allows the users to examine the potential sulfide oxidation rate and resulting seepage quality
over periods of tens to hundreds of years in the future. The greatest promise of deterministic models
is that they may allow the user to predict AMD as it evolves over time under the changing influence
of rate controlling factors. Existing models have built upon earlier work on acid releases from coal
mine spoils as well as work on leachate quality in metals heap leach operations (Nicholson 1992).
The models may rely solely on the causal relationships described in the equations, or may include
empirical data as exogenous drivers (outside the model structure) to solve for certain aspects of the
system (Nicholson 1992; Broughton and Robertson 1991). The most important differences between
the models lie in the particular causal mechanisms (e.g., oxygen diffusion, changing particle size,
temperature variations due to exothermic reactions) addressed within each model structure.
»»
' « .
Nicholson presents a review of AMD models. In that review, Shumate (1971)1 is credited with first
recognizing that diffusion of oxygen within mine rock limits the overall rate of oxidation of sulfides
(Nicholson 1992). The first working models to incorporate this process (Morth 19721, Rica and
Chow 19741) used the acid generation rate to calculate resulting drainage water quality. Rittchie
(1977)1 added to this concept by explicitly accounting for the removal of oxidized sulfur from the
store of available unreacted sulfide. Other models have included convection as a means of oxygen
transport within waste piles (Lu and Zhang undated). Convection may be influenced by changes in
barometric pressure or by the release of heat from the exothermic oxidation of sulfides. Some
'As cited in Nicholson 1992.
27
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Acid Mine Drainage Prediction
researcher's have modelled the feedback mechanisms operating between temperature and biological
and chemical oxidation rates, noting that the mechanism is only significant where waste permeabilities
are high enough to allow convective oxygen transport to occur (Nicholson 1992).
More recent models have addressed the hydrologic and geochemical conditions in waste unit matrices,
as well as reaction product transport, to more realistically represent changes in seepage quality
(Nicholson 1992). Bennett (1990)' and others found that water flow through the waste pile strongly
influences sulfide oxidation rates by acting as a heat sink and removing heat produced by oxidation.
Jaynes et al. (1986)' and Schafer (1991)1 have incorporated chemical equilibrium relationships of
varying complexity to model the mobilization and attenuation of oxidation and dissolution products
within the waste pile. These relationships drive the residence times of various constituents within
"mixing cells" of the waste matrix, and, along with allowing for consumption of acid by alkaline
materials, result in changes in effluent, chemistry as conditions within the matrix evolve (Nicholson
1992).
f
Model developments such as those listed above have significantly contributed to understanding of the
processes controlling AMD. For instance, explicit consideration of oxygen diffusion reveals that, in
instances where diffusion is restricted, fast processes such as biologically catalyzed oxidation can be
unimportant to the overall rate of oxidation. Similarly, consideration of hydrologic flow within the
waste matrix shows that the rate of release of oxidation products from waste piles depends strongly on
the flow characteristics within the wastes (Nicholson 1992). More recent models have corroborated
the proposition that waste dump geometry can be important to oxidation rates by influencing the
surface area exposure and air infiltration rates (Nicholson 1992).
2.6 Conclusions
Notwithstanding the understanding that existing models have provided, AMD models to date have not
found extensive applications in predicting oxidation rates and effluent quality at operating or proposed
sites (Ferguson and Erickson 1988). As stated above, models are simplifications of reality, and
consequently are subject to a high degree of uncertainty. Among the sources of uncertainty are
incomplete or invalid model structure; natural variability of certain parameters; and lack of parameter
calibration and model verification (British Columbia AMD Task Force 1989).
Among the greatest concerns facing the reliability of predictive deterministic models are model
calibration and validation. Model parameters must be adjusted to match the conditions prevailing at
an actual site. Therefore, reliable waste characteristics, hydrologic and geochemical data must be
collected and incorporated into the model structure. Validation requires comparison of model .
predictions with actual field sampling results. To date, the availability of field data for validation is
very limited.
28
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Acid Mine Drainage Prediction
3. CURRENT REGULATORY REQUIREMENTS
Regulatory programs for five states were reviewed for their requirements concerning acid generation
prediction. A summary of the regulatory requirements for California, Minnesota, Idaho, Montana,
and Nevada is presented in Table 7. In addition, the U.S. Forest Service is developing a protocol.
In the meantime, the acid generating potential associated with mines on Forest Service land is
considered on a case-by-case basis as part of their review of proposed plans of operation.
29
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Table 7. Summary of State Regulations for Add Generation Prediction Testing (August 1994)
Regulations/Guidance
Sampling
Analysis
California1
Regulations: Waste classification under
§2571C of Chapter 15, of the California
Mining Code. No other requirements
specified.
No specific requirements indicated. Each site is considered on
a case-by-case basis.
California is considering adoption of new regulations. These
new regulations require testing of rock using a procedure for
predicting AMD. Neither specific static or kinetic testing
procedures are identified. Test results would be analyzed and
interpreted by the mining company or its contractor.
The state sets a trigger level at a 3:1 ratio (NP/AP) with a 95
percent confidence interval. If samples do not meet mis
requirement, kinetic tests are required, or the mine has the
option to develop a management plan for waste disposal.
Minnesota1
Regulations: Under $6132.1000 Mine
Waste Characterization:
Sample types include material generated from exploration, pre-
production sampling, and process testing. An outline of
chemical and mineralogical analyses and laboratory tests must
be conducted and presented to the commissioner for use in
evaluating mining and reclamation plans. Mine waste
characterization data submitted must include laboratory tests
describing acid generation and dissolved solids release from
mine waste. (Minnesota §6132.100)
Based on results of analyses and tests, additional mine waste
characterization may be required. May include laboratory
dissolution tests to describe a material's acid-producing and acid
consuming mineral content. (Minnesota §6132.100)
Results of mine waste characterization data should be submitted
throughout the life of the operation to regulatory agencies
establishing water quality and compliance monitoring standards.
(Minnesota §6132.100)
Idaho1
Regulations: There are no formal policies
or regulations that specifically address
AMD. Under Chapter 47-1513 of Idaho's
Surface Mining Act and Dredge and Placer
Mining Act, reclamation and operating plans
are required that are protective of Idaho's '
water resources. (Schuld 1993)
Protocols based on BC Acid Mine Task
Force Report "Acid Rock Drainage
Technical Guide." Also, a proposed "Policy
Guidance Memorandum" has been submitted
to the DEQ, but has not been signed by
Administrator as of April 1993. (Schuld
1993) Under mis policy, Idaho may request
Federal land managers (BLM. USFS) to
determined AGP for sites on Federal lands.
Sampling should begin during exploration. The state requires
that exploration plans stipulate mat half the samples collected
should be kept in storage. Storage should minimize potential
for sample weathering.
Materials selected for sampling should include waste rock,
overburden, and ore/subore. Composites of core samples
should be obtained as samples.
The number of samples obtained for AMD prediction testing
should be based on the size of probable ore reserves and
overburden. Consult "Acid Rock Drainage Guide" or other
technical guidance document, in addition to best professional
judgement, to determine minimum number of samples. (Schuld
1993) The sampling interval is based on lithology and changes
in units.
Reporting should occur prior to excavation and continue after
mining has commenced. Results of static tests must be
reported in order to prepare permit. '
Idaho does not require the use of a specific static or kinetic test,
however, the state must be informed of, and approve, the test
methodology selected. Tests are conducted by U.S. EPA
approved CLP laboratories only.
Idaho uses BMPs in place of monitoring requirements to prevent
any contact of AMD with groundwater or surface waters of the
State. BMPs must function to avoid AMD generation, or should
collect and treat AMD until it no longer exists.
Future goals for BMPs will include (1) Leachate
Detection/Collection Systems, (2) Closure requirements and
monitoring, and (3) Bonding.
If the ratio of acid potential (AP) to neutralization potential (NP)
is greater man 2:1, the State requires a kinetic test to be
performed.
If State waters are impacted, an NOV and/or Consent Order may
be issued, and other site specific requirements may be imposed.
I
I
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Table 7. Summary of State Regulations for Add Generation Prediction Testing (August 1994) (Continued)
Regulations/Guidance
1
SampBng
1
Analysis
Montana4
Regulations: Under Title 82, "Metal Mine
Reclamation", J 82-4-336 (7), reclamation
plans are required to provide for
"...reclamation of disturbed land to
comparable utility and stability,...". The
Slate interprets this to mean both chemical
and physical stability. Review and approval
of reclamation plans gives the State the
authority to reject plans that do not
adequately assess AMD potential.
Guidance: DRAFT, "Geochemical
Characterization Checklist", Montana
Department of State Lands. This provides
specific recommendations mat are only
summarized in this table.
Sample collection is preferred during the advanced exploration
stage of project. This allows data to be compiled and long term
leachate extraction tests to be performed before submittal for a
mining permit. Information requested includes descriptions of
climate, topography, hydrology, vegetation, geology,
mineralogy especially iron sulfides, and total element content
for mineralized and unmineralized lithologies.
The number of samples to collect is dependent on the variability
of the lithology/alteration assemblage. The British Columbia
Draft Task Force Guide (SRK, 1989) and the Saskatchewan
Mine Rock Guidelines (SRK, 1992) are used as references for a
rough guide as to methods and sample sizes needed to
characterize mine waste.
Samples should not be composited if possible. Samples should
be split 4 ways. For each sample please record: sample
location, sample description including mineralogy/petrology
especially sulphur fractionation and carbonates, grain size and
crystal form of iron sulfides, particle size distribution, paste pH,
and slaking characteristics after Brodie et al, 1991.
Two suites of samples should be collected.
1) Collect representative samples from each lithology for a
reference suite. This suite should incorporate samples from
unmineralized areas to establish regional background. The
"representativeness" for each sample should be determined by
the geologist who is most familiar with the site. This sample
set will be biased.
2) Collect random samples over the entire deposit to limit bias.
This sample set will be much larger man the representative
suite. At least 8 samples should be analyzed for each lithology
or alteration assemblage. A statistical analysis of the data
should be compiled.
For splits from each sample provide analyses for total element
or trace element and any static test. After data is reviewed and
compared to average crustal abundances and/or regional
background values, a carefully picked subset of samples should
be analyzed using any humidity cell test method and/or any field
leachate extraction test method to help establish limits for
suitable and unsuitable material. The definition of "suitable* and
"unsuitable" may vary-with each site depending on the regional
geology. Any laboratory and/or method may be used but must
be approved by the agency prior to use. Rationale must be given
as to why certain methods were used.
After all information has been compiled and reported for each
suite, decisions are made as to what materials are suitable for
reclamation purposes and which materials are unsuitable and
need to be isolated. The above mentioned testing sequence will
not predict whether a material will produce contaminants. It will
define which lithologies/alteration assemblages are suspect with
respect to contaminant production and should be segregated from
the suitable waste materials. Independent interpretations of the
data set can be forwarded but the agencies' interpretation will
comprise the effective recommendation. Testing should be
ongoing throughout mine life to better substantiate preliminary
conclusions made during the permitting process.
More detail and references are given in "Permitting Guidelines
for Mine Rock Characterization" available from the Department
of State Lands, Hard Rock Bureau, 1625 llth Ave, Helena, MT
59620. (406)444-2074.
(References: Brodie, 1991; Steffen et al, 1989; Steffen et al,
1992)
I
!
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Table 7. Summary of State Regulations for Acid Generation Prediction Testing (August 1994) (Continued)
KJ
RegulaUoitt/GuJdance
Sampling
Nevada9
Regulations address process components,
Nevada regulations, § 445.242
Guidance documents include the Nevada
Division of Environmental Protection's
(NDEP's) "Waste Rock and Overouiden
Evaluation" document, dated September 14,
1990.
This evaluation document requires the use of
the Meteoric Water Mobility Test (MWMT)
to determine a sample's potential to release
pollutants. This test does not test for AGP,
but is required as a precursor to acid
generation tests. Procedural requirements
for the MWMT are provided in NDEP's
September 19, 1990 guidance document
tided, "Meteoric Water Mobility Procedure,"
dated September 19, 1990.
Wasterockand overburden must be evaluated for its potential to
release pollutants and its acid generation potential. (NDEP
1990)
Drill core samples should be collected during initial orebody
definition, and used to characterize materials. Samples should
be sent to an assay lab. During active mining operations,
samples can be collected from remaining, saved, assayed
materials to be "representatively composited" (not defined) on a
quarterly basis for on-going evaluations. Samples are also
required of waste materials that were not subject to assaying.
(NDEP 1990)
A representative sampling program must consider lithological
and mineralogical variation, the extent of "sulfide"
mineralization, color variation, degree of fracturing and of
oxidation, and extent of secondary mineralization. (NDEP 1990)
(Source: Based on phone conversations with State personnel and collected documents)
(Source: 'Humphries, 1994)
(Source: 2Lapakko, 1994)
(Source: 'Schuld, 1994)
(Source: 'Miller, 1994)
(Source: 'Gaskin, 1994)
The Meteoric Water Mobility Procedure should be used to
determine the potential release of pollutants from samples.
Consult NDEP's "Meteoric Water Mobility Procedure", dated
September 19, 1990 for specifics regarding the procedure
requirements. Acid generation potential must be evaluated using
the Static Test, Acid/Base Accounting procedure, to determine
neutralization potential (NP). (NDEP 1990)
Acidification potential (AP) should be determined based on two
alternatives: (1) determining total sulfur content, or
(2) determining peroxide oxidizable sulphur. For alternative (1),
compare results to NP. If NP exceeds AP value by 20%,
material is considered non-acid generating. If less man 20%,
determine total sulfide sulfur content according to Standard
Methods of Chemical Analyses, or equivalent procedure. If NP
is less than 20% greater than AP, kinetic testing.must be
initiated. For alternative (2), if NP value exceeds value by .
100%, material is non-acid generating. If less man 100%,
initiate kinetic testing. (NDEP 1990)
Operating facilities with positive acid generating results from
static testing, must notify the NDEP and begin kinetic testing
within 10 days. Kinetic testing is required to be conducted
according to procedures identified in attachment I. If kinetic
testing confirms acid generating potential, containment/
neutralization methods must be evaluated on she specific basis
and proposed to the NDEP for approval. (NDEP 1990)
-------
Acid Mine Drainage Prediction
4. CASE HISTORIES
Presented below are selected case studies for mines where acid drainage from mine wastes or mine
works has occurred. Both active sites and sites on the National Priorities List are described. The
active sites were selected to represent sites where the potential to generate acid was either not
considered, or not expected, but later developed. Case histories for the Newmont Rain facility in
Nevada, Cyprus Thompson Creek in Idaho, and the LTV Steel Mining Company Dunka site in
Minnesota are presented below in Sections 4.1, 4.2, and 4.3, respectively. EPA visited each of these
sites to further its understanding of the mining industry. Each site has experienced acid generation
problems; however, it is important to note that each is also taking corrective action to mitigate the
problem. The companies are working with appropriate State and Federal agencies to determine long-
term treatment needs.
The EPA (1991) has prepared National Priorities List (NPL) Site Summary Reports for the mining
sites on the NPL. NPL sites were selected from these reports if acid generation was identified as a
problem. Using this criteria, seven of 56 mining-related sites were selected for review. The purpose
of the review was to determine if acid generation predictive tests were conducted at individual sites,
and if such tests were conducted, how the data were used. The review included examination of
available literature on each site and interviews with each site's Remedial Project Manager (RPM).
Based on incomplete information for the seven sites studied, tests for either acid prediction or pH
prediction have not been conducted at Silver Bow Creek, Eagle Mountain Mine, Tar Creek, and
Whitewood Creek. Eagle Mountain Mine and Silver Bow Creek have not conducted prediction tests
because acid generation is such a clear and extreme problem (Taylor 1993, Forba 1993, and Overbay
1993).
Sites that have assessed the acid generation potential include Clear Creek/Central City, California
Gulch, and Iron Mountain. At Clear Creek acid/base potentials were calculated for waste materials
and potential acid generation testing is being required by the City of Clear Creek for any new
development that disturbs the ground. Both the RPM and State contact for Iron Mountain indicate
that acid generation predictive tests have been done while mucking out tunnels. Acid generation
prediction has also occurred at California Gulch. Further details on the sampling and analytical
methods usedlo.predict acid generation have not been obtained. Sections 4.4 through 4.7 provide
details on acid generation prediction experiences at these three NPL sites (Fliniau 1993, Hyman 1993,
and Sugarek 1993).
4.1 Newmont Rain Facility, Elko County, NV
4.1.1 Introduction
EPA visited Newmont Gold Company's Rain facility in September of 1991 (U.S. EPA 1992b). The
facility is located on approximately 627 acres, 9 miles southeast of Carlin in Elko County, Nevada.
33
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Acid Mine Drainage Prediction
The facility is a mining-milling-leaching operation for beneficiating disseminated gold ore. Ore and
waste rock are mined from an open pit. Of the ore removed from the mine, over forty percent is
milled and beneficiated by the carbon-in-leach method at a current rate of about 840,000 tons per year
(TPY). The remaining ore (about 1,000,000 tons per year) is leached using a modified heap method
referred to as a valley leach. An average of 35,000 tons of material was being removed from the
mine each day as of late 1991. Of this, 5,500 tons was ore grade, 29,500 tons was waste rock. This
rate varies between 7,000 and 40,000 tons per day, respectively.
/
Most of the ore-grade material is taken from the oxidized sediments of the Webb Formation, proximal
to the Rain fault. Gold cpncentrations in this material range from 0.01 to 0.15 ounces of gold per ton
of rock. According to Newmont, sulfide-bearing rock does not contain gold hi sufficient quantity to
be economically recoverable, and is therefore disposed of as waste rock.
4.1.2 Waste Rock
Projected waste rock tonnage was estimated to be 41.4 million tons by the end of 1990, and 62.5
million tons during the life of the mine. In late 1991, the waste rock dump covered 211 acres north
and east of the pit. Waste jock production from the pit averaged 29,500 tons per day. Of this, 7,500
tons were sulfidic and 22,000 tons oxide. Newmont had estimated that by mine closure in 1995,
there will be 62.5 million tons of waste rock; of this, 77.8 percent was expected to be mostly
oxidized mixed sedimentary material of the Webb Formation (some of which will contain sulfide
mineralization), 15.4 percent carbonaceous and potentially sulfidic, 4.3 percent limestone of the
Devil's Gate Formation, and 2.5 percent alluvium from surface deposits.
Prior to the spring of 1990, sulfide, oxide, and calcareous waste rock were disposed of together. On
May 8,1990, acid drainage was observed flowing from the base of the waste rock dump and into the
unnamed drainage above Emigrant Spring, toward Dixie Creek. Inspection of the drainage
downstream of the dump revealed that approximately two miles of the channel contained a red-brown
precipitate. Discharge to the drainage was estimated by Newmont to be 3 gpm. According to
Newmont, snow removed from the roads was disposed of on the waste dump. As the snow melted, it
infiltrated the waste rock pile, oxidizing sulfur-bearing minerals and generating acid. The solution
migrated along pre-mining topography and discharged at the toe of the dump.
Surface-water samples were taken along 5 points hi the drainage above and below Emigrant Spring in
May, June, and July of 1990. They showed pH values ranging from 2.37 to 3.21 near the base of
the waste rock at the discharge point, and from 6.5 to 8.64 about 4,000 feet downstream. Arsenic
levels near the effluent point were 46 ppm in May and 1.5 ppm in July; at the distant sampling point,
arsenic levels were 0.023 ppm in May and 0.005 ppm in July. Mercury levels near the discharge
point were 0.19 ppm in May and 0.0019 ppm in July; at the distant sampling point, mercury levels
were <0.0001 ppm hi May and 0.0003 ppm in July.
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Acid Mine Drainage Prediction
4.1.3 Acid Generation Prediction .
Following detection of the acid generation in 1991, Newmont's Rain facility Water Pollution Control
Permit was revised. As part of the revised Permit, Newmont is required to report quarterly on results
of Meteoric Water Mobility testing and Waste Rock Analysis. The meteoric water mobility test is an
extraction procedure that determines moisture content of the waste, percent of a sample passing -200
mesh, pH of deionized water, and final pH of extract (following 24-hour extraction time). Following
the meteoric water mobility test, total carbon, organic-carbon, and sulfur assays are obtained on the
composite waste sample by combustion-infrared analysis to measure sulfur and sulfide contents, and
to estimate carbonate content. Acid neutralization potential is then measured using titration. The
extracted solution is analyzed for nitrate, phosphorous, chloride, fluoride, total dissolved solids,
alkalinity, sulfate, and metals. Waste rock analysis is intended to determine the net acid generation
potential of the material placed in the waste rock dump during the quarter.
Data for the third and fourth quarters of 1990 and the first quarter of 1991 were examined by EPA
following the site visit (U.S. EPA 1992b). Third quarter'results for the waste rock analysis indicated
a net acid generation potential of -10.6 tons of CaCO3 for each 1,000 tons of waste. This suggests
that the wastes generated.during this quarter had sufficient buffering capacity to neutralize any acid
solution generated by sulfidic material. Fourth quarter results showed a large shift, with an acid
generating potential of 5.35 tons of CaCO3 for each 1,000 tons of waste. The total acid generating
potential of waste rock disposed during this quarter was equivalent to the amount of acid neutralized
by 5.35 tons of CaCO3 for each 1,000 tons of waste rock. For the first quarter of 1991, waste rock
analysis data showed a net acid generating potential of 8.57 tons. In these circumstances, Newmont
is required to perform kinetic testing according to State of Nevada protocol. Results of this analysis
were not available; however, in the third Quarterly Monitoring Report for 1991, Newmont indicated
that column studies were underway to fulfill this requirement.
4.1.4 Treatment
*
-In response to the drainage^ Newmont took the following actions. By May 9 (one day after the
drainage was noted), a small pond was constructed to collect the flow from the dump. On May 11,
an HOPE liner was installed in the pond, and on May 18, Newmont constructed a cutoff trench
across the channel downstream of the collection pond to collect subsurface solution. The trench was
twenty feet deep and forty feet across and included a HOPE liner. Inflow to this trench was pumped
to the collection pond and then trucked to the tailings impoundment for disposal.
*
The State and BLM approved Newmont's long-term mitigation plan with construction beginning in
November of 1990, and completed in March of 1991. The solution collection and return system
consists, of surface and subsurface water collection and recovery system. Surface water is collected in
a ditch and drains to a sump located at the toe of the waste rock pile. Drainage collected in the sump
drains by gravity to a 200,000-gallon capacity, double-lined pond. Subsurface flow is recovered in an
35
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Acid Mine Drainage Prediction
HDPE-lined trench and also drains to the double-lined pond. Flows average 23.8 gpm with a
maximum of 183 gpm. In the event of a power failure, the pond has a capacity to retain in excess of
65 hours of inflow at the maximum projected flow rate. In addition, storm water from the surface of
the waste rock dump and surrounding area is collected in a single-lined, 600,000-gallon pond located
just below the double-lined pond. Solution from both ponds is pumped to the mill area and added to
the tailings pipeline.
As a long-term mitigation/prevention measure, Newmont began encapsulating sulfidic waste rock
within oxidized and/or calcareous waste rock that has either no net acid generating potential or some
acid neutralizing potential. As of late 1991, this was being accomplished by placing a pervious layer
of coarse oxidized waste rock on the native soil. On this, five feet of compacted oxidized ore was
placed. Additional oxide ore was placed against the natural hillslope to act as a barrier. These layers
were to act as barriers to water movement into and out of the sulfidic waste rock. Following these
steps, sulfidic waste rock was placed on and in front of the oxide ore. Several lifts were expected to
be added to the sulfidic waste pile. In addition, haul trucks follow random routes during construction
to compact the material, thereby reducing its permeability. Eventually, the front edge and top will be
covered with IS feet of oxidized material to complete the encapsulation. Prior to encapsulation,
sulfide waste rock will be mixed with oxidized material or the limited quantity of calcarious material
available to buffer any acidic solution generated. The sulfidic materials are fine to coarse grain
sedimentary rocks extracted primarily from the Webb Formation.
Neither the draft nor the final Environmental Assessment prepared for the Rain Facility discussed the
potential for sulfidic material to generate acid drainage.
4.2 Cyprus Thompson Creek, Challis, ID
4.2.1 Introduction
EPA conducted a site visit of the Cyprus Minerals Corporation Thompson Creek (Cyprus) facility in
September 1991 (U.S*. EPA 1992c). Cyprus mines molybdenite (molybdenum disulfide, MoSz) from
an open pit mine near Challis in central Idaho. Cyprus staked its first mineral claims at Thompson
Creek in 1967.,In 1981, mining operations began and the first concentrates were produced in 1983.
In late 1991, the Cyprus Thompson Creek Mine site consisted of (1) an open pit mine and two waste
rock dumps; (2) a primary in-pit crusher; (3) a mill with grinding and flotation, and (4) a tailings
impoundment.
Cyprus has been conducting a study to investigate the potential for the waste dumps and the tailings
impoundment to generate AMD. The results of the AMD study of the waste rock and tailings were
to be provided to USFS by March 1, 1992. Proposed revisions to the facility's reclamation plan were
also to be submitted to the U.S. Forest Service (U.S.FS). According to USFS personnel, the
revisions to the Plan of Operations were to be subjected to the environmental review requirements of
36
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Acid Mine Drainage Prediction
National Environmental Policy Act (NEPA). This review may include preparation of a supplemental
Environmental Impact Statement (U.S. EPA 1992c). The AMD study has been requested.
4.2.2 Waste Rock
When mining began hi 1981, approximately 130 million tons of overburden were initially removed as
"pre-production stripping." Most of the overburden was placed in two waste rock dumps (the
Buckskin and Pat Hughes dumps) located adjacent to the pit. In 1990, approximately 16.2 million
cubic yards of waste rock were generated, consisting mainly of metasediment, quartz monzonite,
challis volcanics, and clayey rock (i.e., decomposed volcanics).
4.2.3 Acid Generation Prediction
Both intrusive and metasedimentary rocks have high sulfur content (up to 1.13 and 1.66 percent,
respectively). Therefore, in 1990, Cyprus began a study of the potential for AMD generation from
the waste rock and tailings, using both static and kinetic test methods. As of late 1991, static testing
had been performed on twenty intrusive rock, and 58 metasedimentary rock samples collected from
both the lower and upper benches of the pit. For each sample, Cyprus calculated the neutralization
potential (NP) and the acid generation potential (AP) to determine the net neutralization potential
(NNP) and the NP/AP ratio. The NNP represents the neutralization potential (the tons of calcium
carbonate required to neutralize 1,000 tons of waste rock) minus acid generation potential (calculated
based on the total sulfur content). According to Cyprus personnel, waste rock with an NP/AP ratio
in excess of 3:1 was considered non-acid generating. According to USFS personnel, a NP/AP ratio
of at least 5:1 should be required before a material is determined to be non-acid forming (U.S. EPA
1992c).
Static testing of eight intrusive rock samples from the lower benches of the pit, close to the ore zone,
yielded an average net neutralization potential (NNP), and neutralization ratio (NP/AP) of 0.53 and
1.88:1, respectively. These results exhibited more AMD potential than the average NNP (4.93) and
average NP/AP (3:80:1) values obtained from 12 intrusive rock samples from the upper bench. They
indicate a greater potential for AMD with intrusive waste rock hi the vicinity of the ore zone. The
AMD potential decreased with distance from the ore zone. The difference between intrusive rock
samples collected from the upper and lower benches was believed to be caused by a relatively
predictable pattern of mineralization and alteration zoning around the ore body.
According to Cyprus, the metasedimentary rocks did not appear to be sources of AMD. Cyprus has
performed static testing on the metasedimentary rock in the lower benches and found average NNP
and NP/AP values of 24.95 and 3.11:1, respectively. It should be noted that, while the
metasedimentary rocks are considered non-acid forming by Cyprus (NP/AP greater than 3:1), the
average NP/AP ratio is less than the minimum (5:1) ratio suggested by the USFS. Metasedimentary
rock samples obtained from the upper benches showed average NNP and NP/AP values of 19.02 and
37
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Acid Mine Drainage Prediction
8.52:1, respectively. Though the average NNP value did not increase in samples from the upper
bench, the NP/AP ratio increased significantly, supporting the theory that AMD potential decreases
with distance from the ore zone.
t
Kinetic testing of intrusive and metasedimentary rock was ongoing in 1991 for those static test
samples showing acid generating potential. Results of these tests were to be incorporated into the
AMD study as they became available.
4.2.4 Tailings
During the ongoing acid drainage study, indications of acid generation were found in the tailings. As
of late 1991, the tailings impoundment covered a total of approximately 150 acres with the
embankment covering about 60-70 acres and the tailings pond behind the embankment approximately
90 acres. According to Cyprus personnel, tailings oxidation to a depth of several feet had been
evident for over two years (U.S. EPA 1992c).
^
4.2.5 Acid Generation Prediction
A- »
In October 1990, ten hollow stem auger borings were completed in the tailings embankment.
Samples collected from the these borings were subjected to humidity cell testing, and showed that the
average sulfur content of the tailings sands was 0.79 percent and the pH ranged from 3.5 to 7.3 s.u.
(Analyses of tailings sands have shown pH levels as low as 3.0 s.u.) In addition, of eight samples
tested, six produced elevated iron and sulfate concentrations, and associated increased acidity, within
a 15-week test period. The kinetic tests affirm the reactive nature of the tailings found in static test
results.
According to Cyprus personnel, the tailings pond and the seepage return pond were not a problem
(pH > 5.7 s.u.). However, in 1991, Cyprus conducted a water quality trend analysis for six surface
water quality monitoring locations in the tailings impoundment area. These locations included the
main drain of the rock toe, springs located on the left and right abutments of the rock toe, the
discharge from the rock foe, the sump below -the seepage return pond dam, and Bruno Creek
(immediately downstream of the sump). This analysis found that during the period 1981-1990,
(1) pH decreased at four locations (but not at the left and right abutment springs), (2) sulfate had
increased at all locations, (3) iron had increased at four locations (not at the left and right abutment
springs), and (4) no trends in zinc, copper, or arsenic were recognized. The increase in sulfate
concentrations was attributed to tailings oxidation and acid generation.
4.2.6 Treatment
Cyprus applied trisodium phosphate (TSP) to tailings embankment sand to address the AMD problem.
Previous column testing had found that TSP addition increased the pH, and reduced iron
38
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Acid Mine Drainage Prediction
concentrations in leachate samples. According to Cyprus's consultant, two TSP tests, humidity cell
tests and large scale tests, were being conducted in 1991 to determine TSP's effectiveness in
controlling AMD from the embankment, and maintaining impoundment water quality. However,
because die tailings impoundment unit has no discharge and water from the impoundment, seepage
return pond, and pump back system is returned to the mill, the TSP application were expected to
cause elevated phosphorus levels in the reclaim water. Cyprus personnel indicated these levels may
adversely affect flotation operations and that this issue was being studied.
Cyprus' original plan for reclamation of the tailings impoundment indicates that Cyprus initially
anticipated that water quality standards could be met by diluting impoundment seepage with natural
runoff. No water treatment beyond sediment control was expected to be required. However, the
original reclamation plan did not consider the AMD issue. According to Cyprus personnel, the AMD
problem could extend well beyond the life of the mine and perpetual care/treatment may be necessary.
Therefore, Cyprus was evaluating remedial alternatives (other than perpetual care) and was preparing
to submit a revised tailings pond reclamation plan (as a modification to their operating plan).
Alternatives to be considered included installing an additional flotation unit to remove pyrite and/or
in-place treatment of tailings with trisodium phosphate as a buffer. Preliminary flotation tests have
been conducted to investigate the possibility of removing sulfldes from the tailings prior to disposal in
the impoundment. Test results indicated that a high percentage of pyrite may be recovered. Limited
static testing performed on a whole tailings sample from which pyrite was recovered indicated a
NP/AP ratio in excess of 4:1 compared to an average value of 0.84:1 for all tailings analyses.
According to Cyprus personnel, oxidation had only been found to occur in the top two to three feet of
tailings (despite the results of analyses of the 1990 borehole samples that showed oxidation at all
depths down to 150 feet). Therefore, an additional alternative under consideration was to encapsulate
the tailings. Information on specific types of cover materials was not provided. Additionally, Cyprus
was investigating the potential use of wetlands treatment.
4.3 LTV Steel Mining Company, The Dunka Site, Minnesota
4.3.1 Introduction
EPA visited the LTV's Dunka site in August 1991 (U.S. EPA 1992d). The site is located
approximately 20 miles northeast of LTV Steel Mining Company's (LTV SMCo.) Hoyt Lakes
facility. The site is on private, State, Bureau of Land Management, and U.S. Forest Service lands;
LTV SMCo. holds surface and mineral leases for the area. The Dunka pit is part of the eastern-most
extension of the Biwabik iron formation and is one of the smaller pits on the Mesabi Range at three
miles in length. Although additional material may be removed from the pit for beneficiation, in
1991, plans called for no further exploration activity at the site or enlargement of the pit.
39
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Acid Mine Drainage Prediction
4.3.2 - The Acid-Generating Duluth Complex
The taconite ore at the Dunka site contacts Duluth Complex material (DCM), which must be removed
to reach portions of the taconite ore deposit. The Duluth Complex is a sulfur-containing, mafic
intrusive rock unit, considered to be one of the largest known sources of copper and nickel resources.
As of late 1991, LTV SMCo. had removed and placed in "gabbro" stockpiles approximately 50
million tons of Duluth Complex material containing an average of more than 0:2 mass percent copper
oxides and/or 0.05 mass percent nickel oxides as gabbro stockpiles.
The remaining Duluth Complex material stockpiles were categorized as waste rock stockpiles and are
made up of material containing less than 0.2 percent copper oxide and less than 0.05 percent nickel.
Since these waste rock stockpiles were constructed hi 1976, monitoring of drainage from the piles has
revealed a decrease in pH levels, as well as an increase in trace metal concentrations. Copper and
nickel concentrations as high as 1.7 and 40 mg/L, respectively, were observed hi seepage/runoff from
Duluth Complex waste rock stockpiles at the site. In addition, during sampling conducted by the
Minnesota Department of Natural Resources between 1976 and 1980, pH values as low as 5.0 at Seep
1 were reported.
4.3.3 Acid Rock Drainage Prediction Methods
To address this drainage, the Minnesota Department of Natural Resources hi conjunction with LTV
SMCo., constructed full scale test piles of the Duluth Complex material to monitor its acid generation
potential. The MDNR continues to monitor the test piles and study acid generation. Lapakko (1988)
conducted kinetic tests of Duluth Complex material using a humidity cell. Nine samples were
selected from core material and one sample from a test stockpile. This experimental method was
selected based on ongoing field test results, which demonstrated a strong correlation between sulfur
content, trace metal mobility, and acid production. Laboratory scale tests provided better control and
simplified analysis. Sulfur content was identified as the independent variable. Samples that had
variable sulfur content were selected. Part of the study was to determine the feasibility of
extrapolating laboratory results to operational conditions.
Each cell was loaded with 75 gram rock samples passing 100 mesh but less than 270 mesh. Samples
were rinsed with 200 ml of distilled-deionized water, which was allowed to remain in contact with the
sample for five minutes. Rinse water was collected and filtered through a 45 micron filter. At the
beginning of the experiment, the samples were rinsed five tunes to remove oxidation products
generated during sample preparation. Two rinses were used each week during the remainder of the
experiment. Between the weekly rinsings, the samples were stored hi a box fined with temperature
and humidity controls.
The laboratory study found that drainage pH decreased as the sulfur content of the sample increased.
Drainage pH also decreased as the experiment time increased. Both of these findings are consistent
40
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Acid Mine Drainage Prediction
with field observations on pH variation correlated with sulfur content and time. Based on the data,
Lapakko (1988) concluded that the small particles (<2.0 mm) have a large influence on field
stockpile drainage quality. The weighted average sulfur content for particles in this fraction is 1
.percent compared to 0.6 percent in the bulk rock. Most of the sulfur occurs as pyrrhotite. The
higher sulfur content combined with the higher surface area of these particles make this fraction
susceptible to more intense oxidation reactions.
4.3.4 Environmental Risks
Toxicity testing of the leachate showed that copper and nickel concentrations exceeded the 48-hour
lethal concentration (LC50) for Daphnia pulicaria; nickel concentrations also exceeded the 96-hour
LC50 for fathead minnow. Concentrations of calcium, magnesium, and sulfate in the stockpile
drainage were also elevated. According to LTV SMCo., there was some question whether the metals
were the toxic agent.
Most of the seepage from waste rock piles at the Dunka site has historically been discharged to
Unnamed dreek. Unnamed Creek flows into Bob Bay, a part of Birch Lake. In a 1976-1977 study
of trace metals in Bob Bay, it was found that concentrations of copper, nickel, cobalt, and zinc in the
waters of the Bay were higher than the regional average concentrations and decreased with distance
from the mouth of Unnamed Creek. Elevated metal concentrations were also observed in the
sediments, as well as in aquatic plant and clam tissue. In the study, it was estimated that the total
discharge from the Dunka watershed into Bob Bay through Unnamed Creek was 500 million gallons
per year. Unnamed Creek contributes more than 90 percent of the trace metals load to Bob's Bay.
Annual loading is over one ton of nickel. Less than 40 percent of this nickel load was found to be
removed from the system through natural lake processes. According to LTV SMCo., carbon dating
of sediment samples from Bob Bay indicates significant metal concentrations which predate mining.
4.3.5 Treatment
*
As of late 1991, the State and LTV SMCo. were working to develop technologies to mitigate leachate
generation and release of trace metals associated with stockpile drainage. The technologies being
tested and employed -included pile capping/channeling to limit infiltration, active treatment in a
neutralization pond to lower pH and remove metals, and use of artificial wetlands to remove metals.
The ultimate goal was a passive treatment system that would require little or no maintenance (U.S.
EPA1992d).
4.4 California Gulch
The California Gulch NPL site is located in the upper Arkansas River Valley in Lake County,
Colorado. It is bounded by the Arkansas River to the west and the Mosquito Mountains to the east,
and is approximately 100 miles southwest of Denver. The study area for the remedial action
41
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Acid Mine Drainage Prediction
encompasses approximately 15 square miles, and includes California Gulch and the City of Leadville.
California Gulch is a tributary of the Arkansas River. Mining for lead, zinc, and gold has occurred
in the area since the late .1800's. This site was added to the NPL in 1983 (U.S. EPA, 1991).
A Remedial Investigation conducted by EPA in 1984 indicated that the area is contaminated with
metals (including cadmium, copper, lead, and zinc migrating from numerous abandoned and active
mining operations). A primary source of the metals contamination in the Arkansas River is acid-mine
drainage from the Yak Tunnel into California Gulch. The Yak Tunnel was built to drain the mine
workings in the area of California Gulch. The acid dissolves and mobilizes cadmium, copper, iron,
lead, manganese, zinc, and other metals. The tunnel and its laterals and drifts collect this metal-laden
acidic water, and drain it to the tunnel portal. The tunnel drains into California Gulch and then to the
Arkansas River. The Yak Tunnel's discharge contributes to the contamination of California Gulch,
the Arkansas River, and the associated shallow alluvial ground-water and sediment systems. From
previous investigations and sampling data, it was concluded that, as of the early 1980's, the Yak
Tunnel discharged a combined total of 210 tons per year of cadmium, lead, copper, manganese, iron,
and zinc into California Gulch, which is biologically sterile (U.S. EPA 1991). Results of acid
generation predictive tests of tailings and waste rock samples were not available for this report.
1
4.5 Clear Creek/Central City
The Clear Creek/Central City NPL site is located approximately 30 miles west of Denver, Colorado,
and includes the Clear Creek mainstem and the North and West Forks of Clear Creek. Active
operations, which began in 1859, include gold, silver, copper, lead, molybdenum, and zinc mining.
Initial investigations at the site focused on the discharges of Acid Mine Drainage (AMD) and milling
and mining wastes from five mines/tunnels in the Clear Creek and North Clear Creek Drainages.
The five mines/runnels of interest are: (1) the Argo Tunnel; (2) the Big Five; (3) the National
Tunnel; (4) the Gregory Incline; and (5) the Quartz Hill Tunnel. The first two are portals along
Clear Creek and the last three are in the North Clear Creek Drainage. They are close to the Cities of
Idaho Springs, Black Hawk, and Central City. Associated with the AMD is contamination of surface
drainages by metals in solution such as cadmium, chromium (VI), lead, manganese, nickel, and silver
(U.S. EPA 1991).
\ /
Acid/base potentials, similar to acid/base accounting, of waste materials were tested as part of the
Remedial Investigation. The acid/base potentials (NNP) were calculated as the neutralization potential
(NP) minus the potential acidity (AP). Results indicated that waste materials at the Gregory Incline,
the Quartz Hill Tunnel, and the Argo Tunnel have the capacity to generate large quantities of acid
leachate. The mill tailings at the Gregory Incline are especially capable of producing acid through the
oxidation of large quantities of pyrite. For example, the average acid potential for the Gregory
Incline mill tailings was -21.5, the waste rock was 1.7, and the alluvium was 11.6 (a negative
acid/base potential indicates acid forming potential). In the waste rock and alluvium, 11 of 18 and 2
42
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Acid Mine Drainage Prediction
of 13 samples showed acid forming potential. Information on the types of sampling and analytical
methods used was not available.
The City of Blackhawk, with guidance from EPA, is requiring, through a city ordinance, acid
generation potential testing of onsite materials prior to any development activities. Central City is in
the process of doing the same. The ordinance requires that, for any excavation or site development, a
sample collection plan that includes chemical analysis of acid-base potential must be prepared. The
ordnance requires that the tests conform to the methods outlined in EPA-670/2-74-070, Mine Spoil
Potential for Soil and Water Quality or an equivalent method, and that sampling must be
representative of the conditions at the property. If the acid/base potential is negative, the applicant
must have a mitigation plan approved by the city (Fliniau, 1993).
4.6 Iron Mountain Mine
The Iron Mountain Mine is a 4,400-acre NPL site in Shasta County, California, approximately nine
miles northwest of the City of Redding. Between 1865 and 1963, the area was used for the mining
and processing of copper, silver, gold, zinc, and pyrite. In 1983, Iron Mountain Mine was added to
the NPL. Acid mine drainage, leaching from both the underground mine workings and from the
tailings piles located at the site, is causing zinc, cadmium, and copper contamination of the Spring
Creek Watershed and the Sacramento River. Environmental damage is primarily in the Sacramento
River and tributaries in the Spring Creek and Flat Creek watersheds, where fishery productivity loss
and periodic fish kills have been observed. Drinking water drawn from the Sacramento River for the
City of Redding (population 50,000) is also threatened (U.S. EPA 1991).
In general, acid mine drainage generation is seasonal and is accelerated during periods of heavy
rainfall. According to EPA, the annual average rate of acid mine drainage at the site is 100 gallons
per minute (gpm) with peak flows of 300 to 600 gpm. The average loading per day to the Spring
Creek Watershed from Iron Mountain Mine is 423 Ibs of copper, 1,466 Ibs of zinc, and 10.4 Ibs of
cadmium (U.S. EPA 1991, Biggs 1991).
According to the Remedial Project Manager acid generation potential tests were conducted while the
tunnels were being mucked out. The procedures used are those required by California State law.
Information on test results and sampling and analytical methods used was not available (Hyman 1993
and Sugarek 1993).
4.7 Silver Bow Creek/Butte Area Site
The Silver Bow Creek/Butte Area NPL site is one of four separate but contiguous Superfund Sites
located along the course of the Clark Fork River in southwestern Montana. The Silver Bow
y
Creek/Butte Area Superfund Site is the largest (450 acres) and most complex of the four sites. The
site was listed on the NPL in 1983. The Silver Bow Creek/Butte Area site includes the Cities of
43
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Acid Mine Drainage Prediction
Butte and Walkerville (population 38,000), the Berkeley Pit (a nonoperating open-pit copper mine);
numerous underground mine workings; the Continental Pit (operated by Montana Resources); Silver
Bow Creek; Warm Springs Ponds (mine tailings); and Rocker Timber Framing and Treating Plant.
In the early 1980s the Berkely Pit open pit mine was closed and dewatering pumps were shut down.
As a result, the interconnected underground workings and the open pit began filling with water. EPA
is concerned with the waters filling Berkeley Pit because they are highly acidic (the RI shows pH
values ranging between 2.5 and 3.3, depending upon at what depth the samples were taken) and
contain high concentrations of copper, iron, manganese, lead, arsenic, cadmium, zinc, and sulfates.
If the water continues to rise in the Berkeley Pit, contaminated water may eventually flow into
shallow ground water (alluvial aquifer) and into Silver Bow Creek, creating the potential for
significant environmental impacts and human health problems (U.S. EPA 1991). There have been no
tests performed to predict pH changes either in Berkely Pit or the drainages that feed the Berkely Pit
(Forba 1993). Total acidity has been tested for some samples collected at the Silver Bow Creek site.
Information on the materials sampled, analytical methods, and results were not available.
44
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Acid Mine Drainage Prediction
5. REFERENCES
Biggs, F., 1991, Remediation Progress at the Iron Mountain Mine Superrund Site, California,
Bureau of Mines Information Circular, 1C 9289.
»
Bradham, W.S., and F.T. Caruccio, 1990. A Comparative Study of Tailings Analysis using
Acid/Base Accounting, Cells, Columns and Soxhelets. Proceeding of the 1990 Mining and
Reclamation Conference and Exhibition, Charleston, WV, April 23-26, 1990. p. 19-25.
British Columbia AMD Task Force, December 1989. Acid Rock Drainage Draft Technical Guide,
Volumes I and n. Report 66002/2. Prepared for the British Columbia AMD Task Force by
SRK, Inc.
British Columbia AMD Task Force, 1990 (August). Monitoring Acid Mine Drainage. Prepared by
E. Robertson in association with Steffen Robertson and Kirsten (B.C.) Inc. Bitech Publishing,
Richmond, B.C.
Brodie, M.J., L.M. Broughton, and Dr. A. MacG. Robertson, 1991. A Conceptual Rock
Classification System for Waste Management and a Laboratory Method for ARD Prediction
From Rock Piles. In Second International Conference on the Abatement of Acidic Drainage.
Conference Proceedings, Volumes 1-4, September 16, 17, and 18, 1991, Montreal, Canada.
Broughton, L.M. and Dr. A. MacG. Robertson, 1991. Modeling of Leachate Quality From Acid
Generation Waste Dumps. In Second International Conference on the Abatement of Acidic
Drainage. Conference Proceedings, Volumes 1-4, September 16, 17, and 18, 1991, Montreal,
Canada.
Broughton, L.M. and Dr. A. MacG. Robertson, 1992. Acid Rock Drainage From Mines - Where
Are We Now. Steffen, Robertson and Kirsten, Vancouver, B.C. Internal Draft Paper.
Bruynesteyn, A. and R" Hackl, 1982. Evaluation of Acid Production Potential of Mining Waste
Materials. Minerals and the Environment 4(1).
«
California Mining Association, 1991. Mine Waste Management. Edited and Authored by Ian .
Hutchison and Richard D. Ellison. Sponsored by the California Mining Association,
Sacramento, CA.
Coastech Research Inc. 1989. Investigation of Prediction techniques for Acid Mine Drainage.
MEND Project 1.16. la. Canada Center for Mineral and Energy Technology, Energy, Mines,
and Resources Canada. 61 pages
diPretoro, Richard S., 1986. Premining Prediction of Acid Drainage Potential for Surface Coal
Mines hi Northern West Virginia. Master of Science thesis submitted to West Virginia
University.
Doepker, Richard D., 1993. Laboratory Determination of Parameters Influencing Metal Dissolution
From Sulfidic Waste Rock. Preprint from American Society of Surface Mining and Reclamation
(Spokane, Washington, May 16 to 19, 1993).
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Acid Mine Drainage Prediction
Duncan, D. and A. Bruynesteyn, 1979. Determination of Acid Production Potential of Waste
Materials. Metaluragy Society, AIME, paper A-79-29. Published by AIME, Littleton, CO.
Duncan, D. and C. Walden, 1975. Prediction of Acid Generation Potential. Report to Water
Pollution Control Directorate, Environmental Protection Service, Environment Canada,
November 1975.
Edgar, A., and K. Lapakko, 1985. Heavy Metal Study Progress Report on the Field Leaching and
Reclamation Program: 1977-1983. MN Dept. Nat. Res., Division of Minerals, St. Paul, MN.
Ferguson, K.D. and P.M. Erickson, 1988. Pre-Mine Prediction of Acid Mine Drainage. In:
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