Updated: August 9, 2026
The violent fires which hit France during the summer of 2026, particularly in Gironde, Landes and Var, naturally pose a less visible question than that of smoke or destroyed forest areas: What happens to water quality after a large fire?
Ash, burned soil and substances released by combustion can be carried by rain into rivers, lakes and reservoirs. But can they also reach groundwater?
The scientific answer is: yes, it is possible, but it is neither automatic nor uniform. The risk depends in particular on the nature of the soil, the depth of the water table, the intensity of the fire, the precipitation following the fire and the substances present in the burned areas.
A summer of 2026 marked by exceptional fires
The 2026 season is particularly severe.
In a publication updated on July 31, 2026, the Ministry of Interior indicated that 119,000 hectares had already burned in France since the start of the year, with 14,320 fires recorded since the start of the season. The ministry then reported significant fires in Gironde, in the Landes, but also in the Var, in Corsica and in Côte-d'Or.
In the South-West, the fires took on an exceptional dimension. According to the Ministry of Ecological Transition, the Saumos fire, started on July 22, affected approximately 39,800 hectares, while that of Biscarrosse, started on July 23, affected approximately 3,600 hectares. More than 240 buildings were damaged and more than 250,000 people had to be evacuated.
At the beginning of August, the Var was also faced with a major fire around Gros Bessillon, with approximately 6,300 hectares covered by fire and 5,700 hectares burned according to the Var prefecture.
These considerable areas do not only represent a forest loss. They temporarily modify the functioning of soils and the water cycle.
How can a fire pollute water?
When a forest burns, trees are not the only elements transformed.
Fire produces ash, mineral particles and carbonized organic matter. It can also modify the availability of certain elements naturally present in the soil.
During the fire, ashes and particles transported by the atmosphere can fall directly into rivers, lakes and reservoirs.
But a particularly important phenomenon occurs after the fire, when it starts to rain again.
Vegetation and forest litter which previously retained part of the water have disappeared. Soils are more exposed to erosion. Rain can then transport large quantities of:
- ash;
- sediments;
- organic matter;
- nutrients;
- iron and manganese;
- other metals and mineral elements.
L'US to watercourses Geological Survey (USGS), an American public scientific organization specializing in particular in water resources, confirms that fires can increase erosion and the supply of sediments, nutrients, dissolved organic carbon, ions and metals in waters located downstream of burned areas.
The soil can even become temporarily less permeable
Another phenomenon is particularly interesting.
Under the effect of high temperatures, some soils can develop hydrophobicity, in other words become temporarily more repellant to water.
Instead of gradually penetrating into the soil, a greater proportion of precipitation can run off the surface.
The USGS explains that the intense heat of a fire can promote this hydrophobic property of the soil and increase runoff. The disappearance of vegetation and the protective layer of the soil simultaneously increases the risk of erosion.
This is why the first heavy rains following a major fire constitute a particularly sensitive period for the quality of surface water.
What contaminants can be found after a fire?
There is no single “fire pollution ".
The composition of the water after a fire depends greatly on what burned and the local geology.
In a forest relatively far from buildings, the main changes may concern suspended matter, nutrients, organic carbon, iron, manganese and various mineral elements.
When the fire also affects homes, vehicles, industrial installations, synthetic materials or infrastructure, the nature of the substances likely to be released becomes much more complex.
The US Environmental Protection Agency (EPA) monitors, in particular, in its post-fire water research, nitrates, arsenic, certain volatile organic compounds and disinfection byproducts.
Polycyclic aromatic hydrocarbons, or HAP, can also be produced during incomplete combustion, particularly during vegetation fires. Their possible presence in the environment then depends on their transport, their adsorption on particles and local conditions.
It would therefore be incorrect to consider all burned areas as having exactly the same contamination.
What about groundwater?
This is probably the most important question.
Yes, contamination of groundwater is possible
Some rainwater naturally infiltrates through soils to groundwater.
After a fire, certain substances deposited on the surface or released into the first horizons of the soil may be dissolved or carried away by this water.
If the groundwater is superficial and poorly protected, certain contaminants can therefore theoretically reach groundwater.
This risk increases in particular when:
- the aquifer is close to the surface;
- soils and geological formations are very permeable;
- precipitation after the fire is significant ;
- the fire was very intense;
- a large proportion of the watershed burned;
- buildings, vehicles or installations containing chemical substances were also destroyed.
Conversely, a deep aquifer protected by several poorly permeable geological layers is generally less directly exposed to pollution coming from the surface.
This risk for aquifers is not only theoretical
The EPA studied public drinking water networks supplied by both surface water and groundwater after fires.
The researchers observed that systems using underground resources could also present, after certain fires, significant increases in several parameters studied, notably nitrates and arsenic.
This demonstrates that an impact on underground resources is possible.
However, we must be careful in the interpretation: these studies were carried out mainly in the United States and do not absolutely prove that the water tables of Gironde or Landes are currently contaminated.
They show a possible mechanism and justify appropriate monitoring.
Why do Gironde and Landes deserve particular attention ?
The geology of the Aquitaine basin is complex and includes several superimposed layers, some deep and protected, others much more superficial.
In the Landes de Gascogne sector, the BRGM describes in particular free aquifers from the Plio-Quaternary associated with sandy formations. Hydrogeological studies also show important relationships between these surface aquifers and the watercourses that drain them.
This does not allow us to conclude that a fire like that of Saumos will necessarily contaminate the aquifer.
On the other hand, the combination of immense burned surfaces, sandy formations, interconnected surface and groundwater and future precipitation scientifically justifies monitoring the water quality in the sectors concerned.
This is a conclusion of hydrogeological prudence, and not the observation of pollution already measured.
Surface waters are generally the first concerned
It is important to distinguish two phenomena.
For rivers, streams, lakes and retained, the impact can be relatively rapid: the ashes can fall directly into the water and the rains can then carry the burned materials towards watercourses.
For an underground water table, the transfer is generally more complex. Water and contaminants must pass through the soil and geological formations before reaching the aquifer.
Consequences may therefore be delayed in time.
The USGS indicates that the effects of fires on the quality of water resources can persist for several months and, in some situations, several years, particularly during heavy rainfall in burned watersheds.
Should we be concerned about drinking water in Gironde today?
As of the date of writing this article, August 9, 2026, we have not identified any official communication establishing that the recent fires in Gironde or Landes have caused widespread contamination of groundwater or drinking water.
It would therefore be incorrect to state that "Gironde water is contaminated".
On the other hand, scientific knowledge clearly shows that after fires of this magnitude, the quality of surface water and, in certain hydrogeological configurations, that of groundwater must be the subject of particular attention.
Analyses carried out by health authorities and water managers are the only ones able to determine whether abstraction specific has really been affected.
Pollution which can appear after the fire
Extinguishing the fire does not necessarily mean the end of its influence on the environment.
There are in reality several stages:
During the fire: smoke, dust and ashes can be deposited directly in the environment.
After the fire: the soils remain bare, weakened and exposed to erosion.
During the first rains: ash, particles and soluble substances can be carried towards ditches, rivers, lakes and infiltration areas.
During the following months : certain effects can continue to be observed at the rate of precipitation and the recovery of vegetation.
It is therefore sometimes after the fire that the most interesting period to monitor from the point of view of water quality begins.
What we can say today
The available knowledge makes it possible to clearly distinguish what is established from what remains to be verified locally.
It is established that:
- large fires can permanently modify the soil, increase runoff and erosion and degrade the quality of water located downstream;
- des ashes, sediments, nutrients, metals and organic compounds can be mobilized after a fire;
- groundwater resources can, in certain circumstances, be affected after fires.
On the other hand, we cannot say today that the water tables of Gironde or Landes were contaminated by the fires of July 2026 without having analyzes demonstrating this contamination.
Conclusion
The gigantic fires of the summer of 2026 remind us that the consequences of a forest fire do not stop when the flames disappear.
The forest participates in the functioning of the water cycle: it protects the soil, slows down runoff and intervenes in the exchanges between precipitation, soil, rivers and water underground.
When a considerable forest area burns, this balance is suddenly modified.
The first resources exposed are generally surface water, particularly during the rains following the fire. But scientific research also shows that an impact on certain underground resources is possible.
For Gironde and Landes, it will therefore be necessary above all to observe what will happen in the months following the fires: precipitation, runoff, quality of watercourses and results of catchment controls.
The right approach is neither to assert that there is no risk, nor to claim that the water tables are already polluted.
The risk exists. Its real importance can only be determined by monitoring and analyzing the water.
Main sources
Ministry of the Interior, “On land and in the air: fighting fire on all fronts”, published on July 24 and updated on July 31, 2026. National report of fires and situation in Gironde.
Ministry of Ecological Transition, August 6, 2026, “Fires in Gironde and Landes: recognition of a large-scale disaster for the affected forests”.
Var Prefecture, August 2026, situation updates on the Gros Bessillon fire.
U.S. Environmental Protection Agency (EPA), “Wildfires and Water Quality Research,” last updated March 18, 2026. Research on the impacts of wildfires on surface water, groundwater, and drinking water.
U.S. Geological Survey (USGS), “Water Quality after a Wildfire.” Summary of the effects of fires on erosion, sediments, nutrients, metals and water resources.
BRGM, hydrogeological work on the aquifers of the Aquitaine basin and the Plio-Quaternary system of the Landes de Gascogne.
