The U.S. Department of Energy acted illegally when it found that two Mexican power plants would not have a significant impact on the air and water quality in the border region between northwestern Mexico and Southern California, a federal judge in San Diego ruled yesterday. That ruling calls into question the legitimacy of U.S. permits granted to the power companies to build cross-border transmission lines and could prevent the plants from exporting electricity to California this summer as planned. It could also bode ill for plans by energy companies to build as many as 20 other power plants in Mexico to sell electricity to the U.S. Environmentalists and human-health advocates in the region welcomed the ruling, which found that the Bush administration’s cursory review of the plants did not adequately consider the potential environmental impact. Environmentalists contend that these impacts include increased salinity in the Salton Sea and emissions of carbon dioxide, nitrogen oxide, particulate matter, and ammonia.