The sudden spike in PFAS contamination in wells serving a south-side water treatment plant is a product of humans and nature, experts say.
Tucson Water could well have increased the buildup of these chemicals in the wells by pumping tainted water out of the wells and toward the plant, said one scientist and a city official. The pumping can pull more heavily contaminated water lying south of those wells into the wells, boosting their PFAS concentrations.
At the same time, other experts say some PFAS compounds, unlike the trichlororethylene that’s been in the south-side’s aquifer for many decades, can move quickly in groundwater, rather than attaching to soil particles that store water in the underground aquifer.
Just the fact that it happened is not that big a surprise to some officials, since Assistant City Manager Tim Thomure says, “We see that type of change occur routinely with PFAS.”
But the latest increases, ranging up to 700% since 2017, have now triggered Tucson Water’s decision to shut the plant down indefinitely. If concentrations keep increasing, they’ll overwhelm the Tucson Airport Remediation Project plant’s ability to treat PFAS at all, officials said last week.
The discovery of these higher concentrations in city wells doesn’t mean there’s been new disposal of PFAS compounds into the aquifer. The nearest known potential pollution source — the Air National Guard Base at Tucson International Airport — stopped all use of PFAS in firefighting foam in 2018. Instead, the higher concentrations are most likely originating from high levels of PFAS lying between the Air Guard Base and the city wells from past PFAS use, and moving northwesterly toward the wells.
The city operates nine south-side groundwater wells that supply polluted water to the treatment plant, known as TARP. The wells also send contaminants TCE and 1,4-dioxane in the same south-side aquifer to TARP.
These wells are downhill from an area just north of the Air National Guard base where PFAS levels have topped 10,000 parts per trillion, far exceeding recommended drinking water limits.
The base for many years used a foam containing PFAS compounds for firefighting and training exercises, stopping in 2018.
The most polluted, neighboring city well shot up from less than 200 parts per trillion in 2017 to slightly more than 800 in 2019 and again early this year. In five other wells, PFAS levels rose nearly 200% to 700% over similar periods.
“I need more information to be sure, but I would guess some type of change in how (the city is) pumping the wells” is part of the cause, said Brown University engineering professor Kurt Pennell. “If you change the pumping regime you can have pretty large jumps like that. It depends on where you’re pulling the water from.”
If you have a spill or other release of a toxic compound that goes deep enough to contact the groundwater, it will slowly migrate through the aquifer, said Pennell, whose research focuses on soil and groundwater remediation including for PFAS.
“But to see a really sharp increase like that, I would suspect it’s kind of a pumping system” causing the increase, he said.
Yolanda Herrera, a longtime south side resident who co-chairs an advisory board overseeing the water cleanup there, agreed that city pumping could have accelerating the increasing PFAS levels, saying, "If we’re pumping groundwater, doesn’t that make the groundwater move? Doesn’t that change the dynamics of our aquifer, or the landscape underground?"
Herrerra, president of the Sunnyside Neighborhood Association, is community co-chair of the Unified Community Advisory Board that meets quarterly to discuss the ongoing cleanup.
However, while pumping can accelerate movement of a pollution “plume,” the closeness of these wells to a pollution source is also likely a factor, said Christopher Higgins, a Colorado School of Mines civil and environmental engineering professor.
“It’s not surprising that the levels are increasing. It’s maybe a little bit surprising the rate they are increasing,” said Higgins, whose research focuses on PFAS’ fate in groundwater. “It really has to do with groundwater flow and how close they are to where the foam was released, and the fact that the chemicals move readily in groundwater.”
The most widely studied PFAS compounds, known as PFOA and PFOS, tend to move at “pretty comparable” rates in groundwater to that of TCE, which moves fairly slowly, he said.
But other PFAS varieties, known as “short chain compounds,” tend to move faster in groundwater, he said.
They’ve been used in place of other PFAS compounds in many cases because scientists believe they don’t build up as much in human and animal tissues as the others.
In those cases, PFAS doesn’t adsorb, or gather onto, surfaces of underground soils. “It moves along with the water. It just moves faster,” said Bill Ellett, a retired Arizona Department of Environmental Quality hydrologist who now serves on an advisory board that oversees the south side’s groundwater cleanup.
Thomure, a former Tucson Water director, said the city has seen such concentration changes in wells north of Davis-Monthan Air Force Base that were shut down by PFAS contamination.
“When high concentrations of PFAS enter a well, the concentration changes can be sudden and large,” Thomure said.
Overall, PFAS is different than prior contamination issues Tucson has dealt with in several ways, he said. First, because PFAS represents a suite of contaminants, not one, their management is much more complicated.
Second, it’s not fully understood how quickly PFAS compounds move in groundwater and whether different compounds move at different rates.
Third, one reason the concentrations appear to be “spiking” right now is that they’re measured in parts per trillion and not parts per million or billion the way many other chemicals are, Thomure said.
“The PFAS contaminated water at 10,000 parts per trillion is equivalent to 10 parts per billion. And we are used to seeing a contaminant go from 8 to 10 parts per billion, and it seems like a gradual change,” he said.
“So, while there is still a lot we don’t know,” the ‘spikes’ of PFAS from one sample to the next are as much a function of the precision of our measurements as anything else,” he said.
