The Great Lakes and their connecting channels form the largest system of freshwater on earth. Covering more than 94,000 square miles, they contain about 84 percent of North America’s surface freshwater and 21 percent of the world’s supply. The lakes provide water for a multitude of activities and occupations, including drinking, fishing, swimming, boating, agriculture, industry, and shipping for more than 30 million people who live in the Great Lakes Basin—which encompasses nearly all of the state of Michigan and parts of Illinois, Indiana, Minnesota, New York, Ohio, Pennsylvania, Wisconsin, and the Canadian province of Ontario. During the 1970s, it became apparent that pollution caused by persistent toxic substances, such as BCCs, was harming the Great Lakes and posing risks to human health and wildlife. On average, less than 1 percent of the Great Lakes’ water recycles or turns over each year, and many pollutants stay in place, settling in sediments or bioaccumulating in organisms. As a result, under the GLWQA of 1978, the United States and Canada agreed to a policy of prohibiting the discharge of harmful pollutants in toxic amounts and virtually eliminating the discharge of such pollutants. The two parties also pledged to develop programs and measures to control inputs of persistent toxic substances, including control programs for their production, use, distribution, and disposal. The concept of virtual elimination recognizes that it may not be possible to achieve total elimination of all persistent toxic substances. Some toxic substances may be produced by or as a result of natural processes, persist at background or natural levels, or cannot be eliminated for technological or economic reasons. In addition to agreeing to a policy calling for the virtual elimination of toxic pollutants, the 1978 GLWQA, as amended, also established a process and set of commitments to address the pollutant problem. Other joint United States and Canada toxic reduction efforts were initiated in subsequent years, in keeping with the objectives of the agreement. These included the 1991 Binational Program to Restore and Protect the Lake Superior Basin—which, among other things, established a goal of achieving zero discharge of designated persistent and bioaccumulative toxic substances from point sources in the Lake Superior Basin. In addition, recognizing the long-term need to address virtual elimination, the EPA Administrator and Canada’s Minister of the Environment signed the Great Lakes Binational Toxics Strategy in 1997, which provides a framework for actions to reduce or eliminate persistent toxic substances, especially those that bioaccumulate in the Great Lakes Basin. Agreements within the two countries also addressed the problem of toxic pollutants and the implementation of the GLWQA. In the United States, the Governors’ Agreement in 1986 developed by the Council of Great Lakes Governors recognized that the problem of persistent toxic substances was the foremost environmental issue confronting the Great Lakes, and they were committed to managing the Great Lakes as an integrated ecosystem. At that time, inconsistencies in state standards and implementation procedures became an increasing concern to EPA and state environmental managers. The Governors agreed to work together to, among other things, establish a framework for coordinating regional action in controlling toxic pollutants entering the Great Lakes Basin, increase federal emphasis on controlling toxic pollution, and expedite the development of additional national criteria or standards for toxic substances to protect both the ecosystem and human health. In Canada, the Canadian and Ontario governments entered into several agreements with each other over the last 30 years to address environmental problems in the Great Lakes. These agreements, each referred to as the Canada-Ontario Agreement Respecting the Great Lakes Basin Ecosystem, included a focus on the control of toxic chemical pollution and runoff. In addition, a 2002 agreement outlines how these two governments will continue to work together to focus efforts to help clean up the Great Lakes Basin ecosystem. Several priority projects are planned under the agreement, including reducing the amount of harmful pollutants, such as mercury, that find their way into the Great Lakes. To further control toxic substances in the United States, efforts on the GLI began in the late 1980s to establish a consistent level of environmental protection for the Great Lakes ecosystem, particularly in the area of state water quality standards and NPDES programs for controlling point sources of pollution. As authorized by the Clean Water Act, the NPDES permit program controls water pollution by regulating point sources that discharge pollutants into U.S. surface waters. Under NPDES, all facilities that discharge pollutants from any point source into U.S. waters are required to obtain a permit that provides two levels of control: (1) technology based limits (discharge limits attainable under current technologies for treating water pollution) and (2) water quality-based effluent limits (based on state water quality standards). Point sources are discrete conveyances such as pipes or constructed ditches. Individual homes that are connected to a municipal system, use a septic system, or do not have a surface discharge, do not need an NPDES permit; however, industrial, municipal, and other facilities must obtain permits if their discharges go directly to surface waters. As of May 2005, there were nearly 5,000 facilities in the Great Lakes Basin that had NPDES permits, and over 500 of these were considered major facilities. In 1989, the Council of Great Lakes Governors agreed to join EPA in developing GLI because it supported the goal of consistent regulations among the Great Lakes states. The effort to develop GLI was under way when Congress amended the Clean Water Act with the passage of the Great Lakes Critical Programs Act of 1990. This act required EPA to publish by June 1992, final water quality guidance for the Great Lakes System that conformed to the objectives and provisions of the GLWQA. It further required the states to adopt water quality standards, antidegradation policies, and implementation procedures consistent with the guidance no later than 2 years after it was published. If the states failed to adopt such water quality standards, policies, and procedures consistent with the guidance, EPA was to promulgate them not later than the end of the 2-year period. In making such a determination, EPA reviewed the states water quality standards, antidegradation policies, and implementation procedures for consistency with the guidance. To control toxic substances and protect aquatic life, wildlife, and human health, GLI sets forth water quality criteria for 29 toxic substances, such as PCBs, mercury, dioxin, and chlordane. These criteria include standards for 9 of 22 BCCs. GLI also contains detailed methodologies for developing criteria for additional pollutants and implementation procedures for developing more consistent, enforceable water quality-based effluent limits in discharge permits for point sources of pollution. The most common of the 22 BCCs currently being discharged from point sources in the Great Lakes Basin is mercury. Because mercury can be highly toxic and travel great distances in the atmosphere, it has long been recognized to have a wide range of detrimental effects for ecosystems and human health. When mercury is deposited within a water body, microorganisms can transform it into a very toxic substance known as methyl mercury. Methyl mercury tends to remain dissolved in water and can bioaccumulate in the tissues of fish to concentrations much higher than in the surrounding water. The primary way people are exposed to mercury is by eating fish containing methyl mercury. Poisoning can result from eating fish contaminated with bioaccumulated methyl mercury, which is dangerous to certain adults, children, and developing fetuses. Three general principles guided the development of GLI: (1) to incorporate the best science available to protect the Great Lakes Basin ecosystem; (2) to promote consistency in standards and implementation procedures in Great Lakes states’ water quality standards while allowing appropriate flexibility; and (3) to reflect the unique nature of the Great Lakes Basin ecosystem by establishing special provisions for toxic substances, such as BCCs. Although improved consistency in Great Lakes states’ water quality standards and NPDES programs was a primary goal of GLI, implementing and supplemental regulations published by EPA provided flexibility to states in adopting and implementing GLI provisions in several areas. These regulations included relief from GLI provisions for point source dischargers through the use of existing NPDES program provisions such as variances, mixing zones, and compliance schedules. For example, provisions in GLI allow the states to grant dischargers variances for up to 5 years from GLI water quality standards, which are the basis of a water quality based effluent limitation included in NPDES permits. According to GLI, variances are to apply to individual dischargers requesting permits and apply only to the pollutant or pollutants specified in the variance. GLI has limited potential to incrementally improve water quality in the Great Lakes Basin because first, it primarily focuses on point sources, which are not the major source of certain toxic pollutants that currently affect the Great Lakes Basin. Moreover, once GLI was implemented, few NPDES permits included limits for BCCs because they were not in discharges, and many of these BCCs were already regulated or banned before the GLI guidance was issued. Finally, for mercury, which is the BCC that is most frequently controlled in NPDES permits, GLI provisions provide flexible implementation procedures, including variances, that under certain circumstances are used by states to allow dischargers relief from the more stringent water quality standards. The stringent water quality standards may be either technically or economically unattainable by dischargers. A primary focus of GLI is to establish consistent water quality standards within the Great Lakes Basin, which apply to all sources of pollutants but mainly to point sources. Thirty-three years ago, point sources of pollution were the basis for the establishment of the NPDES program and the major cause of poor water quality in the Great Lakes Basin. In implementing this program, it was recognized that controlling point sources was an important means of reducing pollutants discharged into waterways by requiring permits that specified allowable levels of pollutants. Since the introduction of the NPDES program there have been significant water quality improvements in the Great Lakes Basin.