About 70 percent of commercial toxic waste landfills are known or believed to be contaminating ground water, posing cleanup costs rivaling the $8.5 billion Superfund effort for abandoned dumps, officials said.

Despite the massive problem, the Environmental Protection Agency has not only failed to adequately enforce ground water monitoring requirements but issued operating permits to landfills with serious deficiencies, congressional investigators told Congress on Thursday.In addition, lawmakers charged that top EPA officials sought to repress critical comments about the agency's ground water program by an internal EPA task force that studied conditions at 24 landfills.

At a House hearing, Rep. John Dingell, D-Mich., released internal Environmental Protection Agency documents that estimated the cost of cleaning up contamination from landfills could go as high as $10 billion.

"Corrective action measures may have to be initiated at 60-70 percent of these facilities," said a "draft corrective action outyear strategy" document authored by EPA regional and headquarters officials.

"Whatever the estimate, it is clear that the (landfill) corrective action program could potentially equal or exceed the magnitude of the Superfund program."

Dingell, chairman of the House Energy and Commerce Committee's subcommittee on oversight and investigation, said EPA officials had told his panel's investigators that 70 to 80 percent of the 1,447 land disposal facilities in the nation are known or suspected of leaking toxic waste into ground water.

"Yet, only a small percentage of such facilities have assessed the rate, magnitude and extent of contamination, and very few have initiated cleanup action," Dingell said.

The hearing focused on commercial toxic waste landfills regulated under the federal Resource Conservation and Recovery Act.

EPA officials say that while federal and state governments have hundreds of millions of dollars to spend on cleaning up Superfund sites, only about $30 million a year is budgeted for addressing contamination at commercial landfills.