ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - POLLUTION FROM MINE DRAINAGE docx

Water pollution from coal mining was known in medieval England and the mines in Wales were known to make the creeks run red.. This similarity is so great that most of the treatment proce

INTRODUCTION

All forms of mining cause some impact on the aquatic

envi-ronment, just as any other earth disturbance will impact the

local hydrology Sometimes this impact is very adverse, in

which case there is usually a considerable disruption of the

natural life cycles in the affected water Other, and less noted

cases may even improve the local waters Unfortunately, the

adverse impacts greatly outnumber the advantageous

cir-cumstances so that the result of mining is to severely degrade

the aquatic environment

A generalized characterization of the impact of mining

on water is rather difficult, as almost any specific change in

the chemical qualities of the affected waters may be found

at some specific point In almost all cases, however, there is

an increase in the total dissolved solids in the mine

drain-age waters Additionally, the acidity of mine draindrain-age is

increased above normal ground water levels for the area,

and the level of dissolved metal is increased In some areas,

however, the alkalinity levels are increased by mining Many

forms of mining also increase the suspended solids content

of water

Coal mining has received the greatest amount of

atten-tion as the mining which causes water polluatten-tion This is

perhaps deserved as the mining of coal has been a major

operation for many years and more coal has been mined

than any other single mineral Water pollution from coal

mining was known in medieval England and the mines in

Wales were known to make the creeks run red This fact

was important to the exploration of the North American

continent, as early explorers deduced the presence of large

deposits of coal from the natural color and character of some

of the streams and creeks Similar conditions were

some-times noted in relation to other mineral deposits in the US

As the outcrop materials come into contact with the

atmo-spheric conditions, oxidation and solubilization take place

and the products are transported into the streams Hence,

the natural production of some mine drainage is a natural

phenomena which has existed almost from the beginning

of time

Coal mine drainage may vary from waters pure enough

to drink without treatment to waters containing more than

20,000 mg/l acidity with commensurate amounts of iron and

other dissolved solids Drainage from metal mines may vary

over almost equally wide ranges of acidity but often

con-tain substantial amounts of dissolved heavy metals In most

respects the acid drainages from metal mines are similar

to acid coal mine drainages This similarity is so great that most of the treatment processes and prevention mechanisms developed and applicable to coal mine drainage can also be applied to metal mine drainage

ORIGIN OF ACID MINE DRAINAGE

The earth strata associated with and superjacent to coal and many other minerals almost always contain the iron sul-fide mineral pyrite (FeS 2 ) Oxidation of the acidforming pyritic material associated with mining is necessary for the formation of mine acids and as oxygen is a necessary part

of the oxidation of these materials, there can be no signifi-cant amount of oxidation until these are exposed to air The process of mining greatly increases the exposure of these materials to atmospheric oxidation Oxidation of the sulfide mineral begins as soon as it is exposed to the air and contin-ues at a rate characteristic of the geologic and atmospheric conditions Usually this oxidation causes spalling of the mineral substance with a progressive increase in the amount

of surface area available for oxidation Time then becomes a significant factor in the amount and rate of formation of acid mine drainage

The exact nature of the pyritic mineral which oxidizes

so rapidly and causes the acid drainage from mining has been sought for many years In appearance, the mineral

is often grey like marcasite, and its oxidation rate is even more rapid than that of marcasite X-ray diffraction stud-ies of the sulfide material associated with coal, however, have confirmed by the crystal structure that the material

is pyrite rather than some other crystalline form of iron sulfide

The intricate mechanism of oxidation of this pyrite and the formation of acid drainage has been the subject of many learned discussions and research efforts extending back over fifty years or so The reaction will occur at normal room temperature and humidity conditions with the release of SO 2 into the atmosphere Under more humid conditions, the reac-tion results in all of the sulfur being converted into sulfate

Typical reactions depicting some of the several paths pos-sible for the reaction of the iron sulfide, air, water and alkali materials are listed

This list is typical rather than all-inclusive to cover all possible reaction routes

4 2

⫹

→

→

Fe (SO )

4 2 2 4 3 2

2 4

⫹ ⫹

4

→ F

2 2 2 4 3

2

2 2 2 4 4

O

→

→

There has been a continuing debate among the scientific

community over the role which bacteria may play in the

for-mation of acid mine damage Bacteria of the Ferrobacillus

ferroxidans family are almost always found in large pools of

acid mine drainage The bacteria have been shown to have

the capability of oxidizing ferrous iron to the ferric state

However, they thrive in a very limited pH range

(approxi-mately pH 3.5) and there is no evidence to indicate that they

contribute directly to the primary oxidation of the pyretic

substance Ferric ion can oxidize sulfide sulfur and this could

possibly provide a mechanism for the bacteria to increase

the rate of oxidation of pyrite in some circumstances Other

recent studies have shown that the transfer of oxygen from

the atmosphere to the pyrite surface is the rate limiting

factor of the reaction, making moot arguments put forth as

to whether bacterial or chemical oxidation is the principal

mechanism of acid drainage formation

Bacteria may play a significant role in the oxidation of

the ferrous to ferric ion in mine drainage This fact can be of

considerable importance in the treatment of mine drainage

as ferrous iron tends to retard the rate of neutralization

reac-tions It almost appears that this is Nature’s first step in the

neutralization of acid drainage The bacterial oxidization of

iron allows the alkalinity of the associated earth strata and

of diluting waters to be more readily reacted with the

acid-ity of the mine drainage waters This concept is rather

radi-cally at variance with former concepts that considered the

sterilization of a mine as a possible method of reducing or

eliminating the formation of acid drainage Bacteria has been

used deliberately as a step in the treatment of mine

drain-age This process allows the ferrous iron to be oxidized and

accelerate the neutralization of the acid salts

The formation of acid drainages from surface and

under-ground mines is essentially the same process, and the two

drainages are indistinguishable on the basis of the

chemi-cal qualities of the waters In general the iron contained in

drainage from surface mines and also from coal refuse piles

is in the ferric state The drainage from surface mining may

contain very substantial amounts of suspended solids or

sedi-ment Because many of the drainage problems of strip and

deep mines are directly interrelated there is almost no rational

way of separating the treatment or preventive measures which may be applied to the two types of mining

EXTENT OF ENVIRONMENTAL IMPACT

The problem of environmental degradation caused by mine drainage is widespread and serious Some form of mining occurs in each of the fifty states and many states are exten-sively mined The aquatic degradation caused by coal mining

in the eastern and Appalachian region is best known and has been best documented For this reason, most statements of the damages caused by mine drainage cite only the degra-dation in the Appalachian area The Appalachian Regional Commission reported that some 10,500 miles of streams in that region are affected by mine drainage and acid drainage continually pollutes nearly 5700 stream miles Since data are not available in many mining areas, particularly in the Rocky Mountain and western areas, firm total statements of the extent of the problem cannot be made However, enough information is available to indicate that it is very substantial

The extent and amount of degradation which may be caused by the presence of mine drainage, and the environ-mental and economic impacts which may be felt, vary, of course, with the type of mine, the strata surrounding it, and other localized conditions

For example, coal mine drainage, which is usually acidic, kills fish and other forms of aquatic biota by lowering the pH

of the water and also may have an adverse economic effect upon the human population The acidity accelerates the cor-rosion of bridges, culverts, boats and navigational facilities, making replacement at shorter intervals necessary High cost water treatment may be required to make the local water supply potable or suitable for industrial use Water contact sports may not be possible in the area, causing a loss of potential tourist industry revenues

Although metal mining may cause the same adverse effects, it usually occurs in less densely populated regions

Additionally, metal mine drainage may render the waters toxic to humans as well as aquatic life by the presence of heavy metals

REMEDIES FOR MINE DRAINAGE POLLUTION

In the past a substantial number of investigators have simply discussed the problem and observed its extent so that more is known about the nature of the problem than about the mech-anisms which may be applicable to correcting or mitigating

it Mine drainage may be characterized on the basis of its source and possible remedies considered by this categoriza-tion even though the chemical nature and biological impact

of the drainage from the several sources is identical For the purposes of this discussion let us consider three types of mine drainage by the source: drainage from active or operat-ing mines, drainage from non-operatoperat-ing (sometimes called abandoned) mines and drainage which will be generated in the future from mines which have not yet begun operation

Separating surface and underground mines is not feasible

as they frequently occur together and interact to add to the

problem

Presently operating mines have certain characteristics

which differentiate mines them from other mines Primarily

because they are now in operation a responsible owner or

operator can be located and people, equipment, machinery

and power are available at the mine site This allows

con-sideration of procedures to treat the mine drainage as well

as procedures to reduce or minimize the amount of

pollut-ants discharged both during the remainder of the mine’s

life and after mining has been terminated Procedures

pres-ently available may be employed to minimize the amount

of mine drainage pollution which issues from an

operat-ing mine, but no procedures are now available which can

totally eliminate it

Non-operating, or abandoned mines, generally do not

have any responsible person readily available, or any other

resources such as personnel and machinery, which makes

abatement techniques more expensive for this type of mine

than for one which is operating

When a mine is still in the planning stage, it is easier

to plan for future prevention of pollution and thereby reduce

it For instance, provisions can be pre-planned for a mine to

have rapid and complete drainage during the mining

opera-tion, thereby reducing the pollutional discharges which

other-wise may need to be treated Additionally, improved mining

methods can provide for minimum void spaces after mining;

also water level control after mining ceases can be provided only during the mine pre-planning stages

The methods for alleviating mine drainage may be divided into the two basic categories of treatment of mine drainage, and prevention of the formation of discharge of pollutants

Treatment removes pollutants by physical/chemical means and generally results in only specific pollutants being removed or reduced The process must continue for as long

as the source produces pollution and usually in the case of mine damage this is tantamount to “treatment in perpetuity.”

Disposal of wastes and constant usage of power or chemicals made such treatment unduly consumptive of both human and physical resources Treatment methods for coal mine drain-age are summarized in Table 1 and many of the methods are applicable to other types of mine drainage pollution

Prevention is the total cessation or massive reduction of the formation or discharge of pollutants during and after the operating life of the mine Prevention of all forms of pollution, including dissolved and suspended materials, is obviously more desirable than simple treatment of pollution with its attendant problems Provisions for prevention of the formation of pollu-tion cab be planned into a mine while it is still on the drawing board Specific techniques for prevention and correction of pollution from mines, both operating and non-operating, are shown in Table 2

TABLE 1 Summary of treatment techniques

R—regenerate.

TABLE 2 Cost and effectiveness of various at-source prevention and corrective techniques

structures Prevents acid formation, erosion control, runoff of dissolved soils Cost and effectiveness controlling factors are nature

of surface and overburden, slope of land, and proposed use of land.

cost of $5000–100,000/seal may be required to control entrance of air through subsidence holes, boreholes, outcrop, etc.

Sealing of an underground mine to prevent entrance of air Prevents acid formation Cost of effectiveness depend on the ability to locate and seal all air paths to the mine, type and condition of mine operating and type of seal Most mines cannot be airsealed.

Additional costs of $5000–

20,000 may be required to control drainage through bore holes, outcrop, etc.

Sealing of an underground mine to completely and permanently flood the working Prevents acid formation and sometimes all discharges

Cost and effectiveness depend on the ability to seal all discharges, size of mine, dip of seam, outcrop condition, condition of mine opening, type of seal, and amount of grouting required Is not applicable to all mines.

siltation and flushing of pollutants Cost and effectiveness depend on ability to divert as much water as possible in properly designed structures.

effectiveness depend on complete and permanent flooding of the material responsible for acid production.

Refuse pile reclamation

(reject material from

mining and processing)

25–75 $1000–3000/acre Stabilizing a refuse pile with soil, chemicals, vegetation, etc Prevents

acid production and siltation Cost and effectiveness depend upon the availability of the land in which the refuse piles can be filled and also upon the availability of impervious materials such as clay, fly ash, or limestone for compaction over the surface of the filled area.

Reject tailing pond (reject

material from mining

and processing in slurry

form)

25–95 $300–2000/acre Stabilizing of tailings by flooding, soil covering, chemicals, vegetation,

etc Prevents air pollution and discharge of suspended and dissolved solids Cost and effectiveness depend upon the size, location.

piles, and pond Prevents erosion Cost and effectiveness depend on the type of cover, soil conditioning, and thickness of cover required.

Controlled pumping and

drainage

25–75 $0.190.23/1000 gallon Involves rapid removal of water from a mine before it gets

contaminated or discharge of contaminated water at regulated rate so that dilution provides minimum contamination effects Cost and effectiveness depend upon the characteristics of the material in the mine, contact time between water and exposed materials, rate of pumping, pumping head, and amount of dilution water available.

Inert gas blanket Under research and development Filling of an underground mine with an inert gas to prevent acid

production Control of the bacteria in a mining environment

Prevents acid production Technique does not show merit at this time.

Internal sealing Under research and development Internal sealing of underground mine to prevent acid production and/or

mine drainage discharge.

roof collapses behind the working face, eliminating “breathing” of the mine.

left before Surface must be reclaimed.

It can be seen from Table 2 and Figure 1 that the cost of

creating or preventing the formation of mine drainage is a

wide ranging variable, dependent on the method(s) selected

for use The estimated costs for the limestone neutralization

treatment of mine drainage are shown in Figure 1 as a

func-tion of acidity and quantity to be treated

As can be seen in the figures, the number of methods of

alleviation and control of mine drainage which are highly

effective are few, and most treatment processes produce

undesirable by-products, as well as being expensive over the

long run It would be highly desirable to lessen the sources

of mine drainage and to be able to treat it effectively with a

gain in desirable by-products

New techniques for the prevention of pollution from

mines are presently in the development and demonstration

stage For example, a new technique known as

“daylight-ing” is being demonstrated This procedure will use strip

mining techniques to remove the residual coal from

shal-low, non-operating mines and consolidate the overburden to

prevent the continued discharge of acid drainage A variety

of abatement techniques related to surface mine

reclama-tion are being demonstrated in the Appalachia region Also

there are two major efforts directed toward the development

of non-pollutional mining techniques These are the mining

of coal under oxygen free conditions within the mine to

$1.00 90 80 70 60 50 40

.30

.20

.10 09 08 07 06

ESTIMATED LIME NEUTRALIZATION TREATMENT COST - COAL MINE DRAINAGE

O.I MGD

1 MGD 2–4 MGD 6–7 MGD

FIGURE 1 Estimated costs for treatment of coal mine drainage waters based upon a composite

of published laboratory, pilot plant, and actual plant data The estimate for the 0.1 MGD plant is preliminary and based upon limited information.

prevent the oxidation of pyrite and the formation of acid drainage and the application of a new mining technique called “longwall stripping.” Longwall stripping will apply longwall mining techniques to shallow cover coals, which are now strip mined, to remove the coal without inverting or dismantling the overlying earth strata

LITERATURE REFERENCES

Most discussions of this type are buttressed by an impressive listing of reference documents citing sources of the many facts contained therein To the casual reader, these references are useless, except for their creation of an impression of author-ity The serious worker will demand to have even more refer-ences and documentation The field of coal mine drainage is somewhat unique in that the literature of the field is regularly collected, abstracted and these abstracts published This pub-lication, entitled “Mine Drainage Abstracts, a Bibliography”

is prepared by the Bituminous Coal Research, Inc for the Commonwealth of Pennsylvania Copies may be purchased from B.C.R., Monroeville, Pennsylvania A listing of the reports of the research in this field may be obtained from the Publications Branch, Office of Research and Monitoring, Environmental Protection Agency, Washington, DC 20460

REFERENCES

1 Ramsey, D.L and D.G Brannon, Predicted Acid Mine Drainage

Impacts to the Buckhannon River, West Virginia, Water, Air and Soil

Pollution, 39, 1, 1988

2 Sobek, A.A., M.A Bambenck, and D Meyer, Soil Sci Soc Am J., 46,

1982

3 Sullivan, P.J., S.V Mattigo, and A.A Sobek, Dissociation of Iron

Sulfates from Pyritic Coal Wastes, Environ Sci Tech., 20, 10, 1986

ERNST P HALL

Environmental Protection Agency

POLLUTION LAW: see ENVIRONMENTAL LAW

POLLUTION METEOROLOGY: see AIR POLLUTION METEOROLOGY

POLLUTION OF GROUNDWATER: see GROUND WATER RESOURCE

PRIMARY TERRESTRIAL CONSUMERS: see ECOLOGY OF PRIMARY

TERRESTRIAL CONSUMERS

PROTECTION OF THE ENVIRONMENT: see ENVIRONMENTAL LAW