Low water treatability efficiency of wildfire-induced dissolved organic matter and disinfection by-product precursors

Biodegradability of unheated and laboratory heated dissolved organic matter

Following wildfires, partially combusted biomass remains on the forest floor and erosion from the landscape can release dissolved pyrogenic organic matter (dPyOM) to surface waters. Therefore, post-fire alterations to dissolved organic matter (DOM) in aquatic systems may play a vital role in DOM stability and biogeochemical cycles. Dissolved PyOM biodegradation remains poorly understood and is expected to vary with combustion temperature and fuel source. In this study laboratory heating and leaching of forest floor materials (soil and litter) were used to compare the biodegradability of unheated, low (250 °C), and moderate (450 °C) temperature leachates. Inoculation experiments were performed with river microbes. Dissolved organic carbon (DOC) and nitrogen (DON), inorganic nitrogen, and DOM optical properties were monitored for 38 days. Inoculation experiments showed significantly greater DOC biodegradation of low and moderate temperature samples (64% and 71%, respectively) compared to unheated samples (32%). The greater DOC biodegradation may be explained by lower molecular weight DOM composition of heated leachates which was supported by higher initial E2/E3 ratios (absorbance at 250 nm/365 nm). Further, the observed decrease in the E2/E3 ratio after incubation suggests biodegradation of smaller compounds. This trend was greater for heated samples than unheated DOM. Specific ultraviolet absorbance increased after incubation, suggesting biodegradation of aliphatic compounds. Inoculated moderate temperature samples showed the greatest DON degradation (74%), followed by low temperature (58%) and unheated (51%) samples. Overall, results suggest that low and moderate temperature dPyOM was more biodegradable than unheated DOM, which may have implications for aquatic biogeochemical cycling, ecosystem function, and water quality in fire-impacted watersheds.

Temperature Thresholds of Pyrogenic Dissolved Organic Matter in Heating Experiments Simulating Forest Fires

Heating temperature (HT) during forest fires is a critical factor in regulating the quantity and quality of pyrogenic dissolved organic matter (DOM). However, the temperature thresholds at which maximum amounts of DOM are produced (TTmax) and at which the DOC gain turns into net DOC loss (TT0) remain unidentified on a component-specific basis. Here, based on solid-state 13C nuclear magnetic resonance, absorbance and fluorescence spectroscopies, and Fourier transform ion cyclotron resonance mass spectrometry, we analyzed variations in DOM composition in detritus and soil with HT (150-500 °C) and identified temperature thresholds for components on structural, fluorophoric, and molecular formula levels. TTmax was similar for detritus and soil and ranged between 225 and 250 °C for bulk dissolved organic carbon (DOC) and most DOM components. TT0 was consistently lower in detritus than in soil. Moreover, temperature thresholds differed across the DOM components. As the HT increased, net loss was observed initially in molecular formulas tentatively associated with carbohydrates and aliphatics, then proteins, peptides, and polyphenolics, and ultimately condensed aromatics. Notably, at temperatures lower than TT0, particularly at TTmax, burning increased the DOC quantity and thus might increase labile substrates to fuel soil microbial community. These composition-specific variations of DOM with temperature imply nonlinear and multiple temperature-dependent wildfire impacts on soil organic matter properties.

Wildfire impact on disinfection byproduct precursor loading in mountain streams and rivers

We investigated short (first post-fire precipitation)- and long-term (11-month) impacts of the Caldor and Mosquito Fires (2021 and 2022) on water quality, dissolved organic matter, and disinfection byproduct (DBP) precursors in burned and adjacent unburned watersheds. Both burned watersheds experienced water quality degradation compared to their paired unburned watersheds, including increases in dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and DBP precursors from precipitation events. DBP precursor concentrations during storm events were greater in the Caldor Fire's burned watershed than in the unburned watershed; precursors of trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), and haloacetamides (HAMs) were 533 µg/L, 1,231 µg/L, 64 and 58 µg/L greater. The burned watershed of the Mosquito Fire also had greater median concentrations of THM (44 µg/L), HAA (37 µg/L), HAN (7 µg/L), and HAM (13 µg/L) precursors compared to the unburned watershed during a storm immediately following the fire. Initial flushes from both burned watersheds formed greater concentrations of more toxic DBPs, such as HANs and HAMs. The Caldor Fire burn area experienced a rain-on-snow event shortly after the fire which produced the greatest degradation of water quality of all seasons/precipitation events/watersheds studied. Over the long term, statistical analysis revealed that DOC and DON values in the burned watershed of the Caldor Fire remained higher than the unburned control (0.98 mg C/L and 0.028 mg N/L, respectively). These short and long-term findings indicate that wildfires present potential treatment challenges for public water systems outside of the two studied here.

Building water resilience in the face of cascading wildfire risks

Severe wildfire is altering the natural and the built environment and posing risks to environmental and societal health and well-being, including cascading impacts to water systems and built water infrastructure. Research on wildfire-resilient water systems is growing but not keeping pace with the scale and severity of wildfire impacts, despite their intensifying threat. In this study, we evaluate the state of knowledge regarding wildfire-related hazards to water systems. We propose a holistic framework to assess interactions and feedback loops between water quality, quantity, and infrastructure hazards as determinants of post-fire water availability and access. Efforts to address the evolving threat of wildfires to water systems will require more interdisciplinary research on the complex relationships shaping wildfire's threat to water availability and access. To support this, we need reliable long-term data availability, consistent metrics, greater research in shared contexts, more extensive research beyond the burn area, and multistakeholder collaboration on wildfire risks to water systems.

Effect of fire on characteristics of dissolved organic matter in forested catchments in the Mediterranean biome: A review

Fires in forested catchments pose a water contamination risk from fire-derived dissolved organic matter (DOM). Fire events are expected to increase under a projection of warmer and drier climatic conditions; therefore, understanding the consequences of fire-derived DOM is critical for water supply and management of drinking water and catchments. This paper addresses how fire regime - the intensity, severity and frequency of fires - influences DOM quantity and composition in surface waters in forested catchments, and how long it takes for water quality to recover to pre-fire levels. A review of post-fire studies in Mediterranean regions reporting on DOM related parameters has been conducted. The literature shows that post-fire DOM composition and reactivity is different from DOM generated under processes of biological degradation, and hence our reliance on DOM 'bulk properties' and surrogate DOM bulk parameters may not provide sufficient information to deal with the potential complexity of the organic compounds produced by a catchment fire. Appropriate measures are important to adequately operate conventional water treatment facilities, for example. Critical parameters for the effects of burning include the alteration of DOM composition, aromaticity, and the relative amounts of labile/recalcitrant organic components. The literature shows mixed information for the influence of both burn severity and fire intensity, on these parameters, which indicates DOM response to fire is highly variable. For fire frequency, the evidence is more unequivocal, indicating that frequent fires change the composition of DOM to components that are less bioavailable, and elevate the degree of aromaticity, which may be detrimental to water quality. In addition, and in general terms, the more recent the fire, the more aromatic and humified DOM components are found, and vice versa. The recovery of surface water quality to pre-fire conditions was variable, with no safe temporal thresholds suggested in the literature. In some cases, fire-induced changes in DOM composition were observable up to 16 years post-fire. The lack of clearly observed trends in post-fire DOM with fire regimes could be attributed to numerous factors such as limited long-term and event-based observations, experimental design challenges, and site-specific biological, physical and hydrological factors. The application of terminologies used to describe fire regimes such as burn severity and fire intensity also creates challenges in comparing the outcomes and results from numerous studies.

Emerging investigator series: physicochemical properties of wildfire ash and implications for particle stability in surface waters

The erosion of wildfire ash from the forest floor to nearby surface waters presents a concern due to potential contamination and alteration of water quality. Meanwhile, the properties of wildfire ash that drive ash particle stability in aquatic systems, mobilization downstream, and transport of contaminants are not well known. Physicochemical properties of ash samples from three wildfires were characterized to understand the relation of ash color and combustion completeness with particle stability and mobilization in aquatic systems. Generally, lighter colored ash, indicative of greater combustion temperatures, had higher pH, electrical conductivity, specific surface area, and zeta potential, and smaller particle size than darker ash and unburned soils. Zeta potential was used as an indication of particle surface charge. White ash had the greatest mean zeta potential (-31.8 ± -11.5 mV), followed by gray ash and dark gray ash. Black ash had similar zeta potential to unburned soils. However, with adjustment to the same pH range the ash and unburned soils had similar mean zeta potentials, although lighter ashes had high variability. Dark gray ash leached the highest organic carbon and nitrogen while white ash leached the lowest C and N, similar to unburned soils. The results suggest that high combustion temperature wildfire ash particles will have greater potential for mobilization downstream and may be more stable in both natural and engineered water systems. However, the high organic matter released from dark gray ashes will likely increase particle stability through steric repulsion. More stable particles have greater potential for downstream transport to aquatic ecosystems or water supplies and increase the possibility of post-fire contamination from ash.

Challenges in quantifying and characterizing dissolved organic carbon: Sampling, isolation, storage, and analysis

Despite the advancements in analytical techniques, there are still great challenges and difficulties in accurately and effectively quantifying and characterizing dissolved organic carbon (DOC) in environmental samples. The objectives of this review paper are (a) to understand the roles and variability of DOC along the water continuum; (b) to identify the constraints, inconsistences, limitations, and artifacts in DOC characterization; and (c) to provide recommendations and remarks to improve the analytical accuracy. For the first objective, we summarize the four ecological and engineering roles of DOC along the water continuum from source water to municipal utility, including nutrients and energy sources, controlling the fates of micropollutants, buffering capacity, and treatability and precursors of disinfection byproducts. We also discuss three major challenges in DOC analysis, including spatial and temporal variations, degradability and stability, and unknown structures and formulas. For the second objective, we review the procedures and steps in DOC analysis, including sampling in diverse environmental matrices, isolation of DOC fraction, storage and preservation techniques, and analyses on bulk chemical characteristics. We list and discuss the available options and evaluate the advantages and disadvantages of each choice. Last, we provide recommendations and remarks for each stage: sampling, isolation, storage, and analysis.

Insight into disinfection byproduct formation potential of aged biochar and its effects during chlorination

Biochar can achieve multiple benefits including solid waste management, polluted water remediation, carbon sequestration, and emission reduction. However, various environmental factors (such as temperature variations and dry-wet alternation) and microbial activity may lead to the fragmentation, dissolution, and oxidation of biochar. These accelerate the dissolution of biochar-derived dissolved organic matter (DOM) and then influence disinfection byproducts formation potential (DBPFP) throughout the water treatment process. In this paper, biochars from six biomass feedstocks with five aging processes were prepared, and the DBPFP of biochar and its derived DOM were first studied systematically. Different aging processes might increase the DBPFP of biochar by increasing DOM content and changing the fraction distribution of DOM derived from biochar. Especially, the DBPFP of biochar increased apparently with the chemical aging process. Coexisting with the environmental concentration of humic acid, even aged biochar showed the potential to reduce DBPFP and integrated toxic risk value of the mixed system. In this study, the DBPFP of biochar-derived DOM during the disinfection process is confirmed, and the results can give information to the selection of biomass feedstocks of biochar and its service life in the water treatment process.

Clair SI, Bratsman S, Merritt M, Norris AJ, Carling GT, Hansen N, St. Clair SB, Abbott BW. Megafire affects stream sediment flux and dissolved organic matter reactivity, but land use dominates nutrient dynamics in semiarid watersheds

Climate change is causing larger wildfires and more extreme precipitation events in many regions. As these ecological disturbances increasingly coincide, they alter lateral fluxes of sediment, organic matter, and nutrients. Here, we report the stream chemistry response of watersheds in a semiarid region of Utah (USA) that were affected by a megafire followed by an extreme precipitation event in October 2018. We analyzed daily to hourly water samples at 10 stream locations from before the storm event until three weeks after its conclusion for suspended sediment, solute and nutrient concentrations, water isotopes, and dissolved organic matter concentration, optical properties, and reactivity. The megafire caused a ~2,000-fold increase in sediment flux and a ~6,000-fold increase in particulate carbon and nitrogen flux over the course of the storm. Unexpectedly, dissolved organic carbon (DOC) concentration was 2.1-fold higher in burned watersheds, despite the decreased organic matter from the fire. DOC from burned watersheds was 1.3-fold more biodegradable and 2.0-fold more photodegradable than in unburned watersheds based on 28-day dark and light incubations. Regardless of burn status, nutrient concentrations were higher in watersheds with greater urban and agricultural land use. Likewise, human land use had a greater effect than megafire on apparent hydrological residence time, with rapid stormwater signals in urban and agricultural areas but a gradual stormwater pulse in areas without direct human influence. These findings highlight how megafires and intense rainfall increase short-term particulate flux and alter organic matter concentration and characteristics. However, in contrast with previous research, which has largely focused on burned-unburned comparisons in pristine watersheds, we found that direct human influence exerted a primary control on nutrient status. Reducing anthropogenic nutrient sources could therefore increase socioecological resilience of surface water networks to changing wildfire regimes.

Characterization of Dissolved Organic Matter from Wildfire-induced Microcystis aeruginosa Blooms controlled by Copper Sulfate as Disinfection Byproduct Precursors Using APPI(-) and ESI(-) FT-ICR MS

Copper-based algaecides are usually used for controlling algae bloom triggered by the elevated levels of nutrients after wildfires, resulting in the promoted reactivity of dissolved organic matter (DOM) in forming disinfectant byproducts (DBPs). To identify the best strategy for handling this source water, we employed Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize the DBPs precursors after 4-d Microcystis aeruginosa bloom cultured with black (BE) and white (WE) ash water extracts under 0, 0.5, and 1.0 mg-Cu/L. The disappeared DOM during disinfections, primarily composed of O_1-14, N₁O_1-14 and N₂O_1-14, had a higher average molecular weight (MW) and double-bond equivalent (DBE), relative to DOM after incubation, regardless of disinfects and Cu²⁺. This result suggests assigned features with larger MW and more double bonds/rings as preferable DBP precursors. We observed a larger number of disappeared assigned features with low DBE of 1-10 in control without Cu²⁺ addition, possibly explaining lower DOM chlorine reactivity in forming carbonaceous and oxygenated DBPs, relative to the treatments with Cu²⁺ addition. We found a larger number of O_1-14 and N₁O_1-14 with DBE=5-16 in the treatments, potentially explaining higher DOM chloramine reactivity in forming N-nitrosodimethylamine (NDMA), compared to the control. Our study suggests removing oxygen- and nitrogen-containing organic compounds with more double bonds/aromatic rings as a preferable strategy for handling source water after controlling post-fire algae blooms with copper sulfate.