Near Ron Irwin's vacation cabin in Ontario, Canada, there are crystal lakes that lie beautiful but dead in the summer sun, their water too acid to support fish or even much bacterial life. At least 140 lakes are like that, Irwin said today, and thousands more are threatened.

Irwin, a member of the Canadian Parliament, blames acid rain born in the smokestacks of American industry. "You feel so helpless when you realize these pollutants are coming from hundreds or thousands of miles away," he said.

Irwin sat with Sen. George J. Mitchell (D-Maine) during the Senate Environmental and Public Works Committee's first field hearing on one of the thorniest problems in the upcoming rewriting of the Clean Air Act: what to do about acid rain.

The Canadian government calls acid rain the most serious problems of its relations with the United States, and Irwin, chairman of a parliamentary committee on acid rain, is disappointed in the Reagan administration's response so far.

"I get no sense that they have assigned it any priority," he said after today's hearing. "There wasn't one word about policy."

Irwin and the scientists on his side say acid rain forms when oxides of nitrogen and sulfur, products of combustion in American cars, power plants and industry, combine with water in the atmosphere to form weak sulfuric and nitric acids that fall hundreds or thousands of miles away.

Leaves begin to brown at the edges, defacing lettuce and other leafy crops. In lakes, the acids kill fish eggs, frogs and bacteria that clean the waters.

"We know what has to be done," Irwin said. "The emissions have to be controlled at the source. The only question is, do we have the political will to do what is necessary?"

Scientists on the other side, however, say it isn't that simple. "Nobody denies there is something going on," said Norman J. Temple, vice president of Central Maine Power Co., "but some of it is from natural causes."

He said he fears "a rush to judgment" that would slap expensive controls on U.S. power plants, raise customer rates, and later prove unnecessary. "Even drastic cuts in power plant emissions might have little noticeable effect" on acid rainfall downwind, he said.

The two views neatly framed a debate involving billions of dollars. The Clean Air Act controls seven air pollutants, including the oxides. Should it be rewritten to include limits for the sulfates that form acids? The law restricts pollution near a smokestack. Should enforcers also count pollution that falls time zones away?

There are only a few ways to get acids out of the rain, none of them cheap. The 1970 Clean Air Act tried to encourage the burning of low-sulfur coal by setting low pollution standards. But when that threatened to idle high-sulfur mines, the 1977 amendments exempted many such places. It also extended deadlines and waived others, especially for auto manufacturers.

Another way, endorsed by environmental groups, is to wash the sulfur out of the coal before burning it. But that leaves its own sludge, and utilities say the process must be done at the mines to be efficient.

Then there are stack scrubbers, massive tangles of pipes, towers, ducts, tanks and ponds that wash the coal smoke with chemicals. But a scrubber for a common 700-megawatt plant costs about $115 million, and operating costs can run $2,000 per hour, according to the Edison Electric Institute.

The Clean Air Act is festooned with efforts to make things easier on utilities, but business groups argue that the law still hinders growth. Nothing should be done, they argue, until results are in from $20 million in research being done by the Environmental Protection Agency and other government agencies.

In the only semblance of a Reagan administration policy statement so far, EPA acting Administrator Walter C. Barber seemed to agree with that point. He told the committee's open hearings in Washington last week that "intensive research" is required.

Mitchell is testing another approach, one that could require a flat 10 percent or greater reduction in all sulfur doxide emissions from power plants. He asked the witnesses if enough is known yet to do that, and most of them said yes.

"Maine is at the end of the geographic exhaust pipe," said Maine Gov. Joseph E. Brennan. Although acid rain effects on forest growths are not certain, he said, "there is enough information at this time to be more than suspicious."

Maine's attorney general James Tierney, called for a six-point program to deal with acid rain: a moratorium on further relaxations of the law, federal subsidies for controls on a few major plants in the Ohio River Valley, emissions standards for sulfates, mandatory coal washing, a ban on high smokestacks and stiff law enforcement. Maine could go to court to get its way, he said.

Mitchell said he could not understand resistance to any policy that would at least control emissions at the current level. That, he said, is "a problem of waiting for a more perfect mousetrap, and meanwhile, the mice are running all over the barn."