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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
