SAFE DRINKING WATER for 600,000 Northern Virginians will be at stake today when Gov. John N. Dalton meets with the Prince William County officials who are trying to torpedo the Upper Occoquan sewage treatment plant. The county group claims that the regional project, set to open in 10 days, is overbuilt and will make Manassas-area sewer rates soar. They want the governor and the State Water Control Board to reassess the Occoquan policy - and put off the scheduled closing of an existing plant.

Gov. Dalton and the board should stand firm. The new plant is so advanced and expensive for one reason: Its effluent flows into the Occoquan Reservoir, from which those 600,000 Virginians drink. State and area officials, led by then-water board, chairman Noman Cole, saw several years ago that undertreated sewage from upstream would threaten the reservoir. Rapid growth in that basin has narrowed the margin of safety since then.

What about the rate-increase claims that have stirred up Manassas-area taxpayers?Mr. Cole, for one, asserts that Prince William officials are blaming the new treatment plant for increases caused by their own poor planning and management. Although Mr. Cole is a partisan, he has a point. Before even considering policy changes, Gov. Dalton should be sure all the facts and figures are clear.