While finding that petroleum wastes contaminate drinking-water supplies and livestock and otherwise damage the environment, the Environmental Protection Agency said yesterday that oil and gas producers should nonetheless maintain their exemption from strict regulations governing the disposal of hazardous substances.

In a report to Congress, the agency said the regulations would be difficult to enforce and would pose a "substantial financial burden" on the petroleum industry, potentially costing consumers $6 billion per year.

The agency concluded that while states have often failed to assure safe disposal of petroleum wastes, they have sufficient regulatory tools to minimize the dangers of the 400 billion barrels of wastes annually generated by exploration and production.

Environmentalists criticized the agency for perpetuating special treatment for the petroleum industry and failing to fill a regulatory breach.

"By EPA's own admission, failure of the states to enforce existing regulations is the singlemost cause of health and environmental damage," said Lisa Steer of the Natural Resources Defense Council. "So, what are they proposing to do? Leave it to the states.

"Why should the petroleum industry be allowed to dodge control on hazardous wastes that other industries are spending billions of dollars to comply with?" she asked.

Numerous carcinogens and neurotoxins contaminate the drilling muds, brine and fluids associated with gas and oil production. But Congress, under intense lobbying by the petroleum industry, temporarily exempted those substances in 1980 from the EPA regulations governing the disposal, storage and handling of hazardous wastes.

Congress directed the agency to set a regulatory strategy for the "special wastes," produced in larger volume in generally less toxic concentrations than the "hazardous" chemicals and heavy metals that have qualified for strict regulation since 1976.

Unlike regulated refuse that must be pretreated before burial in special landfills or deposited in ponds lined by two layers of clay or synthetics, petroleum wastes are managed in ways that the EPA said in its report to Congress "have been associated to some degree with endangerment of human health and damage to the environment."

The report described the negative effects of dumping petroleum wastes into unlined pits, noting that the practice has contaminated the ground water of "many areas," threatening the drinking and irrigation water supplies with toxic substances such as cadmium, arsenic and lead.

Brine and fluids used to lubricate drill bits have been discharged into bays and the Gulf of Mexico, harming aquatic life, according to the report. "Of greater concern is the potential for heavy metals, taken up the food chain, possibly endangering human health through the consumption of contaminated fish and shellfish," said the EPA.

Releases of oily wastes into intermittent streams and unlined ponds in California has resulted in wildlife deaths, said the report, while the human carcinogen benzene, dumped into unprotected pits in other western states, has migrated into ground water. The discharges were permitted by the states.

Among the damages cited in the report are contamination of drinking water supplies and crops "above levels considered safe for consumption, chemical contamination of livestock, reduction of property values, damage to native vegetation."

Nevertheless, the agency ruled out widespread dangers of the contamination, maintaining that of the hundreds of chemical substances contained in petroleum wastes, "only a few . . . appear to be of concern to human health and the environment."

In its recommendations to Congress, which ordered the study to determine the need for federal controls, the EPA estimated that strictly regulating petroleum wastes as "hazardous" substances could reduce domestic oil production by up to 18 percent by the year 2000, which would lead to increased imports and trade deficits. Oil prices would rise and cost consumers up to $6.4 billion per year.

Acknowledging that states have been lax in enforcing their own environmental laws and that violations are common, the agency concluded that states are already armed with "adequate" regulations to protect ground and surface waters. Most states, for example, require liners for disposal pits.

Concluding that strictly regulating petroleum wastes is "neither desirable nor feasible," the agency suggested that it work with states to encourage enforcement of their regulations.