Fire impacts on river water chemistry, microbial diversity, and watershed hydrological connectivity

Michelle Newcomer, Susan Hubbard and Craig Ulrich, Lawrence Berkeley National Laboratory, Jennifer Underwood, Ronald Harvey, Hannah Rigoni, Jeramy Jasmann, Sheila Murphy, David Roth and Paul Bliznik, US Geological Survey, Todd Schram, Marcus Trotta, Jay Jasperse and Donald Seymour, Sonoma Water

Background/Question/Methods
Wildfires are a natural yet changing component of many landscapes around the world. From 2017-2020, a series of multiple fires devastated many forested, agricultural, and urban areas within Sonoma, Mendocino and Napa counties, California, U.S.A, and across the Russian River Watershed. The compounding burn areas provide significant sources for ash-leachate runoff flowing to the Russian River, Lake Mendocino, Lake Sonoma, and nearby tributaries during subsequent rainfall events. In addition, leachate infiltrate to local groundwater basins supports groundwater discharge to the Russian River during the dry season. Beginning in 2017, we developed a long-term monitoring program across three partner agencies: Sonoma Water, Lawrence Berkeley National Laboratory, and the U.S. Geological Survey to address two main knowledge gaps within fire research: 1) how compounding fire events impact hydrological, chemical, and microbial conditions of the river and watershed, and 2) how post-fire storm events impact hydrological, chemical, and microbial connectivity across the watershed including the tributaries, and main stem. Our work will discuss the statistical changes observed relative to historical baseline data gathered by a diverse number of agencies. We developed extensive watershed and rapid-response monitoring protocols using standard and novel assessment metrics including dissolved organic carbon (DOC), total dissolved nitrogen (TDN), total and dissolved trace metals, total and dissolved mercury, major anions, major cations, and DNA for microbial community abundance and structure.
Results/Conclusions
Our work reveals that changes include increased loading of sediments, trace metals (Fe, Mn, Hg, Cr), and DOC, and more basic pH conditions during storm events in the first post wildfire water year. Analysis of water DNA indicated an increase in the Shannon Diversity Index in burned tributaries, indicating that wildfire may play an important role in increasing and restructuring instream aquatic and riparian biodiversity. We find that trace metals emerge in the stream one to two years post-fire. These, and other important findings, will be summarized into pre-and-post fire monitoring considerations, strategies, and needs at local and state levels. We will present a framework for successful inter-agency collaboration to help with understanding fire impacts on watershed processes and develop tools to monitor and assess drinking water sources.