Mercury Accumulation in Alpine Lakes, Colorado David Manthorne, USGS Mark Williams, CU-Boulder.

Mercury Accumulation in Alpine Lakes, Colorado

David Manthorne, USGS

Mark Williams, CU-Boulder

Problem Statement• Concern for mercury arose from human health

effects caused by mercury ingestion from freshwater and marine fish, in which mercury bioaccumulates

• Documented bioaccumulation of mercury in fish has occurred in many high elevation lakes in Sweden (Johansson et al., 1995)

• In 1998, five water bodies in Colorado were put on the EPA section 303(d) list for impaired water bodies – Hg content in fish exceeded advisory levels of 0.5mg kg-1

MERCURY and BRAIN FOOD

• Mercury is toxic to the developing fetal brain

• Exposure to mercury in the womb can cause learning deficiencies and delay mental development in children

Mercury in the Environment

• In a recent EPA report to Congress (1994), they pointed to coal fired utilities as the major anthropogenic source of mercury

• It is suggested that deposition of mercury and organochlorines may increase with elevation because of cold condensation and orographic precipitation (grasshopper effect)

• This may be cause for concern in warm mid-latitude climates where water resources fall mainly in the form of snow (70%)

NOT A PROBLEM IN THE WEST?

Mercury Loading in Wetfall

MOUNTAINS ENHANCE MERCURY PROBLEMS

• Snowfall increases with elevation– Mercury deposition may increase with altitude

• Mercury transport associated with carbon– Carbon transport increases during snow melt– Mercury transport from soils to lakes may be

greater than in catchments without snow melt

• Increasing nitrogen deposition– May increase lake productivity– May increase mercury sequestration in lakes

Study Objectives

1) To compare wet vs. freeze-dried methods2) To evaluate trends in mercury accumulation

in alpine lakes of Colorado3) Place these results in context by comparing

to other sites4) Evaluate other factors that may worsen Hg

accumulation in Colorado lakes:1) Cold-condensation2) Increasing Hg deposition with elevation3) Snow melt4) Increased carbon production from N deposition5) Fires

Colorado Lakes Cored

Black LakeGreen Lake 4, 5

Pristine Lake

Navajo Lake

Pristine Lake

Navajo Lake

Black LakeGreen 4,5

Denver

Navajo, San Juan PP

Hayden, Craig

Sulfate/Mercury emissions (1,000/yr SO2)

Lake Sediment Cores - Sample Collection

• Lake sediment cores were collected with gravity corer from each lakes deepest point

• Samples were extruded in the field in .5 to 1cm intervals

• Samples were kept cold until they could be frozen

• Wet and freeze-dried samples were digested and analyzed with CVAFS

Methods

• Lake sediments were dated with 210Pb activity

• Sediment mass accumulation was calculated as the dry weight per section and years each section represented

• Hg mass flux per section calculated as Hg concentration times sediment accumulation

• Hg flux ratios were calculated as surface Hg flux divided by background Hg flux

Digestion Comparison - Concentrations

Concentration (ng g-1

)

0 40 80 120 160 200 240

Dep

th (c

m)

0

5

10

15

20

25

30

Dry Sediment Wet Sediment

Black Lake Concentrations

Concentration (ng g-1)

0 40 80 120 160D

epth

(cm

)

0

4

8

12

16

20

Dry SedimentWet Sediment

Navajo Lake Concentrations

Concentration (ngg-1)

0 50 100 150 200 250

Dat

e

1500

1600

1700

1800

1900

2000

Black LakePristine LakeGreen Lake 5Green Lake 4Navajo Lake

Hg Concentrations in Sediment

Accumulation Rate (gm-2y-1)

0 100 200 300 400 500 600

Dat

e

1800

1840

1880

1920

1960

2000

Black LakePristine LakeGreen Lake 5Green Lake 4Navajo Lake

Sediment Accumulation Rates

Black Lake Mass Flux

Mass Flux (µgm-2y-1)

0 15 30 45 60

Ye

ar

1650

1700

1750

1800

1850

1900

1950

2000

1825

Hg Flux Ratio = 4.4

Green Lake 4 Mass Flux

Mass Flux (µgm-2y-1)

0 15 30 45

Yea

r

1650

1700

1750

1800

1850

1900

1950

2000

1821Hg Flux Ratio = 4.0

Green Lake 5 Mass Flux

Mass Flux (µgm-2y-1)

0 15 30 45

Yea

r

1700

1750

1800

1850

1900

1950

2000

1841

Hg Flux Ratio = 3.8

Pristine Lake Mass Flux

Mass Accumulation (µgm-2y-1)

0 15 30 45

Ye

ar

1650

1700

1750

1800

1850

1900

1950

2000

1856

1984

Hg Flux Ratio = 3.2

Navajo Lake Mass Flux

Mass Flux (µgm-2y-1)

0 15 30 45 60 75

Ye

ar

1600

1650

1700

1750

1800

1850

1900

1950

2000

1856

1968

1988

Hg Flux Ratio = 3.0

Flux Ratio Comparisons to Remote Sites

1.3

2.12.0

4.1

0

1

2

3

4

5

Colorado FrontRange

Glacier Bay. AK. Remote Sweden Wonder Lake, AK

Hg

Mas

s F

lux

Rat

io

INDUSTRIAL SITE COMPARISON

4.1

7.0

3.53.7

0

1

2

3

4

5

6

7

8

Colorado FrontRange

Northern Minn. &Wis.

Adirondacks, NY Industrial Sweden

Hg

Mas

s F

lux

Rat

ios

Year

1800 1840 1880 1920 1960 2000

% o

f S

urf

ac

e M

as

s F

lux

0

20

40

60

80

100

120

Black LakeGreen Lake 5Green Lake 4

Front Range Lakes – Historical trends

MERCURY DEPOSITION INCREASING IN FRONT

RANGE

• Highest rates of mercury accumulation in history are now

• Mercury accumulation will get worse before it gets better

• Front Range lakes and reservoirs at risk

• Brown cloud?

Year

1800 1840 1880 1920 1960 2000

% o

f S

urf

ace

Mas

s F

lux

0

25

50

75

100

125

150

175

200

Pristine LakeNavajo Lake

West Side Lakes – Recent Decline?

POWER PLANTS ???

• Unclear why max rates have declined since mid-60’s

• Difficult to obtain information on emissions

• Stack heights increased?

• Future trends unclear

SOUTHWEST CO: COMPROMISED DATA

• Mining activity comprised data

• USGS snow survey: highest mercury content in SW CO

• Need additional samples

NUTRIENT ENRICHMENT PRISTINE LAKE

NUTRIENT ENRICHMENT ENHANCES Hg IN LAKES

POWER PLANT EMISSIONS

• Mercury sources• Nitrogen sources• Stimulate algal

production in lakes• Enhanced lake

productivity increases mercury sequestration

• More mercury enters the food chain

Mercury and DOC in StreamwatersMast (unpublished), 2001

CARBON and MERCURY

• TOC/DOC increases mercury transport to lakes and reservoirs

• DOC increases production of MMHg

• DOC mobility increases during snow melt

• Alpine areas at risk

MERCURY AND WILDFIRES: A SMOKING GUN?

• 95% of mercury stored in biomass volatilized

• 90% as elemental Hg• 10% with aerosols• Biomass burning may

account for 25% of global emissions

• Large increase after wildfires?

CABALLO RESERVOIR: NM

• 2,930 ha burned in ’95• THg increased 650%• MMHg up 3,000 %• TOC up 600%• Ratio of MMHg to

THg up 1,000 %• FIRES INCREASE

DELIVERY OF Hg TO RESERVOIRS

Summary

• Mercury loadings are elevated in alpine lakes geographically distributed throughout Colorado

• Mercury loadings in all 5 lakes are more than 2x current global background

• Mercury loadings in lake sediments are comparable to impacted states such as Minnesota and Wisconsin

Summary 2

• Mercury accumulation in Front Range lakes is increasing with time

• Mercury accumulation on Western Slope more difficult to evaluate

Summary 3

• Snow melt runoff increases mercury and DOC transport to lakes

• Nitrogen fertilization from atmospheric deposition increases lake production of DOC

• DOC is coupled with mercury transport to lakes, mercury sequestration in lakes, and production of MMHg

Summary 4: FIRE

• Increases mercury availability

• Produces DOC

• Produces nutrients: N, P

• Changes hydrologic flowpaths to increase transport of DOC, N, P, and Hg to lakes

• Causes lake eutrophication, enhancing MMHg production and THg sequestration

Suggested Future Research Objectives

1) More extensive investigation of atmospheric Hg deposition in lake sediment cores and fish bioaccumulation; SW Colorado emphasis

2) Whole lake and basin Hg accumulation rates from multiple sediment cores (Engstrom et al., 1994), compare these values to wet-fall collectors for determination of dry deposition

3) Analysis of spheroidal fly ash particles in sediments to assess power plant input: role of power plants

More suggestions

4) Importance of TOC versus DOC for mercury transport

5) Hg and DOC during snowmelt

6) Hg measurements in burned basins

7) Amount of MMHg vs THg

8) Atmospheric sources of Hg