Justices seemed skeptical of claims that a Los Angeles-area clean air agency can go beyond the federal Clean Air Act to impose tougher antismog restrictions for city buses, airport shuttles and other vehicles.
The court is looking at cases from opposite coasts that challenge pollution regulations, part of the court's unusually in-depth review this year of environmental issues.
In cases involving the Florida Everglades and California smog, justices were considering whether lower courts were too protective of the environment. The Bush administration wants the high court to overturn both decisions.
In the air pollution case, an attorney for oil companies and diesel engine manufacturers argued that local pollution rules conflict with the federal Clean Air Act.
Washington lawyer Carter Phillips said the justices "have to consider the possibility that all 50 states or every local jurisdiction could follow suit" if the local district gets to set its own rules.
The rules, adopted in 2000 and 2001 by the South Coast Air Quality Management District, aim to replace diesel-fueled buses, street sweepers, trash trucks and airport shuttles with cleaner-burning models.
Seth Waxman, representing the district, told justices the rules don't conflict with federal regulations because they do not force manufacturers to change emission standards. They merely direct fleet operators to buy cleaner-burning vehicles that are already on the market, he said.
Justice Stephen Breyer suggested that requiring purchasers to buy cleaner-burning vehicles would have the effect of telling manufacturers what to make.
The second case argued is being closely watched by the nation's water managers. It pits the 500-member Miccosukee Indian tribe and an environmental group against a water district the Indians accuse of illegally dumping pollutants into Florida's Everglades.
The tribe says a South Florida Water Management District pump west of Fort Lauderdale dumps as much as 423,000 gallons a minute of polluted runoff from suburban lawns, farms and industrial yards into the Everglades, including 189,000 acres the state leased to the tribe and promised to keep in its natural state.
Breyer told the district's lawyer that their system "takes this filthy, absolutely disgusting water" and dumps it in the pristine Everglades.
Timothy Bishop responded that they are only moving water that already has pollutants, part of an effort to protect nearby areas from flooding.
Justice Antonin Scalia agreed. "No pollutants have been added to the navigable waters," he said.
Western states worry that if the Supreme Court should rule in favor of the Miccosukees, regional water diversion efforts might become subject to expensive federal anti-pollution requirements.
The Bush administration supports the water district, even though such a ruling could limit government authority in clean water cases.
The Miccosukees and Friends of the Everglades sued the water management district in 1998 under the federal Clean Water Act.
Lawyers for the water district told the Supreme Court in a filing that the hundreds of thousands of similar public water diversion facilities that exist in the United States are not responsible for the pollution that they transport.
The cases are South Florida Water Management District v. Miccosukee Tribe, 02-626; and Engine Manufacturers Association v. South Coast Air Quality Management District, 02-1343.