A Wildfire's Mark on the Water Supply Can Last Fourteen Years

✅ Key Takeaway: Wildfire can degrade catchment water for over a decade; with 70% of UK drinking water from the uplands, post-fire monitoring here is overdue.

A severe wildfire can drive the pollutant load in a catchment's streams to many times its normal level and hold it there for more than a decade. That matters to Britain's uplands more than almost anywhere.

Around 70% of UK drinking water comes from catchments dominated by peatland habitat, and University of Leeds research puts 72.5% of UK reservoir storage capacity down to peatlands-fed water.

The finding comes from a new review in Science of the Total Environment, in which researchers at the University of British Columbia drew together 23 studies covering 28 burned watersheds across North America, Europe and Australia. They found that fire causes large, durable increases in pollutants across rivers and reservoirs, including the headwaters that public supply depends on.

The measured peaks are not marginal. Suspended sediment after fire reached 1,142 milligrams per litre and turbidity climbed to around 145 NTU, well past the loads conventional treatment is built for.

In the worst cases some metals and toxic organic compounds were recorded thousands of times above environmental safety limits. And these readings did not subside quickly: many stayed elevated beyond five years, and after Colorado's Hayman Fire nitrate was still high 14 years on.

How fire wrecks a catchment

The mechanism is one any hill manager will recognise. Fire strips the vegetation that anchors the soil and bakes the ground until it sheds water. The first heavy rain or snowmelt then carries ash and exposed soil straight into the watercourses, and material that settles in the riverbed is stirred back up by storms for years afterwards.

What the studies found in the water:

• Sediment was the largest and most stubborn effect, clouding water, smothering riverbeds and silting up reservoir capacity.

• Nitrate rose by almost 400 times in one catchment; after the Hayman Fire it climbed by more than 700% and remained elevated 14 years later, feeding the risk of algal blooms.

• Lead increased more than 700-fold and copper more than 100-fold in some catchments, with iron, manganese, zinc and lead all breaching safety standards before settling into sediment to be released again in later floods.

• Polycyclic aromatic hydrocarbons (PAHs) and other persistent, carcinogenic compounds were widespread, and smoke plumes carried them more than 700 kilometres, contaminating catchments far from any fire.

Even the firefighting carries a cost

One result cuts against the assumption that suppression is consequence-free. Long-term retardants such as Phos-Chek are built largely from fertiliser compounds, so when they wash off they load streams with nutrients and have proved lethal to tadpoles above one gram per litre. Some firefighting foams have also carried PFAS, the persistent "forever chemicals." The fire and the response to it both leave a residue in the water.

The bill lands on treatment

Conventional treatment is not designed to absorb a simultaneous surge of sediment, organic matter and metals. After one Colorado wildfire the coagulant dose rose sharply, yet the works still stripped out less than a tenth of the dissolved organic carbon.

The cost figures the review cites are blunt: roughly £24 million removing post-fire sediment from a single Denver reservoir, and an extra £400,000 a year on treatment chemicals for one Canadian town after a 2016 fire.

That bill is not hypothetical for Britain. Removing peat sediment and dissolved organic carbon already represents the largest single cost in raw-water treatment for UK utilities, and dissolved organic carbon has doubled across many UK catchments in recent decades as peat condition has declined. UK treatment works are paying for catchment state before a single fire is added to the equation.

What it means for the working uplands

The implication for upland Britain is hard to avoid. These catchments are a national water asset, their treatment costs already rise and fall with peat condition, and a severe moorland wildfire would be a step-change worse on every measure the review tracks.

How that risk is managed, including the contested question of controlled burning to reduce fuel load, has to be settled on evidence rather than assertion - the Moorland Association's consistent position, and the premise of Natural England's 2025 evidence review on managed burning in the uplands.

The sharpest gap is closer to home than the dollar figures suggest. The UK has almost no post-fire water-quality monitoring of its own. Until it does, every decision about protecting the catchments that supply most of the country's drinking water rests on data borrowed from forests on another continent.