Wildfire impacts on surface water quality parameters: Cause of data variability and reporting needs

Assessing the leaching of nutrients and trace metals from wildfire ashes and heated soils through soil column tests: Potential impacts on groundwater

Wildfires are a significant diffuse source of pollution to water systems. Ash deposited after forest fires contains hazardous contaminants that can leach into groundwater, posing risks to water quality. While wildfire effects on surface water are well-documented, the processes and risks of contaminant leaching into groundwater remain poorly understood. This study evaluated the leaching behavior of nutrients (N and P) and trace metals (V, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) in ash-soil systems using soil column tests with two soil types (granite [SG] vs. schist [SX]) and two heating temperatures (150 °C vs. 500 °C). Significant differences in leaching were observed based on soil type, fire severity, and ash deposition. Ash addition increased leachate concentrations of N, P, Mn, Zn, Co, and V, with Mn often exceeding drinking water standards. In general, SG soils exhibited greater contaminant mobilization, except for N, Cd, V, and Zn, which were found in higher concentrations in SX soil leachates. Higher temperatures decreased soil organic matter and altered pH and conductivity. These findings highlight the critical role of soil texture, heating, and ash deposition in post-fire contaminant mobility and underscore the urgent need for targeted management strategies to protect groundwater in wildfire-prone regions.

Modeling Wildfire Effects on Ecosystem Services in two Disparate California Watersheds and Communities

Ecosystem services are important for human well-being and for sustaining environmental quality objectives. Growing concern over extreme wildfire events in various watersheds necessitates understanding their impacts on regulating ecosystems services. Past studies have documented how wildfires regulate ecosystem services, but the distributional impacts of such ecosystem services across various human settlements (i.e. communities) remains understudied, despite renewed focus on how they are increasingly at risk from and being impacted by wildfires. We used the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) model to examine how two wildfires that occurred in California, USA in 2017 impacted water provisioning, soil loss and sediment delivery, carbon sequestration services, and nutrient delivery in two watersheds and their respective communities. Regression analyses were used to determine the differences in the distribution of ecosystem services before and after the fires, and whether these wildfires exacerbated the differences in impacts to ecosystem services across communities in the watershed. We find that a year following the fires, the amount of biomass in forestland, woodland, and chaparral declined in both studied watersheds, while the amount of grassland increased. The model revealed that the changes in vegetation resulted in losing about 200,000 tons of carbon from the Mark West subwatershed and about 160,000 tons of carbon from the southern California watersheds. The expected mean annual water yield for both watersheds increased by 5% and 42%, respectively post-fire. Expected post-fire phosphorus and nitrogen export also increased. Finally, we found evidence of human community-level differences in the distribution of pre-fire ecosystem services but no evidence that post-fire conditions either exacerbated or alleviated these impacts.

Wildfires in Alaskan Boreal Forests Release Elevated Levels of Polyaromatic Hydrocarbon

Despite the increasing intensity of wildfires in Alaska's extensive and flammable boreal forests, there remains a critical gap in understanding their environmental impacts, particularly on water systems. This study examines water, soil, sediment, and ash samples from the North and Middle Fork regions of the upper Chena River watershed, which faces recurring annual wildfires. Polyaromatic hydrocarbon (∑PAH) concentrations range from 41.1 to 1455.3 ng/L in water and 14.5 to 63.2 μg/kg in sediments, topsoil, and ash. Water primarily contains phenanthrene and trace levels of pyrene and fluorene, while solid samples exhibit elevated levels of naphthalene and phenanthrene, along with trace amounts of acenaphthene, anthracene, and fluorene. Runoff and terrain slope patterns derived from topographical analyses of the Chena River watershed might explain some of these elevated PAH levels, which surpass those found in other wildfire-affected watersheds worldwide. These findings underscore the risks to aquatic ecosystems, as PAHs are toxic, bioaccumulative, and can lead to the formation of harmful disinfection byproducts. Thus, the increasing frequency of wildfires in Alaska poses a significant threat to local communities relying on wildfire-impacted freshwater for fishing and drinking.

Timing since deforestation for pastures implementation in the western Amazon: Impacts on stream water biogeochemistry

Water quality degradation is a global concern and land use changes is one of the main causes of water pollution globally. In the Brazilian Amazon, large-scale deforestation is mostly associated to pasturelands, with strong implications for soils and stream waters biogeochemistry and resulting in environmental degradation. Studies addressing soil biogeochemistry in Amazonian pastures of different ages documented a consistent pattern of elements peaking on the first years after deforestation, followed by a continuous decrease until reach depletion in older, degraded pastures. The same pattern is expected on the adjacent stream water biogeochemistry due to the land-water connection; however, this issue has not being explored. We assessed how timing since deforestation impacts water biogeochemistry in streams of Rondônia State, Brazil, a region that showed a relatively fast colonization process associated to large-scale deforestation for pasture implementation since 1970. Mapping deforestation from 1984 to 2011 and collecting water samples in 2012 for 41 headwater catchments, we (i) compared water biogeochemistry among streams draining undisturbed catchments with primary forests (PRI) and streams draining pasture catchments deforested mostly in a recent (NEW) and in a distant past (OLD); and (ii) related water biogeochemistry with land use intensity (pasture permanency along years), also considering the effect of covariates (soils, forest cover, and catchment area) in our analysis. Our results indicate that pasture implementation altered the concentrations of ions and nutrients in the stream ecosystem, with distinct water quality impacts between newer and older pastures. In general, the longer the area stayed under pasture use, the larger were the alterations in the water biogeochemistry (lower NO3-, DOC, and DOX and higher Ca2+, Mg2+, DIC and COND). Pasture degradation is a widespread process in the Amazon and it is strongly associated to increasing deforestation. Thus, pasture reclamation and intensification is urgently necessary to achieve the long-term productivity and sustainability of Amazonian pasturelands, avoiding new deforestation and environmental degradation. Adopting Best Management Practices (riparian forested buffers) is another action to protect water resources in the region.

Pollution and toxicity of heavy metals in wildfires-affected soil and surface water: A review and meta-analysis

Wildfires, both natural and man-made, release and mobilize hazardous substances such as heavy metal(loids) (HM), which are known carcinogens. Following intense rainfall events, HM bound to soil organic matter are transported from the soil to surface water, resulting in water quality degradation. This study reviews the pollution status of HM in wildfire-affected soil and surface water, as well as their toxic effects on aquatic organisms and humans. The rate of HM release during wildfires depends on factors such as the type of tree burned and fire severity. The mobility of HM from soil to surface water is influenced by soil pH, organic matter content, rainfall intensity, and duration. The risk priority number (RPN) analysis indicates that both wildfire-affected soil and surface water require remediation to address HM contamination. HM concentrations in both soil and surface water decrease over time due to soil erosion, wind, storm events, and the depletion of burnt residues. The greatest percentage changes in HM concentrations in burned soils compared to unburned soils were observed for vanadium (340%), nickel (260%), and arsenic (110%). In surface water, the highest increases were seen for iron (740%), vanadium (530%), and aluminium (510%). Wildfire-affected water has been shown to cause toxic effects in aquatic organisms, including DNA damage, oxidative stress, and lipid peroxidation. The consumption of HM-contaminated water and fish poses significant health risks to humans. Therefore, post-fire monitoring of wildfire-affected areas is essential for designing treatment plants, assessing risks, and establishing maximum allowable HM concentrations in water.

Heavy metal(loid) contamination in forest fire affected soil and surface water: pollution indices and human health risk assessment

Forest fires, whether natural or anthropogenic, release and mobilize heavy metal(loids) (HM). Following intense rainfall events, soil-bound HM are transported from soil to surface water through surface runoff, leading to water quality deterioration. Pollution and ecological risk indices are effective tools for assessing HM contamination. Most forest fire-affected soils and surface water exhibited a degree of contamination greater than 3 and 8 (high and moderate pollution), with associated high and extremely high ecological risks (165 and 2389, respectively). Pollution indices revealed that soils were highly contaminated with Ni, Cu, Cr, and Pb, while Ni, Cu, Hg, Cd, and As posed significant ecological risks. Surface water was heavily contaminated with Pb, Mn, Al, and Fe, with Ni and V contributing to extremely high ecological risks. This study highlights that trace HM also requires substantial removal efforts to make water potable, with removal efficiencies needed for Sb (94.49%), Be (85.83%), Ba (70.75%), V (68.19%), and Se (65.51%). Fire-affected surface water poses an elevated cancer risk to both children (0.18 and 4.5 × 10-3) and adults (0.39 and 1.53 × 10-3) through oral and dermal exposure, respectively. Children are more vulnerable to dermal cancer and noncancer risks compared to adults. Low-cost treatment methods, such as the application of immobilizing agents combined with compost, straw mulching, and seeding, can be implemented to control soil erosion in forest areas, thereby reducing the transport of soil-bound HM to surface water. These findings can aid government agencies in developing new soil and water quality standards and implementing effective treatment measures.

Mitigating Wildfire Impact on Water Quality through Climate-Based Financing: A Case Study of the Provo River Watershed

Following wildfires, riverine water quality in forested watersheds is prone to degradation, impacting drinking water treatment and potentially causing increased carbon emissions because of additional electricity consumption during treatment. We explore the potential for climate-based financing to support wildfire mitigation and watershed restoration by reducing potential water treatment energy demand following a fire within the Provo River watershed, Utah, USA. We model pre- and post-wildfire erosion and water quality in the Provo River using GeoWEPP. We use energy data from a water treatment plant in the watershed and literature data to estimate the increase in energy use for treating degraded water. We find that most watershed areas are not subject to large treatment demand changes, but a few hotspots are prone to increased sediment loads. In the Provo River watershed, on average, a fire in a single 12-digit hydrologic unit code (HUC) subwatershed corresponds to an additional 350 metric tonnes of carbon-dioxide-equivalent (CO2e) emissions for one year following a wildfire event due to increased energy required by the water treatment plant. If wildfire risk is reduced, the avoided emissions can generate a potential of $88,500 annually in carbon credit revenue (at $10/CO2e credit) for the contributing HUC8 sub-basin.

Effects of wildfire ash on the fatty acid and sugar profiles of bivalves - A comparative study of a freshwater and a marine species

Wildfires can impact both freshwater and marine ecosystems through post-fire runoff, but its effects on bivalves, particularly those living in marine habitats, remain largely overlooked. While evidence exists that wildfire ash can alter the fatty acid (FA) and sugar profiles of aquatic biota, its influence on the biochemical profiles of bivalves have not been addressed to date. This study aimed to assess the effects of ash exposure on the FA and sugar profiles of two bivalve species used for human consumption: a freshwater clam (Corbicula fluminea) and a marine bivalve (Cerastoderma edule), additionally evaluating potential effects on their nutritional value. Both species were exposed to environmentally relevant concentrations of aqueous extracts of Eucalypt ash (AEAs) for 96 h. Results showed species-specific responses to ash extracts exposure, with more pronounced effects on C. edule. This species exhibited a trend for reduced FA content, statistically significant for C17:0 but also evident for unsaturated FAs, which is relevant for human health as they represent a decrease in the nutritional value. Conversely, an increase in the sugar content of this species was observed with increasing AEA concentrations, despite only statistically significant for galactose and xylose. In contrast, the clams exhibited only minor effects, showing a trend for increased FA and decreased sugar contents, but only significant for the monounsaturated FA content. This study suggests a higher sensitivity of marine bivalves to wildfire ash compared to their freshwater counterparts. Moreover, it highlights, for the first time, the potential of post-fire runoff to alter the biochemical profiles of bivalve species, raising concerns about broader impacts on aquatic trophic webs and human health, an issue that becomes particularly relevant given the forecasted increase in wildfire's frequency and extension due to global warming.

The heterogenous molecular characteristics of biomass-pyrogenic smoke dissolved organic matters (BPS-DOMs) binding with PAHs: Novel insights from combined analysis of FT-ICR MS and fluorescence variation

Biomass-pyrogenic smoke dissolved organic matter (BPS-DOM) can co-deposit with polycyclic aromatic hydrocarbons (PAHs), thereby altering their environmental behavior and fate in surface environments. However, the heterogeneous molecular characteristics of BPS-DOM binding with PAHs remain unclear. This study systematically elucidates the binding characteristics of PAHs (phenanthrene and pyrene), with various molecular compositions in BPS-DOM, utilizing FT-ICR MS and fluorescence variation analysis. CHO compounds in BPS-DOM, characterized by high aromaticity and abundant CO bonds, significantly enhance PAHs binding by promoting π-π electron donor-acceptor interactions. In contrast, CHON compounds with higher aliphaticity inhibit pyrene binding by competing for binding sites on BPS-DOM. Furthermore, the binding sequence of different fluorescent molecules follows the order of CHO→CHOS→CHON for phenanthrene and CHO→CHON→CHOS for pyrene. This was primarily due to the larger conjugated aromatic structures of CHO compounds, which provide stronger π-π interaction sites for PAHs binding. The difference in binding sequences between phenanthrene and pyrene is primarily attributed to phenanthrene's reliance on π-π electron donor-acceptor interactions induced by -SO and -N = O, while pyrene binding depended on π-π interactions driven by larger conjugated aromatic structures. These results provide an important theoretical foundation for further understanding the molecular-level interactions between BPS-DOM and PAHs.

Intersection of Wildfire and Legacy Mining Poses Risks to Water Quality

Mining and wildfires are both landscape disturbances that pose elevated and substantial hazards to water supplies and ecosystems due to increased erosion and transport of sediment, metals, and debris to downstream waters. The risk to water supplies may be amplified when these disturbances occur in the same watershed. This work describes mechanisms by which the intersection of mining and wildfire may lead to elevated metal concentrations in downstream waters: (1) conveyance of metal-rich ash and soil to surface waters, (2) increased dissolution and transport of dissolved metals due to direct contact of precipitation with mine waste, (3) increased erosion and transport of metal-rich sediment from mining waste, (4) remobilization of previously deposited metal-contaminated floodplain sediment by higher postfire flood flows, and (5) increased metal transport from underground mine workings. Predicted increases in wildfire size, frequency, and burn severity, together with the ongoing need for metal resources, indicate that improved mapping, monitoring, modeling, and mitigation techniques are needed to manage the geochemical hazard of the intersection of wildfire and mining and implications for water availability.

Impacts of ash-induced environmental alkalinization on fish physiology, and their implications to wildfire-scarred watersheds

Changes in land use, a warming climate and increased drought have amplified wildfire frequency and magnitude globally. Subsequent rainfall in wildfire-scarred watersheds washes ash into aquatic systems, increasing water pH and exposing organisms to environmental alkalinization. In this study, 15 or 20 °C-acclimated Chinook salmon (Oncorhynchus tshawytscha) yearlings were exposed to an environmentally-relevant ash concentration (0.25 % w/v), increasing water pH from ∼8.1 to ∼9.2. Salmon experienced significant disturbance to blood plasma pH (pHe) and red blood cell intracellular pH (RBC pHi) within 1 h, but recovered within 24 h. Impacts on plasma ion concentrations were relatively mild, and plasma glucose increased by 2- to 4-fold at both temperatures. Temperature-specific differences were observed: 20 °C salmon recovered their pHe more rapidly, perhaps facilitated by higher basal metabolism and anaerobic metabolic H+ production. Additionally, 20 °C salmon experienced dramatically greater spikes in plasma total ammonia, [NH3] and [NH4+] after 1 h of exposure that decreased over time, whereas 15 °C salmon experienced a gradual nitrogenous waste accumulation. Despite pHe and RBC pHi recovery and non-lethal nitrogenous waste levels, we observed 20 % and 33 % mortality in 15 and 20 °C treatments within 12 h of exposure, respectively. The mortalities cannot be explained by high water pH alone, nor was it likely to be singularly attributable to a heavy metal or organic compound released from ash input. This demonstrates post-wildfire ash input can induce lethal yet previously unexplored physiological disturbances in fish, and further highlights the complex interaction with warmer temperatures typical of wildfire-scarred landscapes.

Metals in Wildfire Suppressants

Frequent and severe wildfires have led to increased application of fire suppression products (long-term fire retardants, water enhancers, and Class A foams) in the American West. While fire suppressing products used on wildfires must be approved by the U.S. Forest Service, portions of their formulations are trade secrets. Increased metals content in soils and surface waters at the wildland-urban interface has been observed after wildfires but has primarily been attributed to ash deposition or anthropogenic impact from nearby urban areas. In this study, metal concentrations in several fire suppression products (some approved by the U.S. Forest Service, and some marketed for consumer use) were quantified to evaluate whether these products could contribute to increased metal concentrations observed in the environment postfire. Long-term fire retardants contained concentrations of toxic metals (V, Cr, Mn, Cu, As, Cd, Sb, Ba, Tl, and Pb) 4-2,880 times greater than drinking water regulatory limits, and potentially greater than some aquatic toxicity thresholds when released into the environment. Water enhancers and Class A foams contained some metals, but at lower concentrations than fire retardants. Based on these concentrations and retardant application records, we estimate fire retardant application in the U.S. contributed approximately 380,000 kg of toxic metals to the environment between 2009 and 2021.

Revealing the impact of wildfires on groundwater quality: Insights from Sierra de la Culebra (Spain)

Wildfires induce changes in soil and vegetation composition, significantly impacting the hydrological cycle and altering future runoff and infiltration patterns. Ash residue on the ground can infiltrate the subsoil along with water, leading to modifications in groundwater hydrochemistry. Climate change and summer heatwaves can create favourable conditions for severe wildfires, such as the one that occurred in Zamora, Spain, in 2022. Fourteen simultaneous points of origin across various locations in Zamora triggered the worst environmental disaster in this province, as well as the largest fire recorded in the history of Spain. Following the severe wildfires in Sierra de la Culebra, Zamora, groundwater samples were obtained to compare the hydrochemistry with pre-fire background data spanning several years. A general decline in pH across all sampling points was observed, most notably at Z1, likely due to its very high permeability and leaching of organic acids from burned vegetation. Increases in major ions such as SO42- and NO3- were detected at Z1-2, while HCO3- levels decreased, indicating possible oxidation of soil organic matter and the introduction of wildfire-derived organic acids into the groundwater system. Elevated concentrations of Na+, K+, Mg2+, and Ca2+ were observed at Z3, suggesting possible ash residue infiltration. Despite the severity of the wildfires, the results indicate that there were no significant long-lasting impacts on groundwater quality overall. This finding suggests that the groundwater systems in the study area are resilient to such environmental catastrophes.

Wildfires Influence Mercury Transport, Methylation, and Bioaccumulation in Headwater Streams of the Pacific Northwest

The increasing frequency and severity of wildfires are among the most visible impacts of climate change. However, the effects of wildfires on mercury (Hg) transformations and bioaccumulation in stream ecosystems are poorly understood. We sampled soils, water, sediment, in-stream leaf litter, periphyton, and aquatic invertebrates in 36 burned (one-year post fire) and 21 reference headwater streams across the northwestern U.S. to evaluate the effects of wildfire occurrence and severity on total Hg (THg) and methylmercury (MeHg) transport and bioaccumulation. Suspended particulate THg and MeHg concentrations were 89 and 178% greater in burned watersheds compared to unburned watersheds and increased with burn severity, likely associated with increased soil erosion. Concentrations of filter-passing THg were similar in burned and unburned watersheds, but filter-passing MeHg was 51% greater in burned watersheds, and suspended particles in burned watersheds were enriched in MeHg but not THg, suggesting higher MeHg production in burned watersheds. Among invertebrates, MeHg in grazers, filter-feeders, and collectors was 33, 48, and 251% greater in burned watersheds, respectively, but did not differ in shredders or predators. Thus, increasing wildfire frequency and severity may yield increased MeHg production, mobilization, and bioaccumulation in headwaters and increased transport of particulate THg and MeHg to downstream environments.

The influence of recent bushfires on water quality and the operation of water purification systems in regional NSW

Over the past decade, escalating extreme weather events have significantly affected New South Wales (NSW), Australia, with unprecedented droughts and intense fires. Yet, the impact on water quality and purification processes remains insufficiently studied. This research focuses on the immediate changes in NSW's environmental water quality and issues in water purification unit operations following the 2019 bushfires. Water samples and maintenance records from affected catchments, intakes, purification units, and reservoirs were analysed. Compared to control samples, post-bushfire water exhibited high turbidity. Sediment and ash shock loads posed significant threats to aquatic ecosystems. Elevated turbidity, suspended sediments, pH, and alkalinity were major concerns for water purification. Raw water samples showed turbidity exceeding 195 NTU, with flocculation and sedimentation most impacted. Immediate measures included sediment traps, aeration, pre-chlorination, and inline monitoring. These findings inform strategies to mitigate bushfire impacts on water quality and optimise water purification in fire-prone regions.

Fire impacts on the biology of stream ecosystems: A synthesis of current knowledge to guide future research and integrated fire management

Freshwater ecosystems host disproportionately high biodiversity and provide unique ecosystem services, yet they are being degraded at an alarming rate. Fires, which are becoming increasingly frequent and intense due to global change, can affect these ecosystems in many ways, but this relationship is not fully understood. We conducted a systematic review to characterize the literature on the effects of fires on stream ecosystems and found that (1) abiotic indicators were more commonly investigated than biotic ones, (2) most previous research was conducted in North America and in the temperate evergreen forest biome, (3) following a control-impact (CI) or before-after (BA) design, (4) predominantly assessing wildfires as opposed to prescribed fires, (5) in small headwater streams, and (6) with a focus on structural and not functional biological indicators. After quantitatively analyzing previous research, we detected great variability in responses, with increases, decreases, and no changes being reported for most indicators (e.g., macroinvertebrate richness, fish density, algal biomass, and leaf decomposition). We shed light on these seemingly contradicting results by showing that the presence of extreme hydrological post-fire events, the time lag between fire and sampling, and whether the riparian forest burned or not influenced the outcome of previous research. Results suggest that although wildfires and the following hydrological events can have dramatic impacts in the short term, most biological endpoints recover within 5-10 years, and that detrimental effects are minimal in the case of prescribed fires. We also detected that no effects were more often reported by BACI studies than by CI or BA studies, raising the question of whether this research field may be biased by the inherent limitations of CI and BA designs. Finally, we make recommendations to help advance this field of research and guide future integrated fire management that includes the protection of freshwater ecosystems.

Evaluation of historical data on persistent organic pollutants and heavy metals in Lake Baikal: Implications for accumulation in marine environments

Lake Baikal, the largest freshwater lake by volume, provides drinking water and aquatic food supplies to over 2.5 million people. However, the lake has been contaminated with recalcitrant pollutants released from surrounding industrial complexes, agriculture, and natural lands, thereby increasing the risk of their bioaccumulation in fish and seals. Yet, a collective analysis of historical concentration data and their bioaccumulation potential as well as what factors drive their accumulation in fish or seals remains largely unknown. We analyzed concentration data from 42 studies collected between 1985 and 2019 in water, sediment, fish, and seals of Lake Baikal. Heavy metals had the highest concentrations in water and biota followed closely by polycyclic aromatic hydrocarbons (PAHs) and organochlorines. Among organochlorines, polychlorinated biphenyls (PCBs) showed the highest levels in water, surpassing hexachlorocyclohexane (HCH) concentrations, particularly after normalizing to solubility. While naphthalene and phenanthrene exhibited the highest average concentrations among polycyclic aromatic hydrocarbons (PAHs), their relative concentrations significantly decreased upon solubility normalization. The analysis confirmed that bioconcentration and biomagnification of organochlorine pesticides, PCBs, PAHs, and heavy metals depend primarily on source strength to drive their concentration in water and secondarily on their chemical characteristics as evidenced by the higher concentrations of low-solubility PCBs and high molecular weight PAHs in water and sediment. The differential biomagnification patterns of Cu, Hg, and Zn compared to Pb are attributed to their distinct sources and bioavailability, with Cu, Hg, and Zn showing more pronounced biomagnification due to prolonged industrial release, in contrast to the declining Pb levels. Dibenzo-p-dioxins were detected in sediment and seals, but not in water or fish compartments. These data highlight the importance of addressing even low concentrations of organic and inorganic pollutants and the need for more consistent and frequent monitoring to ensure the future usability of this and other similar essential natural resources.

Wildland-urban interface wildfire increases metal contributions to stormwater runoff in Paradise, California

The 2018 Camp Fire was a large late-year (November) wildfire that produced an urban firestorm in the Town of Paradise, California, USA, and destroyed more than 18 000 structures. Runoff from burned wildland areas is known to contain ash, which can transport contaminants including metals into nearby watersheds. However, due to historically infrequent occurrences, the effect of wildland-urban interface (WUI) fires, such as the Camp Fire, on surface water quality has not been well-characterized. Therefore, this study investigated the effects of widespread urban burning on surface water quality in major watersheds of the Camp Fire area. Between November 2018 and May 2019, 140 surface water samples were collected, including baseflow and stormflow, from burned and unburned watersheds with varying extent of urban development. Samples were analyzed for total and filter-passing metals, dissolved organic carbon, major anions, and total suspended solids. Ash and debris from the Camp Fire contributed metals to downstream watersheds via runoff throughout the storm season. Increases in concentration up to 200-fold were found for metals Cr, Cu, Ni, Pb, and Zn in burned watersheds compared to pre-fire values. Total concentrations of Al, Cd, Cu, Pb, and Zn exceeded EPA aquatic habitat acute criteria by up to 16-fold for up to five months after the fire. To assess possible transport mechanisms and bioavailability, a subset of 18 samples was analyzed using four filters with nominal pore sizes ranging from 0.22 to 1.2 μm to determine the particulate size distribution of metals. Trace and major metals (Al, Ba, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn) were found mostly associated with larger grain sizes (>0.45 μm), and some metals (Al, Cr, Fe, and Pb) also included a substantial colloidal phase (0.22 to 0.45 μm). This study suggests that fires in the wildland-urban interface increase metal concentrations, mainly through particulate driven transport. The metals with the largest increases are likely from anthropogenic disaster materials, though biomass ash also is a major contributor to water quality. The increase in metals following WUI burning may have adverse ecological impacts.

Resilience of stormwater biofilters following the deposition of wildfire residues: Implication on downstream water quality management in wildfire-prone regions

Stormwater treatment systems such as biofilters could intercept and remove pollutants from contaminated runoff in wildfire-affected areas, ensuring the protection of water quality downstream. However, the deposition of wildfire residues such as ash and black carbon onto biofilters could potentially impair their stormwater treatment functions. Yet, whether and how wildfire residue deposition could affect biofilter functions is unknown. This study examines the impact of wildfire residue deposition on biofilter infiltration and pollutant removal capacities. Exposure to wildfire residues decreased the infiltration capacity based on the amount of wildfire deposited. Wildfire residues accumulated at the top layer of the biofilter, forming a cake layer, but scraping this layer restored the infiltration capacity. While the deposition of wildfire residues slightly changed the pore water geochemistry, it did not significantly alter the removal of metals and E. coli. Although wildfire residues leached some metals into pore water within the simulated root zone, the leached metals were effectively removed by the compost present in the filter media. Collectively, these results indicate that biofilters downstream of wildfire-prone areas could remain resilient or functional and protect downstream water quality if deposited ash is periodically scraped to restore any loss of infiltration capacity following wildfire residue deposition.

Assessing post-fire water quality changes in reservoirs: Insights from a large dataset in Portugal

Wildfires in the Mediterranean basin, especially in Portugal, have increased in extent and frequency over the last few years. One of the impacts of wildfires on humans and ecosystems is on the water quality of surface waters. Ashes and increased erosion rates might, for example, change oxygen levels and elevate the influx of sediments, nutrients, or other water quality-related components like metals and polycyclic aromatic hydrocarbons (PAHs), possibly affecting water supply. In this study, time series of eight water quality parameters: biological- and chemical oxygen demand (BOD and COD), electrical conductivity (EC), total phosphorous (TP), nitrate (NO3-), total suspended sediments (TSS), dissolved oxygen (DO), and pH, were assessed via changepoint analysis to identify events of post-fire water contamination in over 60 Portuguese reservoirs. Further, possible fire, watershed, reservoir, and climate-related drivers were linked with the occurrence of these contamination events through logistic regression using generalized additive models. All measured parameters exhibited post-fire changes, with some being more frequently affected than others. The concentrations of TP, NO3-, and TSS showed a noticeable increase following 9.6 %, 12.6 %, and 13.6 % of all wildfires, respectively. Most changes fell into the unusually large fire seasons of 2003-2005 and 2017. The most significant impacts could be seen in southern reservoirs after the fire seasons of 2003-2005. The burned area ratio of the watershed was identified as the main driver of post-fire water contamination, while reservoir and climate-related characteristics like water levels also played a significant role in some parameters. Increased levels of suspended sediments were identified as a potential threat to water supply, especially when large wildfires coincide with drought-induced low reservoir water levels. The identification of post-fire water contamination events and their drivers from large datasets can inform water managers about potential threats to water supply.

Impact of wildfire ash on bacterioplankton abundance and community composition in a coastal embayment (Ría de Vigo, NW Spain)

Wildfire ash can have an impact on coastal prokaryotic plankton. To understand the extent to which community composition and abundance of coastal prokaryotes are affected by ash, two ash addition experiments were performed. Ash from a massive wildfire that took place in the Ría de Vigo watershed in October 2017 was added to natural surface water samples collected in the middle sector of the ría during the summer of 2019 and winter of 2020, and incubated for 72 h, under natural water temperature and irradiance conditions. Plankton responses were assessed through chlorophyll a and bacterial abundance measurements. Prokaryotic DNA was analyzed using 16S rRNA gene partial sequencing. In summer, when nutrient concentrations were low in the ría, the addition of ash led to an increase in phytoplankton and bacterial abundance, increasing the proportions of Alteromonadales, Flavobacteriales, and the potentially pathogenic Vibrio, among other taxa. After the winter runoff events, nutrient concentrations in the Ría de Vigo were high, and only minor changes in bacterial abundance were detected. Our findings suggest that the compounds associated with wildfire ash can alter the composition of bacterioplanktonic communities, which is relevant information for the management of coastal ecosystems in fire-prone areas.

Trends and patterns of polycyclic aromatic hydrocarbons (PAHs) in forest fire-affected soils and water mediums with implications on human health risk assessment

Forest fires are extreme natural/artificial events releasing polycyclic aromatic hydrocarbons (PAHs), which are carcinogenic. Most of the released PAHs are trapped in burnt ash, a part of which is transported and settle on different mediums like soil and water. After strong rainfall events, PAHs enter into surface water bodies through surface runoff, thereby deteriorating water quality. Changes in PAHs levels during the post-fire duration and human health risks due to PAHs released from forest fires need attention. This study aim to explain the trends and patterns of PAHs and health risks due to exposure to soil and water contaminated with PAHs. Forest fires release a higher percentage of low molecular weight PAHs (LMW PAHs) than high molecular weight PAHs (HMW PAHs). Ash and burnt soils contain a higher percentage of LMW PAHs since biomass burning releases huge amounts of LMW PAHs. Whereas, sediments contain a higher percentage of HMW PAHs since most of the LMW PAHs are already degraded. HMW PAHs were causing higher risk to humans (both cancer and non-cancer) due to their higher oxidation potential. Exposure to water contaminated by PAHs resulted in higher health risks for both BaP equivalent and a mixture of PAHs. Exposure to sediment produced the highest health risk due to a higher percentage of HMW PAHs, followed by surface water, burnt soil, ash, and unburnt soil. Cancer and non-cancer risk due to dermal exposure was more elevated than oral exposure. The mixture of PAHs in sediment produced a higher average dermal risk for children (2.21E+00 for cancer and 7.69E+03 for non-cancer risk) and oral cancer risk for adults (7.11E-03). However, exposure to BaP equivalent in sediment produced higher oral non-cancer risk (7.01E+02) for children. Thus, effective PAHs monitoring is required in both soil and surface water mediums for ensuring proper treatment in water supply systems.

Chemical tracers for Wildfires-Analysis of runoff surface Water by LC/Q-TOF-MS

A quantitative methodology using high resolution mass spectrometry was developed for the identification of organic compounds derived from wildfires in surface water samples. The methodology involves the use of solid-phase extraction (SPE) followed by detection using liquid chromatography-quadrupole time of flight-mass spectrometry (LC/Q-TOF-MS) for a group of fourteen chemical compounds (pyridine, benzene, naphthalene and biphenyl polycarboxylic acids). All compounds were successfully separated chromatographically using a reversed phase column and they were identified by accurate mass using the deprotonated species and their main fragment ions. The method produced excellent accuracies (>95%) and precisions (3-10%) for all the compounds studied. This methodology was successfully applied to the identification of fourteen compounds in runoff surface waters impacted by wildfires in Colorado in 2020. Concentrations of individual compounds ranging from 0.1 to 59.5 μg/L were found in wildfire impacted waters, with totals of ∼200 μg/L, thus showing these compounds as chemical tracers of wildfire events at significantly high concentrations. In addition, non-target analysis using chromatography patterns and mass spectrometry identification by MS-MS revealed other polycarboxylic acid isomers were also present in runoff surface water samples.

Unique challenges posed by fire disturbance to water supply management and transfer agreements in a headwaters region

As a headwaters region, Colorado is a critical source of water for surrounding states and Mexico. But fuel densification and shifts in hydrometeorological processes, such as climate aridification and precipitation sharpening, are causing increasingly severe and erratic wildfire behavior and post-disturbance geomorphic hazards in and downstream of its forested source water areas. Human development patterns and inter and intra-state water rights agreements further complicate resource management. This is prompting land managers to consider progressive planning and management tools to mitigate fire-related degradation of water supply and irrigation systems. This narrative review examines aspects of Colorado's geography, demography, and hydrology that make its water supply systems and transfer agreements particularly vulnerable to landscape disturbance and then provides hazard mitigation recommendations. Readers are introduced to Colorado's water supply portfolio including how water is moved, stored, treated, and consumed; why those systems are vulnerable to wildfire disturbance; and how risk can be reduced before and after fires occur. Lessons learned are applicable to other source water areas facing similar challenges. By synthesizing our review findings, we identified numerous research and programmatic gaps including the need for more interdisciplinary studies; a lack of explicit research into how disturbance-driven hydromodification may hinder the ability of headwater regions to exercise their water rights and fulfill water transfer agreements (crucial for reducing potential future water conflict); an unresolved debate regarding the potential effects of forest treatments on water yield; and the need for additional funding to roll out tools and educational programs to communities experiencing severe wildfire activity for the first time.

The Rural Fires of 2017 and Their Influences on Water Quality: An Assessment of Causes and Effects

As water is facing increasing pressures from population and economic growth and climate change, it becomes imperative to promote the protection, restoration and management of this resource and its watersheds. Since water quality depends on multiple factors both natural and anthropic, it is not easy to establish their influences. After the October 2017 fires that affected almost 30% of the Mondego hydrographic basin in Central Portugal, 10 catchments were selected for periodic physical-chemical monitoring. These monitoring campaigns started one month after the fires and lasted for two hydrological years, measuring the electric conductivity (EC), pH, dissolved oxygen (DO), turbidity (Turb), alkalinity (Alk), major and minor ions, and trace elements. The obtained data were then statistically analysed alongside the geomorphological characteristics of each catchment coupled with features of land-use and occupation. From the results, it was possible to establish that fire-affected artificial areas, through the atmospheric deposition and surface runoff of combustion products, had the most impact on surface water quality, increasing As, K^-, Ca²⁺, Mg²⁺, NO₃^-, SO₄^2- and Sr, and consequently increasing electrical conductivity. Agricultural land-use seems to play a major influence in raising the water's EC, Cl, K^- and Na²⁺. Regarding natural factors such as catchment geology, it was found that the extent of igneous exposures influences As, and the carbonate sedimentary units are a source of Ca²⁺ and HCO₃^2- concentrations and impose an increase in alkalinity. Rainfall seems, in the short term, to increase the water concentration in Al and NO₃^-, while also raising turbidity due to sediments dragged by surface runoff. While, in the long-term, rainfall reduces the concentrations of elements in surface water and approximates the water's pH to rainfall features.