Scientists reviewing over a decade's worth of studies on the fate of notorious pollutants – dubbed 'forever chemicals' for the way they persist in waterways, soils, and sea ice – have unearthed where environmental hotspots of contamination lie.

The review, led by hydrologist Xueyan Lyu of Nanjing University of Information Science and Technology, focused specifically on the Earth's crust: the soil it's made of and the waters that percolate through it.

Much work has been done to detect 'forever chemicals' in contaminated water sources and elucidate the impacts on human health, which arise even at very low levels of exposure, prompting health and environmental authorities to revise their safety guidelines.

PFASs (per- and polyfluoroalkyls) – synthetic chemicals highly effective as non-stick and waterproofing agents – have been in use since the 1950s. Since the early 2000s, these compounds have been widely detected in global groundwater.

But grasping where and how much PFAS substances accumulate beneath Earth's surface is an ongoing challenge – especially when new PFAS are being manufactured to replace phased-out, first-generation compounds.

Groundwater and subsurface soil layers may be hard to sample and tricky to remediate more so than snaking rivers and waterways, but by no means should the problem be out of sight, out of mind. What goes down underground comes back around.

Studies have shown how PFAS substances have leached out of landfill, chemical plants and military sites, and into groundwater systems, which supply drinking water to millions of people who tap into aquifers and boreholes.

As rainwater soaks into soils and seeps into cracks between rocks, the water dissolves and carries with it all sorts of minerals, nutrients and pollutants. The point where water pools in permanently saturated sediments and rocks is called the water table; below this is groundwater that runs ever so slowly through deep earthen layers.

People can get exposed to PFAS substances in other ways – in food crops, firefighting foams and cosmetics – but water supplies are one of the most pervasive routes.

Amalgamating studies published over 12 years (2010–2022) on the fate and transport of PFAS in subsurface environments from Sweden to Australia, Lyu and colleagues identified the portion of soil above the water table as a hotspot of PFAS accumulation.

Their work looked at how the chemical properties of PFAS substances – which vary in length, molecular weight and polarity – and geological factors such as soil and water chemistry interact to influence the transport and retention of these pollutants in various subsurface layers.

It builds on past analyses by incorporating those newer PFAS chemicals that older studies had not been able to consider.

Of the PFAS studied, those with long carbon chains were retained in soils, whereas more mobile, shorter PFAS molecules were able to rapidly infiltrate groundwater. Negatively charged PFAS were also most likely to move through soil and contaminate the groundwater.

Understanding the fate and transport of these synthetic chemicals in subsurface environments is "pivotal to assess PFAS-related risks as well as develop effective remedial strategies for contaminated site cleanup," Lyu and colleagues write.

Alarmingly, although perhaps not surprisingly, the review found only just a handful of PFAS chemicals have been studied, even though upwards of 12,000 PFAS substances have been singled out by the US Environmental Protection Agency (EPA).

So much more work needs to be done to connect the dots between where forever chemicals enter, flow through and accumulate in the environment.

In particular, the fate of soilborne PFAS chemicals when exposed to thermal processes such as wildfires has thus far "received little attention or been overlooked altogether," Lyu and colleagues note.

Remediation of contaminated sites is costly but essential. The stakes are dangerously high, with chemical manufacturers pushing them higher still with each new PFAS substance they make.

"The rate at which these pollutants are appearing in the environment far exceeds the capacity of governments to assess global and regional risks, let alone control any potential problems," ecotoxicologist Bethanie Carney Almroth from the University of Gothenburg said earlier this year when warning we've already breached the safe planetary limit of synthetic chemicals.

Encouragingly, scientists have made progress not only in developing methods to filter PFAS substances out of water sources but also in figuring out a simple way to degrade them into a few innocuous by-products.

But health concerns and environmental harms will undoubtedly linger until we identify, trace, remove and dispose of every last ounce of these persistent pollutants.

The study was published in Reviews of Geophysics.