WASHINGTON - The Supreme Court ruled Monday against the Obama administration's attempt to limit power-plant emissions of mercury and other hazardous air pollutants, but it may be only a temporary setback for regulators.

The justices split 5-4 along ideological lines to rule that the Environmental Protection Agency did not properly take costs into account when it first decided to regulate the toxic emissions from coal- and oil-fired plants.

The EPA did factor in costs at a later stage, when it wrote standards that are expected to reduce the toxic emissions by 90 percent. But the court said that was too late.

The rules, which took effect in April, will remain in place while the case goes back to a lower court for the EPA to decide how to account for costs, environmental advocates say.

They were supposed to be fully in place next year. At issue was whether health risks are the only consideration under the Clean Air Act.

"I'm hopeful the EPA will be able to prove the benefits to the American people far outweigh the cost to the industry," said Gwen Lachelt, chairwoman of the La Plata County Board of Commissioners. "If they take a look at health impacts and the cost of health care over time, I think they'll be able to. The goal isn't to overburden the industry, but make sure they protect public health."

Studies have shown abnormally high levels of mercury in the soil at Mesa Verde National Park and in the Molas Pass area. The heavy metal has been an air-quality priority for La Plata County after advisories were issued by the Colorado Department of Public Health and the Environment regarding mercury pollution in several Southwest Colorado reservoirs, including Vallecito and Navajo Lake.

In the Mercury Contamination in Southwestern Colorado Study conducted by a team from the University of Colorado in 2004, about one-third of the northern pike at Vallecito Reservoir had concentrations of 0.71 milligrams of mercury per kilogram of fish. Advisories go into effect at 0.5 milligrams.

"They've limited how much fish people should eat per week and per month from our fisheries because of the mercury," Lachelt said.

Local environmentalists point to power plants in northern New Mexico as the source.

"We don't get any of our power from them, but we do get a lot of their pollution," said Dan Olson, executive director of the San Juan Citizens Alliance.

In 2013, the Four Corners Power Plant closed down three of its five power generators and implemented Best Available Retrofit Technology in order to be in compliance with a different set of EPA rules to improve visibility and reduce haze. But that technology doesn't address mercury pollution.

Four Corners is one of two plants with majority ownership by Arizona Public Service.

"We're still reviewing the ruling because we're not sure what this means for us," APS spokesman Alan Bunnell said.

APS was not party to the lawsuit ruled on by the Supreme Court on Monday.

Mercury is especially dangerous to young children and pregnant or breast-feeding women because of concern that too much could harm a developing brain.

In the case of the mercury rules, the costs of installing and operating equipment to remove the pollutants before they are dispersed into the air are hefty - $9.6 billion a year, the EPA found.

But the benefits are much greater, $37 billion to $90 billion annually, the agency calculated. The savings stem from the prevention of up to 11,000 deaths, 4,700 nonfatal heart attacks and 540,000 lost days of work, the EPA said. The agency estimates its rule would have prevented up to 140 premature deaths in Colorado while creating up to $1.1 billion in health benefits in 2016.

The challenge was brought by industry groups and 21 Republican-led states, which argued that the regulations were too costly for coal miners, businesses and consumers.

Writing for the court, Justice Antonin Scalia said the EPA was unreasonable in refusing to consider costs at the outset.

"The agency must consider cost - including, most importantly, cost of compliance - before deciding whether regulation is appropriate and necessary," Scalia said.

In dissent, Justice Elena Kagan said it was enough for EPA to consider costs later in the process.

"Over more than a decade, EPA took costs into account at multiple stages and through multiple means as it set emissions limits for power plants," Kagan said. She was joined by the court's liberal members.

The EPA said it is reviewing the court's decision, and will determine any appropriate next steps once a review is completed.

"EPA is disappointed that the Supreme Court did not uphold the rule, but this rule was issued more than three years ago, investments have been made, and most plants are already well on their way to compliance," said EPA spokeswoman Melissa Harrison.

Indeed, more than 70 percent of power plants have already installed controls to comply with the rules, said Vicki Patton, an attorney at the advocacy group Environmental Defense Fund.

"EPA already has an economic analysis that it can rely on to demonstrate that the public-health benefits of the standards far outweigh the costs," Patton said.

The case is the latest in a string of attacks against the administration's actions to use the Clean Air Act to rein in pollution from coal-burning power plants.

Senate Majority Leader Mitch McConnell, R-Ky., called the ruling "a cutting rebuke to the administration's callous attitude." He said it "serves as a critical reminder" to state governors, who are awaiting more EPA rules this summer aimed at curbing pollution from coal-fired power plants that is linked to climate change.

States have already challenged those rules even before they are final, and Congress is working on a bill that would allow states to opt out of any rules clamping down on heat-trapping carbon dioxide.

White House spokesman Josh Earnest called out McConnell for reprising his suggestion that governors should flout EPA regulations. He said McConnell was not advocating in the best interests of the American public.

House Speaker John Boehner, R-Ohio, suggested that the high-court ruling gives opponents an opening "to protect the jobs and energy that are still at risk under this administration."

A disproportionate share of the 600 affected power plants, most of which burn coal, are in the South and upper Midwest.

"This just feels like a big win for polluters and a big loss for communities downwind of them," Olson said.

Herald Staff Writers Mariam Baksh and Ann Butler contributed to this story.