Sierra Club, Imperial County Air Pollution Control District, Intervenor v. United States Environmental Protection Agency Gale Norton

We must decide whether the U.S. Environmental Protection Agency lawfully concluded that a Southern California county would have achieved the 24-hour air quality standards required by the Clean Air Act but for the negative effects of transborder emissions from Mexico.

I

Imperial County encompasses approximately 4,600 square miles in Southeastern California and is bordered by Riverside County to the north, Mexico to the south, Arizona to the east, and San Diego County to the west. The county shares approximately 80 miles of border with Mexico. Calexico, one of three significant population centers in the county, sits close by the border, not far from the Mexican city of Mexicali. Moving north from the border, the county’s other two major population centers are El Centro and Brawley. As of 1999, the county had approximately 142,-000 inhabitants.

The Imperial Valley runs roughly through the center of the county, from the northwest to the southeast, and across its southern border into Mexico. Most of Imperial County, save for a small stretch of land on the county’s eastern end, falls within the Imperial Valley Planning Area (“Imperial Valley”), and Intervenor Imperial County…