Wildfire and drinking water
A wildfire rarely contaminates a water supply while it burns. The damage arrives with the first heavy rain on a burned catchment, often months later.
Wildfire is increasingly recognised as a drinking water problem rather than only an air quality and land management one. Where a fire burns a water supply catchment, the consequences for the treatment works downstream can last years.
What fire does to a catchment
- Removes the vegetation that intercepted rainfall and held soil in place
- Leaves a layer of ash rich in nutrients, carbon and metals
- Creates a water-repellent hydrophobic soil layer where intense heat volatilised organic compounds that then condensed below the surface
- Destroys the root structure that gave the soil cohesion
The combination is severe. Rain that would previously have infiltrated now runs straight off a bare, water-repellent surface, mobilising ash and soil. Post-fire runoff and erosion can exceed pre-fire levels by more than an order of magnitude, and debris flows are common on steep burned slopes.
What arrives at the reservoir
| What increases | Consequence for treatment |
|---|---|
| Turbidity and suspended sediment | Can exceed intake or filtration capacity, forcing a works offline |
| Dissolved organic carbon | Reacts with chlorine to form more disinfection by-products |
| Nitrogen and phosphorus | Fuels algal blooms in the reservoir, including cyanobacteria |
| Manganese and iron | Discoloured water and, for manganese, a health parameter |
| Sedimentation in the reservoir | Permanently reduces storage capacity |
Denver Water’s experience after the Buffalo Creek and Hayman fires is the most documented case: sediment delivery to Strontia Springs Reservoir required dredging costing tens of millions of dollars, and elevated treatment costs persisted for years. Several utilities now invest directly in catchment fuel reduction on the basis that it is cheaper than the treatment it avoids.
Sources
- Copernicus Atmosphere Monitoring Service — Global Fire Assimilation System. Open — attribution required · Copernicus licence
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted