2006 Progress Report: Pathways of Mercury Evasion from Contaminated Wetlands: A Globally Important Source of Atmospheric Mercury?EPA Grant Number: GR832214
Title: Pathways of Mercury Evasion from Contaminated Wetlands: A Globally Important Source of Atmospheric Mercury?
Investigators: Peters, Stephen , Morris, Donald P. , Windham, Lisamarie
Institution: Lehigh University
EPA Project Officer: Carleton, James N
Project Period: January 15, 2005 through January 14, 2008
Project Period Covered by this Report: January 15, 2006 through January 14, 2007
Project Amount: $298,907
RFA: Greater Research Opportunities: Persistent, Bioaccumulative Chemicals (2004) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Safer Chemicals , Hazardous Waste/Remediation , Health Effects
The objectives of this research project are to: (1) determine the relative importance of two different mercury evasion pathways; (2) investigate the fundamental processes governing chemical interactions within each pathway; and (3) evaluate the net contribution of evasion from mercury (Hg)- contaminated wetlands to the global Hg budget. These objectives will be studied by experimentally and empirically testing hypothesized relationships between measured environmental parameters and Hg behavior in a wetland.
Two pathways exert primary control over the release of Hg0 from estuarine environments: the abiotic reduction of Hg(II) to Hg0(aq) as moderated by complex interactions of UV radiation (UVR), dissolved organic carbon (DOC), salinity, and pH in the water column; and the diffusive release of bacterially reduced Hg0 from plant leaf surfaces during transpiration.
In this second year, we continued to make progress on two of the three primary objectives: (1) evaluate the relative importance of each mercury evasion pathway; and (2) investigate the fundamental processes governing chemical interactions within each mercury evasion pathway. Our efforts have centered on method refinement and data collection in both field and laboratory settings. We have also presented preliminary results at several national scientific meetings and have one manuscript submitted for publication and three more in preparation.
The first field efforts focused on the investigation of the effects of DOC, water chemistry, and UV light on mercury evasion from the water column. We tested hypotheses that relate to how mercury evasion rates are affected by DOC quality and quantity, pH, and salinity. Continuous measurements of mercury flux, water chemistry parameters (pH, conductivity, turbidity, DO, chlorophyll a, temperature, depth), and meteorological conditions (temperature, wind speed/direction, precipitation, solar and UV irradiance) were performed for 52 consecutive hours in July 2006, in conjunction with hourly water sample collection. Measurements and samples were collected in Berry’s Creek, adjacent to the Ventron/Velsicol Superfund site in the Hackensack Meadowlands.
Measured mercury flux rates from the water surface ranged from near zero to 64 ng/m2/h. Daytime fluxes were significant and positively correlated with solar radiation and UVR. The highest fluxes (> 60 ng/m2/hr) were observed prior to sunrise on July 19, although it is not clear what variables control these isolated events.
Mercury emission from plants was measured using both whole-plant and single- leaf chambers, depending on the species. Maximum mercury flux from plants was found to vary widely from species to species, with the lowest fluxes observed in Salicornia europaea (Pickleweed) and the highest fluxes observed in Typha angustifolia (Narrow-leaf Cattail). Mercury emission from two plant species measured adjacent to each other demonstrates peak emission rates that differ by one order of magnitude (37.28 vs. 388.37 ng/m2/hr). Species-specific plant physiology appears to be an important factor for determining emission rates. Furthermore, measurements of multiple individuals of Scirpus fluviatilis (River bulrush) and multiple plots of S. europaea were similar within species, suggesting that mercury emission rates for a given species may not vary substantially between individuals.
Iron plays a role in the photoreactivity of DOC and absorption of UVR in freshwater systems, and may therefore be important in dissolved gaseous mercury (DGM) production. We performed a laboratory experiment to assess the effects of UV, DOC optical characteristics, and iron on the formation of DGM. Results suggest that iron increases the production of DGM in the absence of DOC. In the presence of both oxidized and unoxidized DOC, iron addition resulted in a decrease in the proportion of total mercury in solution as DGM. The effects of iron were significantly more pronounced in the unoxidized DOC treatments, suggesting that the DOC-Fe interaction is not as strong for oxidized DOC as it is for fresh DOC. Replication and refinement of this experiment is planned for 2007 to further clarify the role of DOC, iron, and UVR in the production of DGM.
In the next year, we plan to continue to examine the variables that control mercury re emission into the atmosphere, paying particular attention to those variables that are synergistic. Identification of these variables could play an important role in remediation strategy for contaminated sites.
In particular, we plan to do several day-long, continuous monitoring experiments at the Meadowlands site to identify the parameters controlling highly variable spikes of mercury emitted from the water surface. This year, much of our field efforts will also focus on measurement of plant emission rates and the variables controlling them.
Experiments conducted in the laboratory will be aimed at clarifying and extending the results discussed herein. Specifically, we will repeat the mercury-iron-DOC experiment with some methodological improvements and will use the isolates tested in the solar simulator to test the importance of specific UV wavelengths on photoreaction rates.
We plan on disseminating our results through several peer-reviewed manuscripts currently in preparation, as well as through a variety of conferences, including the American Geophysical Union, and the Science To Achieve Results (STAR) Grantees Conference.