PNNL

The Science                                

In many regions, such as in the Pacific Northwest, wildfires are increasing in frequency and intensity. Rain events after wildfires mobilize large amounts of charred material from the ground and transport these materials to local water bodies after a fire. Wildfires therefore can affect local water chemistry, but the in-stream responses can be variable. To predict how more frequent and intense fires affect water chemistry, researchers must first understand what controls in-stream responses after a fire. In this study, differences in the dissolved materials delivered to the streams were studied to determine how severely the fire burned on the landscape. Short-term differences in the dissolved organic matter in streams after wildfire and rain events were found to be directly related to the severity of the wildfire.

The Impact

Most studies capture longer-term impacts (months to years) of wildfires on aquatic ecosystems, missing the immediate in-stream responses. Findings from this study captured short-term water chemistry variability within a single burn perimeter during the first major storm event post-fire, addressing a key knowledge gap in scientific understanding of post-fire ecosystem responses. In-stream responses of the organic matter content are highly likely to evolve as the ecosystem and streams continue to recover post-fire.

Summary

After a wildfire, the transport of charred material to local water bodies after rain events raises major concerns for how increasing fire activity in the Pacific Northwest will affect aquatic ecosystems. A multi-institutional team of scientists collected water samples at 1-hour intervals for 24 hours from multiple streams affected by the 2020 Holiday Farm Fire that burned 700 square kilometers of forested hillslopes directly upstream of Eugene, Oregon. The team characterized these water samples for their dissolved organic matter chemistry using advanced capabilities available from EMSL, the Environmental Molecular Sciences Laboratory, a Department of Energy user facility.

Differences in the dissolved organic matter chemistry delivered to the streams were directly related to how severely the fire burned in the surrounding landscapes. Specifically, the type of organic matter found in streams after rain events was more light absorbing and nitrogen-rich in more severely burned areas than in areas where fire temperatures were lower. These differences in the type of dissolved organic matter found in streams could alter microbial activity in those streams, which could have implications for aquatic ecosystem functions and subsequent drinking water treatment processes. The ability to capture short-term water chemistry responses post-fire will improve the ability of researchers to predict how stream chemistry will respond to more frequent and intense fires.

PNNL Contact

Allison Myers-Pigg, Pacific Northwest National Laboratory, Allison.Myers-Pigg@pnnl.gov 

Funding

This research was supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program, Environmental System Science program through the River Corridor Science Focus Area project. A portion of this research was performed using capabilities at the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility.