William D. Ruckelshaus was sworn in as administrator of the Environmental Protection Agency yesterday as President Reagan heralded an end to scandal within the EPA and the start of a "new chapter in the history of this agency."

"Too much time has already been wasted in fault-finding, recrimination and innuendo," said Reagan after Ruckelshaus was sworn in by Supreme Court Justice William Hubbs Rehnquist. "Bill, I'm counting on you, in your daily performance of your duty, to reaffirm this administration's firm commitment to a sound and safe environment--an EPA that is trusted and respected by all."

The president set forth four "areas of immediate concern" for Ruckelshaus to deal with as he takes control of the EPA.

Reagan directed Ruckelshaus to improve management of the $1.6 billion Superfund for cleaning up the most hazardous dump sites in the nation. Allegations of political use of the Superfund dogged Ruckelshaus' predecessor, Anne M. Burford, until she resigned in the face of several congressional investigations.

"Let's pledge that no American will be held hostage or exposed to danger because of bureaucratic snafus or legal disputes over responsibility," the president said.

Reagan also asked Ruckelshaus, 50, who is resigning as a senior vice president of the Weyerhaeuser Co., a major forest products company, to focus his efforts on three other environmental concerns: acid rain, "vigorously" enforcing environmental laws and separating the proper roles of the federal and local governments in handling environmental laws.

The president asked him to develop a strategy for dealing with acid rain, saying people in both the United States and Canada "must understand that we are doing what's right and fair in this area."

Ruckelshaus served as the first administrator of the EPA when it was formed by President Nixon. After three years, he left to become a deputy attorney general, where he cemented a "Mr. Clean" reputation by refusing Nixon's orders to fire Watergate special prosecutor Archibald Cox. He lost his own job as a result in the "Saturday Night Massacre."

"I cannot imagine anyone who is more qualified or better suited to be at the helm once again than 'Mr. Clean' Bill Ruckelshaus," said Reagan.

Ruckelshaus pledged that as he pursues "the public interest which is often so elusive at EPA . . . , I pledge to you and the American people that I will never break your trust."