AS IS THE CASE with most unamimous Supreme Court rulings, this week's decision dealing with the Environmental Protection Agency's duties in considering costs and benefits under the Clean Water Act turns on a narrow legal point rather than on broad matters of policy. Nevertheless, the decision is an important one; for had it gone the other way, EPA would have been forced into a case-by-case assessment of the economic health of individual plants -- a sure-fire formula for regulatory chaos.

The Clean Water Act set up a two-stage process for reaching its ultimate goal of cleaning up the nation's water. For the stage -- at issue in this case -- EPA was directed to examine the best plants of each major water-polluting industry and to determine a rough average of how much pollutant could be removed, at how great a cost and for how great a benefit. It was then to issue a regulation that would apply to all plants in that particular industry.

A few years ago, the Supreme Court ordered EPA to allow as well exceptions for individual plants that did not fit the range of circumstances considered in arriving at the regulation. Though the law does not require such variances, this was a sensible step toward allowing needed flexibility in regulations designed to fit an imaginary average. But certain industries then tried to take these variances one step further, arguing that a variance should also be allowed if the economic impact of the regulation on one particular plant were so great that the plant would be forced to close down. It was on this point that the Supreme Court said no in an 8-0 ruling.

The court's decision does not affect EPA's duty to consider the costs and benefits of its water pollution control regulations. The agency must still calculate these numbers as best it can. Its figures may be challenged in court and the regulation invalidated if they are shown to be inaccurate. Not does the decision affect EPA's duty to allow variances if an individual plant is clearly different in some substantial way from the industry as a whole. But it says that a plant cannot receive an exemption just because it cannot afford to meet the pollution control standard.

In writing the law, Congress realized the implications of what it was doing. The certainty that a number of already marginal plants would be forced to close down because of the added costs of pollution control was debated and agreed to be an acceptable cost of cleaning up the country's waterways. In fact, the number of facilities that have closed as a result of the Clean Water Act has turned out to be a small fraction of what was predicted.

In its ruling, the Supreme Court has not made any judgment on the necessity of considering costs and benefits in environmental regulation. It has merely endorsed Congress' sensible intent to make a certain minimal level of water pollution control an unavoidable cost of doing business: companines that cannot afford the cost cannot stay in operation or water quality cannot be improved. Nor does the EPA, or any other government agency, have the resources to decide whether thousands of individual plants can or cannot afford to meet the regulations. That way lies a true regulatory nightmare.