How wildfire affects drinking water

Fire changes a catchment’s hydrology as much as its ecology. The water quality problem usually arrives with the first heavy rain after the fire, not during it.

The timing: The fire itself rarely contaminates supply. The damage arrives with the first intense rainfall on a burned, bare, water-repellent catchment — often months later.
Organic carbon: Ash and burned vegetation raise dissolved organic carbon, which reacts with chlorine to form more disinfection by-products.
Duration: Elevated sediment and nutrient delivery can persist for several years until vegetation re-establishes.

Text description

A wildfire burns vegetation in a catchment, leaving bare soil, ash and in some cases a water-repellent hydrophobic soil layer. The first heavy rain after the fire produces greatly increased runoff carrying ash, sediment, nutrients and metals into the reservoir. This raises turbidity, organic carbon, nutrients and manganese, which in turn increases disinfection by-product formation, overwhelms filtration, and can force a treatment works offline.