HAVING FOCUSED new public attention on the financial tug of war between local governments' spending on services and the amount of money coming to these governments from taxes and other fees, a special task force on the region is now getting down to tough, specific questions for taxpayers. Given all the costly demands from, as well as on, taxpayers, how much are people prepared to pay for a clean Potomac River, and for modern, safe water and sewer service?

The answer is another question: how clean is clean enough? If the people and their local governments are content to live with current procedures for treating pollution and sewage -- "acceptable" quality, according to official standards -- the region can save millions of taxpayer dollars that would be necessary to meet the highest level of treatment procedures. That is the decision set forth in the latest report from the Greater Washington Research Center, which suggests that this adjustment to the status quo would not be dangerous to your health.

How much are we talking about here? Perhaps $25 million to $50 million a year in operating expenses and hundreds of millions of dollars in long- term spending for construction of treatment plants and other projects. Though there may not be precise agreement on how much the region should forgo, federal and local experts do seem to agree that some modification of the highest standards could be made without inviting damage.

Their findings are not merely for the sake of frugality, either. The task force report cites a dramatic improvement in the water quality of the Potomac since 1970, and argues that moving to the more expensive treatment would produce insignificant additional improvements.

To accommodate future demands for water and to slow the flow into treatment plants, the report urges continued conservation programs and -- remember, nothing's free anymore -- higher user fees that continue to set higher charges for heavier users.

So here, as in the other areas scrutinized by the task force, the choices are not attractive, but they are important and they involve enormous amounts of money. Here, too, the importance of a regional approach becomes both practically and financially obvious. Thanks to cooperation so far, there seems to be an adequate supply of water for the region through 2030. To assure the quality of that supply and the money it takes to maintain it, still greater and more sophisticated agreements will have to be struck between the local governments--and therein lies a serious political challenge for all elected officials