Environmental groups yesterday denounced the Environmental Protection Agency for proposing legislation they said would seriously impair enforcement of the federal clean-water act.

They charged that EPA is seeking changes under which industry could stretch out for years the corrective steps now required to limit discharge of pollutants, including toxic chemicals, into waterways and public sewage treatment plants.

A statement by seven national groups called it "a cynical attempt to undermine one of the country's best environmental laws" and compared it to the efforts to trim back enforcement of the Clean Air Act.

The criticism was the first round in what may be a protracted congressional struggle this year over amendments to the 1972 water law. Private industry is seeking relief from pending cleanup deadlines it says are impossible to meet.

The issues have not been debated extensively in Congress for several years, although they have been the subject of lengthy court suits. Congressional sources predicted yesterday that the Senate is not likely to approve major changes in the current law. The outlook in the House is not clear, they said. Hearings will begin late in May.

The environmental groups' criticism was directed against several amendments now being circulated within the administration but not officially announced.

They said the administration is seeking to put off industry's compliance deadlines and in one case is abandoning cleanup requirements for industrial waste that is pumped into municipal sewage plants.

J. Taylor Banks, attorney for the Natural Resources Defense Council, charged that the effects of the changes would permit "millions of pounds of toxic chemicals to be released every year the amendments are in effect."

Disputing that charge, EPA deputy administrator John Hernandez described the proposed amendments as "minor" changes intended to give the agency "some flexibility" in writing regulations.

A major argument turns on extending industry's deadline for installing what is known as the "best available technology" to prevent the discharge of toxic chemicals into the nation's waterways. A summary of the EPA amendments obtained by the groups indicates the deadline would be extended from 1984 to 1988.

EPA has not yet promulgated the guidelines that will govern the technological installations and is asking Congress to postpone from this year until 1984 the final deadline for issuing all the guidelines.

The combination of the new deadlines would mean that in some instances an industry would have from four to six years to comply with technology requirements, twice the amount of time needed, the environmental groups charged.

Hernandez said that because EPA has failed to put any technology regulations into law yet "it is not possible for industry to meet guidelines by the 1984 date. Industry does not know what it has to do. It is just common sense to extend the date."

The other main issue concerns the discharge into municipal sewage systems of toxic substances from about 60,000 industrial sources, many of them metal-working plants.

The environmental groups contend these discharges damage operations of the sewage treatment plants and leave metal residues in the treated sludge.

Amendments passed in 1977 required industry to install new technology to prevent many of the toxic materials from being discharged into municipal plants.

The EPA's proposed amendments would make some of the pre-treatment standards for those substances effective by 1984 but would leave others up to the agency's discretion instead of being mandatory as under present law. That would leave the task of enforcing some standards up to local authorities who are not equipped to handle them, the environmental groups charged.