LOS ANGELES — For most Californians, the sight of aircraft spewing neon pink liquid over flaming trees and brush has become a hallmark of aggressive wildfire suppression campaigns — if not a potent symbol of government's struggle to control increasingly destructive forest fires.

But as the use of aerially delivered retardant has soared in recent years, some forest advocates say the substance does more harm than good. They claim wildfire retardant drops are expensive, ineffective and a growing source of pollution for rivers and streams.

"There's no scientific evidence that it makes any difference in wildfire outcomes," said forester Andy Stahl. "This is like dumping cash out of airplanes, except that it's toxic and you can't buy anything with it because it doesn't work."

Now, a federal lawsuit in Montana that seeks to stop the U.S. Forest Service from dropping retardant into water could reshape how the agency battles wildfires throughout the western United States.

The case is being watched particularly closely by officials in California, where an extremely wet winter is likely to stoke the growth of so-called connecting fuels — grasses that can carry small flames from a spark on a roadway to chaparral and forested areas.

"This is going to destroy towns and many communities in California, if they allow this to go through," said Paradise Mayor Greg Bolin, whose town was razed by the Camp fire in 2018. "To maybe save a few fish, really?"

The lawsuit, filed by the Forest Service Employees for Environmental Ethics, of which Stahl is executive director, accuses the Forest Service of violating the Clean Water Act, which prohibits the discharge of pollutants into U.S. waters without a permit.

The action comes as more retardant is being dropped from the air than ever before amid longer, more active fire seasons. In 2021, 52.8 million gallons of retardant were dumped on federal, state and private land, compared with a 10-year average of about 39 million gallons per year, according to figures provided by the Forest Service. More than half of the retardant the Forest Service used on national forest lands was dropped in California — more than any other state.

The Forest Service primarily uses ammonium phosphate-based retardant, which is intended to coat vegetation and other fuels around the edges of a fire to deprive advancing flames of oxygen. The goal is to slow fire spread and lessen its intensity so crews on the ground get a chance to directly attack it.

But the chemical, which is also used as fertilizer, can kill aquatic life. For example, in Santa Barbara County, dozens of endangered steelhead trout were killed in Maria Ygnacio Creek during the 2009 Jesusita fire. UC Santa Barbara scientists documented elevated ammonia levels in the water and concluded the fish kill was likely due to retardant drops.

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