Why was Rita M. Lavelle fired?

She thought she was doing a good job as manager of the "Superfund" at the Environmental Protection Agency.

Her boss often bragged about her work on Capitol Hill.

Lavelle seems typecast as a woman appointee of the Reagan administration. She's a savvy, plump, take-charge, 35-year-old blonde who has all the managerial lingo about "power curves," "interaction" and "cross-fertilization."

She's out of California, earned her ribbons in the Reagan campaign. She has a benign view of big business, derived from four years with Aerojet-General Corp. She has friends in the White House--presidential counselor Edwin Meese III held the Bible at her swearing-in.

She has the Republican slant on Washington as a place where people in agencies "always think you're trying to set policy or undermine your boss."

But something went wrong. At a terrible session on Feb. 4 with her boss, EPA Administrator Anne M. Gorsuch--Anne M. Burford since her weekend wedding--she was accused of ghastly acts. She was called "a liar," accused of writing a memo that baldly identified the business community as "the primary constituency of this administration," told she was "not a team player" and charged with "keeping book on other presidential appointees." Her appeals to the White house were not heeded.

She denied it all yesterday to the Senate Environment and Public Works Committee, before which she chose to make her first Capitol Hill appearance, although she has a sheaf of invitations from other curious committees.

Her 4 1/2 hours on the stand provided no solution to the mystery that has gripped Washington, especially since the shredders showed up at the EPA.

Her performance was a tribute to the coaching skills of her lawyer, the veteran James Bierbower. Her tactics were disarming--"I don't want to avoid the question," she would say before not answering one. Or, after an especially long, whisper-filled pause, she explained, "Well, the reason I was conferring with counsel . . . I didn't want to hurt anybody's character." She sighed over her lack of notes on disputed matters, and was most convincing until Sen. George J. Mitchell (D-Maine), her most patient and persistent questioner, elicited from her the fact that she had made no request for her papers.

The White House would have us think Mary McGrory DUMPED --that the Burford-Lavelle conflict was just a matter of bad chemistry between two strong-minded women, both of whom are devoted to cleaning up America's toxic waste dumps.

Surely nothing Lavelle told the commitee would have shocked anyone of consequence in either the EPA or the White House.

She lunched and dined profusely with industry representatives who had dumped the chemicals on which so many Americans are choking. They went to the best places: the Lion d'Or, Romeo & Juliet, the Jockey Club. Her calendar, the only document she was able to rescue from the carnage of her firing, shows a steady diet of parleys with big guns from big corporations. She was just being a "saleswoman for the negotiating approach." She says she was betimes traipsing off to dump sites, but her one meeting with environmentalists was held in the privacy of her office, where nobody in town could see what was going on.

Chairman Robert T. Stafford (R-Vt.), one of three New England senators who were not swallowing the "negotiating mode," said sternly that she had created "the impression your agency may be in bed with polluters."

Lavelle replied, "I was meeting with people, not with polluters."

There is, it turns out, even in EPA, a directive that forbids officials to meet with representatives of firms in trouble over their dumps. Lavelle said she never discussed litigation. She was simply selling negotiation, an option that the companies, with their million-dollar batteries of hotshot lawyers, apparently would know nothing about had she not been out peddling it.

Sen. John H. Chafee, the Yankee Republican from Rhode Island, pointed out that Congress had provided the EPA with a blunter instrument than persuasive lunches and dinners to wield over the likes of Dow and Monsanto. Under the law, the EPA can move in, clean up and then charge the polluters triple the costs.

But Lavelle, in the spirit of the administration, did not wish to harass the corporations in the "traditional adversarial mode" she found at work in the agency when she got there.

"The idea that industry is all bad and government all good was put to rest with Watergate," she declared firmly.

Sentiments like that might get her in trouble with environmentalists, but hardly with the Reagan administration.

No, it was something else. The mystery of why Rita Lavelle was dumped is still unsolved.