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

Physical and chemical factors controlling mercury and methylmercury concentrations in stream water Mark E. Brigham and Dennis A. Wentz5th National Monitoring ConferenceSan José, CaliforniaMay 7-11, 2006 U.S. Department of the Interior U.S. Geological Survey

USGS NAWQA mercury study areas Western Lake Michigan Drainages Willamette Basin Georgia-Florida Coastal Plain Reference stream Urban stream

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) 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 Wet deposition Dry deposition Evasion (Hg°) Watershed soils: storage / runoff methylation demethylation Channel sediments: storage / resuspension methylation demethylation resuspension fluvial transport

Wet Hg & MeHg deposition: Mercury Deposition Network (MDN) sites Load: ∑ (weekly [Hg] x precip volume), expressed as μg/m2/yr

Hg in precipitationPopple River, WI site (WI09—Mercury Deposition Network) Jan ’04 Jan ’03 Jan ’05 Oct ‘02

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

+++ Whole waterMeHgTHg Mercury in stream water: sample processing === FilteredFMeHgFTHg ParticulatePMeHgPTHg 0.7 μm QFF

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 1000 THg yield: 4.4–48% of wet deposition 900 800 700 MeHg yield: 22–926 % of wet deposition (excludes site where MeHg < MDL*) 600 500 2003-04 Fluvial yield as % of wet dep load 400 300 200 100 * * 0 FL-Urb WI-Urb OR-Urb FL-Ref-L WI-Ref-L FL-Ref-H WI-Ref-H OR-Ref-L

THg yield vs precip Hg deposition, 2003-2004 Florida Urban Reference Fluvial THg yield, μg/m2/yr Oregon 1:10 line Wisconsin Wet THg deposition, μg/m2/yr, 2003-04

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) Log10 [FMeHg] (ng/L) Log10 [FTHg] (ng/L) Log10 [DOC] (mg/L) Log10 [DOC] (mg/L) Log10 [DOC] (mg/L)

Santa Fe River, Florida 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 Log10 [FTHg] (ng/L) Log10 [FMeHg] (ng/L) Log10 [Q] (cfs)

St Mary’s River, Florida Negative relation between MeHg and flow? Evidence for in-channel methylation? Or, high [MeHg] in wetlands during low-flow periods? Log10 [FTHg] (ng/L) Log10 [FMeHg] (ng/L) Log10 [Q] (cfs)

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

DOC and Suspended Sediment—a potential screening tool for total mercury… R2=0.62 Log10THg concentration (ng/L) Log10 Susp Sed (mg/L) Log10 DOC (mg/L)

…and methylmercury. Log10MeHg concentration (ng/L) Log10 Susp Sed (mg/L) Log10 DOC (mg/L)

What have we learned about THg and MeHg in streams?

SummaryPrecipitation 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

SummaryRole 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

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