IntroductionThis report presents an overview and discussion of the use of the ambient water quality criterion for methylmercury developed by the U.S. Environmental Protection Agency (EPA) in 2001 (66 FR 1344). Unlike all other previous water quality criteria for the protection of human health@ the criterion for methylmercury was issued as a fish tissue concentration (i.e.@ 0.3 mg methylmercury/Kg wet weight fish tissue)@ as the fish consumption pathway is the main route of exposure to mercury in the environment. As a result@ enforcement of water quality standards based upon this criterion requires either fish tissue sampling or the conversion of the criterion to a concentration in water@ especially for the development of National Pollutant Discharge Elimination System (NPDES) permits.Because many permitted dischargers will be faced with meeting the methylmercury criterion@ this report focuses on (1) available analytical methods for evaluating mercury in fish and water;; (2) the proposed methods for translating a fish tissue concentration for mercury into a concentration in water; and (3) implementation of the mercury criterion in the development of Total Maximum Daily Loads (TMDLs) and water quality-based effluent limits (WQBELs). EPA (2002a@ 2004a) draft Guidance for Implementing the January 2001 Methylmercury Water Quality Criterion addresses these issues and is discussed below in each relevant section.Issues regarding mercury are significant because of mercury??s complex cycling in the environment@ high bioaccumulative potential@ and high toxicity. Generally@ inorganic mercury is deposited to water bodies through atmospheric deposition (the major pathway)@ runoff@ stormwater discharge@ or other industrial or municipal discharges. Once in the water body@ mercury deposited in sediments or dissolved in water may be methylated by bacteria to form methylmercury. Methylmercury is highly bioaccumulative and is the form of mercury that is accumulated up the food web. EPA??s criterion for methylmercury assumes@ therefore@ that all mercury in fish is methylated. While this may be true@ as discussed in Section 3.1@ use of appropriate analytical methods for both total mercury and methylmercury are critical to fully understand site-specific mercury cycling. In addition@ as discussed in EPA (2002a) guidance@ several methods are available to translate the fish tissue criterion to a water concentration. Although site-specific data may provide the most reliable results@ other default options are available@ if needed. Finally@ EPA (2002a; 2004a) attempts to provide recommendations for using the mercury criterion in the development of TMDLs and WQBELs@ and the implementation of these in the NPDES permitting program. Several different approaches have been used for mercury TMDLs@ as no single approach is universally applicable given the site-specific factors that influence mercury deposition@ transformation@ and accumulation and the nature of the contributing sources to a water body. The approaches that have been taken and their acceptability to EPA@ or lack of acceptability@ are illustrated with examples.To address these issues@ Section 2 provides a discussion of the analytical methods for evaluating total mercury and methylmercury in fish tissue and water. Section 3 discusses the various methods for conducting a translation of the methylmercury criterion. Section 4 addresses the use of the criterion in developing TMDLs and WQBELs. Finally@ Sections 5 and 6 present a discussion of the applicability of the issues to other metals and conclusions/ recommendations@ respectively. Appendix A provides a bibliography of recent studies that can be used to update default mercury translators. Appendix B describes several mercury TMDLs that are used to illustrate the different approaches.