Metal toxin threat in wildland fires determined by geology and fire severity

Domain: earth_science
Published: 2023-12-12
Editorial outcome: accept

Abstract
Accentuated by climate change, catastrophic wildfires are a growing, distributed global public health risk from inhalation of smoke and dust. Under-recognized, however, are the health threats arising from fire-altered toxic metals natural to soils and plants. Here, we demonstrate that high temperatures during California wildfires catalyzed widespread transformation of chromium to its carcinogenic form in soil and ash, as hexavalent chromium, particularly in areas with metal-rich geologies (e.g., serpentinite). In wildfire ash, we observed dangerous levels (327-13,100 µg kg−1) of reactive hexavalent chromium in wind-dispersible particulates. Relatively dry post-fire weather contributed to the persistence of elevated hexavalent chromium in surficial soil layers for up to ten months post-fire. The geographic distribution of metal-rich soils and fire incidents illustrate the broad global threat of wildfire smoke- and dust-born metals to populations. Our findings provide new insights into why wildfire smoke exposure appears to be more hazardous to humans than pollution from other sources.

Introduction
Global wildfire activity represents a rising distributed health risk from smoke and dust inhalation1–6. While fire is a fundamental ecological process on which many ecosystems are dependent for vegetation composition, structure, and function, extreme wildfires can devastate human and ecosystem health7. Wildfire smoke is a complex, dynamic mixture of gases and particulate matter (PM) dependent on fire fuels and severity that activate inflammatory pathways or DNA damage responses8–12. Respiratory exposure to fine particulate matter (diameter ≤ 2.5 µm; PM2.5) is particularly problematic owing to its ability to penetrate deep in lungs13–15. Recent work has demonstrated that wildfire PM2.5 is more harmful than urban sources16 and prescribed fires17, each of which differ in physical and chemical properties; however, the chemical composition of wildfire PM, and its compound-specific health effects, remain poorly characterized. Many wildfire pollutants in dust and smoke are classified respiratory carcinogens; commonly studied are organic aerosols and gases derived from incomplete combustion of vegetation8. Increased heavy metals in PM have been documented during wildfire episodes and may induce cytotoxicity, increase lung cancer risks, and greatly contribute to oxidative stress18–29. In particular, chromium in its hexavalent form [Cr(VI)] represents an unforeseen health concern.

Metals in soils and ash are commonly linked to structural burning within wildland-urban interfaces (WUI)30–33, with negligible awareness of wildland landscapes (soils and ash) as an alternative and highly distributed source30. Coinciding with highly populated areas globally, landscapes with ultramafic and mafic geology have soil Cr concentrations ranging from 200 up to 60,000 mg kg−1 and average 2650 mg kg−134 (Supplementary Fig. 1). Chromium is naturally abundant in its trivalent form [Cr(III)], presenting a limited hazard to human health. However, the otherwise sluggish rate of Cr(III) oxidation by molecular oxygen accelerates at temperatures above 200 °C, leading to rapid generation of toxic Cr(VI), even from mineral-bound Cr(III) in soils35. On the basis of heating experiments, up to 35% of soil Cr may be oxidized to Cr(VI) within fires35. Urban ash collected after a wildfire in southern California, USA, contained Cr(VI) concentrations up to 3200 µg kg−1 and naturally illustrates heat-catalyzed Cr(VI) generation36,37. Further, an 8.6-fold increase in Cr(VI) concentrations was observed after a controlled vegetation fire relative to pre-burn concentrations in topsoil (0-3 cm depth)38.

Fire-affected ultramafic and mafic areas occur within all continents (excluding Antarctica), including tropical and temperate climate regions across the western U.S., Brazil, Australia, South Africa, Europe, and Indonesia, and therefore may be a natural source of metals during fire conditions (Fig. 1). The frequency of large (greater than 2023 ha, Fig. 1) and small (Supplementary Fig. 2) fire occurrences within global mafic-ultramafic regions illustrate the propensity for global exposure to Cr(VI) within wildfire dust and smoke. Following wildfires, severely burned areas are often barren and blanketed with ash and loose, rough topsoil leading to enhanced post-fire wind and water erosion4,39–42. The resuspension of Cr(VI)-bearing soil dust and ash presents an acute respiratory exposure risk during and after wildfires43,44. Depending on PM size and Cr(VI) solubility, Cr(VI) can cause localized pulmonary toxicity and translocate to other organs45,46, increasing risks of lung, paranasal sinuses, and nasal cavity cancers47–49. An excess lifetime risk of 1:100,000 is associated with an airborne Cr(VI) concentration of 0.25 ng m−350.

Fire-induced Cr(VI) production in soils and ash may represent a major pathway for human exposure to a severe toxin. Here, we examine the combination of soil properties and fire severity governing Cr(VI) formation within fire-impacted landscapes and the persistence of the toxin in surficial soil layers post-fire. The recent wildfires within California’s North Coast Range, namely the 2019 Kincade Fire and the Hennessey Fire, within the 2020 LNU Lightning Complex, provided us the exceptional ability to explore Cr(VI) generation in fire-impacted landscapes of differing parent geology and under differing canopy-dependent fire severity and duration of Cr(VI) within surface soils. The generalities of our findings, however, are globally transferable.

Here, we show that fire severity, geologic substrate, and ecosystem type influence landscape-scale production of Cr(VI) in particulates during recent wildfires. We find that Cr(VI) concentrations were highest in soils derived from metal-rich geologies, including (ultra) mafic rocks and their metamorphic derivatives. Further, soil particulates from severely burned areas exhibited 6.5-fold greater Cr(VI) concentrations than from unburned soil. Reactive Cr(VI) concentrations are remarkably higher in surficial soil and ash, with up to 13,100 µg kg−1 detected in wind-dispersible particulates (<53-µm diameter) and remained a threat in surface particulates nearly a year post-fire. Our results demonstrate that Cr(VI) persists in particulates composing smoke and post-fire dust, which presents concern for respiratory exposure of local and distal communities.

Results
We analyzed reactive Cr(VI) concentrations in burned and unburned soils from four ecological preserves (Supplementary Fig. 3) that provide variation in ecosystems (grassland, chaparral, and forest; Supplementary Fig. 4) resulting in a gradient in burn severity (a combination of fire intensity and duration) (Supplementary Table 1)40. Importantly, they also span contrasting geologies (felsic to ultramafic, or metamorphic equivalents) having highly variable total Cr concentrations that range from 102 to 4070 mg Cr kg−1 soil (Supplementary Fig. 5). Both wildfires examined in this study occurred during the late fire season (August−November), when soils were driest, and within a multi-year drought across northern California.

Impact of geology
The geology of soil parent material affected the amount of reactive Cr(VI) concentration in fire-impacted soil (Fig. 2). We assessed burned and unburned soil profiles within rhyolitic (felsic and low Cr content; Fig. 2a), mélange (intermediate Cr content; Fig. 2b), and serpentinite (ultramafic equivalent and high Cr content; Fig. 2c) derived soils (Supplementary Table 1; Supplementary Fig. 5). The amount of reactive Cr(VI) produced in fire-affected soils increased with total Cr content and was highest in serpentine soils (Fig. 2c). Average Cr(VI) concentrations generated in soils derived from mélange (Fig. 2b) were more than double the respective levels in rhyolitic soil (Fig. 2a) regardless of overlapping ranges in total Cr content, 152-954 and 102−338 mg kg−1, respectively, reflecting the potential contribution of differing mineralogy to Cr(VI) generation (Supplementary Fig. 6)35. Except in near surface depths, reactive Cr(VI) concentrations were below the detection limit in rhyolitic soils (Fig. 2a).

Reactive Cr(VI) concentrations were higher at near-surface depths (0−2 cm) in burned soil across all geology types with respect to near surface unburned soil and within the soil profile at control depths (10−16 cm). Depending on burn severity, combustion and heat transfer drive the development of steep thermal gradients, with the highest soil temperatures occurring in the top 5-cm, below which temperatures rarely exceed 150 °C40. Despite a range of fire severity conditions (Fig. 3), serpentine soils had the greatest difference in reactive Cr(VI) concentrations at near surface depths in burned versus unburned soils (p-value=0.041, Supplementary Table 2). Reactive Cr(VI) concentrations at control depths did not significantly differ between burned and unburned serpentine soils (p-value=0.25, Supplementary Table 3) and neither concentration within unburned soil profiles (p-value=0.37, Supplementary Table 4). In fact, Cr(VI) concentrations showed little variability between burned and unburned sites and between geologic conditions at deeper control depths despite differences in total Cr content (Supplementary Tables 2 and 3). Moreover, rhyolitic and mélange sites experienced low severity fire conditions during the Kincade Fire and the concentration differences between near surface burned and unburned soil were not statistically significant (Supplementary Table 2).

Impact of fire severity
We examined a gradient in fire severity conditions within a Cr-rich serpentine chaparral landscape (Fig. 3). Average Cr(VI) concentrations in severely burned sites exceeded 200 µg kg−1 in near surface soil (0−2 cm), which was 6.5-fold greater than Cr(VI) concentrations in soil of unburned chaparral landscapes (35 µg kg−1, p-value=0.0001, Supplementary Table 2) and 3.6 times more than at control soil depths below 10 cm within burned soil cores (63 µg kg−1, p-value=0.0109) (Fig. 3a, Supplementary Table 5). In low severity soils, the average reactive Cr(VI) concentration was slightly elevated compared to soil in unburned areas, 73 and 35 µg kg−1 within the top 2-cm (Fig. 3a), respectively, though this difference was not statistically significant (p-value=0.173, Supplementary Table 2). Similarly, Cr(VI) concentrations were not statistically different across depths within low severity soil cores (Supplementary Table 5). Average reactive Cr(VI) levels within unburned soil cores ranged from non-detectable to 64 µg kg−1 and were consistent with water-soluble concentration profiles in near surface soil from a proximal serpentine grassland sampled in 2014 with up to 27 µg Cr(VI) kg−151.

Independent of geology, we observed low fire severity results in minimal Cr(VI) generation, and the reactive fraction was not significantly different from unburned soil profiles (Figs. 2, 3a). Our field observations suggest that low severity fires do not generate Cr(VI) concentrations of concern even in soils with high total Cr concentrations (Supplementary Table 1). At the contrasting end of the fire temperature range, previous work on soil35 and industrial ash has documented that mineral and Cr transformations above 800 °C can also limit (reactive) Cr(VI) production52. In a Knobcone pine (Pinus attenuata) forest that experienced extreme soil heating (>800 °C), we observed lower reactive Cr(VI) concentrations in near surface soil (Supplementary Fig. 7). The effects of fire severity on chemical differences highlight the temperature dependence of soil Cr(VI) production.

Contribution of ash
Blanketing the ground in the burned serpentine chaparral was a surface layer that contained disaggregated ash and soil minerals highly susceptible to wind erosion compared to the underlying, structured soil53. Rhyolitic and mélange sites did not have distinguishable surficial soil-ash layers because of the lower severity fire conditions. Total Cr content varied from 999 to 4829 mg kg−1 (Supplementary Table 6). We observed reactive Cr(VI) concentrations as high as 3335 µg kg−1 in the bulk composition (particles less than 2-mm diameter) of the surficial soil-ash layers (Fig. 3b). At moderate-high fire severity sites, surficial soil-ash layers contained an average reactive Cr(VI) concentration of 1073 µg kg−1, which was nearly 5-fold greater than the average concentration in the top 1-cm of soil (219 µg kg−1; Fig. 3a). These Cr(VI) concentrations exceeded the United States Environmental Protection Agency (USEPA) recommended screening level for residential soils, 300 µg Cr(VI) kg−1, based on its carcinogenicity54. At site A7, the average reactive Cr(VI) concentration was more than three times greater than other moderate-high fire severity sites (Fig. 3b). We suspect that longer burning duration and fire intensities with greater biomass combustion contributed to the relatively high levels of Cr(VI), as this was a severely burned forested area. Importantly, ash from severely burned areas concentrate alkali (Na, K) and alkaline earth (Ca, Mg) metals (often from biomass combustion) that are key for the thermal oxidation of Cr(III)9,52. For example, CaCrO4 was noted after agricultural soil amended with composted Cr(III)-rich tannery sludge was heated to 500 °C55.

At low burn severity sites, average reactive Cr(VI) concentrations in the surficial soil-ash layer were 36 and 82 µg kg−1 (Fig. 3b), approximately 3−40 times less than bulk measurements collected from moderate-high severity sites that ranged from 222 to 3335 µg Cr(VI) kg−1. The physicochemical conditions within the soil-ash layer also contribute to the longevity of Cr(VI)56. Due to incomplete biomass combustion, ash from low severity fires is characteristically rich in pyrogenic carbon, which increases Cr(VI) reducing capacity and promotes Cr immobilization as mineral-bound Cr(III)9,57,58. Reactive Cr(VI) concentrations in low severity surficial soil and ash were consistent with levels present in the near-surface depths of low severity and unburned soil (Fig. 3).

Persistence of chromium (VI)
With minimal rainfall, reactive Cr(VI) persisted within the surface soil-ash layer ten months after post-fire sampling (July 2021) (Fig. 3c). At McLaughlin Natural Reserve, precipitation was merely 25.1 cm (September 2020−July 2021) since the Hennessey Fire, which is smaller than approximately a third of the average precipitation (74.3 cm) during these months59. After nearly a year, the serpentine chaparral remained covered with loose surface soil and ash (approximately 2−5 cm thick), illustrating the absence of significant water erosion or leaching of reactive Cr(VI). Reactive Cr(VI) concentrations ranged from 64 to 1060 µg kg−1, with a median concentration of 257 µg kg−1, and remained elevated compared to concentrations (5−64 µg kg−1) within the near surface depths (0−2 cm) of unburned serpentine soil (Fig. 3c); however, these differences were not statistically significant (p-value=0.164).

Wind-dispersible fraction
Within severely burned landscapes, wind velocities and dust flux are enhanced post-fire, and conditions may persist for multiple years during droughts and depending on revegetation rates4,44,53. Wind-driven suspension of dust is a major mechanism of soil-borne contaminant transport in semiarid environments after wildfires5,43,44. Similar to soil nutrients and other contaminants, the reactive fraction of Cr(VI) is often associated with fine, wind-dispersible particles, primarily clay-sized particles less than 2-µm diameter, due to their high surface area that dominates adsorption sites5. Moreover, post-fire soil pH may exceed 8.5, depending on ash content and fire severity, which increases Cr(VI) stability and enhances extended Cr(III) oxidation by O2 under ambient temperature conditions56. Burning of a Pinus forest on ultramafic terrain illustrates the susceptibility of fine particles to wind transport post-wildfire, where surficial soil and ash high in Cr (and Ni) were lost to wind erosion60.

Surface soil-ash layers across sites contained 12−22% particles smaller than 53-µm diameter, representing wind-dispersible silt- and clay-sized particles (Supplementary Table 6). Respirable particles less than 10-µm diameter made up approximately 27−30%, of which 8−12% were less than 2.5-µm diameter. Total Cr content (946−1643 mg kg−1) was less than half the total Cr concentration measured in respective bulk compositions (up to 2-mm diameter). Chromium has been detected in nanoparticles of structural ash ranging from 90 to 270 nm in size61. Concentration ranges of additional metals enriched in ultramafic rocks, Ni, Fe, and Mn, were 2294−3489 mg kg−1, 82.7−105 mg g−1, and 1910−2547 mg kg−1, respectively. A recent study found greater total Fe content in wildland ash (with variable geologic substrate) than structural ash, with median concentrations of 36.1 mg g−1 and 18.5 mg g−1, respectively61.

Using micro-scale X-ray techniques, Cr(VI)-containing soil and ash particulates were identified in a high fire severity sample but not in a low fire severity sample (Fig. 4; Supplementary Fig. 8). Here, Cr(VI) was associated with mineral surfaces (e.g., adsorbed) or enriched in relatively low-Cr particles with Ca and K (Fig. 4, Supplementary Fig. 9). Consistent with particle analysis, reactive Cr(VI) concentrations spanned from 327 to 13,100 µg kg−1 (Fig. 3b). Reactive Cr(VI) concentrations were similar or higher than respective concentrations in bulk soil and ash (up to 2-mm particle size) from moderate to high fire severity sites (Fig. 3b) in the serpentine chaparral, and all of which exceeded the US EPA screening level (300 µg kg−1). Interestingly, we detected 9 to 14 times higher reactive Cr(VI) concentrations in the inhalable size fractions of surface soil and ash from low fire severity sites (Fig. 3b).

Suburban fires illustrates the impacts of inhaling Cr(VI)-containing ash within the respiratory tract by measuring Cr(VI) leached with a simulated lung fluid36,37 and discerning Cr mineralogy within nano-sized particulates (<100 nm)27,29. Our results indicate that comparable toxic levels of reactive (water-soluble) Cr(VI) are formed within a serpentine wildland landscape as previously observed from structural burning36,37 (Fig. 3b), and stresses that Cr(VI) is enriched in wind-dispersible particles of surface soil and ash. Exposure to particulate Cr(VI) is thus a concern both during and subsequent to high-severity wildland fires on mafic/ultramafic substrates. In addition to local (and possibly distal) communities that may be exposed by transport of Cr(VI)-bearing PM during and after wildfires, wildland fire crews and first responders are particularly at risk. Given the persistence of Cr(VI) after wildfires, continued dust exposure serves as a continued human health threat to land workers, recreationalists, and local residents.

While recognized for urban fires, threats from metal exposure in smoke and dust need to be recognized within wildland fires arising on metal-rich geologies. Across tropical climate regions, deeply weathered lateritic soils are common, in which Cr is predominantly found within crystalline Fe oxides (e.g., hematite, goethite), and soil Fe content may exceed 500 mg g−162. Past work has quantified the Cr oxidation capacity of Cr-bearing Fe oxides and lateritic soils during heating simulations35,62. For example, the greatest Cr(VI) formation (more than 40% of total Cr) upon heating hematite occurred at temperatures less than 400 °C, while up to 100% of total Cr in goethite transformed to Cr(VI) at 800 °C. In Mediterranean or temperate climates (similar to this study’s region), ultramafic soils are relatively more enriched in Cr-bearing phyllosilicate minerals (e.g., serpentine), Fe oxide content is moderate (typically less than 100 mg g−1) with more amorphous secondary phases, and soil pH is neutral to alkaline pH. In our study, and under natural wildfire conditions, we observed that up to about 0.015% of total Cr was reactive Cr(VI) in burned serpentine soils (Supplementary Fig. 6). Chromium(VI) generation during wildfires depend on fire conditions and host mineralogy; thus, the extent of Cr(VI) formation in lateritic soils may differ from temperate serpentine soils, but field observations of Cr(VI) in burned lateritic soils following wildfires are currently lacking.

Similarly, the generation of Cr(VI) within PM is particularly concerning and is long-lived within surficial soil-ash layers, leading to a persistent exposure risk during and after fires. With wildland fires expected to increase in frequency and severity in many geographic regions due to a combination of climate change and past fire management63,64, post-fire dust emissions are likely to rise41. Thus, recognizing chemical-specific health risks beyond general PM2.5 is crucial. Information about environmental drivers of fire-catalyzed Cr(VI) generation in wildland areas can help guide the development of prediction tools to mitigate exposure risks to first responders and local communities. Further, the threat is global, with all vegetated continents being threatened by the combined impacts of increasingly severe wildfires across metal-rich landscapes.

Methods
Study area
Soils and ash were collected from the North Coast Range, which has a Mediterranean climate, across Sonoma, Napa, and Lake Counties, California, U.S.A. after the Kincade (38.7925°, −122.7801°) and Hennessey Fires (within the LNU Lightning Complex; 38.4819°, −122.1486°) in November 2019 and September 2020, respectively. Our field sites were located at four natural preserves that were partly or completely burned during one of the wildfires (Supplementary Fig. 2): Pepperwood Preserve (1295 hectares; 38.5698°, −122.6917°), Sonoma Land Trust’s White Rock Preserve (37 hectares; 38.7082°, −122.8332°), Audubon Canyon Ranch’s Modini Preserve (1214 hectares; 38.7069°, −122.7625°), and University of California’s Donald and Sylvia McLaughlin Natural Reserve (2833 hectares; 38.8712°, −122.4215°). Pepperwood, White Rock, and Modini Preserves are located within the 31,468-hectare burn scar of the Kincade Fire. The McLaughlin Natural Reserve burned during the Hennessey Fire (123,692 hectares) (Supplementary Fig. 2). Burn severity at each sampling site was determined based on landscape fire history, post-fire observations of aboveground vegetation, ash and soil color, and was further supported by laboratory measurements, including bulk soil mineralogy and ammonium (NH4+) concentrations (Supplementary Fig. 10), a direct combustion product40. Pepperwood Preserve, Modini Preserve, and McLaughlin Natural Reserve had partly or completely experienced at least one wildfire since 198465. Pepperwood Preserve completely burned (95%) during the 2017 Tubbs Fire, which was an near identical footprint to the preceding Hanly Fire in 1964. Modini Preserve was previously burned during the 2004 Geysers Fire, and McLaughlin Natural Reserve previously burned in 1999 and 2015.

Vegetation classifications at each field site were determined using countywide map databases (Supplementary Fig. 3, Supplementary Table 1)66,67. At McLaughlin Natural Reserve, the sampled serpentine chaparral was dominated by post-fire seed-regenerating species, including whiteleaf manzanitas (Arctostaphylos viscida) and Macnab’s cypress (Hesperocyparis macnabiana), and post-fire resprouting species, including leather oak (Quercas durata) and musk brush (Ceanothus jepsonii). The chaparral experienced two recent, major wildfires in 2015 and 2020, which resulted in overlapping mosaic burn patterns. Low severity sampling sites had burned twice in the five-year period and contained a limited shrub cover in 2020. Surface temperatures were estimated to not exceed 250 °C40. Sites that were classified as moderate to high burn severity consisted of dense whiteleaf manzanita stands that did not burn in 2015, resulting in an accumulation of highly volatile fuels. Within the moderate-high severity areas, bare, disaggregated reddened soil crosscut a characteristic thick layer of black and white ash39. Soil temperatures at the surface likely exceeded 700 °C as reported in a previous study on soil heating in chaparral ecosystems during various fire severities68. Within the surface soil-ash layer, forsterite was detected (Supplementary Fig. 11), indicating that surface temperatures exceeded 575−600 °C for sufficient time to thermally decompose serpentine minerals (e.g., lizardite) present within surface soil69,70.

Sampling sites at Modini Preserve were located within grasslands and a densely vegetated forest. The grasslands are perennial with native purple needle grass (Stipa pulchra) and blue wildrye (Elymus glaucus) and invaded by annual non-native species. The forest contained fire obligate knobcone pines (Pinus attenuata) with an understory dominated by whiteleaf manzanitas, chamise (Adenostoma fasciculatum) and resprouting madrone trees (Arbutus menziesii). Like Modini, two field sites at Pepperwood Preserve were within grasslands that are primarily annual exotics (Bromus hordeaceus, Avena barbata, Festuca perennis, Elymus caput-medusae, and Festuca bromoides) with an abundant diversity of native perennial grasses. Since 2013, cattle grazing (up to 250−300 heads annually since 2018) has played a major role in reducing fuel loads. While grassland fire intensities can be high (depending on their moisture content), fires often burn rapidly (short duration) limiting overall burn severity and the exposure of soils to high temperatures39. Fire progression and its effects on grasslands is also often uniform, unlike the mosaiced burn pattern observed in chaparral. Grassland sites examined in this study had low severity fire as exhibited by post-fire charred litter and grass stems that were not completely combusted. Additional field sites at Pepperwood Preserve were located along the boundaries of a mixed hardwood forest and a savanna of blue oak (Quercus douglasii) underlain by grasses, and within a forest of Douglas fir (Pseudotsuga menziesii). At White Rock Preserve, soil sampling sites were located within grasslands and in a valley oak (Quercus lobata) forest underlain by grasses.

The four natural preserves in this study comprise contrasting geologies, including felsic (rhyolite, rhyodacite), mélange (consisting of mixed geology, including greenstone and metabasaltic blueschist, along with graywacke), and ultramafic (serpentinite) rocks, which are characteristic of the North Coast Range (Supplementary Fig. 4, Supplementary Table 1). The geology at each sampling location was determined based on surface bedrock characterization using detailed regional geologic maps when available71–75. Regional sandstone (Kfss) comprises the Devils Den Terrane of the Franciscan Complex and contains a multitude of rocks, including graywacke, derived from oceanic crust of the Late Cretaceous71. Further, serpentinite results from the Coast Range Ophiolite, which reaches 320 km across the Coast Range. Sandstone and serpentine soils contained 686–1518 and 1137−4070 mg Cr kg−1, respectively (Supplementary Table 1). Mélange (fsr, KJfs) of the Franciscan Complex consists of a complex mixture of rocks due to tectonic activity, and associated soils (152−954 mg Cr kg−1) were located at White Rock and Modini Preserves. At Pepperwood Preserve, fire-affected soils with low Cr content (102−338 mg Cr kg−1) were collected from areas with silica-rich rhyolitic and rhyodacitic rocks (Tsr, Tst, and proximal surface deposits).

Major soil types are described in the U.S. Department of Agriculture Natural Resources Conservation Service’s Soil Survey76. Soil sampling sites within the serpentine chaparral at McLaughlin Natural Reserve were clay loam to very gravelly clay loam within the top 30-cm. At Pepperwood Preserve, rhyolitic soils were very gravelly loam and clay loam. Mélange-derived soils at White Rock and Modini Preserves were clay loam, and sandstone soils at Modini Preserve were gravelly loam to clay loam.

Soil and ash sample collection
The three preserves (Modini, White Rock, and Pepperwood Preserve) that burned during the Kincade Fire were sampled twice, once immediately following the fire’s complete containment (October 23−November 6, 2019) on November 15th and 19th, 2019, and then approximately 11 months post-fire on September 18th, 2020. Sampling sites at McLaughlin Natural Reserve were sampled once on September 19th, 2020, before full containment of the LNU Lightning Complex (August 17−October 2, 2020), and bulk surface soil and ash were resampled on July 29th, 2021. Immediate post-fire soil and ash sampling was completed before any major precipitation events to minimize Cr(VI) transport and prevent blurring of fire effects on the physicochemical composition within the top 30-cm of soil profiles.

Near surface soil samples were collected from fire-affected and unburned locations as intact cores using a slide hammer soil core sampler (5×30 cm; AMS Inc., ID, USA) and cased in plastic liners with caps. Soil cores ranged from 15 to 30 cm and depended on the refusal depth at each sampling location. A total of 38 cores were collected across the four preserves, 22 from burned areas and 16 from unburned locations. At McLaughlin Natural Reserve, unburned soil cores were collected from locations that hadn’t been burned in at least 5 years. Unburned controls at Pepperwood, White Rock, and Modini Preserves were collected approximately 11 months post-fire from sampling sites that burned at low severity during the Kincade Fire. Among all sampling locations, 19 cores were collected from grasslands, 10 cores were from chaparral, and 9 cores from mixed forests. Ten cores were from serpentine (ultramafic) soil, 7 from rhyolitic (felsic) soil, 16 soil cores were from mélange (similar chemistry as mafic) parent material, and 5 were from sandstone (similar chemistry as mafic). Within the severely burned serpentine chaparral at McLaughlin Natural Reserve, bulk soil and ash were collected from the loose, surface soil overlying the coring. Depending on fire severity, this layer contained charred plant material, black to white ash, and disaggregated reddened soil. Prior to laboratory analysis, field moisture was retained by storing cores and bulk samples within air-tight containers at 4 °C.

Sample preparation and solid-phase analysis
The top 6-cm of the soil cores were manually divided into 1-cm intervals and the remaining depths were sliced in 2-cm intervals to an analysis depth of 16-cm. Except for extremely severe burned soils (Supplementary Fig. 7), we considered soil depths below 10 cm as a control depth36,38. Each soil section and bulk sample from surface soil-ash layers was homogenized, and soil aggregates were lightly ground in an agate mortar and pestle to pass through a 2-mm sieve. Gravimetric water content was determined by the weight difference of field-moist and oven-dried soil (105 °C for 24 h). Within the top 16-cm of the soil, the gravimetric water content ranged from 1 to 11%.

We passed representative samples of bulk surface soil-ash layers through a series of sieves to separate ash/soil solids into two size fractions: sand-sized (0.053−2 mm) and silt/clay-sized particles (less than 53 µm). The additional sieving to collect particles less than 53 µm allowed us to further quantify the percentage of this inhalable size fraction and analyze its physicochemical characteristics. We did not gently grind soil prior to sieving so as to preserve sand-sized micro-aggregates that would otherwise result in an overestimation of the fine particle fraction highly susceptible to dust resuspension. Particle-size distribution (0.4−53 µm) of the silt/clay-sized particle fraction was determined using a laser diffraction particle size counter (Coulter LS-230) for select moderate and high severity field site samples (A4, A6, A7). Homogenized, air-dried samples were prepared in a suspension with 2 g L−1 sodium hexametaphosphate solution and vortexed for 25 min. Particle-size distributions were determined by averaging two measurements for each sample.

Total major and trace element concentrations of finely ground oven-dried soil samples were quantified by energy dispersive X-ray fluorescence (XRF) spectrometry (Spectro XEPOS). To assess thermal alterations in bulk mineralogy within the surface soil ash layers and soil profiles from burned sites, a subset of soil aliquots was dried at room temperature and qualitatively analyzed by powder X-ray diffraction (XRD; Rigaku MiniFlex 600) with Cu-Kα radiation (40 kV, 15 mA). Diffraction patterns were collected in the 2θ range of 3−90° with a 0.01° step size, 1.2 s time count per step. The XRD patterns were evaluated using PDXL2 software (Rigaku).

Micro X-Ray fluorescence analysis
Fine particulates (<53 µm) from two surface soil-ash layers (representing high and low fire severity) were fixed in epoxy resin (EPO-TEK 301-2FL) and thin sections (30-µm thickness) were prepared on quartz slides (Grindstone Laboratory LLC, Portland, OR). Thin sections were analyzed in ambient air conditions and in fluorescence mode at the bending magnet beam line 2−3 at Stanford Synchrotron Radiation Lightsource (SSRL, Stanford, CA) with a double Si(111) crystal monochromator to tune the X-ray beam and a Si Drift Detector (Vortex-90EX, Hitachi) for data collection. The size of the X-ray beam was focused to 1-µm diameter (Sigray). Data processing and visualization was completed using the Microanalysis Toolkit software package77.

Micro-XRF maps were collected with a 10-µm (diameter) spot size across approximately 2−4 mm grids of each thin section at multiple incident energies to examine the micron-scaled distribution of Cr, Fe, Mn, and other light elements (e.g., Ca, K). Chromium speciation maps were generated at 5993, 6003, and 6010 eV, energies characteristic of Cr(VI), Cr(III), and total Cr, respectively. The X-ray beam was calibrated by measuring the X-ray absorption near-edge structure (XANES) spectral region of a Cr metal foil, and by setting the position of the first maximum of its first derivative to 5989 eV. The pre-edge absorption peak of K2CrO4 was set to 5993 eV. We also collected a coarse-scale map above the Mn and Fe K-edges at 7250 eV. We selected regions within the coarse-scale maps to collect finer resolution µ-XRF maps (1×1 µm spot size) at 5993, 6003, and 6010 eV. These regions were chosen based on principal component analysis and correlations with other elements.

The relative abundance of Cr(VI) in particulates of surface soil-ash layers was approximated within mapped regions. For each image, we removed pixels along edges and dead pixels, and applied a smoothing function (“blur”) to each energy channel, which takes a Gaussian distribution of pixel values within a 5×5 pixel area (standard deviation of 0.8) to minimize background noise. Using the map generated at 6010 eV, we defined Cr-bearing particulates using an inverse-binary threshold of approximately 1% (0.56−1.69%), which assigns any pixels with intensities less than 1% of the highest intensity as background in the mapped region. Thereafter, we calculated the ratio of fluorescence intensities at 5993 and 6010 eV within identified particulates to estimate the presence of Cr(VI). Previous work has shown that the height of the Cr(VI) pre-edge peak is proportional to the total concentration of Cr(VI)78. The presence of Cr(VI) yields a strong absorption peak at 5993 eV; however, distortion of the octahedral Cr(III) environment can result in two weaker peaks at 5990 and 5993 eV, with the peak intensity being relatively larger at 5990 eV.

To corroborate bulk measurements of reactive Cr(VI), we combined multi-energy mapping with µ-XANES of select spots on particles to confirm the presence of Cr(VI). We acquired Cr µ-XANES spectra for points across the high-resolution particle maps, which were collected from −230 to 270 eV around the Cr K-edge. Up to 9 spectral scans were collected to increase the signal-to-noise ratio for particles with relatively low Cr concentrations. XANES spectra were processed in the Microanalysis Toolkit and ATHENA software packages.

Although µ-XRF imaging provides a unique ability to spatially speciate Cr-bearing particles, it is not without limitations. First, the size fraction of greatest concern for respiratory exposure are particulates less than 2.5 µm, which is near or below the pixel resolution for µ-XRF imaging. By setting an inverse-binary threshold to isolate Cr-bearing particulates, it’s likely that we removed inhalable Cr-bearing particles (<10 µm) of relatively low intensities. Second, we underestimate Cr(VI) for particles highly enriched in Cr, having an overwhelming abundance of Cr(III).

Aqueous extractions and chemical analysis
Reactive Cr(VI) concentrations (most available fraction, including dissolved and adsorbed Cr(VI)) in soil-ash samples (bulk and particle size fraction <53 µm) and within soil cores were extracted with 10 mM K2HPO4/KH2PO4 (buffered at pH 7.2)79,80. Phosphate effectively competes with Cr(VI) ions for surface adsorption sites. At circumneutral to alkaline pH ranges in natural soils, it’s expected that nearly all aqueous Cr is present in the hexavalent form, and that Cr(VI) concentrations will be primarily limited by adsorption81. The clay size fraction (less than 2-µm diameter) typically has a dominant influence on species retention given their high surface areas and greater number of adsorption sites; therefore, it is likely that the reactive Cr(VI) concentrations measured here largely represent the fraction of Cr(VI) associated with clay particles. Triplicate samples were agitated in a 1:4 soil/solution ratio for 24 h, centrifuged (30 min, 2272g, 4 °C), and filtered through 0.22-µm filters. A subsample of unacidified filtrate was used to quantify aqueous Cr(VI) concentrations using the diphenylcarbazide (DPC) method on a UV-Vis spectrophotometer (Shimadzu UV-1601)79,82. The detection limit was 3 µg L−1 (approximately 12 µg kg−1)82. Total Cr concentrations were determined with inductively coupled plasma mass spectrometry (ICP-MS, Thermo Scientific XSERIES 2), and confirmed that approximately all aqueous Cr was in the form of Cr(VI) in unburned soil and burned soil and ash, similarly observed in previous studies36,37. An aliquot of each soil extract was immediately acidified post-filtration and stored in 2% nitric acid at 4 °C until ICP-MS analysis.

To determine relative differences in K+-extractable NH4+ concentrations (mg NH4+-N kg−1) within burned and unburned soils (Supplementary Fig. 10), additional unacidified samples (after K2HPO4/KH2PO4 extraction) from 30 of the 38 total soil cores (21 fire-affected and 9 unburned soil cores) were frozen at −20 °C until chemical analysis. Ammonium is a direct combustion product and will be elevated in the near surface soil after wildfires depending on burn severity40. Ammonium concentrations in the top 6-cm were measured in triplicate (when sample volume allowed) using a flow injection analyzer (Westco SmartChem 200 Discrete Analyzer), with a detection limit of 0.05 mg L−182.

Statistical analyses
Means and standard errors were calculated for aqueous and solid-phase chemical measurements in all cores using replicates described below. Half the detection limit was used when measured concentrations were below detection limits. Total elemental concentrations were measured in 3−4 solid-phase aliquots from each soil core (Supplementary Table 1). At each soil depth interval (1-cm from 0 to 6 cm; 2-cm from 6 to 16 cm), triplicate aqueous extractions were conducted to evaluate reactive Cr(VI) and exchangeable NH4+ concentrations (Fig. 2, Fig. 3a, Supplementary Fig. 10). In select soil depths within cores, replicates were limited (less than 3) due to solid mass or post-extraction aqueous volume.

To assess data normality, we applied the Shapiro-Wilk test and reported W statistics and p-values (Supplementary Table 7). If data met normality assumptions at the 95% confidence interval (p-value=0.05), we used two-sided parametric tests; otherwise, we utilized two-sided nonparametric tests. Likewise, we used the f-test to determine equal variance. Unpaired t tests were used to compare mean reactive Cr(VI) concentrations at the 95% confidence interval in near surface soil (0−2 cm) of fire-affected and unburned sites based on geology. If one or both datasets were not normally distributed, such as in burned and unburned soils at control depths (10−16 cm), Mann−Whitney U test was used. Within a soil core, we compared mean reactive Cr(VI) concentrations in surface soil (0−2 cm) versus control depths (10−16 cm) using either paired t test or Wilcoxon signed rank test. Detailed information about and results for each statistical analysis is provided in Supplementary Tables 2−5 and 7. All statistical analyses were executed using the stats package in R (v. 4.1.3).

Global fire frequency map
Wildfires burn millions of wildland hectares each year. To approximate fire frequency globally within mafic and ultramafic regions, we combined a generalized geologic map with bedrock domain classification83 and GlobFire Database84 within a geographic information system (QGIS 3.10.4). Total global population, including urban and rural areas, was visualized at 1-km resolution using a projection model for 2020 based on a “middle-of-the-road” socio-economic scenario (SSP2) (Supplementary Fig. 1b)85,86.

Among other themes, the generalized geologic map designates a mafic-ultramafic igneous theme in which contains mafic and/or ultramafic igneous domains (or their metamorphic equivalents) that are classified as intrusive or extrusive and subdivided by era (Supplementary Fig. 1a). At the global scale, domains representing small outcrops of mafic and ultramafic rocks may be exaggerated in size while clusters of mafic-ultramafic domains appear as single polygon units. Geologic substrate can be further resolved by integrating regional and statewide geologic maps (rather than a generalized global map) to better assess exposure risks of geogenic metals based on bedrock type.

The GlobFire Database (version 2, Jan. 2021) contains individual fire perimeters from 2001 to 2020 that are derived from the satellite MODIS burned area collection 6 product (MCD64A1). Fire events were overlaid on mafic-ultramafic geologic units and filtered by final area size. Large and small fires were respectively defined if the area equaled or exceeded and was less than approximately 2023 hectares, the size distinction for Class G wildfires87. Importantly, devastating wildfires within WUIs range in fire sizes. Smaller fires in a densely populated WUI may pose a greater risk to PM exposure than expansive wildland fires in remote areas that burn at lower fire intensity; therefore, fire size does not simply correlate with exposure risks. Using a grid (5×5 km pixel resolution) within mafic-ultramafic regions, we calculated the frequency (count) of fire events within each grid pixel. Global maps are presented using the Robinson projection.

Fire regimes vary across ecosystem types, and the extent of exposure risks to extreme wildfires in certain regions cannot be completely captured within the available satellite-derived global fire database, particularly considering future shifts from climate change7,88. For example, many forest ecosystems (e.g., boreal, temperate) have fire frequencies on the decadal and century timescales. Savannah and grassland ecosystems burn at a higher frequency (1-5 years) but at a lower fire intensity and thus severity because of smaller fuel loads. Regional trend assessments of fire activity, where historical fire records extend earlier than 2001, are critical for understanding and predicting fire frequency and severity7.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of manuscript 395109 submitted to Nature Communication\nMetal toxin threat in wildland fires determined by geology and fire severity\nAlandra Marie Lopez, Juan Lezama Pacheco and Scott Fendorf\nThis manuscript presents the results of a study aimed at emphasizing the health threat\npotentially arising from chromium transformation to its most harmful Cr(VI) form in soils\nand ashes upon wildfires. To reach their objective, the authors analyzed 38 cores\ndrilled in more or less severely burned soils at four Preserves that experienced large\nwildfires in the North Coast Range of California (USA). The results obtained confirm\nthe catalytic effect of high temperatures generated by wildfires on chromium oxidation\nin soil, and they point to control of soil geology and fire severity on this effect. They\nalso interestingly indicate that reactive Cr(VI) can reach dangerous levels in wind-\ndispersible particulates found in the surficial layers of the soil at ultramafic settings,\nand that this harmful form of chromium can persist in the soil/ash system up to one\nyear after the wildfire if rainfalls are not significant. These results yield to the conclusion\nthat more work is required to further evaluate this potential risk around the world.\nI found this paper very pleasant to read. The subject is well introduced, the sites and\nsamples are well described and fit with the objective of the study, the results are well\npresented and they support the discussion that gives proper attention to the existing\nliterature on the topic and yield to the novel conclusions that geology and fire intensity\nare important drivers of harmful hexavalent chromium in soils and wind-dispersible\nsoil/ash surface particles, and that the associated health risk can persist up to one year\nafter fire.\nAlthough the scale of the study (North Coast Range of California, USA) could at first\nbe considered too short to extend its findings at the global scale, I agree with the\nauthors that the number and diversity of sites studied can be considered enough to\naddress this issue. I also agree with the concluding remark on the importance of further\nevaluating the potential risk of wildfire-induced harmful hexavalent chromium in wind-\ndispersible soil/ash particles at the global scale. Finally, I agree with the authors that\nthe results provided in this study deserve to be shared with a large audience, ranging\nfrom scientists working on the topic to policy makers, public administrators and nature\nmanagers.\nFor all these reasons, I consider that this manuscript deserves to be published\nin Nature Communications. I have listed below few issues that I would be\ninterested to see addressed, although only few of them are mandatory for\npublication.\nFigure 1\nI agree that a large fraction of ultramafic/mafic areas are concerned by wildfires at\nthe global scale. Figure 1 shows that these areas are mainly located in the tropical region\nwhere most soils are deeply weathered (Ultisols and Oxisols, according to the USDA\nclassification). However, the Fe contents reported in Table S6 suggest that the soils\nstudied in this work do not correspond to these soil types. This raises the question of\nthe actual representativeness of the results regarding tropical areas. I would\nrecommend that the authors comment on that point, and maybe further consider it in\ntheir concluding remarks.\nFigure 3\nMaybe, the authors could add the results of comparative statistics tests on barplots\nand boxplots (either directly report the p-values or show corresponding labels as *** or\n**) ?\nDo they have any hypothesis to explain the much higher concentration in reactive C(VI)\nmeasured in the A7 surface soil-ash sample from the serpentine chaparral landscape\n? The data provided in Table S6 indicate about twice more total Cr in this sample\ncompared to sample A6 (for instance), but the concentration in reactive Cr(VI) is more\nthan 3 times higher in the bulk fraction and more than 25 three times higher in the\n<53µm fraction. Are there any mineralogical differences with the other moderately-\nhighly burned serpentine soil-ash samples (A3-A6) that could help to explain that ?\nFigure 4\nI wonder if this figure should be maintained in the main text. First, I am not convince\nthat it really supports the assumption that ‘’total Cr(VI) was most abundant in wind-\ndispersible soils and ash particulates after high fire severity conditions compared to a\nlow severity sample’’. Second, I do not understand why the two figures are plotted on\ndifferent sizes (A is larger than B).\nIt could maybe be replaced by a figure similar to Figure S7 (maybe use only panels A\nand B or panels C and D from Figure S7 and add two similar panels from a low fire\nseverity burn sample from a serpentine chaparral soil) ?\nWhatever, if Figure 4 is maintained in the main text, I would recommend that the\nauthors add some histograms with the estimated number of Cr(VI) and total Cr\nparticles on the two figures, in order to help the reader to better assess the relative\nproportion of both types of particles. At least, I would recommend that they change for\nmore contrasted colors, in order to help the reader to better visually decipher between\nCr(VI) and total Cr particles.\nFigure S6\nI would recommend to change the letters for A, B and C in the legend to fit with the\nletters reported on the figure.\nTable S6\nTotal concentrations in bulk surface soil-ash samples (and some <53 µm fractions) are\ndisplayed, but total concentration in bulk soil samples across the whole soil cores are\nnot provided. I would recommend that the authors provide these data (at least those\nfor Cr) as mean total concentration for rhyolite, mélange and serpentine soils in SI,\neither in the form of a Table or as a figure similar to Figure 2. Such a figure would better\nshow that the fractions of reactive Cr(VI) is very low compared to total Cr concentration.\nIn the same way, I would have been interested to see a figure similar to Figure 2 that\nwould have depicted the fraction of reactive Cr(VI) as a function of the total Cr\nconcentration. Indeed, such a figure would have helped to check if the fraction of\nreactive Cr(VI) is really higher in serpentine soils. Even if I agree with the authors that\nthe concentration of reactive Cr(VI) is the most relevant parameter to assess a\npotential environmental and/or health risk, the fraction of reactive Cr(VI) could further\ninform on the actual mechanism(s) and/or soil characteristic(s) that favor Cr(III) to\nCr(VI) oxidation in burned soils. But, maybe this question is beyond the scope of the\npaper...\nFigure S7\nLegend : … from a serpentine chaparral soil…\nWhy did the authors not tried to analyze more some Cr(VI) areas on panel A ?\nThe XANES spectrum at point 1 on panel C shows a well-marked Cr(VI) pre-edge peak\nbut the color code indicate rather low amounts of Cr(VI) at this point. Could the authors\nexplain that ?\nSuggested additional references\nRascio et al., 2022. Evidence of hexavalent chromium formation and changes of Cr\nspeciation after laboratory-simulated fires of composted tannery sludges long-term\namended agricultural soils. Journal of Hazardous Materials, 436, 129117.\nhttps://doi.org/10.1016/j.jhazmat.2022.129117\nTerzano et al., 2021. Fire effects on the distribution and bioavailability of potentially\ntoxic elements (PTEs) in agricultural soils. Chemosphere, 130752.\nhttps://doi.org/10.106/j.chemosphere.2021.130752\nRé et al., 2021. Cytotoxic effects of wildfires ashes : In-vitro responses of skin cells.\nEnvironmental Pollution, 285, 117279. https://doi.org/10.1016/j.envpol.2021.117279\nJahn et al., 2021. Metallic and crustal elements in biomass-burning aerosols and ash:\nPrevalence, significance, and similarities to soil particles. ACS Earth and Space\nChemistry, 5, 136-148. https://dx.doi.org/10.1021/acsearthspacechem.0c00191\nXu et al., 2020. Wildfires, global climate change, and human health. The New England\nJournal of Medicine, 383, 2173-2181. https://doi.org/10.1056/NEJMsr2028985", "reviewer_2": "The authors have to highlight the novelty of their manuscript. The abstract should be\nrevised to attract the reader’s attention. The Introduction section should be improved by\nadding references dealing soil contamination issues. However, the problem is that the\nEnglish and the whole organisation of the present version are definitely below an acceptable\nstandard for an international scientific journal. Analytical quality control is missing.\nDetection limits of the applied methods should be reported. The main problem for this\nmanuscript is its structure. Major parts are missing from the manuscript. My suggestion is to\nreject this manuscript and encourage the authors to submit a more mature manuscript."}, "author_response": "Response to Review Comments\nReviewer #1\nThis manuscript presents the results of a study aimed at emphasizing the health threat potentially\narising from chromium transformation to its most harmful Cr(VI) form in soils and ashes upon\nwildfires. To reach their objective, the authors analyzed 38 cores drilled in more or less severely\nburned soils at four Preserves that experienced large wildfires in the North Coast Range of\nCalifornia (USA). The results obtained confirm the catalytic effect of high temperatures\ngenerated by wildfires on chromium oxidation in soil, and they point to control of soil geology\nand fire severity on this effect. They also interestingly indicate that reactive Cr(VI) can reach\ndangerous levels in wind- dispersible particulates found in the surficial layers of the soil at\nultramafic settings, and that this harmful form of chromium can persist in the soil/ash system up\nto one year after the wildfire if rainfalls are not significant. These results yield to the conclusion\nthat more work is required to further evaluate this potential risk around the world.\nI found this paper very pleasant to read. The subject is well introduced, the sites and samples are\nwell described and fit with the objective of the study, the results are well presented and they\nsupport the discussion that gives proper attention to the existing literature on the topic and yield\nto the novel conclusions that geology and fire intensity are important drivers of harmful\nhexavalent chromium in soils and wind-dispersible soil/ash surface particles, and that the\nassociated health risk can persist up to one year after fire.\nAlthough the scale of the study (North Coast Range of California, USA) could at first\nbe considered too short to extend its findings at the global scale, I agree with the authors that the\nnumber and diversity of sites studied can be considered enough to address this issue. I also\nagree with the concluding remark on the importance of further evaluating the potential risk of\nwildfire-induced harmful hexavalent chromium in wind- dispersible soil/ash particles at the\nglobal scale. Finally, I agree with the authors that the results provided in this study deserve to be\nshared with a large audience, ranging from scientists working on the topic to policy makers,\npublic administrators and nature managers.\nFor all these reasons, I consider that this manuscript deserves to be published in Nature\nCommunications. I have listed below few issues that I would be interested to see addressed,\nalthough only few of them are mandatory for publication.\nResponse: We thank the reviewer for their constructive feedback and review of the\nmanuscript's findings and implications. We have reviewed and addressed in detail below\nthe reviewer's suggestions.\nChanges: Please see our specific changes to the reviewer's suggestions below.\nFigure 1: I agree that a large fraction of ultramafic/mafic areas are concerned by wildfires at\nthe global scale. Figure 1 shows that these areas are mainly located in the tropical region where\nmost soils are deeply weathered (Ultisols and Oxisols, according to the USDA classification).\nHowever, the Fe contents reported in Table S6 suggest that the soils studied in this work do not\ncorrespond to these soil types. This raises the question of the actual representativeness of the\nresults regarding tropical areas. I would recommend that the authors comment on that point,\nand maybe further consider it in their concluding remarks.\nResponse: We agree with the reviewer that the soils in this study are not as deeply\nweathered as Oxisols and Ultisols, like lateritic soils, common across tropical climate\nregions (e.g., New Caledonia, Cuba, Brazil, Malaysia, Indonesia, Madagascar, northern\nAustralia). In our study, the average Fe concentration across burned and unburned bulk\nultramafic (serpentine) soil was 8.71 wt. %, while lateritic soils can contain more than 5x\nmore Fe, typically in the form of crystalline Fe oxides (e.g., hematite, goethite).\nChromium is predominantly found within these crystalline Fe oxides. Additionally,\nlateritic soils are often depleted in Ca, Mg, and Si. The Fe concentrations of our study\nsoils are representative of serpentine soils in Mediterranean or temperate climates (e.g.,\nCalifornia, Oregon, Washington, Turkey, Balkans) with moderate Fe oxide content and\nneutral to alkaline pH. We have added a paragraph within the Discussion section that\naddresses the characterization of ultramafic soils globally and the limitations of soil types\nwithin our study site when considering mechanisms in lateritic and highly weathered\nmetal-rich soils in the tropical regions. Furthermore, we place our findings from field\nanalysis in the context of laboratory studies that examine Cr(VI) generation from Cr(III)\nsolids common to lateritic soils.\nChanges: Within the Discussion we now state:\n“While recognized for urban fires, threats from metal exposure in smoke and dust need to\nbe recognized within wildland fires arising on metal-rich geologies. Across tropical\nclimate regions, deeply weathered lateritic soils are common, in which Cr is\npredominantly found within crystalline Fe oxides (e.g., hematite, goethite), and soil Fe\ncontent may exceed 50 wt. % 62. Past work has quantified the Cr oxidation capacity of\nCr-bearing Fe oxides and lateritic soils during heating simulations 35,62. For example, the\ngreatest Cr(VI) formation (more than 40% of total Cr) upon heating hematite occurred at\ntemperatures less than 400°C, while up to 100% of total Cr in goethite transformed to\nCr(VI) at 800°C. In Mediterranean or temperate climates (similar to this study’s region),\nultramafic soils are relatively more enriched in Cr-bearing phyllosilicate minerals (e.g.,\nserpentine), Fe oxide content is moderate (typically less than 10 wt. %) with more\namorphous secondary phases, and soil pH is neutral to alkaline pH. In our study, and\nunder natural wildfire conditions, we observed that up to about 0.015% of total Cr was\nreactive Cr(VI) in burned serpentine soils. Chromium(VI) generation during wildfires\ndepend on fire conditions and host mineralogy; thus, the extent of Cr(VI) formation in\nlateritic soils may differ from temperate serpentine soils, but field observations of Cr(VI)\nin burned lateritic soils following wildfires are currently lacking.”\nFigure 3: Maybe, the authors could add the results of comparative statistics tests on barplots\nand boxplots (either directly report the p-values or show corresponding labels as *** or **) ?\nResponse: We performed a one-way ANOVA for Figure 3c and found no statistically\nsignificant difference between the three groups (p-value = 0.164). In Figure 3b, we are\nillustrate reactive concentrations within surface soil-ash based on fire severity (low\nversus moderate-high) and particle size (bulk soil less than 2 mm versus the silt and clay-\nsized fraction less than 53 µm). We are unable to run a two-way ANOVA because burn\nseverity sample sizes are not equal.\nChanges: For Figure 3, we now state \"Reactive Cr(VI) concentrations ranged from 64 to\n1,060 µg/kg, with a median concentration of 257 µg/kg, and remained elevated compared\nto concentrations (5-64 µg/kg) within the near surface depths (0-2 cm) of unburned\nserpentine soil (Figure 3c); however, these differences were not statistically significant\n(p-value = 0.164). \"\nDo they have any hypothesis to explain the much higher concentration in reactive C(VI)\nmeasured in the A7 surface soil-ash sample from the serpentine chaparral landscape ? The data\nprovided in Table S6 indicate about twice more total Cr in this sample compared to sample A6\n(for instance), but the concentration in reactive Cr(VI) is more than 3 times higher in the bulk\nfraction and more than 25 three times higher in the <53μm fraction. Are there any mineralogical\ndifferences with the other moderately- highly burned serpentine soil-ash samples (A3-A6) that\ncould help to explain that ?\nResponse: At site A7, the soil experienced longer burning duration and fire intensities\nwith greater biomass combustion that may further contribute to the high-levels of Cr(VI).\nUnlike A7 (original Figure S9; revised Figure S11), we did not observe mineralogical\nchanges in bulk composition for samples A3-A6 in the surface soil and ash compared to\nunderlying burned soil. We suspect that high temperatures did not persist for sufficient\ntime to alter bulk mineralogy in the latter samples.\nChanges: Within the Results we state:\n\"At site A7, the average reactive Cr(VI) concentration was more than three times greater\nthan other moderate-high fire severity sites (Figure 3b). We suspect that longer burning\nduration and fire intensities with greater biomass combustion contributed to the relatively\nhigh-levels of Cr(VI), as this was a severely burned forested area. Importantly, ash from\nseverely burned areas concentrate alkali (Na, K) and alkaline earth (Ca, Mg) metals\n(often from biomass combustion) that are key for the thermal oxidation of Cr(III) 13,43.\nFor example, CaCrO was noted after agricultural soil amended with composted Cr(III)-\n4\nrich tannery sludge was heated at 500°C 55.\"\nFigure 4: I wonder if this figure should be maintained in the main text. First, I am not convince\nthat it really supports the assumption that ‘’total Cr(VI) was most abundant in wind- dispersible\nsoils and ash particulates after high fire severity conditions compared to a low severity sample’’.\nSecond, I do not understand why the two figures are plotted on different sizes (A is larger than\nB).\nIt could maybe be replaced by a figure similar to Figure S7 (maybe use only panels A\nand B or panels C and D from Figure S7 and add two similar panels from a low fire\nseverity burn sample from a serpentine chaparral soil) ?\nWhatever, if Figure 4 is maintained in the main text, I would recommend that the authors add\nsome histograms with the estimated number of Cr(VI) and total Cr particles on the two figures,\nin order to help the reader to better assess the relative proportion of both types of particles. At\nleast, I would recommend that they change for more contrasted colors, in order to help the\nreader to better visually decipher between Cr(VI) and total Cr particles.\nResponse: We appreciate the reviewer's feedback regarding Figure 4. We agree that our\nresults of greater Cr(VI)-bearing particles in high severity soil-ash versus low-severity\nconditions can be clearer, especially by including a histogram of particles containing\nCr(VI). Regarding the reviewer's concern about the different size plots, the sample area\nfor large-scale micro-XRF maps of each thin section were not held constant during data\ncollection, resulting in different mapped areas. Due to limitations of the analysis\nsoftware, we are unable to change for more contrasted colors, but we can update the\nmin/max values for color brightness.\nWe have revised Figure 4 to focus on a 1-µm resolution XRF image containing particles\nfrom a high fire severity sample with and without measurable Cr(VI) by XANES\nanalysis. We then moved the original Figure 4 to the Supplementary Information as\nFigure S8, and added another example of Cr(VI)-containing particles in a 1-µm\nresolution XRF image as Figure S9. We also revised the main text related to the figures\nto highlight the presence of Cr(VI)-containing particles within the high fire severity\nsamples, which was not apparent in low fire severity samples.\nChanges: See revised Figure 4, Figure S8, and Figure S9 below.\n“Using micro-scale X-ray techniques, Cr(VI)-containing soil and ash particulates were\nidentified in a high fire severity sample as opposed to a low fire severity sample (Figure\n4; Figure S8). Here, Cr(VI) was associated with mineral surfaces (e.g., adsorbed) or\nenriched in relatively low-Cr particles with Ca and K (Figure 4, Figure S9). Consistent\nwith particle analysis, reactive Cr(VI) concentrations spanned from 326 to 13,000 µg/kg\n(Figure 3b).”\nFigure S6: I would recommend to change the letters for A, B and C in the legend to fit with the\nletters reported on the figure.\nResponse: Based on the reviewer’s recommendation, we have revised the figure,\naccordingly, by changing uppercase letters to lowercase, in addition to the other figures\nwith sub-panels (similarly identified by Reviewer #2).\nChanges: We changed the letters to lowercase, as requested.\nTable S6: Total concentrations in bulk surface soil-ash samples (and some <53 μm fractions)\nare displayed, but total concentration in bulk soil samples across the whole soil cores are not\nprovided. I would recommend that the authors provide these data (at least those for Cr) as mean\ntotal concentration for rhyolite, mélange and serpentine soils in SI, either in the form of a Table\nor as a figure similar to Figure 2. Such a figure would better show that the fractions of reactive\nCr(VI) is very low compared to total Cr concentration.\nResponse: We thank the reviewer for their recommendation, which we have addressed in\nthe revision. Please refer to our response and changes to the reviewer's next comment\nrelated to including a figure showing the fraction of reactive Cr(VI) to total Cr\nconcentrations.\nChanges: Using total Cr concentrations reported for soil cores in Table S1, we have\nrevised Table S6 to include the mean total element concentrations for Cr, Fe, Mn, Ni, Ca,\nMg, Na, and K in addition to the surface soil-ash sample data so that the reader can\ncompare elemental concentrations in surface soil and ash to bulk soil from different\ngeologies.\nIn the same way, I would have been interested to see a figure similar to Figure 2 that would have\ndepicted the fraction of reactive Cr(VI) as a function of the total Cr concentration. Indeed, such\na figure would have helped to check if the fraction of reactive Cr(VI) is really higher in\nserpentine soils. Even if I agree with the authors that the concentration of reactive Cr(VI) is the\nmost relevant parameter to assess a potential environmental and/or health risk, the fraction of\nreactive Cr(VI) could further inform on the actual mechanism(s) and/or soil characteristic(s)\nthat favor Cr(III) to Cr(VI) oxidation in burned soils. But, maybe this question is beyond the\nscope of the paper...\nResponse: We thank the reviewer for their recommendation. We agree that the reactive\nCr(VI) fraction of total Cr in soil and soil-ash is low; however, the fraction is relatively\nhigher in near surface soil depths compared to past studies quantifying natural Cr(III)\noxidation in unburned soils, including a 2017 study at McLaughlin Natural Reserve.\nMoreover, as the reviewer notes, the hazard imposed by the particulates is related to the\nreactive Cr(VI). The percentage of total Cr that was reactive Cr(VI) in unburned\nserpentine soil was consistent with previous measurements (McClain et al., 2017).\nInterestingly, the reactive Cr(VI) fraction differs based on geology (rhyolite, mélange,\nand serpentinite). The reactive Cr(VI) fraction in rhyolitic and mélange soils composed\nmore of the total Cr content than the relative fraction within serpentine soils. In order to\nhighlight these variations across soil depth in fire-affected and unburned sites, we added a\nfigure to the supplementary information.\nChanges: Figure S6 (below) was added to the Supplementary Information. Succeeding\nfigure numbers were updated based on this addition. Within the Results and Discussion\nsections, we now state:\n“Average Cr(VI) concentrations generated in soils derived from mélange (Figure 2b)\nwere more than double the respective levels in rhyolitic soil (Figure 2a) regardless of\noverlapping ranges in total Cr content, 152-954 and 102-338 mg/kg, respectively,\nreflecting the potential contribution of differing mineralogy to Cr(VI) generation (Figure\nS6) 35.”\n“In our study, and under natural wildfire conditions, we observed that up to about 0.015%\nof total Cr was reactive Cr(VI) in burned serpentine soils (Figure S6).”\nFigure S7: Legend : ... from a serpentine chaparral soil...\nResponse: We agree with the reviewer’s recommendation and have made the appropriate\nchanges. Based on the reviewer’s previous recommendations, this figure has revised as\nFigure 4 and Figure S9.\nChanges: (Figure 4 and Figure S9 caption) \"... particles (< 53 μm) from a serpentine\nchaparral that experienced high fire severity ...\"\nWhy did the authors not tried to analyze more some Cr(VI) areas on panel A ?\nThe XANES spectrum at point 1 on panel C shows a well-marked Cr(VI) pre-edge peak but the\ncolor code indicate rather low amounts of Cr(VI) at this point. Could the authors explain that ?\nResponse: Our XANES analysis was used to corroborate our bulk measurements of\nreactive Cr(VI). Particles within the XRF map were used to denote the presence and\nabundance of Cr(VI), corroborating (and visualizing) the reactive fraction measurements.\nChanges: “To corroborate bulk measurements of reactive Cr(VI), we combined multi-\nenergy mapping with µ-XANES of select spots on particles to confirm the presence of\nCr(VI).”\nSuggested additional references\nRascio et al., 2022. Evidence of hexavalent chromium formation and changes of Cr speciation\nafter laboratory-simulated fires of composted tannery sludges long-term amended agricultural\nsoils. Journal of Hazardous Materials, 436, 129117.\nhttps://doi.org/10.1016/j.jhazmat.2022.129117\nTerzano et al., 2021. Fire effects on the distribution and bioavailability of potentially toxic\nelements (PTEs) in agricultural soils. Chemosphere, 130752.\nhttps://doi.org/10.1016/j.chemosphere.2021.130752\nRé et al., 2021. Cytotoxic effects of wildfires ashes : In-vitro responses of skin cells.\nEnvironmental Pollution, 285, 117279. https://doi.org/10.1016/j.envpol.2021.117279\nJahn et al., 2021. Metallic and crustal elements in biomass-burning aerosols and ash:\nPrevalence, significance, and similarities to soil particles. ACS Earth and Space Chemistry, 5,\n136-148. https://dx.doi.org/10.1021/acsearthspacechem.0c00191\nXu et al., 2020. Wildfires, global climate change, and human health. The New England Journal\nof Medicine, 383, 2173-2181. https://doi.org/10.1056/NEJMsr2028985\nResponse: We thank the reviewer for sharing additional references. We agree that these\nreferences are relevant to the study.\nChanges: We have added these references suggested above, in addition to a few other\nrecently published and relevant studies, to discussions within the main text.\n“Global wildfire activity represents a rising distributed health risk from smoke and dust\ninhalation 5–10.”\n10. Xu, R. et al. Wildfires, Global Climate Change, and Human Health. N. Engl. J. Med.\n383, 2173–2181 (2020).\n“Increased heavy metals in PM have been documented during wildfire episodes and may\ninduce cytotoxicity, increase lung cancer risks, and greatly contribute to oxidative stress\n19–30.”\n“Suburban fires illustrates the impacts of inhaling Cr(VI)-containing ash within the\nrespiratory tract by measuring Cr(VI) leached with a simulated lung fluid 36,37 and\ndiscerning Cr mineralogy within nano-sized particulates (< 100 nm) 22,30.”\n21. Boaggio, K. et al. Beyond Particulate Matter Mass: Heightened Levels of Lead and\nOther Pollutants Associated with Destructive Fire Events in California. Environ. Sci.\nTechnol. 56, 14272–14283 (2022).\n22. Alshehri, T. et al. Wildland-urban interface fire ashes as a major source of incidental\nnanomaterials. J. Hazard. Mater. 443, 130311 (2023).\n28. Jahn, L. G. et al. Metallic and crustal elements in biomass-burning aerosol and ash:\nPrevalence, significance, and similarity to soil particles. ACS Earth Sp. Chem. 5, 136–\n148 (2021).\n29. Ré, A. et al. Cytotoxic effects of wildfire ashes: In-vitro responses of skin cells.\nEnviron. Pollut. 285, 117279 (2021).\n“Metals in soils and ash are commonly linked to structural burning within wildland-urban\ninterfaces (WUI) 1,31–33, with negligible awareness of wildland landscapes (soils and ash)\nas an alternative and highly distributed source 1.”\n32. Alexakis, D. E. Suburban areas in flames: Dispersion of potentially toxic elements\nfrom burned vegetation and buildings. Estimation of the associated ecological and human\nhealth risk. Environ. Res. 183, 109153 (2020).\n33. Alam, M. et al. Identification and quantification of Cr, Cu, and As incidental\nnanomaterials derived from CCA-treated wood in wildland-urban interface fire ashes. J.\nHazard. Mater. 445, 130608 (2023).\n“Following wildfires, severely burned areas are often barren and blanketed with ash and\nloose, rough topsoil leading to enhanced post-fire wind and water erosion 8,39–42.”\n41. Yu, Y. & Ginoux, P. Enhanced dust emission following large wildfires due to\nvegetation disturbance. Nat. Geosci. 2022 1511 15, 878–884 (2022).\n42. Shakesby, R. A. & Doerr, S. H. Wildfire as a hydrological and geomorphological\nagent. Earth-Science Rev. 74, 269–307 (2006).\n“For example, CaCrO was noted after agricultural soil amended with composted Cr(III)-\n4\nrich tannery sludge was heated at 500°C 55.”\n55. Rascio, I. et al. Evidence of hexavalent chromium formation and changes of Cr\nspeciation after laboratory-simulated fires of composted tannery sludges long-term\namended agricultural soils. J. Hazard. Mater. 436, 129117 (2022).\nReviewer 2\nThe authors have to highlight the novelty of their manuscript. The abstract should be revised to\nattract the reader’s attention. The Introduction section should be improved by adding references\ndealing soil contamination issues. However, the problem is that the English and the whole\norganisation of the present version are definitely below an acceptable standard for an\ninternational scientific journal. Analytical quality control is missing. Detection limits of the\napplied methods should be reported. The main problem for this manuscript is its structure.\nMajor parts are missing from the manuscript. My suggestion is to reject this manuscript and\nencourage the authors to submit a more mature manuscript.\nResponse: We appreciate the reviewer's feedback and have sought to make the abstract,\nand the manuscript, have more pizazz. With that said, the manuscript was formatted\nspecifically to the Nature guidelines. Further, the authors are all native speakers, and the\nsenior author has published several hundred articles, including ones in Nature and\nScience. The present manuscript holds to those same standards. It is also worth noting\nthat counter to Reviewer 2, Reviewer 1 stated “I found this paper very pleasant to read.\nThe subject is well introduced, the sites and samples are well described and fit with the\nobjective of the study, the results are well presented and they support the discussion…”.\nFurther, several established authors at Stanford have read the manuscript and all support\nthe writing and presentation. Thus, while we don’t want to dismiss the comments of the\nReviewer, we do see them as anonymous in comparison to others.\nBased on the reviewer’s feedback, we have revised our Methods section to include more\ninformation regarding our analyses. We have added detection limits for Cr(VI) and NH\n4\nmeasurements, and have revised related Figures and Tables to reflect these changes. With\nthe exception of Na (detection limit of 0.1 mg/g), total concentrations for all elements\nreported using XRF were significantly greater than respective detection limits. We\nperiodically analyzed certified reference material, NIST 2711a, with bulk soil samples to\nconfirm accuracy of the XRF instrument. For aqueous extractions and associated\ninstrument analyses, we tracked quality assurance in multiple ways. For each round of\naqueous extractions using 10 mM K HPO /KH PO solution, we included at least two\n2 4 2 4\ncentrifuge tubes containing the phosphate buffer solution and no soil/ash that were\nanalyzed similar to samples for Cr(VI) and NH . Unless soil mass was limited,\n4\nextractions were conducted in triplicate to assess sample heterogeneity. On the UV-Vis\nand ICP-MS, we analyzed instrument blanks every 15-20 samples and multiple quality\ncontrol standard solutions prepared with certified Cr reference solutions throughout each\nanalysis.\nChanges: We have modified the Abstract, added references to the Introduction section,\nand have sought to ensure the format and writing are consistent with the expected quality\nof the Nature journals. We have also revised the Methods to include detection limits,\nwhere relevant, and have similarly revised Figures and Tables to reflect non-detectable\nsample concentrations throughout the manuscript.\nWithin the Methods section, we now state:\n\"Aqueous Extractions and Chemical Analysis\nReactive Cr(VI) concentrations (most available fraction, including dissolved and\nadsorbed Cr(VI)) in bulk soil ash samples (bulk and particle size fraction < 53 µm) and\nwithin soil cores were extracted with 10 mM K HPO /KH PO (buffered at pH 7.2) 79,80.\n2 4 2 4\nPhosphate effectively competes with Cr(VI) ions for surface adsorption sites. At\ncircumneutral to alkaline pH ranges in natural soils, it’s expected that nearly all aqueous\nCr is present in the hexavalent form, and that Cr(VI) concentrations will be primarily\nlimited by adsorption 81. The clay size fraction (less than 2-µm diameter) typically has a\ndominant influence on species retention given their high surface areas and greater number\nof adsorption sites; therefore, it is likely that the reactive Cr(VI) concentrations measured\nhere largely represent the fraction of Cr(VI) associated with clay particles. Triplicate\nsamples were agitated in a 1:4 soil/solution ratio for 24 h, centrifuged (30 min, 4000 rpm,\n4°C), and filtered through 0.22-µm filters. A subsample of unacidified filtrate was used to\nquantify aqueous Cr(VI) concentrations using the diphenylcarbazide (DPC) method on a\nUV-Vis spectrophotometer (Shimadzu UV-1601) 79,82. The detection limit was 3 µg/L\n(approximately 12 µg/kg) 82. Total Cr concentrations were determined with inductively\ncoupled plasma mass spectrometry (ICP-MS, Thermo Scientific XSERIES 2), and\nconfirmed that approximately all aqueous Cr was in the form of Cr(VI) in unburned soil\nand burned soil and ash, similarly observed in previous studies 36,37. An aliquot of each\nsoil extract was immediately acidified post-filtration and stored in 2% nitric acid at 4°C\nuntil ICP-MS analysis.\nTo determine relative differences in K+-extractable NH + concentrations (mg NH +-N/kg)\n4 4\nwithin burned and unburned soils (Figure S10), additional unacidified samples (after\nK HPO /KH PO extraction) from 30 of the 38 total soil cores (21 fire-affected and 9\n2 4 2 4\nunburned soil cores) were frozen at -20°C until chemical analysis. Ammonium is a direct\ncombustion product and will be elevated in the near surface soil after wildfires depending\non burn severity 40. Ammonium concentrations in the top 6-cm were measured in\ntriplicate (when sample volume allowed) using a flow injection analyzer (Westco\nSmartChem 200 Discrete Analyzer), with a detection limit of 0.05 mg/L82.\nStatistical Analyses\nMeans and standard errors were calculated for aqueous and solid-phase chemical\nmeasurements in all cores using replicates described below. Half the detection limit was\nused when measured concentrations were below detection limits. Total elemental\nconcentrations were measured in 3-4 solid-phase aliquots from each soil core (Table S1).\nAt each soil depth interval (1-cm from 0-6 cm; 2-cm from 6-16 cm), triplicate aqueous\nextractions were conducted to evaluate reactive Cr(VI) and exchangeable NH +\n4\nconcentrations (Figure 2, Figure 3a, Figure S10). In select soil depths within cores,\nreplicates were limited (less than 3) due to solid mass or post-extraction aqueous volume.\nTo assess data normality, we applied the Shapiro-Wilk test and reported W statistics and\np-values (Table S7). If data met normality assumptions at the 95% confidence interval\n(p-value = 0.05), we used two-sided parametric tests; otherwise, we utilized two-sided\nnonparametric tests. Likewise, we used the f-test to determine equal variance. Unpaired t\ntests were used to compare mean reactive Cr(VI) concentrations at the 95% confidence\ninterval in near surface soil (0-2 cm) of fire-affected and unburned sites based on\ngeology. If one or both datasets were not normally distributed, such as in burned and\nunburned soils at control depths (10-16 cm), Mann-Whitney U test was used. Within a\nsoil core, we compared mean reactive Cr(VI) concentrations in surface soil (0-2 cm)\nversus control depths (10-16 cm) using either paired t test or Wilcoxon signed rank test.\nDetailed information about and results for each statistical analysis is provided in Tables\nS2-S5 and Table S7 the Supplementary Information. All statistical analyses were\nexecuted using the stats package in R (v. 4.1.3).\"\nFrom comments made on the manuscript pdf\nIntroduction section needs a short paragraph at the beginning to discuss elements distribution\nissues in wildfire impacted areas worldwide. More papers related to this paragraph will be\nbeneficial for the paper.\nMay I suggest, among others, the following articles, e.g.:\n1) Wildfire effects on soil quality. Application on a suburban area of West Attica (Greece).\nGeosciences Journal, 25 (2), 243–253 (https://doi.org/10.1007/s12303-020-0011-1).\n2) Suburban areas in flames: Dispersion of potentially toxic elements from burned vegetation\nand buildings. Estimation of the associated ecological and human health risk. Environmental\nResearch, 183, 109153, https://doi.org/10.1016/j.envres.2020.109153.\n3) Elements’ Content in Stream Sediment and Wildfire Ash of Suburban Areas in West Attica\n(Greece). Water 2022, 14, 310. https://doi.org/10.3390/w14030310\nResponse: We thank the reviewer for the suggestion to add a paragraph discussing\nelemental concentrations of soil and ash after wildfires. We address past work that has\nquantified elemental concentrations in soils and ash in the first two paragraphs of the\nIntroduction. We have added the second reference the reviewer suggested to our main\ntext, in addition to a few recent studies on metals and their prevalence as a function of\nstructural burning. References 1 and 3 address elemental concentrations within stream\nsediments of WUIs, and are ancillary to the introduction on metals within airborne\nparticulate matter, and soil and ash, which are the focus of our study.\nChanges: Based on both reviewers' recommendations, we have added the following\nreferences within the main text:\n10. Xu, R. et al. Wildfires, Global Climate Change, and Human Health. N. Engl. J. Med.\n383, 2173–2181 (2020).\n26. Jahn, L. G. et al. Metallic and crustal elements in biomass-burning aerosol and ash:\nPrevalence, significance, and similarity to soil particles. ACS Earth Sp. Chem. 5, 136–\n148 (2021).\n27. Ré, A. et al. Cytotoxic effects of wildfire ashes: In-vitro responses of skin cells.\nEnviron. Pollut. 285, 117279 (2021).\n29. Boaggio, K. et al. Beyond Particulate Matter Mass: Heightened Levels of Lead and\nOther Pollutants Associated with Destructive Fire Events in California. Environ. Sci.\nTechnol. 56, 14272–14283 (2022).\n30. Alshehri, T. et al. Wildland-urban interface fire ashes as a major source of incidental\nnanomaterials. J. Hazard. Mater. 443, 130311 (2023).\n32. Alexakis, D. E. Suburban areas in flames: Dispersion of potentially toxic elements\nfrom burned vegetation and buildings. Estimation of the associated ecological and human\nhealth risk. Environ. Res. 183, 109153 (2020).\n33. Alam, M. et al. Identification and quantification of Cr, Cu, and As incidental\nnanomaterials derived from CCA-treated wood in wildland-urban interface fire ashes. J.\nHazard. Mater. 445, 130608 (2023).\n41. Yu, Y. & Ginoux, P. Enhanced dust emission following large wildfires due to\nvegetation disturbance. Nat. Geosci. 2022 1511 15, 878–884 (2022).\n42. Shakesby, R. A. & Doerr, S. H. Wildfire as a hydrological and geomorphological\nagent. Earth-Science Rev. 74, 269–307 (2006).\n55. Rascio, I. et al. Evidence of hexavalent chromium formation and changes of Cr\nspeciation after laboratory-simulated fires of composted tannery sludges long-term\namended agricultural soils. J. Hazard. Mater. 436, 129117 (2022).\n80. James, B. R. & Bartlett, R. J. Behavior of Chromium in Soils: VII. Adsorption and\nReduction of Hexavalent Forms1. J. Environ. Qual. 12, 177 (1983).\n81. Rai, D., Eary, L. E. & Zachara, J. M. Environmental chemistry of chromium. Sci.\nTotal Environ. 86, 15–23 (1989).\n82. McClain, C. N., Fendorf, S., Johnson, S. T., Menendez, A. & Maher, K. Lithologic\nand redox controls on hexavalent chromium in vadose zone sediments of California’s\nCentral Valley. Geochim. Cosmochim. Acta 265, 478–494 (2019).\nFigure 2: Change A, B, and C in Figure plot panels to a, b, and c, reflected in the figure caption\nResponse: We appreciate the reviewer's feedback and have made the salient changes.\nChanges: We have revised the figure panels (see below) in addition to references to the\nfigure throughout the main text.\nPlease avoid using \"we\". Apply this type of revision all over the manuscript.\nResponse: We appreciate the reviewer's concern for the use of \"we\" within the\nmanuscript. According to the editor and style guidelines for Nature Communications, the\nuse of \"we\" is allowed and in some places encouraged.\nChanges: No changes were made.\nPlease avoid using \"we\".\nResponse: Please refer to previous response.\nChanges: No changes were made.\nFigure 3: Change A, B, and C in Figure plot panels to a, b, and c, reflected in the figure caption\nResponse: We appreciate the reviewer's feedback agree with the suggestions.\nChanges: We have revised the figure panels (see below) in addition to references to the\nfigure throughout the main text.\nFigure 4: Change A and B in Figure plot panels to a and b reflected in the figure caption\nResponse: We appreciate the reviewer's feedback and agree with the suggestions.\nChanges: We have revised the figure panels (see below) in addition to references to the\nfigure throughout the main text. Please note that this figure was moved to the\nSupplementary Information (revised Figure S8), based on the recommendation of\nReviewer 1.\nWhere are the sections \"Results and Discussion\" and \"Conclusions\"?\nResponse: We have followed the format for Nature, which often do not have specific\nsections denoted as \"Results and Discussion\" nor \"Conclusions\". Further, within the\nauthor guidelines for Nature Communication submissions: \"Nature Communications is\nflexible with regard to the format of initial submissions. Within reason, style and length\nwill not directly influence consideration of a manuscript. We also do not require a\nparticular structure or format at first submission. If and when revisions are required, the\neditor will provide detailed formatting instructions at that time.\" Based on the editor's\ninstructions for manuscript revisions, we have defined \"Introduction\" and \"Results\"\nsections in the manuscript according to the Nature Communications formatting\ninstructions.\nChanges: Based on the editor's instructions for manuscript revisions, we have defined\n\"Introduction\" and \"Results\" sections in the manuscript according to the Nature\nCommunications formatting instructions."}]

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