James W. Sanderson, who represented a large landfill operator while working as a high-level adviser at the Environmental Protection Agency, participated last year in several meetings on landfill regulations that clearly affected his client, several EPA officials said yesterday.

Sanderson, a Denver attorney, has repeatedly denied that he participated in any EPA decisions that affected his client, Chemical Waste Management Inc., a subsidiary of the nation's largest waste disposal firm.

Sanderson's attorney modified that statement yesterday, however, saying it was proper for Sanderson to discuss broad EPA regulations that affected a number of waste-disposal or landfill companies.

He said Sanderson did not get involved in any specific matter involving Chemical Waste Management while working as a paid, part-time adviser to EPA Administrator Anne M. Burford from March, 1981, to June, 1982.

Three current and former EPA officials said yesterday that Sanderson participated in at least two agency meetings in early 1982 at which officials discussed proposed regulations for waste disposal at landfills, including several operated by Sanderson's client.

Rep. James J. Florio (D-N.J.) said yesterday that the latest disclosures "could be the smoking gun" of the EPA controversy and should cause the White House to fire Burford.

Florio said this was an obvious case of "an outside special interest helping to make policy" at EPA.

Burford, by participating in the meetings with Sanderson, had demonstrated "a degree of insensitivity as to the propriety of relations between regulators and the regulated community," Florio charged.

The generally stringent landfill regulations, which EPA adopted last July, do not require landfill operators to install protective liners and take other steps to prevent groundwater contamination until the agency issues them final operating permits, which could take years.

Critics said the delay may benefit firms such as Chemical Waste Management, which operates the large Lowry Landfill in Denver and others across the country.

In recent days, The Washington Post has reported that during and after Sanderson's service at the EPA, Chemical Waste Management benefited from several agency decisions.

In one case, after Sanderson talked to Burford and other senior EPA officials last month, the agency reversed a staff decision and speeded up consideration of a permit for Chemical Waste Management to burn hazardous wastes at sea.

Sanderson remains under Justice Department investigation for allegations that while working at EPA last year, he improperly influenced a water quality standard affecting another of his clients, the Denver Water Board.

"Obviously, anyone who comes from the private sector is going to be involved in general regulatory matters that are going to affect their clients," Sanderson's lawyer, Paul Cooper, said yesterday.

Cooper said Sanderson, on advice from EPA lawyers, "walled himself off" from particular matters affecting his clients but participated in discussions of more general agency rules. Acknowledging that some of these EPA rules would affect such clients as Chemical Waste Management, Cooper added: "You have to rely on the good faith of people that they're not coming into government to benefit their client companies."

Former EPA solid waste director Gary Dietrich said Sanderson was "an active participant" in two meetings on landfill regulations and that this may have been "to the benefit of one of his clients, Chemical Waste Management."

Dietrich added that on another occasion Sanderson snapped at him for suggesting that Burford announce tougher hazardous waste regulations in order to improve her image.

Former EPA hazardous waste chief Rita M. Lavelle also told a Senate committee yesterday that Sanderson "was definitely in some of the meetings" at which agency officials discussed lifting a ban on disposing of liquids in landfills, a decision that later enabled Chemical Waste Management to dispose of hazardous liquids at several of its landfills.

She said Sanderson "was rather indiscreet in continuing to represent clients while he was soliciting a nomination at the agency" for its No. 3 job.

EPA spokesman Clay Jones said Sanderson "played a very small part, if any, in the development of the landfill regulations."

Jones said Sanderson excused himself from some meetings and that "we have no evidence and cannot remember an incident where he was present at a meeting that would have directly affected one of his clients."