An Examination of the Factors that Control Methylmercury Production and

Bioaccumulation in Maryland Reservoirs

Draft Final ReportJune, 2006

Cynthia C. Gilmour Smithsonian Environmental Research CenterEdgewater, MD 21032

Robert P. Mason Dept. Marine Sciences University of ConnecticutGroton, CT 06340

Andrew Heyes Chesapeake Biological LaboratoryUniversity of Maryland, Center for Environmental ScienceSolomons, MD, 20688

Mark CastroAppalachian Laboratory University of Maryland, Center for Environmental ScienceFrostburg, MD 21532

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g/kg

0

200

400

600

800

1000Coastal

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Piedmont

Ridge &Valley

AppalachianPlateau

Mercury concentrations in largemouth bass in Maryland reservoirs, size-normalized to a 370 mm fish (data from Gilmour et al. “An Examination of the Factors that Control Methylmercury Production and Bioaccumulation in Maryland Reservoirs” MD DNR in review).

What drives the differences in MeHg in fish among Maryland reservoirs?What can be done to limit MeHg bioaccumulation in Maryland?

EPA’s Ambient Water Quality Criterion (AWQC) for Hg in fish tissue is 235 ug/kg

Parameters used to examine controls on Hg in Maryland fish

Factors that affect Hg deposition and transport: Watershed sizeWatershed:lake areaLandusePhysiographic provinceReservoir capacityHydraulic residence timeWater chemistrySediment

Factors that affect net methylation:Water chemistry

Hg, pH, sulfate, DOC, conductivity, DO, TSS, nutrients

Sediment chemistryPore waters: Hg, pH, sulfate, sulfide, Fe, Mn, DOC Bulk phase: Hg, density, organic matter, reduced sulfur (AVS/CRS)

Sediment water partitioning

Factors that influence MeHg bioaccumulation: Water chemistry

MeHg, pH, DOC, TSS, nutrients

Deposition

Transport

Watershed retention

Deposition

Methylation Bioaccumulation

Statistical Analysis

Three separate models:• Factors that affect Hg deposition and transport:

– Dependant variable: Hg in water and sediments• Factors that affect net methylation:

– Dependant variable: MeHg in water and sediments • Factors that influence MeHg bioaccumulation:

– Dependant variable: BAF (MeHg in Largemouth bass: MeHg in water)

Stepwise multiple regression

Significant parameters in each model component

Factors that affect Hg deposition and transport: Dependant variable: Hg in water and sedimentsHg deposition (+)Landuse: % developed (+)Water chemistry: Cl, DOC (+)Sediment: organic content (+)Reservoir flow (-), water depth (-)

Factors that affect net methylation:

Dependant variable: MeHg in water and sediments

Water chemistry Hg (+), pH (-), sulfate (+), DO (-)

Sediment chemistryHg (+), sulfide (-), organic matter (+)

Factors that influence MeHg bioaccumulation: Dependant variable: BAF DO (-), pH (+), surface:volume ratio (+)

Deposition

Transport

Watershed retention

Deposition

Methylation Bioaccumulation

Watershed factors: Hg deposition

From MD DNR (Sherwell) 2006

Watershed factors: Hg deposition

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use,

%

0

20

40

60

80

100DevelopedAgricultureForestWetland

Watershed factors: Landuse

% M

eHg

(filte

red)

0

20

40

60

80

100

120

Surface water Bottom water

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% M

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(unf

ilter

ed)

0

20

40

60

80

100

Surface water Bottom water

Coastal Plain

Piedmont

Ridge &Valley

AppalachianPlateau

Methylation: %MeHg as the dependant variable

Varies strongly among reservoirs

Bottom water DO mg/L

0 2 4 6 8 10

% M

eHg

0

20

40

60

80

100

Surface waterBottom water

Controls on methylation: bottom water anoxia

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DO

, mg/

L

0

2

4

6

8

10

12

Surface water Bottom water

DO

C, m

g/L

0

5

10

15

20

25

Surface water Bottom water

Coastal Plain

Piedmont

Ridge &Valley

AppalachianPlateau

Controls on methylation:

pH

Sulfate

pH

5 6 7 8

% M

eHg

in w

ater

col

umn

0.0

0.2

0.4

0.6

0.8

1.0Sulfate

2 4 6 8 10 12 14

% M

eHg

in w

ater

col

umn

0.0

0.2

0.4

0.6

0.8

1.0

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Surface water Bottom water

SO

4, m

g/L

0

2

4

6

8

10

12

14Surface water Bottom water

Coastal Plain

Piedmont

Ridge &Valley

AppalachianPlateau

Significant parameters in each model component

Factors that affect Hg deposition and transport: Dependant variable: Hg in water and sedimentsHg deposition (+)Landuse: % developed (+)Water chemistry: Cl, DOC (+)Sediment: organic content (+)Reservoir flow (-), water depth (-)

Factors that affect net methylation:

Dependant variable: MeHg in water and sediments

Water chemistry Hg (+), pH (-), sulfate (+), DO (-)

Sediment chemistryHg (+), sulfide (-), organic matter (+)

Factors that influence MeHg bioaccumulation: Dependant variable: BAF DO (-), pH (+), surface:volume ratio (+)

Deposition

Transport

Watershed retention

Deposition

Methylation Bioaccumulation