Both us, humans, and forests sometimes end our lives transformed into ashes. Although everything’s already dead, where those ashes end up matters. People often scatter the ashes of their loved ones in meaningful places. Forests, nevertheless, do not have that much of a choice. It is then our responsibility to deal with those ashes.

After a wildfire, the ashes left behind might find their way into rivers, lakes, and reservoirs, where they can significantly impact water quality and the ecosystems that rely on it. But what exactly happens when wildfire ashes enter the water? The effects of those ashes can be more complicated—and more dangerous—than we think.

Historically, farmers burned fields after harvests, using the ashes to enrich the soil with nutrients in preparation for the next planting season. However, wildfire ashes have a very different impact, especially for water bodies. To unpack this complexity, we turn to Stefan Doerr, a researcher at Swansea University and co-creator of WEPPcloud—a revolutionary tool for predicting fire risks and assessing environmental impacts. In this blog post, he will help us explore the often unexpected effects of ash on water.

To demonstrate the depth of the issue, wildfire-derived ashes are not always beneficial for the soil. “Imagine you have heavy contamination in an industrial area, from iron smelting. You would get deposition of those heavy metals in the vegetation. If you then burn that vegetation, you can immediately get a high amount of metals,” Doerr says. Also, within the FirEURisk project he adds: “Think of the Red Forest in Chornobyl, a very, very high accumulation of radionuclides. If you burn that forest, you’re releasing those metals, and the soil can become even more contaminated than it already is”.

However, there’s no need to imagine such dystopic scenarios—ashes from any wildfire often end up in rivers, lakes, and reservoirs, where they can seriously impact water quality.

What makes ashes so harmful? “It’s all a matter of concentration,” Doerr highlights. Certain compounds, like polycyclic aromatic hydrocarbons, or heavy metals are extremely toxic for the wildlife if highly concentrated, though they pose less of a danger when diluted in large amounts of water.

Conversely, the ashes also bring nutrients which can initially trigger algal blooms. While these blooms might seem harmless at first, as Doerr explains “the simple presence of sediment from ashes can cause a high level of turbidity. That means there’s not enough light coming through and aquatic organisms who require light have a problem.” Therefore, algae can initially quickly grow, depleting the oxygen present in water, but then die releasing toxins. This process is known as eutrophication and can severely damage aquatic ecosystems.

Yet, the effects of ash on water quality are anything but predictable. For instance, the impact of ashes can differ significantly between a river and a lake. Rivers, with their flowing water, may dilute and disperse contaminants more effectively, while lakes can trap sediments and pollutants. As Doerr notes “if you’re talking about a drinking water reservoir or a lake, the effects can last between weeks and months, and in some cases even last for years”.

And it’s not just about where the ash ends up, but also where it comes from. Ashes from an Australian forest, for instance, might have entirely different nutrients and chemical compositions than those from Mediterranean woodlands, triggering unique ecological consequences in each setting. As part of FirEURisk project, Doerr and his team have gathered and analysed over 100 ash samples worldwide, building a clearer picture of the typical ash components in each region and providing the knowledge needed to respond appropriately.

Moving to a human perspective, when ashes reach water reservoirs used for consumption, the consequences can be severe, though not in the way one might expect. As Doerr states “the main problem is simply that the water changes colour and has sediment in it, and that makes it quite challenging to clean. In many cases the water would still be drinkable”. In this case, heavy metals are usually quite diluted, but the turbidity makes ultraviolet filtration, which is a required health standard, impossible.

What can be done to prevent water pollution, then? Prevention of fires should be the first thing, but once they have happened there’s still something you can do, and this is “putting an erosion barrier into the landscape, which would help prevent the ash from moving down the hill and accumulating in the rivers and the reservoirs,” Doerr explains. Erosion barriers are usually made of straw or biodegradable chemicals, and, in both cases, they act by avoiding landslides. While these approaches are surprisingly effective, they are also expensive. Therefore, all the ground can’t be covered. A decision needs to be made.

This is where WEPPcloud comes in, an accessible online model developed within the FirEURisk project. WEPPcloud predicts how fires impact water quality, estimates how much ash will wash down slopes, evaluates potential heavy metal contamination in water, and so much more. “You can then make the decision. You could say, ‘Oh this hill slope is at really high risk of generating erosion, so I will put in some erosion measures here’,” Doerr illustrates.

In the end, the journey of ash—especially as it enters our rivers, lakes, and reservoirs—underscores the lasting impact of fires on our most essential resources. Apart from the ritual of scattering our loved ones’ ashes in nature, perhaps it’s time we adopt a similar ritual of care for the ash left behind by wildfires.