HOW FAST SHOULD Congress expand the Superfund? The case for fast-as-possible is simple: Superfund pays for cleaning up dumps of dangerous wastes. But the Environmental Protection Agency argues that not-too-fast will be more effective in a new field with intricate technical requirements. Superfund's staff has tripled in the past two years, observes Lee M. Thomas, EPA's administrator. At its present scale -- spending about $1 billion a year -- it can handle some 600 to 800 sites at a time. Going much beyond that, Mr. Thomas says, may strain the agency's capacity to manage the job as skillfully as the nature of these hazards requires.

Last summer, the House passed a bill to raise the Superfund authorization to $10 billion over the next five years, and the Senate Environment Committee apprved a figure of $7 billion. The Reagan administration's bill, which Mr. Thomas defended before the Senate committee yesterday, would authorize $5.3 billion over five years. In effect, it would continue Superfund at its present rate of spending through the rest of the decade.

It's an interesting bill carrying an important change in the source of the clean-up money. Most of it, so far, has come from a tax on the petrochemical feedstocks that are the ingredients of the chemical wastes in these dumps. The administration proposes to rely on this feedstock tax for one-third of future Superfund revenue, and to put a new tax on the actual wastes themselves for the other two-thirds. Going a step farther, it intends to impose the waste tax at two rates -- a lower rate for companies that use the best technologies for waste disposal, and a higher rate for companies that don't.

In principle, those changes are exactly right. As a practical matter, the appeal of taxing the petrochemical industry was its simplicity. It meant collecting the tax from hardly more than a dozen big oil companies. But there are some 15,000 companies that generate substantial amounts of chemical waste, and taxing them efficiently is a much more ambitious proposition. EPA says that its regulation of waste disposal has now reached a point at which it knows with some confidence who is creating these wastes, and how much. But that claim deserves further attention in the congressional hearings now beginning.

A number of environmental organizations have denounced the administration's bill for its refusal to move farther and faster. Perhaps some of that denunciation is deserved. But it's worth noting that the administration has finally sent to Congress an environmental bill that would neither cut back the government's powers to control hazards nor diminish the money for enforcement and clean-up. Lately Mr. Reagan seems to be giving better support to the EPA. Perhaps that is because he's becoming more familiar with its work.