What started as good news for water managers in New Mexico’s largest urban area has led to a headache for the Air Force and state environmental officials in charge of cleaning up a decades-old jet fuel spill that contaminates part of Albuquerque’s aquifer.

Earlier this year, conservation efforts by the local water authority led to a surprisingly quick rise in the aquifer beneath Albuquerque – normally a good thing. But that quick rise ended up submerging dozens of well screens used to monitor the top of the fuel spill that started at Kirtland Air Force Base on the southern edge of the city.

For the past several years, the water table beneath Albuquerque has risen as the Albuquerque Bernalillo County Water Utility Authority has “banked” water in the aquifer. Increased use of water from the San Juan-Chama Drinking Water Project, which diverts water from those two rivers, has decreased the need for water from groundwater wells, causing the water table to rise about 2–3ft a year since 2009.

“Then, all of a sudden, the water table in the first quarter of this year jumped 5ft,” said Kathryn Lynnes, the senior adviser for the Bulk Fuels Remediation Project at Kirtland Air Force Base.

“That was unexpected to everybody,” said Rick Shean, the Water Utility’s water rights program manager.

While everyone knew the water table had been rising steadily for years, no one thought it would come up this fast. It means the monitoring wells at the top of the table will need to be replaced about two years earlier than planned. And while there are dozens of other monitoring wells lower in the water table (including the recently submerged wells), there is now a gap in monitoring the size and chemical composition of the top of the fuel plume.

That’s critical because the highest concentrations of ethylene dibromide (EDB) – a highly toxic fuel additive – and other fuel components occur at the top of the water table.

“We need to be able to have early indicators of what is happening with that EDB plume – and that’s done through these water table wells,” said Diane Agnew, project technical lead for the New Mexico Environment Department, which approves and then monitors the Air Force cleanup plans.

The Air Force and the Environment Department are still hashing out plans for the replacement wells. Meanwhile, important data about the leak is going unmonitored.

The fuel leak was first detected in 1999 in underground pipes at a fueling facility on the base. “It wasn’t a constant, gushing leak,” Lynnes said. “It was a drip-drip-drip every time there was a fueling episode.”

No one knows how much fuel spilled in all, but estimates range up to 24 million gallons. The Air Force kept fueling records for only a couple of years at a stretch, back when the leak was detected. Those records indicated the pipes leaked as much as 162,000 gallons in 1999 alone. Early testing found that the plume contains EDB, which the military stopped using in 1975. Its presence indicated the leak had been going on for decades.

The leaking fuel, “played Plinko on the way down” as it filtered through rocks and soil, then hit a layer of clay, and traveled north, and eventually dropped approximately 480ft to the water table, said Scott Clark, an Air Force restoration program manager. Over time, that drip-drip-drip grew to a mile-long plume of toxic aviation fuel that extends for a mile underground, both on base and in the surrounding residential neighborhoods.

The plume sits between water wells on base and one at a neighboring Veterans Affairs facility, but it’s slowly moving toward a pair of wells that supply local drinking water. So far, a monthly testing regimen has found no fuel in any drinking-water well, on base or off.

The Air Force has installed “sentinel wells” between the leading edge of the plume and the Water Utility’s municipal wells as an early warning system. “The sentinel well is 1,400ft away. And at that time we thought that it could give us five years of warning,” said Shean. “There hasn’t been a detection at that well either.”

Such fuel spills aren’t uncommon. In fact, Agnew says California has several similar incidents, but it’s much easier to retrieve data on those. There, monitoring wells need only go down 20ft, and the wells can be dug in less than a day. In Albuquerque, wells must be dug at least 450ft, which takes up to five weeks (and it’s often in residential neighborhoods). That increases both cost and planning time.

Leaking fuel first passed through the vadose zone­ – the layer of soil and rock below the surface – before hitting the water table. Between 2003 and 2015 the Air Force extracted more than 780,000 gallons of spilled fuel from the zone, but no one knows how much remains. One worry is that the water table could rise and reach undetected pockets of fuel in the vadose. But there is no way to watch for that without monitoring wells atop the water table.

Planning meetings for the new wells are slated to begin in January. Meanwhile, the Air Force is continuing a series of interim cleanup measures. It has drilled three wells into the plume and extracted and treated more than 313 million gallons of contaminated water. Testing indicates that these are shrinking the concentration of EDB in the aquifer. A fourth well is to come online in January.

The Air Force is also pumping oxygen into the vadose zone to encourage growth of naturally occurring bacteria that break down fuel and related chemicals. And it’s starting a project to add sugars to the soil to further energize those bacteria.

Still the water table keeps rising. Shean says that the Water Utility anticipates it will rise another 50ft over the next 20 years.

Lynnes says she is a big fan of the Water Utility’s conservation plans and water banking.

“I think it’s awesome,” she said. “It’s just made our life a little more interesting.”