Pursuant to the Clean Water Act, the Environmental Protection Agency (EPA) (defendant) established a total maximum daily load (TMDL) of certain contaminants, capping the daily amount of each containment that could be released into the Chesapeake Bay. The regulation allocated permissible discharge levels of nonpoint-source pollutants, including nitrogen, phosphorus, and sediment among different sources and sectors (i.e., agriculture, forest, urban, etc.). The act directed states to regulate nonpoint sources pursuant to their traditional land-use role. However, the act required states to provide the EPA with waters for which point-source caps were insufficient to achieve satisfactory water quality. The American Farm Bureau Federation (Farm Bureau) and other groups representing agricultural interests (plaintiffs) sued, challenging the regulation. The district court upheld the regulation. Farm Bureau appealed, arguing that the act contemplated TMDL as simply a total number without specific allocations among point and nonpoint sources.