The Safe Drinking Water Act required the Environmental Protection Agency (EPA) (defendant) to promulgate regulations to promote safe drinking water. In many of its regulations, the EPA set a maximum contamination level (MCL) for the contaminant, subject to the regulation. For lead, however, it was difficult to determine the exact level of lead in water, for a variety of reasons. Lead entered water systems mostly through corroded service lines and privately owned plumbing materials that were outside of the EPA’s jurisdiction and control. Further, system-wide lead testing was difficult due to the variations with respect to plumbing materials. In fact, consecutive tests from the same water source could result in significantly different lead levels. Additionally, implementing aggressive corrosion control techniques that could decrease lead amounts actually increased levels of other toxins in water systems. In light of these difficulties, the EPA chose not to establish an MCL for lead. Rather, the EPA promulgated regulations requiring water systems to implement certain limited corrosion-control treatments. The National Resources Defense Council and the American Water Works Association (plaintiffs) brought suit, challenging the regulations on the ground that establishing an MCL for lead was feasible.