workshop paper geology & mine water quality part 1

8/12/2019 Workshop Paper Geology & Mine Water Quality Part 1

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Geologic Clues Useful in Predicting Coal Mine Water Quality:

A Perspective from Pennsylvania, USA

Keith B.C. Brady

Department of Environmental Protection

P.O. Box 8461Harrisburg, Pennsylvania USA 17105

Introduction

Coal mining in the state of Pennsylvania fueled the United States of Americas

Industrial Revolution during the second half of the 1800s and first half of the 1900s.About 15 billion metric tons of coal has been extracted from Pennsylvania. Most of this

production occurred before there were environmental regulations. The legacy of

unregulated mining left Pennsylvania with 4,000 kms of polluted streams (out of 87,000total kms) and more than 1,000 km

2of abandoned surface mines with dangerous

highwalls and water filled pits. Lawmakers realized that if things continued as they were,the economic future of Pennsylvania would be in jeopardy. The experience presented in

this paper is derived from more than a century of doing things improperly.

Papers published as early as the 1920s recognized that not all coal mines

produced pollution. Since there were no regulations preventing pollution little effort wasspent in figuring out how to predict post-mining water quality or how to treat pollution

discharges. Pollution from coal mines was made illegal by laws implemented in the

1960s and 1970s. These resulted in a need to be able to predict post-mining waterquality.

The development of static and kinetic tests (discussed by Eric Perry in a

companion paper) was one consequence. It was also recognized that understanding the

geology of a mine can provide valuable clues to predict water quality. This paper looksat these geologic indicators. Many of the geologic clues to post-mining water quality

seen in Pennsylvania are probably applicable in other parts of the world, but some

undoubtedly are not. Each coalfield will yield its own set of rules that will help in mine

drainage prediction. This knowledge is gained through experience. The examples sharedhere are provided to illustrate the types of indicators that may be useful in other

coalfields.

Mineralogy

Surface coal mining accelerates weathering by exposing fresh rock surfaces to airand water, and increasing the surface area of rocks through breakage. The weathering of

two minerals groups, sulfides and carbonates, dominate coal mine water quality. Pyrite,

which is stable in a reduced premining environment, upon exposure to oxygen willdecompose into iron, sulfate and acid (Equations 1 and 2):

FeS2(s) + 3.5 O2+ H2O = Fe2+

+ 2 SO42-

+ 2 H+ (1)


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The overall reaction, including the oxidation of iron from Fe2+

to Fe3+

is:

FeS2(s) + 3.75 O2+ 3.5 H2O =Fe(OH)3+ 2 SO42-

+ 4 H+ (2)

The H

+

ions are acidity and the Fe(OH)3is iron hydroxide precipitate that coats thesubstrate of polluted streams.

Calcareous carbonate minerals (such as calcite and dolomite) neutralize acid(Equation 3) and can create alkaline conditions under which many metals have low

solubility.

CaCO3+ H+= Ca2++ HCO3

- (3)

The sulfide and carbonate mineral groups need only be present in an abundance of

a few percent or less to dominate post-mining water quality. By comparison the other

95% or so of coal overburden minerals play a minor role. The single most importantfactor as to whether or not a site will produce alkaline or acidic drainage is not the lack of

pyrite, but the abundance of calcareous minerals. Pyrite, although necessary for acidformation, is of secondary importance, because if sufficient carbonates are present

postmining water will be alkaline. Therefore, an understanding of the geologic processes

that determine the amount of pyrite and carbonates that will occur on a coal mine site canhelp with predicting the type of water that will result from a mine and provide insights

into how to prevent problems from occurring.

There are three types of sulfur that occur in coal-bearing sediments: sulfide (pyriteand marcasite), sulfate (solid pyrite weathering products), and organic (organically bound

sulfur). Pyrite is created under reducing conditions. Coal-bearing rocks are often

favorable to pyrite formation because organic matter, which is typically abundant in coal-bearing rocks, can act as an oxygen sink. The sulfate minerals present on coal mines are

often salts that form from the weathering products of pyrite oxidation. A simple example

is the creation of melanterite from an evaporating mine water (Equation 4).

Fe2+

+ SO42+

+ 7 H2O = FeSO47H2O (4)

These secondary minerals tend to be very soluble and when they dissolve the iron canhydrolyze and produce acid:

Fe2+

+ 2.5 H2O + 0.25 O2== Fe(OH)3+ 2 H+ (5)

Equations 4 and 5 show that sulfate sulfur should not be discounted as an acid producer

unless it can be demonstrated that the sulfate minerals that are present will not produceacid. An example of a non-acid-generating sulfate mineral would be gypsum

(CaSO4H2O). Organic sulfur is generally presumed to produce little or no acid.


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The acid generated from pyrite oxidation can attack other minerals, which release

the ions such as aluminum, manganese and magnesium. If these ions hydrolyze they toocan create acidity:

Al3+

+ 3 H2O == Al(OH)3+ 3 H+ (6)

The most reactive carbonates in terms of their ability to neutralize acid are calcite

and dolomite. Calcite and dolomite can be the main ingredient in a rock (such as

limestone and dolostone) or be present to a lesser extent as clasts or cement. Calcite anddolomite not only neutralize acidity, but as mentioned earlier, many metals have low

solubility under alkaline conditions. Alkaline conditions can also inhibit pyrite oxidation

by eliminating ferric iron from solution (which otherwise can oxidize more pyrite) and bycreating conditions that are hostile to pyrite-oxidizing bacteria. Siderite (FeCO3), a

common mineral in many coal fields, does not generate alkalinity.

Geologic Processes in the Appalachian Basin of the Eastern US

The presence or absence of calcareous minerals and pyrite is a function of

geologic processes that operated at the time of coal deposition and the more recentgeologic process of near-surface weathering. The chemistry of coal and the surrounding

rocks in Pennsylvania has been related to influences from the depositional environment

and climate during the Pennsylvanian (Upper Carboniferous) Period, the of geologic timein which the coals were deposited. During the Pennsylvanian Period, the state of

Pennsylvania was located essentially at the same latitude as that of Indonesia today

(Edmunds, et al., 1979) (Figure 1) and had a tropical environment (Cecil et al., 1985).Figure 2 shows the entire Appalachian Basin. The rocks of the central and southern

Appalachians are, in general, slightly older than the rocks of the northern Appalachians.The differing geology of the different regions influences water quality. This is discussed

below.

Cecil et al. (1985) have suggested the differences in the sulfur and ash content of

Appalachian Basin coals is related to seasonality of rainfall. When the climate was

ever-wet the peat formed domed deposits, which became low-sulfur coal. When the

climate had seasonal rain the peat formed plainer (flat) deposits, which became highersulfur coal. The sulfur and ash concentrations of these coals are illustrated in Figure 3.

The Lower and Middle Pennsylvanian low-sulfur coals Cecil proposes were formed in

domed swamps, whereas the high sulfur coals of the Upper Pennsylvanian were formedin plainer swamps. A modern analogue for the domed swamps are the peat deposits on

eastern Sumatra (Cecil, et al., 1993; Zeuzil et al., 1993, and Supardi et al., 1993).


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Figure 1. Paleogeographic map showing Pennsylvania during the Pennsylvanian (Upper Carboniferous)

Period. Modified from Edmunds, et al., 1979.

Figure 2. The Appalachian Coal Basin of the eastern United States. The line separating the Central

region from the Northern region is called the hinge line and separates the high sulfur coals (>2% S) ofthe northern region from the low sulfur coal of the southern regions (


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Figure 3. Stratigraphic variation of sulfur and ash content of 34 coal beds of the central and northern

Appalachians, including Pennsylvania. From Cecil et al., 1985.

The differing geology of the northern and central/southern Appalachians is also

evident in the post-mining water quality produced by coal mines. The southern

Appalachian rocks, in addition to having low concentrations of pyrite, also evidently haveenough calcareous minerals to produce alkaline drainage. The northern Appalachian

rocks, by contrast have a wide spread of pyrite concentrations and variable concentrations

of calcareous minerals. Figure 4 shows that more streams in the northern Appalachianshave been impacted by low pH and high sulfate mine drainage than in the southern

Appalachians. This reflects the more common acid mine drainage problems in thenorthern Appalachians.


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Figure 4. Percentage of surface water sample stations that had pH 75 mg/L by

watersheds in the Appalachian Basin (data from Wetzel and Hoffman, 1983).

Paleoclimate influenced the amount of pyrite in the peat and likely in theoverlying rock. However, depositional environment has a major influence on what rocks

get deposited above the coal (Figure 5). The mineralogy of these overlying rocks is

generally what influences the post-mining water quality. Rocks deposited in marine

environments often have higher sulfur and higher neutralization potential. Brackishenvironments generally produce high sulfur with no calcareous minerals (although

siderite can be present). High sulfur occurs in marine and brackish environments becauseof abundant sulfate ions in marine waters. This sulfate served as the source of pyritic

sulfur. Marginally brackish environments are generally lower in sulfur and have little

neutralization potential. Freshwater (terrestrial) deposits generally have lower sulfur andare often calcareous (Figure 6) (Brady et al., 1998). These influences are reflected in the

water quality. Figure 7 shows the average sulfur in coal for three differentpaleoenvironments. Studies of the shale above these coals show a similar trend. Water

associated with the brackish environments is generally very acidic with pH often in the

range of 2.5 to 3.5. This is due to a combination of high sulfur shales overlying the coal

and a lack of neutralizing minerals. Mines in the area of marine rocks range from acidicto alkaline, depending on the abundance (or absence) of calcareous minerals. Mines in

the marginally brackish area tend to be slightly acidic with pH between 4.0 and 5.5.

Another geologic factor that can affect water quality is the percentage of

sandstone in the overburden. Figure 8 shows the post-mining net alkalinity (alkalinity

acidity) for a coal seam in northern West Virginia. Where sandstone comprises less than50% of the overburden the water is typically alkaline. Where it is greater than 50% of the


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overburden the water is almost always acidic. We have found this relationship to be true

for mines in the Allegheny Group coals, but sandstones in some of the other coal-bearingformations are often calcareous and those mines will produce alkaline drainage.

Figure 5. Reconstruction of the depositional environments of the rock above the Pittsburgh coal, UpperPennsylvanian age. Freshwater limestone was deposited in the Sea-Lake, calcareous shale in the Lakesand Mud Flats, shale, carbonaceous shale and coal in the Lakes and Swamps, sandstone and siltstone in

the Subdeltas, and sandstone in the Alluvial Plain. Figure from Donaldson and Shumaker (1979).

Figure 6. Schematic summary of paleodepositional environmental controls on acid mine drainage

production in the northern Appalachians. Modified from Hornberger (1985).


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Figure 7. Paleodepositional environment map of the Lower Kittanning coal seam in western Pennsylvania.

Note increase in sulfur as a function of salinity. Low salinity water would contain less sulfate than highsalinity water. Modified from Brady et al., 1998.

Percent Sandstone

Figure 8. This figure shows the effect of percent sandstone on post-mining water quality for a coal seam inthe Upper Allegheny Formation. Where sandstone was less than 50% of the overburden the water was

generally alkaline. Where the sandstone was greater than 50% the water was almost always acidic. Net

alkalinity is alkalinity minus acidity. Negative numbers indicate acidic water, positive numbers indicate

alkaline water. Data from diPretoro (1985).

Surface weathering of rocks removes pyrite by oxidation and removes carbonatesby dissolution. The relative depth of removal of these two mineral groups can be an


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important factor in determining the water quality potential of a mine site. Weathering

depth is influenced by lithology, orientation (dip) of strata, time of exposure, and climate.Strata that are near-vertical will weather deeper than horizontal strata, all other things

being equal. Figure 9 shows the effect of weathering on pyrite (measured as sulfur) and

calcite (measured as calcium) in coal overburden from West Virginia and Pennsylvania.

A sharp chemical demarcation between weathered and unweathered rock is common. Inthe northern Appalachians the rock has typically lost pyrite and carbonates to a depth of

~5 m. This weathered material is essentially inert and will not produce water with low

dissolved solids.

Figure 9. Depth of surface weathering and its effects on pyrite (measured as sulfur) and calcite (measured

as calcium) concentrations. Figure 9A is of four drill holes penetrating a sandstone overburden in northernWest Virginia (Singh et al., 1982). Figure 9B is the percent sulfur of a shale just above a coal, as measured

in 12 drill cores (each symbol represents the value from a different drill hole). Site B is in southwesternPennsylvania (Brady et al., 1998).

The geologic controls on coal overburden that have and are operating in

Pennsylvania have resulted in overburden that can contain a wide range of concentrationsof pyrite and calcareous minerals. Mine drainage quality reflects this variable

mineralogy; water can be significantly alkaline to severely acidic (Figure 10). In fact, the

pH of coal mine drainage is bimodal, reflecting the dominent influence of pyrite andcalcite. Pyrite controls the low pH mode and calcite controls the circum-neutral pH

mode.

Within Pennsylvanias Bituminous Coal Field, mine drainage problems differ by

stratigraphic horizon and geographic region. The stratigraphy of the Pennsylvanian strataof western Pennsylvania is shown in Figure 11. Acid drainage problems are more

significant in the lower Allegheny Group than in higher strata. As seen in Figure 11, the

lower Allegheny has abundant marine units and often contain abundant pyrite. Figure 10

shows the distribution of pH of coal mine drainage by stratigraphic horizon. Note thatthe Pottsville Formation coals are generally acid producing. The Upper Allegheny coals

typically produce alkaline drainage.


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Regional variations are often due to paleodepositional environment. For example,lower Allegheny overburden deposited in marine, brackish-marine, to marginally-

brackish environments, will differ in the amount of pyrite and the amount and type of

carbonates present. Alkalinity-producing mines are associated with the thick freshwater

limestone sequences of the Monongahela Group in the southwestern corner ofPennsylvania. The thick marine Vanport limestone of the lower Allegheny Group in the

central to northwestern area of the bituminous region, and the freshwater limestones and

calcareous rocks of the upper Allegheny Group. The highest acidity concentrations areassociated with overburden where black brackish shales and thick marine shales are

predominate in the overburden, in areas where carbonate strata are absent or lack

appreciable thickness, and where non-calcareous sandstone predominates. The widerange of pH and the range in alkalinity and acidity concentrations for each stratigraphic

group documents that some strata within each group has the potential to produce alkaline

and acidic drainage.

The effects of marine waters influence on the rocks composition is shown inFigure 12. The example shown is from the Illinois Basin of the central United States. In

areas where the freshwater Energy Shale overlies the coal and is more than 5 m thick,the coal has


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Figure 11. Pennsylvanian stratigraphy of

western Pennsylvania. The Pottsville formation,

which lies below the Allegheny is not shown.Economic coals are typically not found in the

Pottsville Formation. Units followed by M are

marine units. Other units are presumedfreshwater. This figure was constructed by the

geology department of the University ofPittsburgh.


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Figure 12. An example of how marine waters can influence the chemistry of a rock (in this case coal).

Where the freshwater Energy Shale is thick, it protected the coal seam from marine water influence and

the sulfur in the coal was comparatively low. Where the Energy Shale was absent the sulfur in the coal is

much higher. The graph on the right shows the relationship between coal quality and proximity to marinedeposits. Figure on left is from Nelson (1983) and figure on right is from Gluskoter and Hopkins (1970).

A common rule of thumb that is probably universal is that the highest sulfur coaloccurs at the top and bottom of a coal seam. Figure 13 is an example of the sulfur

distribution in a coal seam in Pennsylvania. The importance of this fact is that this is thecoal that is most likely to be left behind during mining. This discarded high sulfur coal

can contribute to acid mine drainage problems.

Figure 13. Cross-section of total sulfur in the Pittsburgh coal of Pennsylvania. Note that the highest sulfur

is at the top and bottom of the coal seam. From Donaldson et al., 1979.


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Conclusions

Geologic information is essential for determining the economic feasibility of a

proposed mine site and is routinely obtained. A careful look at the types of rocks and the

minerals contained therein can provide information as to the acid or alkaline producing

potential of strata. When the distribution of these materials is known and the amount ofthe various acidity and alkalinity generating rocks are known, this knowledge can be used

to develop site-specific pollution prevention techniques such as material handling and

calculation of alkaline addition rates. The precise clues to acid generation may differfrom region to region, but clues should nonetheless exist.

References

Brady, K.B.C., R.J. Hornberger and G. Fleeger, 1998. Influence of geology on postmining water quality:

Northern Appalachian Basin. In: Coal Mine Drainage Prediction and Pollution Prevention in

Pennsylvania. Pennsylvania Department of Environmental Protection, Harrisburg, PA, pp. 8.1 to

8.92.

Cecil, C.B., R.W. Stanton, S.G. Neuzil, F.T. DuLong, and B.S. Pierce, 1985. Paleoclimate controls on Late

Paleozoic sedimentation and peat formation in the central Appalachian Basin. Intern. J. of CoalGeol., v. 5, pp. 195-230

Cecil, C.B., F.T. Dulong, J.C. Cobb, and Supardi, 1993. Allogenic and autogenic controls on

sedimentation in the Central Sumatra Basin as an analogue for Pennsylvanian coal-bearing strata.

In: Modern and Ancient Coal-Forming Environments, Cobb, J.C. and Cecil, C.B. Editors.Geological Society of America Special Paper 286, p. 3-22.

diPretoro, R.S., 1986. Premining prediction of acid drainage potential for surface coal mines in northern

West Virginia. M.S. Thesis, WV Univ., 217 p.

Donaldson, A.C., J.J. Renton, R. Kimutis, D. Linger, and M. Zaidi, 1979. Distribution pattern of totalsulfur content in the Pittsburgh coal. Carboniferous Coal Guidebook, A. Donaldson, M. Presley

and J.J. Renton, eds., WV Geol. and Econ. Survey Bull. B-37-3, pp. 143-181.

Donaldson, A.C. and R.C. Shumaker, 1979. Late Paleozoic molasse of Central Appalachians. In:

Carboniferous Coal Guidebook, WV Geol. And Econ Survey Bull. B-37-3 Supplement, pp. 1-42.

Edmunds, W.E., T.M. Berg, W.D. Sevon, R.C. Piotrowski, L. Heyman and L.V. Rickard, 1979. The

Mississippian and Pennsylvanian (Carboniferous) Systems in the United States - Pennsylvania and

New York. USGS Professional Paper 1110-B, pp. B1-B-33.

Gluskoter, H.J. and M.E. Hopkins, 1970. Distribution of sulfur in Illinois coals. In: DepositionalEnvironments in Parts of the Carbondale Formation - Western and Northern Illinois. Illinois State

Geological Survey Guidebook 8, pp. 89-95.

Hornberger, R.J., 1985. Delineation of Acid Mine Drainage Potential of Coal-Bearing Strata of thePottsville and Allegheny Groups in Western Pennsylvania. M.S. Thesis, Penn State Univ., 558 p.

Nelson, W.J., 1983. Geologic disturbances in Illinois coal seams. Illinois State Geol. Survey Circular 530,

47 p.


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Neuzil, S.G., Supardi, C.B. Cecil, J.S. Kane and K. Soedjono, 1993. Inorganic geochemistry of domed peat

in Indonesia and its implication for the origin of mineral matter in coal. In: Modern and Ancient

Coal-Forming Environments, Cobb, J.C. and Cecil, C.B. Editors. Geological Society of America

Special Paper 286, p. 23-44.

Singh, R.N., W.E. Grube, Jr., R.M. Smith and R.F. Keefer, 1982. Relation of pyritic sandstone weathering

to soil and mine spoil reclamation. In: Acid Sulfate Weathering. Soil Science Soc. of America

Special Publication No. 10, pp. 193-208.

Supardi, A.D. Subekty, and S.G. Neuzil, 1993. General geology and peat resources of the Siak Kanan and

Bengkalis Island peat deposits, Sumatra, Indonesia. In: Modern and Ancient Coal-Forming

Environments, Cobb, J.C. and Cecil, C.B. Editors. Geological Society of America Special Paper

286, p. 45-62.

Wetzel, K.L. and S.A. Hoffman, 1983. Summary of surface-water-quality data, Eastern Coal Province,

October 1978 to September 1982. US Geol. Survey Open-File Report 83-940, 67 p.

workshop paper geology & mine water quality part 1

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