Forests are our most important source of high-quality water. In the southwestern United States, approximately half of the drinking water is supplied by forests, while fully 80 percent of freshwater in the rest of the country originates on forested land. More than 3,400 public drinking water systems are located in watersheds containing national forest lands (USDA, 2006).
Forested land – this vital water resource, is subject to catastrophic and ongoing damage from wildfires. More than 12 million acres of land, including vital forest watersheds, have burned in the southwestern U.S. in the past 30 years. These impacted watersheds have suffered both flooding and erosion, resulting in impaired water supplies.
Unfortunately, wildfires can compromise water quality both during active burning, and for months and years after the fire. During active burning, ash can settle on lakes and reservoirs contaminating drinking water. In addition, storms following wildfires increase erosion of the burned land, promoting downstream accumulation of sediment in streams, rivers, and reservoirs which negatively affects water quality. In short, the potential impacts from past, current, and future wildfires on the quantity and quality of runoff are considerable, and may greatly impact domestic, agricultural, and ecological water supplies.
Flooding and erosion affecting burned watersheds can have both short and long-term impacts on water supplies, such as increased treatment costs, need for alternative supplies, and diminished reservoir capacity (Smith et al 2011). The degree to which wildfire degrades water quality and supply depends on multiple factors, including the extent and intensity of the wildfire, post-wildfire precipitation, watershed topography, and local ecology.
Implications for Drinking-Water Treatment
Drinking-water utilities strive to provide safe drinking water for their communities. Unfortunately, the unpredictable nature of wildfire makes it challenging to develop specific strategies for treating source water degraded by the effects of wildfire. High-intensity rainfall events in steep, burned watersheds are likely to move large amounts of suspended and dissolved material into downstream water supplies.
Potential effects of wildfire on municipal water supplies and downstream aquatic ecosystems include:
- Changes in the magnitude and timing of snowmelt runoff, which influence filling of water-supply reservoirs
- Increased sediment loading of water-supply reservoirs, shortened reservoir lifetime, and increased maintenance costs
- Increased loading of streams with nutrients, as well as dissolved organic carbon, major ions, and metals
- Post-fire erosion and transport of sediment and debris to downstream water-treatment plants, water-supply reservoirs, and aquatic ecosystems
- Increased turbidity (cloudiness caused by suspended material), or heightened iron and manganese concentrations, both of which may increase chemical treatment requirements and produce larger volumes of sludge, raising operating costs
- Changes in source-water chemistry that can alter drinking-water treatment
High-intensity storms can affect water quality for years after a wildfire. Monitoring source water downstream of burned watersheds allows water managers to minimize adverse water-quality effects through such means as temporarily diverting compromised water or changing source water.