One of the premises of President Reagan's approach to government is that a less aggressive federal regulatory role will prompt states, localities and industry to come up with their own solutions to national problems. Last week, the National Governors Association and the Conservation Foundation announced the start of such an approach to an urgent problem -- preventing further contamination and depletion of the country's ground water.

Roughly half the country depends on ground water -- rather than rain-deposited surface water -- for drinking. The U.S. Geological Survey warns that water supplies are rapidly depleting in 35 of the 48 contiguous states. The problem is most extensive in the fast-growing Southwest, where federal policy hastens the process by heavly subsidizing both water for crop irrigation and the surplus crops thus produced.

As ground water is depleted, the many pollutants that seep into it become more concentrated and, hence, potentially more harmful. Thus far, only a few localities' ground-water supplies are irreversibly polluted. But Arizona Gov. Bruce Babbitt, who will lead the new cooperative effort, points out that scientists now realize that everything that gets dumped on the ground -- agricultural pesticides, industrial wastes, household chemicals and so on -- ultimately ends up in underground water supplies. Trying to curb ground-water use and pollution will arouse concern and competition among states, municipalities and industries. To minimize strife, Gov. Babbitt would like to see the federal government play a stronger role in setting drinking water standards, gathering data and developing regional approaches. But the administration has not yet set up a National Ground Water Commission mandated by Congress last year, and it decided last summer to let the states take the initiative in this area.

Only a few states -- notably Florida, Connecticut and Wisconsin -- have made much progress. But the Governors Association and the Conservation Foundation hope to accelerate action by other states -- especially in the Southwest -- by bringing federal, state and local officials together with representatives of the major environmental groups and corporations.

Keeping this cooperative effort going -- and translating its findings into enforceable policies -- won't be easy. But as members of the new Groundwater Policy Forum stress, it will be far more difficult to deal with the staggering effects of massive ground-water pollution in the future than to take steps now to prevent such a crisis.