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
