An EPA plus

The Reagan administration has just taken a good step forward in preserving the quality of the nation's fresh water. In a political year it is a useful move , too, in combatting the frequent perception that this administration is anti-environment. The challenge is for it to be similarly alert to other legitimate needs of present and future Americans for an environment free from potentially harmful pollution.

In recent years the United States has made significant progress in cleaning up fresh water. Now it is about to embark on a new phase. Subject to approval by the federal Environmental Protection Agency, individual states will establish minimum quality standards for their rivers and lakes, based on the use each will have: swimming, fishing, industrial. Then the states will set the maximum level of pollutants allowable under these standards.

Each state must justify its decisions to the EPA, which is where the new administration action comes in. In a reversal of his predecessor's decision, EPA administrator William Ruckelshaus has decided to insist that the states adhere to high standards.

Environmentalists believe this is a particularly important decision, since the states soon will begin to decide what the safe levels are for toxic chemicals fed into waterways. Setting standards for toxics is ''the next generation of (water) pollution control,'' one specialist says.

Beyond this EPA decision, loom others. One expected soon deals with ways to clean up the air pollution that causes acid rain. Beyond that is the question of whether William Clark will bring significant changes to the environmental policies of the Department of Interior.