Hundreds, perhaps thousands, of chemical dumps across America contain toxic and disease-causing wastes that could invade much of the nation's drinking water supply over the next 10 to 20 years, a leading government scientist testified yesterday.

These wastes, often leaking from corroded or inadequate containers into ground water, represent a "time bomb" that could carry the dangers of the Love Canal and other such dumps far beyond their immediate surroundings, according to Dr. David Rall, director of the National Institute of Environmental Health Sciences.

The testimony came at a Senate subcommittee hearing devoted mostly to hearing from people who live near chemical dumps tell of their pain and frustration in trying to get government assistance.

Environmental officials have studied fewer than 1,000 dump sites, but fear that there may be as many as 30,000 sites capable of causing "significant" health problems.

James G. McCarthy, a resident of a New Jersey neighborhood near a dump site, cried into his hands as he said: "I am 33 years old. I don't care what happens to me in the future. I have two children. Are they going to live? Somebody's got to help us."

His words were typical of what victim after victim of health problems from upstate New York's Love Canal and at four other sites -- in New York, New Jersey, Massachusetts and Tennessee -- told a joint Judiciary Committee hearing chaired by Sen. Edward M. Kennedy (D-Mass.).

Kennedy and other senators, as well as Rep. John LaFalce (D-N.Y.), of the Niagara Falls-Love Canal area, flayed the Carter administration for what they called "inaction," "delay," confusion" and "inexplicable failure" to include adequate funds to help victims in President Carter's current request for a $1.6 billion, four-year "superfund" to seek and clean up chemical wastes.

But Rall's testimony added a new dimension. He said he fears "very serious problems" as some of "the 300 billion pounds of chemicals we dispose of each year" begin to "migrate into the ground water.

"Forty percent of the American people get their drinking water from the ground water supply," Rall explained.

The water in aquafers (water-bearing layers of material) moves very slowly, typically only a tenth of a mile to a mile a year, he said, but "75 percent of the waste dumps are either over (such) aquifers or over wet areas."

This means, Rall said, that there is a considerable chance of "contaminating the water we drink," in many major cities and towns "10 or 20 miles or farther" from toxic waste sites.

So far, he said, the Environmental Protection Agency has not "put together the ground water maps and the toxic dump maps" so "we can begin to identify" the areas most vulnerable to water contamination.

Rall, whose environmental research institute is part of the Department of Health and Human Services, not EPA, called this identification "a critical next step."

But it is just one of many critical steps, it became clear, as a parade of witnesses -- citizens and officials -- told how they have barely begun to cope with the growing problem of toxic wastes, as well as toxic chemicals generally.

"There are around 10,000 chemicals we are all exposed to," Rall reported. "We have tested 2,000 to 3,000. Some 500 more being introduced each year, and we have the capacity to test only 250 to keep up."

Pressed hard by Kennedy, Assistant Secretary of Defense George Mariental confessed that the Defense Department still does not have a running inventory of all the 250,000 tons of chemical wastes it disposes of or stores each year, though 40 percent becomes landfill.

EPA officials said the "superfund" President Carter seeks will primarily find and contain wastes, not help victims or care for their illnesses ranging from cancer to kidney destruction to miscarriages to birth defects.

"This is a very unpleasant tradeoff" to have to make, said Swep Davis, an EPA associate administrator. But the cleanup alone will cost billions, he told Kennedy, and "I think you'll agree" that preventing future illnesses is the most important priority.