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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.