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Seasonal Changes in Biogeochemistry of a Natural Wetland Receiving Drainage from an Abandoned Mine
Diane McKnight and Eric August – University of Colorado at Boulder, Institute of Arctic and Alpine Research
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Acid Mine Drainage Formation
FeS2 (s) + 7/2O2 + H2O Fe2+ + 2SO42- + 2H+
Fe2+ + 1/4O2 + H+ Fe3+ + 1/2H2O
FeS2 (s) + 14Fe3+ + H2O 12Fe2+ + 2SO42- + 16H+
Fe2+ Fe3+ + e- or
Fe2+
Fe3+
FeS2 (s)
FeS2 (s)
FeS2 (s)
FeS2 (s)Iron, sulfate and acid released into water
Additional metals dissolve that are
associated with pyrite or in
surrounding rock: Zn, Pb, Mn, Cu, Ag, Al
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Conventional Treatment of AMD
Difficulties and Challenges
* Mining in Colorado essentially stopped after the 1950’s
* AMD Referred to as the greatest water quality problem in Western U.S. (Mineral Policy Center, 1997)
* $$$ needed for chemical usage and maintenance
* Requires accessibility and infrastructure in relatively isolated areas
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Wetlands as a treatment to AMD
Advantages
* low cost – construction costs can be recovered within one year of operation, through savings in chemical usage (Kleinmann, 1989)
* no continuous maintenance
Disadvantages
* long-term potential unknown
* limited knowledge concerning AMD wetlands effect on watershed scale
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Biogeochemical Processes in AMD wetlands
1. Ion exchange and adsorption
2. Complexation with organic matter
• Sedimentation
4. Plant uptake
5. Oxidation and precipitation of metal oxides
6. Sulfate reduction and precipitation of metal sulfides
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Dinero Mine operated 1891-1939part of Sugar Loaf Mining District Dinero Tunnel and all
Lake Fork Creek sites sampled 30 times: Sept.1999-2000 (avg. every 12 days)
Samples collected 3 times on each sampling date(8 am, noon, 5 pm)
Sites within wetland sampled during summer
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Dis
char
ge (L
/s)
0
1
2
3
4
5
6
7
8
snowmelt
wetland frozen & snow covered
Groundwater comprises majority of flow from Dinero Tunnel (Westside Eng., 1983)
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Conce
ntr
atio
n (
mg/L
)
0
10
20
30
40
50 IronZincManganese
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
200
250
300
350
400
450
500
Sulfate
snowmelt
snowmelt
Fe: 90% Fe2+
85% dissolvedMn: 100% dissolvedZn: 100% dissolved
Al: 15 ug/LCu: NDCd: 12 ug/LPb: 31 ug/L
DOC: 0.7 mg/LpH: 6.4
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Iron
Mas
s F
low
(g/
hr)
0
200
400
600
800
1000
2800
3000
Dinero Tunneldelta-LFC
snowmelt
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Research Site Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO
50 m
Site Fe (mg/L) pH inflow 34 6.2U1 25 5.8U2 1.1 3.9M1-3 0.9 3.9L1-4 0.7 3.7
Fe3+ + 3OH- Fe(OH)3
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snowmeltDate
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Zin
c M
ass
Flo
w (
g/hr
)
0
200
400
600
800
1000
1200
Dinero Tunneldelta-LFC
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Research Site Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO
50 m
Site Zn (mg/L) pHinflow 13 6.2U1,2 12.2 5.8,3.9M1,2 9.3 3.8M3 6.9 4.0 L1-3 6.8 3.7L4 0.05 6.9
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Research Site Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO
4 m2 vegetation plots
Divided into 4 sections and sampled for metals (Fe, Zn, Mn) and biomass – sampled every 30 days
Concentration & biomass data combined to estimate mass of metal bound in vegetation
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Bound Zinc in Wetland Vegetation
0
50
100
150
200
May-00 Jun-00 Jul-00 Aug-00 Sep-00
Bo
un
d Z
n (
mg
/g)
L1
L2
L3
L4
M1
M2
M3
U2
U1
Total Mass: 1.1 kg 2.2 kg 2.4 kg 3.0 kg 3.1 kg
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High zinc and sulfur sediment concentrations indicate precipitation of ZnS – mostly in mid and lower reaches of wetland
core depth to Fe (mg/g) Zn (mg/g) Mn (mg/g) Sulfur (mg/g)location clay (cm)
U1 22 45.9 0.58 1.79 0.59U2 55 23.3 0.28 1.59 0.81M1 25 20.4 1.52 1.28 5.29M3 25 7.91 0.52 0.95 0.74L1 70 17.1 2.03 1.79 1.32L2 57 32.4 2.44 1.95 4.15L4 15 14.0 0.24 0.22 2.50
~ 46,000 kg of zinc in sediment (peat) of wetland
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Total mass of zinc stored in wetland on par with amount retained during 1999-2000.
Fe (kg) Zn (kg) Mn (kg)
July - May June - November June - October
3300 450 650
December - March November - March
90 750
June April - June April - June
590 640 1300
110 210 400
Net Sink
Net Source
Snowmelt
Estimated Load from Tailings
during Snowmelt
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snowmeltDate
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Zin
c M
ass
Flo
w (
g/hr
)
0
200
400
600
800
1000
1200
Dinero Tunneldelta-LFC
Wetland became source of Zn in winter and snowmelt through tailings became source in May and June
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High mass loading of zinc calculated during snowmelt period (early April – mid June)
Fe (kg) Zn (kg) Mn (kg)
July - May June - November June - October
3300 450 650
December - March November - March
90 750
June April - June April - June
590 640 1300
110 210 400
Net Sink
Net Source
Snowmelt
Estimated Load from Tailings
during Snowmelt
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date pH ± sd Fe ± sd (mg/L) Zn ± sd (mg/L) Mn ± sd (mg/L) Est. Flow (m3/day)
4/8/00 3.3 ± 0.1 9.95 ± 1.39 33.3 ± 1.71 76.4 ± 3.25 145/5/00 2.9 ± 0.1 26.1 ± 2.62 49.3 ± 4.17 102 ± 6.49 1405/14/00 3.0 ± 0.1 21.1 ± 1.29 29.9 ± 1.87 54.9 ± 3.25 1205/26/00 2.9 ± 0.1 11.6 ± 3.24 31.7 ± 5.42 46.7 ± 2.08 796/8/00 3.1 ± 0.1 9.09 ± 8.50 26.2 ± 6.78 49.3 ± 4.51 356/17/00 3.2 ± 0.1 2.65 ± 0.61 18.0 ± 0.94 43.3 ± 0.53 10
Loading estimated from tailings only accounts for 1/3 of difference between ÄLFC and Dinero Tunnel mass flows
• Tailings source underestimated
• All inflows not measured
• Flushing in wetland by snowmelt
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Man
gane
se M
ass
Flo
w (
g/hr
)
0
300
600
900
1200
1500
1800
2100
Dinero Tunneldelta-LFC
snowmelt
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Research Site Lake Fork Creek and El Rojo Wetland located 5 miles west of Leadville, CO
50 m
Site Mn (mg/L) pHinflow 40 6.2U1,2 39 5.8,3.9M1,2 37 3.8M3 34 4.0 L1-3 36 3.7L4 2.1 6.9
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Total Mass: 11 kg 23 kg 24 kg 31 kg 33 kg
Bound Manganese in Wetland Vegetation
0
500
1000
1500
2000
May-00 Jun-00 Jul-00 Aug-00 Sep-00
Bo
un
d M
n (
mg
/m2 )
L1
L2
L3
L4
M1
M2
M3
U2
U1
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High manganese and sulfur sediment concentrations indicate precipitation of MnS possible – with no clear spatial variation observed
~ 58,000 kg of manganese in sediment (peat) of wetland
core depth to Fe (mg/g) Zn (mg/g) Mn (mg/g) Sulfur (mg/g)location clay (cm)
U1 22 45.9 0.58 1.79 0.59U2 55 23.3 0.28 1.59 0.81M1 25 20.4 1.52 1.28 5.29M3 25 7.91 0.52 0.95 0.74L1 70 17.1 2.03 1.79 1.32L2 57 32.4 2.44 1.95 4.15L4 15 14.0 0.24 0.22 2.50
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Total mass of manganese stored in wetland on par with amount retained during 1999-2000.
Fe (kg) Zn (kg) Mn (kg)
July - May June - November June - October
3300 450 650
December - March November - March
90 750
June April - June April - June
590 640 1300
110 210 400
Net Sink
Net Source
Snowmelt
Estimated Load from Tailings
during Snowmelt
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Stoichiometry indicates S and Mn + Zn would fall on line of slope = 1
Sulfur (mmol/g)0.00 0.03 0.06 0.09 0.12 0.15 0.18
Mn
+ Z
n (m
mol
/g)
0.00
0.03
0.06
0.09
0.12
0.15
0.18
Precipitation of MnS and ZnS likely
Form of Mn and Zn in sediment not fully tested
Other possibilities: metal-organic complexes, metals sorbed to inorganic species, metals sorbed to other metal oxides
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Dis
char
ge (
L/s)
0
200
400
600
5000
10000
15000
20000
snowmelt
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Dis
char
ge (
L/s)
0
3000
6000
9000
12000
15000
Arkansas RiverHalf Moon Creek
snowmeltDate
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Dis
char
ge (
L/s)
0
3000
6000
9000
12000
15000
Arkansas RiverHalf Moon CreekLake Fork Creek
snowmelt
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Date
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Fe
ma
ss f
low
(g
/hr)
0
1000
2000
3000
4000
5000
snowmelt
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct
Fe
co
nce
ntr
atio
n (
ug
/L)
20
40
60
80
100
120
140
160
180
LF
C d
isch
arg
e (
L/s
)
0
3000
6000
9000
12000
15000
18000
Fe LFC-4 concLFC discharge
Fe concentration relatively constant through seasons
20 times less than chronic toxicity level
Mass flow controlled by LFC discharge
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Conclusions
• Wetland retained over 50% of all metals flowing from Dinero Tunnel during summer months.
• Aerobic precipitation of iron oxides and anaerobic precipitation of manganese and zinc sulfides were the dominant removal mechanisms.
• High loading of metals from the tailings during snowmelt was a critical time of year – intensified by unnatural flow conditions downstream of Sugar Loaf Dam.
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General Conclusions
• Seasonal trends (mine, wetland, stream) are important in understanding ecosystem impacts, making water management decisions, and planning remediation.
2. Wetland efficiency in removing metals may decrease with age.
3. In remediation of AMD sites, the role of mine tailings should be considered in design.