Despite a fast-approaching deadline for a defensible plan to upgrade protection of drinking water sources near its Red Hill fuel farm, the U.S. Navy has continued to drag its feet.

The U.S. Environmental Protection Agency and state Department of Health issued a letter on June 7 rightly criticizing the Navy, which has been conducting an environmental investigation for almost two years, for providing too little information on water flow modeling that tracks where previously spilled fuel might end up.

The call for flow modeling is part of an administrative order on consent agreed to by the Navy, state Health Department, EPA and Defense Logistics Agency to minimize the threat of fuel releases, following a 27,000-gallon spill in 2014 and periodic spills before that at the World War II-era facility.

In response to the letter, which referenced repeated chiding of the Navy for “insufficient understanding of the expertise and level of effort necessary” to meet the demands of the consent order, Rear Adm. John Fuller, head of Navy Region Hawaii, presented a promising can-do reply.

In a June 20 letter to Red Hill “stakeholders,” he said: “We can and we will do better. While no one likes getting a progress report that essentially says, ‘Navy, you need to work harder and smarter to meet a future requirement,’ their letter illustrates the (consent order’s) power and value.”

But Fuller, who took the helm in Hawaii two years ago, is slated to relinquish command of Navy Region Hawaii and Naval Surface Group Middle Pacific on Friday. It’s imperative that his successor follow through with a plan that provides a better buffer against an ever-present public health threat.

Fuller asserted that the 2014 leak was “due to human error, not simple material failure,” and that the facility’s steel-lined tanks — now more than 70 years old — are in “great condition” and undergoing continual modernization. Regardless, the call for a retrofit or even possible relocation is justified because the Red Hill Underground Fuel Storage Facility is perched just 100 feet above Oahu’s primary aquifer, which supplies drinking water to more than 600,000 residents, from Moanalua to Hawaii Kai. That proximity is too close for drinking-water comfort in the world’s most isolated island chain.

Constructed as a bombproof reserve, the Navy-operated facility houses 20 aging tanks — each 250 feet tall and 100 feet in diameter, situated vertically underground. Each is large enough to bottle Aloha Tower (184 feet tall). The site’s fuel capacity is 250 million gallons, enough fuel to fill 379 Olympic-size swimming pools.

The finalized fuel tank upgrade plan is due Dec. 8. At least six options are under review — three envision what’s now considered the closest thing to a sure-bet seal: a double-walled retrofit. The rest opt for a single-wall. Both types present daunting engineering challenges. In that respect, some foot-dragging, while not acceptable, is understandable.

Red Hill was a state secret until the early 1990s, when the facility was declassified. By the late ’90s, the Navy had initiated various environmental probes and monitoring. Given that nearly two decades have passed since then, it’s puzzling that the Navy does not have water flow modeling down cold.

In all, reports suggest that since 1947 there have been more than 30 leaks, with at least 170,000 gallons of fuel seeping away from tanks.

The military needs the facility’s flow of fuel to support vessels and aircraft in its Pacific theater. Oahu residents, however, are due for updated protection against catastrophe scenarios. For example, the city Board of Water Supply estimates that structural failure — triggered by an earthquake, for example — could drop more than 1 million gallons of fuel pollution into groundwater and potentially several million gallons into Halawa Stream and Pearl Harbor.

Both military and civilian sectors are counting on the Navy to produce a plan for Red Hill that can stand up to scientific and engineering scrutiny. Over the next several months, it can — and must — do better.