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The amount of toxic "forever chemicals" flowing into the River Mersey in north-west England has reached some of the highest levels recorded anywhere in the world.

My team's research links much of this to old landfills, waste facilities and past industrial activity. Even if these chemicals were banned tomorrow, they would continue polluting our rivers for decades, possibly centuries.

But there is a path forward. We've developed to track and prioritize the largest sources for clean-up, giving regulators a clearer picture of where to act first.

Per- and polyfluoroalkyl substances (PFAS), more commonly known as "forever chemicals," are a large family of human-made chemicals found in everyday products like , and . They are valued for their ability to resist very high temperatures and to repel water and oil, but these same properties make them extremely persistent.

Once released, some PFAS could take thousands of years to break down. They accumulate in the , build up—with different compounds accumulating at —inside the bodies of and , and have been associated with harms to health. The most studied types have been , hormone disruption and immune system problems.

Last year, my research team discovered that the amount of two potentially cancer-causing PFAS chemicals washing off the land and into the Mersey was among the . In our follow-on research, we traveled upstream to try and locate where these PFAS are coming from. But with hundreds of potential PFAS sources, how do we isolate the largest ones?

The secret is measuring something called the PFAS load—the total amount of PFAS flowing through the river at a given point, rather than just the concentration in the water.

Here's why that matters: a small stream can have high concentrations but carry only a small total amount, while a large river with lower concentrations can be transporting far more PFAS overall. If we only look at concentration, we risk missing the really heavy polluters.

By measuring PFAS loads at multiple points along the Mersey system, we could see exactly where the largest increases occurred. That told us both the location and the scale of PFAS inputs.

We detected PFAS chemicals at 97% of our sample sites, even in supposedly pristine streams draining from the Peak District national park. But the big breakthroughs came when we matched the largest PFAS load increases to specific areas.

PFBS (a type of PFAS) was coming in huge amounts from land draining old landfills in the Glaze Brook watershed near Leigh, west of Manchester. PFOA, a and , appeared to originate from a waste management facility on the River Roch, north of Manchester. PFOS, another banned PFAS, was entering the River Bollin, with strong evidence pointing to historic firefighting foam use at Manchester Airport.

What's most striking to me is that all these sources are rooted in the past—old landfills, waste sites or historic industrial use. These chemicals are no longer in production, but they are still escaping into the environment, decades later.

This is where PFAS load measurements make a real difference. Instead of chasing the highest concentrations—which might lead to cleaning up small streams that contribute little overall—we can target the sites releasing the largest total amounts of PFAS into our rivers.

It's a simple idea with major implications. In a world where environmental regulators face tight budgets and limited monitoring capacity, knowing exactly which sites are the biggest sources is vital.

The Mersey is just one example. , PFAS contamination follows a similar pattern: numerous potential sources scattered across the landscape, many of them historical. The chemicals' extreme persistence means they will continue cycling through rivers, soils and wildlife for generations unless active steps are taken to remove or contain them.

shows that measuring PFAS load can help solve one of the toughest challenges in managing chemical pollution: working out where to start. By identifying and prioritizing the biggest sources, regulators have a realistic chance of reducing the flow of forever chemicals into our rivers—and perhaps one day, making that nickname a little less true.

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