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The Chemistry of Fires at the Wildland-Urban Interface (2022)

Chapter: 5 Water and Soil Contamination

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Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
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5

Water and Soil Contamination

As earlier chapters in this report have discussed, combustion reactions for materials at the wildland-urban interface (WUI; e.g., household components such as siding and plastic, as well as biomass; see Chapter 3) result in emissions of chemical species to the surrounding environment. Although part of these emissions will remain in the air and be further transformed in the downwind plume, as described in Chapter 4, pathways also exist for the mobilization (or partitioning) of some of these chemical species into nearby soils and water streams, potentially impacting ecosystem health or even public health. The possibility also exists of pollutants in the plume being deposited downwind of the fire area, as long-term studies on the impacts of wildfires have observed. Finally, the active process of firefighting could add pollutants to the immediate area, in the form of flame retardants and other compounds (McGee et al., 2003).

All these processes can impact the immediate conditions after the fire, which are critical as first responders and community residents arrive back in the area to assess damage. There is a dearth of peer-reviewed data documenting the specific impacts of WUI fires on water and soil contamination, with only a few studies providing critical information on potential impacts (as will be discussed below). In addition, a number of reports commissioned by the State of California after major fires over the last few years document the potential for contamination from pollutants in ashes.

Perhaps the most widely known case for water contamination observed in the recent past is the case of the contamination of water distribution systems after two significant fires in California, the Tubbs Fire in 2017 and the Camp Fire in 2018 (see Chapter 2 for more details about the Camp Fire). As has been well summarized elsewhere, in these cases, benzene and other small-molecular-weight toxicants (e.g., volatile organic compounds, or VOCs) were measured in the potable water distribution systems. Although limited samples were taken across areas where communities had been burned, levels of benzene above 5 ppb (the US Environmental Protection Agency’s (EPA’s) maximum contaminant level recommendation) were measured in several samples collected from water mains, hydrants, and service connections (Proctor et al., 2020; Solomon et al., 2021). Subsequent investigations indicated that the source of contamination was more than likely the combustion of household materials, as opposed to contamination via other sources (e.g., hydrocarbon storage tanks). Researchers hypothesized that ingress into the distribution system occurred through negative pressures, in piping systems relative to external pressures (Proctor et al., 2020). In these cases, contamination persisted for months until remediation of the situation, which required extensive efforts by the water providers. Additionally, reports of data from WUI fires in California document levels of metals measured in ash samples collected in impacted neighborhoods, with elevated levels of antimony and other species (TetraTech Inc., 2019).

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
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Although the recent cases of water contamination after a WUI fire have become widely known and received significant press coverage, it has been known for decades that wildland fires can impact water and soil quality (Bladon et al., 2014; Hohner et al., 2019a; Olivella et al., 2006; Rhoades et al., 2019b; Smith et al., 2011). These issues will continue to be a concern as the frequency of WUI fires continues to increase.

The focus of this chapter is to provide a description of some of the concerns regarding water and soil contamination after WUI fires. These two media environments are discussed separately. One common theme in the subsequent sections is the lack of detailed information on the specific contamination of water and soils under conditions where the area burned included the built environment, for example, structures (see Chapter 2 for a more extensive definition of WUI fires). Therefore, the discussion cites studies that provide information originally collected for wildland fires, which can be extrapolated to WUI fires. As with other chapters, the last section provides a series of research needs.

Box 5-1 defines key terms that are used throughout this chapter.

IMPACTS TO WATER QUALITY RELATED TO COMMUNITY WATER SYSTEMS

This section discusses impacts to centralized community water systems (CWSs; Figure 5-1), generally in the vicinity near a fire, followed by potential long-range impacts. A CWS is defined as a system serving populations greater than 25 people, year round (EPA, 2021b). The committee does recognize that smaller communities can also be impacted by fires, and the concerns raised will also apply to those communities’ systems, including groundwater systems. The contaminants considered will include largely organic materials and metals, but will not consider what it is typically referred to as black carbon or pyrogenic organic carbon (Jaffé et al., 2013). Also not explicitly covered is fire-impacted or fire-produced carbon in streams. This ill-defined mixture on its own is not considered toxic, even if some of its components (e.g., polycyclic aromatic hydrocarbons, or PAHs) have health considerations.

CWSs rely on stable and predictable water sources. Treatment operations are designed with an expectation that their systems will not be subject to abrupt changes in conditions. CWSs are regulated by federal statutes and therefore are required to meet stringent water quality standards, for example related to disinfection by-product formation levels (EPA, 2021a).

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×
Image
FIGURE 5-1 WUI fires can impact soil and water, ultimately also impacting CWSs. SOURCE: Hohner et al. (2019a).

A WUI fire can potentially impact the operations of a CWS in both the short and long term. Initially, operational concerns arise to address the immediate aftermath of the fire, focusing on returning potable water service to the impacted communities and possibly issuing notices to communities in and near the incident area (e.g., boil water advisories). In the aftermath of the fire, impacts related to water quality occur; different types of chemical species can contaminate the water, with different entry points.

Contamination to a CWS can occur via source water contamination (i.e., streams or reservoirs, some of which are in urban settings, and groundwater) or through critical failures in distribution systems, for example, negative pressure events that may draw in contamination (potentially gas phase or in the form of ashes). Source water issues relate directly to the runoff of contamination during or after the fire. For example, the use of firefighting chemicals can result in mobilization that ultimately impacts source waters, although the use of these firefighting chemicals is restricted near surface waters (Nolen et al., 2022). Mobilization of ashes and runoff from incident areas can impact surface waters as well.

Ashes and Sediments

The types of contamination considered important for CWSs include mobilization of ashes and sediments. These substances increase the turbidity in the water and can also act as a source of other pollutants, as chemicals partition between solid particles and the aqueous environment. The turbidity in the final water produced by a CWS is regulated, and for the most part, high turbidity (>100 NTUs [nephelometric turbidity units, the units from a calibrated nephelometer]) represents a challenge for CWSs (Becker et al., 2018). The particles leading to the turbidity can be mobilized by rain events in the aftermath of a fire and can impact CWS source waters.

Although a challenge to CWSs, turbidity is not directly related to potential health effects. However, it is used as a surrogate for pathogenic contamination (Fewtrell and Bartram, 2001; Sinclair et al., 2012). Microbiological contamination due to failures in drinking water–treatment systems (e.g., reduced disinfectant exposure) can cause immediate health concerns for communities. Treatment operations need to be able to remove the particles causing turbidity, most commonly using the process of coagulation. Work on the efficacy of coagulation at removing these particles suggests that higher coagulant doses are needed to achieve a target turbidity level in finished water (Hohner et al., 2016, 2019b). The physicochemical properties of the particles, including particle size distribution

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

and zeta potential (charge at the surface of a particle), impact the performance of coagulation. Limited work exists on the characterization of these particles after wildland fires, and to the best of the committee’s knowledge, no work has been done on any particles emanating from urban fires or WUI fires.

Ultimately, the particles that mobilize in water serve as a substrate from which the water can leach off other potential pollutants. Wildland fires impact the levels of dissolved organic matter (DOM) in surface waters (Hohner et al., 2019a). For example, recent studies in several burned watersheds provided evidence of changes in the concentration of DOM (Hohner et al., 2019a; Santos et al., 2016). The overall impact in the mobilization of DOM is expected to be a function of combustion temperature. The same studies indicate that mid-level combustion temperatures (~250°C–350°C) produce the highest levels of DOM in water after wildland fires. Once again, the committee found a dearth of information on DOM from particles (from sediment and ashes) after WUI fires. Given the different materials that combust in WUI fires, it is expected that the DOM mobilization from ashes in urban settings will be greater compared to that of wildland settings, although this hypothesis needs to be tested.

DOM is not directly toxic, but it is a substrate for the formation of disinfection by-products (DBPs) upon water disinfection (e.g., chlorination), which is why CWSs pay attention to this parameter. DBPs include a collection of halogenated compounds that are formed via the reaction between electron-rich components of DOM and the hypochlorous acid (or chloramines) used in water treatment (Richardson, 2003). DBPs include trihalomethanes and haloacetic acids, the two most studied classes of compounds in water disinfection, since they are regulated by the EPA at 80 and 60 ppb, respectively (EPA, 2006).

Changes in the concentration of DOM, as well as in the inherent reactivity of its components, can cause issues related to DBP formation. Post-fire, DBP levels can continue to change. Recent reports indicate that DOM levels impacted by fires can either increase or decrease the formation potential of different DBPs (Hohner et al., 2016; Wilkerson and Rosario-Ortiz, 2021). Work has focused mostly on wildland fires, and it is unclear what the impact would be under WUI fire conditions. Ultimately, the impact will relate to the kinds of compounds found in the overall DOM matrix, which is mostly unknown for WUI fires.

Organic Chemicals and Nutrients

The next level of contamination of concern for CWSs relates directly to specific chemicals impacting both surface waters and distribution systems. As one example, research on the impact of wildland fires on water quality has established that nutrient levels are impacted and can be elevated for years after the event. For example, Rhoades and coworkers examined the long-term mobilization of nutrients (e.g., nitrate) after fires, with data suggesting effects even beyond 10 years (Rhoades et al., 2019a). They attributed this observation to changes in the dynamics of nitrogen within the watershed, with changes in the amount of nitrogen retained after deposition. Other studies have also summarized the mobilization of nitrogen (and phosphorus); these studies predicted overall levels below that of any potential health concerns, but high enough to promote eutrophication in reservoirs (Smith et al., 2011). Eutrophication can result in issues with algal blooms, which can lead to the release of algal toxins and impacts to water quality.

In the context of WUI fires, limited data exist on the potential mobilization of nutrients from the combustion of urban materials; measurements are needed in this area. However, concerns related to eutrophication in reservoirs and surface waters are not an immediate concern in the aftermath of a fire; nitrate, for example, has a maximum contaminant level of 10 ppm, and it is unclear whether such levels can be reached in the immediate aftermath of a fire, even near the incident area.

As discussed above, recent reports demonstrated the potential for VOC contamination after WUI fires. Proctor et al. and others reported on the contamination of water distribution systems and households after recent fires in California (Proctor et al., 2020; Solomon et al., 2021). Table 5-1, taken from Proctor et al. (2020), shows the level of contamination measured after the Tubbs and Camp Fires. They provide concentrations for numerous VOCs measured, with benzene the most likely species to be found. Solomon and coworkers also reported benzene detection in samples collected after the 2018 Camp Fire (Solomon et al., 2021). They also detected methylene chloride, although the data suggest that this compound may be formed via other pathways not necessarily related to the wildfires. Work by Schulze and Fischer (2021) posits that local burn severity drives VOC contamination. The main

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

TABLE 5-1 VOCs Detected in Drinking Water Samples from the Tubbs and Camp Fires in California

Chemical Exposure and Public Notification Limits Tubbs Fire (21 months post-fire) Camp Fire (8 months post-fire)
Long-term Limits Short-term Limits City of Santa Rosa PID SWR CB in PID DOWC (three systems)a
USb Californiac HAd NLe n Max n Max n = 1 n Max
Benzene 5 1 200, 26i 8,387 40,000 1,699f 923 >2,217 200g
40/20/140
530
8.1/5.3/530
Dichloromethane 5 200 10,000 6,254 41 NAh 28 NA
Naphthalene 100 500 17 661 6,800 NA 278 693 NA
Styrene 100 100 20,000 6,227 460 NA 6,800 378 NA
Tert-Butyl alcohol 12 339 29 NA 600 NA
Toluene 1,000 20,000 8,387 1,130 NA 1,400 676 NA
Vinyl chloride 2 3,000 6,227 16 NA 0.8 NA

NOTE: All units are μg/L. DOWC, Del Oro Water Company; PID, Paradise Irrigation District; SWRCB, State Water Resources Control Board.

a The three systems shown are Lime Saddle/Magalia/Paradise Pines.

b Federal maximum contaminant level (MCL) (USEPA, 2018)

c California MCL of the Office of Environmental Health Hazard Assessment (OEHHA) (SWRCB, 2019d).

d USEPA 1-day health advisory (HA) for a 10-kg child drinking 1 L/day (USEPA, 2018).

e California notification level (NL) (SWRCB, 2018a).

f This count does not include 98 blanks the were processed.

g This count does not include samples that were reported but later omitted due to long holding times.

h NA indicates that it is unknown how many samples were analyzed for this analyte because the utility did not record this information in their database (i.e., nondetects look the same as not-measured).

i California OEHHA (2019) reported that a benzene concentration of greater than or equal to 26 ppb may cause acute adverse health effects.

SOURCE: Table reproduced from Proctor et al. (2020, Table 2).

concern regarding VOC contamination is ingress through distribution systems, as recent work has rejected other sources (such as liquid fuel contamination).

These recent publications raise several concerns regarding water quality in the immediate aftermath of a WUI fire, as well as long-term (up to a year) concerns with the exposure of communities to unhealthy levels of VOCs. It is not clear what the formation mechanisms for these compounds are. Chapter 3 describes in general terms the complexity that is expected from combustion during WUI fires. These compounds may form in the gas phase and then partition to water or into ashes. If ashes are the main source, negative pressures in the distribution system could draw these materials into pipes, then mobilize them and raise concerns regarding long-term issues.

Given the potential contamination due to the species discussed in this section, CWSs need to have plans in place to isolate areas that may burn, with additional plans to limit the exposure to these compounds and to introduce higher sampling frequency after WUI fire incidents.

The VOC contamination after the Tubbs and Camp Fires raises the concern of what other chemicals, beyond those already reported, can be found in water after fires. Based on the complexities of the combustion reactions that are occurring, one might expect many other compounds to form. Recent work found numerous compounds in surface runoff and mobilized from ashes after wildland fires (Ferrer et al., 2021; Thurman et al., 2020). For example, researchers measured benzene polycarboxylic acids in both simulations of soil combustion and surface runoff samples (Thurman et al., 2020). The combustion reactions described in Chapter 3, and the emission factors measured for a wide range of organic compounds in gas-phase emissions, suggest that some of the compounds formed can partition to water or deposit downstream. Recent work also measured other compounds, in addition to benzene polycarboxylic acids, from ashes and runoff samples collected at different fires (Ferrer et al., 2021). Table 5-2 shows some of the chemical structures recently measured in runoff from wildland fires.

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
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TABLE 5-2 Identifications of Compound Classes Found in Ash Leachates and Water Samples from Wildland Fires

Compound Chemical Structure
Quinoline monocarboxylic acids image
Quinoline dicarboxylic acids image
Naphthoic acid image
Naphthalene dicarboxylic acids image
Naphthalene tricarboxylic acids image
Benzofuran monocarboxylates image
Benzofuran dicarboxylates image
Benzofuran tricarboxylates image

SOURCE: Adapted from Ferrer et al. (2021).

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×
Image
FIGURE 5-2 Total ion chromatogram of aqueous leachate of an ash sample from Cold Springs, collected in Nederland, Colorado. Although over 200 molecular species were detected, only 14 compounds were identified with standards using accurate mass fragmentation with liquid-chromatography quadrupole time-of-flight tandem mass spectrometry. This unpublished data illustrates the complexity of these types of samples and the need for non-targeted analyses to fully characterize them. SOURCE: Figure courtesy of Thurman and Ferrer, University of Colorado Boulder.

Although not every chemical structure is potentially toxic at the levels expected in water (at or below ppb levels), these results indicate that more work is needed on organic compounds in surface waters after fires, with an emphasis on fires at the WUI. Many other compounds will probably be found in surface streams near fires. Figure 5-2 is a total ion chromatogram showing the analysis of one aqueous sample collected in Colorado after a wildland fire. More than 200 molecular structures are observed that have expected concentrations above 0.3 ppb. Additional work to better understand the extent of potential contamination is needed; to better assess potential health impacts, researchers need information on the presence of compounds, their concentrations, and their dynamics.

Finding: VOCs have been found in distribution systems in two incidents related to WUI fires in California, and other chemicals have been identified in water systems after wildland fires.

Research need: Research is needed to further characterize potential chemical contamination to water resources (both surface waters and distribution systems) from WUI fires, and to better understand the formation pathways.

Metals and Other Organic Contaminants

Inorganic species can also be mobilized after fires. For example, many studies show the impact of fires on the mobilization of mercury and other metals after wildland fires (Caldwell et al., 2000; Kelly et al., 2006). In these cases, mercury mobilization can yield elevated levels of methylmercury, which presents a concern in water bodies. Studies have also measured different levels of metals and PAHs in runoff from wildland fires. For example, Stein and coworkers measured elevated concentrations of copper, lead, zinc, and PAHs in runoff after a wildland fire, compared to control sites (Stein et al., 2012). Other reports, as summarized in the review by Smith and coworkers, also indicate substantial mobilization of metals after wildland fires (Smith et al., 2011). Again, most of the published work regards wildland fires.

One source of important information regarding potential contamination from the combustion of urban materials comes from several reports commissioned by the State of California after fires. Although these reports are not peer reviewed, they were conducted following established methods, and the analysis of the samples was conducted using EPA methodologies in certified laboratories.

One of these reports characterized ash samples collected after the 2018 Camp Fire. It provides evidence of the potential for WUI fires to contaminate surface waters (TetraTech Inc., 2019). This study sampled 41 residential

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

sites for a total of 150 individual samples of ash. Of these 41 sites, 37 sites had at least one metal above the screening level. The study identified a total of eight metals over the appropriate screening level, including antimony, arsenic, cobalt, copper, lead, nickel, and zinc. This report did not assess potential contamination in nearby source waters, although it is expected that these metals will partition into water. Given the content of modern homes and vehicles, it is reasonable to expect that ash will have metal contents that could result in contamination to both source waters and distribution systems.

Other similar reports, including those from fires in 2007 and 2015 in California, also confirm the presence of metal and PAHs in ash collected from urban settings. For example, ash samples collected from fires that impacted Southern California in 2007 (the Slide Fire in San Bernardino County and the Witch Creek Fire in San Diego County) showed elevated levels of arsenic, cadmium, copper, lead, and other metals, as well as PAHs including benzo[a] anthracene and naphthalene (Geosyntec Consultants, 2007). Dioxins have also been detected in runoff from burned wildland areas, although at low concentrations (no data are available from WUI fires; Gallaher and Koch, 2004).

Finding: Significant levels of metals were measured in ash samples collected from WUI fire incidents in California.

Research need: Research is needed to further characterize the potential for chemical contamination stemming from ash mobilization into both soil and water after WUI fires.

Although numerous studies characterize gas-phase emissions from the combustion of both urban materials and biomass (as summarized in Chapter 3), limited information exists on the potential mobilization of contaminants to water. Current work is largely observational, and conclusions are hard to generalize. Even though benzene was measured in both distribution systems and combustion studies with pipes, the main contamination mechanism is not clear. It is also not clear whether other compounds are present, even though studies from wildland fires suggest that many types of compounds could be present.

Fire Retardants

Potential contamination of surface waters and groundwaters by firefighting chemicals is also a concern. Fire suppressants are widely used in areas of active wildfires. The US Forest Service evaluates chemical products used in wildland firefighting activities for toxicological impacts and risks to human health (USFS, n.d.). Two major types of chemicals are commonly used: long-term retardants and foams (including Class A foam suppressants and water enhancers). Long-term fire retardants are used to decrease fire intensity and slow fire spread, and can be used preemptively ahead of an approaching fire.

Typical fire suppressants used for wildland and WUI fires are the Class A suppressants. Class A fire suppressants contain foaming and wetting agents. Perhaps the most common fire suppressant is Phos-Chek, which has been deployed by planes combating wildland fires. These Class A foams contain mostly inorganic salts (including ammonium, phosphates, and sulfates), in addition to other ingredients. Furthermore, as part of the application protocols, these suppressants are applied away from streams, although the substances could potentially reach streams. As such, limited impacts of these compounds are expected to surface waters. Mobilization of organic salts has the potential to result in eutrophication and changes to trophic levels (Angeler and Moreno, 2006; Angeler et al., 2005). But limited impacts to human health are expected since water enhancers or gels, composed of polymers and hydrocarbons, are used (Carratt et al., 2017).

Class A fire suppressants do not contain perfluorinated compounds, but Class B suppressants do. These substances, commonly known as PFASs, can cause adverse health effects. A significant number of cases of groundwater contamination with PFASs have been reported in different locations, including near sites where firefighting training occurs and the use of aqueous film–forming foams is documented. Class B fire suppressants could raise serious issues for CWSs in the aftermath of their use. For example, an incident in Galveston Bay in 2019 at a petrochemical plant resulted in significant contamination of water around the site (Nolen et al., 2022). Class B foams, which may contain fluorinated surfactants, have been used for liquid fuel fires but are not included in the

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

US Forest Service Wildland Fire Chemical Systems program and are unlikely to be used in a wildland fire situation, or for residential structures in a WUI fire.

Long-term fire suppressants are composed of some form of ammonium phosphate (monoammonium phosphate, diammonium phosphate, or ammonium polyphosphate), often mixed with dyes, wetting agents, anticorrosive agents, or other performance additives. Recently, durable, long-term ammonium polyphosphate fire retardants have been marketed for preventative application in areas prone to wildland fires, including WUI areas (Perimeter Solutions, 2022). Widespread use of preventative fire retardants may occur in areas not imminently threatened by wildland fire, which might change the chemicals of concern involved in a WUI fire, as well as the fire behavior in high-risk WUI areas. Furthermore, given that the chemical composition of these products is largely proprietary, it is difficult to determine what, if any, chemicals of concern these fire-suppression products contain.

IMPACTS TO GROUNDWATER AND SOIL CONTAMINATION

The impact of WUI fires on groundwater supplies needs to be considered. Mansilha and coworkers reported PAH contamination in groundwaters after a wildland fire (Mansilha et al., 2014). In their study, the total sum of PAHs in burned areas ranged from 23.1 to 95.1 ppt, about six times higher than for control samples. Once again, a dearth of information exists regarding groundwater contamination after a WUI fire, and environmental measurements need to be conducted to guide the research.

Soil contamination is also possible after a wildfire. As with water contamination, published data on soil contamination directly related to WUI fires are scarce. The literature on wildland fires and soil contamination includes examples of contamination due to different organic and inorganic compounds, such as PAHs and metals. As an example, García-Falcón et al. (2006) showed elevated levels of PAHs after wildland fires. More recently, Chen et al. (2018) investigated the levels of PAHs in soils and ash after fires. Recently, a metadata analysis of published PAH data in soils after wildland fires presented several conclusions, including how wildland fires increase PAH concentrations by approximately 205 percent compared to unburned soils (Yang et al., 2022). The same report demonstrated how the significant increase in PAHs overall in soils impacted by wildland fires corresponded to only a 73 percent increase in toxicity equivalence, mostly due to the production of low-molecular-weight PAHs. Finally, the study showed that the increase in PAH content was mostly observed in ash and topsoils, and that mild-intensity fires created higher PAH concentrations compared to both moderate and high-intensity events.

Once again, limited information exists regarding the expected levels of soil contamination after a WUI fire, though it is expected that a wide range of these organic compounds can be found.

Regarding fluorinated compounds, substantial evidence indicates widespread PFAS contamination in soils near sites where these compounds are used. These sites tend to be near firefighting training facilities, where aqueous film–forming foams are used for practice; or in cases where Class B foams were used for liquid fuel fires, such as at the petrochemical plant in Galveston Bay in 2019. Aqueous film–forming foam contamination results in mobilization to soils (Maizel et al., 2021).

As mentioned in the previous section on water quality, the State of California sponsored testing of ash in soils after the Camp Fire in 2018 (TetraTech Inc., 2019) and found numerous contaminants in soil. In addition, other chemicals are expected to be found in soils, although information is scarce regarding WUI fires. The studies from large-scale fire incidents also apply to soil contamination.

IMPACTS OF ATMOSPHERIC WET AND DRY DEPOSITION

Atmospheric deposition is a major contributor of nutrients (e.g., reactive nitrogen), acidic species (e.g., acidic sulfate, nitric acid, hydrochloric acid, acetic acid, formic acid), and toxicants (e.g., PAHs, mercury, chlorinated organics, PFASs) to soils, lakes, estuaries, and the remote ocean (Clark et al., 2018; Degrendele et al., 2020; Leister and Baker, 1994; McVeety and Hites, 1988; Shimizu et al., 2021; Valiela et al., 2018; Wiener et al., 2006; Wright et al., 2018; Young et al., 2007). The largest fluxes to terrestrial and aquatic bodies occur close to the source. However, atmospheric deposition can substantially impact ecological systems in remote locations as well, including pristine systems (e.g., alpine lakes and the arctic; Wright et al., 2018).

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

Deposition mechanisms (and chemical mechanisms) are different for gases and airborne particles, and thus the partitioning of a species between the gas and particle phases has a substantial impact on the species’s atmospheric lifetime and the geographic distribution of the deposited species (Wania et al., 1998). Scavenging by precipitation (wet deposition) and gravitational settling (dry deposition) generally limits the atmospheric transport of particle-phase species to several thousands of miles (1–2 weeks of transport; NASA EO, 2017), but when particles are lofted into the upper troposphere, as often happens with wildland and WUI fires, interhemispheric transport can occur (NASA EO, 2017; Stohl et al., 2002). Wet deposition of gas-phase species occurs via precipitation after in-cloud or below-cloud scavenging, and can be an effective deposition mechanism for water-soluble gas-phase species (e.g., acid gases). Dry deposition via diffusion of vapors to terrestrial and aquatic surfaces (Wesley and Hicks, 2000) is also an important mechanism for many volatile species (Pan et al., 2012; Totten et al., 2006). VOCs and semi-VOCs with low atmospheric reactivities can be transported globally.

Wildland fires have been identified as a substantial contributor to the atmospheric deposition of some species. They are the largest contributor to the dry deposition of reactive nitrogen (ammonia and nitrogen dioxide), which is a nutrient for plant growth and a contributor to eutrophication. For example, Kharol et al. (2018) estimated deposition fluxes of 1.35 Tg of reactive nitrogen over the United States during the 2013 fire season, two to three times larger than deposition from other sources. Tang et al. (2021) argue that the anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the iron-limited Southern Ocean resulted from atmospheric iron deposition from Australian wildland fires. Wildland fires are one of many contributors to mercury deposition, contributing about 10 percent (15 Mg/year) of the annual atmospheric flux of mercury to the Arctic (Kumar and Wu, 2019). Although wildland fires clearly contribute to PAH deposition, an increase in the PAH flux from wildland fires can be hard to discern, given the abundance of other anthropogenic sources (e.g., downwind of the 2016 Fort McMurray Fire in the oil sands region of Canada; Zhang et al., 2022). In cases like this where multiple sources exist, receptor modeling techniques that make use of differences between sources in the relative concentrations of many chemical tracers (source profiles; e.g., Positive Matrix Factorization) can be quite valuable in estimating source contributions.

While there are studies examining the impacts of wildland fires on atmospheric deposition, a paucity of data exists quantifying water and soil contamination caused by wildland fire–associated atmospheric deposition in the near field. One example is a study that concluded atmospheric deposition led to a 5-fold to 60-fold increase in stream phosphorus and nitrogen levels above the background in the vicinity of the 1988 Red Bench Fire in Glacier National Park (Spencer et al., 2003). Atmospheric deposition of WUI fire toxicants, nutrients, and acidic species (e.g., metals, reactive nitrogen, HCl, formaldehyde, isocyanic acid, chlorinated organics, phenols) could be a significant contributor to downwind soil and water contamination; however, deposition fluxes from WUI fires are largely unknown. Identification and measurement of unique atmospheric tracers of WUI fires could help address this gap.

Finding: Data that accurately characterize the (chemical) composition and concentration of WUI fire emissions present in water runoff and soil are missing.

Lessons Learned from Other Types of Disasters

Although significant data gaps exist regarding WUI fires and water contamination, reports do exist on contamination in water and soil from runoff of other types of large-scale fire disasters (see Table 5-3). These reports include the collapse of the World Trade Center and the Grenfell Tower fire in the United Kingdom. In these studies, as described by Guillaume (2020), the impact that any runoff has on the water and soil depends on a wide variety of factors, including the volume of runoff produced, the time of travel from the site of the fire to the receptor, and the dilution in the receiving water body. The chemical composition of the runoff, influenced to a great extent by the source of the fire, is probably the most important factor, as it may include soot, ash, and other suspended solids; the decomposition products of combustion of the building, storage vessels, and substances stored on site; the stored chemicals and their thermal-decomposition products, washed off the site by the runoff; and, if used as a firefighting agent, the firefighting foam.

Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

TABLE 5-3 Published Results on Large-Scale Fire Incidents

Fire Incident Details (Source) Fire Emissions
World Trade Center, September 2001 The fire in and subsequent collapse of the World Trade Center released a wide range of ecotoxicants into the environment (Nordgrén et al., 2002) Analysis of the dust revealed the presence of PAHs, PCBs, PCDDs/PCDFs, pesticides, phthalate esters, heavy metals, brominated diphenyl ethers, synthetic vitreous fibers, and asbestos
Buncefield, December 2005 A fire occurred in an oil storage depot and burned for several days (Lönnermark, 2005) The groundwater under the site and up to 2 km to the north, east, and southeast of the site was heavily contaminated with hydrocarbons and firefighting foams
Grenfell Tower fire, June 2017 A high-rise fire broke out in the 24-story Grenfell Tower block of flats in London (Stec et al., 2019) Analysis of soil revealed the presence of char fragments from building materials, PAHs, PCDDs/PCDFs, flame retardants, and synthetic vitreous fibers

NOTE: PCB = polychlorinated biphenyl; PCDD = polychlorinated dibenzo-p-dioxin; PCDF = polychlorinated dibenzofuran.

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Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

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Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
×

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Suggested Citation:"5 Water and Soil Contamination." National Academies of Sciences, Engineering, and Medicine. 2022. The Chemistry of Fires at the Wildland-Urban Interface. Washington, DC: The National Academies Press. doi: 10.17226/26460.
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Wildfires in America are becoming larger, more frequent, and more destructive, driven by climate change and existing land management practices. Many of these fires occur at the wildland-urban interface (WUI), areas where development and wildland areas overlap and which are increasingly at risk of devastating fires as communities continue to expand into previously undeveloped areas. Unlike conventional wildfires, WUI fires are driven in part by burning of homes, cars, and other human-made structures, and in part by burning vegetation. The interaction of these two types of fires can lead to public health effects that are unique to WUI fires.

This report evaluates existing and needed chemistry information that decision-makers can use to mitigate WUI fires and their potential health impacts. It describes key fuels of concern in WUI fires, especially household components like siding, insulation, and plastic, examines key pathways for exposure, including inhalation and ingestion, and identifies communities vulnerable to exposures. The report recommends a research agenda to inform response to and prevention of WUI fires, outlining needs in characterizing fuels, and predicting emissions and toxicants.

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