FELICIA FONSECAFLAGSTAFF, Ariz. (AP) â¿¿ Federal rules aimed at limiting mercury emissions from coal-fired power plants factored into a plan by Arizona Public Service Co. to shut down three of five generating units at a northwestern New Mexico facility that it operates. The 2,040-megawatt Four Corners Power Plant is one of the largest of its kind in the United States. It provides electricity to about 300,000 households in New Mexico, Arizona, California and Texas. Arizona Public Service, a unit of Pinnacle West Capital Corp., plans to retire the three units in 2013, a decision that came toward the end of last year and that the utility partly attributed to rules targeting mercury and other toxic pollutants that the U.S. Environmental Protection Agency is expected to announce soon. Dozens of coal-fired plants nationwide already meet at least some of the standards, but the EPA said about 44 percent of all such plants lack advanced pollution controls. The proposed rule unveiled earlier this year would give owners up to four years to meet the new standards by installing scrubbers, bag houses or other modern technology. Four Corners is one of three coal plants on or near the Navajo reservation. The San Juan Generating Station, also in northwestern New Mexico, said it stands ready to meet the new rules, already having seen dramatic reductions in mercury emissions since completing an upgrade in 2009. Mercury emissions dropped from nearly 500 pounds per year in 2006 to 66 pounds in 2010, said Don Brown, a spokesman for the plant's operator, Public Service Company of New Mexico. The operator of Navajo Generating Station to the west in Page, Ariz., said the 2,250-megawatt plant will run as long as the owners are convinced there isn't a better alternative. But spokesman Scott Harelson said the plant is facing some challenges, the most pressing of which are EPA regulations, and negotiating coal supply agreements and a site lease â¿¿ "any of which could put the plant at risk of closure."