After three days of hearings in which William D. Ruckelshaus was quizzed on everything from toxic soup to environmental nuts, the Senate Environment and Public Works Committee had only one question left.

As Sen. David F. Durenberger (R-Minn.) framed it: "Why would Bill Ruckelshaus risk his good name, to say nothing of his economic status, to come back to work with this EPA, in this Congress?"

Ruckelshaus, who will give up a $300,000-a-year business career to take charge of the Environmental Protection Agency, a $69,800-a-year job where the headaches come at high-volume discount rates, responded: "I think EPA's crippled. I think it's in trouble. And if I didn't believe that, I wouldn't be here."

The EPA, low on staff, low on funds and low on morale, has been under attack almost since the day the administration took office. More than a dozen high-ranking officials have resigned or been fired in recent months, and the agency now rests in the hands of temporary caretakers, waiting for an infusion of new confidence, new enthusiasm and new direction.

But the situation on Capitol Hill isn't been much better. Sen. George J. Mitchell (D-Maine), who told Ruckelshaus that he was in an enviable position because "the president needs you a lot more than you need him," might well have added that the same holds true for Congress.

In the last two years, with environmental goals running afoul of economic recovery efforts, the bipartisan majorities that enacted the nation's major environmental laws in the 1970s have largely disappeared.

Eight of the 10 environmental laws that the EPA administers have expired, in a technical sense, while Congress foundered in attempts to reauthorize them. A ninth, the Toxic Substances Control Act, is scheduled to expire this September.

Only the "Superfund" hazardous-waste cleanup law can wait--until 1985. But some members of Congress say that law needs immediate attention as well, because nearly everyone agrees that the $1.6 billion fund will not be sufficient to deal with even half of the 419 dump sites on the EPA's high-priority list.

While some critics of the Reagan administration are watching the EPA's budget and its enforcement tally as key measures of Ruckelshaus' impact on environmental policy, others say the real test will be whether he is able to create enough consensus between the administration and Congress to get environmental legislation moving again.

While the chances are looking better for action on some of the laws, notably the Clean Water Act and the the Resource Conservation and Recovery Act, the hazardous-waste disposal law, others remain hostage to an increasingly bitter fight over how much industry should be expected to pay to curb pollution.

Sen. Lloyd Bentsen (D-Tex.) pointedly noted last week that the last time Ruckelshaus appeared before the Environment Committee, in April, 1981, "We were talking about the Clean Air Act. We still are. Almost every compromise is opposed by some interest group."

The act technically expired several months after that hearing. But despite 60 days of hearings and markups on the Senate side, and endless hours of closed-door negotiating in the House, neither side is envisioning success this year.

"I am pessimistic," said a Senate aide. "What's the point of pushing a bill that the House won't even look at? Nobody has the appetite for it."

There are also indications that environmentalists and their allies, whose legislative victories in the past two years have consisted largely of maintaining the status quo, will try to convert the public outcry over the EPA's highly publicized problems into congressional support for tougher environmental laws.

The National Audubon Society has backed off its endorsement of the Senate's compromise Clean Air Act, for example, saying that "increased public awareness of the clean air issue" and more Democratic votes in the House "now bring a stronger bill within reach."

Industry officials, who are anticipating "negative offshoots" from the EPA furor, are girding up for a renewed battle that will probably find them on the defensive.

"We're going to be tough. We're going to be smart," said William M. Stover, chief lobbyist for the Chemical Manufacturers Association. "We'll use our lawyers to make the case the best way we can, just as other parties on other sides do."

Into this arena, where the rhetoric occasionally rivals a hazardous waste dump for poison, comes Ruckelshaus, who coasted through his confirmation hearings on his reputation for integrity and what senators hope will be a knack for achieving compromises within the administration.

Referring to Ruckelshaus' stint as the EPA's first administrator from 1970 to 1973, Sen. John H. Chafee (R-R.I.) warned him: "The task you face today, in restoring credibility to the agency you founded, will be even more difficult."

Even so, most observers are putting their bets on Ruckelshaus, who has already begun to put together a staff that will include many familiar faces from his previous tenure at the EPA.

Alvin Alm, staff director of the White House Council on Environmental Quality when Ruckelshaus was last at the EPA, is expected to be his deputy. James Barnes, a former Ruckelshaus aide who is now at the Agriculture Department, will probably become his general counsel.

Former aide Phillip Angell is already in place at the agency as a special assistant, as is Fitzhugh Green, an EPA veteran who will take over the international affairs office.

"He's a folk hero in this town," said the chemical industry's Stover. "He has a bulletproof vest in dealing with the Congress."

But some environmental groups, who neither endorsed nor opposed the nomination, are not quite as euphoric.

"It is a mistake to think that one person can be some sort of Marshal Dillon who can come in and take care of everything," said David Hawkins of the Natural Resources Defense Council, who served at the EPA in the Carter administration. "He is capable of making mistakes and bad decisions. He is a mortal like the rest of us."