Last spring, a mining company consultant handed Senate aides what appeared to be a straightforward proposal to reform the selection process for mining sites in the Superfund toxic-waste cleanup program.

The aides, convinced by the consultant that if this proposal failed, a version more generous to mining interests would pass, redrafted the measure and with little fanfare tacked it onto legislation to extend the Superfund program. The bill and amendment passed the Senate in September.

Today, the amendment is one of the most controversial sections of the Superfund bill. The Environmental Protection Agency contends that it would all but free mining, oil and utility companies from the sizable financial obligations of cleaning up their toxic debris, including such serious public health threats as arsenic, cyanide, asbestos, radium, cadmium, lead, barium and benzene.

EPA officials say the amendment would give those industries treatment not afforded other corporate polluters regulated by Superfund and liable for waste-site cleanups that average $10 million each.

The story of Amendment 639 illustrates the hidden intricacies of legislation and the influence of interest groups in creating the laws by which they must live.

"Certain special interests want to be above the law," Hugh Kaufman, assistant to the director of EPA's Hazardous Site Control division, said. "The way to do that is to get some senators to write exemptions into the law that will cover them."

Superfund legislation has been stalled since late last year when House-Senate conferees failed to resolve their differences on numerous issues, including the amendment which was sponsored chiefly by Sen. Max Baucus (D-Mont.) and supported by colleagues from other western mining and oil states.

Staff members on the Senate Environment and Public Works Committee who wrote the amendment in meetings last year with the mining company consultant, Jeffrey Schwartz, and oil and utility lobbyists say they underestimated its implications.

On Thursday, Senate conferees, under pressure from EPA, environmentalists and several governors, removed some of the most controversial portions of the measure and offered a new Superfund package to their House counterparts.

A senior EPA official called this a "movement in the right direction" but said "there are still a significant number of problems."

At issue is Superfund's jurisdiction over a special category of wastes left behind by mining, oil and gas production and the combustion of coal. Congress, hoping to foster development of energy resources during the fuel crisis of the 1970s, required EPA to conduct studies on such industrial debris before designating any of it as hazardous waste subject to strict regulations on treatment, storage and disposal.

Superfund, which began in 1980, has ignored the status of these "special study wastes" in selecting sites for cleanup and forcing corporate polluters to foot the bill.

The National Priority List (NPL), which ranks the most serious sites for possible cleanup under Superfund, contains dozens of locations contaminated by mining, utility or oil production wastes, including some of the nation's most severely polluted places, EPA officials said.

Baucus' amendment evolved from concerns of the mining industry that the NPL's scoring system is biased against mining wastes. Mine operators argue that their operations generate massive amounts of largely nontoxic and nonvolatile waste and that EPA overstates the risk by focusing on quantity rather than concentration of the waste.

EPA responds that its scoring system amounts to a rough and inexpensive estimate of places deserving more detailed examination and that the NPL is merely a catalog of possible sites for cleanup.

The Baucus measure, approved by Senate conferees Thursday, would raise the NPL threshold for "special study wastes" until EPA revises its scoring system. EPA would be prohibited from listing special waste sites unless it gathered detailed findings on the quantity, toxicity and concentration of hazardous substances there.

Moreover, EPA would be required to evaluate the extent of risk that hazardous properties posed to the environment and public health.

The amendment also would require EPA to come up with such findings before it could order a producer of mining, utility and oil wastes to clean up its mess, and before the agency could recover cleanup costs from the polluter.

No special treatment would be required of EPA for sites where special wastes are present in "significant quantity" or in emergencies.

EPA officials say amendment requirements would make listing special waste sites prohibitive, costing hundreds of thousands of dollars and months of work even though the statistical likelihood of including a site in the Superfund program is low -- one of every 10 sites tested ends up on the priority list.

EPA's Kaufman said the measure would place "bureaucratic hoops" around those sites and that EPA would find it "too time-consuming, too expensive" to jump through them.

Baucus replied that EPA's concerns represent "a bureaucratic response, to be reluctant to do anything different."

He said his measure was not motivated by corporate constituents, but grew out of his "philosophical" concerns that a scoring system that "works well for chemical-dump states" may not work fairly and objectively in mining states.

Congressional aides say Baucus headed off possible moves by Sens. Lloyd Bentsen (D-Tex.) and Alan K. Simpson (R-Wyo.) to exempt "special study wastes" from Superfund control. The two senators ended up supporting Baucus' measure.

Baucus opposed a blanket exemption for such wastes, and his aides directed the mining industry to come up with a more moderate approach.

A rough draft was proposed to the staff of the Environment and Public Works Committee last spring by Schwartz, an environmental consultant with Asarco Inc., which mines copper, lead, zinc and silver in several western states and overseas. Earlier, Schwartz had been counsel to the House Energy and Commerce Committee and the EPA in the Nixon administration.

"Industry representatives generally show up with something less than statutory language, and you begin to discuss it," a committee staff member said. "Jeff supplied a series of drafts. He was part of the discussions right up to the very end."

Schwartz had what one aide called a "gorilla in the closet" to bolster his case. He warned that unless his compromise approach was approved, conservative senators would rally behind a measure more favorable to industry, according to three aides to senators who favor strong environmental protections.

"He said if we didn't buy what Baucus had to offer, then we were going to face Simpson and others," recalled one staff member. "There's a huge Senate constituency out there for the special interests."

Schwartz declined to discuss his role in the creation of Amendment 639. He said, however, that "there were no threats I'm aware of."

At its birth, the amendment seemed like a small wrinkle in the complex Superfund bill, aides said. It was only after protests by EPA, environmentalists and a number of governors that committee staff members began to see the measure's potential impact.