Grantee Research Project Results
2001 Progress Report: Photo Induced Reduction of Mercury in Lakes, Wetlands, and Soils
EPA Grant Number: R827632Title: Photo Induced Reduction of Mercury in Lakes, Wetlands, and Soils
Investigators: Nriagu, Jerome O. , Keeler, Gerald J. , Zhang, Hong , Lehman, John , Lindberg, Steve
Current Investigators: Nriagu, Jerome O. , Keeler, Gerald J. , Zhang, Hong , Lehrnan, John , Lindberg, Steve , Qang, Xia-Qin
Institution: University of Michigan , Oak Ridge National Laboratory
EPA Project Officer: Chung, Serena
Project Period: September 1, 1999 through August 31, 2002 (Extended to September 30, 2003)
Project Period Covered by this Report: September 1, 2000 through August 31, 2001
Project Amount: $865,771
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
The objective of this research project is to undertake a comprehensive evaluation of photoinduced formation of Hg(0) in a watershed. Experimental studies will involve field measurements, in situ incubation experiments, shipboard experiments, and laboratory studies under controlled conditions. Study sites will include wetlands, soils, and a lake. Photodependent processes that will be investigated may be biotic, abiotic, physical, or chemical, and reaction mechanisms that we will study may be homogeneous or heterogeneous. We will compare similarities and differences in pathways and rates of Hg(0) formation in different segments of a watershed. The Saginaw Bay to Lake Huron system has well-defined gradients in chemical parameters (including chlorophyll-a, dissolved organic carbon [DOC], phosphorus, nitrogen, chloride, and suspended particulates), trophic conditions, taxonomic composition, and biomass, and provides a unique study site. We will monitor diurnal and seasonal variations in a cross-gradient generation of Hg(0), and we will estimate the total release of Hg(0) from surface waters of the bay. We also will conduct laboratory experiments to test the following hypotheses: (1) oxidation of Hg(0) as an important factor in its accumulation and release from aquatic environments; (2) solid phases can activate the reduction of Hg(II) and Hg(I) compounds in soils and wetlands; (3) extracellular and intracellular pathways provide a link between DOC and accumulation of Hg by aquatic biota; and (4) degradation of organic matter under light plays a critical role in the formation of Hg(0) and possibly other mercury species.
Progress Summary:
During the past 12 months, our effort focused on defining the photochemistry of mercury in waters of Saginaw Bay at selected stations. We made field measurements of: (1) Hg levels in the Saginaw Bay water column and the tributary rivers of its watershed; (2) diel and seasonal changes of dissolved gaseous mercury (DGM) in nearshore waters of the bay; (3) DGM levels in the offshore waters of the bay; and (4) air/water exchange of Hg in a coastal wetland of the bay. In addition, both field tests and laboratory experiments were conducted to study the kinetics and mechanism of photochemical reduction of Hg in the lake waters. We also initiated laboratory experiments on the photoreduction of mercury in the presence of various amino acids.
Saginaw Bay generally has low levels of total dissolved Hg (around 0.2 ng L-1), which is unusual for one of the more polluted areas in the Great Lakes Basin. The DGM concentrations in the nearshore waters of Saginaw Bay at Tawas Bay exhibited both diel trends with peaks around noontime and seasonal variations with the levels in June being higher than in August. The DGM concentrations in the offshore waters of Saginaw Bay showed similar seasonal trends generally with the DGM levels in June being higher than in August. These diel and seasonal variations in DGM concentrations in Saginaw Bay indicate that the dissolved Hg pool, though small, is being actively recycled in the water column.
In a short-term field study conducted in August 2001 in Tobico Marsh, a lagoon wetland at the coast of Saginaw Bay in Bay City, MI, we measured very low Hg evasion fluxes using both the dynamic flux chamber method (~1-2 ng h-1 m-2) and a micrometeorological method, even in strong sunlight. Also, very low DGM concentrations (~1-4 pg L-1) were found there, implying that the level of Hg in the marsh water is controlled by evasional loss to the atmosphere. Assuming average water depth of 1.0 m, the lifetime of DGM in the marsh is estimated to be about 1 hour.
Nearshore lake water photoexposure samples from Saginaw Bay spiked with Fe(III) showed strong enhancement in the production of DGM, similar to what was previously observed with similar tests using Whitefish Bay water of Lake Superior. The enhancement of photochemical production of DGM by Fe(III) spike seems consistent in all the field tests performed in August 2000, and in June and August 2001. Field photoexposure of nearshore lake water of Saginaw Bay after removal of dissolved O2 appeared to lead to more production of DGM either with or without Fe(III) spikes in studies performed in both June and August 2001. This indicates that dissolved (photoactive) O2 promotes the oxidation of produced DGM.
Preliminary laboratory tests using lake water and UV light yielded rates of DGM production similar to the in situ photochemical tests previously described. However, we found that addition of higher levels of Fe(III) led to significant enhancement of photochemical production of DGM under 365 nm UV radiation. It is hypothesized that photochemical reduction of Hg(II) in freshwater is believed to occur along two major pathways: dissolved humic matter-mediated reduction, and dissolved small organic acids-mediated reduction. Our results suggest that Fe(III) may enhance photoreduction of Hg(II) along either pathway. Ferric ions also may facilitate photochemical decomposition of dissolved humic matter and production of small organic acids. Experiments are underway to better define the role of iron to gain a better understanding of photochemical reduction of Hg(II) in freshwater.
Amino acids also are common photoactive constituents in aquatic ecosystems. Their role in the mercury redox cycle has not been systematically investigated. We have started to screen various amino acids for their potential to reduce Hg(II) in distilled water in the dark and under UV light. Our preliminary experiments show that some amino acids (including tryptophan, alanine, leucine, histidine, glutamine, proline, methionine, and aspartic acid) were able to photo-induce the reduction of Hg(II), others such as arginine, phenylalanine, cystine, and homocystine could accomplish the same reaction. We are continuing the experiments to determine why some amino acids can photo-induce the reduction of Hg(II), while others cannot.
Future Activities:
We will complete measurements and experiments on: (1) Hg levels in water column of Saginaw Bay and the tributary rivers of its watershed; (2) diel and seasonal changes of dissolved DGM in nearshore waters of the bay; (3) DGM levels in the offshore waters of the bay; and (4) air/water exchange of Hg in a coastal wetland of the bay. In addition, we will complete laboratory experiments started on the kinetics and mechanism of photochemical reduction of Hg in the lake waters. We also will complete experiments on the photoreduction of mercury in the presence of various amino acids.
Journal Articles:
No journal articles submitted with this report: View all 4 publications for this projectSupplemental Keywords:
photochemistry, mercury photolysis, wetlands, soils, mercury re-emission, mercury volatilization, organomercury complexes, heterogeneous reaction, dissolved gaseous mercury, DGM., RFA, Scientific Discipline, Air, Water, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Nutrients, Hydrology, Ecology, particulate matter, Ecosystem/Assessment/Indicators, Ecosystem Protection, Chemistry, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Ecological Indicators, Great Lakes, Mercury, anthropogenic stress, aquatic ecosystem, fate and transport, hydrological stability, nutrient supply, ecological exposure, wetlands, anthropogenic disturbances, aquatic, re-emission, bioavailability, mercury loading, photo induced reduction, mercury cycling, soils, lakes, soil, photo induced reduction of mercury, methylation, suspended particulates, aquatic ecosystems, ecosystem, mercury in lakes, ecosystem stress, Lake Huron, lake ecosystem, wetland, heavy metalsRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.