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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 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.

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 mass basis, 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, "Acid Neutralizing 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 (neutralization potential) 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 CaCO3 are 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 CaCO3 and "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 CaCO3 produce 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 CaCO3 produce 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 CaCO3 do not produce acid.

Tailings with NNP less than 0 ppt

CaCO3 produce 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's bituminous 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)

   N  e   t   A   l  a   k   l  a   i  n   i   t  y   (  m  g   /

   C  a   C   O   3   E  q   )

 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)

   N  e   t   A   l   k  a   l   i  n   i   t  y   (  m  g   /   L   C  a

   C   O   3

 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

  o   t  a   l   M  e   t  a   l  s   (  m  m  o   l  e

 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 >21 Net

Alkaline7.0 135 2.15 5.1 344

10<NP<21 Variable 6.0 79 1.97 7.1 408

 NP<10 Acid 4.1 0 15.55 19 756

 NNP> 12 Net

Alkaline7.07 194 2.08 4.7 358

0<NNP<12 Variable 6.0 56 1.73 8.3 413

 NNP<0 Acid 4.1 0 61.7 30.1 971

(1) ABA data in ppt, pH in standard units, all others in mg/L

Table 2 shows trends similar to figure 4. The highest median concentrations of metals

and sulfate are from mines containing little neutralizing capability. Sulfate concentrations

are 2 to 3 times greater in acid waters compared to alkaline waters. Iron and manganeseconcentrations are 4 to 7 times greater in acid waters compared to alkaline waters. Thus

the worst water quality can be expected to form on mines lacking adequate neutralizers.

Table 2 also shows that postmining sulfate concentrations decrease as NP increases.

These data are interpreted as showing that pyrite oxidation is inhibited by the presence of

carbonate minerals in amounts as low as 1 to 3 percent (10 to 30 ppt NP).

Carbonates, with the exception of siderite, have at least three different inhibiting effectson pyrite oxidation and acid generation. 1) Carbonates are acid reactive, with their

dissolution rate a function of H+ activity (pH) and the partial pressure of CO2 (Plummer

et.al,1978). As acidity increases, the rate of carbonate dissolution increases. Conversely,

under alkaline conditions, carbonate dissolution slows until equilibrium is reached.

2) Alkaline conditions created by carbonate dissolution are not conducive to bacterial

catalysis of ferrous iron oxidation. Singer and Stumm (1970) showed that the activity of

Thiobacillus species could increase the rate of ferrous to ferric iron conversion by six

orders of magnitude. These bacteria are most active in the pH range of about 2 to 4.Ferrous to ferric iron conversion is the "rate determining step" in the overall sequence of

acid generation from pyrite (Singer and Stumm, 1970). Thus, inhibiting bacterial activityslows pyrite oxidation.

3) Alkaline conditions greatly limit the activity of dissolved ferric iron. Removal of

dissolved ferric iron by alkaline conditions is important since it interrupts the self propagating acid cycle. Dissolved ferric iron is capable of rapidly oxidizing pyrite as

follows:

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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

   C  a   l  c   i   t  e   S  a   t  u  r  a   t   i  o  n   I  n   d  e

 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 quality parameters 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-generating processes, 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

   C  o  n  c  e  n   t  r  a   t   i  o  n   (  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 rock pile 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 not predict mine water concentrations of metals or sulfate, however.

Several studies comparing Acid Base Accounting and postmining water quality have been 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 CaCO3 equivalent, 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 poor predictor 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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