Three months into the job, William D. Ruckelshaus says he is finding the task of running the Environmental Protection Agency a lot tougher than it used to be.

"The difficulty is in getting a general political consensus of what we mean by cleaning up," Ruckelshaus said yesterday in an informal discussion with reporters. " . . . At some point the nation has to make a judgment about that: how clean is clean?"

The session was the first of what EPA officials say will be regular meetings between Ruckelshaus and the news media. He suggested yesterday that the press may be at the root of some of the agency's problems.

The EPA, he said, was "not in total control" of its workload, because the press covers environmental problems that might otherwise not rank high on the agency's list of priorities.

"There are those issues that events carry along," Ruckelshaus said, mentioning dioxin contamination as an issue that has commanded headlines and a significant chunk of the EPA's time and resources.

"We don't have any choice but to respond to it," he said. Asked if that meant he thought dioxin was not a problem, Ruckelshaus quickly responded: "No."

But he declined to name his priorities for an agency that, in the decade since he served as its first administrator, has increasingly turned its focus away from the conventional pollutants of air and water and toward ever more complex issues of public health and hazardous substances.

Ruckelshaus suggested that the public, not the EPA, will have to set those priorities and help make those complex decisions.

Along those lines, he defended his controversial decision to involve the citizens of Tacoma, Wash., in setting rules for arsenic emissions at a copper smelter. The EPA announced in July that it would give residents of Tacoma and nearby Vashon Island statistical information on their likelihood of getting lung cancer from a proposed level of arsenic emissions, and would then ask them if the risk was acceptable.

The smelter contends that it cannot reduce the emissions further, so the residents are being asked, in effect, to choose between the smelter's 550 jobs and the risk to their own health.

The Tacoma experiment has drawn sharp criticism from public health officials and environmentalists, who contend that it amounts to an abdication of the EPA's responsibility under the Clean Air Act to protect the public "with an ample margin of safety."

Ruckelshaus admitted to "reservations" about efforts to involve the public in regulatory decisions, but he said it was important for them to "begin to perceive the true nature of these choices."

And at the least, he said, "I think we're going to get a sharper definition of the problem."

On other issues, Ruckelshaus:

* Said he has "a lot of sympathy" for industry and city officials who have been unsuccessfully seeking waivers from the Clean Water Act's requirements that toxics be removed from industrial discharges before they are passed on to municipal waste treatment facilities.

Ruckelshaus said he counseled the Senate against the waivers because he didn't believe the agency could design a system that would be acceptable to Congress. "It put me in the position of pleading for time with a Congress that has run out of patience," he said.

* Called a proposed national ground water strategy a "motherhood kind of statement" of questionable value in protecting the nation's underground aquifers. Ruckelshaus' predecessor, Anne M. Burford, was unable to persuade the Cabinet council headed by Interior Secretary James G. Watt to approve the strategy. Ruckelshaus' comment suggested that he won't try again, although he said a task force was reviewing the proposed policy.

* Said he was undecided whether he will campaign for President Reagan, should Reagan decide to run again. "It would depend on whether I thought it would be helpful," he said.

Asked if he might play the role of adviser--and perhaps prevent Reagan from repeating his now-famous statement about trees being at fault in air pollution--the former Weyerhaeuser lumber company executive smiled.

"I thought that was an attack on Weyerhaeuser," he said.