United States
Environmental Protection
Agency
ineering Issue
Management and Treatment of Water
from Hard Rock Mines
Index
1.0 PURPOSE
2.0 SUMMARY
3.0 INTRODUCTION
3.1 Background: Environmental Problems
at Hard-Rock Mines
3.2 Conceptual Models at Hard-Rock
Mines
3.3 The Process of Selecting Remedial
Technologies
3.4 Resources for Additional Information
4.0 TECHNOLOGY DESCRIPTIONS
4.1 Source Control
4.1.1 Capping and Revegetation for
Source Control
4.1.2 Plugging Drainage Sources and
Interception of Drainage by
Diversion Wells
4.1.3 Prevention of Acid Drainage via
Protective Neutralization
4.1.4 Passivation of Sulfidic Rock
4.2 Treatment of Contaminated Water
4.2.1 Treatment of Acidic Waters
4.2.2 Treatment of Neutral and
Alkaline Waters
4.2.3 Treatment of Mine Water with
Microbial Processes
4.3 Mine Pit Lake Management
4.3.1 Backfilling and Neutralization
4.3.2 Bioremediation and Induced
Stratification of Mine Pit Lakes
5.0 CONCLUSION
6.0 ACKNOWLEDGMENTS
7.0 ACRONYMS AND ABBREVIATIONS
8.0 REFERENCES
1.0 PURPOSE
The U.S. Environmental Protection Agency (EPA) Engineering Issues
are a new series of technology transfer documents that summarize the
latest available information on selected treatment and site remediation
technologies and related issues. They are designed to help remedial
project managers (RPMs), on-scene coordinators (OSCs), contractors,
and other site managers understand the type of data and site charac-
teristics needed to evaluate a technology for potential applicability to
their specific sites. Each Engineering Issue document is developed in
conjunction with a small group of scientists inside the EPA and with
outside consultants, and relies on peer-reviewed literature, EPA re-
ports, Internet sources, current research, and other pertinent informa-
tion. The purpose of this document is to present the "state of the sci-
ence" regarding management and treatment of hard-rock mines.
Internet links are provided for readers interested in additional infor-
mation; these Internet links, verified as accurate at the time of publi-
cation, are subject to change.
2.0 SUMMARY
Contaminated water draining from hard rock mine sites continues
to be a water quality problem in many parts of the U.S. The types of
water range from strongly acidic water laden with metals, to variable
water quality in mining pit lakes, to alkaline water being released from
closed cyanide heap leach operations.
Prevention of water contamination at mine sites is usually the best op-
tion and can sometimes be realized by appropriate management of waste
material, or by hydrologic control in underground systems, or by using
a variety of capping methods for waste rock dumps or closed heaps.
However, long-term (decades and beyond) treatment is, and will con-
tinue to be, required at many sites. Once a contamination source is
established (e.g., reactive waste rock dumps), elimination of these as
sources of water is often very expensive and technically challenging.
Acid drainage remains the most problematic water quality, in large
part due to the ability of acidic water to dissolve a variety of toxic
metals (e.g., cadmium, zinc, nickel) and release of that water to sur-
face or ground water. The most common treatment is neutralization
using lime, or another suitable alkaline agent, followed by oxidation
and precipitation of metals. This will also reduce sulfate to near the
gypsum solubility limit (approximately 2,000 mg/L, depending on
-------
calcium concentrations). A variety of methods have been tive in many cases, further research is required to reduce
utilized to add lime to acidic water, and, particularly for water treatment costs and increase the reliability of these
large flows (>100 gal/min) and/or high acidity/metals technologies.
loadings, this option is usually the most cost effective.
Other methods for treatment of acidic water include a
variety of wetland systems and bioreactors that are based
on sulfate reducing bacteria that reduce sulfuric acid to
hydrogen sulfide, which consumes acidity and allows
precipitation of metals as metal sulfides. These systems
can either utilize the wetland organic carbon or an exog-
enously supplied carbon source (e.g., ethanol) for sulfate
reduction. These systems show particular promise where
the flows and acidities are relatively low. The advantage
of these systems is that they commonly do not require
the same level of monitoring and operational expense as
the lime systems. They also can reduce sulfate levels to
well below the gypsum solubility limits, depending on the
characteristics of the bioreactor/wetland system utilized.
Drainage from precious metals heaps and tailings facili-
ties offers a different set of challenges. While most of the
precious metals heaps and tailings are not acid generating,
several examples of acid generating processing wastes ex-
ist in western states. In most cases in mine closures, the
residual water used in cyanide extraction of precious met-
als remains net alkaline, and was continuously recycled
during operation. The soluble constituents were concen-
trated as water evaporated, and often contain elevated so-
dium from the sodium cyanide used in the process. Thus,
land application of these fluids should be limited, due to
salts, arsenic and other constituents. Other than ion re-
moval technologies (e.g., reverse osmosis), few cost effec-
tive methods for treatment and release of these water are
available. In arid regions, evaporation is often the only
option available for such heaps and tailings facilities.
Mining pit lakes that are derived from open pit mines that
penetrated ground water are particularly prevalent in the
precious metals and copper mines in the western United
States. The water quality can vary from a highly acidic sys-
tem in high sulfide host rock to slightly alkaline, better
quality water in carbonate host rock. While treatment of
pit lakes is potentially expensive, at least one example of
neutralization of an acidic pit lake (Sleeper Pit Lake in Ne-
vada) has demonstrated that this is technically possible.
Because of the long-term nature of many of these drainag-
es, methods for cost-effective treatments are still needed.
Many of the presently available technologies have been
derived from coal mine drainage research. While effec-
3.0 INTRODUCTION
This Engineering Issue document on treatment of min-
ing waters is a practical guide to understanding and se-
lecting technologies for the environmental management
of waste materials and effluents at hard-rock mines. For
the purposes of this discussion, hard-rock mining primar-
ily refers to open pit and underground mines that produce
base metals (e.g., copper, zinc, lead) and precious metals
(e.g., gold and silver). While drainage from coal mines has
similar water quality issues, coal drainage has been consid-
ered extensively in other publications. It responds to the
need for environmental management at new and aban-
doned hard-rock mines by providing guidance for select-
ing among available technologies for the stabilization of
mine waste, treatment of mine water, and management of
mine pit lakes. Target audiences are operators, regulators,
stakeholders, and technical consultants involved in select-
ing technologies for environmental management of hard-
rock mines. The general contents of this Engineering Issue
document are listed above in the Table of Contents.
The goal of this document is to increase the efficiency
of decision makers in defining the scope of mine-related
water quality problems and selecting the least expensive
effective management technology. It begins with technical
overviews and conceptual models of contaminant sources
(i.e., environmental behavior in the dominant hard-rock
mining facilities—waste rock and heap leach facilities,
tailings impoundments, and pit lakes). A general overview
of remedial technologies (acid neutralization, biologically
induced sulfide treatment, and pit lake management) fol-
lows. With these technical foundations reviewed, specific
remedial technologies are presented individually and de-
scribed using the context of the feasibility study process—
a practical framework for selecting remedial technologies
based on implementability, effectiveness, and cost.
3.1 Background: Environmental Problems at
Hard-Rock Mines
Few environmental problems are as widely documented as
the legacy of historic hard-rock mines. Small mines, oper-
ating in the era before environmental regulation, removed
and milled ore primarily from vein deposits, leaving un-
vegetated spoils, unsealed adits, and often-acidic seepage
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
laden with metals. Modern U.S. hard-rock mining is in
sharp contrast, with closure designs and associated finan-
cial bonds for environmental management commonly re-
quired even before operations begin and water discharges
permitted only within the constraints of the Clean Water
and Safe Drinking Water Acts. Environmental issues re-
main, of course, and economies of scale have produced
larger operations, but today's mines are better managed—
water is treated, and waste is capped and revegetated. Clo-
sure requirements for modern mines depend on the reg-
ulatory requirements and the environmental capabilities
and risk of the associated mine.
Collectively, the economic liabilities and technical
challenges of hard-rock mining are immense (if poorly
constrained):
Although no global estimation of the impact of acid
drainage exists, total liability costs for potentially acid-
generating wastes at mining sites is estimated to be
US$530 million in Australia, between US$1.2 and
20.6 billion in the USA, and US$1.3 and 3.3 billion
in Canada. Effectively dealing with acid drainage has
been—and continues to be—a formidable challenge for
which no global solutions currently exist. Acid drain-
age is one of the most serious and potentially enduring
environmental problems of the mining industry. Left
unchecked, it can result in such extensive water quality
impacts that it could well be this industry's most harm-
ful legacy. (INAP,2004)
There remains an enormous need for development and
evaluation of effective low-cost technologies for stabiliza-
tion and treatment of mine waste.
3.2 Conceptual Models at Hard-Rock Mines
The main processes responsible for water quality deg-
radation at hard-rock mines are reviewed here briefly
to provide a foundation for understanding remediation
technologies. Mine water contamination comes from two
sources: release of constituents contained in rock that
has been mined and chemical reagents used in mining,
milling, extraction, and ultimate recovery of the valuable
metal or mineral. The largest source of water contamina-
tion is nearly always the material being mined. Ore and
waste rock has generally been isolated from oxygen and
water for geologic time frames, and bringing the material
to the surface potentially results in reactions that release
contaminants that degrade water quality.
Acidic drainage is the dominant environmental problem
associated with hard-rock mining. Many valuable metals
in ore deposits are bound to sulfide sulfur, forming spar-
ingly soluble sulfide minerals such as sphalerite (ZnS),
covellite (CuS), or galena (PbS). Acidic drainage forms
primarily when iron sulfide, pyrite (FeS2), comes in con-
tact with water and oxygen, producing dissolved sulfuric
acid and iron. The three steps summarizing this overall
reaction are:
FeS
2(S)
H20 = Fe2++2S042-
2HH
(3-1)
(3-2)
(3-3)
Fe2+ + -|02 + H+ = Fe3+ + |H20
Fe3+ + 3H ,0 = Fe(OHLq, + 3H+
L 0(b|
Pyrite oxidation liberates soluble iron (Fe2+) and acidity
(H+) and sulfate, with the primary limit on the oxidation
rate being the availability of oxygen. The oxidation pro-
cess also liberates other sulfide-bound metals (e.g., cadmi-
um, zinc, copper, lead, uranium) and metalloids (e.g., ar-
senic, antimony, selenium). In addition, most metals are
more soluble under acidic conditions (i.e., at low pH), so
oxidation and acid production tend to be associated with
increasing metal concentrations. The result is that acidic
conditions (low pH) in mine effluent tend to be highly
correlated to elevated heavy metal concentrations.
The primary offset to acid production in natural systems
is the consumption of acidity by calcite (CaCO ):
(3-4)
CaC0
C0
2(G|
H20
and precipitating sulfate as calcium sulfate (CaSO4).
S0,+ Ca2+ = CaSO
4(S)
(3-5)
Acidic mine water that is neutralized by reaction with
calcite generally contains 1,500-2,200 mg/L sulfate. Fur-
ther, the consumption of acidity in Equation 3-4 increas-
es the pH, which tends to decrease total heavy metal con-
centrations as these constituents precipitate or adsorb to
surfaces.
Figure 3-1 is a model simulation illustrating how pore
water in waste rock or tailings will change as pyrite and
calcite are consumed during oxidation. The top graph
shows the amount of acid-generating potential (AGP) (as
pyrite) and acid neutralizing potential (ANP) from cal-
cite (CaCO3) remaining, with increasing oxidation (also
related to increasing time) shown along the graph from
left to right. As long as calcite remains, pore water pH re-
mains near neutral and sulfate concentrations are limited
to below 3,000 mg/L. Once all calcite is consumed, acid
buffering ceases, the pH drops to below 3, and dissolved
sulfate increases. Results indicate the dramatic change in
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
-------
water quality that can occur when excess AGP remains in
materials.
Oxidation of Mine Waste
0.1 1
SULFIDE OXIDIZED
Figure 3-1. Oxidation of mine waste and production of sulfate. This
idealized 10% sulfide rock contains both pyrite and calcite and
demonstrates how sulfate concentration and pH are changed as
the sulfides in the rock are oxidized and the capacity to neutralize
the acid is consumed.
Contaminated mine waters may also be neutral to alka-
line, depending on the type of rock being mined and the
reagents used to selectively extract the valuable substance.
Thus rock with excess calcite will produce pH-neutral ef-
fluent. However, neutralized mine waste effluent can still
contain elevated metalloids, such as selenium, arsenic, and
antimony. Zinc and other heavy metals have also been ob-
served in pH-neutral mine waters (e.g., the Burleigh Tun-
nel and Wellington Oro Mine discharges in Colorado).
Metal recovery reagents that may present water quality
issues include a variety of flotation agents for concentra-
tion of metals, as well as lixiviants, particularly cyanide. In
the latter case, cyanide can form complexes with a variety
of metals that are very weak (e.g., zinc cyanide) to strong
complexes (e.g., cobalt, iron, and mercury cyanides) and
also transformation products of cyanide, particularly thio-
cyanate and nitrate.
3.3 The Process of Selecting Remedial
Technologies
Selection of an optimal technology for a specific reme-
diation problem would, ideally, follow from tightly con-
strained algorithms or flow charts. Regrettably, critical de-
cision variables, such as cost per cubic meter to treat water,
net percolation through caps, etc., are generally too de-
pendent on site-specific conditions to allow direct trans-
fer between projects. Design methods are transferable across
sites, but not specific designs. In response, remedial tech-
nologies discussed in this document are presented in a for-
mat that supports the EPA's feasibility study process. An
overview of each technology (Tables 4-1, 4-5, and 4-6) is
provided to facilitate early screening of inappropriate op-
tions. Details of each technology are presented in the text,
with a focus on identifying those parameters most criti-
cal in evaluating implementability, effectiveness, and cost.
Where possible, specific examples of cost and effectiveness
under pilot- or full-scale implementation are provided.
The feasibility study process provides a framework for se-
lecting from a range of remedial technologies for specific
site conditions amidst the interests of regulators, stake-
holders, and technology developers. The process begins
with a characterization of the problem (e.g., chemicals of
concern, risks, exposure paths, identification of remedial
goals, etc.), then identifies potential technologies (screen-
ing process), and finally evaluates the feasibility of a short
list of technologies to select a remedy.
The primary technical evaluation criteria for feasibility
under Superfund (EPA, 1988) are:
• Effectiveness—the potential for the alternative to
achieve remedial goals established for the site.
• Implementability—the ability to comply with tech-
nical and administrative issues and constraints in-
volved in implementing a technology at a specific site.
• Cost—typically an estimate of net-present cost for
each technology.
In practice, implementers identify the technologies that
can meet their water quality goals ("effectiveness"), elimi-
nate those that can't be applied for practical reasons ("im-
plementability"), then implement the least expensive op-
tion ("cost"). This document is intended to support this
technology selection process, providing descriptions of
the common environmental technologies for hard-rock
mining and identifying the critical components affecting
the feasibility of each.
Selecting a technology can be more difficult than imple-
menting it. The critical components in the evaluation and
selection of a technology include:
• Source definition—water flow rates, material mass,
solute concentrations, expected duration, etc.
• Identification of environmental goals—discharge
standards, compliance points, and human or ecologi-
cal risk thresholds.
• Identification of applicable technologies—those
technologies potentially capable of meeting goals.
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
• Identification of critical parameters—early deter-
mination of values for parameters that typically drive
cost or effectiveness.
• Impartial evaluation—a feasibility analysis that is
completely independent from technology vendors.
Other feasibility study evaluation criteria that should be
considered during the technology selection process are
community and regulatory acceptance. These criteria are
covered to a lesser degree in this document than the tech-
nical criteria, but can have a great impact on the final
selection.
3.4 Resources for Additional Information
Below are prominent organizations dedicated to research
on the causes and remedies for management of drainage
at hard-rock and coal mines:
• Mine Environment Neutral Drainage (MEND):
http://www.nrcan.gc.ca/mms/canmet-mtb/mmsl-
Imsm/mend/default e.htm.
• Acid Drainage Technology Initiative (ADTI):
http://www.unr.edu/mines/adti/.
• International Network for Acid Prevention (INAP):
http://www.inap.com.au/. Includes clear overview of
topics and reports on INAP-funded research.
• Australian Center for Mining Environmental Re-
search (ACMER): http://www.acmer.com.au/.
• The U.S. Geological Survey Toxic Substances Hy-
drology Program section on Hard-Rock Mining
Contamination: http: //toxics, usgs.gov/topics/
minelands.html.
• The Restoration of Abandoned Mine Sites Technol-
ogy Database (RAMS tech): http://www.unr.edu/
mines/ramstech/techintro. asp.
• U.S. Army Corps of Engineers Restoration of Aban-
doned Mines Sites (RAMs) Western Region:
https://www.nwo.usace.army.mil/html/rams/rams.
html. Includes project summaries and documents for
nine western states
• West Virginia University Extension Service Land
Reclamation Program: http://www.wvu.edu/
-agexten/landrec/land.htm#ACID. Focuses on acid
drainage from coal sites.
• EPA's Web site for abandoned mine lands:
http://www.epa.gov/superfund/programs/aml/. Gives
current technology updates on application of new
treatment methods.
• EPA's Web site for the EPA/DOE Mine Waste
Technology Program: http://www.epa.gov/
minewastetechnology.
The following publications provide additional informa-
tion on innovative acid mine drainage treatment technol-
ogies and a comprehensive summary of recent closure and
bonding practices and related costs at hard-rock mines,
respectively:
• "Acid Mine Drainage: Innovative Treatment Tech-
nologies," by Christine Costello for the EPA (Costel-
lo, 2003): http://clu-in.0rg/s.focus/c/pub/i/1054/.
• "Hardrock Reclamation Bonding Practices in the
Western United States," by the Center for Science in
the Public Interest for the National Wildlife Federa-
tion (Kuipers, 2000): http://www.csp2.org
/REPORTS/Hardrock%20Bonding%20Report.pdf.
4.0 TECHNOLOGY DESCRIPTIONS
The strategy for management and protection of water
quality at mine sites varies on a site-specific basis. The
range of contaminants that is released into water from
one ore may be very different from another ore, and the
management methods utilized will depend on the volume
and environmental characteristics. However, there are
common threads that are fundamental to water manage-
ment, and sufficient similarities exist that a generalized
discussion is useful. While a large variety of metals are
mined in the U.S., nearly 90% of the value of metals (ex-
cluding iron ore) consists of a group including gold, cop-
per, zinc, lead, silver, and molybdenum (USGS, 2006).
As such, this discussion will primarily consider waters re-
leased from these important metals mines.
Specific remedial technologies are divided into three
categories:
1. Source Control: typically the chemical stabilization
of reactive rock, or physical isolation or diversion of
water away from the mine waste.
2. Water Treatment: methods of reducing
contaminants in mine waters or otherwise managing
contaminated water to reduce impacts to humans
and the environment.
3. Pit Lake Management: treated separately here
due to the unique physical characteristics of lakes,
although many elements of source control and
treatment also apply.
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
-------
The descriptions of individual technologies under each
category are intended to provide enough information
to design a feasibility study at a particular site. This in-
cludes specific information on target analytes, treatment
efficiency, examples of field-scale applications, critical pa-
rameters, and references for additional information. Key
information on the feasibility of each technology is also
summarized in tables. References are provided for sup-
porting information, and links to reliable Web pages are
included throughout.
4.1 Source Control
Treatment of contaminated mine waters is very often a
long-term commitment, and resources will be required for
the duration of the flows. Reducing or eliminating these
flows through source control methods clearly has long-
term benefits and should always be considered for deter-
mining the appropriate method for surface and ground
water protection. However, the uncertainty of successful-
ly implementing source control and costs of preventing
releases of contaminated water also needs to be evaluated
as a component of the water quality management deci-
sion. In some cases, source control needs to be considered
early in the mine development; in other cases, it can be
applied to mine sites that have long been closed.
Source control can be applied to two broad categories of
drainage waters: drainage from surface waste facilities and
drainage from underground workings (e.g., adits). Both
are ultimately supplied by meteoric (precipitation runon/
runoff) water, so flow rates are related to precipitation.
However, surface waste facilities generally have a more lo-
cal response to rainfall and can be managed by appropriate
caps, while drainage from underground workings requires
consideration of the regional ground water system.
4.1.1 Capping and Revegetation for Source Control
Capping and revegetation technologies seek to reduce or
eliminate the flow of water and oxygen into surficial mine
waste, producing a corresponding decrease in the produc-
tion and transport of solutes out of these potential sourc-
es. "Store-and-release" caps are simply vegetated surface
layers of material with a high moisture-retention capac-
ity that store water in the cap until it can be transpired
or evaporated. The goals are to minimize net percolation,
support vegetation, reduce erosion, and isolate acid-gen-
erating rock from the surface. These caps can dramatically
reduce, although not eliminate, net percolation of water
into mine waste.
Suitable cap materials include topsoil, run-of-mine waste
rock, or waste rock amended to improve its performance
(e.g., with nutrients to enhance plant growth, with fine
material or tailings to increase water retention, or with
alkali reagents to offset acid-generating potential). Selec-
tion criteria include moisture retention characteristics
(measured directly or estimated from particle-size distri-
bution), shear strength (measured in a laboratory), and
acid-generating potential (AGP) (chemical analysis). Be-
nign waste rock, with or without amendment, is a par-
ticularly attractive cap material because it is typically read-
ily available and often strong enough to resist erosion on
slopes. Vegetation type is entirely site specific, but seed
mixtures typically focus on perennials that are efficient at
extracting water, have deep roots, are drought resistant,
and are consistent with post-reclamation land use.
Inhibition of oxygen is often cited also as a goal of mine
waste caps, as oxygen flux is approximately proportional
to acid rock drainage (ARD) formation. Diffusive oxygen
flux into waste rock facilities typically produces several kg
sulfate/m2-yr for at least tens of years. Measurements in-
side waste rock indicate that oxygen advection through
coarse zones may be a larger oxygen source (Andrina et
al., 2003), so total oxidation rates in mine waste facilities
may be several times higher. However, long-term oxygen
exclusion has been demonstrated only with subaqueous
disposal. Void space in waste rock is typically 40% (Wil-
son et al., 2000b), and the high air diffusivity of oxygen
(-10,000 times greater than in water) allows rapid oxygen
transport. Oxygen-consuming layers (e.g., wood chips)
are effective, but have a very short life. Although models
indicate that water-saturated zones could be maintained,
even in semi-arid climates, other variables often make
them impractical. Short of subaqueous disposal, no prac-
tical cap designs currently provide complete long-term
barriers to oxygen. Wet covers are facilities that maintain
a permanent water body above reactive mine waste—a
form of subaqueous storage. They have been found to
minimize oxidation and release of contaminants; a nu-
merical analysis study utilizing modeling concluded, "a
water cover alone leads to a reduction of approximately
99.1%, in the [sulfide] oxidation rate relative to uncov-
ered tailings" (Romano et al., 2003). However, such caps
generally require perpetual management to ensure con-
tinued water saturation of the surface.
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
One important caveat for cap effectiveness is that reduc-
ing net percolation in acid-generating material may delay
the onset of impacts, but not the magnitude, as pore wa-
ters become more concentrated in slower-flowing waste
(Ritchie, 1994). Decreasing flow in mine waste does in
fact allow more time for attenuation reactions, including
silicate mineral buffering and precipitation of sulfate salts,
so reducing net percolation may in many cases reduce en-
vironmental impacts from mine waste. However, reduced
infiltration is not a guarantee of reduced impacts.
Finally, two theoretical technologies potentially offer
walk-away designs. An umbrella design with sloping lay-
ers of fine material—the most conductive in unsaturated
waste (Wilson et al., 2000a)—could shed water around
net acid-generating rock (Barbour, 2000). Potential draw-
backs are that this design still requires select handling of
acid-generating rock and that low shear strength of fine
materials may limit its applicability on steep slopes. Sec-
ond is a tailing and waste rock blend design (- -| tailings and
-} waste rock). This material has shear strength comparable
to waste rock, but with moisture retention high enough to
maintain saturated conditions, providing a long-term bar-
rier to oxygen introduction (Wilson et al., 2000b; Wilson
et al., 2003). Potential limits may include high blending
costs, long-term physical stability of blends, and suitabil-
ity of such blends for revegetation. Neither has been dem-
onstrated in field-scale tests, and they are not considered
further in this document.
Characterization requirements for mine waste caps in-
clude the following:
• Climate (daily temperature, precipitation, humidity,
potential evaporation, and insolation)
• Reclaimed vegetation mix (post-reclamation species
and their root depth and leaf area index)
• Availability of suitable cover (waste rock, soil, tail-
ings, and limestone)
• Physical characteristics of cap (particle-size distribu-
tion, Atterburg limits, specific gravity, compaction
curve)
• Hydraulic characteristics of cap (saturated hydraulic
conductivity and soil water characteristic curve)
• Moisture-retention characteristics of proposed cap
material (can be estimated from particle-size distri-
bution or determined more reliably with pressure-
plate laboratory hydraulic tests)
Design and analysis of store-and-release caps can be con-
ducted with models (e.g., HELP for screening-level analy-
sis and SoilCover or Vadose/W for more refined analy-
sis—see the review by O'Kane and Barbour, 2003).
Key Web Site References
• Overview of dry covers for mine waste, available on-
line at INAP (O'Kane Consultants, 2003):
http://www.inap.com.au/completed research
projects.htm
• "Design, Construction, and Performance Monitor-
ing of Cover System for Waste Rock and Tailings," a
comprehensive, five-volume design and monitoring
report (MEND Report 2.21.4): http://www.nrcan.
gc.ca/ms/canmet-mtb/mmsl-lmsm/mend/
mendpubs-e.htm
4.1.2 Plugging Drainage Sources and Interception of
Drainage by Diversion Wells
Because these systems often intercept ground water and
can even change the hydrologic system, source control
is limited, complicated, and uncertain. Two general ap-
proaches are often considered: interception of water from
the underground workings and plugging the drainage
routes from the underground workings.
Plugging of drainage routes: Plugging of adits and
grouting of drainage pathways have sometimes been
demonstrated to be effective in reducing the volume of
contaminated water from underground mines. The goal
is to retain the contaminated water in the underground
workings and allow the ground water table to rise. This
approach also is coupled with the expectation that the lo-
cal ground water level will cover the underground work-
ings to prevent continued oxidation of the rock. While
release of contaminated water through new routes is often
observed, further management of these sources can po-
tentially cover the historic workings and reduce the con-
taminant load in the water considerably. If successful, and
assuming the adit plugs and grouting are stable, the costs
of treatment can consequently be substantially reduced.
The implementability of this technology is highly site
specific and requires an understanding of the hydrologic
system, as well as the mine workings. While adit plugs can
work well under favorable conditions of geology, hydrol-
ogy, and mine development, such favorable combinations
have been found to be rare.
Interception of ground water: In some cases, drainage
patterns of surface and ground water can be altered to
keep good-quality water away from reactive underground
Management and Treatment of Water from Hard Rock,
Engineering Issue
-------
workings, pits, or waste rock dumps to reduce the vol-
ume of contaminated water that is produced. Each case
requires an extensive study of the hydrologic system and
the associated contaminant source. For surface water, sim-
ple diversions via channels over areas of infiltration (e.g.,
faults and slopes) can reduce the amount of contaminated
water that is generated. Ground water diversions can po-
tentially involve two general techniques. The first is to
establish passive drainage systems that take advantage of el-
evation differences and ground water system opportuni-
ties. In this case, water is drained away from reactive rock
by drilling water conduits that change hydrologic gradi-
ents to limit the amount of water that rinses reactive rock.
The second is to establish in-perpetuity pumping programs
to keep good-quality water from the reactive rock under-
ground workings. In this case, wells are drilled upgradi-
ent of reactive rock surfaces to lower the water table to
reduce the contaminant load in the surface or ground wa-
ter. Such proposals have been developed for maintaining
dry pits, reducing flows of water from springs that exist
under reactive waste rock dumps, and reducing flows that
pass through underground workings. These techniques
can potentially reduce or eliminate the need for water
treatment. While almost always expensive, these types of
pumping systems, under certain circumstances, can be
less costly than water treatment. However, in establish-
ing programs that require very long-term pumping, it is
necessary to recognize that if the pumping is discontinued
and ground water flows return to the pre-pumping con-
dition, the contaminants in the underground workings
will again be mobilized. Additionally, long-term pumping
upgradient of the source area, and resultant dewatering,
may increase the release of heavy metals from a negative
geochemical effect.
4.1.3 Prevention of Acid Drainage via Protective
Neutralization
The detrimental effects of sulfide oxidation in mine waste
can be offset when the material contains excess acid-neu-
tralizing minerals, such as calcite (CaCO3). Neutralizing
minerals react in situ with acidic leachate to neutralize acid-
ity, precipitate most sulfate (as gypsum, CaSO4-2H2O, or
other calcium sulfate compounds) and iron (as oxides or
sulfates), and reduce dissolved trace metal by inducing ad-
sorption to surfaces. As a result, sulfidic mine waste that
contains excess neutralizing potential can, theoretically,
weather into perpetuity without releasing acidic water.
This section describes the technologies for in-situ source
control of net-acid - generating waste using the addition
of neutralizing materials and identifies those factors most
critical in assessing their feasibility.
In-situ acid neutralization technologies are based on the
acid base accounting (ABA) of a material. The ABA is the
balance between total acid-generating potential (AGP),
which is the total amount of acidity that would be pro-
duced if all sulfide in a material is completely oxidized,
and total acid-neutralizing potential (ANP), which is the
amount of acid that could be consumed by neutralizing
minerals. There are numerous methods for analyzing for
ABA, allowing the flexibility to tailor testing to site con-
ditions and budgets. Unfortunately, there are also several
systems of ABA nomenclature in use, with no clear stan-
dard emerging. In this document, the convention in which
ANP and AGP are converted to CaCO3 equivalents and
reported in g CaCO /kg rock (i.e., parts per thousand,
%o) is used. ABA is described using net-neutralizing po-
tential (NNP), defined as ANP - AGP. Thus, NNP has
units of %o CaCO3 and is negative for net acid-generating
material and positive for net-neutralizing material.
ABA is typically calculated from analysis of sulfide S
and carbonate C, assuming a 1:1 molar ratio of sulfide S
(AGP) and carbonate C (ANP).
Converting chemical analysis for sulfide S (S(FeS2)) and car-
bonate C (C(CaCo3)):
AGP = S(FeS|x(10)x(3.12)
ANP = C(caco3| X (10) X (8.33)
NNP = ANP-AGP
Where
S(FeS| = Concentration sulfide sulfur in sample (weight % S)
3.12 = molecularweightof CaC03/molecular weight of sulfur
^icaco3) = Concentration carbonate carbon in sample (weight % C)
8.33 = molecularweightof CaC03/molecular weight of carbon
Basic silicate minerals may also contribute to ANP with
"silicate neutralization," consuming acidity in the process
of dissolving. Acid neutralization by silicate minerals is
typically much slower than reactions with carbonate, and
reaction rates depend strongly on pH, particle size, and
surface area.
Pore water neutralized in-situ by calcite does not ensure
perfect water quality. Neutralization of sulfuric acid by
calcite can still leave sulfate concentrations greater than
2,000 mg/L. Under oxidizing conditions, iron will pre-
cipitate from neutralized water, forming hydroxide and
sulfate minerals that are effective adsorption substrates for
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
trace metals, but trace metals in pore water may remain
above remedial goals. However, solute reductions upon
neutralization can be dramatic, with 10-fold to 1,000-
fold reductions in concentration common. Lime amend-
ment of acid-generating waste when mixing, dispersion,
and other attenuation processes are considered can, under
favorable conditions, produce mine waste that meets ex-
posure-point water quality standards. However, if lime is
consumed prior to exhausting the acid-generating capac-
ity or is inefficiently mixed, contaminated acidic water
can begin to drain from these sites long into the future.
Fortunately, studies find that ANP is generally a good in-
dicator of long-term acid release. A review of 281 kinet-
ic tests (various humidity cell and column tests from 53
different mines) found no net-neutralizing samples (i.e.,
NNP > 0) that produced acidic leachate (Morin et al.,
1995). A separate comparison of 307 samples from nine
hard-rock mines found similar results—NNP (using car-
bonate carbon for ANP) was a reliable predictor of ac-
tual acid release under simulated weathering conditions
(see Figure 4-1). Acid production rates in sulfidic mine
waste vary enormously with intrinsic oxidation rates (i.e.,
oxidation rate under atmospheric conditions), with low
rates being -10~8 kgO /m3 — s and high rates being -10~6
kgO2/m3 — s (Bennett, 1998). These results indicate that
when there is an excess of naturally occurring carbonate
minerals in mine waste, the neutralizing reactions gener-
ally keep pace with the acid production.
0 200
NNP (%> CaCQ3)
Figure 4-1. Humidity cell results from nine hard-rock mines: NNP
vs. final humidity cell pH. (Source: Exponent, 2000)
Estimating the acid/base accounting of mine waste has
two components: (1) obtaining sufficient sampling to
generate a representative sample of the target material and
(2) conducting chemical analysis that accurately indicates
material ABA.
The number of samples required to adequately define the
distribution of acid/base accounting in mine waste de-
pends on the size of the unit targeted for treatment, the
variability in the ABA of the material, and the desired ac-
curacy. A 1989 guidance document (SRK, 1989) is one of
the few references to recommend a fixed number of ABA
samples based on the size of each geologic unit. When
large waste rock or tailings facilities are targeted for treat-
ment, geostatistical analysis may be warranted to identify
spatial correlations in ABA.
The dominant analytical methods for acid/base account-
ing in mine waste, in order of increasing complexity, are:
1. Net acid-generating test (NAG) (Miller et al.,
1997). This is the simplest ABA analysis, reacting
a sample with hydrogen peroxide to completely
oxidize all sulfide minerals, then noting the pH after
reaction as an indicator of whether the material
is net-acid generating (pH < 4.5) or net acid-
neutralizing (pH > 4.5). It is rapid and inexpensive,
can be conducted in simple field laboratories, and
can be modified slightly to yield more quantitative
information or excess AGP
2. Leco furnace method (ASTM, 2003). This is a
rapid method that requires sophisticated equipment
but relatively little labor. Results of this method are
generally consistent with comparison tests using
long-term kinetic tests.
3. Sobek titration method (Sobek et al., 1978).
This is the original ABA analytical method, based
on titration of samples to determine acid and base
concentrations directly. It is labor intensive and thus
generally more expensive to conduct than the two
methods previously described, but it is generally
regarded as the most reliable indicator of long-term
acid release potential.
Lime and limestone are the most commonly used amend-
ment materials. Lime—in both the processed (CaO) and
hydrated [Ca(OH)2] forms—is more soluble and reacts
more rapidly than calcite [i.e., limestone (CaCO )], and
is thus considered to be more effective in controlling ARD
(Evangelou and Zhang, 1995). However, due to their high
solubility, lime amendments can be washed quickly from
waste rock, thus limiting their long-term effectiveness in
unsaturated conditions, where acid production can con-
tinue after the lime is leached out. Thus, an effective waste
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
-------
rock amendment strategy would be to use calcite for the
cap, where long-term maintenance is required, and CaO
or Ca(OH)2 for subaqueous waste, where oxidation will
dramatically slow after emplacement of the waste. While
oxygen diffusion is slowed, it is usually not completely
eliminated, and in most cases, the rate of contaminant
release will depend on this rate of oxygen penetration to
the reactive surfaces.
Mine waste amendment is suitable for any materials that
can be accessed and subjected to complete mixing with
the amendment. In practice, amendment is generally con-
sidered for mine waste that (1) can be treated as it is being
excavated; (2) is near the surface (< 1 to 2 meters deep),
which can be amended by surface application followed by
ripping to mix at depth (may be amenable when vegeta-
tion of acid-generating waste is considered); and (3) is to
be moved for additional purposes.
Excavation of large waste rock and tailings and use of
amendment to ensure perpetual in-situ ARD neutral-
ization is often more expensive than other alternatives,
including perpetual collection and treatment of acidic
seepage.
Field-scale tests indicate that mixing neutralizing minerals
with acid-generating waste may need to be nearly ideal to
prevent ARD formation (Mehling et al., 1997) and that
neutralizing amendments should be 2 mm in diameter or
smaller. Field- and large-scale test plots indicate that there
may need to be as much as a 100% excess of amendment
to ensure perpetual acid neutralization. Amendment rates
should exceed those estimated solely on the basis of an
ABA (Day, 1994; Cravotta et al., 1990). However, the
amount of excess neutralization capacity required to en-
sure pH-neutral effluent varies from site to site. Mehling
et al. (1997) summarized three wide-ranging guidelines
taken from successful waste rock blending schemes at his-
torical coal mining sites:
1. NNP > 80 %o CaCO3 (Erickson and Hedlin, 1988)
2. NNP > 10 %o CaCO3 and ANP > 15 %o CaCO3
(Brady et al., 1990)
3. ANP/AGP > 2 (Day, 1994)
Not surprisingly, U.S. regulatory guidelines for classify-
ing waste as non-acid generating also vary widely. Some
state guidelines consider waste to be non-acid generat-
ing without additional kinetic testing if it has 20 percent
excess neutralizing capacity [i.e., a safety factor of 1.2,
ANP:AGP ratio > 1.2:1 (NDEP, 1990)]. Bureau of Land
Management (BLM) guidelines set this criterion at 300%
excess ANP (i.e., a safety factor of 3, ANP:AGP ^ 3) and also
suggest an ANP greater than 20 %o CaCO3 (BLM, 1996).
Cost considerations are listed below. These costs are esti-
mates based on current quoted costs in 2005. However,
the costs will vary, based on availability of raw materials,
energy costs, and other specific requirements at a site.
Limestone (crushed to < 2 mm, assuming local
source):
• Delivered from off-site source: $US30-50/tonne
• Mined and crushed from on-site source at operating
mine: $US2-3/tonne
Lime, variable, depending on source and haulage:
• Hydrated lime: $60-l40/tonne
• Lime (CaO): $80-$240/tonne
Safety factor for neutralized waste:
• ANP greater than 20 %o CaCO3 (BLM, 1996)
• ANP/AGP > 1.2 (i.e., 20% excess-neutralizing
potential)
Cost to mix amendments into waste rock (mixing
costs only):
• Complete mixing (Grizzly to separate waste rock and
pug mill to mix): $US0.75-1.50/tonne
• Surface mixing by ripping in amendment with
bulldozer (maximum depth -6 ft.): $US0.04-0.06/
tonne-treated rock
Performance Data
Laboratory and field-scale studies demonstrate that the ef-
fectiveness of mine waste amendment is affected primarily
by the mixing efficiency and the particle size of neutral-
izing materials. Specifically, mixing at less than complete
homogenization can allow acid production, followed by
migration of acidic leachate in preferential flow paths;
neutralizing amendments, particularly limestone, greater
than approximately 2 mm in diameter are significantly
less effective at neutralizing acidity. Following are a few
studies from the literature that illustrate these conclusions.
Further, many states have guidance on ABA requirements
for mine waste, suggesting that blending programs are
generally accepted. However, the failure of several field-
scale, neutralization-blending tests is likely to be a cause
of concern for the scientific community and possibly for
experienced representatives in industry and the regulatory
community. Two cases of mixed success are presented in
the following table.
10
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
Site Name
and Location
Experimental
Design
Results
Site Name
and Location
Experimental
Design
Results
Samatosum Mine in south-central BC (Mom
andHutt, 1996; Mehling etal., 1997)
Field-scale horizontal layers of acid-generating
waste rock; NNP/ANP ratio of 3.
The waste rock pile produced acidic leachate
despite being amended to obtain a three-fold
excess ANR Hydraulic short-circuiting was cit-
ed as the probable cause of failure of a waste
rock amendment scheme.
Kutcho Creek Project, BC (Mehling et al., 1997)
Field-scale test constructed with 10-cm thick
horizontal layers of acid-generating waste rock
with net-neutralizing rock to achieve an ANP/
AGP ratio of 1.1. Two-year duration.
Partial success, with ARD released from net-
acid - generating comparison, but not from the
net-neutralizing material. However, projections
indicate that ARD was likely from amended lay-
ers, leading to the conclusion that blending was
not effective for preventing ARD in material with
an ANP:AGP of only 1.1. Cost analyses of com-
plete blending suggested thatthis method might
be prohibitively expensive on a large scale.
4.1.4 Passivation of Sulfidic Rock
In recent years, emerging technologies have been exam-
ined that are designed to limit the release of acidic com-
ponents by forming a thin protective layer on the reactive
rock surface. This "passivation" has the potential to reduce
or eliminate oxidation of the rock and thus reduce or elim-
inate release of contaminants from the rock. Each of the
technologies examined utilizes liquids that can be applied
to pit walls or reactive waste rock. The following three
technologies are examples that are being investigated:
1. Potassium permanganate: Pyritic surfaces are first
rinsed with a solution of lime, sodium hydroxide,
and magnesium oxide at a pH > 12, followed
by treatment with potassium permanganate. The
manganese/iron/magnesium surface formed is
resistant to further oxidation and substantially
reduces the amount of acidic water draining from
the treated rock.
2. Ecobond™: MT2 (http://www.metalstt.com) uses a
phosphate-based solution to coat acid-generating rock
to form a stable, insoluble coating on the surface. The
technology forms stable iron phosphate complexes
that resist hydrolysis and prevent further oxidation.
3. Silica Micro Encapsulation (SME): KEECO has
a patented process that treats acid-generating rock
with a solution of silica. It encapsulates metals in an
impervious microscopic silica matrix and prevents
additional acid generation or metals migration. This
technology has been utilized previously for metals-
contaminated soils.
These technologies are being tested at several sites, but
their efficacy has not been thoroughly established. The
EPA/DOE (Department of Energy) Mine Waste Tech-
nology Program has examined these technologies (http://
www.epa.gov/minewastetechnology/). The future utiliza-
tion of these methods for waste rock requires consideration
of the following:
• Hydrologic characteristics of the waste rock: For
pre-existing waste rock dumps, how can complete
(or near-complete) coverage of the reactive surfaces
be ensured? Is it sufficient to treat the top of the
waste rock dump (1-5 meters), or does the entire
waste rock dump require treatment?
• Longevity: Each type of coating is thin, and the
reactive bulk rock remains reactive. How long will
these coatings prevent release of contaminants?
• Cost: What are the site characteristics that will
change the cost of these treatments? How do these
costs compare to conventional water treatment (e.g.,
lime precipitation)?
Table 4-1 on pages 12 and 13 provides an overview of the
source control technologies covered in this section, tech-
nology selection factors, and limitations.
4.2 Treatment of Contaminated Water
As discussed previously, waters draining from mine sites
can vary dramatically, and the methods used to treat those
waters will similarly vary. For this discussion, mine waters
can be put in three groups, although the distinction be-
tween the groups is sometimes not clear.
• Acidic water (pH < 5.5): Most commonly, these
waters are contaminated as a result of pyrite oxida-
tion and contain elevated metals and sulfate. While
any water with a pH < 7 is acidic, the most prob-
lematic waters are those with a pH < 4 since metals
loadings increase substantially. Because of the prob-
lems with acidic waters, extensive research has been
conducted on cost-effective methods of treatment.
Chemical neutralization consumption and sludge
management are large factors for selecting a treat-
ment method.
(Continued on page 14)
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
11
-------
Table 4-1. Source Control Technologies for Hard-Rock Mine Waste
Technology
Name
Capping and
Revegetation
Wet Covers
Hydrologic
Controls
Technology
Description
Cover waste rock or tailings with
suitable growth medium and estab-
lish vegetation. ("Suitable" means
soil, waste rock, tailings, or blend
of these materials that is non-acid
generating and contains heavy
metals below levels that are phyto-
toxic or that may cause ecological
or human health risk.) A "Store and
Release Cap" retains meteoric wa-
ter long enough for plants to uptake
and transpire the water, minimiz-
ing the "net percolation" (i.e., the
flux of water from the cap into the
waste) and associated rate that
solutes in waste are flushed out.
(No specific chemical reactions.)
Storage of acid-generating rock
and tailings under water to prevent
(or minimize) oxidation and release
of contaminants
Drainage is controlled by direct-
ing water flow from reactive rock
surfaces, by diverting ground water
flows, or by pumping ground water.
A second option is to plug adits and
shafts to allow water to fill under-
ground workings and cover reac-
tive surfaces. Water and/or oxygen
reaction with surfaces is minimized.
Rerouting surface water around
surface or underground disturbanc-
es is also sometimes an option.
Target
Analytes
All soluble
constituents
in the solid
waste
All
All
Critical Feasibility Factors
Implementabilty
• Stability of waste or
cap to slope failure
and erosion
• Availability of suit-
able cap material
(i.e., non-acid gen-
erating, low met-
als, high moisture
retention)
• Access to slopes
for seeding and
grading
• Commonly per-
formed, with often
good but variable
results
Can be utilized when
reactive rock can be
submerged in a per-
manent water body,
including a perma-
nent tailings facility
pitlake orflooding
underground
workings
• Requires an un-
derstanding of the
hydrologic system
surrounding the
reactive rock
• Not commonly un-
dertaken due to dif-
ficulties in obtaining
a full understanding
of the hydrologic
system
Effectiveness
• Moisture retention
capacity of cap
material
• Cap thickness
• Potential for metals
uptake from cap
by plants (affects
ecological risk)
• Fire frequency
• Fraction of precipi-
tation as snow
Will generally reduce
oxidation rates of
reactive rock
• Elimination of water
from the reactive
surfaces can ef-
fectively stop acidic
drainage
• Effectiveness is de-
pendent on the abil-
ity to control water
in the underground
and surface water
systems
Cost
• Availability of local
cap material, either
topsoil, benign
waste rock or tail-
ings, or mixture of
these
• Need to possibly
amend cap with
neutralizing agent
or nutrients
• Cap thickness (at
least 1 m in semi-
arid climate to
sustain plants; may
be thinner in wetter
P 1 1 n~13 tQ C\
U II 1 1 Id Leo/
• Life expectancy of
cap
Highly variable, de-
pending on the avail-
ability of water and
site-specific
considerations
• Highly variable
depending on site-
specific factors
• Long-term pump-
ing has 0/M costs,
while an effective
drainage system
can be highly cost
effective
Important
Limitations
• A zero-net-percolation
cap has not been
demonstrated
• Precipitation as snow
greatly increases net
percolation
• In acid-generating waste,
reduced flow delays im-
pact, but may not reduce
contaminant load rate
• Highly engineered caps
(e.g., liners, capillary
breaks) have finite life
• Oxygen barrier to perma-
nently stop ARD pro-
duction is theoretically
possible, but has not been
demonstrated for long-
term applications
• Requires in-perpetuity
coverage of the reactive
rock and management of
the water body
• Previously oxidized rock
surfaces may still release
problematic contaminants
• Effectiveness depends
on the ability to control
water, either by plugging
or draining the adits
• This method, while it al-
ways should be
considered, is not often
successful
• Plugging of underground
workings has resulted in
blowouts of the plugs in
certain cases
1
i-
-------
Table 4-1. Source Control Technologies for Hard-Rock Mine Waste (continued)
Technology
Name
Acid Potential
Neutralization
Passivation
Technology
Description
Waste rock or tailings that are
net acid-generating are amended
with neutralizing agents (e.g.,
crushed calcite [CaCO3] or lime
[CaO], alkaline industrial wastes),
producing waste thatwill remain
permanently net neutralizing.
Amendments can be applied
surficially to existing waste and
mixed (-0.5 -2m depth-typical
root-zone depth for cap only
treatment) or added to new waste
and mixed during emplacement.
Reduces or halts the oxidation
of reactive surfaces, primarily
pyrite. Application methods vary,
but usually involve coating rock
surface with fluids and allowing
the specific passivation reaction to
occur. These technologies are still
in a research and demonstration
mode.
Target
Analytes
Heavy metal
cations
(e.g., Cu,
Cd,Pb,Zn),
acidity,
sulfate
(pore water
reduced
to -1,500-
2,000 mg/L)
Acidity and
soluble
constituents
in the solid
waste
Critical Feasibility Factors
Implementabilty
• Access to all acid-
generating waste
rock targeted
for blending in
neutralizing agents
• Stability of slope
during blending
• Fnr ran
rui u d |J
amendment,
sufficient access
and slope to
permit distribution
and mixing of
neutralizing
amendment into
near-surface (-2 m
depth)
The effectiveness of
passivation remains
to be established.
Laboratory and pilot-
scale treatments
show promise for the
various treatments,
but full-scale
applications have not
been undertaken.
Effectiveness
• Ability to uniformly
blend neutralizing
agents with acid-
generating waste
• Crushing
neutralizing agents
small enough to
ensure reaction
(e.g., 2mm)
Unknown. The
costs need to be
evaluated against the
probability of long-
term treatment using
more conventional
methods.
Cost
• Availability of
local source of
limestone or other
neutralizing agent
• Cost to mine,
crush, and deliver
neutralizing agent
(ideally
2 mm diameter for
blending)
• Cost to spread and
mix neutralizing
agentinto cap
• For existing waste,
cost to excavate
and uniformly
amend
Costs are highly
variable, depending
on the technology
utilized and the need
for periodic treatment
Important
Limitations
• Surficial amendment of
acid-generating waste
does not stop ARD
production below the cap
• Typically, it is not
economic to excavate
and amend existing buried
waste
• Layering or sequential
placement of neutralizing
waste with acid-
generating waste often
does not stop acid release
Full passivation of waste
rock dumps is difficult,
due to the problems with
delivering fluids in such a
way that all surfaces are
contacted. Also unknown
is the longevity of these
treatments. Requires further
investigation.
-------
• Near-neutral water (pH 5.5-9): These waters are
common at many non-acid - generating sites, partic-
ularly those with high net neutralization in the waste
rock. Sulfate concentrations are generally less than
2,000 mg/L, but may contain elevated concentrations
of certain metals (e.g., zinc, copper, or nickel), oxy-
anions, or arsenic, antimony and selenium, particu-
larly at the higher pH ranges. Common examples are
drainage from carbonate-hosted waste rock dumps,
closed precious metals heaps, and pit lakes.
• Alkaline water (pH > 9): With few exceptions,
these are commonly associated with process fluids,
and the elevated pH is due to chemical reagent addi-
tion (e.g., sodium cyanide plus lime). The solubility
of a variety of oxyanions can be enhanced at alkaline
pH. Over time, the pH of these waters is reduced
when atmospheric carbon dioxide dissolves.
4.2.1 Treatment of Acidic Waters
Acidic water is generally considered the most problematic
mine-related drainage water, and it offers the greatest po-
tential for degradation of surface and ground water. While
prevention of acid drainage is a common goal for manage-
ment of acid-generating rock, treatment of acidic drainage
at many mine sites will be required far into the future.
Methods for treatment of acidic drainage vary consider-
ably, but most focus on increasing the pH to above pH 7,
which will subsequently reduce the solubility of a variety
of contaminants in the drainage water. This is especial-
ly true for the divalent metals and aluminum, which are
precipitated as hydroxides. The literature on treatment of
drainage from coal mines has examined the various types
of neutralizing agents in detail, and although differences
exist between coal and hard-rock mine waters, treatment
of coal mine waters has been extensively examined in the
past 25 years (http://www.wvu.edu/~-agexten/landrec/
chemtrt.htm). Five chemicals that have commonly been
used for treatment of acidic water are listed in Table 4-2.
Ammonia has also been utilized for treatment of coal
mine waters, but is uncommon for treatment of hard-rock
mine waters and will not be considered further here. This
leaves two general types of neutralization agents, the cal-
cium- and sodium-based systems. Of these, the calcium-
based systems are generally preferable to sodium due to
the ability of calcium to remove sulfate as calcium sulfate
compounds (e.g., gypsum). Calcium will also ultimately
precipitate as calcite when the water is equilibrated with
carbon dioxide in air if the pH is slightly elevated. Alterna-
tively, while sodium-based neutralization agents are effec-
tive in raising the pH, elevated sodium in irrigation water
causes soil structure to collapse (sodic soils). Also, particu-
larly when handled in bulk, lime is generally less expensive
than either sodium carbonate or sodium hydroxide.
Table 4-2. Neutralizing Reagents for Treatment of Acidic Water from Mines (Source: Modified from Skousen in
http://www.wvu.edu/~agexten/landrec/chemtrt.htm.)
Common Name
Limestone
Hydrated lime
Lime (quicklime)
Soda ash
Caustic soda
Chemical Name
Calcium
carbonate
Calcium
hydroxide
Calcium oxide
Sodium
carbonate
Sodium
hydroxide
Formula
CaC03
Ca(OH)2
CaO
Na2C03
NaOH
Conversion
Factor*
1.0
0.74
0.56
1.06
0.8
Comments
Inexpensive chemical cost, but difficult to dissolve — tends
to armor and reduce effectiveness. Utilization of only 30%
of neutralizing capability.
Relatively inexpensive chemical cost and most utilized form
of lime as a slurry. Requires control to maintain suspension.
Neutralizing efficiency of 90%.
Also commonly utilized, although more effort is required
to maintain a suspension. Requires slakerto convert to
hydrated lime. Neutralizing efficiency of 90%.
Dissolves rapidly; less caustic alternative to sodium
hydroxide. Does not remove sulfate effectively. Increases
sodium content of treated water. Neutralizing efficiency of
60%.
Does not remove sulfate effectively and increases sodium
content of treated water. Neutralizing efficiency of 100%.
* The conversion factor is the relative amount of weight of each material (compared to limestone) to neutralize a given amount of acid.
The estimated tons of acid/year can be multiplied by the conversion factor to get the tons of chemical needed for neutralization.
74
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
4.2.7.7 Conventional Physical/Chemical Treatment of
Acidic Water Using Lime
Use of lime (calcium hydroxide or calcium oxide) for neu-
tralization is the accepted conventional water treatment
for most hard-rock mine acidic waters, particularly when
the acidity and/or flows are high. Not only does this treat-
ment raise the pH in a cost-effective manner, it also reduc-
es the sulfate concentrations to below 2,000 mg/L due to
the relatively low solubility of gypsum (calcium sulfate).
(See INAP, 2003 for summary of methods for treating
sulfate in water.) For this discussion, "conventional wa-
ter treatment" refers to fixed facilities of pipes, metering
pumps, reaction vessels, clarifiers, and solid management
mechanical fixtures (e.g., filter presses). Lime is mixed
with acidic water as a 10-15% slurry that is most com-
monly generated on site from a storage tank of hydrated
lime using a slurry mixer.
Conventional acidic water treatment using alkaline sourc-
es is used for removal of Al, Fe, Cu, Cd, Pb, Zn, Ni, and
Mn as metal hydroxides. The dissolved concentrations of
the oxyanions, including Cr, Se, Sb, Mo, As, and U, can
also be substantially reduced by the co-precipitation with
the metal hydroxide.
Oxidation of soluble ferrous iron to ferric iron is required
for treatment of most mineral acidic waters and utilizes
atmospheric oxygen at an elevated pH > 7. Ferric ox-
ide rapidly precipitates at neutral or alkaline pH. Mixing
of the acidic water with lime slurries requires aeration to
ensure good contact with atmospheric oxygen. Most con-
ventional treatment systems utilize a stirred aeration basin
to accomplish this oxidation (see Figure 4-2).
A relatively new method for oxidation uses the Rotating
Cylinder Treatment System (RCTS) to provide rapid oxy-
gen transfer to the solution and efficient utilization of the
lime slurry (http://www.iwtechnologies.com). The RCTS
uses shallow trough-like cells to contain the impacted wa-
ters and rotating perforated cylinders for improved atmo-
spheric oxygen transfer and improved agitation during
treatment of the water.
Following oxidation and neutralization, agitation of the
suspension and addition of flocculants allows the metal
oxide solids to settle out by growth of precipitants to suf-
ficiently large particles to form sludge. The sludge pro-
duced is generally of low density and requires thickening
or filter presses to decrease the water content. Addition-
ally, management of the sludge generally requires a de-
Bin vent
filter
-Turbulence box
High level
sensor
Re-order
sensor
Low level
Bin discharger
Clear water
pump
Sump
o o o o
im
&&&&
/ \
Electrical panel
SCR
controller
Slurry mixer
Flash mixer
Volumetric'
screw feeder
Slurry mix
tank
Distribution
trough
Flash tank
Aeration basin
%.. HUH
•: .• ".•;:.• \ *:: \t'.-:>. >.:•: ;:/._.•".;.;;
Thickener or
settling pond
Figure 4-2. Conventional water treatment utilizing lime. (Source: http://amd.osmre.gov/Cost.pdf)
Management and Treatment of Water from Hard Rock ML
Engineering Issue
15
-------
termination of its contaminant leachability to decide
whether the sludge can be managed on-site or needs to be
transported to an off-site hazardous waste management
facility. The Toxicity Characteristic Leaching Procedure
(TCLP) is usually used to determine if a material is Re-
source Conservation and Recovery Act (RCRA) - haz-
ardous due to its leaching characteristics. The Synthetic
Precipitation Leaching Procedure (SPLP) is a better indi-
cator of leaching behavior under natural environmental
conditions. State regulations also apply to how sludge is
managed, on site or off site.
Because of the wide range of flows, the amount of lime
required, the length of time for each type of reaction, the
method of settling or filtering out the solids from the wa-
ter, and the method for residual (sludge) management,
site-specific information is required for the design of each
system. Shakedown operations and modifications of orig-
inal design are often performed to meet target discharge
requirements and to optimize operations to reduce costs
or volume of residual (sludge) produced.
The costs for construction of active lime treatment facili-
ties can be substantial due to the requirements of power,
pumps, lime addition systems, tanks, and sludge manage-
ment equipment. Several organizations have developed
guidelines. An example of such guidelines is one devel-
oped by the Office of Surface Mining (2000) (http://amd.
osmre.gov/Cost.pdf). While this document is focused on
costs for treatment of acidic drainage from coal mines, the
same approach can be used for estimating costs for treat-
ment of acidic drainage from hard-rock mines. Because
the characteristics of water quality, flows, remoteness, and
reagent costs, as well as other factors, can vary substantial-
ly, it is difficult to provide a reliable estimate for treatment
at a specific site until a careful engineering estimate is de-
veloped. However, estimates for treatment cost vary from
less than $1/1,000 gallons to well over $10/1,000 gallons
on an annual operating and maintenance basis.
While the newer designs for lime treatment systems are
increasingly automated, these systems still require fre-
quent monitoring and oversight due to the caking and
scaling problems common with the use of lime. Addition-
ally, these active systems utilize pumps and mixing sys-
tems that require routine maintenance. Thus, operation
of a lime treatment plant has inherent fixed construction
and operation/maintenance costs that make these treat-
ment systems expensive on a cost per volume of water
treated when the flows are low. However, as flows increase
(e.g., > 100 gal/min) or the acidity and metals load-
ings increase, the fixed costs become a smaller fraction of
the total cost, and lime treatment is generally the most
cost-effective method for treating large volumes of acidic
drainage from mines. Comparison of costs of treatment
at different flows (using reagent costs in 1996) is avail-
able at http://www.wvu.edu/-agexten/landrec/chemtrt.
htm#Chemical. While the reagent costs change with time
and location, as well as the implementation of a treatment
system at a specific location, this example provides an in-
dication of the non-linear cost differences with differing
flows. Each treatment alternative needs to be evaluated
relative to the total costs and intended characteristic of
the effluent water.
Using an engineered system of conventional water treat-
ment requires proper road access, a power supply, stable
land area, and manpower. In remote areas of the west-
ern U.S., access may be difficult and expensive during the
winter months, and a conventional lime treatment system
may not be appropriate.
Key Web Site References
• Overview of chemicals available to treat AMD:
http://www.leo.lehigh.edu/envirosci/enviroissue/
amd/links/cheml .html
• AMD abatement cost-estimating tool developed co-
operatively by the Pennsylvania Department of Envi-
ronmental Protection, the West Virginia Department
of Environmental Protection, and the Office of Sur-
face Mining (OSM) Reclamation and Enforcement:
http://amd.osmre.gov/
• The MEND manual is a set of comprehensive work-
ing references for the sampling and analyses, predic-
tion, prevention, control, treatment, and monitoring
of acidic drainage. The document provides informa-
tion on chemistry, engineering, economics, case stud-
ies, and scientific data, http://www.nrcan.gc.ca/mms/
canmet-mtb/mmsl-lmsm/mend/mendmanual-e.htm
• UK summary of active and passive treatment:
http://www.parliament.uk/commons/lib/research/
rp99/rp99-010.pdf
• Britannia Mine Water Treatment Plant Feasibility
Study. An example of a feasibility study for a site-
specific conventional system: http://www.agf.gov.
bc.ca/clad/britannia/downloads/reports/tech
reports/WTP feasibility.pdf.
16
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
• Example of evaluating options for sludge manage-
ment for a conventional water treatment system:
http://www.agf.gov.bc.ca/clad/britannia/reports.html
• Detecting change in water quality from implementa-
tion of limestone treatment systems in a coal-mined
watershed of Pennsylvania: http://www.mbcomp.
com/swatara/Cravotta.pdf
• Abandoned mine remediation clearinghouse for
treatment of acidic drainage in Pennsylvania:
http://www.amrclearinghouse.org/Sub/
AMDtreatment/ZZTreatmentStrategies.htm
• National Lime Association Web site. A wealth of in-
formation about neutralizing acidic water with lime:
http://www.lime.org
• Army Corps of Engineers document "Engineering
and Design: Precipitation/Coagulation/Flocculation":
http://www.usace.army.mil/inet/usace-docs/eng-man-
uals/emlllO-l-4012/toc.htm
4.2.1.2 Physical/Chemical Treatment in Alkaline Ponds
and Lagoons
Physical/chemical treatment in alkaline ponds and lagoons
is very similar to conventional treatment as described in
the preceding section. Using ponds and lagoons for aera-
tion, settling, and solids accumulation has the benefits of
exploiting natural processes. Lime (calcium hydroxide) is
added using the same type of equipment that is used in
conventional plants.
Physical/chemical treatment in alkaline ponds and la-
goons is used to remove metals, including Al, Fe, Cu, Cd,
Pb, Zn, Ni, Mn, and the oxyanions Cr, Se, Sb, Mo, As,
and U. Depending on the water chemistry, the oxyanions
are reduced in dissolved concentrations by co-precipita-
tion with metal oxides and calcite. Formation of metal
hydroxide precipitates and formation of calcium carbon-
ate with flux from the atmosphere result in solids settling
out in the ponds. While the primary neutralization of the
acidic drainage is through lime addition, the ponds and
lagoons can improve the overall water treatment and met-
als reduction by the photosynthetic activity in the water
(see Figure 4-3).
A larger area of land is needed for physical/chemical treat-
ment in alkaline ponds and lagoons than for conventional
treatment plants. Pond or lagoon treatment systems are
often easier to construct if existing settling ponds, tailing
ponds, or excavated areas are available for use. Site-specific
information is critical for design of these systems because
Diversion
structure and
spillway
Heavy dashed lines show
underground flow or pumping path
Figure 4-3. Use of aeration ponds for polishing lime treatment pro-
cess. (Source: http://www.facstaff.bucknell.edu/kirby/RCr.html)
of the wide range of flows, the amount of lime required,
the length of time for each type of reaction, the method
of settling or filtering out the solids from the water, and
the method for residual (sludge) management. Alkaline
ponds and lagoons have been used to effectively remove
metals, metalloids, and uranium from mine waters when
designed to account for variations in flow and composi-
tion. Removals at a treatment lagoon in Butte, Montana,
are presented in Table 4-3, and a photograph of the polish-
ing pond is shown in Figure 4-4 on the next page. Shake-
down operations and modifications of original design are
often performed to meet target discharge concentrations
and to optimize operations to reduce costs or the volume
of residual (sludge) produced.
The construction cost for physical/chemical treatment in
alkaline ponds and lagoons is usually similar to the addi-
tional chemical components of a conventional treatment.
Due to land availability and status of land relative to the
hydraulic profile of proposed system, site-specific factors
can make ponds or lagoons less expensive than clarifi-
ers and reaction vessels. Due to the relative larger size of
ponds and lagoons than most conventional treatments,
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
77
-------
fewer operator hours are required to account for system
variations and to physically manage solids produced.
Smaller systems can be designed with cleanout and sludge
management at frequencies of a few years to decades. An
additional advantage of using alkaline ponds and lagoons
is the buffering capacity of the lagoons, which corrects
minor process upsets or variations.
Table 4-3. Influent and Effluent Concentrations for Treatment
Lagoon in Butte, Montana, for Year 2003
Basis
Total
Total
Total
Total
Total
Total
Total
Total
Total
Total
Analyte
Ag
Al
As
Cd
Cr
Cu
Fe
Mn
Pb
Zn
Untreated
Concentration
(ppb)
5
155
35
15
11
388
1,499
2,478
6
4,526
Treated
Concentration
(ppb)
5
34
7
0.3
10
15
41
72
1
107
%
Removed
Detection
Limit
78%
80%
98%
Detection
Limit
96%
97%
97%
Detection
Limit
98%
Using an engineered system of physical/chemical water
treatment in ponds and lagoons requires proper road ac-
cess, a power supply, stable land area, and manpower. One
additional advantage of a lagoon system is the attractive-
ness and wildlife attributes of a wetland, although these
features need to be weighed against metals bioavailability
and insect breeding issues.
Figure 4-4. Final pond in the treatment lagoons in Butte, Montana,
used for polishing and robustness of system.
Key Web Site References and Pictures
• Silver Bow Creek/Warm Springs Ponds One-Page
Summary: http://www.epa.gov/superfund/programs/
recycle/success/1 -pagers/bowcrk.htm
• Pictures of a lime lagoon at Leviathan Mine, Cali-
fornia, which has no biological component due to
limited size of the pilot project. Filter bags are used
to capture and manage the majority of the sludge,
while a lined pond is used for settling and polishing:
http://yosemite.epa.gOv/r9/sfund/sphotos.nsf/0/
75c4f97d7640242488256e98006656ab/$FILE/Le-
viathan 04%20p7-22.pdf
4.2.1.3 Low-Flow/Low-Acidity Chemical Treatment
Options
While conventional lime treatment has distinct cost and
treatment advantages, the costs of treating lower flows on
a per-gallon basis can potentially be reduced using alter-
native neutralization methods in certain cases. Examples
include the following:
Automatic lime addition using an Aquafix system: As
discussed previously, addition of lime to acidic water in
a controlled and efficient manner requires lime addition
technology that increases the fixed costs and is often infea-
sible for small streams. Jenkins and Skousen (1993) have
shown the utility of an Aquafix pebble quicklime (CaO)
water treatment system that utilizes a water wheel con-
cept for coal mine drainage waters. The concept is that
these systems can be operated without intensive manage-
ment, and the rate of addition of lime can be controlled
by the flow rate of the acidic stream. For this system, the
amount of chemical utilized is controlled by a water wheel
attached to a screw feeder that dispenses lime directly into
the flowing acidic drainage. This system was initially de-
veloped for small flows from coal mines of high acidity
because calcium oxide is very reactive. Recently, however,
water wheels have been attached to large bins or silos for
high-flow/high-acidity situations. These systems have re-
ceived only limited applicability at hard-rock mine sites in
the western U.S., although additional testing is warrant-
ed. Controlling the rate of application of the quicklime
without operator attendance and problems with remote
cold weather operation have somewhat limited the inter-
est for many mineral mine sites. These systems also may
require settling basins and sludge management for the
metals-laden precipitates (http://www.wvu.edu/-agexten/
landrec/chemtrt.htm).
18
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
Open limestone channels: While limestone beds/chan-
nels have been used with some success in neutralizing
mildly contaminated coal mine acidic drainage, the rate
of release of alkalinity is difficult to control, and the lime-
stone tends to armor with aluminum and iron oxide coat-
ings. Open limestone channels are constructed simply by
laying limestone rock in a channel and allowing the acidic
solution to pass over the rock or by laying limestone di-
rectly in a channel of acidic drainage (Ziemkiewicz et al.,
1997: http://www.dep.state.pa.us/dep/deputate/minres/
bamr/amd/science of amd.htm). Because of the armor-
ing that occurs, this method has shown best treatment
when the channel is sloped to allow rapid movement of
the water and scouring of the coatings on the limestone.
However, in mineral-mining applications, limestone
channels have not been shown to be successful, and the
applicability may be limited to iron-free, aluminum-free
waters that only contain metals that can be removed by
chemical precipitation at pH < 7. Depending on the re-
quirements, these systems can be lined or unlined. Settling
basins may be used under certain conditions to collect
precipitates. Limestone treatment is generally not effec-
tive for acidities exceeding 50 mg/L (http://www.osmre.
gov/amdtcst.htm). A somewhat more effective method for
limestone treatment utilizes pulsed, fluidized bed reactors
in which acidic water is injected in an upward manner at
high velocity into limestone columns. This method can
improve the scouring of the limestone and increase the
release of alkalinity. Carbon dioxide (either from tanks or
by utilization of CO2 released from the limestone) aids
the process by reducing the rate of iron oxidation in the
reactors (http://www2.nature.nps.gov/pubs/yir/yir2000/
pages/07 new horizons/07 02 reeder.html).
Anoxic limestone drains: An anoxic limestone drain
(ALD) is similar to an open limestone channel, except the
limestone is buried under a cap and designed to exclude
oxygen and reduce the amount of iron oxidation prod-
ucts that coat the limestone. This will tend to improve
the release of alkalinity from the limestone. These systems
have been used to decrease the acidity of drainage waters
prior to aerobic wetlands or sulfate-reducing bioreactors
(SRBs). The downside to using these systems is that if the
limestone becomes armored, uncovering the limestone
requires excavation of the cap. Because acidic drainage
from hard-rock sites often contains appreciable amounts
of aluminum that coat the limestone, these systems are
not commonly utilized. Limestone dissolution can in-
crease the pH sufficiently to precipitate oxidized iron and
aluminum, but does not effectively remove most heavy
metals. Limestone has the most potential as a pretreat-
ment method for passive microbial-based systems where a
decreased dissolution rate from armoring can be incorpo-
rated into the design.
Sodium hydroxide: Addition of solutions of 25% sodium
hydroxide to acidic water can be accomplished by either
gravity flow or small solar-powered pumps. This system
can be very inexpensive to construct, depending on the
site conditions, although the cost of sodium hydroxide is
higher than a similar amount of calcium-based neutraliza-
tion agents. The sodium hydroxide solution is complete-
ly utilized and an effective neutralization agent. Because
50% solutions will solidify under cold conditions, a 25%
solution is generally utilized and is available in bulk solu-
tions. However, the volumes that one will need to use will
require either frequent refilling or large storage capacity.
For example, a flow of 100 mL/min will utilize 52,500
liters per year (-13,900 gallons) of solution. Two other
disadvantages are the safety issues associated with using
sodium hydroxide, as well as the increase in sodium con-
centrations that remain in the treated water.
Sodium carbonate: A less caustic alternative to sodium
hydroxide is the use of sodium carbonate (Na2CO3). So-
dium carbonate briquettes are available and can be uti-
lized by simply diverting a small stream of the acidic wa-
ter through (or over) a bed of the briquettes and allowing
that solution to mix with the acidic water. Control of the
diversion can be managed with a weiring system. Sodi-
um carbonate tends to cement together and change the
amount of surface available for dissolution. Temperature
changes can also affect the amount of delivered alkalinity.
Sodium increases in the treated water are also an issue.
4.2.2 Treatment of Neutral and Alkaline Waters
Contaminated neutral or alkaline mine drainage waters
are present at sites that have sufficient neutralization (gen-
erally from calcite) in the rock such that any acid produc-
tion is offset by the neutralization available. These acidic
waters are also commonly generated from precious met-
als ore processing using cyanide for mill circuits or heap
leach processing.
Neutral and alkaline drainage from mine sites is gener-
ally less of a water quality problem than acidic drainage
since the solubility of many of the problematic metals is
low at neutral or alkaline pH. Neutral and alkaline drain-
age contaminants generally are most problematic for the
oxyanions of selenium, arsenic, and antimony since the
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
19
-------
solubility of these constituents increases with higher pH.
In addition, nitrate, sulfate, and other salts, as well as cy-
anide species, may be elevated at cyanidization facilities
and exceed discharge requirements.
4.2.2.1 Arsenic and Antimony
Total arsenic concentrations in drainage water can vary
from less than 10 ug/L to several mg/L. Antimony is gen-
erally found at lower concentrations. Because these ele-
ments are closely related (group 5A in the Periodic Table),
treatments for removal are similar and will be considered
together. In general, methods to remove arsenic from wa-
ter also are effective for antimony.
Arsenic treatment technologies have received the greatest
focus in recent years, primarily due to the need for arsenic
removal in drinking water. An extensive recent EPA ar-
senic treatment review (EPA, 2002: http://cluin.org/con-
taminantfocus/default.focus/sec/arsenic/cat/Treatment
Technologies/) and the U.S. Geological Survey Web site
(http://arsenic.cr.usgs.gov/) provide a more detailed dis-
cussion of the treatment options than is provided here.
The most common arsenic treatment systems are briefly
discussed below.
For specific application to mine-related waters, arsenic
removal from large volumes of water (e.g., pit lakes, dis-
charge water from pit dewatering) most often utilizes iron
precipitation/co-precipitation methods. For these systems,
ferrous or ferric salts are added to the water and allowed to
precipitate. Arsenic, particularly in the + 5 valence state,
sorbs strongly to the surface of the precipitates and is ef-
fectively removed from the water. When arsenic in the
+ 3 valence state is present in appreciable concentrations,
a pre-oxidation step may be required since it sorbs less
strongly to iron oxides than in the + 5 state.
A recently developed method for arsenic treatment uti-
lizes zero-valent iron (Su and Puls, 2001; Nikolaos et al.,
2003). For this technology, arsenic-containing waters are
passed over iron filings that generally have been mixed
into sand at a ratio of 10-20% iron. Iron oxidizes to iron
oxide, and arsenic is sorbed to the iron oxide surface. Al-
though the iron is ultimately mobilized (albeit slowly)
and the treatment system will need to be replaced, the
arsenic that is sorbed is generally not available. The iron
oxide/arsenic residue is generally not hazardous, although
its classification is dependent on the results of site-specific
waste characterization testing. Depending on the design,
these systems can remain effective for an extended period
of time (months to years). Zero-valent iron systems have
also been effectively applied as permeable reactive barri-
ers (PRBs) in subsurface systems for remediation of arse-
nic-containing ground water at a mill tailings site (EPA,
2000). PRBs are described in Section 4.2.3.2. Arsenic can
also (at least partially) be removed from mine waters by
sulfate-reducing bacterial systems as described below.
4.2.2.2 Heap Effluent
The use of heap leach technology for recovery of precious
metals has evolved over the past 25 years and is commonly
employed for low-grade ore (typically 0.015-0.06 ounces
per ton-equivalent of gold) at many sites throughout the
world. The tonnage of ore processed in this manner in
Nevada, for example, is estimated to be on the order of 2
billion tons. In this process, ore is placed on high-density
polyethylene sheets and rinsed with dilute concentrations
of sodium cyanide. In arid regions of the world, these sys-
tems are operated in a zero-discharge mode: the amount
of water evaporating is greater than the rainfall, and ad-
ditional water is required to make up the difference of the
amount lost to evaporation and the amount of rainfall.
When precious metals recovery is completed, the process
for closure of the heaps begins.
For arid sites, the most common method for initial re-
duction of water volume is to continue to recirculate the
water to the heap using enhanced evaporation methods:
water is sprayed into the air over the heaps and allowed to
evaporate, subsequently increasing the concentration of
soluble constituents in the remaining water. The rate of
water that is recirculated will decrease over time from op-
erational flows of several hundred to several thousand gal-
lons/minute to residual flows that decrease to 0-50 gal-
lons/minute. During this time, carbon dioxide dissolves
in the water and reduces the pH to between 8 and 9. This
process also allows volatilization/oxidation of cyanide and
also enhances the activity of microorganisms that can con-
vert the nitrogen in a variety of cyanide species to nitrate.
Because mercury is mobilized as a mercury-cyanide com-
plex, removal of the cyanide is also reasonably effective in
reducing mercury concentrations in the drainage water.
The amount of water that drains from heaps will vary
depending on the site conditions, but it will also depend
on the amount of meteoric water, the type of cap (if any)
that is placed on the top of the heap, and other site-spe-
cific conditions that may be present. However, low-flow
drainage from heaps has been observed at most sites and
will continue for the foreseeable future. Discharge from
20
Engineering Issue
wagement and Treatment of Water from Hard Rock Mines
-------
heaps can be reliably eliminated only for sites in a highly
arid region or those that have a very efficient store-and-
release cap.
In higher-rainfall regions, where rainfall on the heap ex-
ceeds the amount of water that evaporates, treatment and
discharge of excess water is required. Although many of
the constituents in these fluids are the same as during clo-
sure, cyanide removal becomes more important and re-
quires specialized treatment.
The constituents present in residual cyanidization flu-
ids differ substantially from acid drainage sites. Drain-
age from three distinct closed heaps is described in Table
4-4. These waters contain elements that have enhanced
solubility at higher pH, as well as residual cyanide com-
ponents. The constituents that are of particular concern
include arsenic, antimony, selenium, nitrate, sodium, sul-
fate, cyanide species [both weak acid-dissociable (WAD)
cyanide as well as total cyanide], mercury, and nickel.
Effective treatment of heap effluent requires consideration
of all of the constituents present in the drainage water (Ta-
ble 4-4). While specific treatment methods are available
for several of these constituents, or even groups of constit-
uents, relatively few methods are available that can remove
all of these to surface water discharge requirements.
Reduction in the volume of water by recirculation and
evaporation on the heaps is generally utilized. However,
the collection pond water volume is usually large, and
treatment is often required for the several millions of gal-
lons typically left after recirculation of the water to the
heaps is discontinued. Since the volume is contained in
a pond, this water can often be treated in a single batch
mode and can utilize intensive techniques (e.g., membrane
separation, ion exchange, or aggressive evaporation).
Because the water quality from these heaps is unlikely
to change substantially for years to decades due to the
slow migration of meteoric water through the heaps, any
treatment process will need to be either continuous or al-
low accumulation of water for periodic batch treatments.
Thus, the more intensive management techniques be-
come very costly on a per-gallon-treated basis, and pas-
sive methods for water management (1-20 gal/min) are
favored. However, few options are available, particularly
for saline waters.
Current methods for residual heap drainage water treat-
ment include the following:
Table 4-4. Heap Drainage Chemistry Profiles of Three Closed
Heaps (Source: NDEP, 2004)
pH
IDS
nitrate
sodium
chloride
WADCN
sulfate
antimony
arsenic
copper
manganese
mercury
nickel
selenium
molybdenum
vanadium
Heapl
Effluent
6/23/98
7.79
3,032
54
340
160
3
1,600
0.023
0.08
0.007
0.051
0.022
0.034
0.18
0.31
< 0.002
Heap 2
Effluent
4Q95
8.17
11,200
171
3,880
1,130
0.11
6,130
0.543
0.028
0.035
0.004
5.84
-
HeapS
Effluent
5/02
9.6
5,670
96
1,640
3,200
14.3
470
< 0.003
0.209
0.515
0.01
0.102
0.535
0.109
0.917
0.642
All units are mg/L, except pH
Land application and French drains: For these meth-
ods, water is simply land-applied via irrigation systems or
passively drained through perforated pipe. In both cases,
the contaminants in the drainage water are released ei-
ther to the land surface or allowed to move downward
in the subsurface. Although this method is very inexpen-
sive, this form of water management carries risks from
whatever contaminants exist in the water. For example,
the land application at Beal Mountain mine resulted in
a near-complete removal of all of the vegetation due to
elevated concentrations of thiocyanate, a soil sterilant.
Elevated selenium and sodium have resulted in potential
plant uptake problems and changes in the soil structure
for land applications from heap effluent from the Zort-
man-Landusky mine in Montana. However, because of
the very low expense of pond volume reduction, land ap-
plication is sometimes used. However, it can in some cases
create serious problems.
Discharge of water to French drains: This method of
disposal of contaminated water has been permitted in
Nevada for sites for which ground or surface water con-
tamination is unlikely. While the risk factors in certain
situations in extremely arid areas are low, the release of
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
21
-------
Treatment of Mine Water with Microbial
Processes
highly contaminated water (some of which meets hazard- of Reclamation literature on desalinization: http://www.
ous waste criteria) into the subsurface has been criticized, usbr.gov/pmts/water/reports.html.
and it is unlikely to be permitted for new applications.
Evaporation: Particularly for those sites that have high 4.2.3
salinity, evaporative methods are one of the few options
available for long-term treatment of residual heap drain-
age water. A recent analysis of alternatives of water man-
agement by consultants for a Nevada mine (see Heap 3
Effluent quality in Table 4-4) (Telesto Solutions, 2003)
indicated that the most cost-effective method was the use
of evaporative ponds. In addition to the sodium load that
resulted from the addition of sodium cyanide, the source
water was a geothermal water high in dissolved salts. As a
result, the number of treatment options was few. Bioreac-
tors would not be effective for treatment of the high salin-
ity, freshwater rinsing would require very large volumes of
water (unavailable), and land application had similar is-
sues with salts. The option of geothermal aquifer injection
was seriously considered, but was found to be much more
expensive than passive evaporation using surface ponds.
Evaporative processes are not completely passive, howev-
er, and require regular monitoring to ensure the integrity
of the pond liner and the piping system to deliver the
water. Most heaps will have a soil cap to limit infiltration
of meteoric water, and monitoring of this cap will be re-
quired to ensure that it retains the design characteristics.
In addition, the salt loading in the heaps can be substan-
tial, particularly when the source water has high salt load-
ing (e.g., from a geothermal aquifer), and it will need to
be managed on a year-to-decade time frame.
Biological treatment: Biological processes can also be
used for heap treatment, particularly when salt concen-
trations are not excessive. SRBs (discussed below) can be
successfully employed for sulfate and nitrate removal, as
well as for treatment of selenium and arsenic.
Membrane processes: Reverse osmosis and nanofiltra-
tion are examples of processes that can also be utilized for
treatment of heap effluent, although the costs for long-
term treatment of low flows reduce the applicability of
these methods that require intensive management and
monitoring. Although one option is to accumulate a larg-
er volume of water and follow this by periodic treatment
using various membrane processes, this technique has not
been utilized extensively. The most extensive literature on
applicable membrane processes is in the large-scale desali-
nization technology. See, for example, the U.S. Bureau
A variety of microorganisms can facilitate the removal
of metals, metalloids, and sulfate from mining-impact-
ed waters in both natural and engineered systems. The
primary removal mechanism is the formation of oxide,
hydroxide, sulfides, or carbonate precipitates. Successful
removal of metals and metalloids from mining-impacted
waters depends on providing appropriate environmental
conditions to promote the desired microbial activity in
conjunction with the appropriate chemistry.
Aerobic environments will promote the oxidation of re-
duced metals, particularly manganese and iron. After oxi-
dation, manganese and iron will precipitate in neutral (or
near-neutral) waters and potentially remove other con-
taminants (e.g., arsenic) by co-precipitation.
Anaerobic environments will promote the reduction of
sulfate, nitrate, oxidized metals, and metalloids (e.g., sele-
nium, arsenic, and antimony). A byproduct of a number of
anaerobic microorganisms is bicarbonate, which increases
the pH and promotes precipitation of metal hydroxides.
The production of bicarbonate also promotes the forma-
tion of metal carbonate precipitates (e.g., Zn, Mn, and
Pb). Biogenic sulfide (produced from sulfate reduction)
will promote the precipitation of metal sulfides (e.g., Cu,
Cd, Zn, Pb, Ni, and Fe) under a wide range of chemical
conditions. Chromium (VI) and uranium (VI) can be re-
duced by a number of microorganisms (fermenters, sulfate
reducers, and iron reducers) under anaerobic conditions to
Cr(III) and U(IV), respectively. Subsequently, Cr(OH)3(s)
and UO2(s) are precipitated from solution. Selenate
(Se(VI)) can be reduced to selenite (Se(IV)), which is sub-
sequently reduced to elemental selenium. Under sulfate-
reducing conditions, As(V), Mo(VI), and Sb(V) can be
reduced and subsequently precipitated as a sulfide mineral
(As2S3, MoS2, Sb2S3). Some metals will also be removed by
co-precipitation with aluminum or iron hydroxides.
The design of microbial treatment schemes needs to
consider:
1. Identification of target compound(s) and desired
effluent limits,
2. Conditions for desired microbial activity,
22
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
3. Conditions for desired chemistry, and
4. Mass transfer and kinetic constraints.
Additional issues that will affect the selection of any treat-
ment process are solids management, operation and main-
tenance requirements, and cost.
As with other treatment technologies for mining-impact-
ed waters, identification of target contaminants and the
associated discharge requirements are necessary for selec-
tion of a microbial system. While microbial systems can
treat a number of types of contaminants effectively, the
microbial treatment options are usually constrained by
the contaminant load in the water, as well as the require-
ments for treatment. When flows are high, and conse-
quently residence time is reduced, insufficient sulfide is
generated to precipitate the metals.
Microorganisms need an electron donor and acceptor cou-
ple for energy generation, a carbon source and nutrients for
cell synthesis, and appropriate environmental conditions.
Most microbial-based treatment systems require organic
material for the electron donor, which then also serves as
the carbon source. The organic material can be supplied
in a water-soluble form (e.g., molasses or ethanol) or in a
solid form (e.g., wood chips or leaf compost). Water-sol-
uble organics have been used for active bioreactor systems
and ground water treatment systems. Solid-phase organics
have been used in active and passive bioreactor systems,
permeable reactive walls, and wetland systems.
Potential electron acceptors used for energy acquisition
include oxygen, nitrate, sulfate, and carbon dioxide. Dis-
solved oxygen is typically insufficient for desired microbi-
al reactions and must be added either actively or passive-
ly. Sulfate is present at adequate concentrations in many
mining-impacted waters, particularly those where pyrite
oxidation has occurred. Carbon dioxide is sufficient for
fermentative reactions involving solid-phase organic mat-
ter hydrolysis and production of organic acids and alco-
hols, which are then available for sulfate reducers.
Nutrient addition (particularly nitrogen) is typically re-
quired when water-soluble organic materials are used as
the carbon source. Solid-phase organic substrates used are
typically a combination of a number of materials (e.g.,
manure, compost, or wood) and can be selected to in-
clude organic material containing sufficient nitrogen.
Mining-impacted waters exist with a range of tempera-
tures, pH, and redox conditions, and microorganisms are
sensitive to all of them. Microbial activity tends to decrease
with temperature, although the overall rates of reaction
(e.g., sulfate reduction) can be kept constant if the num-
ber of active bacteria increases proportionally. Most of the
desired microbial processes have optimal rates at neutral
pH. However, many microorganisms can adapt to lower
and higher pH values (5-9) or may be protected from
bulk solution-phase pH in microenvironments. Redox
conditions are important relative to the electron acceptor
used by an active consortium of bacteria. In general, the
highest energy couple is used first, followed by those of
decreasing energy. However, the presence of a microenvi-
ronment and microorganisms with different metabolisms
allows concurrent usage of multiple electron acceptors.
Microorganisms alter the chemical environment to pro-
mote conditions conducive to desired precipitation or
co-precipitation reactions. The changes in chemical envi-
ronment can include pH, redox, and reactant formation.
The theoretical predictions by chemical equilibrium pro-
grams, such as PHREEQC (see http://wwwbrr.cr.usgs.
gov/projects/GWC coupled/phreeqc), provide a useful
estimate of the potential of precipitates to form, but the
added interactions of the microorganisms can alter the
expected distribution of precipitates formed.
The rate of precipitation tends to be controlled by the rate
of the microbial function of interest (e.g., sulfate reduc-
tion). One way the rate of sulfate reduction is controlled
is by the rate-limiting step of the microbial community
providing growth substrates for sulfate reduction. Models
developed to describe the rate of precipitation range from
empirical constant-rate models to models that couple
microbial kinetics with a selected reactor configuration.
Mass transfer is also important in describing the overall
rate of precipitation for biotreatment systems. Mass trans-
fer limitations are particularly important for biofilm sys-
tems (any system with solid-phase growth media) and will
be a function of linear velocity, media size, and biofilm
thickness. In biofilm systems, mass transfer can control
the observed rate of reaction.
4.2.3.1 Sulfate-Reducing Systems
Sulfate-reducing systems promote the microbial-facilitat-
ed reduction of sulfate, production of sulfide, generation
of alkalinity, and reduction of redox active metals, metal-
loids, or radionuclides. A carbon source, such as lactate or
ethanol, is required to promote the growth of sulfate re-
ducers in these systems. Solid-phase organic material can
also be used to indirectly provide a carbon source for sul-
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
23
-------
fate reducers from the actions of cellulolytic and ferment-
ing bacteria. Wide ranges of microbial species are able to
catalyze sulfate reduction. (See INAP [2003] for a sum-
mary of water treatment methods designed specifically for
removal of sulfate.)
The range of reactions promoted in a sulfate-reducing sys-
tem depends in part on the type of carbon source selected.
The use of more complex organic compounds results in
a greater diversity of microbial population in addition to
sulfate reducers. The resulting number of reactions that
control sulfate reduction also increases in complexity. The
primary reactions of interest are shown in Figure 4-5 at
the bottom of this page.
Sulfate-reducing systems may be implemented in ac-
tive or relatively passive treatment configurations. Rela-
tively passive configurations include anaerobic wetlands,
compost-based bioreactors, and PRBs. Relatively passive
systems with soluble carbon input include permeable re-
active zones (PRZs) and rock-filled bioreactor ponds. Ac-
tive systems include a number of patented configurations
that may include partial sulfate removal as gypsum and
recovery of excess sulfide as elemental sulfur. Suspended
reactor systems require the highest level of operation and
maintenance. A method of removing metal precipitates
and excess biomass must be included as part of the over-
all system. Sulfide precipitation is very effective in reduc-
ing a number of metals to low levels. Reduced metalloids
also may be effectively removed. The overall effectiveness
is dependent on the capture of precipitated metals and
metalloids and the stability of the microbial community
as a whole. Note that high-flow events, if not bypassed,
may damage the microbial community and disperse
precipitates downstream, where the precipitates can be
dissolved.
4.2.3.7.7 Anaerobic Wetlands
An anaerobic wetland is a subsurface water body that sup-
ports the growth of emergent plants, such as cattails and
reeds. The vegetation and sediment provide surfaces for
the growth of attached bacteria. Anaerobic removal pro-
cesses control the treatment of metals and the neutraliza-
tion of acid. The contaminated water is intercepted and
diverted through the wetland system (see Figure 4-6). A
minor aerobic component of this system is the surface
vegetation, which allows the release of carbon dioxide and
hydrogen sulfide, and oxidation of iron on the surface.
Anaerobic wetlands utilize sulfate-reducing bacteria to
immobilize metal cations (Fe, Cu, Cd, Pb, Zn), oxyani-
ons (Cr, Se, Sb, Mo, As), and U. In addition, the produc-
tion of alkalinity allows for the neutralization of excess
acid present in target mine waters.
Large areas with a relatively flat topography are required
for wetland treatment systems. The area required is a
function of the mass loading of each target contaminant.
The removal of metals as metal sulfides is typically based
on the expected rate of sulfate reduction (sulfide produc-
tion). The rate of removal for metalloids and uranium is
not as well established and may require bench- and pilot-
scale testing.
Optional inlet
manifold warm
climates
Water
surface
Vegetation
Outlet zone
2" to 3"
gravel
t \
Inlet zone Inlet manifold
2" to 3" cold climates
gravel
\
Membrane liner
or impermeable
soils
Treatment Outlet
zone 1/2" to manifold
11/2" gravel
Figure 4-6. Schematic of anaerobic wetland with subsurface flow.
Bioreaction:sulfate + organic carbon => sulfide + alkalinity (bicarbonate)
Chemicalreaction:sulfide + metal => metal-sulfide and carbonate + metal => metal-carbonate
HS- + Me2+^MeS + H+ and/or
Bioreaction: oxidized metalloid + organic carbon => reduced metalloid (e.g., Selenate -» Elemental selenium)
Chemical reaction: reduced metalloid precipitate formation
Figure 4-5. Examples of the multiple reactions that can occur under sulfate-reducing condition.
24
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
The construction cost for anaerobic wetlands might be
lower compared to active treatment. In addition, the op-
eration and maintenance effort and cost are proportion-
ally lower. Costs cannot be generalized on a per-mass ba-
sis for target contaminants because of the effect of other
important factors such as flow, temperature, and pH.
The precipitation of metals modifies the pore structure
within the wetland subsurface and may reduce the effec-
tive hydraulic retention time; thus, the design hydraulic
residence time should include a suitable safety factor. Low
temperature will reduce the bacterial activity and hence
the rates of sulfate, metalloid, and uranium reduction. A
larger wetland for colder climates will be required relative
to more temperate areas. Highly variable flow may result
in the flushing of collected precipitates; thus, subsequent
polishing ponds are required. Collection of precipitates
and sediments, and loss of permeability, will lead to the
periodic need to rebuild the entire system.
4.2.3.7.2 Bioreactors
SRBs can be designed in a number of configurations (see
Figure 4-7). Configuration A simply treats the influent
acidic water and allows the precipitated sludge to settle in
the bioreactor, which will ultimately need to be removed,
probably by flushing, and appropriately managed. Config-
uration B allows a more convenient settling of the sludge
in a settling basin, which is more easily removed and man-
aged from the bioreactor system. In this configuration a
portion of the effluent from the settling basin is recycled
to the front of the bioreactor where SRBs reduce sulfate to
sulfide. The sulfide-containing water is then mixed with
the influent acidic mine drainage. The metals precipitate
(mostly as sulfides) in the settling basin, and the pH is
raised. The flow rate of discharge is the same as the flow
rate of the influent. The pumps used to recycle the water
require a power source, although the ease of sludge man-
agement will usually outweigh the power costs.
Solid-phase organic material can also be used to provide
a carbon source and a surface for bacterial attachment.
SRBs have been used to immobilize metal cations (Cu2+,
Cd2+, Fe2+, Pb2+, Zn 2+) as metal sulfides, oxyanions (Cr,
Se, Sb, Mo, As), and U. They also are effective in reduc-
ing sulfate concentrations. The effectiveness for removal
via sulfide precipitation is dependent on both the pH and
sulfide concentration. For typical sulfide concentrations
(10~3 — 10~4 M), the effective pH for removal of iron
sulfide is above pH 6.5.
The use of solid-phase substrate in a packed bed system is
affected by the precipitation of metals, which may reduce
the hydraulic retention time and exclude flow through
portions of the reactive zone. Thus, the initial sizing must
take this and the replacement frequency into consider-
ation. Biofilm systems constructed of rock or plastic me-
dia may allow the release of precipitated metals, and thus
the effluent from these reactors should be polished via
gravity settling or filtration. These flushable bioreactor
systems can allow continuous use, providing the precipi-
tated metal sulfides, calcite, and biomass are flushed into
a collection basin at a frequency that eliminates hydraulic
plugging in the bioreactor.
Influent
. Effluent
Bioreactor I >•
Sludge
Recycle
j Sludge
T
Figure 4-7. SRB configurations. A is a simple flow-through system
that requires periodic removal of solids from main reactor. B is a
modification that includes a separate tank or pond for precipita-
tion and sludge collection.
The optimum pH for SRB systems is 7-7.5, and the ef-
fectiveness of SRB systems can be substantially reduced
when the influent acidity is high. Experience at the Levia-
than bioreactor, which utilizes ethanol as a carbon source
to treat an influent flow of 40 L/min, has shown that the
system is most effective when the influent pH is adjusted
to above pH 4.5 or higher (Tsukamoto et al., 2004). This
has been accomplished by the addition of a 25% sodium
hydroxide solution, which can be added using a solar-
driven pump. While ethanol can be added by a simple
gravity flow, the more viscous sodium hydroxide requires
a positive pumping system.
Alternatively, the bioreactor can be operated as a sulfide-
generating system (Figure 4-7) in which a portion of the
bioreactor effluent is recycled back to the front of the biore-
actor. Ethanol is added to the bioreactor, and sufficient sul-
fate remains in the water to allow the SRB system to gener-
ate sulfide and add alkalinity. The acidic drainage is then
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
25
-------
mixed with the effluent from the bioreactor in a settling
pond, and the metal sulfide precipitates and is effective-
ly captured in this pond. This configuration allows better
management of the sludge and maximizes the SRB activity
by keeping the pH close to optimal. However, this con-
figuration also requires pumping approximately 30 - 40
gal/min from the settling pond to the front of the bioreac-
tor and requires an energy source of approximately 0.5 hp.
SRBs offer the advantage of a lower sludge management
requirement since the sulfides are precipitated as metal
sulfides or as sulfur. Bioreactors can also be managed
more effectively at remote locations, with visitations of
1-2 times per month, rather than daily management, as is
usually the case with conventional lime/treatment facili-
ties. Site-specific criteria will determine which treatment
option provides the most cost-effective approach.
The cost of SRBs varies widely from site to site and is a
function of both the system type and the size required to
treat the site-specific concentrations and types of contam-
inants. Simple, flushable lined systems to treat up to 50
L/min can be constructed for under $200,000. The cost
of the carbon source (e.g., ethanol at $US2/gal) is gener-
ally a relatively small component (< 20%) of the cost of
operating a bioreactor. The cost of adding base (gener-
ally sodium hydroxide or sodium carbonate) will vary, de-
pending on the acidity. If the acidity (or flow) of the water
is sufficient that the cost of raising the pH dominates the
cost of treatment, lime treatment will, at some point, be-
come a more cost-effective and reliable option.
4.2.3.1.3 Alkalinity-Producing Systems
Alkalinity-producing systems (APSs) are an integration of
ALD systems with anaerobic sulfate-reducing biosystems.
Two configurations of APSs have been developed: the suc-
cessive APS (SAPS) and the reducing APS (RAPS). The
SAPS consists of an ALD overlaid with organic material
(e.g., hay and manure); the RAPS consists of an ALD in-
tegrated with organic material. Under certain conditions,
these systems can help increase the pH of influent water
sufficiently to allow SRB systems to better thrive, as dis-
cussed previously.
APSs target acidity and metals that precipitate as hy-
droxides or carbonates at slightly alkaline pHs. Relative
to active treatment, APSs are inexpensive and have low
operation and maintenance costs. However, while they
have shown success in certain drainages from coal sites,
the applicability in hard-rock mine sites is complicated by
surface precipitation of aluminum and iron oxide coat-
ings on the surface that limits the availability of the cal-
cium carbonate for neutralization. Many hard-rock acidic
drainage sites have high aluminum (> 30 mg/L) concen-
trations, and even when iron oxidation is inhibited by
having anoxic conditions, aluminum coating alone will
reduce the effectiveness since precipitation only requires
a slight increase in pH to near pH 5 to result in armoring
of the limestone.
4.2.3.2 Permeable Reactive Barriers
A permeable reactive barrier (PRB) is a zone of reactive
media emplaced in the flow path of contaminated ground
water (see Figure 4-8). The reactive media promotes the
removal of metals and radionuclides by precipitation,
sorption, or ion exchange. The contaminants are retained
in the barrier and eventually are removed by excavation. A
related subsurface technology is a permeable reactive zone
(PRZ) that is created by the injection of a reactive solution
in a series of wells that transect a ground water plume.
Water table
Permeable reactive barrier
Figure 4-8. Schematic of PRB system. (Source: EPA/600/R-98/125)
The reactions promoted in a PRB depend on the reac-
tive media selected and the target contaminant. The main
types of reactive media used include organic material (to
promote biogenic sulfide production) and zero-valent
iron. However, media that promote sorption or ion ex-
change can also be found. Reactive media types may be
mixed to promote the removal of multiple contaminants
by different reaction mechanisms.
• Sulfate-reducing biozone reactions
• Zero-valent iron:
oxidation/reduction: Fe(0) + Cr(VI) = Fe(lll) + Cr(lll)
and precipitation: Cr(lll) + 30H = Cr(OH)3(s)
26
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
• Sorption:
Me2+ + surface site
• Ion exchange: Me2+
» Me2+ -surface complex
R-Ca = Ca2+ + Me-R
Sulfate-reducing biozones have been used to immobilize
heavy metal cations (Cu, Cd, Pb, Zn), oxyanions (Cr, Se,
Sb, Mo, As), and U. Zero-valent iron barriers can reduce
and immobilize redox active compounds that include Cr,
U, Mo, Sb, Se, and As. Media promoting sorption or ion
exchange can be selected to target cations (Cu2+, Cd2+,
Pb2+, Zn2+, UO22+) or anions (Cr, As, Mo, Se, Sb).
PRZs have been achieved by the injection of reactive com-
pounds into the subsurface area to be treated. Organic
compounds, such as acetate, can promote biogenic sulfide
production or biological metal reduction. Inorganic com-
pounds, such as sodium dithionite, can form Fe2+ from
Fe3 + on aquifer material. The Fe2 + can then participate
in the reduction of Cr(VI) to Cr(III).
The most important implementation issue for PRBs and
PRZs is the ability to capture the contaminated ground
water flow within the reactive zone. The second issue is
the ability to promote the desired chemical reactions, giv-
en the chemical composition of the target water. A third
issue is the availability of cost-effective reactive media and
the frequency of media replacement.
At a minimum, column experiments are required to de-
termine the effectiveness of a specific reactive media con-
figuration and a specific ground water. The microbial and
chemical complexity of processes in the PRB and PRZ
precludes the use of a cookbook design protocol. The hy-
drologic and geologic properties of the subsurface also
must be adequately characterized to assess potential ef-
fectiveness. Uniform mixing and emplacement of the re-
active media is another critical factor, as is the ability to
maintain an acceptable hydraulic conductivity through-
out the reactive zone.
The cost for PRBs and PRZs varies widely from site to
site. The cost is a function of both the media type and the
barrier or zone size required to treat the site-specific con-
centrations and types of contaminants. Organic materials
are the least expensive, zero-valent iron is more expen-
sive, and the most expensive are specially designed sorp-
tive or ion exchange materials. The frequency and cost of
replacement will also vary with media type and the level
of contamination. Media has been demonstrated to have
a life of about 7 years, but theoretically its life could be 10
years to several decades.
The precipitation of metals modifies the pore structure
within the reactive zone and may reduce the hydraulic re-
tention time and exclude flow through portions of the re-
active zone. Solutes may be released from the dissolution
of solid-phase materials, ion exchange, or desorption.
Key Web Site References
• EPA remediation technologies development forum:
http://www.rtdf.org/public/permbarr/default.htm
• EPA hazardous waste cleanup information:
http://clu-in.org/techfocus/default.focus/sec/
Permeable Reactive Barriers/cat/Overview/
• University of Waterloo, Department of Earth Sci-
ences, Groundwater Geochemistry and Remediation,
Permeable Reactive Barriers: http://www.science.
uwaterloo.ca/research/ggr/PermeableReactiveBarri-
ers/PermeableReactiveBarriers.html
• An example of the use of a PRB for treatment of ura-
nium in ground water (Chapter 16) and acid reme-
diation in ground water (Chapter 17): http://www.
image-train.net/products/proceedings first/
4.2.3.3 Other Bioremediation Systems
A number of non-sulfate-reducing biosystems have been
used for removal of contaminants from mine water. Aer-
obic wetlands have been commonly used for coal mine
drainage and, to a lesser extent, for hard-rock mine drain-
age. New systems are constantly being developed. The
microbial oxidation of elemental sulfur to sulfide for met-
al sulfide precipitation is a recently developed proprietary
process, as is a patented process for the microbial oxida-
tion of manganese. New proprietary systems require inde-
pendent verification of effectiveness before selection.
4.2.3.3.1 Aerobic Wetlands
An aerobic wetland is a shallow water body (less than 2 ft.
deep) with a free water surface that supports the growth
of emergent plants, such as cattails and reeds (see Figure
4-9). The vegetation and the sediment provide surfaces
for the growth of attached bacteria. Aerobic removal pro-
cesses control the treatment of metals. The contaminated
water is intercepted and diverted through the wetland
system.
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
27
-------
Water table -
Liner'
Soil'
Figure 4-9. Schematic of a free water surface aerobic wetland.
The reactions promoted in an aerobic wetland are primar-
ily the oxidation of iron and manganese. The rate of abi-
otic oxidation is increased by the presence of bacteria.
Bio-oxidation: Fe(ll) + oxygen => Fe(lll) + water
and chemical precipitation Fe(lll) + 30H~ = Fe(OH)3(s)
Bio-oxidation: Mn(ll) + oxygen => Mn(IV) + water
and chemical precipitation Mn(IV) + 02 = Mn02(s)
Aerobic wetlands for iron and manganese removal are
most amenable to near-neutral and net-alkaline waters.
Large areas with a relatively flat topography are required
for wetland treatment systems. The area required is a func-
tion of the mass loading of both iron and manganese. The
removal of manganese requires a larger area per unit- mass
of manganese removed than for iron.
The construction cost for aerobic wetlands is relatively
low compared to active treatment. In addition, the opera-
tion and maintenance effort and cost are proportionally
lower. Costs cannot be generalized on a per-mass basis for
iron or manganese because of the effect of other impor-
tant factors such as flow, temperature, and pH.
Low temperature will reduce bacterial activity and hence
the rates of iron and manganese oxidation. Ice covers will
also limit the rate of oxygen transfer to the wetland. High-
ly variable flow may result in the resuspension of settled
iron and manganese precipitates. Most of the successful
application of aerobic wetlands has been for coal mine
drainages, not metals mine drainages.
Key Web Site References
• A general discussion of passive mine water treat-
ments: Jittr2^/wwwbim.go^/mt£/library/p_dfy
TN409.PDF.
• The science of acid mine drainage and passive treat-
ment: http://www.dep.state.pa.us/dep/deputate/
minres/bamr/amd/science of amd.htm.
Table 4-5 on the following pages provides an overview
of water treatment technologies covered in this section,
technology selection factors, and limitations.
4.3 Mine Pit Lake Management
Lakes are typically "windows to the ground water"—
where the land surface drops below the water table, we
"see" the water table as the surface of the lake. Mine pit
lakes are special cases of this phenomenon, forming in
open pit mines that are excavated to below the water ta-
ble. In practice, excavation below a water table requires
dewatering to lower the water table, leaving the open pit
(or "void") within the ground water cone of depression.
With cessation of dewatering, ground water flows to the
center of the cone of depression, forming a lake. Steady-
state mine pit lakes can have (1) throughflow to ground
water (in some cases, evaporation produces concentration
of ground water), (2) outflow to surface water and ground
water, or (3) zero outflow (a "terminal" lake, where all in-
flows are balanced by evaporation).
The ultimate quality of the pit lake is strongly affected
by the surrounding wall rock. Wall rock affects pit lake
water quality primarily by leaching solutes released by the
oxidation of sulfide minerals exposed in the pit. Further,
dewatering of sulfide zones can pull air into surrounding
aquifers, potentially inducing regional oxidation in aqui-
fers and increasing, temporarily at least, solutes in ground
water. The depth of rapid oxidation may be limited to
a few meters into the face. Rock that is net neutralizing
will produce a pit lake that is relatively benign since the
problematic divalent metals concentrations will be low.
Arsenic, antimony, and selenium, which are mobilized at
elevated pH, can be a concern under conditions where the
pH is alkaline and insufficient iron is present to cause co-
precipitation of these constituents.
Pit mines are by definition in areas of elevated metals, and
groundwaters often contain elevated trace metals or sul-
fate. When ground water is a dominant source of inflow,
the effect of ground water quality on pit lake quality tends
to increase with increasing lake size. Ground water quality
can change over time, particularly given the long times re-
quired to fill lakes. Where evaporation is large and ground
water outflow small, lakes will concentrate solutes and can
eventually becomes sources of ground water exceeding
water quality standards for TDS or other solutes.
(Continued on page 32)
28
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effluent
Technology
Name
Conventional
Lime
Treatment
Limestone
Ponds and
Open
Limestone
Channels
Anoxic
Limestone
Drains
Technology
Description
Lime or hydrated lime is mixed as
10-15% slurry and added to acidic
water to raise the pH of the water
and precipitate metals as metal
oxides and sulfate as gypsum
Establish open ponds or channels
that can receive acidic water. The
limestone neutralizes the acids and
allows precipitation of a variety of
metals as metal oxides.
Intercept acidic water that pri-
marily has ferrous iron and pass
this water through limestone beds
under anoxic conditions. This limits
the amount of oxidation of the iron
and limits the amount of precipita-
tion on the limestone.
Target
Analytes
Acidity.
Most diva-
lent metals.
AI,As,Sb,
sulfate (to
2,000 mg/L).
Acidity, Al,
Fe, Mn. Par-
tial metal
removal.
Acidity, Al.
Some metal
reduction is
observed.
Critical Feasibility Factors
Implementabilty
• Requires engi-
neered system to
efficiently utilize
lime, including pow-
er, pumps, tanks,
mixers, and lime ad-
dition systems
• Acidic water is
(generally) pas-
sively added to the
limestone pond or
channel and al-
lowed to react
• Turbulent systems
improve release of
alkalinity
• Care must be taken
to maintain anoxic
conditions
• Generally need
sloping topography
and passive trans-
port of water
Effectiveness
• Generally con-
sidered the most
proven method for
acid drainage
treatment
• Depends on types
of metal loading
• Can treat the most
concentrated acidic
drainages
• Variable, depending
on the aluminum,
iron, and acidity
• Armoring is a
problem
• Usually low
maintenance
• Shown to be use-
ful for coal acidic
drainage, but less
so for hard-rock
mine drainage and
heavy metals
• Decreased overall
rate of reaction
• Longer residence
times provide better
neutralization and
decrease in target
analytes
Cost
The most cost-ef-
fective method for
treating large flows
or highly contaminat-
ed water. Less cost
effective for small
streams due to fixed
costs.
• Relatively inex-
pensive and low
maintenance
• Depends on the
availability of lime-
stone and con-
struction costs
Relatively inexpen-
sive. Some main-
tenance cost is
required if a biologi-
cal system is used
to maintain anoxic
conditions.
Important
Limitations
• Requires frequent
monitoring and sludge
management
• Arsenic treatment effec-
tive only with a high iron-
to-arsenic ratio
• High aluminum and iron
waters will armor the
limestone and reduce
effectiveness
• Precipitated sludge may
require management,
depending on location and
regulations
• May not treat certain di-
valent metals well (Cd, Cu,
and Zn)
• High aluminum-containing
waters will armor lime-
stone and decrease the
rate of alkalinity addition
• It is difficult to remove all
of the oxygen, so some
iron is oxidized and tends
to armor the limestone
• Unless sized appropri-
ately, these systems will
not respond well to large
fluctuations in volume or
influent water quality
i
(b
I
s/r
1
-------
CO
Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effluent (continued)
S1
i-
Technology
Name
Anaerobic
Wetlands
Sulfate-
Reducing
Bioreactors
Alcohol
Amended
Sulfate-
Reducing
Bioreactor
Technology
Description
Intercept surface water flow and
distribute through one or more sub-
surface water wetlands
Collect flow with pumps or natural
hydraulic gradient and distribute
through a vessel containing growth
substrate (manure, wood chips,
other organic waste) and sulfate-
reducing bacteria. SRBs reduce
sulfate, raise the pH, and precipi-
tate metals.
Alcohols (e.g., ethanol) and base
added to lined impoundments con-
taining rocks, wood chips, or other
physical support. Bacteria use the
alcohols as reducing sources for
sulfate. The system is designed to
manage sludge efficiently.
Target
Analytes
Fe,Zn, Cu,
Cd, Pb,As,
Cr, Mo,
Sb,Se,U,
sulfate, low
levels of
acidity
Fe,Zn, Cu,
Cd, Pb,As,
Cr, Mo,
Sb,Se,U,
sulfate, low
levels of
acidity
See above
Critical Feasibility Factors
Implementabilty
• Steepness of slope
• Sufficient land area
• Availability of in-
expensive organic
substrates
• Power availability
for active systems
• Accessibilityfor
system
maintenance
• Sufficient land area
for passive systems
See above
Effectiveness
• Sensitive to low
temperatures
• pH > 5 and moder-
ate metal loading
• pH>5
• Moderate metal
loading
• Method of retaining
metal precipitates
• Longevity is de-
pendent on carbon
source and the
ability of SRB to
maintain a pH suffi-
ciently high to sup-
port SRB activity
Alcohol and base
addition can be
controlled and allow
better treatment of
varying flows and
contaminant loads.
Sludge manage-
ment and hydraulic
control are improved
compared to the more
passive SRB systems.
Cost
• Excavation
• Plants and support-
ing soil
• Hydraulic
structures
• Growth substrate
• Bioreactor
• Additional tanks or
ponds for process
modifications
• Higher initial costs
for construction, as
well as the costs of
alcohols, base, and
nutrients
• Allows substantially
improved longev-
ity of the bioreactor
due to lack of plug-
ging and a continu-
ous carbon source
Important
Limitations
• Relatively low flows
• Large land areas and flat
topography
• Periodic sediment remov-
al and wetland reestab-
lishment required
• Oxidation and release
of metals and sulfides is
probable if the wetlands
become dry
• Difficult to control metal
migration
• Best for water above pH
5; effluent metal concen-
tration may exceed dis-
charge limitations when
flows or contaminant
concentrations are high
• Systems with media that
create small pores sizes
(mm) are more prone to
clogging by metal
precipitates
• Longevity is dependent
on carbon available to the
microbial consortium
• Although these systems
are more adaptable to
variations in flow and
contaminants, monitoring
is required to maintain the
bioreactor operation
• Requires a continuous
source of carbon, base,
and planned sludge
management
-------
Table 4-5. Water Treatment Technologies for Hard-Rock Mining Effluent (continued)
Technology
Name
Alkalinity-
Producing
Systems
Permeable
Reactive
Barriers
("Reducing
Reactive
Walls")
Aerobic
Wetlands
Technology
Description
Intercept surface water flow and
distribute through a series of
shallow drains containing both
limestone and reducing organic
material. Metals are precipitated as
metal oxides and metal carbonates.
Intercept contaminated ground wa-
ter plume with a permeable barrier
constructed of reactive material.
Water flows through and contami-
nants are retained.
Intercept contaminated surface
water and flow through one or
more free water surface wetlands.
Iron and manganese oxidation form
species thatare less soluble and
tend to precipitate as Fe(OH)3 and
as Mn02 , respectively. Arsenic can
be removed by co-precipitation
with iron hydroxides.
Target
Analytes
Acidity, Al.
Some metal
reduction is
observed.
See above
Fe, Mn,As
Critical Feasibility Factors
Implementabilty
• Steepness of slope
• Sufficient land area
• Stability of trench
wall during
installation
• Plume width
• Depth to ground
water and bottom
of aquifer
• Steepness of slope
• Sufficient land area
Effectiveness
• Flow
• Acid-loading rate
• Metal-loading rate
• Homogeneous
emplacement of
barrier material or
injection of reactive
solution
• Column studies
required to assess
potential
effectiveness
• pH > 5 and moder-
ate metal loading
• Near-neutral pH re-
quired to maximize
oxidation reactions
• High-flowvariations
may re-suspend
metal precipitates
Cost
• Excavation
• Limestone
• Reducing organic
material
• Hydraulic structure
• Reactive material
• Excavation and
dewatering during
excavation
• Soil and ground
water disposal from
construction
• Thickness along
flow line to achieve
residence time
• Excavation
• Plants and support-
ing soil
• Hydraulic structures
Important
Limitations
• Experience primarily
based on coal mine
• Improves water quality,
but may not meet strin-
gent discharge standards
• Periodic exchange of sub-
strate required, but time
frame not well established
• Uncertainty in PRB life
-affects cost of
technology
• Periodic replenishment of
reactive media expected,
butfrequency not well
established
• Concurrent iron reduc-
tion may mobilize metals
sorbed to iron mineral
surfaces
• Sulfate reduction rates
-50 mg/L-d. Rate affects
cost of technology.
• Periodic sediment and
precipitate removal and
wetland reestablishment
required
• Experience primarily
based on coal mine
drainage
i
(b
I
s/r
1
-------
Numerous studies of mine pit lakes indicate that they
behave in accordance with well-understood processes of
limnology (e.g., Atkins et al., 1997). The fundamental
physical process is mixing, which is a balance between
wind shear acting on the surface, which tends to increase
mixing, and stable density stratification caused by tem-
perature and salinity gradients, which tend to inhibit
mixing. As a result, pit lakes that mix annually (i.e., most
U.S. lakes) can be approximated as stirred reactors, where
ground water inflow is mixed into the lake each year, and
in most cases, the water is oxygenated at least part of the
year. A chemical mass balance on solutes needs to incor-
porate loads from inflow and outflow of ground water,
precipitation, surface flows, and loss to precipitation and
adsorption. This analytical solution is typically used in
predictive lake models.
Biological productivity in lakes is superimposed on the
physical stratification. In natural lakes, this is primarily
the use of light energy by phytoplankton to convert car-
bon dioxide into cell mass and oxygen. Productivity in
natural lakes is typically limited by nutrients, particularly
phosphate. Highly productive lakes can also become an-
oxic at depth as dissolved oxygen is consumed in reac-
tions with organic detritus from the productive surface.
In mine pit lakes, where sulfate concentrations are elevat-
ed, the presence of anoxic conditions induced by elevated
organic carbon will generally result in reduction of sulfate
to produce alkalinity and hydrogen sulfide.
Finally, several studies of existing mine pit lakes demon-
strate that they generally respond as predicted by estab-
lished limnologic studies. Detailed measurements of sea-
sonal profiles in mid-latitude pit lakes show that even in
steep-sided lakes with high walls, the lakes stratify from
surface warming during the summer, then completely
mix in fall and spring. As important, observed physical
stratification and biological productivity in these pit lakes
matched accurately with predictions using a numerical
model (Atkins et al., 1997). Field-scale nutrient addition
has demonstrated that pit lake productivity can be reli-
ably increased through the addition of limiting nutrients
(e.g., Martin et al., 2003).
These fundamental characteristics of lakes manage-
ment—the ability to isolate denser deep layers, to induce
biological productivity and sulfate reduction, and to re-
liably simulate these phenomena with models—lay the
foundation for remedial strategies that treat in-situ met-
als-contaminated pit lakes and even use pit lakes as reac-
tors to treat mine effluent from other facilities.
4.3.1 Backfilling and Neutralization
Backfilling pits completely with waste rock or tailings can
preclude the formation of a pit lake and can also provide
permanent stable disposal of sulfidic waste rock below a
water table. However, backfilling with reactive rock typi-
cally produces a plume of sulfate and other solutes re-
leased by partial oxidation caused by handling, particu-
larly if the pH of the backfilled material is not controlled.
Backfilling is often eliminated based on cost, generally
over $US1/tonne, depending on site conditions, but can
greatly exceed this cost at challenging sites.
Partial pit backfilling is an option that is becoming in-
creasingly common in precious metals pits. Particularly
for large pits, partial pit backfilling can be done as part of
a mine plan and reduces the haulage costs of waste rock
out of the pit. Reactive rock placed appropriately in the
bottom of a pit during mining will then be flooded when
mining is complete and effectively eliminate further oxi-
dation of the rock that is placed below the water table.
Handling of sulfide rock produces some oxidation: reac-
tion of sulfide minerals with the oxygen in the pore space
of backfilled waste rock will produce -500 mg/L sulfate
in the first flush of water, and any additional handling-in-
duced oxidation adds to this baseline. In this case, lime or
some other neutralization agent can be added to maintain
a neutral pH as the acids are rinsed off the rock as the
water table recovers. The effectiveness of this option was
demonstrated by treatment of a large acidic pit lake using
lime at the Sleeper Mine in Nevada.
Treatment of acidic pit lakes can be achieved using di-
rect addition of powdered lime (CaO), hydrated lime
(Ca(OH)2) or limestone (CaCO ), and treatment costs
can be very low (if a local source is available, limestone
crushed to < 2 mm can typically be obtained for $US5 -
10/tonne, yielding lake neutralization costs of a few cents
per cubic meter). However, the local type of limestone
near hard-rock mining is usually not as reactive as other
forms of process neutralization agents, which could in-
crease costs. Neutralization precipitates iron and typically
removes most heavy metals by co-precipitation or adsorp-
tion. However, a neutralized pit lake typically contains
below 3,000 mg/L sulfate.
32
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
4.3.2 Bioremediation and Induced Stratification of
Mine Pit Lakes
Water in hard-rock mine pit lakes can in some cases be
acidic, and regardless of pH, they can contain concen-
trations of sulfate or metals that may be problematic.
Pit lakes vary enormously in size, from a few acre-feet
to over 400,000 acre-feet. Remediation requirements
include monitoring only (where water quality is good),
single or infrequent treatment (e.g., where sulfidic wall
rock is eventually inundated by the lake and oxidation
ceases), or perpetual treatment (e.g., where sulfidic wall
rock remains above the lake, loading solutes in runoff or
by direct sloughing). In-situ treatments include stratifica-
tion, which isolates deep lake water from oxygen at the
surface and potential exposure to terrestrial animals, and
biotreatment technologies, which induce mineral forma-
tion, adsorption, and/or chemical reduction reactions
that remove metals from solution. These technologies,
often combined, offer lower-cost options for closure and
management of mine pit lakes.
In-situ bioremediation induces chemically reducing con-
ditions in lakes that remove target analytes by either
transforming them to another form (e.g., acidity, sulfate)
or inducing them to precipitate as insoluble minerals that
settle out of solution (e.g., heavy metal sulfides). Biore-
mediation is a relatively well-established alternative for
treatment of mine pit lakes (Castro and Moore, 2000)
and has been successfully demonstrated in microcosm
(Frommichen et al., 2004) and full-scale (Poling et al.,
2003) applications. Specific reactions include biologically
induced reduction of sulfate to sulfide in a lake (Castro et
al., 1999), which leads to precipitation of dissolved met-
als as sulfide (CdS, CuS, PbS) and reduction to a less sol-
uble reduced form [U(VI) to U(V), Sb(V) to Sb(III), or
Cr(VI) to Cr(III)]. Chemical reactions involve reduction
of a target analyte by organic carbon. Example reactions
(using CHO to represent organic carbon source) include
reduction of sulfate to sulfide, which can also be used to
neutralize acidity (Frommichen et al., 2004):
2CO
2(aq)
2H20
and reduction of metals to a less soluble form:
Sb(OH)
3(S)
Target analytes are then removed from solution by being
converted to a reduced form that precipitates as oxides
(e.g., UO2), hydroxides [e.g., Sb(OH)3, Cr(OH)3)], or
metallic sulfide (e.g., FeS, CdS, CuS, and ZnS). In addi-
tion, enhanced biological productivity increases biomass,
which can effectively remove metals such as zinc and cad-
mium, which adsorb and settle with detritus (Martin et
al., 2003). Ideally, the long-term fate of precipitated sol-
utes is burial in sediments in a chemically stable form.
Two fundamentally different approaches are used to in-
troduce organic carbon to pit lakes:
• Organic carbon addition: the direct addition of
soluble organic carbon reagents, typically alcohols,
sugars, or organic waste, to the lake (e.g., Castro et.
al., 1999)
• Nutrient addition: typically phosphate and nitrate,
which stimulate the growth of aquatic biota (algae,
phytoplankton, and zooplankton) near the surface
of the lake, producing biologic detritus that induces
reducing conditions at depth as it settles through the
lake (Pederson et al., 2003; Poling et al., 2003)
Both treatments can result in rapid production of bio-
mass, producing organic detritus that can adsorb and set-
tle out dissolved metals.
Where sulfide production is desired, anoxic conditions
must be created and maintained long enough to allow the
biologically induced reactions between organic carbon
and sulfate. This is where stratification is required—an-
oxic conditions generally require that a lake be stratified
(thermally and/or chemically) during at least part of a year
so that a deep anoxic zone can form in isolation from the
atmosphere. Thermal stratification generally occurs each
summer in temperate climates and can be a long-term
natural condition in very cold or tropical climates. More
stable stratification can also be induced by actively main-
taining a layer of less dense water [e.g., warmer and/or less
saline than the deep water (Poling et al., 2003)] on a lake.
Direct carbon source addition has higher material costs,
but the treatment is generally rapid (reactions completed
over a few seasons) and may thus be best where infrequent
treatment is required. Nutrient addition has much lower
material costs, but it generally requires longer treatment
times and a more detailed analysis of lake limnology, and
it may be more practical where long-term management
is anticipated. Both methods have been demonstrated in
full-scale applications.
Finally, developing technologies such as metal-specific
microbes that precipitate arsenic and selenium as sulfides
in very low-volume sludges may offer potential for more
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
33
-------
targeted treatment of metalloids. These are noted as pos-
sible future remedies for these often difftcult-to-treat met-
alloids, but are not addressed further here.
For nutrient addition, the effectiveness of inducing organ-
ic carbon formation with nutrients can be estimated using
standard engineering relationships for lakes (Thomann
and Mueller, 1987; Martin et al., 2003). In practice, the
effectiveness of nutrient addition will be limited in part
by the organic carbon production rate, and the efficiency
of sulfate reduction depends on the reaction rate, tem-
perature, detritus settling rate, and reactivity of organic
carbon in the sediment. Application of nutrient-addition
treatment should anticipate a site-specific pilot test, nu-
merical modeling to estimate dose rates, and several years
of active treatment.
For organic carbon addition, dosing depends on the stoi-
chiometry between organic carbon and the desired target
reactions, adding sufficient organic carbon to remove ox-
ygen and then producing sufficient sulfide to precipitate
the heavy metals in solution. Direct reduction of specific
elements to less soluble forms (e.g., U, Cr, Se, As, Sb) is
less widely described and may require pilot-scale demon-
stration. If the water quality in the pit is sufficiently poor
that the microbial community cannot thrive, alternative,
pre-biological treatments may be necessary.
For induced stratification, a supply of less dense water for
maintaining a capping layer is generally required. This
can be warmer water (e.g., power plant cooling water) or
less saline water (e.g., fresh water over a saline lake). The
viability of maintaining an isolated deep layer of dense
water can be evaluated with a numerical limnologic mod-
el [e.g., CEQUAL/W2 (Cole and Buschek, 1995)] using
site-specific parameters for bathymetry, water salinities,
and climate.
Performance and Cost Data
Examples are provided at right that demonstrate the tech-
nology in practice for carbohydrate additions (commonly
sugar industry byproducts) and nutrient additions (typi-
cally nitrate and phosphate).
Carbohydrate Addition
Site Name and Location
Experimental Design
Results
Site Name and Location
Experimental Design
Results
Gilt Edge Pit Lake, South Dakota,
USA. (Arcadis, Inc., not published)
In-situ pit lake (volume = 65 million
gallons, dimictic lake). NaOH (125
tons, to increase pH), alcohol, and
sugar in three stages over a summer
(producing -100 mg/L initial dissolved
organic carbon in the lake). Duration
of monitoring: Syears.
Cadmium, copper, lead, nickel, arse-
nic, selenium, and zinc decreased
from above to below treatment
objectives after treatment, including
copper from 20 to 0.05 mg/L, cad-
mium from 0.2 to 0.02 mg/L, and zinc
from 5 to 0.9 mg/L. Monitoring for
excess sulfide in the pit lake during
treatment was identified as an impor-
tant issue.
Koyne/Plessa lignite field, Germany
(Frommichen et al., 2004)
Laboratory microcosm. Ethanol, sug-
ar industry byproduct (Carbocalk),
and wheat straw dosed at 3. 9 kg/m2
Carbocalk and 9.3 kg/m2 wheat straw.
Duration of monitoring: 1 year.
pH increased from 2.6 to 6.5, neutral-
ization rate 6 to 15 equiv/m2-yr
Nutrient Addition
Site Name and Location
Experimental Design
Results
Island Copper Mine Pit Lake (Poling
etal.,2003)
Field-scale pit lake; water volume
241,000,000 m3 (and ~5 million m3 ARD
added to deep layer); permanently
stratified with seawater hypolim-
nion; brackish epilimnion [a 5-m thick
brackish layer is maintained over a
more saline (seawater) hypolimnion].
The contaminant load was moderate,
with a range of 5- 10 mg/L heavy
metals and 500 - 2,000 mg/L sulfate.
Liquid nitrate and phosphate (N:P =
6:1) added every 10 days to surface
using a small boat. Duration of moni-
toring: 6 years.
Treatment produced effective remov-
al of zinc, copper, and cadmiumfrom
the lake while maintaining accept-
able water quality in the epilimnion
layer. Ongoing treatment is estimated
at$100,000/yrand is treating be-
tween 4 and 6 million m3/yr of acidic
inflow.
34
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
Nutrient Addition (continued)
Site Name and Location
Experimental Design
Results
Equity Silver Mine, British Columbia,
Canada (Martin et al., 2003)
Microcosm using "limnocorals" in
a dimictic existing pit lake. Addition
of 0.7,1.4, and 14 mmole P/m2/week.
Duration: 1 year.
High nutrient loading produced dra-
matic increase in algal productivity in
the epilimnion and efficient removal
of metal cations (e.g., zinc from
150 to 20 ug/L; copper from 3 to 0.1
ug/L, cadmium from 6 to 2 ug/L, and
nickel from 15 to 5 ug/L). The removal
mechanism is adsorption of metals to
biogenic particles, which then settle
out.
Following several successful full-scale applications, in-
situ bioremediation of mine pit lakes appears to be rela-
tively well accepted by the scientific community, indus-
try, and regulators. Successful carbohydrate treatments
have been demonstrated using natural organic car-
bon (Frommichen et al., 2004) and alcohols plus sugar
(http://www.arcadis-us.com). Nutrient addition with in-
duced stratification is providing ongoing treatment at the
Copper Island Mine (Poling et al., 2003). Limnologic
models are mature and have demonstrated the ability to
reliably predict physical mixing and biological productiv-
ity in mine pit lakes. In-situ bioremediation of pit lakes
offers the potential in some cases for much lower cost
treatment, particularly using nutrient addition. However,
this remains a research area, with site-specific conditions
dramatically affecting implementability. Potential cost
savings thus need to be weighed against current uncer-
tainty and associated higher potential costs for research
and characterization.
Table 4-6 on pages 36 and 37 provides an overview of pit
lake treatment technologies covered in this section, tech-
nology selection factors, and limitations.
5.0 CONCLUSION
Each mine disturbance that is the source of contami-
nated water requires careful consideration of site-specific
characteristics prior to choosing a strategy to manage the
water. The large majority of mine drainages will require
long-term treatment, on the order of decades and be-
yond. Few walk-away options are available, and financial
requirements for in-perpetuity treatment are a significant
component to the decision on which treatment option to
use. Site characterization is critical and should address the
following questions:
• What is the potential for reducing the flow of the
water?
• What is the highest volume of water that will need
to be treated during major events?
• What is the water quality, and how does it vary
seasonally?
• What are the regulatory discharge requirements?
Many types of rock will only go acidic after several years,
and the rate of acid generation will change over time, of-
ten increasing for several years as the oxidizing bacteria
become widespread. What level of data are required to
accurately predict these changes?
Finally, each treatment technology presents different fi-
nancial and treatability considerations that may require
pilot-scale testing in the field, in order to demonstrate
that it will indeed treat the mine water to acceptable dis-
charge limits over the long term. The state and federal
regulatory agencies, public interest organizations, and the
mining industry all are increasingly focused on issues re-
lated to mine water treatment. This emphasis is unlikely
to go away, since the long term treatment costs can be
very high. Confidence that a treatment option will actu-
ally do the job requires continual technical and financial
evaluation of each option, public release and dissemina-
tion of treatment data, and continued research on new
methods for mine water treatment.
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
35
-------
Table 4-6. Pit Lake Treatment Technologies for Hard-Rock Mining
S1
i-
Technology
Name
Induced
Stratification
and Bio-
remediation
Backfilling:
Partial or
Complete
Accelerated
Filling
Technology
Description
Lake is stratified, using either a
low-salinity cap layer over saline
lake or warm-layer cool hypolimni-
on. Organic carbon can be created
by adding nutrients, and metals
adsorb and settle with organic de-
tritus. Alternatively, direct carbo-
hydrate addition can produce H2S,
precipitating metals as metallic-sul-
fide minerals.
Waste rock and/or tailings are used
to partially or completely fill the pit.
Removes open surface water and
access to humans and wildlife. Re-
active backfill may require amend-
ment to reduce acidity or other
solute release.
Surface or ground water is used to
rapidly refill pit. This reduces the
time for sub-aerial wall rock oxida-
tion and may reduce the rinsing of
reactive rock surfaces into the pit
lake.
Target
Analytes
Primarily
heavy met-
als: Cd,Zn,
Cu, Pb,Ni,
U. Possibly
metalloids
As, Sb, Se.
Possibly
S04.
Eliminates
surface ex-
posure to all
analytes
All analytes
associated
with wall
rock oxida-
tion, i.e., sul-
fate, heavy
metals, and
metalloids
Critical Feasibility Factors
Implementabilty
• Most lakes strat-
ify naturally each
summer, simplify-
ing isolation of the
hypolimnion
• Salinity stratifica-
tion requires saline
lake and fresh-wa-
ter source
• Production and re-
lease of excess H2S
to the atmosphere
can be a health risk
• Access to lake is
required for reagent
addition
Requires proximal
source of waste rock
ortailingsfor
backfilling
Requires access to
source of water. River
diversion can allow
rapid filling, while
ground water pump-
ing is typically slower
and more expensive.
Effectiveness
• Metal cation re-
moval is typically
effective
• Removal by adsorp-
tion to detritus may
require several
seasons
• Metalloid removal
mechanisms are
not well known
• Long-term stability
of metals in sedi-
ments uncertain;
periodic re-treat-
ment may be
required
• Reduces or elimi-
nates further oxida-
tion of rock below
water table
• Reduces the
water volume in the
pit — important in
arid areas
Can significantly
improve water quality
over what would have
existed by refilling by
ground water recov-
ery. However, a rapid
volume increase
could force treat-
ment earlier in time,
increasing costs.
Cost
• Materials can be
significant cost
• Carbohydrate (sugar
or alcohols): ~$0.5
- 1.0 per kg
• Ammonium poly-
sulfate (10-34-0)
solution and urea
ammonium nitrate
(28-0-0) solution
prices depend on
local availability
• Depends strongly on
the mine plan
• Costs are low if
partial backfill oc-
curs during mining.
Backfilling from
rock outside the
pitat$1/tonne or
higher.
During refilling, con-
tinuous operating
ground water pumps
are typically the pri-
mary cost
Important
Limitations
• Inducing and maintain-
ing stratification requires
dense deep water (saline
or cold) and a supply of
low-density water (fresh
and/or warm) for surface
layer. Inducing reducing
conditions can mobilize
metals in sediments.
• Several seasons of treat-
ment may be required
• Carbohydrate addition is
patented
• Sulfide production must
be closely controlled to
avoid health risk
Sometimes difficult to ac-
curately predict the water
quality that will result from
rinsing backfilled mate-
rial. Can degrade quality
in throughflowing ground
water.
• Rapid pit lake refilling
may force poor water
back into the ground wa-
ter system
• Appropriate monitoring is
required to fully under-
stand the impacts to aqui-
fer surrounding the pit
-------
Table 4-6. Pit Lake Treatment Technologies for Hard-Rock Mining (continued)
Technology
Name
Neutralization
Technology
Description
Lime or other neutralizing agents
are added to the pit lake. Adequate
mixing (natural turnover or multi-
level injection) is required to com-
pletely mix in oxygen (to oxidize Fe)
to neutralize acidity throughout the
lake depth profile.
Target
Analytes
Primarily
heavy met-
als: Cd,Zn,
Cu, Pb, and
Ni. Possibly
metalloids
As, Sb, Se,
and sulfate.
Critical Feasibility Factors
Implementabilty
Well-demonstrated
technology using
lime addition from
either floating barge
or amended inflow
water
Effectiveness
Metals removal is
similar to what is
observed with con-
ventional lime treat-
ment — very effective
for acidity and metal
cations, less effective
for oxyanion metal-
loids (e.g., As, Sb, Se)
Cost
Highly variable cost
depending on lake
acidity, cost for de-
livered lime, and the
method used to add
the lime to the lake
Important
Limitations
• Lime added to the surface
may become coated, re-
ducing efficiency
• Oxidation of ferrous iron
is necessary for effective
iron removal
• Sulfate concentrations
are below- 3,000 mg/L
-------
6.0 ACKNOWLEDGMENTS
7.0 ACRONYMS AND ABBREVIATIONS
This Engineering Issues document was prepared for the
U.S. Environmental Protection Agency, Office of Re-
search and Development, National Risk Management
Research Laboratory by Science Applications Interna-
tional Corporation (SAIC) under Contract No. 68-C-02-
067. Mr. Doug Grosse served as the EPA Work Assign-
ment Manager. Mr. Ed Bates acted as the EPA Technical
Project Manager. Ms. Lisa Kulujian was SAIC's Work As-
signment Manager and Mr. Kyle Cook served as SAIC's
technical lead. The primary authors of this document
were as follows: Glenn C. Miller (lead author), University
of Nevada, Reno; Houston Kempton, Integral Consult-
ing, Inc., Boulder, Colorado; Linda Figueroa, Colorado
School of Mines; and John Pantano, Consultant, Butte,
Montana.
Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufac-
turer, or otherwise, does not necessarily constitute or im-
ply its endorsement, recommendation, or favoring by the
United States Government. The views and opinions of
authors expressed herein do not necessarily state or reflect
those of the United States Government, and shall not be
used for advertising or product endorsement purposes.
For additional information, contact the ORD Engineer-
ing Technical Support Center (ETSC):
David Reisman, Director
U.S. EPA Engineering Technical Support Center
NRMRL
26 W Martin Luther King Drive MLK-489
Cincinnati, OH 45268
(513) 487-2588
ABA acid base accounting
ACMER Australian Center for Mining
Environmental Research
ADTI Acid Drainage Technology Initiative
AGP acid-generating potential
ALD anoxic limestone drain
AMD acid mine drainage
ANP acid-neutralizing potential
ANSTO Australian Nuclear Science and Technology
Organization
APS alkalinity-producing system
ARD acid rock drainage
ASTM American Society for Testing and Materials
BC British Columbia
BLM U.S. Bureau of Land Management
CN cyanide
DOE U.S. Department of Energy
EPA U.S. Environmental Protection Agency
ETSC U.S. EPA Engineering Technical Support
Center
ICARD International Conference on Acid Rock
Drainage
INAP International Network for Acid Prevention
MEND Mine Environmental Neutral Drainage
NAG net acid-generating (test)
NDEP Nevada Division of Environmental
Protection
NMA National Mining Association
NNP net-neutralizing potential
NRC National Research Council
O/M operation/maintenance
OSC on-screen coordinator
OSM U.S. Office of Surface Mining
PIRAMID Passive In-Situ Remediation of Acidic Mine/
Industrial Drainage
PRB permeable reactive barrier
PRZ permeable reactive zone
RAMS Restoration of Abandoned Mine Sites
RAPS reducing alkalinity-producing system
RCRA Resource Conservation and Recovery Act
38
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
RCTS Rotating Cylinder Treatment System
RPM remedial project manager
SAIC Science Applications International
Corporation
SAPS successive alkalinity-producing system
SME Silica Micro Encapsulation
SPLP Synthetic Precipitation Leaching Procedure
SRB sulfate-reducing bioreactor
SRK SRK Consulting Engineers and Scientists
TCLP Toxicity Characteristic Leaching Procedure
TDS total dissolved solids
Tonne metric ton
UNR University Nevada-Reno
WAD weak acid dissociable
8.0 REFERENCES
Andrina, J., S. Miller, and A. Neale. "The design, con-
struction, instrumentation and performance of a full-
dcale overburden stockpile trial for mitigation of acid
rock drainage, Grasberg Mine, Papua Province, Indone-
sia." In Proceedings of the 6th International Conference on
Acid Rock Drainage, Cairns, Australia, 2003.
ASTM. "Standard Test Methods for Analysis of Metal
Bearing Ores and Related Materials by Combustion In-
frared Absorption Spectrometry, Method E1915-01,"
2003. http://www.astm.org
Atkins, D., J. H. Kempton, and T Martin. "Limnologic
conditions in three existing Nevada pit lakes: observations
and modeling using CE-QUAL-W2." In Proceedings of
the 4th International Conference on Acid Rock Drainage,
Vancouver, BC, Canada, 1997.
Barbour, L. Personal communication. Short course: "Un-
derstanding waste rock facilities in dry and wet climates."
In Proceedings of the 5th International Conference on Acid
Rock Drainage, Denver, CO, Society for Mining, Metal-
lurgy and Exploration Inc., 2000.
Bennett, J. W. Personal communication. Australian Nu-
clear Science and Technology Organization, Menai, New
South Wales, Australia, 1998.
BLM. Acid Rock Drainage Policy for Activities Authorized
under 43 CFR 3802/3809, Instructional Memorandum
no. 96-79. U.S. Bureau of Land Management, 1996.
Brady, K., M. W. Smith, R L. Beam, and C. A. Cravotta.
"Effectiveness of the use of alkaline materials at surface
coal mines in preventing or abating AMD: part 2. Mine
site case studies." In Proceedings of the 1990 Mining and
Reclamation Conference, West Virginia University, Mor-
gantown,WV, 1990.
Castro, J. M., and J. N. Moore. Pit lakes: their charac-
teristics and the potential for their remediation. Environ-
mental Geology 39 (11): 1254-1260 (2000).
Castro, J. M., B. W. Wielinga, J. E. Gannon, and J. N.
Moore. Stimulation of sulfate-reducing bacteria in lake
water from a former open-pit mine through addition of
organic wastes. Water Environmental Resources 71 (2):
218-223 (1999).
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
39
-------
Cole, T. M., and E. M. Buchak. CE-QUAL/W2: A Two-
Dimensional, Laterally-Averaged, Hydrodynamic and Water
Quality Model, Version 2.0, Instruction Report EL-95-X.
Vicksburg, MS: U.S. Army Corps of Engineers Water-
ways Experiment Station, 1995.
Costello, C. Acid Mine Drainage: Innovative Treatment
Technologies. U.S. Environmental Protection Agency
Technology Innovation Office, October, 2003. http://
clu-in.0rg/s.focus/c/pub/i/1054/
Cravotta III, C. A., K. B. C. Brady, M. W. Smith, and R.
L. Beam. "Effectiveness of the addition of alkaline mate-
rials at surface coal mines in preventing or abating acid
mine drainage—part 1, geochemical considerations." In
Proceedings of the 1990 Mining and Reclamation Confer-
ence and Exhibition, Charleston, WV, April 23-26, 1990;
West Virginia University, Morgantown, WV, 1990.
Day, S. J. "Evaluation of acid generating rock and acid
consuming rock mixing to prevent acid rock drainage."
Paper presented at the International Land Reclamation
and Mine Drainage Conference and the Third Internation-
al Conference on Abatement of Acidic Drainage, Pittsburgh,
PA, April 24-29, 1994. Norecol Dames and Moore, Inc.,
Vancouver, BC, Canada.
EPA. Guidance for Conducting Remedial Investigations and
Feasibility Studies under CERCLA, Interim Final, EPA/ 540/
G89/004. U.S. Environmental Protection Agency, 1988.
http://www.epa.gov/superfund/action/guidance/remedy/
rifs/overview. htm
EPA. Permeable Reactive Barrier Technologies for Contami-
nant Remediation, EPA/600/R-98/125. U.S. Environ-
mental Protection Agency, 1998. http://www.epa.gov/
ada/download/reports/reactbar.pdf
EPA. Innovative Remediation Technologies: Field Scale Dem-
onstration Projects in North America, 2nd Edition, EPA-
542-B-00-004. U.S. Environmental Protection Agency,
Office of Solid Waste and Emergency Response, 2000.
http://www.epa.gov/tio/download/remed/nairt 2000.pdf
EPA. Arsenic Treatment Technologies for Soil, Waste, and
Water, EPA 542-R-02-004. U.S. Environmental Protec-
tion Agency, 2002. http://www.epa.gov/tio/download/
remed/542r02004/arsenic report.pdf
Erickson P. M. and R. Hedin. "Evaluation of overburden
analytical methods as a means to predict postmining coal
mine drainage quality." In U.S. Bureau of Mines 1C 9183,
U.S. Bureau of Mines, 1988.
Evangelou, V P., and Y. L. Zhang. A review: pyrite oxida-
tion mechanisms and acid mine drainage prevention. En-
vironmental Science ^Technology (25):l4l-199 (1995).
Exponent Inc. Hydrochemical Characterization of the Pro-
posed Phoenix Project, Lander County, Nevada. Boulder,
CO: Battle Mountain Gold Company, Battle Mountain,
NV, August, 2000.
Frommichen, R, K. Wendt-Potthoff, K. Friese, and R.
Fischer. Microcosm studies for neutralization of hypolim-
nic acid mine pit lake water (pH 2.6). Environmental Sci-
ence & Technology 38 (6): 1877-1887 (2004).
INAP. Treatment of Sulfate in Mine Effluents. October
2003. http://www.inap.com.au/public downloads/Re-
search Projects/Treatment of Sulphate in Mine Efflu-
ents - Lorax Report.pdf
INAP. 2004. http://www.inap.com.au/
Jenkins, M., and J. Skousen. "Acid mine drainage treat-
ment with the Aquafix system." In Proceedings, Fourteenth
Annual West Virginia Surface Mine Drainage Task Force
Symposium, West Virginia University, Morgantown, WV,
1993.
Kuipers, J. R Hardrock Reclamation Bonding Practices in
the Western United States. Boulder, CO: National Wild-
life Federation, February 2000. http://www.csp2.org/
REPORTS/Hardrock%20Bonding%20Report.pdf.
Martin, A. J., J. Crusius, J. J. McNee, P. Whittle, R. Piet-
ers, and T. F. Pedersen. "Field-scale assessment of biore-
mediation strategies for two pit lakes using limnocorrals."
In Proceedings of the 6th International Conference on Acid
Rock Drainage, Cairns, Australia, 2003.
Mehling, P. E., S. J. Day, and K. S. Sexsmith. "Blending
and layering waste rock to delay, mitigate, or prevent acid
generation: a case study review." In Proceedings of the 4th
International Conference on Acid Rock Drainage, Vancou-
ver, BC, Canada, 1997.
Miller, S., A. Robertson, and T. Donohue. "Advances in
acid drainage prediction using the net acid generation
(NAG) test." In Proceedings of the 4th International Con-
ference on Acid Rock Drainage, Proceedings Vol. II, Vancou-
ver, BC, Canada, 1997.
40
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
Morin, K. A., N. M. Hutt, and K. G. Ferguson. "Mea-
sured rates of sulfide oxidation and acid neutralization
in kinetic tests: statistical lessons from the database." In
Proceedings Sudbury '95—Mining and the Environment, T.
P. Hynes and M. L. Blanchette, eds., Canmet, Ottawa,
Canada, 1995.
Morin, K. A., and N. Hutt. Control of Acidic Drainage
in Layered Waste Rock at the Samatosum Minesite: Labora-
tory Studies and Field Monitoring. Draft MEND Report
2.37.3. 1996.
NDEP. Waste Rock Evaluation. Carson City, NV: Bureau
of Mining Regulation and Reclamation, Nevada Division
of Environmental Protection, 1990.
NDEP. Quarterly Reports for the Sleeper Pit Lake. Car-
son City, NV: Bureau of Mining Regulation and Recla-
mation, Nevada Division of Environmental Protection,
1998-2004.
O'Kane, M, and S. L. Barbour. "Predicting field perfor-
mance of lysimeters used to evaluate cover systems for
mine waste." In Proceedings of the 6th International Con-
ference on Acid Rock Drainage, Cairns, Australia, 2003.
O'Kane Consultants. Evaluation of the Long-Term Per-
formance of Dry Covers: Final Report. OKC report 684-
02. March 2003. http://www.inap.com.au/completed
research projects.htm
OSM. Methods for Estimating the Costs of Treatment of
Mine Drainage. Prepared by Tetra Tech EM Inc. U.S.
Office of Surface Mining, 2000. http://amd.osmre.gov/
Cost.pdf
Pedersen, T. E, J. Crusius, A. J. Martin, J. McNee, and P
Whittle. "Pit lakes as mine-waste remediation cells: pluses
and pitfalls." Presented at the Annual Meeting of the Geo-
logical Society of America, Seattle, WA, November 2-5,
2003. http://gsa.confex.com/gsa/2003AM/finalprogram/
abstract 64346.htm
Poling, G. W., C. A. Pelletier, D. Muggli, M. Wen, J.
Gerits, C. Hanks, and K. Black. "Field studies of semi-
passive biogeochemical treatment of acid rock drainage at
the Island Copper Mine pit lake." In Proceedings of the 6th
International Conference on Acid Rock Drainage, Cairns,
Australia, 2003.
Ritchie, I. A. M. "Rates and mechanisms that govern pol-
lutant generation from pyritic wastes." In Environmental
Geochemistry of Sulfide Oxidation, C. N. Alpers and D. W
Blowes (editors), Washington, D.C.: American Chemical
Society, 1994.
Romano, C. G., K. Ulrich Mayer, D. R Jones, D. A.
Ellerbroek, and D. W. Blowes. Effectiveness of various
cover scenarios on the rate of sulfide oxidation of mine
tailings. Journal of Hydrology (271): 171-187 (2003).
Sobek, A. A., W. A. Schuller, J. R. Freeman, and R M.
Smith. Field and Laboratory Methods Applicable to Over-
burdens and Mine Soils, EPA 600/2-78-054. Cincinnati,
OH: U.S. Environmental Protection Agency, 1978.
SRK Consulting. Draft Acid Rock Drainage Technical
Guide, Volume I. British Columbia Acid Mine Drain-
age Task Force Report. Prepared by Steffen Roberson and
Kirsten (B.C.) Inc. Vancouver, BC, Canada: SRK Con-
sulting, 1989.
Su, C., and R W. Puls. Arsenate and arsenite removal by
zero-valent iron: kinetics, redox transformation and im-
plications for in situ groundwater remediation. Environ-
mental Science 6-'Technology (35): 1487-1492 (2001).
Telesto Solutions. Heap Leach Closure Plan, Florida Can-
yon Mine. A report presented to Florida Canyon Mine as
part of the permitting process. Nevada: Available from the
Winnemucca District of the BLM, 2003.
Thomann, R V, and J. A. Mueller. Principles of Sur-
face Water Quality Modeling and Control, New York, NY:
Harper & Row, Inc., 1987.
Tsukamoto, T. K., H. Killian, and G. C. Miller. Column
experiments for microbiological treatment of acid mine
drainage; low temperature, low pH, and matrix investiga-
tions. Water Research (38): 405-1418 (2004).
USGS. Value of Nation's Mineral Production Still on the
Rise in 2005. United States Geological Survey, 2006.
http://www.usgs.gov/newsroom/article.asp?ID= 1431.
Wilson, J. A., G. W. Wilson, and D. G. Fredlund. "Nu-
merical modeling of vertical and inclined waste rock lay-
ers." In Proceedings of the 5th International Conference on
Acid Rock Drainage, Vol. 1, Denver, CO: Society for Min-
ing, Metallurgy and Exploration Inc., 2000a, 257-268.
Management and Treatment of Water from Hard Rock Ml
Engineering Issue
41
-------
Wilson, G. W, L. L. Newman, and K. D. Ferguson. "The
co-disposal Of waste rock and tailings." In Proceedings
from the 5th International Conference on Acid Rock Drain-
age, Vol. 2, Denver, CO: Society for Mining, Metallurgy
and Exploration Inc., 2000b, 789-796.
Wilson, G. W, H. K. Plewes, D. Williams, and J. Robert-
son. "Concepts for co-mixing of tailings and waste rock."
In Proceedings of the 6th International Conference on Acid
Rock Drainage, Cairns, Australia, 2003.
Ziemkoewicz, P. E, J. G. Skousen, D. L. Brant, P. L. Stern-
er, and R. J. Lovett. Acid mine drainage treatment with
armored limestone in open limestone channels. Journal of
Environmental Quality (26): 1017-1024 (1997).).
42
Engineering Issue
igement and Treatment of Water from Hard Rock Mines
-------
SEPA
United States
Environmental Protection
Agency
Office of Research and Development
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA/625/R-06/014
October 2006
www.epa.gov
PRESORTED STANDARD
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% post-consumer fiber content
processed chlorine free
------- |