The first estimate of the cost of treating the Bethpage underground plumes is out, and it is staggering. The state Department of Environmental Conservation says containing and removing contaminants from the two plumes, the longest of which stretches four miles, could cost as much as $587 million and take up to 100 years.

And we all know what can happen tomorrow to estimates made today. The DEC and Gov. Andrew M. Cuomo say they will seek reimbursement from Northrop Grumman and the U.S. Navy, the latest in a series of forceful moves for accountability by the state. But there also is a cautionary tale in this decades-long saga — about what we don’t know what’s happening underground. That’s particularly important on Long Island with our sole-source aquifer.

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Consider Gabreski Airport in Westhampton Beach, where Suffolk County discovered the toxic chemical PFOS in nearby water wells. It came from the firefighting foam used by the county’s Air National Guard, and exceeded federal safe levels in five of the 33 private wells for which results are known. PFOS also was used in Teflon, carpet treatments, waterproofing and stain repellents, and is on the Environmental Protection Agency’s annual list of contaminants of emerging concern.

Is PFOS elsewhere? What other substances lurk underground that we don’t know to test for? We must learn from our past to ensure our water stays clean. Testing must be exhaustive. Vacancies for chemists and other important Suffolk County Health Department positions must be filled. The primary factor in determining where and how often to test, and what to test for, must remain the public’s health and not the state of budgets at government agencies. — The editorial board