Environmental groups fighting a new, federally approved cleanup plan for the Navajo Generating Station say the Environmental Protection Agency is wrong to say that the new plan would clean the plant up more in the long run than an earlier plan that the agency shelved.

That issue -- which cleanup plan is the cleanest? -- will likely be argued in court if four environmental groups who oppose the newly approved plan decide to sue.

The groups, including the Sierra Club, challenged EPA's conclusion in its earlier, public comments on the cleanup plan approved Monday. EPA calls that plan "Better than BART" because it's supposed to reduce the total amount nitrogen oxide emissions over 35 years by more than would have happened under an earlier EPA plan that would have been based on the use of the best available retrofit technology for the plant.

The enviros used computer models to try to show that EPA is wrong, and that the earlier EPA plan would generate fewer nitrogen oxide emissions than the plan that ultimately was approved on Monday.

EPA disagreed with the criticism, obviously, in its response to public comments that it published yesterday as part of its final decision. The argument is fairly dense and centers in part on the use of various kinds of input information that's plugged into computer models by both parties to the dispute.

The EPA's approved plan, as outlined in today's Star, would delay implementation of much of the reduction in nitrogen oxide emissions at the coal-fired power plant from 2023 under its earlier plan to 2030. By 2020, the plant's operators would have to slice its energy use and emissions by 33 percent, either by shutting down one of the units or reducing total emissions from all three if they're all kept going.

But the three units -- or two, as the case may be -- wouldn't have to receive $500 million worth of selective non-catalytic reduction technology that would really cut heavily into emissions -- about 80 percent -- until 2030. The earlier EPA plan had triggered alarm bells among the plant's operators, including electric utilities such as the Salt River Project and the Navajo Tribe, whose members have hundreds of jobs there, as well as officials of the Central Arizona Project who use Navajo's power to pump water uphill to Phoenix and Tucson.

The fear was that the owners wouldn't be able to afford to install the controls as early as 2023, leading to closure of the plant, eliminating jobs and forcing water users to find what they expect to be more expensive power on the open market. Those concerns led to a coalition of various interest groups that negotiated the proposed alternative cleanup plan -- the one that EPA largely adopted Monday.

 EPA spreadsheets show that the newly approved plan would reduce Navajo's cumulative emissions by 12.2 percent more by 2044 than the earlier plan would have done. Total emissions under the earlier plan would be about 499,000 tons from 2009 to 2044, compared to about 433,000 tons under the approved plan, EPA says.

The environmentalists make two points in reply. Their comment to EPA in January 2014 was written by three attorneys for the enviro-law firm Earthjustice. The lawyers represent the Sierra Club, the Natural Resources Defense Council, the National Parks and Conservation Association and the Grand Canyon Trust.

First, the attorneys wrote  that EPA violates the law by even using the approach it did. Second, the number-crunching by EPA used to total up emissions "Is riddled with errors and improper emissions calculations," wrote the Earthjustice attorneys.

Looking at eight possible arrangements for a "better than BART" alternative, the Earthjustice analysis concluded that they would generate a range of total emissions of 436,000 to 494,000 from 2009 to 2044. Looking at two possible BART scenarios, the same analysis pegged their total emissions at either 379,000 or 280,000 tons over those 35 years.