Mitigation of Metal Mining Influenced Water, Volume 2 - Management Technologies for Metal Mining Influenced Water

184 Pages·2009·3.907 MB·184\184

by  Gusek, James J., Figueroa, Linda A. (Eds.)

Management Technologies for Metal Mining Influenced Water
Mitigation of Metal Mining
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Influenced Water
Volume 2
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Published by
Society for Mining,
Metallurgy, and Exploration, Inc.
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Preface
The Mitigation of Metal Mining Influenced Water is the “how-to-fix-it” volume in a series of six
handbooks on technologies for management of metal mine and metallurgical process influenced
water. The other five handbooks in the Management Technologies for Metal Mining Influenced
Water series are Basics of Metal Mining Influenced Water; Mine Pit Lakes: Characteristics, Predic-
tive Modeling, and Sustainability; Geochemical Modeling for Mine Site Characterization and
Remediation; Techniques for Predicting Metal Mining Influenced Water; and Sampling and Moni-
toring for the Mine Life Cycle.
These handbooks are a volunteer project of the Acid Drainage Technology Initiative–Metal
Mining Sector (ADTI-MMS). The work was directed by the ADTI-MMS Steering Committee,
a technically focused consensus group of volunteer representatives from state and federal govern-
ment, academia, the mining industry, consulting firms, and other interested parties who are
involved in the environmentally sound management of metal mine wastes and drainage quality.
The mission of ADTI is to identify, evaluate, develop, and disseminate information about cost-
effective and environmentally sound methods and technologies to manage mine wastes and
related metallurgical materials for abandoned, inactive, active, and future mining and associated
operations, and to promote understanding of these technologies.
These handbooks describe the technical aspects of sampling, monitoring, mitigation, and
prediction programs of the mine life cycle. The audience for these technical handbooks includes
planners, regulators, consultants, land managers, researchers, students, stakeholders, and anyone
with an interest in mining influenced water (MIW).
Although numerous handbooks, both technical and nontechnical, are available about acid
drainage and the technologies used to sample, monitor, predict, mitigate, and control acid drain-
age and other mine wastes, most of these handbooks relate primarily to acid drainage from coal
mines. But not all adverse drainage from metal mines is acidic; some neutral pH waters can be
detrimental to the environment. In the introduction (Chapter 1), we explain that the use of the
term mining influenced water refers to all waters affected by mining and metallurgical processing,
which includes wastes from historic mining operations. This term resolves much of the confu-
sion that exists from using acid mine drainage for cases in which drainage comes from mines but
is not acidic. The ADTI-MMS handbooks address all MIW, not just acid drainage.
This ADTI-MMS mitigation handbook embraces two, sometimes overlapping, approaches
to resolving environmental issues associated with MIW: prevention and treatment. Using the
information provided in Volume 1, Basics of Metal Mining Influenced Water, as a foundation, the
editors and contributors focused on MIW prevention measures that disrupt the geochemical
relationship termed the acid rock drainage tetrahedron that involves pyrite, water, air, and bacteria
and how they naturally interact to produce acidic MIW. The reader should understand that
there is no single magic bullet to completely prevent acidic MIW formation; even a barrage of
preventive bullets may only slow pyrite oxidation to manageable levels. This is the point where
engineers and designers need to begin considering treatment technology options that fall into
three main categories: active, passive, and semi-passive.
This mitigation volume reflects the state of the practice in MIW problem solving as of the
first few years of the new millennium. Regardless, the first step in the mitigation process now and
in the future will involve gaining a better understanding of the physical environment (Chapter 2)
in the mine vicinity and how it can influence MIW situations. This chapter should serve as a
checklist of issues that designers need to address, including participation by stakeholders who
may be affected by the mitigation strategy implemented after the mine closes.
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viii MITIGATION OF METAL MINING INFLUENCED WATER
Chapter 3, “Planning and Design Considerations,” provides both real and hypothetical case
histories and examples of what works and what does not in mitigating MIW before a future
mine even opens. Although mitigation costs are provided, they should be considered for com-
parative purposes only.
“Source and Migration Control” (Chapter 4) focuses on the proven technologies that have
been used to mitigate MIW in a wide range of metal mining situations, including capping and
covering of tailings and waste rock, surface water diversion, underground and pit backfilling, and
acid rock drainage tetrahedron–disrupting amendments. Individually or in combination, these
techniques comprise the “armory” of MIW prevention technologies that should be deployed as
the first line of defense.
Chapter 5, the longest of the volume, introduces the array of technologies available to treat
residual MIW after all the practical source and migration controls have been implemented at a
given site. Selection of a given technology will depend mostly on the MIW chemistry and treat-
ment flow rate (discussed in the volume on sampling and monitoring) which in combination
influence the size and cost of the treatment system.
The methods for mitigating MIW will continue to evolve as mining companies, regulators,
researchers, and engineers publicize their successes and failures at conferences, forums, and in the
technical literature. To stay current with the latest developments, the reader is encouraged to par-
ticipate in ADTI-MMS sponsored activities, join professional societies that promote the free
exchange of MIW mitigation experience, and attend conferences that focus on this challenging
issue.
This mitigation handbook benefited from expert review and multiple revisions by past and
present members of the ADTI-MMS Mitigation Committee and present members of ADTI-
MMS. Development of its initial draft was supported by generous grants from the Office of Surface
Mining and the U.S. Army Corps of Engineers’ Restoration of Abandoned Mine Sites program.
Special thanks go to associate editor Charles Bucknam for his patience and constant encourage-
ment to stay the course.
Contents
PREFACE vii
Chapter 1 Introduction 1
Approaches to MIW Mitigation 2
Water Quality Standards 3
Measuring MIW Mitigation Performance 3
Chapter 2 The Physical Environment 5
Geology 5
Climate 7
Geomorphology 7
Hydrology 8
Historic Mine Workings 8
Background Conditions 10
Cultural Features 11
Soils 12
Air 13
Flora and Fauna 13
Public Participation 13
Chapter 3 Planning and Design Considerations 15
Open Pit Mining 15
Underground Mining 22
Cyanidation Heap Leach Facilities 25
Special Case—In Situ Mining 26
Materials Handling 31
Waste Rock Disposal 31
Tailings Disposal Design 32
Co-Disposal of Tailings and Waste Rocks as a Preventive Strategy 36
Backfilling Reactive Mine Wastes 40
Closure and Reclamation 42
Chapter 4 Source and Migration Control 53
Water Control 53
Air Exclusion 60
Pyrite and Metal Sulfide Modification 68
Chapter 5 Treatment of Metal Mining Influenced Water 81
Introduction 81
Active Treatment Technologies 84
Passive Treatment Technologies 109
Semi-Passive Treatment Technologies 139
INDEX 157
iii
CHAPTER 1
Introduction
This handbook is the second in a series of handbooks describing management technologies for
metal mining influenced water. The term mining influenced water (MIW) was introduced in the
first handbook in the series, Basics of Metal Mining Influenced Water. MIW is inclusive of a wide
range of potential water-related issues that arise from the water–rock interactions that are com-
mon to mining operations—in contrast to traditional terms such as acid rock drainage and acid
mine drainage, which refer to specific interactions and may imply a falsely narrow range of possi-
ble chemical characteristics. Simply defined, MIW is water that has been affected, adversely or
not, by mining and metallurgical processes.
The mitigation of MIW is a concern for mining companies, regulatory and other govern-
ment agencies, and private entities such as watershed associations or citizen groups. Each group
often has specific interests that impact their approach to mitigation and whether they consider a
given mitigation effort successful. Such interests may include
• Minimizing risk;
• Minimizing costs, optimizing profit, and getting bonds released;
• Discharging water of a quality and quantity similar to that present before mining;
• Discharging water that meets water quality standards;
• Abating impacts of historic mining practices;
• Creating “walk-away” situations that require little, if any, long-term maintenance;
• Avoiding litigation and fines; and
• Minimizing off-site impacts.
In reviewing this list, it is easy to see how different parties might prioritize different aspects
of MIW mitigation, but one vision of success that all parties usually share is the goal of minimiz-
ing long-term impacts from mining activities and not leaving current problems to future genera-
tions. The parties also usually share an understanding of the importance of mining—most
countries, both developed and undeveloped, rely on mining to provide the raw materials that are
the foundation of the global economy and are essential for everyday life. Mitigating MIW
impacts helps societies reap the full benefits of mining. It is exciting that the current generation is
arguably the first in history to truly recognize the scope and breadth of the MIW issue and to
have the rudimentary technical and scientific tools to confront the problem. Agricola recognized
the impacts of mining on streams and rivers more than 500 years ago (Agricola 1556), but
couldn’t have imagined the technology and tools we are able to use today to mitigate these
impacts (Figure 1.1).
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2 MITIGATION OF METAL MINING INFLUENCED WATER
Source: Agricola 1556.
FIGURE 1.1 Mining circa 1500s
APPROACHES TO MIW MITIGATION
Recognizing the worldwide importance of mining and the overarching goal of minimizing long-
term impacts from mining, it is common to begin a design of an MIW mitigation with a risk
assessment of some sort, be it qualitative or quantitative. The receptors (human and ecological)
at risk, the magnitude of risk, and the consequences of failure may be components of a risk assess-
ment. Water quality standards (see the following section) are usually a component of MIW miti-
gation, and these are generally based on toxicity studies that are also risk based. It is important to
clarify the performance end point required of a mitigation effort for a specific site before consid-
ering MIW mitigation options.
This Acid Drainage Technology Initiative—Metal Mining Sector handbook embraces two,
sometimes overlapping, approaches to resolving environmental issues associated with MIW: pre-
vention and treatment. MIW prevention measures focus on the disruption of the geochemical
relationship termed the acid rock drainage tetrahedron that involves pyrite, water, air, and bacteria
and how they naturally interact to produce acidic MIW. There is no single magic bullet to com-
pletely prevent acidic MIW formation; even a barrage of preventive bullets may only slow pyrite
oxidation to manageable levels. This is the point where engineers and designers need to begin
INTRODUCTION 3
considering treatment technology options that fall into three main categories: active, passive, and
semi-passive.
WATER QUALITY STANDARDS
Metal mine operators in the United States are subject to a number of surface water regulations.
States and tribal governments impose environmental and land reclamation performance stan-
dards, and the federal government’s Clean Water Act of 1972 (CWA) regulates discharges into
surface streams, wetlands, and oceans. In addition, mining operations must secure National Pol-
lutant Discharge Elimination System (NPDES) permits for discharges to surface waters. Accept-
able discharge concentration levels are determined by the U.S. Environmental Protection
Agency’s technology-based standards or toxicity-based water quality standards established for
stream uses. NPDES permits for metal mines usually require monitoring of pH, several regu-
lated metals, and a few other chemical and physical parameters. Permit limits tend to be site spe-
cific and may incorporate a waste load allocation based on total maximum daily load. These
standards are discussed in more detail in Chapter 3, where designs and operational strategies for
successful mine closures are addressed.
Groundwater regulations in the United States differ considerably from surface water regula-
tions, although they too are technology and toxicity based. Groundwater quality is regulated
under the federal Safe Drinking Water Act (SDWA). An important distinction between the
SDWA and the CWA is that the SDWA requires that discharges to groundwater meet the use
standard or the ambient condition, whichever is of higher quality.
MEASURING MIW MITIGATION PERFORMANCE
Measuring the performance of MIW mitigation measures is seldom simple or straightforward.
Decisions about how often samples should be collected, and from where, may be driven by eco-
nomics, political interests, convenience, common sense, or regulations. Once samples are col-
lected, their analysis for parameters of concern raises separate problems associated with detection
limits, standard procedures, repeatability, and other data quality issues. Critical performance
measures are dictated by the proposed mitigation goal. If pH and copper are the critical mitiga-
tion targets for a specific site, then a data quality assurance plan should provide sufficient con-
trols to ensure a high degree of confidence in the critical parameters.
REFERENCE
Agricola, G. 1556. De Re Metallica. Translated by Herbert Clark Hoover and Lou Henry Hoover. New
York: Dover Publications. 1950.
CHAPTER 2
The Physical Environment
Elements of the physical environment, both natural and human-made, largely dictate whether
and how water will be influenced by mining activities. Understanding the physical environment
is therefore imperative to designing and implementing successful mining influenced water
(MIW) mitigation measures.
GEOLOGY
Physical and chemical aspects of local geology and climate control the formation and movement
of MIW. Mineralogy of rocks affected by mining exert most of the chemical controls, but the
formation of MIW is also affected by such factors as the availability of water and oxygen, the
final placement of rock after its disturbance, and the rock’s physical characteristics—such as
grain size and permeability. When MIW enters groundwater, its movement is controlled mostly
by geologic factors, including
• Structures,
• Fractures, and
• The porosity and permeability of the ground.
Structures
Geologic structures broadly affect the path and rate of groundwater flow. Consideration of their
effect is a useful first step in designing mitigation measures for MIW. Transmissive faults and
fractures are primary geological features that can control the migration of MIW in groundwater
(see Figure 2.1), independent of the porosity and permeability of the rock mass.
While faults are generally depicted as single, somewhat straight line segments on geologic
maps, they are actually much more complex in orientation and character. Most faults are not a
single plane but inscribe a zone of broken rock of variable thickness and depth. Depending on
how they formed (either under compressional or tensional stresses) and how they were affected
chemically after formation, fault zones can be either permeable or impermeable, forming either
barriers or preferential pathways to groundwater movement. For example, geological alteration
of rocks associated with some faults creates clay-filled zones or silicified or cemented zones that
can act as aquicludes. Conversely, faulting in other cases creates preferential pathways of perme-
able brecciated rock. Understanding the role of faulting in the groundwater regime is required in
some MIW mitigation situations.
Folding may alter natural pathways of groundwater flow, enhancing permeability in some
areas and limiting flow in others—usually in a regular way. For instance, if anticlinal axes present
more permeable paths than anticlinal limbs in the area considered, this condition probably will
be present in all structures that formed under these same stresses, and this information can be
useful to mitigation planners.
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6 MITIGATION OF METAL MINING INFLUENCED WATER
6
4
9
8 2
7
10
3
11
1
5
1 Anticline 4 Normal Fault 7 Dike 10 Discontinuity
2 Syncline 5 Fractures/Joints 8 Sill 11 Formation Contact
3 Batholith 6 Breccia Pipe 9 Vein
FIGURE 2.1 Major geological structures
Fractures
The term solid rock is misleading if taken literally. Virtually all rock formations—whether sedi-
mentary, igneous, or metamorphic—contain fractures. These are the conduits that control
groundwater flow, including MIW migration. Fractures form in response to tensional and com-
pressional stresses and often have regional trends. Fractures may occur in “sets” that complement
each other in the sense that they are oriented in usually two or more directions and inclinations.
Modeling groundwater flow through fracture systems is difficult because of the intrinsic
heterogeneity of fractured rock formations. Nonetheless, understanding the distribution of frac-
tures in an area, and the hydraulic conductivity of the ground (see the following section), is nec-
essary to help determine the rate and direction of groundwater movement.
Porosity, Permeability, and Hydraulic Conductivity
The porosity of a rock is defined as the “percentage of rock or soil that is void of material” (Fetter
1988). Voids are typically filled with gas and fluids, including air, methane, natural gas, hydrogen
sulfide, carbon dioxide, groundwater, and petroleum.
Permeability is defined as “the property or capacity of a porous rock, sediment or soil for
transmitting a fluid; it is a measure of the relative ease of fluid flow under unequal pressure”
(Driscoll 1986). If voids are well connected, as in the case of an alluvial sand or gravel, the relative
permeability of the formation is probably high. A highly porous rock is not necessarily highly
permeable and vice versa.
Hydraulic conductivity is the most useful measurement for characterizing the ability of a
porous medium in the subsurface to transmit water. Hydraulic conductivity is defined as the
“coefficient of proportionality describing the rate at which water can move through a permeable
medium” (Fetter 1988). The density and kinematic viscosity of water may need to be considered
in determining hydraulic conductivity, especially in high-temperature or dissolved-solid situations,