Sailors have long complained about jet fuel seeping into their water on US Navy aircraft carriers and other warships. Here's how that can even happen.

• The crew of a US Navy aircraft carrier recently discovered jet fuel contaminating the water supply.
• Multiple Navy and Marine Corps veterans told Insider they experienced the same issue on other ships.

The US Navy recently found jet fuel in the water sailors drink, cook, and shower with on the aircraft carrier USS Nimitz, and veterans have said this has been a problem on warships for decades. An expert told Insider this week how this potentially hazardous contamination can happen.

A former Navy officer with naval engineering experience explained that this can occur when the pipes between a vessel's potable water tanks and fuel oil tanks are not aligned or connected properly.

Last month, the Navy acknowledged that the Nimitz's water supply had been contaminated with what it referred to as "traces" of jet fuel. A sailor on the ship later told Insider that the issue appeared way worse than what the service described and that they were "exposed to an unhealthy amount" of jet fuel.

Days after the jet fuel was first discovered on the carrier, a Navy official told USNI News that the water contamination wasn't a result of a system failure or leak, but was actually caused by a "procedural 'line up' issue." As an expert explained to Insider, failure to appropriately line up the fuel and water tanks can create a situation in which fuel oil can seep into the water system.

Following reports on the Nimitz, veteran sailors and Marines who served on aircraft carriers and amphibious assault ships told Insider that they also drank or bathed in water contaminated with jet fuel at some point during their service. These veterans served on different Navy warships across four decades.

Here's how jet fuel can contaminate a ship's water

On a carrier like the Nimitz, there are tanks used to store jet fuel for military aircraft — elements of the carrier air wing — that need to be periodically inspected to ensure there's no contamination, Bryan Clark, a former Navy officer who is now a defense expert at the Hudson Institute, told Insider.

To inspect them, these tanks need to be drained of fuel and then flushed out with fresh water from the ship's potable water system so there's no fuel residue inside the tanks and engineers can safely review them. In doing this, the crew has to physically connect the ship's potable water system to its fuel oil system through piping.

"You don't want that to be something that's normally connected, to avoid the problem that we're talking about," Clark said in a recent interview.

Aligning the potable water system and the fuel system involves opening valves that are normally locked shut in order to allow the two systems to connect. The fuel system is supposed to be depressurized, and the potable system should be pressurized. This way, when the valves open, water gets pushed into the fuel tank instead of the other way around.

"The lineup is really important," Clark said. "It's not complicated, but it's a methodical set of steps you have to undertake to connect the potable water system with the fuel oil system and do it in a way that does not back-flush oil into the potable water system. If you do the steps in the wrong order, then you end up having this problem."

A mistake can happen in multiple ways, he explained.

For example, if the piping connecting the two systems are in place and the valves are open, but the fuel oil tanks aren't depressurized, then fuel can get into the potable water system. Or, if the valves are not properly closed after a flush, that can lead to fuel seeping into the water.

In the case of the Nimitz, Navy officials previously said they had identified the source of contamination and isolated it to one of the ship's 26 potable water tanks. By late September, its potable water system was being cleaned, flushed, and tested at San Diego's Naval Air Station North Island, an official told Insider at the time.

The Nimitz spent about two weeks in port in San Diego as the crew worked to fix the water contamination problem, a Navy official confirmed to Insider at the time. The official said that since the contamination was first discovered, a total of 11 sailors reported symptoms that could be attributed to jet fuel exposure — including rashes, headaches, and diarrhea.

Very little is known about the health impacts of jet fuel exposure, according to a 2017 report by the Department of Health and Human Services' Agency for Toxic Substances and Disease Registry. However, using accounts from people who were exposed to kerosene — a compound found in jet fuel — the agency cited reports of "harmful effects" that included symptoms consistent with those described by the Navy.

Clark said the same contamination issue that apparently affected the Nimitz could also impact other warships, like other carriers or even amphibious assault ships, because they have fuel tanks on board that are flushed with potable water systems.

'This is a problem you should be looking for'

Clark said that while it is possible to improperly align the piping and notice the error immediately, allowing the crew to quickly address the problem, the issue may also surface without anyone realizing or proactively checking to make sure that it wasn't happening to begin with. A ship's potable water system can get contaminated even with a low concentration of fuel — making it smell, taste, and look strange.

"Normally, if you flush out your fuel oil tanks — or your fuel oil system, any part of your fuel oil system — you should be watching for this problem, because you've connected the two systems. And once you've changed the lineup back to where it's supposed to be — this is a problem you should be looking for," Clark said.

This isn't an issue that would happen frequently. A ship doesn't find itself flushing its fuel tank very often and only happens during tank inspections — which may only happen every few years or if there's an issue.

Mitigating the contamination, however, is a time-consuming and difficult process. The crew has to turn off the ship's potable water system, fully flush all its sources out to avoid residual fuel in the tanks, and check all the connections between the water and fuel tanks to find misalignments.

"It's a very painstaking process to clean up and to diagnose the source, to then make sure it doesn't recur. And I can see where, if you're not really methodical in how you go about it, you could end up not fixing a problem," Clark said.

After the contamination was detected, it was weeks before the Nimitz was ready to set sail again, though it did so in early October once the water was determined to be within acceptable standards.