Regional Approach for Mercury TMDLs: Quantifying Atmospheric Deposition for Coastal Louisiana

Despite growing national interest in pursuing alternatives to total maximum daily loads (TMDL) for addressing fish consumption advisories for mercury, the U.S. Environmental Protection Agency (USEPA) Region 6 was compelled by a settlement agreement to prepare TMDLs for coastal bays and Gulf waters of the entire Louisiana coastline. A comprehensive paper summarizing the technical approach is not available for inclusion in the WEF TMDL 2005 Conference Proceedings because the TMDL Report was not finalized to the level required by USEPA Region 6. However, the presentation will highlight the key elements of the TMDL technical approach.More than 44 states list waterbodies as impaired as a result of mercury levels in fish tissue concentrations high enough to warrant consumption advisories. To adequately address mercury sources potentially contributing to the fish consumption advisory for king mackerel in Louisiana, a basin-scale assessment approach was deemed necessary to evaluate the Coastal Bays and Gulf Waters and their contributing watersheds. Given the ubiquitous distribution of mercury and the large geographic area considered in this assessment, a regional rather than a waterbody-specific approach was used to develop the TMDLs. Recognizing the need to conduct a more thorough identification and quantification of mercury loading from point and nonpoint sources, Region 6 relied on Parsons for technical support to link recent advancements in modeling atmospheric deposition using the Regional Modeling System for Aerosols and Deposition (REMSAD) with the Pollutant Load Application (PLOAD) of the Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) watershed loading model. It is well-known that a major source of mercury deposition to many of our nation's waters is from the atmosphere. To support development of these TMDLs for coastal Louisiana, REMSAD atmospheric deposition modeling was used to segregate wet and dry deposition into local, regional, and global air sources by calculating wet and dry loading estimates to both terrestrial and aquatic surfaces. Because of the large surface water area in southern Louisiana, this was a critical step in refining the quantification of mercury loading into receiving waters.Since more than 90 percent of mercury in receiving waters is attributable to air emissions, special emphasis was placed on distributing the burden of reducing mercury loading on facilities with mercury air emissions and not just on municipal and industrial wastewater dischargers. Alternative approaches were identified to provide the State with flexibility in its permitting program so mercury limitations for NPDES dischargers could be established or lowered only when warranted. This approach also addressed any potential localized impact of this persistent bioaccumulative pollutant by a point source discharger. While the fate and transport of mercury through the aquatic environment is very complex and not completely understood, certain simplifying assumptions about mercury loading were made so required reductions could be identified and a TMDL determined.This technical presentation summarizes the data compilation, assessment results, and policy decisions associated with developing draft mercury TMDLs for the fish consumption advisory placed on king mackerel (Scomberomorus cavalla) in the coastal bays and Gulf waters of Louisiana. Data compilation and assessment results occurred between September 2003 and April 2005. The technical presentation provides an overview of the various issues and methods considered and utilized in developing TMDLs for mercury using a regional or basins-cale approach. While information provided in the presentation is derived primarily from development of mercury TMDLs in coastal Louisiana, a number of questions and issues that apply to a 303(d)-listed water body anywhere in the U.S. with a fish consumption advisory is addressed.