Two years of post-wildfire impacts on dissolved organic matter, nitrogen, and precursors of disinfection by-products in California stream waters

Abstract

We investigated the effects of two California wildfires (Rocky and Wragg Fires, 2015) compared to an unburned reference watershed on water quality, dissolved organic matter (DOM), and precursors of disinfection by-products (DBPs) for two years' post-fire. The two burned watersheds both experienced wildfires but differed in the proportion of burned watershed areas. Burned watersheds showed rapid water quality degradation from elevated levels of turbidity, color, and suspended solids, with greater degradation in the more extensively burned watershed. During the first year's initial flushes, concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), ammonium (NH 4 þ /NH 3), and specific ultraviolet absorbance (SUVA 254) were significantly higher (67 ± 40%, 418 ± 125%, 192 ± 120%, and 31 ± 17%, respectively) in the more extensively burned watershed compared to the reference watershed. These elevated values gradually declined and finally returned to levels like the reference watershed in the second year. Nitrate concentrations were near detection limits (0.01 mg-N/L) in the first year but showed a large increase in fire-impacted streams during the second rainy season, possibly due to delayed nitrification. Changes in DOM composition, especially during the initial storm events, indicated that fires can attenuate humic-like and soluble microbial by-product-like (SMP) DOM while increasing the proportion of fulvic-like, tryptophan-like, and tyrosine-like compounds. Elevated bromide (Br À) concentrations (up to 8.7 mM]) caused a shift in speciation of trihalomethanes (THMs) and haloacetic acids (HAAs) to brominated species for extended periods (up to 2 years). Wildfire also resulted in elevated concentrations of N-nitrosodimethylamine (NDMA) precursors. Such changes in THM, HAA, and NDMA precursors following wildfires pose a potential treatability challenge for drinking water treatment, but the effects are relatively short-term (1 year).

Turbidity and fecal indicator bacteria in recreational marine waters increase following the 2018 Woolsey Fire

“…While the effects of wildfire on turbidity in ocean ecosystems have not been previously documented, investigations on the effects of wildfire on turbidity in stream ecosystems have associated heightened turbidity levels to burn severity 29 and precipitation events 23 , 25 – 28 , 30 , 31 . In addition, when compared to plumes from urban stormwater runoff in the Santa Monica Bay 50 , 51 , plumes from burned area surface runoff were greater.…”

“…Due to slow watershed recovery, post-fire sediment yields can remain elevated for 3 months to 10 years 19 . Accordingly, studies have reported short-term 23 , 24 , 26 , 28 , 30 and long-term 25 , 27 , 29 , 31 turbidity increases in streams post-fire. The current study did observe short-term increases in turbidity, within 3 months of the Woolsey Fire.…”

“…Wildfires have been shown to alter the physical and chemical water quality of receiving streams. For example, elevated turbidity levels in stream waters post-wildfire are well documented 23 – 31 . However, wildfire impacts on microbial water quality remain elusive 32 .…”

Impact of simulated wildfire on disinfection byproduct formation potential

“…In addition to the loss of human life and property, wildfires can degrade water quality, thus threatening the supply of safe drinking water (Hohner et al, 2016;Uzun et al, 2020;Writer et al, 2014). In 2018, the 5-and 10-year average for acres burned in the United States was exceeded, spurring concern and skepticism over the future of wildfire's anthropogenic and ecological influence (Doerr & Santín, 2016;National Interagency Fire Center, 2018).…”

“…However, oftentimes, it is difficult to separate the effects of a wildfire from routine hydrologic events (Mast et al, 2016). Similar to DOC, post fire dissolved organic nitrogen (DON) trends are also largely variable, with some studies reporting little to no change and others observing increased DON concentrations in surface waters from burned catchments (Bladon et al, 2008;Santos et al, 2019;Uzun et al, 2020). Detritus is mineralized as a result of wildfire heating, effectively lowering the elemental carbon and nitrogen content available for transport.…”