In 1972, Congress amended the Clean Water Act (CWA) to prohibit the discharge of any pollutant to waters of the United States from a point source unless the discharge is authorized by an National Pollutant Discharge Elimination System (NPDES) permit. Initial efforts to improve water quality under the NPDES program focused on reducing pollutants in industrial process wastewater and municipal sewage. These discharge sources were easily connected to poor water quality. As pollution control on industrial process wastewater and municipal sewage were implemented and refined, it became evident that other sources of water pollution were still contributing to water quality impairment, and that stormwater runoff was a major suspect.
In 1987, Congress again amended the CWA to require implementation, in two phases, of a comprehensive national program for addressing stormwater discharges. Phase I required NPDES permits for stormwater discharge form a large number of priority sources including medium and large Municipal Separate Storm Sewer System (MS4s) and several categories of industrial activity, including construction activity that disturbed 5 or more acres of land. The second phase of the stormwater program required permits for stormwater discharges from certain small MS4s and construction activity generally disturbing between 1 and 5 acres. Phase II remained to be further clarified by future EPA analysis and decisions.
EPA provided this clarification in August 1999 through the proposal of a draft final rule that addressed the Phase II aspects of the NPDES stormwater program. Of relevance to Yavapai County was the application of the program to small MS4s. Phase II municipalities were defined as those not included in the Phase I program but located within a U.S. Census Bureau “Urbanized Area.” In practical terms, this category includes urbanized areas with a population between 10,000 and 100,000 by the 1990 Census. EPA also reserved the authority for the permitting agency to designate Phase II municipalities based upon other stormwater impacts on waters of the United States. The Phase II Final Rule was published December 8, 1999 in the Federal Register.