Bandung Nbsp Perry Nbsp Paper

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Estimating Mine Water Composition from Acid Base Accounting and Weathering Tests; Applications from U. S. Coal Mines Eric F. Perry (1)  Mine water quality for coal mines in the United States is estimated using whole rock analysis for Acid/Base Accounting (ABA), or with simulated weathering tests. ABA compares the quantity of acidity that can be generated from pyrite oxidation to the amount of bases, mostly carbonates, that are available to neutralize acid. Studies of surface mine drainage and overburden rocks in the Appalachian region show that the quantity of acid neutralizers is the most important factor controlling mine water quality. Mines producing net alkaline drainage (alkalinity >acidity) contain more than 2 to 3 % neutralizers in overburden rocks, and had an excess of neutralization potential compared to acid production potential. A ratio of about 2:1 or greater of neutralization potential to  potential acidity also produces net alkaline mine drainage. Most mines ca n be classified as to potential to generate acid or alkaline waters. There is a small range of ABA  properties where both acid and alkaline waters occur, and interpretation from ABA alone is uncertain. These relationships are consistent across different coalbeds and overburden rocks. Similar ABA classifications have been proposed for base and precious metal mines. Concentrations of metals or sulfate cannot be determined directly from Acid Base Accounting, however. Simulated weathering tests have the capacity to estimate mine water composition including pH, and relative amounts of metals, sulfate, and trace elements. The relative rates of acidity and alkalinity production can also be estimated from weathering tests. Products of pyrite oxidation are soluble and are produced rapidly, while production of alkalinity is limited by carbonate solubility. Weathering tests are especially useful where ABA results are inconclusive, or the rocks contain sulfide minerals other than pyrite. Different test protocols including columns, cells and soxhlet extractors are in use, so test results must be evaluated against the specific test procedure. Rock to water ratio, flushing frequency, pore gas composition and test length influence the results. A scaling factor relating the laboratory results to mine site conditions is usually required, and appears to be site specific. Examples of mine drainage prediction and actual mine water quality are given for both Acid Base Accounting and simulated weathering tests. Both overburden test methods should be used in conjunction with other geologic, hydrologic and mine site data to estimate post-mining water quality. Eric Perry is a Hydrologist, U.S. Dept. of Interior, Office of Surface Mining, 3 Parkway Center, Pittsburgh, PA, 15220, USA. E mail [email protected] 
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    Estimating Mine Water Composition from

    Acid Base Accounting and Weathering Tests; Applications from U. S. Coal Mines

    Eric F. Perry(1)

    Mine water quality for coal mines in the United States is estimated using whole rockanalysis for Acid/Base Accounting (ABA), or with simulated weathering tests. ABAcompares the quantity of acidity that can be generated from pyrite oxidation to the

    amount of bases, mostly carbonates, that are available to neutralize acid. Studies of

    surface mine drainage and overburden rocks in the Appalachian region show that thequantity of acid neutralizers is the most important factor controlling mine water quality.

    Mines producing net alkaline drainage (alkalinity >acidity) contain more than 2 to 3 %

    neutralizers in overburden rocks, and had an excess of neutralization potential comparedto acid production potential. A ratio of about 2:1 or greater of neutralization potential to

    potential acidity also produces net alkaline mine drainage. Most mines can be classified

    as to potential to generate acid or alkaline waters. There is a small range of ABA

    properties where both acid and alkaline waters occur, and interpretation from ABA aloneis uncertain. These relationships are consistent across different coalbeds and overburden

    rocks. Similar ABA classifications have been proposed for base and precious metal

    mines. Concentrations of metals or sulfate cannot be determined directly from Acid BaseAccounting, however.

    Simulated weathering tests have the capacity to estimate mine water compositionincluding pH, and relative amounts of metals, sulfate, and trace elements. The relative

    rates of acidity and alkalinity production can also be estimated from weathering tests.

    Products of pyrite oxidation are soluble and are produced rapidly, while production ofalkalinity is limited by carbonate solubility. Weathering tests are especially useful where

    ABA results are inconclusive, or the rocks contain sulfide minerals other than pyrite.Different test protocols including columns, cells and soxhlet extractors are in use, so test

    results must be evaluated against the specific test procedure. Rock to water ratio,

    flushing frequency, pore gas composition and test length influence the results. A scaling

    factor relating the laboratory results to mine site conditions is usually required, andappears to be site specific.

    Examples of mine drainage prediction and actual mine water quality are given for bothAcid Base Accounting and simulated weathering tests. Both overburden test methods

    should be used in conjunction with other geologic, hydrologic and mine site data to

    estimate post-mining water quality.

    Eric Perry is a Hydrologist, U.S. Dept. of Interior, Office of Surface Mining, 3 ParkwayCenter, Pittsburgh, PA, 15220, USA. Email [email protected]

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    Introduction

    Mining of coal and minerals in the United States (U.S.) can sometimes produce acid

    drainage and elevated concentrations of metals, dissolved solids, and sulfate in surface

    and ground waters. To prevent water pollution by mining operations, testing ofoverburden and waste rock is conducted in advance of mining. The purpose of testing is

    to identify rocks with potential to generate acidic drainage, and determine which rockscan neutralize acidity and generate alkalinity. Some mines also test rock and soil to selectmaterials that can be used for reclamation and plant growth.

    Geochemical test methods are of two general types; static or whole rock analyses, andkinetic or simulated weathering tests. Static tests include Acid Base Accounting(ABA),

    X-ray diffraction for mineral identification, elemental analyses, exchangeable acidity,

    cation exchange capacity and others. Acid Base Accounting compares the quantity ofacidity that can be generated from pyrite oxidation to the amount of bases, mostly

    carbonates that are available to neutralize acid. It is the most common static test used for

    testing overburden and waste rock at U.S. coal mines.

    Kinetic or simulated weathering tests include various leaching protocols and batch

    extract tests. Kinetic tests attempt to simulate chemical weathering of rocks in contact

    with leach water. Mine water composition, including pH, metals, acidity, and alkalinityis estimated from the leachate chemistry. Column leaching tests are the most frequently

    used kinetic test method. Kinetic tests are especially useful where ABA results are

    inconclusive, or the rocks contain more than one sulfide mineral. They are often used toevaluate the acid drainage potential of waste rock and tailings from base and precious

    metal mines.

    The purpose of this paper is to review the use, assumptions and limitations of static and

    kinetic test methods for predicting mine drainage quality. Examples of mine drainageprediction and actual mine water quality are given for both Acid Base Accounting and

    simulated weathering tests. Both overburden test methods should be used in conjunction

    with other geologic, hydrologic and mine site data to estimate post-mining water quality.

    The U.S. has major coal deposits ranging from lignite to anthracite grade in several

    fields. Most coal mined is either subituminous or bituminous. Figure 1 shows the

    location of major U.S. coal deposits and the approximate percentage of mines in eachfield encountering acid forming materials. The most severe acid drainage associated with

    coal mining occurs in northern Appalachian and the Eastern Interior region. Rocks in

    these regions are Upper Pennsylvanian age, consisting of cyclothems of coal, shale,limestone and sandstone. The rocks generally contain moderate amounts of pyrite and

    carbonates. Annual precipitation is about 1000 to 1500 millimeters per year in these

    areas, and the climate is humid continental. The southern Appalachian region, which has

    less acid drainage, contains Lower Pennsylvanian age rocks, which generally contain lowconcentrations of pyrite and carbonates. The Powder River Basin is semi-arid and

    receives about 250 to 375 millimeters of precipitation per year. Coal bearing rocks are

    Cretaceous age, and generally contain low amounts of pyrite and moderate concentrationsof carbonates.

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    Figure 1. Extent of Acid Drainage From U.S. Coal Mines.

    Most active base and precious metal mining is concentrated in the western U.S. Aciddrainage from old metal mining occurs in Colorado, Montana, California and several

    other states. Geologic, geochemical, and hydrologic conditions largely determine the

    potential for acid drainage form coal and metal mines.

    Static Test Methods

    Origin of Acid Base AccountingAcid Base Accounting is the most frequently used static test for estimating acid drainage

    potential. ABA was developed at West Virginia University by soil scientists interested inreclamation (Skousen et al., 1990). The approach came from early attempts at classifying

    mine spoils for revegetation potential, based on acidity or alkalinity, and rock type. From

    these classifications, they could determine if plants could grow on the mine spoil, andwhether lime should be applied.

    In 1971, West Virginia University began to formally develop a system of balancing the

    acid and alkaline producing potential of rocks. This work included coal overburden rocks

    throughout the Appalachian and Interior coal basins. The importance of acid neutralizingminerals was recognized and quantified, and the term "neutralization potential" (NP) was

    44%

    17%

    12%

    23%0%

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    introduced. This work was published in a series of reports, including a manual of

    recommended field and laboratory procedures (Sobek et al.,1978).

    ABA, as originally developed, consists of measuring the acid generating and acidneutralizing potentials of a rock sample. These measurements of Maximum Potential

    Acidity (MPA) and Neutralization Potential (NP) are compared to obtain a Net

    Neutralization Potential (NNP), or net Acid-Base balance for the rock as follows:

    Net Neutralization Potential (NNP) = NP MPA (1)

    The measurements are usually reported in tons per thousand tons of overburden or parts

    per thousand(ppt). The units designation reflects the agricultural origins of ABA. One

    acre (0.40 hectatres) of plowed agricultural soil weighs about 1000 tons (907 kilograms).Liming requirements are usually expressed in tons per acre (kg/hectare). The units of

    measure for ABA are therefore comparable to lime requirement designations for

    agricultural lands.

    Maximum Potential Acidity(MPA)

    The acid generating potential, MPA, is calculated from a measurement of the total sulfurcontent of the rock by combustion in a sulfur furnace. It is assumed that sulfur is present

    in the form of pyrite (FeS2). For most coal overburden rocks, this is a good

    approximation, and potential acidity calculations are valid. If the rocks have undergonesignificant chemical weathering and contain some sulfate minerals such as gypsum

    (CaSO4* 2 H2O ) melanterite (FeSO4* 7 H2O) and others, total sulfur content may not

    accurately reflect potential acidity.Alkaline earth sulfate salts like gypsum are nonacid

    formers. Metal sulfate salts, however, are intermediate products of pyrite oxidation, andrepresent "stored acidity". These minerals can undergo dissolution and hydrolysis withacid generation. Sulfate sulfur cannot be ruled out as a potential acid source unless the

    mineralogy is known, and the common lab procedures for sulfur fractionation do not

    identify the specific minerals present.

    If samples are suspected of containing significant amounts of sulfate or organic forms ofsulfur, sequential extractions can be used to separate the components (Sobek et al, 1978).

    Organically bound sulfur is generally considered to be to nonacid forming and is found in

    coals, carbon rich shales, partings, "bone coal", etc. In these cases, a revised calculationof potential acidity is made based on the pyritic sulfur content.

    Ore bodies and waste rock at metal mines usually contain different sulfide minerals suchas sphalerite (ZnS), galena (PbS), and others, in addition to pyrite. Not all sulfide produce

    acidity when oxidized, so a measure of total sulfur will probably over estimate potentialacidity. For these mines, identification of specific sulfide minerals is helpful, using X-ray

    diffraction (XRD) and x-ray florescence (XRF) or optical techniques. The samples may

    also be tested using kinetic methods.

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    Neutralization Potential(NP)

    The neutralization potential, NP, is determined by reacting the sample with a known

    quantity and strength of HCl, and measuring the amount of acid consumed by backtitration. It is a modification of a test method designed to measure the calcium carbonate

    content of agricultural lime. The value reported for NP is assumed to represent mostly

    carbonates, exchangeable bases and readily soluble silicate minerals.

    The iron carbonate, siderite (FeCO3) can interfere with the determination ofneutralization potential. Siderite will initially neutralize acid because it is a carbonate.

    However, iron hydrolysis of the iron that is released will produce acid will produce a net

    neutralization of zero as shown below.

    FeCO3 + 2 H+ Fe

    2++ CO2+ H2O (2)

    Fe2+

    + 0.25 O2 + H2 O + H+ Fe

    3++ 1.5 H2O (3)

    Fe3+

    + 3H2 O Fe(OH)3 + 3H+

    (4)

    ------------------------------------------------------------------------------

    FeCO3 + 1/4 O2 + 3/2 H2O Fe(OH)3+ CO2 Summary reaction (5)

    Skousen et al (1997) tested rocks of known mineralogy with four variations of the

    neutralization potential test. Some of the rocks contained significant amounts of siderite.

    They found that interference from siderite was reduced if hydrogen peroxide wasincluded in the test protocol. This modified test procedure is now being used by some

    U.S. laboratories.

    Net Neutralization Potential(NNP)

    The measurements and calculations of NP, MPA, and NNP are based on the following

    assumed stoichiometry of pyrite oxidation and followed by calcite neutralization

    (Cravotta et al.,1990):

    FeS2+ 2 CaCO3+ 3.75 O2+ 1.5 H2O 2 SO42-

    + Fe(OH)3+ 2 Ca2+

    + 2 CO2 (6)

    For each mole of pyrite reacted, 4 moles of acidity are produced by sulfur oxidation and

    iron hydrolysis. Two moles of calcite are required to neutralize the acidity. On a massbasis, 200 grams of calcium carbonate are required for each 64 grams of pyritic sulfur, or

    a ratio of 3.125. If the Acid Base Accounting data is expressed in parts per thousand, the

    mass ratio is 31.25. This factor is used to convert sulfur content into potential acidity ascalcium carbonate equivalent.

    The components of ABA measurements are sometimes referred to by other terms, as they

    have been adapted for use in metal mining and other applications (Miller and Murray,

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    1988, British Columbia Acid Mine Drainage Task Force, 1989). In Australia and

    Canada, the term "Acid Production Potential" (APP) is equivalent to MPA, "AcidNeutralizing Capacity" (ANC) is equivalent to NP; and "Net Acid Producing Potential"

    or NAPP is the same as NNP.

    Estimating Water Quality from Acid Base Accounting

    Sobek et al(1978) suggested that an NNP value of less than -5 parts per thousand(ppt),

    could be used to identify materials unsuited for use in reclamation. It was soon realizedthat ABA could also be used to identify rocks likely to generate acid drainage and

    develop some estimate of mine drainage quality before mining actually began. The

    question was how to use ABA results to classify rocks as acid producers or acidneutralizers?

    There have been numerous attempts to define numerical criteria or levels of significance

    for classifying ABA results and expected rock behavior. These numeric criteria have

    taken the form of (1) boundaries on NNP values; (2) ratios of NP to MPA; and (3)boundaries on values for NP or MPA. Some of these criteria, and their geologic and

    geographic applications, are presented in Table 1. Values in the table refer tocharacteristics of individual rock samples. Variation exists in the reported values, whichare drawn from diverse geologic settings and climates. Some general conclusions are

    summarized as follows:

    A deficit of carbonate material or NP increases the likelihood of acid drainage. Theseinclude rocks with less than about 20 ppt NP and rocks with NNP less than zero, or

    ratio of NP to MPA of less than 1.

    Conversely, excess carbonate lessens the potential for acid drainage. These include

    rocks containing more than about 30 ppt NP and rocks with NNP greater than about10 ppt, or ratio of NP to MPA greater than 2

    A range of ABA values exists where drainage quality is variable. Both acid andalkaline waters can occur within a small range of Acid Base Accounting properties.

    Ratios of NP to MPA between 0 and 1 are often classified as variable.

    A universal ABA criteria for separating acid and alkaline producing rocks on all types

    of mines does not exist.

    The lack of universal criteria is not surprising since mine drainage quality is a product of

    the interaction of many geologic, hydrologic, climatic, and mining factors.

    Acid-Base Accounting and Coal Mine Drainage Studies in AppalachiaABA and mine drainage quality relations have been evaluated in Pennsylvania and

    northern Appalachia in four studies, including projects by the Pennsylvania Department

    of Environmental Protection, West Virginia University, and the U.S. Bureau of Mines.

    These studies have shown that carbonate content of the overburden (neutralizationpotential) is a very important factor controlling mine drainage quality. In each study, net

    alkalinity (alkalinity minus acidity) was used as the primary index of postmining drainage

    quality. The parameters acidity, alkalinity, and net alkalinity are measures of the

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    complete acidity or alkalinity generating capacity of a water. They are also the aqueous

    analogues of the ABA rock parameters of MPA, NP, and NNP. In this paper, I discuss

    Table 1

    Summary of Suggested Criteria for Interpreting Acid-Base Accounting(1)

    CRITERIA APPLICATION REFERENCE

    Rocks with NNP less than

    -5 ppt CaCO3are considered

    potentially toxic.

    Coal overburden rocks in northern

    Appalachian basin for root zone

    media in reclamation; mine

    drainage quality.

    Smith et al., 1974, 1976;

    Surface Mine Drainage Task

    Force, 1979; Skousen et al.,

    1987

    Rocks with paste pH less than 4.0

    are considered acid toxic.

    Coal overburden rocks in northern

    Appalachian basin for root zone

    media, mine drainage quality.

    Base and precious metal mine

    waste rock in Australia and

    southeast Asia.

    Smith et al., 1974, 1976;

    Surface Mine Drainage Task

    Force, 1979

    Miller and Murray, 1988

    Rocks with greater than 0.5%sulfur may generate significant

    acidity.

    Coal overburden rocks in northernAppalachian basin, mine drainage

    quality.

    Base and precious metal mine

    waste rock in Australia and

    southeast Asia.

    Brady and Hornberger, 1990

    Miller and Murray, 1988

    Rocks with NP greater than 30

    ppt CaCO3and "fizz" are

    significant sources of alkalinity.

    Coal overburden rocks in northern

    Appalachian basin, mine drainage

    quality.

    Brady and Hornberger, 1990

    Rocks with NNP greater than 20

    ppt CaCO3produce alkaline

    drainage.

    Coal overburden rocks in northern

    Appalachian basin. Base and

    precious metal mine waste rock and

    tailings in Canada.

    Skousen et al., 1987;

    British Columbia Acid Mine

    Drainage Task Force, 1989;

    Ferguson and Morin, 1991Rocks with NNP less than

    -20 ppt CaCO3produce acid

    drainage.

    Base and precious metal mine

    waste rock and tailings in Canada.

    British Columbia Acid Mine

    Drainage Task Force, 1989;

    Ferguson and Morin, 1991

    Rocks with NNP greater than 0

    ppt CaCO3do not produce acid.

    Tailings with NNP less than 0 ppt

    CaCO3produce acid drainage.

    Base and precious metal mine

    waste rock and tailings in Canada.

    Patterson and Ferguson, 1994;

    Ferguson and Morin, 1991

    NP/MPA ratio less than 1 likely

    results in acid drainage.

    Base and precious metal mine

    waste rock and tailings in Canada.

    Patterson and Ferguson, 1994;

    Ferguson and Morin, 1991

    NP/MPA ratio is classified as less

    than 1, between 1 and 2, and

    greater than 2.

    Base and precious metal mine

    waste rock and tailings in Canada.

    Ferguson and Robertson, 1994

    Theoretical NP/MPA ratio of 2 is

    needed for complete acid

    neutralization.

    Coal overburden rocks in northern

    Appalachian basin, mine drainage

    quality.

    Cravotta et al., 1990

    Use actual NP and MPA values

    as well as ratios to account for

    buffering capacity of the system.

    Base metal mine waste rock,

    United States.

    Filipek et al., 1991

    (1) Criteria in this table were developed for classification of individual rock samples

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    the results of a study that included about 40 surface mines from Pennsylvania'sbituminous coal field (Brady et al., 1994, and Perry and Brady, 1995).

    Each mine had two or more ABA drill holes and multiple postmining water quality

    samples from seeps, springs, or monitoring wells. Raw ABA data were processed into a

    summary value for the entire mine using mass weighting procedures described by Smith and Brady (1990). Summary ABA data were compared to median water quality values.

    Results of Pennsylvania Acid Base Accounting Study

    Mines with neutralization potential (NP) greater than about 21 ppt produced net alkaline

    water (Figure 2). Eight of eleven sites with NP less than 10 ppt had negative net alkaline(net acid) water. NP values between 10 and 21 tons/1000 tons included both net acid and

    net alkaline sites (variable water quality). Ten of 17 mines (58%) in this category

    produced alkaline water. Two low NP sites with net alkaline water were anomalous. Theanomalies could result from nonrepresentative overburden sampling, an influx of alkaline

    ground water from offsite, or alkalinity production from noncarbonate sources.

    PLOT OF NET ALKALINITY vs NEUTRALIZATION POTENTIAL

    -1000

    -800

    -600

    -400

    -200

    0

    200

    400

    0 10 20 30 40 50 60 70 80

    Neutralization Potential (ppt)

    NetAlaklainity(mg/

    CaCO3Eq)

    Acid

    Water

    Variable

    Alkaline Water

    Figure 2. Plot of Overburden Neutralization Potential and Mine Drainage Net Alkalinity.

    Figure 3 is a plot of Net Neutralization Potential and mine drainage alkalinity for the

    same mines. For NNP, all sites with NNP greater than about 12 ppt produced net alkaline

    water. Seven of nine sites with NNP less than 0 produced net acid water, and variable

    results were obtained between NNP 0 and 10 ppt. Twelve of 19 sites (63%) of mines inthis category produced alkaline water.

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    Plot of Net Alkalinity vs Net Neutralization Potential

    -1000

    -800

    -600

    -400

    -200

    0200

    400

    -20 0 20 40 60 80

    Net Neutralization Potential (ppt)

    NetAlkalinity(mg/LCa

    CO3

    Alkaline Water

    Variable

    Acid

    Water

    Figure 3. Plot of Net Neutralization Potential and Mine Drainage Net Alkalinity

    Figure 4 is a plot of the sum of the metals iron, manganese and aluminum in mine

    drainage and overburden neutralization potential. All mines that produced alkaline water

    contain low concentrations of metals, usually less than 0.5 mmoles. Acid waters, howevercontain low (less than 1 mmole) to high concentrations (greater than 5 mmoles) of

    metals, with overall worse water quality. Most of the mines classified in the variable

    category contain less than 1 mmole of metals. Thus highest metal concentrations are inmines with little neutralization potential, while mines with more neutralization potential

    and alkaline waters usually contain the least amount of metals. Even in alkaline waters

    however, metal concentrations may not meet all water quality standards without some

    Plot of Total Metals vs. Neutraliztion Potential

    0.0

    1.0

    2.0

    3.0

    4.0

    5.0

    6.0

    7.0

    0 10 20 30 40 50 60 70 80

    Neutralization Potential (ppt)

    T

    otalMetals(mmole

    Alkaline Water

    Acid

    Water

    Variable

    Figure 4. Plot of Overburden Neutralization Potential and Metals, Iron, Manganese andAluminum.

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    additional treatment. Median water quality values, classified by Acid Base Accounting

    data for pH, alkalinity, metals and sulfate are summarized in Table 2 .

    Table 2Median Water Quality for Mines Classified by Acid Base Accounting Analysis

    (1)

    ABA Data WaterQuality

    pH Alkalinity(mg/L)

    Fe (mg/L) Mn(mg/L)

    SO4(mg/L)

    NP >21Net

    Alkaline7.0 135 2.15 5.1 344

    10

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    FeS2 + 14 Fe3+

    + 8 H2O 15 Fe2+

    + 2 SO2-

    4 + 16 H+

    (7)

    We also determined if active carbonate dissolution is occurring, by examining mineral

    saturation indices for waters with sufficiently detailed analyses. All waters wereundersaturated with respect to calcite; that is calcium carbonate will dissolve.

    Equilibrium calculations determined from PHREEQC (Parkhurst and Appelo,1999) areexpressed as a logarithm of the ratio of ion activity product to equilibrium constant.Values less than zero indicate undersaturation (mineral is expected to dissolve), values of

    zero indicate saturated conditions (equilibrium), while values greater than zero indicate

    oversaturation (mineral could precipitate). The highest saturation index obtained on anywater was -0.13 or about 73% of saturation. Most waters were one or more orders of

    magnitude below saturation. Calcite saturation indices are shown in figure 5.

    Calcite Saturation Indices For Some Mine Waters

    -6

    -5

    -4

    -3

    -2

    -1

    0

    1

    2

    CalciteSaturationInde

    Figure 5. Calcite Saturation Indices for Selected Mine Waters, Pennsylvania Study.

    Kinetic Testing

    Kinetic tests, also called simulated weathering can be useful for estimating mine watercomposition where Acid Base Accounting is inconclusive, or multiple sulfide minerals

    are present in the rock. One advantage of kinetic tests is that they produce an effluent of

    simulated mine drainage quality. The effluent may be tested for the same water qualityparameters that will be applied to the mine. For U.S. mines, these parameters usually

    include pH, acidity, alkalinity, sulfate, iron, manganese and aluminum. Other analysesfor major and trace elements can also be included as needed.

    A limitation of kinetic tests is the interpretation of the results and extrapolation to the

    actual conditions of the proposed mine. There is no single standard test protocol for

    kinetic testing. The results are therefore somewhat dependent on the chosen test method.Some of the physical and chemical variables that influence kinetic testing include:

    particle size distribution of the sample, degree of saturation of the sample (immersed,

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    capillary fringe, unsaturated), solid to liquid ratio, leaching frequency, mineralogy of the

    rock; reaction kinetics and solubility controls on the acidity- and alkalinity-generatingprocesses, and the composition of gaseous phases (e.g. partial pressures of oxygen and

    carbon dioxide). A complete discussion of these factors is beyond the scope of this paper

    but is discussed elsewhere (Hornberger and Brady, 1998; Kleinmann et al, 2000). The

    influence of gas composition is illustrated with a simple example, however.The influence of gas composition on leachate chemistry is shown in Figure 6. The graph

    displays alkalinity and sulfate concentrations for a rock sample containing about 20%

    carbonate. The sample was tested under two conditions, atmospheric (CO2=0.03%), and10% CO2. The increased CO2 concentration was selected to simulate subsurface

    conditions often found in waste rock piles and ground waters. Alkalinity concentration

    under atmospheric conditions quickly drops to about 60 mg/L, representing the maximum

    solubility of calcite under these conditions. Calcite solubility and concentration ofalkalinity increase as partial pressure of CO2 increases (Langmuir, 1997; Appelo and

    Postma, 1992) and alkalinity quickly increases to about 200 to 300 mg/L under 10% CO2.

    Sulfate production from pyrite oxidation is not influenced by carbon dioxide

    concentration. Values are virtually the same for both treatments. For samples containingabundant carbonate, the choice of atmospheric composition during the test could

    influence expected alkalinity production.

    Influence of Atmosphere on Leachate Quality

    0

    100

    200

    300

    400

    500

    600

    700

    800

    900

    1000

    0 2 4 6 8 10 12

    Leaching Cycles

    Concentration(m

    g/

    Alkalinity,10%CO2

    Sulfate,10%CO2

    Alkalinity, air

    Sulfate, air

    Figure 6. Influence of Atmospheric Composition on Leachate Production of Alkalinity

    and Sulfate.

    Kinetic Test Data and Mine Water Case Study

    A coal mine site in West Virginia included a disposal area for waste rock from coal

    cleaning. The waste rock contains abundant pyrite, and little neutralization potential. An

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    Acid Base Accounting test showed the waste rock had a net neutralization potential

    (NNP) of 82.5 ppt. Therefore the waste rock was expected to be acid producing. The

    mining company conducted column leaching tests to estimate the composition of

    drainage from the pile. Five leaching cycles were conducted by passing distilled waterthrough a column packed with waste rock sample. The quantity of water added to the

    column during testing was equal to about one year of precipitation. Results of theleaching cycles and composition of two ground water samples collected at the waste rockpile are shown in table 3.

    Leachate from 5 five cycles of testing produced strongly acid drainage, as expected.Concentrations of all parameters were high in the first cycle, then began to decline, but

    increased again at the end of the test cycle. More leaching cycles are needed to determine

    if water quality would continue to decline or improve. Comparing the leaching data to theactual ground water analyses, concentrations of manganese, sulfate and aluminum are

    similar for the test data and field conditions.

    Iron, and consequently acidity are predicted to be much higher in the leaching columnthan actually exists in the ground water. Iron and acidity are overestimated by a factor of

    about 4 to 10 times actual field conditions, and laboratory pH is one unit or more lower

    than in the ground water. The results can be interpreted in one of two ways. First is toconclude that column leaching test is a more severe chemical weathering environment

    than actually exists on the mine, and that the test will over predict iron and acidity

    concentrations. Second, is to conclude that the ground water has already undergone somein-situ neutralization, reducing acidity and iron levels. The ground water sampling sites

    were located at the pile, where the flow path would only contact the waste rock.

    Therefore it seems unlikely that much in-situ neutralization could have taken place, and ascaling factor is needed to relate the lab and field data. Hood(1984), using a different

    column leaching technique, concluded that his lab results needed a scaling factor of about4.5 to simulate actual mine drainage quality. He also concluded that one cycle of his test

    was equivalent to about 3 years of natural weathering.

    Table 3

    Example Comparison of Kinetic Test Cycles to Actual Mine Water Quality(1)

    Cycle pHSp.

    Cond.Acidity Fe Mn SO4 Al Cu Ni Zn

    1 2.6 4960 2030 564 7.1 1695 68.5 1.21 1.22 2.96

    2 2.9 3750 835 200 6.7 1026 40.4

    3 2.7 3600 994 256 5.37 902 35.8 0.76 0.04 1.744 2.4 4300 1610 438 4.91 1445 41.3

    5 2.5 6460 2492 750 6.83 2240 56.5 0.92 0.04 1.53

    Sample

    GW-13 3.79 2000 431.7 71.7 17.7 1852 35.7 0.01 0.37 0.53

    GW-15 3.69 2900 414.5 48.3 5.16 2603 47.8 0.38 0.82 0.74

    (1) pH in standard units, Specific Conductance in umhsos/cm, acidity in mg/L CaCO3

    Eq., all others in mg/L.

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    Summary

    Mine water quality can be estimated using static tests like Acid-Base Accounting, or

    kinetic test methods. Acid Base Accounting compares the acid producing potential

    against the acid neutralizing potential, to arrive at a net balance for the rock. These dataare interpreted to indicate whether the rock will produce alkaline or acidic drainage. The

    method has been in used for over 25 years at mines in the United States and elsewhere.Test methods are relatively simple and reproducible. Acid Base Accounting does notpredict mine water concentrations of metals or sulfate, however.

    Several studies comparing Acid Base Accounting and postmining water quality havebeen conducted on Appalachian coal mines. These studies have shown that carbonate

    content, or neutralization potential is a very important control on the quality of mine

    drainage. Similar results have been reported for metal mines (Ferguson and Morin 1991).NP contents of as little as 20 to 30 ppt CaCO3equivalent, or 2 to 3 % of the rock mass,

    are effective in producing alkaline drainage.Pyrite must obviously be present for acid

    generation to occur. However, potential acidity of the rock is unrelated to water quality

    parameters, except in the absence of carbonate. By itself, potential acidity is a poorpredictor of mine drainage. Carbonate dissolution consumes (neutralizes) acidity and

    inhibits pyrite oxidation. Alkaline conditions suppress two key components of the acid

    generating process. Bacterial catalysis of ferrous iron oxidation is inhibited and ferriciron activity is also greatly reduced.

    Kinetic tests are useful for samples where Acid Base Accounting data are inconclusive,

    or where more than one sulfide mineral is present in the rock. Relative, if not absolute,mine water composition, and rate of chemical weathering can be estimated from kinetic

    test data. A scaling factor may be needed to relate laboratory and field data, and the factor

    may be site specific. Different kinetic test protocols are used, and the leachate results are

    somewhat dependent on the test method. Production of alkalinity is sensitive tocomposition of the gas phase (Kleinmann et al, 2000; Hornberger et al, 2004) in rocks

    containing significant carbonate.

    Static and kinetic geochemical test methods should be used along with information on

    geology, hydrology, mining method, and reclamation practices to estimate mine waterquality.

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