President Clinton announced plans yesterday to tighten environmental reporting rules by requiring firms that handle even relatively small quantities of certain toxic materials to provide fuller public disclosure when they discharge potentially dangerous chemicals into the air or water.

The new rules, which go into effect on Jan. 1, will significantly lower the amount of 27 dangerous chemicals--persistent bioaccumulative toxics (PBTs)--a company can use before it must report discharges to the public.

"By requiring industries to tell communities how much they pollute the air and water, we empower citizens to fight back and create a powerful incentive for industry to pollute less," Clinton said in his weekly radio address.

The new regulations, for the first time, cover dioxin, a dangerous byproduct of many industrial processes--including waste incineration and chemical and pesticide manufacturing. Under the new requirements, firms that produce as little as a tenth of a gram of dioxin a year would be required to report potentially dangerous discharges to the public.

The president also used his radio address to announce federal acquisition of 14,000 acres of desert land in California's Joshua Tree National Park as well as the $101 million purchase of a 95,000-acre ranch in New Mexico, which contains one of the nation's largest herds of wild elk.

In broadening its chemical reporting rules, the Clinton administration pointed out that over the years it has expanded the number of industries required to report toxic releases and nearly doubled the number of chemicals subject to reporting. But many environmental activists say current loopholes allow significant levels of pollution to go unreported.

In a report released last year, the U.S. Public Interest Research Group, a national watchdog organization, said 90 percent of PBT pollution goes unreported. Moreover, the report said, as little as 30 percent of facilities that use or release PBTs were required to report discharges as recently as 1996, the report said.

Under current rules, companies that handle more than 25,000 pounds or use more than 10,000 pounds of toxic chemicals a year are required to report discharges. The new rules will require reports by companies that use 100 pounds a year, or, in the case of some particularly dangerous chemicals, 10 pounds a year.

"This is a huge step toward advancing the public's right to know," said Jeremiah Baumann, an environmental advocate with PIRG. "But there are a few flaws in the proposal." He called the absence of lead, a substance known to hinder children's development, from the list of substances covered by the reporting guidelines a "major shortcoming" in the new rules.