At California’s McCormick and Baxter Superfund site, a pilot study used hybrid poplars to treat polluted groundwater; runoff from the planted system held 99% less contamination

On a riverbank in Stockton, California, a row of ordinary-looking poplar trees is quietly doing something most passersby would never guess. According to a 2021 EPA Science Matters report, the amount of contamination in the runoff water was 99% less than the polluted water that was fed into the planted systems that year. If this seems like too easy to be true, it is. But that doesn't mean the problem is solved. Here's the whole, truthful story.

The site has a genuinely odd backstory

The McCormick & Baxter Superfund site is a 29-acre former wood-treatment facility that EPA materials describe as roughly 29 to 32 acres near the Port of Stockton — older agency fact sheets and news releases cite 29 acres, while the most recent Five-Year Review lists the site at approximately 32 acres, likely reflecting a later, more precise survey of the boundary. From 1942 through 1991. It was found to have contaminated the soil, sediment, and groundwater for decades, as waste oil had been disposed of there and was so toxic it was listed by the EPA in 1992 as part of the Superfund National Priorities List. This is not a what-if scenario about plants' role in pollution remediation. It's a genuine industrial mess, the type that America abounds in, hiding out in the neighborhoods near residential areas and water supplies.


Why this particular fix is a big deal

The contamination of groundwater is bad enough at the surface level, but here the pollution is about 250 feet below the surface and far deeper than the roots of a typical tree. A 2020 peer-reviewed pilot study titled “Phytoremediation of slightly brackish, polycyclic aromatic hydrocarbon-contaminated groundwater from 250 ft below land surface: A pilot-scale study using salt-tolerant, endophyte-enhanced hybrid poplar trees at a Superfund site in the Central Valley of California, April‒November 2019,” published in the journal Remediation, provides valuable insights. The researchers found that a solar-powered pump was used to bring the slightly brackish, PAH-contaminated groundwater up from that depth to the surface, into aboveground totes and containers, it was only there, at the surface, that salt-tolerant hybrid poplars planted in the containers could actually reach it with their roots. Some of the trees had also been inoculated with a naturally occurring, pollutant-degrading bacterium called Pseudomonas putida PD1.

The trees took up the water, and the contaminants were processed inside the trees by their root system and internal enzymes, and runoff from the trees was significantly cleaner. It's simply a transfer of chemistry to biology, and letting a tree's own metabolism do the job instead of expensive pump-and-treat plants that work around the clock. And that cost angle is more significant than it may appear. EPA has publicly acknowledged that the deeper the contamination is buried, the more expensive the technology of cleaning up the land with traditional methods gets, and the less effective they will be when the deepest part of the contamination has been completely removed. Solar pumps and planted lots are, by comparison, refreshingly low tech and low maintenance.

Some real caveats

This project was a pilot-scale project, not a full site clean-up. This is not a year-round solution as the trees are only active during their growing season. It also isn't a replacement for the soil capping and sediment containment and stabilization work that is separately, and already, part of the site's official EPA-approved remedy, those measures address surface soil and river-bottom sediment, a different part of the contamination than the deep groundwater the poplars are treating. Phytoremediation is one piece in a much larger and more extensive remediation process, and Superfund cleanups tend to be lengthy, lasting for decades. It would be an understatement to use the term “silver bullet” for a smart, promising add-on.

Another question that hasn't been fully addressed yet is the question of scale. This worked at one site, at one contamination profile, at one growing season. According to a 2019 review paper titled “Ecosystem services of poplar at long-term phytoremediation sites in the Midwest and Southeast, United States,” published in the journal WIREs Energy and Environment, the efficiency of poplar phytoremediation is genotype- and site-dependent, so what worked in Stockton may not necessarily work anywhere else. The same review reports that poplars planted on contaminated sites throughout the US Midwest and Southeast have produced benefits to the ecosystem, such as biomass and carbon storage, that are similar to those produced by poplars planted on clean land, which is a positive indicator of how much a contaminated site can start looking like normal land.

Why should this matter

There is a massive inventory of polluted industrial property in the US. Most of the sites happen to be located in proximity to urban centers where there is a desperate need for residential units. Every acre that phytoremediation helps bring back into usable condition is an acre that doesn't have to stay fenced off indefinitely, whether it eventually gets redeveloped for industrial, commercial, or mixed use. (The McCormick & Baxter site itself, for instance, remains zoned industrial and sits in a federal Opportunity Zone for future redevelopment, not a residential rezoning). It's not flashy, it's not quick, and it is certainly not a cure-all. However, this technology that is so quiet, so inexpensive, and so literally “natural” needs a great deal more attention than it is currently getting.