The Environmental Protection Agency has seriously underestimated the number of high-threat hazardous-waste dumps and may be worsening the problem by trying to clean up the dumps too fast, according to the congressional Office of Technology Assessment.

The OTA, in an analysis of federal policy released yesterday, estimated that 10,000 or more dumps will end up on the high-priority list for the "Superfund" toxic waste cleanup program, at an eventual cleanup cost of more than $100 billion. The EPA has estimated that Superfund will be needed to deal with 2,000 sites, at a cost of up to $22 billion.

But in a conclusion that is certain to draw fire from both sides of the Superfund debate under way in Congress, the OTA also urged the agency to devote less attention to cleanups in favor of action to prevent the spread of dangerous contamination.

Joel Hirschhorn, who directed the analysis team, said the recommendation stemmed from evidence that EPA cleanups are exacerbating the problem, moving toxic wastes to new disposal areas and creating additional Superfund sites that will require federal action.

"Some sites get worse, and repeated costs are almost inevitable," the OTA said. "Environmentally, risks are often transferred from one community to another, and to future generations."

The report argues that the EPA should work to contain hazardous wastes, in above-ground storage vaults if necessary, until there are proven ways to destroy the waste.

"Very little focus has been placed on cleanup technology," Hirschhorn told reporters at a news briefing, "but there is a lot of evidence already that land disposal is not the way to go."

The report comes amid an escalating congressional debate over the $1.6 billion Superfund program, which has nearly exhausted its funds and is scheduled to expire in September.

The Reagan administration supports renewal at an expanded funding level of $5.3 billion over five years. The House and Senate are considering versions that would provide more money.

At the center of that debate, however, is the question of how quickly the EPA is moving to clean up hundreds of toxic dumps thought to be threatening drinking water supplies and the health of nearby residents.

Since 1983, when Congress made management of the Superfund the key issue in a controversy that swept most of the EPA's top officials out of office, the agency has accelerated its cleanups.

Only six of nearly 800 priority sites are considered "clean," but EPA officials say long-term cleanups are in progress at more than 300 sites, at least at the engineering stage. In most cases, the cleanup consists of removing waste and contaminated soil and trucking it to a licensed disposal site.

In recent months, some of those licensed sites have been found to be leaking, drawing more congressional complaints that the EPA is playing a "shell game" with hazardous waste.

The OTA also reported that toxic wastes were left in place at some of the "cleaned" sites and may still be contaminating water supplies.

"Permanent remedies, which destroy, detoxify or otherwise treat wastes will be necessary to any cost-effective, long-term Superfund program," the report said.

In response to the OTA report, the EPA said it is "confident that its approach to identifying, assessing and cleaning up hazardous-waste sites is the correct one."

But EPA Administrator Lee M. Thomas, who was in charge of Superfund programs before taking over as head of the agency earlier this year, has expressed concern in the past about creating new problems by shifting waste. Last year, the agency loosened its cleanup policy to permit consideration of more expensive options, such as incineration, in Superfund cleanups.

Community pressure also plays a part in the EPA's cleanup strategy, and agency officials say that neighbors of a dump are often more eager to see the waste trucked away than contained where it is.

"The last thing we want to do is move the problem from one site to another," said agency spokesman Russ Dawson. "But if you go for an on-site remedy, the community is not very happy."

The OTA report does not suggest a dollar level for Superfund. But it says the program will need to be "substantially larger" than it is now, and it takes sharp issue with the EPA's estimate of the number of toxic dumps that will need attention.

According to Hirschhorn, the EPA did not consider sanitary or municipal landfills in its estimate, despite evidence that many of the nation's 600,000 solid-waste landfills received toxic waste.

The OTA is assuming that at least 5,000 of those, less than 1 percent, will produce toxic contamination.

Similarly, the office estimates that about 1,000 dumps now licensed to take hazardous waste will become Superfund sites, a category that the EPA did not include in its estimate.

The OTA also expects about 2,000 more sites to be added to Superfund through "an improved site identification and selection process." According to Hirschhorn, the agency's scoring systems often exclude rural sites because such sites are thought to pose risk to few people.