A HANDBOOK OF
CONSTRUCTED WETLANDS
a guide to creating wetlands for:
AGRICULTURAL WASTEWATER
DOMESTIC WASTEWATER
COAL MINE DRAINAGE
STORMWATER
in the Mid-Atlantic Region
Volume
COAL MINE DRAINAGE
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ACKNOWLEDGMENTS
Many people contributed to this Handbook. An Interagency Core Group provided the initial impetus for the Handbook, and later provided "
guidance and technical input during its preparation. The Core Group comprised:
Carl DuPoldt. USDA - NRCS. Chester. PA
Robert Edwards, Susquehanna River Basin Commission.
Harrisburg, PA
Lamonte Garber, Chesapeake Bay Foundation. Harrisburg, PA
Barry Isaacs, USDA - NRCS, Harrisburg, PA
Jeffrey Lapp, EPA, Philadelphia, PA
Timothy Murphy, USDA - NRCS, Harrisburg, PA
Glenn Rider, Pennsylvania Department of Environmental
Resources, Harrisburg. PA
Melanie Sayers, Pennsylvania Department of Agriculture, Harrisburg, PA
Fred Suffian, USDA - NRCS, Philadelphia, PA
Charles Takita, Susquehanna River Basin Commission, Harrisbur°, PA
Harold Webster, Penn State University, DuBois, PA.
C°ntributed by pr°vidin§ informa«™ and by reviewing and commenting on the Handbook. These
Robert Bastian, EPA .Washington, DC
William Boyd, USDA - NRCS, Lincoln, NE
Robert Brooks, Penn State University,
University Park, PA
Donald Brown. EPA, Cincinnati, OH
Dana Chapman, USDA - NRCS, Auburn, NY
Tracy Davenport, USDA - NRCS, Annapolis
MD
Paul DuBowy, Texas A & M University,
College Station, TX
Michelle Girts, CH2M HILL, Portland, OR
Robert Hedin, Hedin Environmental,
Sewickley, PA
William Hellier, Pennsylvania Department of
Environmental Resources, Hawk Run, PA
Robert Kadlec, Wetland Management
Services, Chelsea, MI
Douglas Kepler, Damariscotta, Clarion, PA
Robert Kleinmann, US Bureau of Mines,
Pittsburgh, PA
Robert Knight, CH2M HILL, Gainesville, FL
Fran Koch, Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Eric McCleary, Damariscotta, Clarion, PA
Gerald Moshiri, Center for Wetlands and
Eco-Technology Application, Gulf Breeze,
FL
John Murtha, Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Robert Myers, USDA - NRCS, Syracuse, NY
Kurt Neumiller, EPA, Annapolis, MD
Richard Reaves, Purdue University, West
Lafayette, IN
William Sanville, EPA, Cincinnati, OH
Dennis Sievers, University of Missouri,
Columbia, MO
Earl Shaver, Delaware Department of
Natural Resources and Environmental
Control, Dover, DE
Daniel Seibert, USDA - NRCS, Somerset, PA
Jeffrey .Skousen, West Virginia University,
Morgantown, WV
Peter Slack, Pennsylvania Department of
Environmental Resources, Harrisburg, PA
Dennis Verdi, USDA - NRCS, Amherst, MA
Thomas Walski, Wilkes University, Wilkes-
Barre, PA
Robert Wengryznek, USDA - NRCS, Orono,
ME
Alfred Whitehouse, Office of Surface
Mining, Pittsburgh, PA
Christopher Zabawa, EPA, Washington, DC.
This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental Protection
Agency-Region III, in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided with
nonpomt source management program funds under Section 319 of the Federal Clean Water Act.
The findings, conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA - NRCS,
EPA - Region III, the Commonwealth of Pennsylvania, or any other state in the northeastern United States concerning the use of constructed
wetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specific
programs and restrictions in this regard.
PS5384RSEZ
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VOLUME 4
TABLE OF CONTENTS
CHAPTER 1. INTRODUCTION 3
Development of Passive Treatment Technology 3
Costs and Limitations of Passive Treatment 4
Information Sources for Handbook 4
CHAPTER 2. BASIS FOR WETLAND TREATMENT OF MINE WATER ;. 5
Introduction 5
Acidity and Alkalinity 5
Iron and Manganese Reactions in Aerobic Environments 6
Iron Oxidation and Hydrolysis 7
Manganese Oxidation and Hydrolysis 7
Iron and Manganese Reactions in Anaerobic Environments 8
Limestone Dissolution 8
Sulfate Reduction 8
Aluminum 9
CHAPTER 3. DESIGN OF PASSIVE TREATMENT SYSTEMS 11
Mine Water Characteristics , 11
Water Quantity 11
Water Quality and Contaminant Loadings 11
Treatment Options 12
Process Sequence 12
Net Alkaline Water 12
Net Acidic Water * 12
Settling Basins and Ponds ;-. 14
Sizing 15
Design Considerations ; 15
System Layout 15
Water Control ' 16
CHAPTER 4. AEROBIC WETLANDS FOR NET ALKALINE WATER , 17
Appropriate Applications 17
Sizing : 17
Configuration ; 17
Long-Term Performance , 17
CHAPTER 5. ORGANIC SUBSTRATE WETLANDS 19
Appropriate Applications 19
Sizing -. , 19
Configuration 1 20
Long-Term Performance 20
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CHAPTER 6. ANOXIC LIMESTONE DRAINS (ALD) 21
Appropriate Applications 2i
Sizing .'.:..."."".'." 21
Type of Limestone 22
Configuration ; 29
Long-Term Performance -; 94
CHAPTER 7. SUCCESSIVE ALKALINITY-PRODUCING SYSTEMS (SAPS) 2
Appropriate Applications .'. '-,.'. 2g
Sizing ! 25
Configuration ; • 2g
Long-Term Performance 26
REFERENCES : 27
LIST OF TABLES
Table 1. Bureau of Mines compliance and abandoned mined land (AML) sizing criteria
for alkaline and acidic mine waters , 15
Table 2: Sample calculations for ALD ^ 22
LIST OF FIGURES
Figure 1. Decision tree for selecting appropriate passive treatment sequence 13
Figure 2. Generalized schematic of an anoxic limestone drain (ALD) 23
Figure 3. Generalized schematic of a successive alkalinity-producing system (SAPS) 26
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CHAPTER 1
INTRODUCTION
This volume is a general guide to the use of
constructed wetlands in treating coal mine drain-
age in northern Appalachia. It is to be used in
conjunction with Volume 1: General Consider-
ations, which provides information on wetland
hydrology, soils, and vegetation, and on the siting,
design, construction, operation, and maintenance
of constructed wetland systems.
If the treatment of mine drainage by con-
structed wetlands is to be effective, the appropri-
ate type of wetland treatment must be chosen and
the wetland must be made large enough to provide
the necessary retention time. This volume dis-
cusses the various types of mine drainage and the
passive treatment the options available, and
presents guidance on how to design systems
correctly.
The use of passive treatment systems for
treating coal mine drainage a developing technol-
ogy. Much is not yet understood and questions
remain regarding the optimal design of systems
and their longevity. As our experience with
passive technologies increases, the information
offered here will probably be replaced by more
refined information.' The Handbook should be
used with this clearly in mind.
DEVELOPMENT OF PASSIVE
TREATMENT TECHNOLOGY
The mining of coal can result in drainage that
is contaminated with high concentrations of
dissolved iron, manganese, aluminum, and sulfate.
The 1977 Surface Mining Control and Reclamation
Act requires that mine drainages from all active
and many inactive mines comply with effluent
quality criteria. Chemical treatment to meet these
criteria imposes a serious financial burden on coal
companies. The high costs of chemical treatment
also limit efforts to treat contaminated water from
abandoned mine lands (AML). Passive treatment
offers a low-cost alternative to conventional
chemical treatment for active mines.
For some mine drainages, constructed u-et-
lands provide the treatment needed to produce
water that meets effluent limitations. For other
drainages, constructed wetlands are used to
pretreat mine water before conventional treatment,
thereby lowering treatment costs, or they are used
as a part-time alternative to full-time treatment. At
pre-1977 sites and abandoned sites, constructed
wetlands offer a low-cost means of improving the
quality of mine water before it is released to
receiving streams, although treatment may not
achieve effluent standards. For abandoned mine
lands, passive treatment may be the only economi-
cally feasible option for treating persistent drain-
ages.
The treatment of mine drainage by wetlands
has evolved from simple surface flow wetlands to
sequential treatment in a variety of wet environ-
ments. Early constructed wetlands were built to
mimic the peat (Sphagnum] wetlands that first
showed that the quality of mine water was im-
proved as it passed through these wetlands.
However, Sphagnum wetlands,proved
to be difficult to establish and maintain and the
design was replaced by one in which emergent
plants, most often cattails, are the dominant
vegetation.
Recently, passive treatment options have been
expanded to include anoxic limestone drains
(ALD), which add alkalinity to the drainage before
wetland treatment, and successive alkalinity- •
producing systems (SAPS), which reduce the
amount of surface area needed to.generate alkalin-
ity. Often, several treatment options are used
sequentially.
Opinions vary on the merits of passive treat-
ment of mine waters by constructed wetlands.
Analyses by the Office of Surface Mining and
others (for instance, Wieder 1989, Wieder et al.
1990) question the feasibility of the constructed
wetland concept. On the other hand, many
constructed wetland systems have worked quite
well for a number of years (Brodie et al. 1993,
Taylor eFal. 1993, Hedin et al. 1994, Stark et al.
VOLUME 4: COAL MINE DRAINAGE
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1994). Hundreds of constructed wetlands are now
being used to.decrease concentrations of contami-
nants from active, reclaimed, and abandoned
mines before the water is released, although not
all of the systems may consistently treat water to
effluent standards.
COSTS AND LIMITATIONS OF
PASSIVE TREATMENT
Compared with conventional chemical treat-
ment, passive methods usually require more land
area but use less costly reagents and require less
operational attention and maintenance.
The costs of wetland treatment are usually
measured in terms of the land required. Con-
structed wetlands take advantage of natural
chemical, physical, and biological processes to
decrease metal concentrations and to neutralize
acidity. Since some of these removal processes
are slower than those used in conventional treat-
ment, particularly if adequate alkalinity is not
provided, the required retention times are longer
and the area needed for treatment is larger for
wetland treatment than for conventional treat-
ment. If wetlands are to produce water that meets
effluent criteria, the wetland must be large enough
to provide the necessary retention times. The land
available for treatment is often limited on mine
sites and the sizing of constructed wetlands
becomes crucial factor. Undersized wetlands
cannot be expected treat water to compliance
levels. At present, however, there is no way to
predict precisely the effects of wetland treatment
on a particular mine drainage and therefore to size
the wetland precisely.
In sizing constructed wetlands at active mines,
the available space and the costs of construction
must be balanced against influent water quality
and chemical treatment costs. Wetland treatment
may be advantageous for alkaline to moderately
acidic mine water; for highly acidic mine water
the wetland may have to be so large that conven-
tional treatment becomes cheaper. If the decision
is made not to use year-round wetland treatment,
wetlands can still play an important role in treat-
ing mine water. Treating mine water passively by
wetlands before the water enters a chemical
treatment system can reduce the costs for conven-
tional treatment. Constructed wetlands can also
be used as a part-time alternative to full-time
chemical treatment.
INFORMATION SOURCES FOR
HANDBOOK
Much of the material presented here has been
summarized from the US Bureau of Mines Infor-
mation Circular 9389, The Passive Treatment of
Coal Mine Drainage (Hedin et al. 1994). The
Circular is an important reference for those inter-
ested in designing passive treatment systems. It
includes detailed information on the systems that
provided the data base for the Circular and pre-
sents an in-depth discussion of mine drainage
chemistry and the theoretical basis for passive
treatment.
The SAPS concept is not covered by the
Circular. The discussion of SAPS is based largely
on the recent work of D. A. Kepler and E. C.
McCleary (Kepler and McCleary 1994, McCleary
and Kepler 1994).
VOLUME 4: COAL MINE DRAINAGE
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CHAPTER 2
BASIS FOR WETLAND TREATMENT OF MINE WATER
INTRODUCTION
A number of natural processes reduce the
impacts of mine drainage on receiving waters. As
water flows through streams, rivers, lakes, and
wetlands, its toxic characteristics decrease through
chemical and biological reactions and through
dilution with uncontaminated water. Metals react
with oxygen in aerated water and precipitate as
oxides and hydroxides. Dissolved iron (Fe) pre-
cipitates as an orange oxyhydroxide, dissolved
manganese (Mn) precipitates as a black oxide or
oxyhydroxide, and dissolved aluminum (Al) as a
white hydroxide. The low pH that is common to
many mine drainages is raised either by mixing
with alkaline or less acidic water, or through
contact with carbonate rocks.
The goal of constructed wetland treatment is
to have these processes occur in the wetland rather
than in the receiving water. Passive treatment
systems function by retaining contaminated mine
water long enough for chemical, physical, and
biological processes to lower contaminant concen-
trations to acceptable levels. Efficient passive
systems create conditions that promote the pro-
cesses that most rapidly remove contaminants.
Thus, the design of efficient passive systems must
be based on an understanding of mine drainage
chemistry and how different passive technologies
affect this chemistry.
The following is a summary of the discussion
of mine drainage chemistry presented in the
Bureau of Mines Information Circular 9389 (Hedin
et al. 1994), which can be referred to for a more
complete discussion.
ACIDITY AND ALKALINITY
Acidity is a measure of the base neutralization
capacity of a volume of water. There are three
types of acidity:
• proton acidity (a measure of free H*ions)
• mineral acidity associated with dissolved metals
• organic acidity associated with dissolved organic
compounds.
Organic acidity is generally low in coal mine
drainages because they contain little dissolved
organic carbon. In mine drainage, acidity arises
from free protons (low pH) and from the mineral
acidity associated with dissolved Fe, Mn, and Al.
Of the many possible reactions of metals with
water, there are four major simplified reactions
that remove dissolved metals from solution. All
produce free protons:
Fez* + 1/4 O2 + 3/2 H2O -> FeOOH + 2H*
Fe3* + 2H2O -> FeOOH + 3H* '
Mn2+ + 1/4O2 + 3/2 H2O -> MnOOH + 2H*
(A)
(B)
(C)
Al3* + 3H2O -> Al(OH)3 + 3H* (D)
These reactions can be used to calculate the total
acidity of a sample of mine water and to partition
the acidity into its various components. The
expected acidity of a mine water is calculated from
its pH and the sum of the milliequivalents of
acidic metals:
Acid^ = 50 [2Fe2*/56 + 3Fe3*/56 + 3A1/27 +
2Mn/55 + 1000(10-PH)j (1)
where all metal concentrations are in milligrams
per liter (mg/L) and 50 is the equivalent weight
of calcium carbonate (CaCO3) and thus trans-
forms milliequivalent per liter of acidity into
mg/L CaCO3 equivalent (Hedin et al. 1994).
The contributions to acidity from free protons
and from dissolved metals vary from mine water to
mine water, and are used as a guide to the type of
passive treatment needed for effective treatment.
. The acidity of many mine waters arises largely
from dissolved metals (mineral acidity) rather than
from free protons (pH).
When the pH of a mine water is greater than
4.5, it has acid neutralizing capacity and is said to
contain alkalinity. Alkalinity neutralizes mineral
acidity and buffers changes in pH. Unless buffered
by alkalinity, the reactions of metals with water
(reactions A - D) will decrease pH.
VOLUME 4: COAL MINE DRAINAGE
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In water, Fe and Mn undergo oxidation and
hydrolysis reactions. Oxidation decreases acidity
while hydrolysis increases acidity. For Fe, the
reactions are:
Fe2' + 1/40, + H'-> Fe3* + 1/2H,0 (oxidation) (E)
Fe3' + 2H:0-> FeOOH + 3H* (hydrolysis) (B)
Alkalinity and acidity are not mutually exclu-
sive. When water contains both alkalinity and
mineral acidity, a comparison of the two measure-
ments determines whether the water is net alkaline
(alkalinity is greater than acidity) or net acidic
(acidity is greater than alkalinity). Net alkaline
water contains enough alkalinity to neutralize the
mineral acidity represented by dissolved Mn and
ferrous Fe.
In mine water, the principal source of alkalinity
is dissolved carbonate, which can be present as
undissociated H2CO3, bicarbonate (HCO30, or
carbonate (CO.,2'). Bicarbonate and carbonate can
neutralize proton acidity:
H« + HC03'-> H20 + C02 (F)
2H* + C032--> H20 + C02 (G)
As Fe and Mn oxidize and hydrolyze, the
resulting proton acidity can be neutralized by
bicarbonate. For waters contaminated with Fe2",
the net reaction is: '
Fe2" +1/402 + 2HCO; -> FeOOH + 1/2H20 +2C02 (H)
This reaction indicates that net alkaline waters
contain at least 1.8 mg/L alkalinity for each 1.0 mg/L
of dissolved Fe. Waters that contain a lesser ratio
are net acidic, since oxidation and hydrolysis of
the total dissolved Fe content results in a net
release of protons and a decrease in pH. The
presence of bicarbonate alkalinity in mine waters
that contain elevated levels of metals is not un-
usual. While all mine drainages are commonly
referred to as "acid mine drainages", in fact many
discharges are net alkaline.
When pH drops below 6, the rate of Fe oxida-
tion slows dramatically and the removal of Mn by
oxidation and hydrolysis virtually stops (Nairn et
al. 1991). Alkalinity is therefore important for
three reasons: it neutralizes mineral acidity, it
buffers against changes in pH, and it enables the
removal of Mn.
Passive treatment systems can be expected to
perform more effectively when the raw mine
water has a pH greater than 6.0 and contains net
alkalinity.
IRON AND MANGANESE
REACTIONS IN AEROBIC
ENVIRONMENTS
When mine water flows through aerobic
environments, the oxidation and hydrolysis
reactions discussed above cause concentrations of
ferrous Fe (Fe2*), ferric Fe (Fe3+), and Mn to de-
crease. Whether these reactions occur quickly
enough to lower metal concentrations to an accept-
able level depends on:
• the availability of oxygen for oxidation reactions
• the pH of the water
• the activity of microbes that catalyze reactions
• the retention time of the water in the treatment
system.
The pH is an especially important parameter
because it influences both the solubility of metal
hydroxide precipitates and the kinetics of the
oxidation and hydrolysis processes. The relation-
ship between pH and metal removal processes in
passive treatment systems is complex because it
differs among metals and also between biotic
(biological) and abiotic (physical/chemical)
processes.
In general, Fe and Mn precipitate sequentially,
not simultaneously. Fe oxidizes and precipitates
much more rapidly than Mn because oxidized Mn
solids are unstable in the presence of Fe2+. Con-
centrations of Fe2+ must be reduced to low levels
before Mn can be converted to a stable solid
precipitate.
VOLUME 4: COAL MINE DRAINAGE
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IRON OXIDATION AND HYDROLYSIS
The most common contaminant in mine
drainage is ferrous iron (Fe2*). Fe2*is removed
from mine water largely through oxidation and
hydrolysis (reactions E and B). In the oxidizing
environments common to most surface waters, Fe2*
is converted to Fe3* through ferrous oxidation. The
conversion occurs both abiotically and as a result
of bacterial activity.
The pH of the mine water affects the kinetics
of the abiotic and biotic processes. For oxygenated
waters with pH less than 4, Fe removal is limited
by the oxidation process (reaction E). When
oxygen is not limiting, the rate of abiotic Fe
oxidation slows 100-fold for every unit decrease in
pH. At pH values greater than 8, the abiotic
process is fast (rates are measured in seconds)
while at pH values less than 5 the abiotic process
is slow (rates are measured in days). In contrast,
bacterial oxidation of Fe2+ peaks at pH values
between 2 and 3, and diminishes at pH less than 5.
Abiotic oxidation processes dominate over bacte-
rial oxidation processes at pH values above 6
while bacterial processes dominate at pH values
below 5.
At pHs between 6 and 7, a range in which
abiotic iron oxidation processes should dominate,
the presence of bicarbonate alkalinity buffers mine
water. Waters that contain no alkalinity have pHs
less than 4.5 and the removal of Fe under oxidiz-
ing conditions occurs primarily by bacterial
oxidation accompanied by hydrolysis and precipi-
tation.
As Fe2*is converted to Fe3*, it is subject to
hydrolysis reactions that can precipitate it as an
oxyhydroxide (FeOOH) or hydroxide [Fe(OH)3].
The hydrolysis reaction occurs abiotically. The
solubility of iron oxyhydroxide solids is such that,
under equilibrium conditions, the amount of
dissolved Fe3* is negligible (<1 mg/L) if the pH is
greater than 2.5. In actuality, the rate of the
hydrolysis reaction is also pH-dependent, and
significant Fe3* can be found in mine water with a
pH less than 2.5. A fourth-order relationship with
pH has been suggested, in which Fe3* hydrolysis
processes shift from a very rapid rate at'pH
above 3 to a very slow rate at pH below 2.5
(Singer and Stumm, cited in Hedin et al. 1994).
The tendency for dissolved Fe to oxidize
and hydrolyze in aerobic environments with
pH greater than 3 results in the precipitation of
FeOOH and Fe(OH)3. Because the net result of
the oxidation and hydrolysis process is the
production of protons, the process can decrease
pH. Thus, the passage of circumneutral net
acidic water through wetlands commonly
decreases both Fe concentration and pH.
MANGANESE OXIDATION
AND HYDROLYSIS
Manganese oxidation and hydrolysis
reactions result in the precipitation of manga-
nese oxyhydroxides (MnOOH) and manganese
oxides (Mn304and MnO,). If the environment is
alkaline, manganese carbonate (MnCO3) can
also form. The specific mechanism(s) by which
Mn2* precipitates from aerobic mine water in
the absence of chemical additions is uncertain.
The processes generally result in the formation
of MnO2, which precipitates.
While the" reactions that remove Mn are
mechanistically similar to those that remove Fe,
Mn removal rates are 20 to 40 times slower than
Fe removal rates under similar pH and Eh
conditions. The kinetics of Mn2* oxidation are
strongly affected by pH. Abiotic reactions are
very slow at pH less than 8. Microbes can
catalyze Mn2* oxidation, but do so only in
oxygenated waters with pH greater than 6.
Although the hydrolysis of Mn produces
protons, the precipitation of MnOOH does not
result in large declines in pH, as can happen
when FeOOH precipitates, because there is no
natural mechanism that rapidly oxidizes Mn2*
under acidic conditions. If the pH falls below
6, Mn2* oxidation virtually stops, the proton-
producing hydrolysis reaction stops, and pH
stabilizes.
VOLUME 4: COAL MINE DRAINAGE
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IRON AND MANGANESE
REACTIONS IN ANAEROBIC
ENVIRONMENTS
Chemical and microbial processes in anaerobic
environments differ from those in aerobic environ-
ments. Because 0, is absent, Fe2* and Mn2* do not
oxidize and their oxyhydroxide precipitates do not
form. Hydroxides of reduced Fe and Mn ions
[Fe(OH)2 and Mn(OH),] do not form because they
are highly soluble under acidic or circumneutral
conditions. The chemistry of mine water in anaero-
bic environments is influenced by the chemical and
biological processes that generate bicarbonate
(HCO3-)and hydrogen sulfide (H2S).
LIMESTONE DISSOLUTION
A major source of HCO3 in many anaerobic
environments is the dissolution of carbonate
materials, such as calcite:
CaCO, + H* -> Ca2* + HCO - (I)
•} J
Carbonate dissolution in anaerobic mine water
environments can result in higher concentrations of
HCO3"than in aerobic environments for two reasons:
• the absence of Fe3* in most anaerobic environ-
ments limits the formation of FeOOH coatings
that armor carbonate surfaces and inhibit further
carbonate dissolution in aerobic environments
• decomposition of organic matter and the neutral-
ization of proton acidity in anaerobic environ-
ments leads to high CO2 partial pressures which
foster the dissolution of carbonate compounds.
The observation that limestone dissolution
is enhanced by contact with mine water has led
to the construction of anaerobic limestone treat-
ment systems, known as anoxic limestone drains
(ALD). In an ALD, mine water flows through a
bed of limestone gravel that has been buried to
limit inputs of atmospheric oxygen. The contain-
ment caused by burial also traps CO2 within the
ALD, fostering the development of high CO2
partial pressures.
The amount of alkalinity that can be generated
by an ALD is limited to a maximum value that is a
function of the C02 partial pressure within the
ALD. When the water in an ALD reaches equilib-
rium with the CaCO3, no further alkalinity can be
generated. The maximum amount of alkalinity
that can be generated is still open to question.
Hedin and Watzlaf (1994) estimate that the maxi-
mum amount of alkalinity that can be generated in
a properly-functioning ALD is about 300 mg/L.
ALD treatment is discussed uvChapter 6.
SULFATE REDUCTION
When mine water flows through an anaerobic
environment that contains an organic substrate,
bacterial sulfate reduction can occur. Sulfate-
reducing bacteria use sulfate in their metabolism,
releasing hydrogen sulfide and bicarbonate in the
process:
2CH20 + SO/- -> H2S + 2HC03 (J)
where CH2O represents organic matter.
Bacterial sulfate reduction results in the
precipitation of dissolved metals as metal sulfide
solids:
M2* + H2S + 2HC03- -> MS + 2H20 + 2CO2 (K)
where M represents metals. For Fe, pyrite forma-
tion is also possible:
Fe2* + H2S + S° -> FeS2 + 2H* (L)
The bacteria require the presence of sulfate,
suitable concentrations of organic compounds, a
pH greater than 4, and the absence of oxidizing
agents such as 02 Fe3*, and Mn4*. These condi-
tions are met in mine drainage wetlands that
contain organic matter and anaerobic substrates.
The precipitation of metal sulfides in an
organic substrate improves water quality by
decreasing the mineral acidity without causing a
parallel increase in proton acidity. Protons re-
leased by H2S dissociation (H2S -> 2H*+ S2-) are
neutralized by an equal release of HCO.,- during
sulfate reduction. For coal mine drainage, where
metal contamination is generally limited to Fe,
8
VOLUME 4: COAL MINE DRAINAGE
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Mn, and Al, the H2S produced by bacterial sulfate
reduction primarily affects dissolved Fe. Alumi-
num does not form any sulfide compounds in
wetland environments and the relatively high
solubility of MnS makes its formation unlikely.
Spent mushroom compost, which consists of a
combination of spoiled hay, horse manure, corn
cobs, wood chips, and calcite, has been used as the
organic substrate at many constructed wetlands in
western Pennsylvania. In wetlands built with
substrates of limestone and spent mushroom
compost, the alkalinity of the pore water appears
to result from a combination of limestone dissolu-
tion and sulfate reduction. In wetlands con-
structed with an organic substrate, the pore water
(the water within the pores of the substrate) often
has a pH of 6 to 8. These alkaline conditions
result in part from microbial reactions involving
H2S that result in the net generation of HCO3'. In
most wetlands (natural and constructed), surface
waters are generally aerobic while the underlying
pore waters are anaerobic. Because pore waters
are circumneutral and strongly buffered by HCO3',
the removal of Fe by oxidation as the pore water
diffuses into aerobic surface zones can occur
rapidly.
ALUMINUM
Aluminum has only one oxidation state in
aquatic syslc::.^: -s-3. Oxidation and reduction
processes, which complicate Fe and Mn chemistry,
do not directly affect concentrations of dissolved
Al. Instead, concentrations of Al are primarily
influenced by the solubility of Al(OH)3. The
passage of mine water through highly oxidized or
highly reduced environments has no effect on Al
concentrations unless the pH changes. When pH
decreases (due to Fe oxidation and hydrolysis),
concentrations of Al increase because of the
dissolution of alumino-silicate clays, aluminum
oxides, or aluminum hydroxides by the acidic
water. At pH levels less than 4, AI(OH)3 is highly
soluble and high concentrations of Al3* and Al ion
complexes are possible. At pH levels between 5
and 8, Al(OH)3is highly insoluble and free Al is
not present in solution. If the pH of acidic mine
water is raised during passage through anaerobic
environments (because of carbonate dissolution or
microbial activity), Al(OH) precipitates.
VOLUME 4: COAL MINE DRAINAGE
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2 0 VOLUME 4: COAL MINE DRAINAGE
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CHAPTER 3
DESIGN OF PASSIVE TREATMENT SYSTEMS
The passive treatment of mine drainage by
constructed wetlands uses chemical and biologi-
cal mechanisms to improve the quality of the
water. Whether or not a constructed wetland
will perform well in treating a specific mine
water depends primarily on two factors;
• selecting the correct treatment process, or
sequence of processes
• sizing the wetland correctly, so that the mine
water is retained long enough for treatment to
occur.
MINE WATER CHARACTERISTICS
WATER QUANTITY
An accurate measurement of the flow rate of
the discharge or seepage is needed. Flow rates can
vary significantly throughout the year and in
response to storm events. Intense storms in
summer, and snowmelt and high groundwater
tables in winter-and early spring can increase
flows to ten times average annual flows. Such
large volumes of water can result in flushing
events that can greatly increase the concentrations
of Fe, Mn, and solids. If a wetland discharge must
consistently meet effluent criteria, the wetland
must be designed to ensure sufficiently long
retention times.
Flow rates are best determined by actual flow
measurements. Measurements of water volume
per unit time can be made with buckets or with
simple weirs and flumes. If accurate flow data
cannot be obtained and the system must produce
water that meets effluent criteria, the system must
be over-designed to assure adequate retention.
WATER QUALITY AND
CONTAMINANT LOADINGS
Water samples for chemical analyses should be
collected at the mine discharge or seepage point.
Water quality analyses should include pH, alkalin-
ity, hot acidity (H2O2 method 2310 4a, APHA
1992), Fe, Mn, and Al. Samples for metal analysis
should be acidified as soon as they are collected.
Samples containing visible particulates should be
filtered before being acidified. If an ALD is being
considered, the acidified sample should be ana-
lyzed for Fe2* and Al, and a field measurement of
the dissolved oxygen (DO) of the mine water
should be made.
Contaminant loading rates must be deter-
mined. Loadings of contaminants (Fe, Mn, and
acidity) are calculated by multiplying contaminant
concentrations by the flow rate. If the concentra-
tions are in mg/L and the flow rate is in gallons
per minute, the calculation is:
loading of (Fe, Mn, acidity)(grams/day) =
flow (gpm) x mg/L (Fe, Mri, acidity) x 5.45 (2)
If the concentrations are in mg/L and the flow rate
is in liters per minute, the calculation is:
loading of (Fe, Mn, acidity)(grams/day) =
flow (L/min) x mg/L (Fe, Mn, acidity) x 1.44 (3)
Concentrations may vary as hydrologic condi-
tions change. Concentrations may be greatest
during low flows if high flows dilute pollutants.
However, if high flows flush materials or raise the
levels of mine pools, concentrations may be
highest during high flows. It is therefore impor-
tant to determine loadings for average conditions
and also for those times when flows and contami-
nant concentrations are high to ensure that the
wetland is adequately sized to accommodate the
range of conditions.
VOLUME 4: COAL MINE DRAINAGE
11
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TREATMENT OPTIONS
Four options for passive treatment are available.
Each is most appropriate for a particular type of
mine water. They can be used in sequence. The
options are:
• aerobic wetlands, which promote oxidation reac-
tions to precipitate metals as oxides and hydrox-
ides. These wetlands typically contain cattails
growing in a soil or spoil substrate. Aerobic
wetlands are surface flow (SF) wetlands.
• organic substrate wetlands, which are often called
compost wetlands. In these wetlands, the water
flows through thick layer of organic material. The
anaerobic conditions in the organic layer promote
chemical and microbial processes that generate
alkalinity and neutralize acidity. Organic material
includes spent mushroom compost, peat, hay
bales, and manure.
• anoxic limestone drains (ALD), which are buried
beds of limestone. The limestone adds alkalinity
to the water, which is then fed to a settling pond
and wetland where the metals are precipitated.
The ALD is sealed to exclude oxygen so that
limestone dissolution can occur without armoring
(the deposition of metal oxyhydroxides on the
limestone) which blocks further dissolution. ALDs
are not wetlands, but a pretreatment to prepare
acidic water for wetland treatment.
• successive alkalinity-producing systems (SAPS),
which place an organic substrate wetland over a
layer of limestone. Water is introduced at the top,
flows down through the layers, and is discharged
from the bottom. As the mine water moves down
through the layers, microbial activity removes
dissolved oxygen and reduces Fe3+ to Fe2*. Alka-
linity is then produced by bacterial sulfate reduc-
tion in the organic layer and by limestone dissolu-
tion in the limestone layer. The strongly reducing
environment of the organic layer prevents the
armoring of the limestone. The water discharges to
a settling pond where the metals are precipitated.
Mine water can be recycled through a SAPS or
passed through several SAPSs as often as neces-
sary to remove the acidity.
PROCESS SEQUENCE
A decision tree for selecting the appropriate
treatment, or sequence of treatments, for a specific
mine water is given in figure 1. The treatment to be
used depends largely on whether the water is net
acidic or net alkaline.
NET ALKALINE WATER
Net alkaline waters are treated with aerobic wet-
lands. Since additional alkalinity is not needed, an
ALD or an organic substrate is not needed. The design
of aerobic wetlands for net alkaline water is discussed
in Chapter 5.
NET ACIDIC WATER
Net acidic waters require that alkalinity be added
to remove metals and raise pH. There are three
options for adding alkalinity: an ALD, a SAPS, and a
compost wetland.
An ALD can be used if DO, Fe3*, and Al concen-
trations are low (<1 mg/L). A properly-sized ALD
can add 150 to 300 mg/L alkalinity to the mine water.
The ALD is followed by a settling pond for removal
of the solids. If the acidity of the mine drainage is
<300 mg/L, the water can be treated with an aerobic
wetland after the settling pond. If the acidity of the
mine drainage is >300 mg/L, the water should be
treated with a SAPS to add further alkalinity since
the ALD will not add sufficient alkalinity.
If the mine water contains >1 mg/L DO or Fe3*,
an ALD should not be used since Fe(OH)3 will
form and armor the limestone. Instead, alkalinity
should be added by a compost wetland or a SAPS.
Treatment by a compost wetland requires a larger
wetland than does treatment by a SAPS. In northern
Appalachia, the treatment of highly acidic waters by
compost wetlands does not consistently transform
these waters into alkaline waters, particularly during
the winter.
Organic substrate wetlands are discussed in
Chapter 5, ALD treatment in Chapter 6, and SAPS
treatment in Chapter 7.
VOLUME 4: COAL MINE DRAINAGE
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determine chemical
composition of raw water
net alkaline water
net acidic water
DO, Fe3*, and Al all <1 mg/L
DO, Fe3*, or Al >1 mg/L
anoxic
limestone
drain
(ALD)
raw water
acidity >300 mg/L
successive
alkalinity
producing
system
(SAPS)
settling
pond
raw water
acidity <300 mg/L
aerobic
wetland
\
recycle
SAPS
as
necessary
settling
pond
1
f
discharge
|_
aerobic
wetland
settling
pond
fe.
r
r ->
i
^
f
settling
pond
1
r
^
r
organic
substrate
wetland
discharge
discharge
Figure 1. Decision tree for selecting appropriate passive treatment sequence
(modified from Hedin et al. 1994, Kepler and McCleary 1994).
VOLUME 4: COAL MINE DRAINAGE
13
-------
Marginally acidic waters (net acidities
of 0 - 100 mg/L) have sometimes been treated
successfully with aerobic wetlands. However, at
present there is no way to predict which margin-
ally acidic waters can be successfully treated with
aerobic systems and ivhich cannot. For marginally
acidic waters, incorporating an alkalinity-generat-
ing component into the design is recommended.
SETTLING BASINS AND PONDS
A settling basin or sediment pond is usually
installed after an ALD or SAPS to remove metal
precipitates (Brodie et al. 1993, Kepler and
McCleary 1994). The advantage of a pond is that it
can be easily dredged. The precipitation of Fe
from newly aerated and highly alkaline water
occurs physico-chemically and does not require
wetland treatment. Hedin et al. (1994) suggest that
about 50 mg/L Fe can be removed by a pond before
additional aeration is needed. For an acidic
drainage with low to moderate Fe concentrations,
and depending on site conditions, the wetland and
pond can be combined into marsh-pond cell with
a large portion devoted to deep water (3-6 ft)
where precipitates can accumulate.
During rainstorms, flows through a wetland
can increase by an order of magnitude or more
with a drop in treatment efficiency to near zero
(Stark et al. 1994). A polishing pond placed
between the wetland and the discharge to the
receiving water can be valuable in preventing 5
discharges of resuspended materials during high
flows. At a site that included three polishing
ponds after a wetland system, discharges remained
in compliance during intense summer storms that
tripled inflow rates, an outcome attributed to the
ponds (Stark et al. 1994).
SIZING
A method for calculating the optimal size of
passive treatment systems for various types of
mine drainage has not yet been developed. Some
systems have been greatly oversized while
others have been greatly undersized. Sizing is
affected by the composition of the mine drainage,
the quantity of water to be treated, and the
specifics of the site. The seasonally-variable
nature of mine discharges and the varying effec-
tiveness of passive treatment during different
times of the year further complicate the assess-
ment of the size needed to achieve a certain level
of water quality. The size of passive treatment
systems has often been determined by the space
available for such treatment rather than by optimal
treatment requirements.
Based on the performance of 13 constructed
wetland systems in Pennsylvania, the Bureau of
Mines (Hedin et al. 1994) suggests two sets of
sizing criteria :
• abandoned mined land (AML) criteria. In many
AML situations, the goal is cost-effective im-
provement in water quality rather than compli-
ance with effluent standards. The criteria are
based on removals that have been observed at
existing sites. Wetland treatment at these sites
significantly improves water quality, although in
many cases the wetlands may not consistently
lower contaminant concentrations to NPDES
effluent standards.
• compliance criteria. These criteria are suggested
for wetlands that must produce effluents that
comply with NPDES effluent standards. The
criteria are conservative and result in wetlands
that are twice as large as AML wetlands.
Table 1 gives recommended wetland sizes for
compliance and AML criteria. For example, to
size a wetland according to the Fe AML criterion
in table 1:
minimum wetland size (ac) =
Fe loading (Ib/day) - 180 (Ib/ac/day)
To remove both Fe and Mn, the size
needed to remove Fe must be added to the
size needed to remove Mn. This is necessary
because Fe and Mn. are removed sequentially
in constructed wetlands.
14
VOLUME 4: COAL MINE DRAINAGE
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Fe removal rates may be a function of Fe
concentration, with removal rates decreasing as
Fe concentrations become smaller, and the
suggested compliance criteria sizing value for Fe
(90 Ib/ac/day, 10 g/m2/day) is conservative to
account for this.
The criteria for Mn removal reflect the large
size of wetland needed to remove Mn. Because
the toxic effects of Mn at moderate concentra-
tions are generally not significant and the size of
the wetland needed to treat Mn-contaminated
water is so large, Hedin et al. (1994) suggest that
AML sites with Fe problems should receive a
higher priority than those with only Mn prob-
lems.
Treatment systems that incorporate more
than one treatment methodology are sized by
summing the treatment areas needed for each of
the components. For instance, to remove Fe and
acidity, a system should be sized for the removal
of Fe by an aerobic wetland plus the removal of
acidity by a compost wetland.
DESIGN CONSIDERATIONS
SYSTEM LAYOUT
The characteristics of the mine drainage and
the site determine the layout of the components of
the treatment system. The source, volume, and
probable variability of the drainage should be
understood before the system is designed. The
mine maps and/or core borings should be re-
viewed to determine the source of the water (shaft.
tunnel, or waste pile), and the area drained by this
source. Information on groundwater and underly-
ing soils is also needed. Site topography affects
cut and fill requirements, drainage and erosion
characteristics, and slope stability. The system
should be designed for gravity flow.
Since precipitation is a major Fe removal
pathway, the configuration should create long flow
paths to provide long retention times. Long flow
paths can be created by building long narrow
channels or by using baffles to create serpentine
flow paths in short, wide cells. Very large cells are
subject to short-circuiting and should be hydro-
logically chambered with simple low or subsurface
finger dikes, logs, riprap baffles, or other struc-
tures. Hay bales can be used but they decompose
readily and must be replaced peridHically. Alter-
natively, irregular or serpentine cells can be built.
The cells should be designed to maximize the
contact of the mine water with the surfaces in the
cell and to avoid channeling and short-circuiting.
The geometry of the site, as well as flow and
treatment considerations, may dictate the use of
multiple cells. The intercell connections can be
ditches, riprapped channels, V-notch weirs, or
lined railroad tie steps. The intercell connec-
tions should dissipate enough kinetic energy so
that the water entering a cell will not cause
erosion or disturb precipitated solids. The
Table 1. Bureau of Mines compliance and
abandoned mine land (AML) sizing criteria
for alkaline and acidic mine waters (from Hedin et al. 1994).
Ib (Fe, Mn, acidity) / ac wetland surface / day
Compliance AML
Alkaline Acidic Alkaline Acidic
Fe • 90 na 180 - na
Mn 4.5 na 9 na
Acidity na 30 na 60
g (Fe, Mn, acidity) / m2 wetland/surface / dav
Compliance AML
Alkaline Acidic Alkaline Acidic
10 ' na 20 na
0.5 na 1 na
na 3.5 na 7
na: not applicable
VOLUME 4: COAL MINE DRAINAGE
15
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connections between the cells can serve as
aeration devices. Pipes should be avoided
because they tend to clog with Fe precipitates.
However, pipes must be used to move water
from an ALD since precipitates will form on
exposure to air, clogging an open channel.
A settling pond should be placed before the
first wetland cell to remove particulate material
that does not need wetland treatment. Effluent
from the wetland system should pass through
another settling pond to settle Fe solids before the
water is discharged to the receiving water.
WATER CONTROL
Mine drainage can come from a variety of
sources, including existing surface drainage
ditches, seeps from backfill or spoil, and openings
such as mine portals or boreholes. These sources
must be plumbed into the treatment system. The
plumbing must be able to handle variations in
flow without leaking.
Seeps and surface drainage can be collected
either in strategically-located collection ponds or
in contour ditches that discharge to a collection
pond or the first wetland cell. These structures
can also serve as pretreatment or primary treat-
ment units to settle precipitates. The advantages
of collection ponds are that they control flow
surges and provide a relatively constant flow to
the system. Inflow surge control is particularly
important if the drainage is a surface flow or is
heavily influenced by runoff since these flows
fluctuate in response to changes in the weather.
A relatively steady flow to the wetland simplifies
design and minimizes hydraulic stresses during
operation.
To collect seepage from spoil, French drains in
the toe area can be used. The drain system should
be designed to minimize mixing with uncontami-
nated water to avoid flow surges to the system.
Pipes can be chronic maintenance problems
because of the clogging that results when Fe
precipitates collect in pipes. If pipes are neces-
sary, as they are for moving water from an ALD,
steps must be taken to exclude oxygen. A gas trap
should be placed at the exit and clean-out plugs
should be provided.
The mine drainage can be collected to yield a
single flow to the wetland. If several widely
different drainages are to be treated, combining
them may or may not be desirable. It may be •
advantageous to combine a low flow, high strength
acid mine drainage (AMD) with a high flow, low
strength AMD to dilute the strong AMD. On the
other hand, adding a low flow, low strength AMD
to a high flow, high strength AMD will only
increase the amount of strong AMD to be treated
and separate systems may be preferable.
Water levels can be controlled with a spillway
or water control structure (weir or swiveling
standpipe). Spillways are simple to construct but
are not adjustable and problems arising from
incorrect water levels can be difficult to remedy.
Spillways should consist of wide cuts in the dike
with sides no steeper than 2H:1V and lined with
erosion-resistant material. Spillways must be be
able to pass the expected high and low flows.
In areas where vandalism could be a problem,
the use of removable boards in weirs or flexible
hoses (see Volume 1) should be avoided. Enclos-
ing pipes and valves in protective enclosures may
be advisable.
16
VOLUME 4: COAL MINE DRAINAGE
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CHAPTER 4
AEROBIC WETLANDS FOR NET ALKALINE WATER
APPROPRIATE APPLICATIONS
Net alkaline mine waters can be treated with
aerobic wetlands since net alkaline water contains
enough alkalinity to buffer the acidity produced
by the metal hydrolysis reactions. The metals - Fe
and Mn - will precipitate, given enough time. The
goal of wetland treatment of net alkaline water is
to aerate the water and to promote the oxidation
and settling of the metals.
SIZING
Aerobic systems for the removal of Fe or Mn
can be sized with the criteria in table 1. Aerobic,
alkaline wetlands function largely through chemi-
cal reactions and performance is not strongly
affected by season. The limiting factor is the slow
settling of metal oxyhydroxides.
CONFIGURATION
A typical aerobic wetland consists of a basin
with cattails growing in soil or in alkaline spoil.
Water depths are generally 6 -18 inches
(10 - 50 cm). These depths are appropriate for
most emergent wetland vegetation. Most wetland
plants cannot tolerate water deeper than about
18 inches (50 cm)(see Volume 1).
The depth of the water may vary, depending
on the needs of the operator. Shallow water
(<6 inches) enhances oxygen levels and oxidizing
conditions but freezes more quickly in winter.
Shallow wetlands fill more quickly with deposits.
Brooks found that saturated sediments rather than
standing water enhanced treatment and recom-
mends depths of 0 (saturated soil) to 6 inches
(R. P. Brooks, Penn State University, pers. comm.).
Deeper water (8 - 24 inches) decreases vegetation
diversity and retards oxygenation near the sub-
strate, but can increase the life span of the cell
since it provides more space for the accumulation
of deposits. Deeper cells may be appropriate for
moderate mine water or as the first stage in a
wetland system to accommodate the rapid precipi-
tation of FeOOH. Hedin et al. (1994) suggest that
designing wetland with shallow and deep marsh
areas plus a few areas of deeper (3-6 ft) open
water will accommodate seasonal and year-to-year
variations in weather and flow.
In many wetlands that treat alkaline xvater, the
removal of Fe appears to be limited by the avail-
ability of dissolved oxygen. To promote Fe re-
moval, aeration of the water should be followed by
passage through quiet areas where the iron can
react with DO and the iron floe can settle out.
Aeration can be provided by waterfalls or V-notch
weirs. Aeration provides only enough DO to
oxidize about 50 - 70 mg/L Fe2*. Mine waters with
higher concentrations of dissolved Fe can be
treated with a series of aeration structures and
wetland cells. The wetland cells allow time for Fe
oxidation and hydrolysis, and provide space in
which the Fe floe can settle out. Many systems
add a sedimentation pond after the SF wetland to
polish the water before discharge.
Some SF wetlands have been built that do
not have typical wetland features. These wetlands
consist of open water ditches or shallow, rock-
filled ponds with few plants. These wetlands
have achieved removal rates similar to wetlands
filled with plants. Although plants may not be
necessary for Fe and Mn removal, plants may
increase the filtration of particulates, prevent flow
channeling, and reduce the resuspension of
sediments during storms.
LONG-TERM PERFORMANCE
In systems treating alkaline water, Fe and Mn
are precipitated by oxidative processes. The rapid
removal of Fe means that alkaline systems can be
expected to fill up with deposits of metal precipi-
tates. At a site in Ohio, Fe sludge is accumulating
at a rate of about 1.25 -1.5 inches (3 - 4 cm) per
VOLUME 4: COAL MINE DRAINAGE
17
-------
year (Stark 1992). Measurements at a number of
sites in Pennsylvania show sludge accumulating at
0.75 -1.25 inches (2 - 3 cm) per year (Hellier and
Hedin 1992). These data suggest that dikes that
provide 3 ft of freeboard should provide sufficient
volume for 25 to 50 years of treatment.
Stark et al. (1994) found that water quality at
some surface mines improved within a decade after
regrading and reclamation were completed. At
these sites, 25 to 50 years of passive treatment may
be enough to mitigate the contaminant problem. At
surface mine sites with continual contaminant
production, or at systems constructed to treat
drainage from underground mines or coal refuse
disposal areas, systems can either be built with
greater freeboard or rebuilt when they fill up. Site
conditions will decide whether it is more economi-
cal simply to bury the wetland in place and con-
struct a new one, or to excavate and haul away the
deposits and reestablish the wetland.
18 VOLUME 4: COAL MINE DRAINAGE
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CHAPTER 5
ORGANIC SUBSTRATE WETLANDS
APPROPRIATE APPLICATIONS
For mine waters containing dissolved oxygen,
Fe3*, Al, or concentrations of >300 mg/L acidity, a
organic substrate wetland can be used. Organic
substrate wetlands generate alkalinity through a
combination of bacterial activity and limestone
dissolution. The desired sulfate-reducing bacteria
require a rich organic substrate in which anoxic
conditions will develop. The anoxic environment
within organic substrate also promotes the dissolu-
tion of the limestone.
Organic substrate wetlands in which water
flows on the surface of the substrate remove acidity
(that is, generate alkalinity) at rates of approximately
2 -12 g/mVd (18 -107 Ib/ac/d). The wide range in
performance is largely the result of seasonal varia-
tion, with lower rates in winter than in summer.
Supplementing the compost with limestone and
designing the wetland so that water flows through,
rather than over, the organic matter has been shown
to improve winter performance.
The three important factors in an organic sub-
strate wetland are:
• a substrate rich in organic matter (to support
sulfate reduction)
• vegetation (to replace consumed organic matter)
• flow contained largely within the substrate (to
promote reducing reactions).
A material often used in organic substrate wet-
lands is spent mushroom compost, which is readily
available in Pennsylvania, although any well-
composted equivalent can be used. Spent mushroom
compost has a high CaCO3 content (about 10% dry
weight). Mixing in more limestone may increase the
amount of alkalinity that can be generated by CaCO3
dissolution. A compost substrate that does not have
a high CaCO3 content should be mixed with supple-
mental limestone. Materials that have been used
where spent mushroom compost is not available
include sawdust, wood chips, peat moss, composted
straw bales, and composted chicken litter.
Compost often contains large amounts of loose
organic matter. As this material is flushed out
during the first few weeks of operation, shock
loading of a stream by high levels of biochemical
oxygen demand (BOD) can kill the aquatic life in
the stream (D. Seibert, Soil Conservation Service,
Somerset, PA, pers. comm.). During system start-
up, it may be prudent to store the wetland effluent
for several weeks in temporary sediment ponds to
reduce BOD concentrations before the water is
discharged.
If the pH of the mine water is greater than 4,
a pond in which Fe can be oxidized and precipi-
tated should be placed before the wetland. A
pond is useful when the influent to the wetland is
circumneutral and rapid removal of Fe can be
expected as soon as the water is aerated. If the pH
is less than 4, iron oxidation and precipitation
reactions are too slow for significant removal of Fe
by a pond. A sedimentation pond can be placed
after the compost wetland to polish the water
before discharge.
SIZING
Compost wetlands can be sized according to
the Bureau of Mines guidelines (table 1). For
example, for an AML site the calculation is:
minimum wetland size (ft2) =
acidity loading (g/day) *• 0.7
The acidity removal rate for compost wetlands
is influenced by seasonal effects that currently
cannot be corrected with wetland design. This is
not a problem for mildly acidic water nor should it
be a problem in warmer climates. However, in
northern Appalachia, compost wetlands do not
consistently transform highly acidic water
(>300 mg/L acidity) into alkaline water. While
considerable cost savings can be realized by using
a compost wetland during warm or mild weather,
such treatment must usually be supported by
conventional treatment during the winter.
VOLUME 4: COAL MINE DRAINAGE
19
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CONFIGURATION
The depth of the compost is usually
12 -18 inches (30 - 45 cm). To encourage water
flow through the compost, it should be laid gently
into the wetland (not compacted). Spent mush-
room compost has a bulk density of about 1,100
lb/yd3. A ton of compost usually covers about
3.5 yd3 to a depth of 18 inches (45 cm). Flow
within the substrate can be encouraged by sloping
or piling the compost a little higher than the free
water surface so that the water must flow through
the substrate.
Cattails or other vegetation are usually
planted in the compost to stabilize it and to
provide additional organic matter to fuel the
sulfate reduction process.
that sulfate reduction is linked, in a dependent
manner, to limestone dissolution. Sulfate-
reducing bacteria are inactive at pH less than 5.
Their activity in a wetland receiving lower pH
water may depend, in part, on the pH buffering
supplied by limestone dissolution. Thus, .
limestone dissolution may create alkaline zones
in which sulfate reduction can proceed and
produce further alkalinity. If this scenario is
accurate, then the long-term performance of a
compost wetland may be limited by the amount
of limestone in the substrate (about 11 years,
according to the above calculation). Under these
conditions, the chemical buffering capacity of
the substrate could be increased by adding
additional limestone during wetland construc-
tion. In fact, this is done at many sites.
LONG-TERM PERFORMANCE
At wetlands that treat acidic water by alkalin-
ity-generating processes associated with an
organic substrate, performance may decline over
time as the alkalinity-generating components
become exhausted. Limestone dissolution is
limited by the amount of limestone present in the
substrate. The limestone content of spent mush-
room compost is approximately 30 kg/m3
(1.9 lb/ft3). If a wetland containing a 40 cm
(16 inch) depth of compost generates CaCO3-
derived alkalinity at an average rate of 3 g/m2/day
(27 Ib/ac/d) (the average rate measured by Hedin
et al. 1994), then the limestone in the compost
will be exhausted in 11 years.
The same volume of compost contains about
40 kg (88 Ib) of organic carbon. If bacterial sulfate
reduction converts 100% of the carbon to bicar-
bonate at a rate of 5 g/m2/day (45 Ib/ac/d), the
carbon will be exhausted in 91 years. This esti-
mate is lengthened by the carbon captured by
plants through photosynthesis and shortened by
non-sulfate mineralization reactions. Also, part of
the carbon is recalcitrant (it does not readily enter
into reactions).
A realistic scenario for a compost wetland is
20
VOLUME 4: COAL MINE DRAINAGE
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CHAPTER 6
ANOXIC LIMESTONE DRAINS (ALD)
APPROPRIATE APPLICATIONS
In an ALD, alkalinity is produced when acidic
mine water dissolves limestone (CaC03). The ALD
is sealed to prevent contact with atmospheric ,
oxygen, which can result in the limestone becom-
ing armored with Fe precipitates, and to promote
the production of carbon dioxide, which increases
the dissolution of the limestone. An ALD is
simply a pretreatment unit to increase alkalinity
and to raise pH before the mine water enters the
wetland. Dissolved Fe and Mn are largely unaf-
fected by flow through the drain and an ALD must
be followed by a settling basin or wetland in
which metals can be oxidized and precipitated.
The amount of alkalinity generated by the ALD
is not easy to predict. ALDs that treat acidic mine
waters with low concentrations of Fe3*, Al3*, and
DO have generally discharged water with alkalini-
ties of 120 - 350 mg/L.
The most important factors limiting the useful-
ness of ALDs are the presence of Fe3*, Al, and DO.
When acidic water containing any Fe3* or Al3*
comes in contact with limestone, metal hydroxide
particulates, such as FeOOH, Fe(OH)3, or Al(OH)3,
will form. No oxygen is necessary. The buildup of
Fe hydroxides armors the limestone and limits its
further dissolution. The buildup of Al hydroxide
particulates within the ALD eventually decreases
the permeability of the ALD and causes it to plug.
The presence of DO in the mine water promotes
the oxidation of Fe2+ to Fe3* within the ALD and
thus potentially leads to armoring and plugging.
The short-term performance of ALDs in treating
water high in Fe3*, Al3*, and DO can be spectacular
(total removal of metals within the ALD). Long-
term performance is questionable because of
armoring and plugging.
Mine water that contains very low concentra-
tions of DO, Fe3*, and Al (that is, each <1 mg/L) is
ideally suited for pretreatment with an ALD. As
concentrations of these parameters increase, the
risk that the ALD will fail prematurely also in-
creases. Two ALDs fed water containing 20 mg/L
Al became plugged within 6 to 8 months (Hedin
et al. 1994).
The suitability of a mine water for ALD treat-
ment can sometimes be made by observing the
mine discharge and measuring the field pH. Mine
waters that seep from spoils and flooded under-
ground mines and that have field pHs above 5
characteristically have concentrations of DO, Fe3*,
and Al3* that are all <1 mg/L. Such sites are a
good candidates for pretreatment with an ALD.
Mine waters that discharge from open drift mines
or have pHs below 5 must be analyzed for Fe3* and
Al. Mine waters with pH below 5 can contain
dissolved Al and mine waters with pH below 3.5
can contain dissolved Fe3*. In northern Appala-
chia, most mine drainages with pH below 3 also
contain high concentrations of Fe3* and Al.
SIZING
As yet, there is no method for calculating the
exact size of an ALD needed to treat a specific
mine water discharge. Theoretical calculations
can estimate the mass of limestone that will be
needed to neutralize a certain discharge for a
specified period of time. An important factor in
the calculations is the concentration of alkalinity
expected to be produced by the ALD.
A maximum value of approximately
275-300 mg/L alkalinity has been observed at a
number of ALDs that have recently been con-
structed. The minimum mass of limestone needed
to treat a year's flow of mine water can be calcu-
lated from the flow rate and the assumption that
the ALD will produce the maximum amount of
alkalinity (300 mg/L):
yearly CaCO3 consumption (tons) =
flow (gpm) x 0.6565
yearly CaCO3 consumption (kg) =
flow (L/min) x 158
To determine_the total mass of limestone
needed in the drain, the above calculation must be
VOLUME 4: COAL MINE DRAINAGE
21
-------
adjusted for the CaC03 content and the projected
life of the ALD. Masses of limestone are con-
verted to volumes by assuming a density for
aggregate limestone of 1.2 - 1.5 ton/yd3.
The amount of alkalinity that can be generated
by an ALD is limited by the solubility of the
calcite. Hedin and Watzlaf (1994) found that a
residence time of about 14 to 23 hours was neces-
sary for mine waters to reach maximum concen-
tration of alkalinity. Retention times longer than
23 hours did not appreciably increase alkalinity
concentrations. Hedin and Watzlaf (1994) recom-
mend a residence time of approximately 15 hours.
To achieve a 15 hour residence time, the required
mass of limestone (m) can be calculated as:
where Q is the flow volume of the mine water (in
L/min), pb is the bulk density of the limestone
(kg/m3), tdis the retention time and is set to
15 hours, and Vvis the bulk void volume ex-
pressed in decimal form. To this mass of lime-
stone, enough must be added to satisfy dissolution
losses during the expected life of the ALD:
M = QCT/x
where Q is the volume flow of the mine water,
C is the predicted concentration of alkalinity in
the ALD effluent (mg/L), T is the design life of the
ALD, and x is the CaCO,, content of the limestone in
decimal form. The sum of the two masses is the
total limestone required.
Table 2 shows a sample calculation.
TYPE OF LIMESTONE
It is important to use limestone with a high
CaCO3 content because of its higher reactivity
compared to limestones with high MgCO3 or
CaMg(CO)3 content. The limestones used in most of
the successful ALDs have had a CaCO3 content
of 85-95%.
Most of the successful ALDs have used #3 or #4
limestone. Larger stone provides settling space for
grit and other particulate matter that may be gener-
ated during construction, but offers less surface area
for contact with the mine drainage (Faulkner and
Skousen 1994).
CONFIGURATION
An ALD is simply a buried bed of limestone
(figure 2). The dimensions of existing ALDs vary
considerably. Many older drains were built as long,
narrow drains that were approximately 1.5 - 3 ft
Table 2. Sample calculation for ALD.
A sample calculation for determining the mass of limestone (M) in metric tons needed to achieve a con-
centration (C) of 300 mg/L alkalinity to a flow (Q) for 25 L/min of AMD for 20 years (T) is calculated as
follows:
Given: 1. bulk density of limestone (pb) = 1600 kg/m3
2. bulk void volume of limestone (Vv) = 50%
3. CaC03 content of limestone (x) = 90%
4. detention time (td) = 15 hr
Solution: M = (Q pb td / Vv) + (Q C T / x)
= £25 L/min x 60 min/hr)(l600 kg/m3 x mVlOOO L x mt/1000 kg)(15 hr)
0.50
+ (25 L/min x 60 min/hr)(300 mg/L x mt/109 mg)(20 yr x 8766 hr/yr)
0.90
= 72.0 mt + 87.7 mt = 159.7 mt
This is equivalent to about 27 tons of limestone for each gallon per minute of flow.
22
VOLUME 4: COAL MINE DRAINAGE
-------
Vegetated Crown
HI - Clay Soil
Plastic Liner
High Quality Limestone
Infiltration
[JUiMmjAi*'' """-^
To Pond or
Wetland
Logitudinal Section
Figure 2. Generalized schematic of an anoxic limestone drain (ALD).
wide. On sites where a linear drain was not fea-
sible, ALDs have been made as wide as 30 - 60 ft;
these ALDs have produced alkalinities similar to
those produced by narrow, conventional beds.
A typical limestone drain is about 3 ft deep
and is capped and covered with soil. The soil
surface should be crowned to encourage surface
runoff and to allow for subsidence as the lime-
stone is consumed over time. The side slopes of
the drain are not critical and are usually made
near vertical to simplify construction.
The ALD must be sealed to minimize inputs of
atmospheric oxygen to the drain and to maximize
the accumulation of carbon dioxide within the
drain. Most ALDs are sealed by burying the ALD
under several feet of clay. One or two layers of
5 -10 mil plastic are often placed between the
limestone and the clay as ah additional gas barrier.
Some ALDs have been completely wrapped in
plastic before burial. The ALD should be designed
so that the limestone will be inundated with water
at all times. Clay dikes within the drain, or riser
pipes at the outflow of the drain will help to
ensure inundation.
A collection system to bring the mine water to
the ALD must be devised. Underground mine
openings can be sealed and flooded, and the
drainage routed to the ALD via a pipe. Alterna-
tively, openings can be backfilled with limestone
to create the ALD. Discrete seeps or springs are
good starting points for excavation into backfill.
Non-point seeps may require more innovative
means of collection, such as specialized rock
drains or the construction of an embankment to
contain the ALD (Brodie et al. 1993).
If the water seeps through backfill or spoil, •
flow paths should be thoroughly investigated
before designing the system. At some sites, prefer-
ential flow paths have developed within backfill
and the water has bypassed the ALD. A number
test pits should be dug several months before the
ALD is to be built to determine where the water
will go so that the collection system can be de-
signed properly.
The collection system should avoid tapping
into sources of uncontaminated water. Some ALDs
have unintentionally collected non-target water,
thereby increasing the volume of water passing
VOLUME 4: COAL MINE DRAINAGE
23
-------
through the systems. Incorporating water that was
more contaminated than the target water led to the
failure of an ALD (Hedin and Watzlaf 1994).
LONG-TERM PERFORMANCE
Most ALDs have been built since the late
1980's and there are no data on the long-term
performance of ALDs. Many aspects of ALDs make
long-term expectations uncertain. ALDs function
through the dissolution, and therefore removal, of
limestone. Questions remain about the ability of
ALDs to maintain unchannelized flow for long
periods of time, whether 100% of the limestone
can be expected to dissolve, and whether the
drains will collapse after a portion of the limestone
has dissolved. In large ALDs, most of the lime-
stone dissolution occurs in the upgradient portion
of the limestone bed. The effects of this preferen-
tial dissolution on the permeability of the system
or its structural integrity are unknown. It may be
prudent to provide contingency for failure, for
instance by including structural supports.
Another aspect that affects long-term perfor-
mance is the fact that ALDs retain Fe3* and Al.
This retention has raised concerns about the
armoring of limestone or the plugging of flow paths
long before the limestone is exhausted by dissolu-
tion. At present, there is no way to predict exactly
how the retention of metals affects ALD perfor-
mance.
24 VOLUME 4: COAL MINE DRAINAGE
-------
CHAPTER 7
SUCCESSIVE ALKALINITY-PRODUCING SYSTEMS (SAPS)
APPROPRIATE APPLICATIONS
Successive alkalinity-producing systems
(SAPS) have been suggested as a means of
overcoming the alkalinity-producing limitations
of ALDs and the large area required for organic
substrate wetlands (Kepler and McCIeary 1994,
McCleary and Kepler 1994). In a SAPS, the mine
water flows down through an organic layer into
limestone beds below the organic layer; the pore
waters are discharged. A SAPS adds alkalinity
through bacterial sulfate reduction and by
limestone dissolution. A SAPS is followed by a
settling pond where the metals are precipitated.
Because DO is removed within the organic layer
of the SAPS before the water comes in contact
with the limestone, the use of SAPS is not
limited by the concentrations of DO, Fe3*, or Al
in the raw mine water. Three SAPSs have been
installed in Pennsylvania since 1990 and all
have performed well (Kepler and McCleary
1994, McCleary and Kepler 1994).
Water containing high acidity levels can be
treated by passing it through a series of SAPSs.
In this case, a SAPS is followed by a settling
pond where the metals are precipitated, and then
an aerobic wetland where oxidizable organic
matter is added, before entering another SAPS.
The sequence can be repeated as often as neces-
sary. SAPS treatment offers two advantages over
other treatment options:
• since a SAPS relies on volume rather than
surface area for treatment contact, the required
surface area can ber smaller than for an aerobic
or organic substrrate wetland.
• the water can be passed through a SAPS as often
as necessary.
Kepler and McCleary (1994) did not encounter
any hydraulic problems due to physical plugging
of the limestone. They suggest that the pressure
exerted by the downward force of the free-standing
pool above the substrate in a SAPS reduces the
risk of physical plugging compared to the lateral
flow in an ALD (Kepler and McCleary 1994,
McCleary and Kepler 1994). The SAPS design,
with adequate freeboard, allows for the buildup of
the static head required to move water down and
through the substrates. This, in combination with
the bottom discharge, maintains vertical flow.
SIZING
A SAPS should be sized to provide the reten-
tion time required to produce the needed alkalin-
ity. The limestone dissolution in ALDs
is viewed as a rate function, with 12 to 15 hours
regarded as a minimum retention time for maxi-
mum alkalinity production (Hedin et al. 1994,
Kepler and McCleary 1994). The detention time
in a SAPS should be similar and the calculation of
the amount of limestone needed should be calcu-
lated similarly to that for an ALD.
CONFIGURATION
The configuration of a SAPS is shown in
figure 3. At the three SAPSs in Pennsylvania, the
depths of the free-standing water are 5 - 6 ft, the
depth of the organic layer (spent mushroom
compost) is 18 inches, and the depths of the
limestone are 18 - 24 inches (Kepler and McCleary
1994). Vertical flow is created by placing the '
discharge pipes at the bottom of the SAPS.
Clogging has not been a problem at any of the
three sites. This is thought to be due to the down-
ward pressure of the freestanding water plus the
lack of free oxygen in the substrate (Kepler and
McCleary 1994).
A SAPS must be followed by a settling pond
where Fe precipitation can occur. A vegetated
aerobic wetland should be placed after the settling
pond to aid in removing suspended solids and to
provide a sustainable supply of oxidizable organic
matter to a subsequent SAPS.
VOLUME 4: COAL MINE DRAINAGE
25
-------
LONG-TERM PERFORMANCE
SAPS treatment is a recent concept and there
are no long-term data. The longevity of effective
SAPS treatment will depend upon the amount of
limestone available for dissolution and the amount
of alkalinity generated by bacterial activity in the
organic layer. Kepler and McCleary (1994) suggest
that additional organic material can be added to a
SAPS by feeding the SAPS with water from an
aerobic wetland.
Organic Material
'1-v.;-.-. r v-.v
Limestone ••
Figure 3. Schematic of a successive alkalinity-producing system (SAPS).
26
VOLUME 4: COAL MINE DRAINAGE
-------
REFERENCES
APHA (American Public Health Association).
1992. Standard Methods for the .Examination of
Water and Wastewater, 18th edition. Washing-
ton, DC.
Brodie, G. A., C. R. Britt, T. M. Tomaszewski, and
H. N. Taylor. 1993. Anoxic limestone drains to
enhance performance of aerobic acid drainage
treatment wetlands: experiences of the Tennessee
Valley Authority, pp 129-138 in Constructed
Wetlands for Water Quality Improvement, G. A.
Moshiri (ed.). CRC Press, Boca Raton, FL.
Faulkner, B. B., and J. G. Skousen. 1994. Treat-
ment of acid mine drainage by passive treatment
systems, pp 250-257 in Volume 2 of Proceedings
of the International Land Reclamation and Mine
Drainage Conference and the Third International
Conference on the Abatement of Acidic Drainage,
Pittsburgh, PA, April 24 - 29,1994.
Hedin, R. S., and R. W. Nairn. 1991. Constructing
wetlands to treat coal mine drainage. Course
notes for National RAMP Workshop, Pittsburgh,
PA, May 8, 1991.
Hedin, R. S., and R. W. Nairn. 1993. Contaminant
removal capabilities of wetlands constructed to
treat coal mine drainage, pp 187-195 in Con-
structed Wetlands for Water Quality Improve-
ment, G. A. Moshiri (ed.). CRC Press, Boca
Raton, FL.
Hedin, R. S., R. W. Nairn, and R. L. P. Kleinmann.
1994. Passive Treatment of Coal Mine Drainage.
Bureau of Mines Information Circular 9389.
US Bureau of Mines, Pittsburgh, PA. 35 pp.
Hedin, R. S., and G. R. Watzlaf. 1994. The effects
of anoxic limestone drains on mine water chem-
istry, pp 185-194 in Volume 1 of Proceedings of
the International Land Reclamation and Mine
Drainage Conference and the Third International
Conference on the Abatement of Acidic
Drainage, Pittsburgh, PA, April 24 - 29,1994.
Hellier, W. W. and R. S. Hedin. 1992. The mecha-
nism of iron removal from mine drainages by
artificial wetlands at circumneutral pH. p 13 in
INTECOL'S IV International Wetlands Conference
Abstracts, Columbus, OH.
Kepler, D. A., and E. C. McCleary. 1994. Succes-
sive alkalinity-producing systems (SAPS) for the
treatment of acidic mine drainage, pp 195-204 in
Volume 1 of Proceedings of the International
Land Reclamation and Mine Drainage Conference
and the Third International Conference on the
Abatement of Acidic Drainage, Pittsburgh, PA,
April 24 - 29, 1994.
McCleary, E. C., and D. A. Kepler/ 1994. Ecological
benefits of passive wetland treatment systems
designed for acid mine drainage: with emphasis
on watershed restoration, pp 111-120 in Volume
3 of Proceedings of the International Land Recla-
mation and Mine Drainage Conference and the
Third International Conference on the Abatement
of Acidic Drainage, Pittsburgh, PA, April 24-29,
1994.
Nairn, R. W., R. S. Hedin, and G. R. Watzlaf. 1991.
A preliminary review of the use of anoxic lime-
stone drains in the passive treatment of acid mine
drainage, in Proceedings of Twelfth Annual West
Virginia Surface Mine Drainage Task Force
Symposium, Morgantown, WV, April 3 - 4, 1991.
Stark, L. R. 1992. Assessing the longevity of a
constructed wetland receiving coal mine drainage
in eastern Ohio, p 13 in INTECOL'S IV Interna-
tional Wetlands Conference Abstracts, Columbus,
OH.
Stark, L. R., F. M. Williams, S. E. Stevens, Jr., and
D. P. Eddy. 1994. Iron retention and vegetative
cover at the Simco constructed wetland: an
appraisal through year 8 of operation, pp 89-98
in Volume 1 of Proceedings of the International
Land Reclamation and Mine Drainage Conference
and the Third International Conference on the
Abatement of Acidic Drainage, Pittsburgh, PA,
April 24 - 29, 1994.
VOLUME 4: COAL MINE DRAINAGE
27
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Taylor, H. N., K. D. Choate, and G. A. Brodie.
1993. Storm event effects on constructed
wetland discharges, pp 139-145 in Constructed
Wetlands for Water Quality Improvement, G. A.
Moshiri (ed.). CRC Press, Boca Raton, FL.
Wieder, R. K. 1989. A survey of constructed
wetlands for acid coal mine drainage treatment
in the eastern United States. Wetlands 9(2):299-
315.
Wieder, R. K., M. N. Linton, and K. P. Heston.
1990. Laboratory studies of Fe, Al, Mn, Ga, and
Mg dynamics in wetlands. Water, Air and Soil
Pollution 51:181-196.
28 VOLUME 4: COAL MINE DRAINAGE
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ABBREVIATIONS AND CONVERSION FACTORS
MULTIPLY
ac, acre ' -,
cfs, cubic foot per second
cfs, cubic foot per second
cm, centimeter
cm/sec, centimeter per second
°F, degree Fahrenheit
ft, foot
ft2, square foot
ft3, cubic foot
ft/mi, foot per mile
fps, foot per second
g/mz/day, gram per square meter per day
gal, gallon
gal, gallon
gpm, gallon per minute
ha, hectare
inch
kg, kilogram
kg/ha/day, kilogram per hectare per day
kg/m2, kilogram per square meter
L, liter
L, liter
lb, pound
Ib/ac, pound per acre
m, meter
m2, square meter
m3, cubic meter
m3, cubic meter
m3/ha/day, cubic meter per hectare per day
mm, millimeter
mi, mile
BY
0.4047
448.831
2.8317 xlO'2
0.3937
3.28 x 10'2
5/9 (°F - 32)
0.305
9.29 x 10'2
2.83 X lO'2
0.1895
18.29
8.92
3.785
3.785 x 10'3
6.308 x 10'2
2.47
2.54
2.205
0.892
0.2
3. 531 x 10'2
0.2642
0.4536
1.121
3.28
10.76
1.31
264.2
106.9
3.94 x 10'2
1.609
TO OBTAIN
ha, hectare
gpm, gallon per minute
m3/s, cubic meter per second
inch
fps, foot per second
°C, degree Celsius
m, meter
m2' square meter
m3, cubic meter
rh/km, meter per kilometer
m/min, meter per minute
Ib/ac/day, pound per acre per day
L, liter
m3, cubic meter
L/s, liter per second
ac, acre
cm, centimeter
lb, pound
Ib/ac/day, pound per acre per day
lb/ft2, pound per square foot
ft3, cubic foot
gal, gallon
kg, kilogram
kg/ha, kilogram perhectare
ft, foot
ft2, square foot
yd3, cubic yard
gallon, gal
gallon per day per acre, gpd/ac
inch
kilometer, km
-------
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