Physical and chemical factors controlling mercury and methylmercury concentrations in stream water

Mark E. Brigham and Dennis A. Wentz

5th National Monitoring ConferenceSan José, CaliforniaMay 7-11, 2006

U.S. Department of the InteriorU.S. Geological Survey

Willamette Basin

Georgia-Florida Coastal Plain

Western Lake Michigan

Drainages

Reference stream

Urban stream

USGS NAWQA mercury study areas

Aqueous methylmercury (MeHg) is a major control on mercury bioaccumulation.

Mean Hg in

forage fish

(μg/g wet wt.)

N ≈ 24 at each site

(2 species x 12 individuals)

R2 = 0.8418

0.00

0.05

0.10

0.15

0.20

0.0 0.1 0.2 0.3 0.4 0.5

Mean aqueous MeHg (ng/L)

N ≈ 35 at each site

What controls aqueous MeHg (and THg) concentrations in

streams?

• Weight-of-evidence approach to assess: – Atmospheric inputs– Watershed processes (methylation

and subsequent delivery to stream)– Methylation in channel sediments

Simplified mass balance

Watershed soils: storage / runoff

methylationdemethylation

fluvial transport

Wet deposition

Channel sediments: storage / resuspension

methylation demethylation

Evasion (Hg°)Dry

deposition

resuspension

Wet Hg & MeHg deposition: Mercury Deposition Network (MDN) sites

Load:

∑ (weekly [Hg] x precip volume),

expressed as μg/m2/yr

Hg in precipitation Popple River, WI site (WI09—Mercury Deposition Network)

Oct

‘02

Jan

’03

Jan

’04

Jan

’05

Methylmercury (MeHg) and total mercury (THg) in stream water

• ~35 samples per site from 2003-05

• Key measure of food-web exposure

• Key component of mass balance

Mercury in stream water: sample processing

0.7 μm QFF

Whole waterMeHgTHg

ParticulatePMeHgPTHg

FilteredFMeHgFTHg

===

+++

Fluvial mercury loads & yields

Fluvial load: • Regress load vs. flow for sampled dates.• Predict to unsampled dates using daily

flows Reference: Runkel et al., 2004, USGS Techniques &

Methods, Book 4, Ch. A5; LOADEST S-Plus program by D. Lorenz, USGS

• Yield = load / watershed area, μg/m2/yr• Examine yield as % of wet depositional

loads to ecosystem…

MeHg deposition unrelated to MeHg yield

0

100

200

300

400

500

600

700

800

900

1000

OR-Urb

OR-Ref

-L

WI-R

ef-H

WI-R

ef-L

WI-U

rb

FL-Ref

-H

FL-Ref

-L

FL-UrbF

luvi

al y

ield

as

% o

f w

et d

ep l

oad

200

3-04

*

THg yield: 4.4–48% of wet deposition

MeHg yield: 22–926 % of wet deposition (excludes site where MeHg < MDL*)

*

Florida

Wisconsin

Oregon

THg yield vs precip Hg deposition, 2003-2004

1:10 line

WI09Pike

OR10Lookout WI22

Oak

FL32LWekiva

GA09 StMary

FL05Santa Fe

OR01Beaverton

WI32Evergreen

0

1

2

3

4

5

6

7

0 5 10 15 20Precip THg load, micrograms/m2/yr, 2004

Flu

via

l T

Hg

yie

ld, u

g/m

2/y

rF

luvi

al T

Hg

yiel

d, μ

g/m

2 /yr

Wet THg deposition, μg/m2/yr, 2003-04

Urban

Reference

Summary of partial mass balance

• Wet MeHg deposition could account for MeHg in most streams– low [MeHg] streams.

• Caveat—Missing key components of mass balance– watershed retention– demethylation– dry deposition

• Must invoke watershed methylation to explain high [MeHg] streams.

Aqueous total Hg and methylmercury correlate strongly to dissolved organic carbon (DOC):

• among all sites (shown here)• within a site (most sites)

Log 1

0 [

FM

eHg]

(ng

/L)

Log 1

0 [

FT

Hg]

(ng

/L)

Log10 [DOC] (mg/L) Log10 [DOC] (mg/L)Log10 [DOC] (mg/L)

Runoff-mobilized Hg-DOC complexes controls: -- THg in most streams -- MeHg in half the study streams.

Evidence for watershed inputs of MeHg

Evidence against in-channel methylation as dominant source

Santa Fe River, Florida

Log10 [Q] (cfs)

Log 1

0 [

FT

Hg]

(ng

/L)

Log 1

0 [

FM

eHg]

(ng

/L)

Negative relation between MeHg and flow?

Evidence for in-channel methylation?

Or, high [MeHg] in wetlands during low-flow periods?

St Mary’s River, Florida

Log10 [Q] (cfs)

Log 1

0 [

FT

Hg]

(ng

/L)

Log 1

0 [

FM

eHg]

(ng

/L)

Aqueous methylmercury strongly linked to wetland density (mean methylmercury; all study sites)

R2 = 0.9224

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 10 20 30 40 50 60

Wetland density (% of total land cover in basin)

FMH

g (

ng/L

)

Log 1

0 T

Hg

conc

entr

atio

n (n

g/L)

DOC and Suspended Sediment—a potential screening tool for total mercury…

R2=0.62

Log10 DOC (mg/L)

Log10 Susp Sed

(mg/L)

…and methylmercury.Lo

g 10 M

eHg

conc

entr

atio

n (n

g/L)

Log10 DOC (mg/L)

Log10 Susp Sed

(mg/L)

What have we learned about THg and MeHg in streams?

Summary

Precipitation and watershed influences

• Precipitation inputs– main source of THg to ecosystem– Could account for all MeHg in

some streams• Watershed inputs

– major vector for MeHg and THg delivery to streams, particularly in wetland-rich basins

Summary

Concentration relationships

• DOC and suspended sediment– Control THg & MeHg in streams

(MeHg picture is noisier)– key explanatory variables– perhaps a useful screening tool– Erosion control—useful to reduce

particulate Hg, and hence THg

Summary

Role of channel sediments

• MeHg source? – At most, a minor source of MeHg to

stream water– Low MeHg at low flow (evidence

against substantial inputs from sediments)…

– …except at one site (either sediment methylation or seasonally high MeHg from wetlands)

• MeHg sink? – Fast demethylation rates in sand, a

dominant substrate in some streams

Implications for monitoring THg & MeHg in streams

• Sample size (N)—depends on objectives…– BAF’s: Perhaps as few as N ≈ 6, well

spaced seasonally (see: Paller and others, 2004, Archives of Environ. Contam. & Toxicology)

– Concentration relationships & fluvial loads: N ≥ 35, well spaced seasonally and hydrologically

Acknowledgements

USGS: Dennis Wentz, Barb Scudder, Lia Chasar, Amanda Bell, Michelle Lutz, Dave Krabbenhoft, Mark Marvin-DiPasquale, George Aiken, Robin Stewart, Carol Kendall, Bill Orem, Rod DeWeese, Jeff Isely, and many others…

USGS: NAWQA and several other USGS programs

MDN site support: USGS, Wisconsin DNR, Oregen DEQ, Forest Service, US Fish & Wildlife Service, St. John’s River Water Management District (FL)

Menomonie Indian Tribe of Wisconsin