&EPA
                           United States
                           Environmental Protection
                           Agency
EPA/540/SR-93/523
September 1993
                          SUPERFUND INNOVATIVE
                          TECHNOLOGY EVALUATION
                           Emerging Technology
                           Summary

                           Handbook for Constructed
                           Wetlands  Receiving Acid
                           Mine Drainage
                             A treatment technology based on
                           constructed  wetlands uses  natural
                           geochemical and biological processes
                           inherent in the aqueous environment
                           and designs a system to optimize pro-
                           cesses best suited to removal of con-
                           taminants specific to  the site. Key
                           features of this wastewater technology
                           are that it is a passive treatment sys-
                           tem, the cost of operation and mainte-
                           nance is significantly lower than that
                           for active treatment processes, and the
                           removal methods try to mock rather
                           than  overcome natural processes. In
                           this study, the contaminant waters were
                           metal-mine drainages with low pH (<3.0)
                           and high concentrations of metals (Al,
                           Mn, Fe, Ni, Cu, Zn, and Pb).
                             From  studies done at  constructed
                           wetlands at the Big Five Tunnel near
                           Idaho Springs, Colorado, the important
                           process for raising the pH and remov-
                           ing metals was found to  be bacterial
                           sulfate reduction followed by precipita-
                           tion of metal sulfides. By optimizing
                           the process and determining how to
                           properly load the wetland with contami-
                           nant drainage, the following results
                           were achieved:
                               • pH was raised from 2.9 to 6.5.
                               • Dissolved AI, Cu, Zn, Cd, Ni, and
                                Pb concentrations were reduced
                                by 98 % or more.
   • Iron removal was seasonal with
     99% reduction in the summer.
   • Mn reduction was relatively poor
     unless the pH of the effluent was
     raised above 7.0.
   • Biotoxicity to fathead minnows
     and Ceriodaphnia was reduced
     by factors of 4 to 20.
  Once it was found that microbial pro-
cesses were primarily responsible for
contaminant removal, a staged design
process comparable to the design pro-
cess used for other wastewater treat-
ment  technologies  was devised.
Laboratory studies determine whether
in principle contaminants could be re-
moved and the best substrate combi-
nation for their removal. Bench scale
studies determine the optimum loading
capacity and treatment system configu-
ration. From these studies design of a
reasonably sized module that is spe-
cific to the site can proceed with the
expectation that it will successfully treat
the contaminanted water.
  This summary was  developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the SITE Emerging Tech-
nology Program that is documented in
a separate report (see ordering  infor-
mation at back).

         OyD Printed on Recycled Paper

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Introduction
  In response to the Superfund  Amend-
ments  and Reauthorization Act of 1986,
(SARA), the U. S.  Environmental Protec-
tion Agency's  (EPA) Office of Research
and Development (ORD) and the Office of
Solid Waste and Emergency  Response
(OSWER) have established a formal pro-
gram to accelerate the development, dem-
onstration, and use of new or  innovative
technologies as alternatives to current con-
tainment systems for hazardous wastes.
This program is called Superfund Innova-
tive Technology Evaluation or SITE.
  The  SITE program is part of EPA's re-
search into cleanup methods for hazard-
ous waste sites throughout the  nation.
Through cooperative agreements with de-
velopers,  alternative or innovative tech-
nologies  are refined at the bench-scale
and  pilot-scale  level  and  then  demon-
strated at actual sites. EPA collects and
evaluates extensive performance data on
each technology to use in remediation de-
cision making for hazardous waste sites.
  The  report summarized here documents
the results of laboratory and  pilot-scale
field tests on the applicability  of sulfate-
reducing bacteria operating in the anaero-
bic zone within a wetland constructed to
remove contaminant  metals  associated
with mine drainages. These metals  can
include Al, Mn, Fe, Co, Ni, Cu, Zn, As, Ag,
Cd, Hg,  and  Pb.  In the  mine drainage
Water used in this study, Mn, Fe, Cu, and
Zn are the  primary contaminants in the
water.  Also, most mine drainages have an
actdic  pH that causes concern and has to
be Increased to effect treatment.  In the
water  used in this study, the average pH
is 3.0.

The Concept of Constructed
Wetlands
   Ecotogists have  long  understood  that
soils in wetlands are often foul  because
they naturally accumulate contaminants by:

   • filtering of suspended and  colloidal
    material from the water;
   • uptake of contaminants into the roots
    and leaves of live plants;
   * adsorption or exchange of  contami-
     nants onto inorganic soil constituents,
    organic solids, dead plant material, or
     algal material;
   •  neutralization and precipitation of con-
    taminants through the generation of
     HCOj'and NH3 by bacterial  decay of
     organic matter;
   «  destruction or precipitation of contami-
     nants in the aerobic zone catalyzed
     by the activity of bacteria; and
  • destruction  or  precipitation  of
    chemicals  in  the anaerobic  zone
    catalyzed by the activity of bacteria.

  With so  many  possible removal pro-
cesses, a wetland,  such as depicted in
Figure 1 , is the typical contaminant treat-
ment  system in a  natural  ecosystem. In
addition,  it operates in a  passive  mode
requiring  no additional reactants and no
continuous maintenance.
  In the last decade, engineers  began to
use wetlands to remove contaminants from
water. In some instances, natural wetlands
were used. A natural system however, will
accommodate  all the above removal pro-
cesses and  probably will not operate to
maximize a certain process. A constructed
wetland, on the other hand, can be  de-
signed to maximize a specific  process suit-
able for the  removing of certain contami-
nants. Engineering and ecological reasons
lead to using  a constructed wetland for
contaminant removal rather than using an
existing natural ecosystem.
  As  an  example of constructing  a wet-
land to maximize  specific removal pro-
cesses, consider the bacterial processes
that are items 6 and 7 in  the above list.
Typical microbially mediated reactions that
are possible in the aerobic zone of a wet-
land include:

  4 Fe2* + O2 + 10 H2O — >

  4 Fe(OH)3 + 8 H+

  2 02 + H2S — > S04- + 2 H*

  2 H20 + 2 N2 + 5 Oz— > 4 NCy + 4 H*

  Typical microbially mediated  reactions
that are possible in the anaerobic zone of
a wetland include:
  4Fe2*+CO2-i-11 H2O

  5 CHO + 4 NO- + 4 H* — >
2 N + 5 C0
               7 H20
SO4- +2 CH2O — > H2S
                       2 HCO-
   In these reactions, "CH2O" is used to
 symbolize organic material in the substrate.
   It is apparent that the anaerobic reac-
 tions are approximately the reverse of the
 aerobic reactions. Both zones  exist in a
 wetland. If removal involves aerobic pro-
 cesses, then the wetland should be con-
 structed so the water remains on the sur-
 face. If removal  involves  anaerobic pro-
 cesses, then the wetland should be con-
 structed so the water courses through the
                                       substrate. In a natural wetland, the water
                                       primarily remains on the surface.
                                         In the important area of microbially me-
                                       diated removal, the wetland must be con-
                                       structed to  maximize removal reactions
                                       and minimize competing  reactions. When
                                       removing contaminants  from  acid  mine
                                       drainage, the removal processes should
                                       consume hydrogen  ions  and,  conse-
                                       quently, anaerobic processes are empha-
                                       sized. The research and development at
                                       the Big Five Tunnel site in Idaho Springs,
                                       Colorado has concentrated on understand-
                                       ing the chemistry and ecology involved in
                                       removal and  designing  structures from
                                       readily available  materials that maximize
                                       these processes.
                                         Although this appears  to be "low tech-
                                       nology", an  intense interdisciplinary effort
                                       and creative engineering  skills are needed
                                       to design and perfect systems that maxi-
                                       mize natural processes.  For more details
                                       on what should  be considered, The full
                                       report cites recent references.

                                       The Big Five Pilot Wetland
                                         The research reported here has involved
                                       studying removal processes from a pilot
                                       constructed wetland designed to receive
                                       metal mine drainage from the Big  Five
                                       Tunnel in Idaho Springs,  CO. The chemis-
                                       try from the adit drainage  is  reasonably
                                       constant throughout the year and is sum-
                                       marized in  Table 1. After a  number of
                                       modifications of  the pilot cells, removal
                                       results were excellent.

                                       Table 1. Contaminant Concentration, Big Five
                                               Tunnel Drainage, Averages.
                                           Constituent
                    Concentration,mg/L
Mn
Fe
Co
Ni
Cu
Zn
Cd
Pb
31
38
0.10
0.15
0.73
9.4
0.03
0.03
   Figure 2 shows the removal trends for
a 2-year period as outflow concentrations
over influent concentrations. Cu  and Zn
are completely  removed;  Fe  removal
changes with the seasons.
  During this  2-year period, analysis  of
chemical data accumulated at the site led
to the conclusion that microbial reduction
of sulfate to sulfide followed by precipita-
tion of heavy  metal  sulfides is the pre-
dominant process accounting for the re-
moval of over 90 % of the Fe, Cu, and Zn
and the rise in the  pH from below 3 to

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above  6. Procedures for construction of
wetland ceils that emphasize  anaerobic
removal processes has been considered.
Another consideration is  how to employ
knowledge of the biochemistry of sulfate
reduction in  the wetland design process.
This has been done by using  two ideas
particularly suited to wetlands-ideas that
employ anaerobic removal processes such
as sulfate reduction: The limiting reagent
concept, and staged design of wetlands.

The Limiting Reagent Concept
  In the design of wetlands for wastewa-
ter treatment, there is a strong emphasis
on  determining the  loading factor which
gives an indication of how large a wetland
should be to remove the contaminants of
concern. This can be stated as the amount
of square feet of wetland per gallon  per
minute of water to be treated  or as  the
grams  of  contaminant removed per day
per square meter of wetland. In our expe-
riences at the  Big Five site, typical mea-
sures of  loading factor  do  not seem to
explain the removal of metals even though
heavy  metals  such as  Cu  and Zn  are
                         Porous
                         Rock
                         Dam
Figure 1. Diagram of a typical wetland ecosystem that emphasizes subsurface flow.
                                           Cell E Removal Trends From Sept. 1989
                    1.00
                                                                                                            MnE
                                                                                                             FeE
                                                                                                             CuE
                                                                                                             ZnE
                                                                                                      ._. SO4E
                    0.00
                                                         Months
 Figure 2. Two year removal trends for a subsurface wetland cell located at the Big Five Tunnel in Idaho Springs, Colorado.

                                                             3

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reduced by greater than 99 %. We have
discovered that a key factor in sulfate
reduction  is to Insure that the optimum
microenvironment for sulfate-reducers  is
maintained. The most important environ-
mental conditions are reducing conditions
and a pH  of around 7. Since the wetland
cell is  receiving mine drainage  with pH
below 3 and Eh  of  above 700  mV, the
water can easily overwhelm the  micro-
environment established by the anaerobic
bacteria. This leads to the limiting reagent
concept for determining how much water
can be treated, as an alternative  to the
use of typical bading factors.
  Consider the following precipitation re-
action:
        Fe^-t-S--
->FeS
  At high flows of mine drainage through
the substrate, sulfide will  be the  limiting
reagent, the microbial environment will be
under stress to produce more sulfide, the
pH of the microenvironment will drop, and
removal will be Inconsistent. At low flows
of mine  drainage through the substrate,
iron will  be the limiting reagent, the ex-
cess sulfide will insure a reducing  envi-
ronment  and a pH  near 7, the microbial
population will remain healthy, and removal
of the metal contaminants will be  consis-
tent and  complete. Using this  idea,  load-
ing factors should be set to insure that the
heavy metal contaminants are  always the
limiting reagents. The question then is how
much sulfide can a  colony of sulfate-re-
ducing bacteria produce per cubic centi-
meter of  substrate per day?
  Studies by the U. S. Bureau of Mines
wetlands group suggests  that a reason-
able figure for sulfide generation is 300
nanomole sutfide/cubic cm/day. This  num-
ber, the  volume of the wetland cell, and
the metals concentrations in the  mine
drainage are used to set the flow of  mine
drainage through the wetland  cell. Using
this concept in a subsurface wetland cell
to determine the loading  factor has  re-
sulted in year round complete  removal of
Cu and Zn (Rgure 2).

Staged Design of Wetlands
  After determining  that precipitation of
metals by sulfide generated from  sulfate-
reducing bacteria is the important process,
H was realized that establishing and main-
taining the  proper environment in the sub-
strate is  the key to  success for removal.
This means that processes operating on
the surface of the  wetland are  not that
Important. In particular, plants are not nec-
essary In a wetland emphasizing  subsur-
face processes. If this is so, then a  large
pilot cell, such as was built at the Big Five
site, is not necessary to determine whether
a wetland that emphasizes anaerobic pro-
cesses  for  removal  will  work. Conse-
quently, the study of  wetland  processes
and the design of optimum systems can
proceed  from laboratory experiments to
bench scale  studies to  design  and con-
struction  of  actual cells.  We call this
"staged design of wetland systems".
  In current  laboratory studies,  culture
bottle experiments  are  used for funda-
mental studies on how to establish simple
tests to  determine the production of sul-
fide by bacteria, and of what substrate will
provide the best initial conditions for growth
of sulfate-reducing bacteria. In these ex-
periments, laboratory production of sulfide
at 18 °C  has  been 1200 nanomole/gm of
dry substrate/day.
  Culture bottle tests  have shown that in
the case  of cyanide, sulfate reduction was
retarded  until the concentration of total
cyanide was  below 10 mg/L and that Cu
concentrations above  100 mg/L would kill
or retard sulfate-reducing bacteria. How-
ever,  other culture bottle tests have also
shown that sulfate reduction was still vig-
orous at Cu and Zn concentrations above
100 mg/L.
  For bench scale studies, plastic gar-
bage  cans are used  to conduct experi-
ments to provide answers necessary to
the design of  a subsurface cell, e.g. deter-
mining the optimum loading factor, sub-
strate, cell configuration,  and  substrate
permeability.  In a recent study,  garbage
cans  filled with  substrate  were used to
determine whether using the sulfide gen-
eration figure of 300 nanomole sulfide/cm3
of substrate/day could be used to set the
conditions for treating severely contami-
nated drainage that flows from the Quartz
Hill Tunnel in Central  City,  CO. Contami-
nant concentrations are shown in Table 2.
  Using the limiting reagent concept de-
scribed  above  and the amount of sub-
strate contained in the garbage can, flow
could not exceed 1  ml/min to ensure that
sulfide would always  be in excess. Con-
taminant concentrations from the outputs
of three  different bench scale  cells are
shown in Table 2. For cell A  the mine
drainage was passed through the cell with
no  delay. For cell  B  the  substrate was
soaked with city water for one  week be-
fore mine drainage started passing through
the cell.  For cell C,  the  substrate was
inoculated with an  active  culture of sul-
fate-reducing  bacteria and soaked with city
water for one week before mine drainage
started passing through the cell. Prepara-
tions on  cells B and C were done to en-
sure that there  would  be a healthy popu-
lation of  sulfate-reducing bacteria  before
mine  drainage  flowed  through  the  sub-
strate. All cells were  run  in a  downflow
mode of the mine drainage through the
substrate. In all three cells removal of Cu,
Zn, Fe, as  well as  Mn  is greater  than
99%.  The increase in  pH is from about
2.5 to above 7. These results were  con-
sistently maintained for over ten weeks of
operation.
  The substrate used  was a mix of 3/4
cow manure and  1/4  planting  soil. The
results from cells B and C show that the
cow manure has an indigenous popula-
tion of sulfate-reducing bacteria that are
quite active. Inoculation with an active cul-
ture of bacteria is not necessary in this
case. Also,  since the results from cell A
are comparable to those of cells B and C,
the population of sulfate reducers can with-
stand immediate exposure to severe  mine
drainage and still produce sufficient quan-
tities  of sulfide. The key  to good  initial
activity is to ensure that the flow of  mine
drainage is  low enough that its low pH
does  not  disturb the  micro-environment
established by the  bacteria.
  These bench scale systems also serve
as  permeameters  and thus  can provide
important information for others  aspect of
wetland design. Determination of soil con-
ductivity and how this physical  property
changes with time is found to be an im-
portant geotechnical parameter for the de-
sign of subsurface constructed wetlands.

Conclusions
  Using constructed  wetlands for waste-
water treatment is  still a developing  tech-
nology. The results from the Big  Five Pilot
Wetland study, however, show promising
removal of heavy metals and increase of
pH for acid mine  drainage.  Conclusions
from the project include:

  • Toxic metals such as  Cu and Zn can
    be  removed  and the pH   of  mine
    drainage can be increased on a long
    term basis.
  • The major removal process is sulfate
    reduction and subsequent precipitation
    of the metals  as  sulfides.  Exchange
    of metals onto organic matter can be
    important during the initial  period of
    operation.
  • A trickling filter type of configuration
    achieves the best contact of the water
    with the substrate.
  • Removal efficiency depends strongly
    on  loading factors. In the  Big  Five
    wetland and in bench scale studies,
    flow of water should not exceed the
    300 nanomoles/cmVday of sulfide that
    can be generated by the microbes in
    the substrate.

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  •  Permeability  of  the  substrate is a
    critical design variable for successful
    operation.  Using laboratory and       5.
    bench-scale tests, a  good indication
    of  the  soil permeability  in  a
    constructed  wetland   can   be
    determined.
  •  As with, any other wastewater removal
    technology, design of a constructed
    wetland  or  passive bioreactor  is
    specific to the site and the water to
    be treated.                              6.
  •  A staged design and  development
    sequence  can  be  used where
    laboratory  studies are  used   to
    determine  the best  conditions  and
    substrate, bench scale experiments
    help to determine loading factors and
    substrate  properties,  and  pilot
    modules test the performance of a
    typical field module.

References                               7.
  1.  Reed,  S. C.,  Middlebrooks, E. J.,
      and Crites, R.  W. Natural Systems
      for Waste Management and Treat-
      ment.  McGraw-Hill,  New York,
      1988. 308pp.
  2.  Hammer,  D. A. Constructed Wet-
      lands  for Wastewater  Treatment.       8.
      Lewis  Publishers,  Chelsea, Michi-
      gan, 1989. 800 pp.
  3.  Kleinmann, R.  L P. (ed.), Proceed-
      ings of a Conference on Mine Drain-
      age and Surface Mine Reclamation.
      Vol. 1. Mine Water and Mine Waste.
      U.  S.  Department of the Interior,
      Bureau of Mines Information Circu-
      lar 1C 9183, 1988, 413 pp.              9.
  4.  Wildeman, T. R., and Laudon, L. S.
      The use of wetlands for treatment
      of environmental problems in min-
      ing: Non-coal  mining applications.
      In: D. A. Hammer (ed.), Constructed
      Wetlands  for  Wastewater Treat-
ment. Lewis Publishers, Chelsea,
Michigan, 1989. p. 221.
Howard, E. A., Emerick, J. C., and
Wildeman, T. R. The design, con-
struction and initial operation of a
research site for passive mine drain-
age  treatment  in Idaho Springs,
Colorado. In: D. A. Hammer (ed.),
Constructed Wetlands for Waste-
water Treatment. Lewis Publishers,
Chelsea, Michigan, 1989. p. 761.
Machemer, S. D.,  Lemke, P.  R.,
Wildeman,  T.  R.,  Cohen  R.   R.,
Klusman, R.  W.,  Emerick, J.  C.,
and  E. R.  Bates. Passive Treat-
ment of  Metals  Mine Drainage
through use of a Constructed Wet-
land. In: Proceedings of the 16th
Annual Hazardous Waste Research
Symposium, U. S. EPA, Cincinnati,
OH, 1990. EPA Document No. EPA/
600/9-90-037, pp. 104-114, 1990.
Machemer, S. D., and Wildeman,
T. R., Organic Complexation Com-
pared with  Sulfide Precipitation  as
Metal Removal Processes from Acid
Mine  Drainage in  a Constructed
Wetland".   Jour.  Contaminant
Hydrology,vol. 9, pp. 115-131,1992.
Hedin, R. S., and R. W. Nairn. Siz-
ing and Performance of Constructed
Wetlands:  Case Studies.  In: Pro-
ceedings of the 1990 Mining and
Reclamation   Conference,   J.
Skousen, J. Scencindiver, and  D.
Samuel  eds., West  Virginia Univ.
Publications,  Morgantown,  WV,
1990. pp. 385-392
Wildeman, T. R., Machemer, S.  D.,
Klusman, R. W., Cohen, R. R., and
P. Lemke. Metal Removal Efficien-
cies  from  Acid Mine  Drainage  in
the Big five Constructed Wetland.
In: Proceedings of the 1990 Mining
and  Reclamation Conference,  J.
   Skousen, J. Scencindiver,  and D.
   Samuel eds., West Virginia Univ.
   Publications,  Morgantown,  WV,
   1990. pp. 417-424.
10. Mclntire, P. E., and H. M. Edenborn.
   The use of Bacterial  Sulfate  Re-
   duction in the Treatment of Drain-
   age  from  Coal  Mines.   In:
   Proceedings of the 1990 Mining and
   Reclamation  Conference,   J.
   Skousen, J. Scencindiver,  and D.
   Samuel eds., West Virginia Univ.
   Publications,  Morgantown,  WV,
   1990. pp. 409-415.
11. Reynolds, J. S., Machemer, S. D.,
   Wildeman,  T. R., Updegraff, D. M.,
   and R. R. Cohen, 'Determination of
   the Rate of Sulfide Production  in a
   Constructed  Wetland Receiving
   Acid Mine  Drainage".  Proceedings
   of the 1991 National Meeting of the
   American Society of Surface  Min-
   ing and Reclamation,  ASSMR,
   Princeton, WV, 1991, pp 175 -182
12. Filas, B., and T. R. Wildeman,  The
   Use of Wetlands for Improving  Wa-
   ter Quality to  Meet  Established
   Standards", Nevada Mining Assoc.
   Annual Reclamation  Conference,
   Sparks, NV, May,  1992.
13. Bolis, J. L., Wildeman, T. R.,  and
   R.  R. Cohen, ' The Use of Bench
   Scale Permeameters for Preliminary
   Analysis of Metal Removal  from
   Acid Mine  Drainage by Wetlands".
   Proceedings of the  1991  National
   Meeting of  the American Society of
   Surface Mining and Reclamation,
   ASSMR, Princeton,  WV, 1991, pp
   123-136.
14. Wildeman,  T. R., Brodie, G. A., and
   J. J.  Gusek, Wetland Design for
   Mining Operations, Bitech Publish-
   ing Co. Vancouver, BC, Canada,
   1992, 300 pp
Table 2. Constituent concentrations in mg/L in the Quartz Hill Tunnel mine drainage and in effluents from the bench scale tests
Sample
Mine Drainage
Cell A
CellB
CellC
Mine Drainage
CellA
CellB
CellC
Mine Drainage
CellB
Cell C
Days
Operated
24
24
24
24
43
43
43
43
71
71
71
Mn
80.0
0.94
0.91
0.99
80.0
0.97
0.64
1.6
70.0
0.48
1.6
Fe
630.0
1.6
1.9
1.0
640.0
0.87
0.96
0.46
820.0
0.40
0.40
Cu
48.0
0.06
<0.05
<0.05
50.0
<0.05
<0.05
<0.05
70.0
<0.05
<0.05
Zn
133.0
0.27
0.17
0.16
135.0
0.18
0.24
0.14
101.0
0.21
0.25
so4
4240
450
70
412
4300
1080
660
1180
NA
NA
NA
pH
2.4
7.4
7.5
7.4
2.5
7.2
7.4
7.2
2.6
8.0
7.9
         •fru.8. GOVERNMENT PRINTING OFFICE: IM3 - 7SO-071/80M*

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   Thomas Wildeman is with the Department of Chemistry and Geochemistry,
     Colorado School of Mines, Golden, CO 80401
   Edward R. Bates is the EPA Project Officer (see below).
   The complete report, entitled "Handbook For Constructed Wetlands Receiving
       Acid Mine Drainages," (Order No. PB93-233914AS; Cost: $36.50, subject
       to change)  will be available only from:
          National Technical Information Service
          5285 Port Royal Road
          Springfield, VA 22161
          Telephone: 703-487-4650
   The EPA Project Officer can be contacted at:
          Risk Reduction Engineering Laboratory
          U.S. Environmental Protection Agency
          Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268

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