When William D. Ruckelshaus takes the helm of the Environmental Protection Agency he may have to excuse himself from a number of regulatory decisions if he is to avoid the kind of conflict-of-interest questions that have tarnished the agency's image in recent months.

As a senior vice president overseeing the legal affairs of Weyerhaeuser Co., Ruckelshaus must decide whether to lift EPA restrictions on herbicides containing dioxin, with which the company has sprayed forests in northwestern and southern states.

He also must decide whether to participate in reviewing water pollution regulations that Weyerhaeuser and the rest of the timber industry had challenged in a lawsuit.

Ruckelshaus, who was the EPA's first administrator, has a reputation for integrity. He has indicated that he plans to disqualify himself from some issues, but the extent of his potential conflicts remains unknown.

Ruckelshaus may be especially sensitive to the question because it has come up once before in his career. One reason Ruckelshaus left town in 1976, a former law partner said, is that he was tired of being criticized for representing vinyl chloride manufacturers and other corporate clients before his former agency.

"Let's say he has the normal complement of potential conflicts, such as pending litigation and pending permits," a key Senate aide said. "I presume he'll say he's not going to handle that. But what if it also has a broad policy implication, such as water pollution regulations?

"Can he disqualify himself from all these matters--and if he does, can he really serve effectively as administrator?"

Weyerhaeuser recently asked the EPA to lift restrictions on the use of 2,4,5-T, a herbicide that contains the most potent form of dioxin. The company wants to resume using 2,4,5-T to spray thousands of acres of forests, as it did until such uses were banned in 1979.

In the meantime, Weyerhaeuser has been spraying forests in Oregon and Washington with another weedkiller, 2,4-D, which has not been restricted by the EPA. Some studies have shown that both herbicides cause cancer and birth defects in animals directly exposed.

As the vice president overseeing Weyerhaeuser's legal division, Ruckelshaus has been involved in various lawsuits involving the EPA. In 1979 the EPA fined the company's Longview, Wash., paper mill nearly $750,000 for water pollution violations. The firm later settled the case out of court for $135,000.

Weyerhaeuser, which had $4.2 billion in sales last year, also has subsidiaries in construction, manufacturing, combustion power, shipping, railroads, mortgage banking and real estate development.

Sources on the Senate Environment and Public Works Committee, which must approve Ruckelshaus' nomination, say that the members also are expected to question whether Ruckelshaus should disqualify himself from matters involving several other companies on whose boards of directors he serves.

While there is no legal requirement to do so, some EPA attorneys said Ruckelshaus may be advised to steer clear of these firms to avoid even the appearance of favoritism, and some congressional committees have asked nominees to make similar pledges in the past.

Corporate records also show that Ruckelshaus is a director of:

* The American Paper Institute, the timber industry's trade association, which sued the EPA in 1979 to block proposed restrictions on discharging wastes into waterways. Weyerhaeuser was a party to that suit, which EPA settled last fall by agreeing to revised water pollution rules that would reduce the industry's cost of compliance.

* Peabody International Corp., which manufactures equipment for the processing and handling of solid and toxic waste. A Peabody division that was sold last fall has received more than $3 million in EPA contracts since 1981 to clean up hazardous waste sites in four states, and the firm is conducting a $200,000 study jointly with EPA on sulfur dioxide emissions.

* Pacific Gas Transmission Co., which transports natural gas. The company recently bought a chemical manufacturing division and is owned by a utility company, Pacific Gas and Electric Co.

* Cummins Engine Co., which produces diesel engines and components for trucks, buses and industrial generators.

Ruckelshaus, 50, has been quick to recognize the potential pitfalls.

"In order to pass all these conflict-of-interest laws, I'm going to have to erase the conflicts that exist," he told a gathering of EPA employes last week.

"I will have to insulate myself against any involvement in EPA, of course, that has anything to do with Weyerhaeuser, and I need your help in doing that. If you identify any area while I'm here where there was any involvement by Weyerhaeuser, I must recuse myself from any involvement."

After heading the EPA from 1970 to 1973 and later resigning as assistant attorney general rather than fire Watergate prosecutor Archibald Cox, Ruckelshaus and two former aides at the EPA founded a Washington law firm that specialized in environmental law.

By 1976, there were press accounts that Ruckelshaus and his partners had made dozens of contacts with EPA officials on behalf of private clients, who ranged from companies producing polyvinyl chloride, a carcinogenic, or cancer-causing, plastic, to a construction firm resisting an EPA audit of its sewage treatment plants.

The 1978 Ethics in Government Act placed new restrictions on a former official's ability to represent clients before his old agency.

Gary H. Baise, Ruckelshaus' chief of staff at the EPA and a partner with the law firm, said that while Ruckelshaus occasionally made phone calls on behalf of clients, he paid little attention to actual cases and spent much of his time lecturing about Watergate.

"He was a guy whose name attracted business," Baise said. "He didn't want to engage in the traditional Washington lobbying. He found it distasteful.

"This whole revolving-door thing was beginning to build. It was a hot issue because of Ruckelshaus and the position he had taken during Watergate. That was something that really bothered him . . . . He finally decided he just couldn't be a lawyer in Washington anymore."

In late 1976, Ruckelshaus moved to Tacoma, Wash., to take a job with Weyerhaeuser. The Senate committee plans to examine Weyerhaeuser's environmental record and possible changes in Ruckelshaus' views on the subject.

At a 1981 Senate hearing, for example, Ruckelshaus said that Weyerhaeuser had been forced to spend $6 million at its Longview plant for pollution equipment that did little to improve air quality. He said that some parts of the Clean Air Act are "unrealistic."

According to EPA figures compiled by Environmental Action--which dubbed Weyerhaeuser one of the nation's "Filthy Five"--the company was cited for 277 air and water pollution violations between 1977 and 1982 and was assessed fines totaling $855,000.

One of the most difficult matters awaiting Ruckelshaus is the EPA's negotiations with Dow Chemical Co., which wants to resume selling 2,4,5-T with more restrictive labeling for forestry use.

When former EPA official Barbara Blum barred the herbicide for such use in 1979, she said there was "remarkable correlation" between the spraying season in Oregon and an unusually high rate of miscarriages there.

In a letter to the EPA last October, Weyerhaeuser complained about the "seeming lack of progress" on 2,4,5-T, saying: "We had hoped that the chemical would be available for the 1983 spraying season." A Weyerhaeuser spokesman said the firm has funded studies on both 2,4,5-T and 2,4-D and has not found any evidence of adverse health effects.

"I'll stack our record up against anybody in a comparable business," said company official Dennis Higman. "In the pulp and paper business, with boilers going 24 hours a day, there are thousands of opportunities for violations. But we have spent $70 million a year in the past decade on pollution control and environmental protection."

Ruckelshaus largely has "stayed in the background" during the company's disputes with environmentalists, said Ruth Weiner, a Sierra Club member in Seattle. The subjects of dispute include plans to build a log-exporting facility near a national wildlife refuge in Puget Sound and repeated efforts to weaken state clean air standards.

"Here's a place where Ruckelshaus could be out front, but he's been very, very low-key," Weiner said. "He has not spoken out publicly on any of the environmental controversies that Weyerhaeuser has been involved in."