This story highlights a current research project at the Hubbard Brook Experimental Forest. To read more about research projects at HBEF, visit Current Research page. Check back regularly to learn about new research projects.
Mercury and methylmercury export in relation to DOC quality in upland landscapes, northeastern USA

 

  Map of site locations  
  Figure 1. Locations of the three study sites.  
  Investigators:
  James B. Shanley, Charles T. Driscoll, Jr., George R. Aiken, Ann Chalmers, Janet Towse and Jason Dittman  
  Contact Info:
  James B. Shanley
U.S. Geological Survey
P.O. Box 628
Montpelier, VT 05601
phone: (802) 828-4466
email: jshanley@usgs.gov
 

Summary
PERVASIVE MERCURY (Hg) contamination poses a threat to human health through fish consumption, even in remote areas of northeastern USA. A central policy question in the debate over regulation of Hg is whether reductions in emissions will translate directly into reductions of Hg in fish. Most Hg deposited on the terrestrial landscape is retained in organic soils. Thus, the soil harbors a large store of Hg from the past century of industrial emissions that may continue to leak Hg into streams (and fish) regardless of future trends in Hg emissions. We are investigating the fate of this stored Hg by quantifying stream transport of Hg and the interactions of dissolved and particulate organic matter with Hg in streamwater. We hypothesize that the landscape characteristics that control the production and mobility of organic matter will likewise control the mobility of Hg.

  Water sample collection  
  Figure 2. Jason Dittman collects samples for analysis of mercury and dissolved and particulate carbon at Archer Creek. Streamwater is pumped through a filter and into a glass amber bottle (for detailed sampling methods see Aiken et al. 1992).
 
  Water sample collection  
  Figure 3. Janet Towse collects a water sample that will be analyzed for mercury at Hubbard Brook. We use clean sampling techniques, including wearing powder-free gloves, to avoid contamination of samples.  

We are working at three sites in three northeastern states that span a range of wetland cover. We have found that hydrophobic organic acid (HPOA), the fraction of dissolved organic carbon (DOC) that is most effective at mobilizing Hg, is produced more in wetlands than in non-wetland soils. While wetlands export the greatest amount of dissolved Hg, landscapes that generate a lot of sediment also mobilize Hg in association with particulate organic carbon (POC). What we do not know is whether this particle-associated Hg is as bio-available to aquatic organisms as dissolved Hg. Most stream export of Hg occurs at high flow, and our sampling is designed to capture export during snowmelt and storms. Wetlands not only export dissolved Hg but are also important producers of methylmercury, the toxic Hg form that enters the food chain.

Preliminary Results
Mercury in the aquatic food chain garners much of the public and scientific attention, but most Hg deposition to the landscape occurs in terrestrial uplands. The northeastern USA is particularly susceptible to Hg contamination on two counts: (1) it is a focal point of atmospheric Hg deposition (Miller et al., 2005), and (2) the landscape is rich in organic matter and sulfate, as well as abundant wetlands that promote the methylation of Hg. Methylmercury (MeHg) is the toxic form of Hg that biomagnifies in food chains. Recent research suggests that much of the Hg in aquatic ecosystems is delivered from headwater landscapes, but research on Hg behavior in headwater environments has been limited. Hg research in headwater systems is needed because: (1) headwaters harbor a large pool of Hg whose fate is uncertain, (2) the bioavailability of this Hg is unclear and needs to be better assessed, and (3) the potential for Hg methylation and export of MeHg from upland landscapes is under-appreciated. The microbially-mediated interactions of Hg, sulfur (S), and carbon (C) to produce toxic MeHg has led our co-P.I. George Aiken to term these three elements the "biogeochemical axis of evil".

  Graph of mercury and DOC  
  Figure 4. There is a fairly linear positive relationship between mercury and DOC concentrations, suggesting that mercury in streamwater is transported in association with DOC. Note that samples from all the sites plot along essentially the same line.  

Our three study sites (Figure 1) represent a range of hydrochemical settings and carbon cycling patterns. Bear Brook (Watershed 6), Hubbard Brook Experimental Forest, NH is relatively poorly buffered and is low in dissolved organic carbon (DOC) (1-4 mg/L). Sleepers River Research Watershed (Watershed 9), VT is well-buffered and is intermediate in DOC (1-10 mg/L). Archer Creek at Arbutus Lake in the Adirondack Mountains, NY is intermediate in buffering capacity and has higher DOC (4-10 mg/L). At Archer Creek (Figure 2), we are sampling above and below a large wetland, but even the upper site has numerous wetlands. The use of watersheds for which long-term data exist, such as at the Hubbard Brook Experimental Forest (Figure 3), provides context and supporting hydrologic and biogeochemical understanding to aid interpretation of Hg, S and C interactions.

Early results confirm the role of DOC in transporting dissolved Hg, with positive linear relations of similar slope at all sites (Figure 4). Hubbard Brook plots are at the low end and the Archer Creek sites at the high end, with Sleepers River spanning the entire range because of large DOC increases at high flows. Dissolved Hg dominates Hg export at Archer Creek, whereas particulate Hg dominates at Sleepers River. Export of both dissolved and particulate Hg are low at Hubbard Brook due to low DOC and low sediment loads. We expect MeHg to be most important in the wetland-dominated Archer Creek system.

References
Aiken, G.R., McKnight, D.M., Thorn, K.A., and Thurman, E.M. 1992. Isolation of hydrophilic acids from water using macroporous resins. Organic Geochemistry 18: 567-573.

Miller, E.K., VanArsdale, A., Keeler, G.L., Chalmers, A., Poissant, L., Kamman, N.C., and Brulotte, R. 2005. Estimation and mapping of wet and dry mercury deposition across northeastern North America. Ecotoxicology 14: 53-70.