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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. Wentz 5 th National Monitoring Conference San José, California May 7-11, 2006. U.S. Department of the Interior U.S. Geological Survey.

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

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  1. 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

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

  3. 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

  4. 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

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

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

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

  8. 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

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

  10. 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…

  11. 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

  12. 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

  13. 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.

  14. 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)

  15. 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)

  16. 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)

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

  18. 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)

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

  20. What have we learned about THg and MeHg in streams?

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

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