FARMINGTON — President Obama on Tuesday directed the U.S. Environmental Protection Agency to establish carbon pollution regulations for coal-fired power plants, a move that could affect San Juan County's two plants.

Four Corners Power Plant and San Juan Generating Station each rank among the nation's top carbon polluters.

The utility companies that own the plants said plans already in place will reduce carbon emissions.

Public Service Company of New Mexico, the operator and largest shareholder at San Juan Generating Station, said an agreement to close two of the plant's four stacks in 2017 will reduce carbon emissions by 50 percent. PNM is planning to install pollution control technology on the two remaining units. The pollution controls will cut emissions of nitrogen oxide, which contributes to regional haze, but the technology does not reduce carbon emissions.

The plan is the result of an agreement PNM reached with the New Mexico Environment Department and the EPA that still faces approval from state and federal regulators.

"PNM is already taking significant steps that will reduce nitrogen oxide emissions to meet federal visibility regulations and bring about additional emission reductions, including carbon dioxide, not called for by the regulations," spokesman Pahl Shipley said in an e-mail message.

PNM supports a national approach to carbon regulations, rather than state-by-state rules, Shipley said.

Arizona Public Service Co., the operator and largest owner of Four Corners, said its plan to close three of the plant's five stacks would reduce carbon emissions by about 30 percent.

"As climate change regulations are considered, it is important that they be implemented nationally and across all industries," APS spokesman Damon Gross said in an email. "They also must contain achievable compliance limits and allow us to continue to provide reliable, affordable electric service to our customers."

APS' plan to close three units at Four Corners is intended to satisfy EPA requirements to cut regional haze. The Phoenix-based utility has asked the EPA to extend a July 1 deadline to commit to the plan, citing uncertainty stemming from an Arizona commission's decision to pursue electric market deregulation. The company announced June 17 that a $294 million purchase of the two remaining units from a California utility is on hold, leaving it unclear when the three units may be powered down.

The specifics of new EPA standards will determine how they affect existing plants, leaving a cloud of uncertainty hanging over the plants. Obama said the EPA would consult with states and industry before issuing the regulations. That process could stretch on for months or years to come.

Four Corners emitted 13.2 million metric tons of carbon dioxide equivalent in 2011, ranking 21st in the U.S. among stationary sources. San Juan Generating Station produced 11.8 million metric tons, coming in 31st in the nation, according to EPA data.

Obama said Tuesday that about 40 percent of U.S. carbon emissions comes from power plants.

"We limit the amount of toxic chemicals like mercury and sulfur and arsenic in our air or our water, but power plants can still dump unlimited amounts of carbon pollution into the air for free," he said. "That's not right, that's not safe and it needs to stop."

Environmental groups cheered Obama's announcement.

"It's long overdue from the perspective of acknowledging the impact of these coal plants," said Mike Eisenfeld, New Mexico energy coordinator for San Juan Citizens Alliance in Farmington.

Eisenfeld said the regulations could help shift the nation and New Mexico toward cleaner forms of energy. He added new federal regulations may alter the plans of utilities that run coal plants that were built decades ago.

"It's further acknowledgement of the challenges that San Juan and Four Corners Power Plant have in continuing to operate long-term," he said.

Carbon pollution is affecting the state now, Camill Feibelman, director of the Sierra Club's Rio Grande chapter, said in a news release

"Climate disruption is hitting New Mexico hard," Feibelman said. "As we've endured one of the worst droughts of the century and witnessed record-setting wildfires, (University of New Mexico) studies predict that this level of drought will become commonplace and wildfires will get worse unless greenhouse emissions are significantly reduced, so President Obama's announcement is welcome news."

Chuck Slothower covers business for The Daily Times. He can be reached at 505-564-4638 and Follow him @Dtchuck on Twitter.