Residents of Hoopa Valley Reservation claim that a Bureau of Indian Affairs spraying program caused a plant-killer linked to cancer and birth defect to drift onto food and water supplies and schoolchildren here, and that, despite their complaints the agency plans to continue its aerial spraying program.

The residents, some Indian and some white, claim that as a result of the BIA program to encourage the growth of fir seedlings by destroying broad-leaf plants, up to 100 residents of the reservation have been exposed to a herbicide known at 2,4,5-TP.

This compound and others closely related to it contain traces of chemicals known as dioxins. Dioxins are potent teratogens - that is, they cause birth defects. They have also been linked to cancer in laboratory animals.

The long-range effect of dioxins on humans are not known, but officials of the California Health Department point out that they are so potent that researchers have been unable to specify a dose small enough to be considered safe.

The BIA says the spraying program is being carried out with the knowledge and permission of the tribal council, and an agency spokesman labels the exposure claims as "preprosterous."

The residents, however, say that on Sept. 21 a helicopter discharged spray about a half-mile from the Weitchpec School. Although the herbicide was mixed with diesel fuel to weigh it down, it hung in the air, drifting across the school and down onto the children.

On the playground some of the children began to coungh, a few complained of headaches, nausea and sore throats. One 9-year-old, covered with a sticky ooze, broke out in a rash. The children soon recovered.

Dr. Mark Carlson, a physician from the California health department gathering data on the Weitchpec incident, believes the children's initial reactions were to the diesel fuel, not the herbicide.

Including the school children, perhaps 100 persons have been exposed to 2,4,5-TP, a sophisticated herbicide that kills broad-leaf plants by causing malfunctions in their growth processes.

It also appears that use of 2,4,5-TP may circumvent an Interior Department policy that bans 2,4,5-TP's parent chemical, 2,4,5-T.

In 1970, use of 2,4,5-T was restricted because of reports of birth defects caused by the defoliant Agent Orange in Vietnam. 2,4,5-TP was a main ingredient of Agent Orange, although it contained larger quantities of dioxin than the chemical currently in domestic use. Dioxin occurs in the herbicide as a manufacturing impurity.

In an August 8 letter, Secretary of the Interior Cecil D. Andrus not only supported the ban but also confirmed that all his agencies - including the Bureau of Indian Affairs - were adhering to that policy.

Andrus was writing to California Assemblyman Barry Keene, supporting a voluntary state park ban on all phenoxy herbicides including both 2,4,5-T and -TP. The policy has since been adopted by seven major timber companies in Northern California.

Ed Winsor, manager for BIA forest land in California, defended the use of 2,4,5-TP, though he acknowledged that it contains a similar amount of dioxin as the banned 2,4,5-T. "It wasn't on the restricted list," he said.

Meanwhile, the Environmental Protection Agency has initiated a program to collect and analyze samples of mothers' milk in logging areas of California and Oregon, including the Hoopa reservation. An EPA-financed study has found measurable traces of the defoliant in water in southern Humboldt County. Spraying on private forest lands in that area was stopped last spring.

Sen. Dale Bumpers (D-Ark.) has introduced legislation to ban aerial spraying of defoliants with dioxin in national forests. Such operations are already prohibited by court order at some parks in Arkansas and Oregon.

The BIA claims that the spray program was approved by the tribal council. The Hoopa tribe owns all timber rights in the 12-square-mile reservation; logging is the main source of income for the tribe's 1,500 members. BIA forestry representatives claim that use of the herbicide increases fir production by up to 40 per cent.

While Hoopa Tribal Chairman Pete Masten defended the spraying, he admitted that he had learned of the September incident only in the course of a reporter's interview. It was later determined that the tribe gave its permission in January, and the BIA gave no further notice of the impending spraying. (The U.S. Forest Service says its spray guidelines call for extensive publicity and posting of warning notices.)

"The information that we have from the chemical company shows that the spray is not harmful," Masten said.

The data sheet of 2, 4, 5-TP, marketed under the trade name "Kuron" by the Dow Chemical Co., makes no mention of dioxin contaminate, which occurs in minute quantities. Instructions do, however, warn of the dangers of drift, contamination of streams and contact with eyes, skin or clothing.

Masten said he doubts the claims that Weitchpec residents were exposed. "The wind always blows the other way," he said.

But more than a dozen adults in the section around the Weitchpec school claimed they were either exposed or made sick by the spraying. Two witnesses claimed that the helicopter pilot swung so wide from his mark that the chemical dropped by a creek where two families take their drinking water. Others complained that the defoliant contaminated an area where local residents traditionally collect acorns, mushrooms and dry grass to make baskets.

"I was all for the spraying," said John Schuchman, who lives 100 yards up the hill from the school. "I worked in the woods all my life, but I'm turning on it now.

"I heard people say it's the hippies, they don't want spraying on their pot gardens. I don't have no pot. But I do have animals, and I'm scared for them."