Areas of Concern: Development of St. Louis River AOC Remedial Targets for Mercury Active
The St. Louis River Area of Concern (SLRAOC) was designated in 1987 owing to the presence of a variety of legacy pollutants, including mercury. Elevated fish-mercury levels lead fish consumption advisories by both Minnesota and Wisconsin, which resulted in Beneficial Use Impairments (BUIs).
The SLRAOC was impacted many decades ago by industrial mercury uses, and those releases are still very apparent in bottom sediments today. However, sorting out how much of this legacy mercury is actively bioaccumulating has been very difficult to determine until recently with the advent of “Hg source fingerprinting” using Hg isotope measurements. The SLRAOC’s Remedial Action Plan has a legacy-related removal target for mercury concentration in fish tissue. This project was designed to provide needed information for the SLRAOC BUI technical team to: (1) determine an ecologically appropriate measure of “regional background” (i.e., identify a reference location within the region); (2) measure whether the mercury concentrations in selected fishes are different between the SLRAOC and the reference location; and, (3) evaluate whether the legacy sources contribution to mercury in SLRAOC fish tissue, should mercury concentrations be elevated in the SLRAOC relative to the reference location.
The St Louis River, like many Great Lakes coastal ecosystems, has complex hydrological, geochemical, and ecological characteristics that affect mercury cycling. Further, multiple sources of mercury are potentially available to the food web. These qualities present a scientific challenge to determine an appropriate reference site and to identify factors affecting mercury methylation and subsequent bioaccumulation in the food web. The goals for this project are to evaluate relative potential for methylation and bioaccumulation between the SLRAOC and the Bad River (proposed reference location); to compare mercury concentrations among select fishes; and to characterize legacy-related mercury contributions (versus contributions from other local and regional sources) to the SLRAOC food web. Ultimately, our objective is to use this information to develop an AOC-specific target for mercury based on the expected change in mercury concentration in fish tissue after the AOC remediation is complete. An important outcome of the project is to provide a methodology for other AOCs to develop a mercury-specific BUI removal target (or provide evidence that the AOC is no different than a chosen reference location).
This study shows for the first time that the combined use of mercury concentrations, mercury isotopes, and a reference site can directly inform long-term difficult questions regarding the impacts of legacy mercury contamination. The SLRAOC site is among hundreds of sites across the US where natural resource managers have been paralyzed to understand whether current day fish mercury burdens are affected by contamination events that may have happened decades ago. In addition, by the inclusion of a reference site, where no known legacy contamination exists, this study demonstrated that the isotope shifts observed within the SLRAOC are not due to unknown ecosystem processes, but rather the existence and impacts of the legacy Hg still residing in bottom sediments.
Contributions
- The development of a tool and process for addressing legacy mercury contamination at AOC sites (there are many) across the Great Lakes basin.
Partners
- U.S. Environmental Protection Agency (USEPA)
- Midcontinent Research Lab in Duluth, MN
The St. Louis River Area of Concern (SLRAOC) was designated in 1987 owing to the presence of a variety of legacy pollutants, including mercury. Elevated fish-mercury levels lead fish consumption advisories by both Minnesota and Wisconsin, which resulted in Beneficial Use Impairments (BUIs).
The SLRAOC was impacted many decades ago by industrial mercury uses, and those releases are still very apparent in bottom sediments today. However, sorting out how much of this legacy mercury is actively bioaccumulating has been very difficult to determine until recently with the advent of “Hg source fingerprinting” using Hg isotope measurements. The SLRAOC’s Remedial Action Plan has a legacy-related removal target for mercury concentration in fish tissue. This project was designed to provide needed information for the SLRAOC BUI technical team to: (1) determine an ecologically appropriate measure of “regional background” (i.e., identify a reference location within the region); (2) measure whether the mercury concentrations in selected fishes are different between the SLRAOC and the reference location; and, (3) evaluate whether the legacy sources contribution to mercury in SLRAOC fish tissue, should mercury concentrations be elevated in the SLRAOC relative to the reference location.
The St Louis River, like many Great Lakes coastal ecosystems, has complex hydrological, geochemical, and ecological characteristics that affect mercury cycling. Further, multiple sources of mercury are potentially available to the food web. These qualities present a scientific challenge to determine an appropriate reference site and to identify factors affecting mercury methylation and subsequent bioaccumulation in the food web. The goals for this project are to evaluate relative potential for methylation and bioaccumulation between the SLRAOC and the Bad River (proposed reference location); to compare mercury concentrations among select fishes; and to characterize legacy-related mercury contributions (versus contributions from other local and regional sources) to the SLRAOC food web. Ultimately, our objective is to use this information to develop an AOC-specific target for mercury based on the expected change in mercury concentration in fish tissue after the AOC remediation is complete. An important outcome of the project is to provide a methodology for other AOCs to develop a mercury-specific BUI removal target (or provide evidence that the AOC is no different than a chosen reference location).
This study shows for the first time that the combined use of mercury concentrations, mercury isotopes, and a reference site can directly inform long-term difficult questions regarding the impacts of legacy mercury contamination. The SLRAOC site is among hundreds of sites across the US where natural resource managers have been paralyzed to understand whether current day fish mercury burdens are affected by contamination events that may have happened decades ago. In addition, by the inclusion of a reference site, where no known legacy contamination exists, this study demonstrated that the isotope shifts observed within the SLRAOC are not due to unknown ecosystem processes, but rather the existence and impacts of the legacy Hg still residing in bottom sediments.
Contributions
- The development of a tool and process for addressing legacy mercury contamination at AOC sites (there are many) across the Great Lakes basin.
Partners
- U.S. Environmental Protection Agency (USEPA)
- Midcontinent Research Lab in Duluth, MN