Acid rain is the most significant threat to the country's national parks and is so serious that in the waters of one preserve, freshly stocked brook trout survived only two days, a conservation group says.

The National Parks and Conservation Association, in a report titled "Acid Rain Invades Our National Parks," found that despite the problem there is insufficient monitoring of acid rain levels by the National Park Service.

At the 27 sites where there is monitoring, the report said, levels of acid have been found that could cause "irreversible damage" to water, soil, plant and animal life.

The parks surveyed in the report include Acadia National Park in Maine, Sequoia and King's Canyon national parks in California, Delaware Water Gap National Recreational Area, Pinelands Reserve in New Jersey and Shenandoah National Park in Virginia.

"The parks are symbols of what's happening to the rest of the environment," said Paul Pritchard, president of the association. "They are contained, controlled units," he said, that suffer from an uncontrolled menace. The report is the first to collect data on acid rain effects in national parks. It queried 87 national parks that registered concern about acid rain with the National Park Service in 1980. These parks were to have been monitored for acid rain by the service.

The report held that, of the 63 parks responding to inquiries, 36 have not been effectively monitored for acid deposition. "The problem is not with the NPS," Pritchard said. "It's absolutely not a concern of this administration."

Acid rain has had serious effects on the vegetation, aquatic species and cultural resources of the national parks. In New Jersey's Pineland Reserves brook trout stocked in lakes have not survived for longer than two days.

"You ask anyone in the field if it's a problem," Pritchard said. "Fish dead in two days is a problem."

The report argued that the National Park Service is not equipped to deal with the acid rain threat and urged Congress to strengthen the Clean Air Act of 1977.

"Acid rain is the No. 1 threat to our national parks," Pritchard said. "The next threat is other government agencies."

The National Acid Precipitation Assessment Program has spent $222 million since it began in 1980 to provide information and research on acid deposition through federal agencies. The National Park Service was allotted $7.6 million.

"We have not done a very large amount of research on it," said Duncan Morrow, a spokesman for the National Park Service.

"One of the prime reasons we have not done more is that there is already a large amount of existing research projects going on in the country in this field," Morrow said. "We don't want to duplicate this research."

Morrow added that the National Park Service focuses on issues that are of greater concern to it than to other organizations. "We're in 1987, in a time where there is a clear sentiment in the country at large that government agencies should be keeping costs down," he said.

Acid rain is an "external threat" since it lies outside the domain of the park service, Morrow said.

It occurs when pollutants from factories, chemical plants and automobiles mix in the air with oxygen and hydrogen, forming sulfuric and nitric acids. The pollutants can travel hundreds of miles before coming down in rainfall.