A massive hazardous-waste landfill in Niagara Falls, N.Y., that received thousands of tons of material from "Superfund" cleanups is leaking and "could endanger the health of the public living downstream," according to an internal Environmental Protection Agency report.

The landfill, a Cecos International facility, was constructed according to the most modern technology for hazardous-waste disposal, with specially designed liners and a collection system that is supposed to catch leaks before contamination occurs.

The facility's two newest units have been open less than three years and have accepted waste from at least seven Superfund toxic waste cleanup sites.

The site has been formally under investigation for several months as a result of tests that showed high levels of contamination in ground water. Cecos and regional EPA officials, however, have attributed the results to "irregularities" in testing procedures.

But the new report, prepared by an investigator from the agency's Washington office, found "substantial circumstantial evidence" that the two newest units are leaking "and probably were leaking at the time Superfund wastes were deposited in them."

According to the report, chemical contaminants from the site, including chloroform, vinyl chloride and industrial solvents, appear headed for a residential area less than one-fourth of a mile away.

"If these homes have basements present, then there is the very real danger that the volatile organics observed in wells at the facility's fence line . . . will be entering these basements at at least trace levels," the report warned, urging that the EPA investigate the homes "as soon as possible."

EPA officials yesterday attempted to play down the report, citing "disagreement" between regional and headquarters investigators. Russ Dawson, a spokesman for the agency's Superfund office, said technicians will try to sort out differences in a meeting this week.

"The regional people hotly dispute these findings and interpretations," Dawson said. "It is possible that the results were due to sampling error and poor analytical technique."

But other EPA officials said the agency considers the site an "imminent hazard" and has ordered officials to "minimize" the amount of Superfund waste sent there.

According to a policy established in January 1983, no Superfund waste is to be sent to a facility "in assessment" -- the formal term for a leak investigation.

Cecos spokesman Thomas L. Moran confirmed that the Niagara Falls facility is in assessment but said he has not seen the EPA's latest report and could not comment on it.

"Cecos is interested in finding out what the story is," he said.

The EPA report found fault with several aspects of the landfill's operation. Among other criticisms, it found its monitoring program inadequate and reporting "sloppy."

The report also suggested that chemical wastes were coming into direct contact with a superstrength material that lines the disposal pits, "a condition that will lead to an early demise of its integrity."

"In short," the report said, " . . . it is probably a safe bet that the unit is leaking, which is not to say that it was not both state of the art and properly installed when it was built. Of course, a question could be raised about the propriety of constructing it in such a hostile environment to begin with."