Major revisions to the 1972 water act, approved by a congressional conference committee Thursday, will make it much easier for the government to control the dumping of dangerous chemicals into the nation's waters, an Enivornmental Protection Agency official said yesterday.

Stung by criticism that the act is being significantly weakened by Congress, EPA Assistant Administrator Thomas Jorling said environmentalists "don't give credit where it is due. The basic structure of the act is intact. We feel pretty good about it."

Environmentalists are criticizing the compromise bill as "a serious step backwards" because it would relax existing industrial pollution controls and exempt federal dams and other water projects from environmental standards.

Jorling praised the conferees, who passed the compromise bill after more than two years of debate, for instituting a provision allowing EPA to regulate "bestmanagement practices" on industrial sites. Under present law, the agency can only restrict what flows out of a plant's pipe into the river.

The new law would give EPA control, for example, over how chemicals are stored in order to prevent runoff. The agency could make a company draw up plans to prevent incidents such as the major spill of carbon tetrachloride, a potentially cancer-causing chemical, into the Ohio River last February.

"Industry fought tooth and nail" against the best management practices provision, Jorling said.

However, industry lobbyists were generally pleased with the bill, a fact which environmentalists attribute to major concessions having been made to them. Lobbyists from paper, timber, steel, oil and chemical companies called the bill "a reasonable compromise"because it allows them an exemption from cleaning up nontoxic pollutants.

According to Jurling, however, the bill is much stricter than present law in dealing with chemicals pollution, which causes cancer, nervous disorders, birth defects and other health hazards - a problem barely r

He said the bill would remove procedural burdens that prevented the agency from regulating more than six chemicals in the last five years. It also would eliminate any waiver for 129 dangerous chemicals which industiness would have control by 1984.

Blake Early of the lobby Environmental Action took issue with Jorling's optimistic assessment of the legislation. "There are possibly 600 toxic chemicals, but Congress has succeeded in focusing EPA on only 129 of them," Early said, deploring a provision in the bill which would allow waivers for possible toxic substances not on the list of 129.

He said the new law would leave it up to EPA whether to control newly discovered toxic chemicals, whereas present law requires the agency to regulate them. Too much control is thus left to the Office of Management and Budget, which is likely to restrict fudning got toxic regulation, he added.

Environmentalists were also keenly disappointed that the conferees exempted congressionally-authorized projects from getting permits to dredge or fill wetlands, the swampy areas around streams which protect fish and wildlife and naturally filter away pollution.

Jorling, however, denied that the exemption would allow ecologically-destructive dams, navigation and stream channel projects to be built as in the past. Although no permits will be required, the administration will carefully review each project to prevent damage, he said.

Jorling also praised the conferees for:

Extending from 12 to 200 miles offshore federal jurisdiction to recover the costs of oil spills from the responsible tanker owners - this despite opposition from the State and Defense departments.

Increasing federal funding for "land treatment systems" in which sewage is sprayed on golf courses and farms for fertilizer, rather than being treated in expensive plants which leave a residue of sludge.

Resisting House efforts to establish congressional veto power over EPA regulations and allow cities to recover sewage treatment costs with property taxes. The agency prefers charges based on water use to encourage conversation.

Jorling acknowledged, however, that he was disappointed that the bill does not include strong restrictions on the size of sewage plants and pipes, which would prevent encouragement of suburban growth.