Alaska oil spill responders have adopted new rules for the rapid use of chemical dispersant but say dispersant will continue to be considered only rarely, when mechanical cleanup is not practical.

Chemical dispersant has been used on an oil spill just once in Alaska in the last 40 years — in tests during the 11-million gallon crude oil spill that followed the 1989 grounding of the Exxon Valdez oil tanker. The preferred method of cleanup is mechanical, usually using boom to corral oil and skimmers to lift it from the water.

Chemical dispersant does not remove oil but breaks it into small droplets that spread throughout the water column, making it more available to natural degradation by oil-eating microbes. Responders acknowledge chemical dispersant use is controversial but say it can be a useful tool if there's a net environmental benefit.

"We are talking about large, enormous spills of crude oil into the waters that completely outpace the ability of mechanical systems of recovery to move that oil from the environment," said the Coast Guard's Mark Everett, co-chairman of the Alaska Regional Response Team. "Those are very rare spills."

The new plan was signed Jan. 27 by representatives of four federal entities involved in oil spill response, the Coast Guard, the Commerce and Interior departments and the Environmental Protection Agency, plus the state of Alaska. The plan replaces rules put in place after 1989 but revoked in 2008.

The plan sets up one "preauthorization zone" that would allow a federal on-scene coordinator to authorize mobilization of dispersant and the elaborate gear needed to spread it. A final decision on actually using dispersant would be made after consultation with wildlife experts, tests of the dispersant and other steps.

Environmental groups oppose dispersants. Anchorage-based Alaska Community Action on Toxics and other plaintiffs in 2012 sued to force the EPA to develop new national standards for dispersant use.

"They are highly toxic to a broad range of marine organisms, from microbes that help to break down the oil to marine mammals, and are toxic to the people who apply them," said Pam Miller, director of Alaska Community Action on Toxics.

Dispersed oil can have a negative effect, Everett said, but can have a more positive effect on the environment than toxic, undispersed crude oil. The aftermath of a nationally significant spill is not the time to begin considering dispersant use, he said, because of short window of time in which it would be effective, the need for trained personnel and the need to have equipment and material staged nearby.

"You cannot pull that all together at the last minute. It's just too complex an operation to be done right and competently," Everett said.

The preauthorization zone covers tankers transiting well offshore after leaving Valdez, the off-load site of the trans-Alaska pipeline, or traveling between the United States and Asia along the Great Circle Route. The zone begins at Cape Suckling east of Cordova and stretches to the end of the Aleutians. Most coastal states have preauthorization zones that start three miles offshore. Alaska's starts 24 miles off shore and extends to open ocean 200 miles south of the Aleutians and 100 miles north of the Aleutians.

In two years, crude oil tankers transiting to or from a U.S. port will be required to have dispersant available when moving in the zone. Most likely will contract with an oil spill removal organization as part of their federally approved vessel response plan, Everett said. Fishing boats and other vessels do not have the same requirement.

The final shape of the preauthorization zone could change. Responders over the next two years will consider areas within the zone for exemption from dispersant use.

Use of chemical dispersant in Alaska waters outside of the zone can be considered on a case-by-case basis.