Environmental Persistence and Toxicity of Weathered Wildland Fire Retardants to Rainbow Trout

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After application, fire retardants may persist on dry stream beds or in riparian habitats before precipitation events flush the retardant into intermittent streams. We exposed juvenile (30-60 days post swim-up) rainbow trout ( Oncorhynchus mykiss ) to fire retardants weathered for 7-56 days on different substrates (duff, gravel, high organic content soil, and low organic content soil) under static conditions for 96 hrs to evaluate the potential toxicity of two current-use long-term fire retardant (LC95A-R and MVP-Fx) products. Trout mortality was greater in LC95A-R treatments compared to MVP-Fx due to higher concentrations of LC95A-R in the applied product than MVP-Fx at the same application rate. Underlying substrate affected fire retardant toxicity, with 31% higher average mortality for products applied to duff and gravel compared to soil. Differences in mortality across substrates and products after weathering may be attributed to differences in the mix-ratio of applied product and interactions of product chemistries with underlying substrate. These interactions resulted in elevated ionic concentrations of the overlying water in duff and gravel treatments. Trout mortality decreased 15% for products weathered 56 days compared to 7 days. Our results suggest that long-term fire retardants may persist in the environment and that underlying substrate may alter the toxicity of these products upon entrance into an intermittent stream. Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction As the likelihood of extreme weather and drought intensify with climate change, both the size and frequency of wildfires are expected to increase (Rind et al. 1990 ; Jones et al. 2020 ). In 2022, over 65 thousand wildfires burned more than 7.4 million acres in the United States, which is above the 10-year average (U.S. National Interagency Fire Center 2023). These wildfires occur across a variety of habitat types such as forests, grasslands, alpine regions, and encroach on urban areas. Each habitat type has a unique combination of vegetation and soil structure. As such, the tools used to combat and control wildfires need to be effective with different fuel types and substrates. Long-term fire retardants are one tool used to combat and control wildfires. These retardants are used, in part, because they can be applied from the air when ground access is restricted (USFS 2011). Long-term fire retardants decrease the intensity and advancement of fire by creating a fuel break that alters the way fuels burn (USFS 2011; U.S. National Interagency Fire Center 2023). These fire retardants are most effective when applied at high concentrations and can continue to suppress fire even after water in the applied retardant has evaporated (Gould et al. 2000 ; Gimenez et al. 2004 ). In 2022, 43.9 million liters of fire-retardant solution was applied on U.S. Forest Service (USFS) lands to combat wildfire (USFS 2023). Given the volume applied annually, it is essential to understand the impacts retardants may have on the environment. Intermittent streams constitute approximately 60% of the total length of rivers in the contiguous United States (Nadeau and Rains 2007 ). Periodically, these streams naturally cease to flow when evapotranspiration exceeds precipitation. Projected changes to temperature and precipitation patterns caused by climate change may have significant and variable effects to these seasonal streams across the United States (Larned et al. 2010 ; Dhungel et al. 2016 ). Intermittent streams are expected to increase in number and length as perennial rivers (i.e., those with continuous flow) shift to periodic flow regimes due to the potential increase in frequency and duration of drought conditions (Döll and Schmied 2012 ). Increased incidence of wildfires, coupled with more intermittent streams, increases the likelihood that long-term fire retardants may be applied to dry streambeds. Long-term retardants that have not been volatized by wildfire likely persist on the landscape for extended periods during drought conditions. The USFS observed from 2015 to 2018 that approximately 25% of long-term retardant applied by airtankers does not interact with wildfire (USDA Forest Service 2020 ). The remaining retardant is exposed to sunlight, wind, humidity, microbes, and other factors that alter the concentration and chemical properties of the retardant while on the ground. Consequently, as retardants weather on dry streambeds, the resulting toxicity to aquatic biota may be affected once the stream is inundated. For example, the cation exchange capacity (CEC), or ability of soils to bind positively charged ions, can affect the bioavailability of ammonium (NH 4 + ) or other toxic components to organisms (Nommik and Vahtras 1982 ; Nieder et al. 2011 ). Organic matter and clay content are the primary factors that impact the CEC to bind and exchange ammonium (Kissel et al. 2008 ). Little and Calfee ( 2002 ) reported the toxicity of previously used fire retardants to aquatic organisms was significantly higher on soils with low organic content compared to soils with high organic content after 21 days of weathering, likely due to differences in CEC. To simulate exposure to fire retardants in an intermittent stream environment following inundation with water, we applied two current-use long-term fire retardants, LC95A-R and MVP-Fx, to four substrate types (duff, gravel, high organic content soil, and low organic content soil). We assessed the toxicity of these fire retardants on juvenile rainbow trout ( Oncorhynchus mykiss ), a native, economically important fish species, after weathering on dry substrates for 7 to 56 days. The objectives of this study were to determine if the toxicity of fire-retardants to rainbow trout was impacted by 1) the product applied, 2) the underlying substrate, or 3) the duration of weathering prior to inundation with water. We hypothesized that the toxicity would decrease as weathering period increased and in treatments of substrates with higher CEC. Materials and Methods Test Fish Erwin/Arlee strain rainbow trout were obtained from the U.S. Fish and Wildlife Service, Ennis National Fish Hatchery (Ennis, Montana) as eyed eggs. Eggs were hatched in culture and raised until 30–60 days post swim-up and acclimated to ASTM soft water (~ 40 mg/L as CaCO 3 using D1126-17 methods; ASTM International 2014 ) three days prior to the start of each assay at 12°C ± 1.5°C. Test Products Two long-term ammonium-based fire retardants (Phos-Chek LC95A-R and Phos-Chek MVP-Fx; Perimeter Solutions, Clayton, Missouri), which are both on the USFS Qualified Products List in 2024 in 2024, were tested (USDA Forest Service 2024 ). LC95A-R is a liquid concentrate formulation composed of ammonium polyphosphate, a gum thickening agent, and iron oxide as a coloring agent (Perimeter Solutions 2019). MVP-Fx is a powder concentrate formulation composed of a combination of monoammonium phosphate and diammonium phosphate, a gum thickening agent, and iron oxide as a coloring agent (Perimeter Solutions 2018). The concentrates are mixed with water according to manufacture specifications, resulting in solutions that are between 11 and 22% product prior to application on the landscape. The formulations of these fire retardants vary, and some information is proprietary; therefore, total ammonia nitrogen (TAN, un-ionized ammonia [NH 3 ] plus ammonium [NH 4 + ]) concentration and specific conductance were used to explore effects on biota between products. Substrate Collection Four substrates were selected for the study: duff, gravel, high organic content soil (HOC), and low organic content soil (LOC). The HOC was collected from a hardwood forest (University of Missouri Baskett Wildlife Research and Education Center, Ashland, Missouri) by removing any deciduous leaf litter and using shovels to collect the top 10 cm of soil. The LOC was a 1:1 ratio by volume mixture of the HOC and sand (HTH Pool Filter Sand; Innovative Water Care, Amboise, France). The tested soils were chosen because the percentage of organic matter content may affect the toxicity of these fire retardants, and wildfire impacts a variety of different soil types across the United States. The duff was a mixture of decomposing woody material (litter) collected near Missoula, Montana in a coniferous pine-fir forest. The gravel substrate was a mixture of river rock with an average diameter of 3.8 cm. The soils (HOC and LOC) were analyzed for total fertility (pH, neutralizable acidity, organic matter, available phosphorous, calcium, magnesium, and potassium), particle size (percent sand, silt, and clay), base saturation, and CEC at the University of Missouri’s Soil and Plant Testing Lab in Columbia, Missouri prior to the start of the assays (Table 1 ). Soil characteristics were measured and assessed using standard protocols outlined in Nathan et al. ( 2012 ). Duff samples were analyzed for pH and percentage of moisture by the same laboratory. Table 1 Characteristics of underlying duff and soil substrates (HOC = high organic content soil, LOC = low organic content soil) used for testing the environmental persistence of two current use long-term fire retardants (LC95A-R and MVP-Fx). Shown are average values with standard deviation in parentheses (n = 3 samples per substrate type). NM indicates not measured. Gravel was not tested for nutrients in this study. Substrate Characteristics Substrate Duff HOC LOC pH 5.3 (0.1) 5.9 (0.1) 6.2 (0.1) Neutralizable Acidity (mEq/100 g) a NM 2.3 (0.3) 0.5 (0.0) Organic Matter (%) NM 6.8 (0.5) 1.5 (0.1) Bray-1 P (mg/kg) b NM 14.8 (1.0) 11.3 (0.6) Ca (mEq/100 g) a NM 9.2 (0.4) 4.4 (0.2) Mg (mEq/100 g) a NM 2.3 (0.1) 1.0 (0.1) K (mEq/100 g) a NM 0.4 (0.0) 0.2 (0.0) Moisture (%) 22.8 (1.4) NM NM Sand (%) NM 25.0 (0.0) 74.2 (1.4) Silt (%) NM 62.5 (0.0) 19.2 (2.9) Clay (%) NM 12.5 (0.0) 6.7 (1.4) Textural Classification NM Silty loam Sandy loam CEC (mEq/100 g) a,c NM 28.1 (0.8) 6.3 (1.1) a mEq/100 g = milliequivalents per 100 grams b Available phosphate c Cation exchange capacity (ammonium acetate distillation method) Experimental Design To simulate environmental weathering of long-term retardants, we applied either LC95A-R, MVP-Fx, or control water (ASTM soft water) to substrate treatments and weathered them outside for 7 to 56 days. We used 4 replicates per treatment [4 replicates x 3 test waters (2 products and control) x 4 substrates x 4 weathering periods (7, 14, 28, and 56 days)] resulting in 192 total containers (i.e., experimental units). Each container (5-gallon [18.9-liter] polyethylene bucket) received 1 L of substrate, which amounted to a depth of approximately 3 cm. Product or control water was then applied to the surface of the substrate using a backpack sprayer to produce an even application of approximately 210 mL (200.1 mL [± 4.33 mL]) to achieve an application rate of 8 gallons/100 square feet (3.3 L/m 2 ), which is the highest application rate the USFS uses (USFS 2011). Containers were covered nightly and during inclement weather to prevent debris from entering and further dilution of product by rain. Upon the completion of a weathering period (7, 14, 28, and 56 days), sets of containers (48 buckets per weathering period) were transferred inside the U.S. Geological Survey, Columbia Environmental Research Center (CERC, Columbia, Missouri) where a 96-hr toxicity assay was conducted. Each container received 10 L of ASTM soft water resulting in nominal concentration of 5360 mg/L LC95A-R and 2300 mg/L of MVP-Fx. Containers were aerated and held in a water bath to maintain a temperature of 12°C ± 1°C during the assay. After substrate settled for 1 hr, 10 juvenile rainbow trout (30–60 days post swim-up) were added. For some treatments (i.e., duff), substrate was still suspended in the water column, hindering daily visual observations. If visible, fish that were dead, immobile with no visible opercular movement, were removed daily. All surviving fish at the conclusion of the 96-hr assay were enumerated and euthanized with an excess dose of buffered tricaine methanesulfonate (MS-222). Water Chemistry Measurements We monitored water chemistry at the start of each assay, as fish were exposed to the highest chemical concentration during this period. Temperature, dissolved oxygen, specific conductance, pH, alkalinity, total hardness, and TAN were measured following established protocols (APHA 2023). Temperature and dissolved oxygen were measured using a YSI Pro20 dissolved oxygen meter (Xlyem Inc., Washington, D.C.). Specific conductance was measured using a portable Orion Star A325 multiparameter meter (Thermo Fisher Scientific, Waltham, Massachusetts). An HQD HQ440d benchtop meter (HACH Company; Loveland, Colorado) equipped with IntelliCAL electrodes was used to measure pH (PHC301), alkalinity (PHC301), and TAN (ISENH3181). Total hardness was measured using the EDTA (Ethylenediaminetetraacetic acid) titration method. Composite samples of all replicates per treatment were collected for pH, alkalinity, and total hardness. Temperature, dissolved oxygen, and specific conductance measurements were collected from one randomly chosen replicate. The TAN measurements were collected from a composite of replicate samples from the first weathering period treatment and each replicate per weathering period treatment thereafter. A duplicated sample was also collected for one random treatment each sampling event. Given that the relationship between TAN and un-ionized ammonia (NH 3 ) is controlled by temperature and pH, proportions of un-ionized ammonia from the TAN concentrations were calculated using the following equation (Emerson et al. 1975 ; Guan et al. 2010 ): $$\:{NH}_{3}=\frac{{NH}_{3}+{NH}_{4}^{+}}{1+{10}^{\left(pKa-pH\right)}}\:,$$ where p K a is a calculated (p K a = 0.09018 + 2729.92/T; T = temperature in Kelvin) ammonia-TAN equilibrium constant value. When comparing our measured TAN concentrations in the overlying water to the acute water quality criteria set by the U.S. Environmental Protection Agency (U.S. EPA), we normalized those concentrations to a pH of 7 using the following equation (US EPA 2013): $$\:A{V}_{t,7}=\frac{A{V}_{t}}{(\frac{0.0114}{1+{10}^{7.204-pH}}+\frac{1.6181}{1+{10}^{pH-7.204}})},$$ where \(\:{AV}_{t}\) is the TAN concentration. All instruments were calibrated prior to sample measurement and certified standards were measured at the beginning and end of each sampling event to verify the accuracy of those measurements. If standards measured outside of certified ranges, the instrument was recalibrated. Statistical Analysis Total mortality of rainbow trout at the end of each assay was summarized. We tested for differences in mortality between products and among weathering periods and substrate types using generalized linear models (GLM) with a binomial distribution and a logit link. Mortality data were logit transformed to meet model assumptions of GLMs using the “glm” function in the “lme4” package (Bates et al. 2015 ). All fixed-effect variables were evaluated using Analysis of Deviance table (Type III tests) with the “Anova” function in the “car” package (Fox and Weisberg 2019 ). For all significant relationships, Tukey’s multiple comparison test was performed to identify differences among treatments. Analyses were conducted in the R statistical programming language (v 4.3.3; R Core Team 2022 ). Trout mortality and water chemistry data are available in Puglis et al. ( 2023 ). Results Mortality The effects of product (F 2,144 = 10.1, p < 0.001), substrate type (F 3,144 = 10.5, p < 0.001), and weathering period (F 3,144 = 6.76, p < 0.001) were significant factors in the model ( R 2 = 0.85). There was a significant interaction among product and substrate type and weathering period (F 18,144 = 2.15, p = 0.007), between product and substrate type (F 6,144 = 4.13, p < 0.001; Fig. 1 ), between substrate type and weathering period (F 9,144 = 3.26, p = 0.001), and between product and weathering period (F 6,144 = 2.81, p = 0.013). Mortality was highest in LC95A-R across all treatments compared to MVP-Fx or control, with near complete mortality on duff and gravel substrates. In general, mortality in MVP-Fx treatments was similar to controls, except in the gravel treatment, where mortality was greater in MVP-Fx. On average, duff and gravel substrates had 31% more mortality compared to the HOC and LOC soils. Also, there was a general trend of decreasing mortality as weathering period increased for each product, suggesting the toxicity of product decreased over time (Fig. 2 ). Average mortality ranged from 0–40% across control treatments, with the highest mortality observed in duff substrate after 7 days of weathering. Water Chemistry Water temperature (10.3–13.4°C) and dissolved oxygen (7.5–9.9 mg/L) were similar across all treatments. Alkalinity was higher in the products than in the control group, while pH was lower in the products (Table S1 ). We only assessed total hardness for control group water samples (38–87 mg/L) as the products interfered with the water hardness method we used to measure hardness. Specific conductance was higher in the products (LC95A-R = 464–2125 µS/cm, MVP-Fx = 398–1859 µS/cm) compared to the control group (166–197 µS/cm). There were differences in specific conductance across substrates, with 696–2125 µS/cm in duff and gravel treatments and 398–654 µS/cm in the soils between both products. The TAN concentrations were 38.6–398 mg of N/L in LC95A-R and 30.3–524 mg of N/L in MVP-Fx across all treatments. The average TAN concentration in the control group was 0.43 ± 0.29 mg of N/L. There was a decrease of 20–47% in TAN concentrations from day 7 to both 14 and 28 days of weathering and an increase of 56% after 56 days of weathering. The TAN concentrations were highest in duff and gravel substrates (0.08–586 mg of N/L) across all treatments compared with the soils (0.20–95.4 mg of N/L). The products corroded the probe membrane, which caused 50% of the ammonia standards to fall outside of the acceptable range. Discussion In this study, the toxicity of long-term wildland fire retardants to rainbow trout was altered by other environmental variables such as the underlying substrate the product was applied to and the length of time product weathered on dry substrate before inundation with water. In previous work, we found that some environmental variables, such as water temperature, alter the toxicity and subsequent impacts of fire retardant to fish (Puglis et al. 2022 ). We also confirmed differences in toxicity between products as described in Puglis et al. ( 2022 ). Lastly, the significant interaction observed between product and substrate highlights the need to consider underlying substrate when evaluating the toxic effects of weathered fire retardant in intermittent streams. Influence of Product Toxicity differed between products, as LC95A-R was always more toxic than MVP-Fx, regardless of substrate (Fig. 1 ). This is consistent with previous studies in larval fish, which attributed the difference in toxicity to higher un-ionized ammonia concentrations in LC95A-R than MVP-Fx at the same application rate of retardant (Puglis et al. 2022 ; Puglis and Iacchetta 2023). In aqueous solution, TAN is present as a ratio of un-ionized ammonia and ammonium (NH 3 :NH 4 + ). Temperature and pH affect this ratio such that increases in temperature and pH increase the concentration of un-ionized ammonia (Emerson et al. 1975 ); therefore, different environments can have varying ratios of un-ionized ammonia to ammonium. Un-ionized ammonia is relatively more toxic to fish than ammonium due to its neutral charge, which allows for greater uptake through membranes, including the gill epithelium (Evans et al. 1989 ). Exposure to elevated concentrations of un-ionized ammonia can cause convulsions, comas, and eventually lead to death in fish (Randall and Tsui 2002 ). Buhl and Hamilton ( 2000 ) reported a 24-hr LC 50 of un-ionized ammonia as 0.125 mg of N/L for 40 days post swim-up rainbow trout larvae. Un-ionized ammonia concentrations in this study were below this threshold (except for one replicate of LC95A-R weathered on duff substrate after 7 days; NH 3 = 0.139 mg of N/L) and were lower than our previously reported values for similar concentrations of fire-retardant in water-only exposures (Puglis et al. 2023 ). This suggests the ammonium, or some other unmeasured component of the products, contributed to the mortality observed across substrates in the current study. The acute water quality criteria for TAN, a concentration set by the U.S. EPA (US EPA 2013) to protect aquatic life, is 24.1 mg TAN/L at pH 7 and temperatures below 14°C (US EPA 2013). All TAN observed in product treatments within this study (n = 160) were above the criterion, except for five replicates in the soil treatments of MVP-Fx and one replicate of LC95A-R in LOC at 14 days of weathering and all control treatments (Fig. 3 ). Influence of Substrate on Product Toxicity For both fire-retardants tested, we observed higher mortality in duff and gravel treatments compared to soil treatments for a given product (Fig. 1 ). Visual inspection of the data indicates that while the un-ionized ammonia concentrations appeared comparable across all substrates (Fig. 4 a), specific conductance varied, with duff treatments showing relatively high values followed by gravel, HOC, and LOC (Fig. 4 b). Specific conductance is an indirect measure of dissolved ions in a solution, including ammonium. Thus, the specific conductance of the overlying water in our treatments was influenced by substrate and is not independent of the TAN concentrations, but also includes other ions, which may contribute to toxicity. Our data suggest that, within a product, the composition of residual ions in the overlying water of duff and gravel are more toxic than in the overlying water of both soil treatments. One mechanism that influences ion availability is CEC, a measure of a soil’s ability to adsorb and retain cations controlled by the negatively charged sites within a soil (Nommik and Vahtras 1982 ; Sumner and Miller 1996 ). Ammonium is a positively charged ion and thus able to bind to negatively charged sites in soils, making it mostly unavailable for cation exchange reactions (Nieder et al. 2011 ). Clay and organic matter content of soil are the primary constituents that influence the CEC to fix added ammonium (Nommik and Vahtras 1982 ; Kissel et al. 2008 ). Generally, higher clay and/or organic matter content equates to greater CEC (Wright and Foss 1972 ; Martel et al. 1978 ). While not measured, the CEC was likely lowest in the gravel treatments and may have resulted in the product remaining suspended throughout the water column, increasing potential exposure to the trout. This may account for the high mortality observed in gravel treatments compared to soil treatments. We would have predicted that the TAN concentration and mortality would be lower in the HOC soil than the LOC soil treatments, given the CEC of the HOC soil was more than four times that of the LOC soil. Yet, we observed similar mortality and measured similar TAN concentrations between the soil treatments within a product. This suggests the concentration of TAN from the product saturated the negatively charged sites in both soils and exceeded the ability of the CEC to measurably affect the TAN concentration in the overlying water. Although not measured here, duff presumably had a higher CEC compared to the soil and gravel treatments. The CEC of duff collected from a coniferous-deciduous forest in Oregon was more than 50 mEq/100 g (Warner 1976 ), which is higher than our soil treatments (HOC = 28.1 mEq/100 g, LOC = 6.3 mEq/100 g; Table 1 ). Yet, the highest mortality was observed in duff treatments, and as previously stated, un-ionized ammonia concentrations did not vary substantially across substrates within a product. It is possible that the duff was not composed of enough decomposing material to elevate the substrate’s CEC, and thus there was a low binding affinity of ammonium. Additionally, trout may have experienced added stress from suspended duff material in the water column. The effect of suspended solids on Salmonids in controlled environments is associated with death, gill trauma, decreased osmoregulatory ability, and increased blood sugar levels (Redding et al. 1987 ; Muck 2010 ). Therefore, potentially lower than expected CEC and additional stress from substrate suspension in the duff treatment may explain the observed mortality. Influence of Environmental Weathering There was a trend of decreasing mortality as weathering period increased (Fig. 2 ). Mortality observed in rainbow trout exposed to product weathered for 56 days was 15% lower than the observed mortality in those exposed to product weathered for 7 days. Physical and microbial processes such as ammonia volatilization, ammonia nitrification (Aislabie and Deslippe 2013 ), or ammonium fixation could have influenced the bioavailability of TAN to fish. As a result, these products on the landscape may become less toxic over time but likely persist in the environment after 56 days. Conclusions Our results suggest that LC95A-R and MVP-Fx may persist in the environment after weathering for 56 days and that the underlying substrate may alter the toxicity of these products under static conditions. Although these data provide a better understanding of the environmental persistence of long-term fire retardants, aquatic organisms are likely to encounter effects from climate change, wildfires, and intrusion of fire retardant in an intermittent stream. The consequences of climate change could have direct and indirect impacts to organisms that utilize these highly productive, dynamic streams for spawning, refuge, and foraging (Mas-Martí et al. 2010; Vander Vorste et al. 2020). Ash inputs (Earl and Blinn 2003; Murphy et al. 2018), sedimentation (Rhoades et al. 2011), and increased temperatures (Dunham et al. 2007) can occur in these streams following wildfire. Additionally, organisms can experience additional stressors from exposure of long-term retardants, either through direct contact into the waterbody or via runoff from precipitation events after application. The combination of these effects may create a cumulative toxic effect to aquatic organisms. This study provides resource managers with key data characterizing potential toxicity to aquatic life in situations where fire retardant is applied to dry intermittent streambeds. Declarations DISCLAIMER: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Acknowledgement: This work was supported by the U.S. Forest Service (USFS) with additional funding through the U.S. Geological Survey (USGS) Contaminant Biology Program and USGS Toxic Substances Hydrology Program. We’d like to thank J. Orech, E. Scott, S. Tidwell, and T. Tidwell for assistance in data collection. Funding: The work was supported using the funds provided by the USFS, U.S. Geological Survey (USGS) Contaminant Biology Program, and USGS Toxic Substances Hydrology Program. Conflict of Interest: The authors declare that there is no financial or non-financial interest to disclose. Author Contributions: All authors commented and edited revisions to the manuscript. Holly Puglis conceptualized and designed the study. Material preparation, experimentation, investigation, and data collection were performed by Christina Mackey and Holly Puglis. Data analysis was performed by Christina Mackey and Michael Iacchetta. The first draft of the manuscript was written by Christina Mackey. All authors read and approved the final manuscript. Ethical Approval: The present study was reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) at the US Geological Survey – Columbia Environmental Research Center under IACUC19-007. Data Availability Statement: The datasets generated during and/or analyzed during the current study are available in the ScienceBase repository, https://doi.org/10.5066/P9C94L2L. References Aislabie J, Deslippe JR (2013) Soil microbes and their contribution to soil services. In: Dymond J (ed) Ecosystem services in New Zealand–conditions and trends. Manaaki Whenua Press, Lincoln, New Zealand. pp 143–161. https://oldwww.landcareresearch.co.nz/__data/assets/pdf_file/0018/77040/1_12_Aislabie.pdf American Public Health Association (APHA) (2023) Standard methods for the examination of water and wastewater. Lipps WC, Braun-Howland EB, Baxter TE (eds.), 24th edn. Washington DC ASTM International (2014) Standard guide for performing acute toxicity tests on test materials with fishes, macroinvertebrates, and amphibians. E729-96. https://doi.org/10.1520/E0729-96R14 Bates D, Mächler M, Bolker B, Walker S (2015) Fitting Linear Mixed-Effects Models Using lme4. J Stat Softw 67(1):1–48. https://doi.org/10.18637/jss.v067.i01 Buhl KJ, Hamilton SJ (2000) Acute toxicity of fire-control chemicals, nitrogenous chemicals, and surfactants to rainbow trout. Trans Am Fish Soc 129(2):408–418. https://doi.org/10.1577/1548-8659(2000)129%3C0408:ATOFCC%3E2.0.CO;2 Dhungel S, Tarboton DG, Jin J, Hawkins CP (2016) Potential effects of climate change on ecologically relevant streamflow regimes. River Res Appl 32(9):1827–1840. https://doi.org/10.1002/rra.3029 Döll P, Schmied HM (2012) How is the impact of climate change on river flow regimes related to the impact on mean annual runoff? A global-scale analysis. Environ Res Lett 7(1):14037. https://doi.org/10.1088/1748-9326/7/1/014037 Dunham JB, Rosenberger AE, Luce CH, Rieman BE (2007) Influences of wildfire and channel reorganization on spatial and temporal variation in stream temperature and the distribution of fish and amphibians. Ecosystems 10(2):335–346. https://doi.org/10.1007/s10021-007-9029-8 Earl SR, Blinn DW (2003) Effects of wildfire ash on water chemistry and biota in South‐Western USA streams. Freshw Biol 48(6):1015–1030. https://doi.org/10.1046/j.1365-2427.2003.01066.x Emerson K, Russo RC, Lund RE, Thurston R V (1975) Aqueous ammonia equilibrium calculations: effect of pH and temperature. J Fish Res Board Can 32(12):2379–2383. https://doi.org/10.1139/f75-274 Evans DH, More KJ, Robbins SL (1989) Modes of ammonia transport across the gill epithelium of the marine teleost fish Opsanus beta. J Exp Biol 144(1):339–356. https://doi.org/10.1242/jeb.144.1.339 Fox J, Weisberg S (2019) An R Companion to Applied Regression, Third edition. Sage, Thousand Oaks CA. https://socialsciences.mcmaster.ca/jfox/Books/Companion/ Gimenez A, Pastor E, Zárate L, et al. (2004) Long-term forest fire retardants: a review of quality, effectiveness, application and environmental considerations. Int J Wildland Fire 13(1):1–15. https://doi.org/10.1071/wf03001 Gould JS, Khanna P, Hutchings P, et al. (2000) Assessment of the effectiveness and environmental risk of the use of retardants to assist in wildfire control in Victoria (Research Report No. 50). Department of Natural Resources and Environment, State of Victoria. https://www.ffm.vic.gov.au/__data/assets/pdf_file/0009/21060/Report-50-Assessment-of-the-Effectiveness-and-Environmental-Risk-of-the-Use-of-Retardants-to-Assist-in-Wi.pdf Guan B, Hu W, Zhang T, et al. (2010) Acute and chronic un-ionized ammonia toxicity to ‘all-fish’ growth hormone transgenic common carp (Cyprinus carpio L.). Chinese Sci Bull 55:4032–4036. https://doi.org/10.1007/s11434-010-4165-5 Jones MW, Smith A, Betts R, et al (2020) Climate change increases the risk of wildfires: January 2020. ScienceBrief Review 116:117. https://ueaeprints.uea.ac.uk/id/eprint/77982 Kissel DE, Cabrera ML, Paramasivam S (2008) Ammonium, ammonia, and urea reactions in soils. In: Schepers S, Raun WR (eds) Nitrogen in agricultural systems, vol 49. Wiley, New York, pp 101–155. https://doi.org/https://doi.org/10.2134/agronmonogr49.c4 Larned ST, Datry T, Arscott DB, Tockner K (2010) Emerging concepts in temporary‐river ecology. Freshw Biol 55(4):717–738. https://doi.org/10.1111/j.1365-2427.2009.02322.x Little EE, Calfee RD (2002) Environmental Persistence and Toxicity of Fire-Retardant Chemicals, Fire-Trol® GTS-R and Phos-Chek® D75-R to Fathead Minnows. USDA Forest Service, Missoula, MT. https://www.fs.usda.gov/rm/fire/wfcs/documents/NWST-2475.pdf Martel YA, De Kimpe CR, Laverdiere MR (1978) Cation‐exchange capacity of clay‐rich soils in relation to organic matter, mineral composition, and surface area. Soil Sci Soc Am J 42(5):764–767. https://doi.org/10.2136/sssaj1978.03615995004200050023x Mas-Martí E, García-Berthou E, Sabater S, et al. (2010) Comparing fish assemblages and trophic ecology of permanent and intermittent reaches in a Mediterranean stream. In: Stevenson RJ, Sabater S (eds) Global Change and River Ecosystems—Implications for Structure, Function and Ecosystem Services, vol 215. Springer, Dordrecht pp 167–180. https://doi.org/10.1007/s10750-010-0292-x Muck J (2010) Biological effects of sediment on bull trout and their habitat – Guidance for evaluating effects. U.S. Fish and Wildlife Service, Washington Fish and Wildlife Office, Lacey, WA. https://pdf.wildearthguardians.org/site/DocServer/USFWS_BIOLOGICAL%20EFFECTS%20OF% 20SEDIMENT%20ON%20BULL%20TROUT_2010.pdf Murphy SF, McCleskey RB, Martin DA, Writer JH, Ebel BA (2018) Fire, flood, and drought: Extreme climate events alter flow paths and stream chemistry. J Geophys Res Biogeosci 123(8):2513-2526. https://doi.org/10.1029/2017JG004349 Nadeau T, Rains MC (2007) Hydrological connectivity between headwater streams and downstream waters: how science can inform policy. J Am Water Resour Assoc 43:118–133. https://doi.org/10.1111/j.1752-1688.2007.00010.x Nathan MV, Stecker JA, Sun Y (2012) Soil testing in Missouri: A guide for conducting soil tests in Missouri EC923. Division of Plant Sciences, University of Missouri, Missouri. https://hdl.handle.net/10355/50590. Accessed 21 March 2024 Nieder R, Benbi DK, Scherer HW (2011) Fixation and defixation of ammonium in soils: a review. Biol Fertil Soils 47(1):1–14. https://doi.org/10.1007/s00374-010-0506-4 Nommik H, Vahtras K (1982) Retention and fixation of ammonium and ammonia in soils. In: Stevenson FJ (ed) Nitrogen in agricultural soils 22:123–171. https://doi.org/10.2134/agronmonogr22.c4 Perimeter Solutions (2018) Phos-Chek ® MVP-Fx Safety Data Sheet. https://www.perimeter-solutions.com/wp-content/uploads/2022/03/SDS-PHOS-CHEK-MVP-Fx-Version-1.4-OSHA-US-EN.pdf Perimeter Solutions (2019) Phos-Chek ® LC95A/Phos-Chek® LC95A-MV Concentrate Safety Data Sheet. https://www.perimeter-solutions.com/wp-content/uploads/2022/03/SDS-PHOS-CHEK-LC95A-PHOS-CHEK-LC95A-MV-Version-1.4-OSHA-US-WHMIS-CA-EN-1.pdf Puglis HJ, Iacchetta M (2024) Toxicity of Wildland Fire Retardants to Rainbow Trout in Short Exposures. Environ Toxicol Chem. https://doi.org/10.1002/etc.5791 Puglis HJ, Iacchetta M, Mackey CM (2022) Toxicity of Wildland Fire‐Fighting Chemicals in Pulsed Exposures to Rainbow Trout and Fathead Minnows. Environ Toxicol Chem 41(7):1711–1720. https://doi.org/10.1002/etc.5347 Puglis HJ, Mackey CM, Iacchetta MG, Scott EL (2023) Water chemistry and biological data of Rainbow Trout following aquatic exposure to weathered wildland fire retardants after application to substrate. U.S. Geological Survey data release. https://doi.org/10.5066/P9C94L2L R Core Team (2022) R: A language and environment for statistical computing Randall DJ, Tsui TKN (2002) Ammonia toxicity in fish. Mar Pollut Bull 45:17–23. https://doi.org/10.1016/s0025-326x(02)00227-8 Redding JM, Schreck CB, Everest FH (1987) Physiological effects on coho salmon and steelhead of exposure to suspended solids. Trans Am Fish Soc, 116:737–744. https://doi.org/10.1577/1548-8659(1987)1162.0.CO;2 Rhoades CC, Entwistle D, Butler D (2011) The influence of wildfire extent and severity on streamwater chemistry, sediment and temperature following the Hayman Fire, Colorado. Int J Wildland Fire 20:430–442. https://doi.org/10.1071/WF09086 Rind D, Goldberg R, Hansen J, et al. (1990) Potential evapotranspiration and the likelihood of future drought. J Geophys Res Atmos 95(7):9983–10004. https://doi.org/10.1029/JD095iD07p09983 Sumner ME, Miller WP (1996) Cation exchange capacity and exchange coefficients. In: Sparks DL, Page AL, Helmke PA, et al (eds) Methods of Soil Analysis, pp 1201-1229. https://doi.org/10.2136/sssabookser5.3.c40 USDA Forest Service (2020) Aerial firefighting use and effectiveness (AFUE) report. https://www.fs.usda.gov/sites/default/files/2020-08/08242020_afue_final_report.pdf USDA Forest Service (2024) Qualified Products List. https://www.fs.usda.gov/rm/fire/wfcs/products/. Accessed 19 November 2024 US Environmental Protection Agency (US EPA) (2013) Aquatic life ambient water quality criteria for ammonia - freshwater (EPA-822-R-13-001). Washington, DC. https://www.epa.gov/sites/production/files/2015-08/documents/aquatic-life-ambient-water-quality-criteria-for-ammonia-freshwater-2013.pdf US Forest Service (USFS) (2011) Nationwide aerial application of fire retardant on national forest system lands: Final environmental impact statement. US Department of Agriculture https://www.fs.usda.gov/sites/default/files/media_wysiwyg/wfcs_final_feis_1.pdf US Forest Service (USFS) (2023) 2022 Aerial fire retardant use on national forest system lands. US Department of Agriculture. https://www.fs.usda.gov/sites/default/files/2023-05/2022-RetardantUse-NFSL.pdf. Accessed 10 Jul 2023 US National Interagency Fire Center (2023) Interagency standards for fire and fire aviation operations (NFES 2724). Boise, ID. https://www.nifc.gov/sites/default/files/redbook-files/RedBookAll.pdf Vander Vorste R, Obedzinski M, Nossaman Pierce S, et al. (2020) Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams. Glob Chang Biol 26:3834–3845. https://doi.org/10.1111/gcb.15116 Warner SA (1976) Cation exchange properties of forest litter as influenced by vegetation type and decomposition. Thesis, Oregon State University. https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj430691c Wright WR, Foss JE (1972) Contributions of clay and organic matter to the cation exchange capacity of Maryland soils. Soil Sci Soc Am J 36:115–118. https://doi.org/10.2136/sssaj1972.03615995003600010027x Supplementary Files RevisedSupplementaryfile1.docx Cite Share Download PDF Status: Published Journal Publication published 14 May, 2025 Read the published version in Archives of Environmental Contamination and Toxicology → Version 1 posted Editorial decision: Accept as is 10 Apr, 2025 Editor assigned by journal 07 Apr, 2025 First submitted to journal 27 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5829115","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":439721496,"identity":"66209402-3d19-4d1c-b951-101ca81a5ac7","order_by":0,"name":"Christina Mackey","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxklEQVRIiWNgGAWjYJACxgYGBjnStRiTriWxgWjl/NMOP/w4o+Jw+obzxy9/YPhlQ1ivxO00Y8kNZw7nbriRUybB2JdGWIuBdIIZ48M2kBaeNAbGnsOEPWUgnf6N8eG/w+kG588kfyBSS44Z48aGwwkGB9IPSDD8OEw4tCVu5xRLzjiWbjjzRg6bRGJDGmEt/LPTN37sqbGW5zt//PGHD39seAhqgYJmIOYxYEhsI1YDA0MdELM/YGD4Q7yWUTAKRsEoGDkAAOOUQ47kYyRcAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-1737-2698","institution":"US Geological Survey Columbia Environmental Research Center","correspondingAuthor":true,"prefix":"","firstName":"Christina","middleName":"","lastName":"Mackey","suffix":""},{"id":439721497,"identity":"fabc08da-4bd6-4fcb-9cba-9d375b7aeaf7","order_by":1,"name":"Michael Iacchetta","email":"","orcid":"","institution":"US Geological Survey Columbia Environmental Research Center","correspondingAuthor":false,"prefix":"","firstName":"Michael","middleName":"","lastName":"Iacchetta","suffix":""},{"id":439721498,"identity":"80e21a94-7fa6-4429-b856-9cfd67d5df20","order_by":2,"name":"Holly Puglis","email":"","orcid":"","institution":"US Geological Survey Columbia Environmental Research Center","correspondingAuthor":false,"prefix":"","firstName":"Holly","middleName":"","lastName":"Puglis","suffix":""}],"badges":[],"createdAt":"2025-01-14 17:50:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5829115/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5829115/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00244-025-01131-y","type":"published","date":"2025-05-14T15:57:02+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":80214293,"identity":"e5b06d2d-d08e-44f8-96a6-fec4800108b3","added_by":"auto","created_at":"2025-04-09 09:18:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":72743,"visible":true,"origin":"","legend":"\u003cp\u003eThe interactive effect of product (Control, LC95A-R, MVP-Fx) and substrate type (Duff, Gravel, HOC [high organic content soil], LOC [low organic content soil]) on the mean 96-hr mortality of juvenile rainbow trout (\u003cem\u003eOncorhynchus mykiss\u003c/em\u003e) across weathering periods (GLM, F\u003csub\u003e6,144 \u003c/sub\u003e= 4.13, p \u0026lt; 0.001). Error bars represent ± standard error. Asterisks indicate a significant difference in pairwise comparisons of mortality between products from the control group within substrate treatment using post-hoc TukeyHSD analysis. Points jittered on the x-axis to improve visibility\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/fe6e451beb1d1325c65fdcbf.png"},{"id":80214295,"identity":"785e747f-28d2-43a4-9ee4-892667864bed","added_by":"auto","created_at":"2025-04-09 09:18:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":76667,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of environmental weathering on the mean proportion of mortality observed in juvenile rainbow trout following 96 hrs of exposure to LC95A-R (green), MVP-Fx (orange), and control water (black; GLM, F\u003csub\u003e6,144 \u003c/sub\u003e= 2.81, p = 0.013). Error bars represent ± standard error. Points jittered on the x-axis to improve visibility\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/14904916d6a3fbd314581133.png"},{"id":80214296,"identity":"8c96f95a-d657-4b32-b22b-940861b2edf8","added_by":"auto","created_at":"2025-04-09 09:18:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":201195,"visible":true,"origin":"","legend":"\u003cp\u003eTotal ammonia nitrogen (TAN), normalized to a pH of 7, measured at the initiation of each assay for each product (Control, LC95A-R, MVP-Fx) after 7, 14, 28, and 56 days of weathering on (\u003cstrong\u003ea\u003c/strong\u003e) duff, (\u003cstrong\u003eb\u003c/strong\u003e) gravel, (\u003cstrong\u003ec\u003c/strong\u003e) high organic content soil (HOC), and (\u003cstrong\u003ed\u003c/strong\u003e) low organic content soil (LOC). TAN was measured in one replicate (n = 4 replicates) per treatment except treatments following 7 days of weathering, where measurements were sampled from a composite of all 4 replicates. The black horizontal dashed line indicates the acute water quality criteria for TAN at 24.1 mg TAN/L set by the U.S. Environmental Protection Agency (US EPA 2013). Points jittered on the x-axis to improve visibility\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/5b984ab9970892307f68a12e.png"},{"id":80215356,"identity":"a62fcd91-0880-4af2-8cd4-133eb5a8dbed","added_by":"auto","created_at":"2025-04-09 09:26:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":87332,"visible":true,"origin":"","legend":"\u003cp\u003eMean concentration of (\u003cstrong\u003ea\u003c/strong\u003e) un-ionized ammonia (mg of N/L, n = 160) and (\u003cstrong\u003eb\u003c/strong\u003e) specific conductance (µS/cm, n = 52) for each substrate (Duff, Gravel, HOC [high organic content soil], LOC [low organic content soil]) and product (Control, LC95A-R, MVP-Fx) measured from the initiation of assays. Error bars represent ± standard error. Points jittered on the x-axis to improve visibility\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/4241c1280099235d7e01bacd.png"},{"id":83067801,"identity":"9e69490e-4615-45a1-bc94-781954ae79a3","added_by":"auto","created_at":"2025-05-19 16:06:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":981190,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/c898386b-c0fe-4d92-a45f-e90b76c1242f.pdf"},{"id":80214306,"identity":"8fbaf769-24aa-47f6-8691-3f4a506c11e7","added_by":"auto","created_at":"2025-04-09 09:18:58","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":26901,"visible":true,"origin":"","legend":"","description":"","filename":"RevisedSupplementaryfile1.docx","url":"https://assets-eu.researchsquare.com/files/rs-5829115/v1/b204000e4fbda2389160f397.docx"}],"financialInterests":"","formattedTitle":"Environmental Persistence and Toxicity of Weathered Wildland Fire Retardants to Rainbow Trout","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAs the likelihood of extreme weather and drought intensify with climate change, both the size and frequency of wildfires are expected to increase (Rind et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Jones et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In 2022, over 65 thousand wildfires burned more than 7.4\u0026nbsp;million acres in the United States, which is above the 10-year average (U.S. National Interagency Fire Center 2023). These wildfires occur across a variety of habitat types such as forests, grasslands, alpine regions, and encroach on urban areas. Each habitat type has a unique combination of vegetation and soil structure. As such, the tools used to combat and control wildfires need to be effective with different fuel types and substrates.\u003c/p\u003e \u003cp\u003eLong-term fire retardants are one tool used to combat and control wildfires. These retardants are used, in part, because they can be applied from the air when ground access is restricted (USFS 2011). Long-term fire retardants decrease the intensity and advancement of fire by creating a fuel break that alters the way fuels burn (USFS 2011; U.S. National Interagency Fire Center 2023). These fire retardants are most effective when applied at high concentrations and can continue to suppress fire even after water in the applied retardant has evaporated (Gould et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Gimenez et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). In 2022, 43.9\u0026nbsp;million liters of fire-retardant solution was applied on U.S. Forest Service (USFS) lands to combat wildfire (USFS 2023). Given the volume applied annually, it is essential to understand the impacts retardants may have on the environment.\u003c/p\u003e \u003cp\u003eIntermittent streams constitute approximately 60% of the total length of rivers in the contiguous United States (Nadeau and Rains \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Periodically, these streams naturally cease to flow when evapotranspiration exceeds precipitation. Projected changes to temperature and precipitation patterns caused by climate change may have significant and variable effects to these seasonal streams across the United States (Larned et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Dhungel et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Intermittent streams are expected to increase in number and length as perennial rivers (i.e., those with continuous flow) shift to periodic flow regimes due to the potential increase in frequency and duration of drought conditions (D\u0026ouml;ll and Schmied \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Increased incidence of wildfires, coupled with more intermittent streams, increases the likelihood that long-term fire retardants may be applied to dry streambeds.\u003c/p\u003e \u003cp\u003eLong-term retardants that have not been volatized by wildfire likely persist on the landscape for extended periods during drought conditions. The USFS observed from 2015 to 2018 that approximately 25% of long-term retardant applied by airtankers does not interact with wildfire (USDA Forest Service \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The remaining retardant is exposed to sunlight, wind, humidity, microbes, and other factors that alter the concentration and chemical properties of the retardant while on the ground. Consequently, as retardants weather on dry streambeds, the resulting toxicity to aquatic biota may be affected once the stream is inundated. For example, the cation exchange capacity (CEC), or ability of soils to bind positively charged ions, can affect the bioavailability of ammonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) or other toxic components to organisms (Nommik and Vahtras \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Nieder et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Organic matter and clay content are the primary factors that impact the CEC to bind and exchange ammonium (Kissel et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Little and Calfee (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) reported the toxicity of previously used fire retardants to aquatic organisms was significantly higher on soils with low organic content compared to soils with high organic content after 21 days of weathering, likely due to differences in CEC.\u003c/p\u003e \u003cp\u003eTo simulate exposure to fire retardants in an intermittent stream environment following inundation with water, we applied two current-use long-term fire retardants, LC95A-R and MVP-Fx, to four substrate types (duff, gravel, high organic content soil, and low organic content soil). We assessed the toxicity of these fire retardants on juvenile rainbow trout (\u003cem\u003eOncorhynchus mykiss\u003c/em\u003e), a native, economically important fish species, after weathering on dry substrates for 7 to 56 days. The objectives of this study were to determine if the toxicity of fire-retardants to rainbow trout was impacted by 1) the product applied, 2) the underlying substrate, or 3) the duration of weathering prior to inundation with water. We hypothesized that the toxicity would decrease as weathering period increased and in treatments of substrates with higher CEC.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eTest Fish\u003c/h2\u003e \u003cp\u003eErwin/Arlee strain rainbow trout were obtained from the U.S. Fish and Wildlife Service, Ennis National Fish Hatchery (Ennis, Montana) as eyed eggs. Eggs were hatched in culture and raised until 30\u0026ndash;60 days post swim-up and acclimated to ASTM soft water (~\u0026thinsp;40 mg/L as CaCO\u003csub\u003e3\u003c/sub\u003e using D1126-17 methods; ASTM International \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) three days prior to the start of each assay at 12\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u0026deg;C.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eTest Products\u003c/h3\u003e\n\u003cp\u003eTwo long-term ammonium-based fire retardants (Phos-Chek LC95A-R and Phos-Chek MVP-Fx; Perimeter Solutions, Clayton, Missouri), which are both on the USFS Qualified Products List in 2024 in 2024, were tested (USDA Forest Service \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). LC95A-R is a liquid concentrate formulation composed of ammonium polyphosphate, a gum thickening agent, and iron oxide as a coloring agent (Perimeter Solutions 2019). MVP-Fx is a powder concentrate formulation composed of a combination of monoammonium phosphate and diammonium phosphate, a gum thickening agent, and iron oxide as a coloring agent (Perimeter Solutions 2018). The concentrates are mixed with water according to manufacture specifications, resulting in solutions that are between 11 and 22% product prior to application on the landscape. The formulations of these fire retardants vary, and some information is proprietary; therefore, total ammonia nitrogen (TAN, un-ionized ammonia [NH\u003csub\u003e3\u003c/sub\u003e] plus ammonium [NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e]) concentration and specific conductance were used to explore effects on biota between products.\u003c/p\u003e\n\u003ch3\u003eSubstrate Collection\u003c/h3\u003e\n\u003cp\u003eFour substrates were selected for the study: duff, gravel, high organic content soil (HOC), and low organic content soil (LOC). The HOC was collected from a hardwood forest (University of Missouri Baskett Wildlife Research and Education Center, Ashland, Missouri) by removing any deciduous leaf litter and using shovels to collect the top 10 cm of soil. The LOC was a 1:1 ratio by volume mixture of the HOC and sand (HTH Pool Filter Sand; Innovative Water Care, Amboise, France). The tested soils were chosen because the percentage of organic matter content may affect the toxicity of these fire retardants, and wildfire impacts a variety of different soil types across the United States. The duff was a mixture of decomposing woody material (litter) collected near Missoula, Montana in a coniferous pine-fir forest. The gravel substrate was a mixture of river rock with an average diameter of 3.8 cm. The soils (HOC and LOC) were analyzed for total fertility (pH, neutralizable acidity, organic matter, available phosphorous, calcium, magnesium, and potassium), particle size (percent sand, silt, and clay), base saturation, and CEC at the University of Missouri\u0026rsquo;s Soil and Plant Testing Lab in Columbia, Missouri prior to the start of the assays (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Soil characteristics were measured and assessed using standard protocols outlined in Nathan et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Duff samples were analyzed for pH and percentage of moisture by the same laboratory.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacteristics of underlying duff and soil substrates (HOC\u0026thinsp;=\u0026thinsp;high organic content soil, LOC\u0026thinsp;=\u0026thinsp;low organic content soil) used for testing the environmental persistence of two current use long-term fire retardants (LC95A-R and MVP-Fx). Shown are average values with standard deviation in parentheses (n\u0026thinsp;=\u0026thinsp;3 samples per substrate type). NM indicates not measured. Gravel was not tested for nutrients in this study.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSubstrate Characteristics\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eSubstrate\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDuff\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHOC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLOC\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.3\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.9\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.2\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNeutralizable Acidity (mEq/100 g) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003cp\u003e(0.3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOrganic Matter (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003cp\u003e(0.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBray-1 P (mg/kg) \u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e14.8\u003c/p\u003e \u003cp\u003e(1.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e11.3\u003c/p\u003e \u003cp\u003e(0.6)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa (mEq/100 g) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.2\u003c/p\u003e \u003cp\u003e(0.4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.4\u003c/p\u003e \u003cp\u003e(0.2)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMg (mEq/100 g) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003cp\u003e(0.1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK (mEq/100 g) \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMoisture (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e22.8\u003c/p\u003e \u003cp\u003e(1.4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSand (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25.0\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e74.2\u003c/p\u003e \u003cp\u003e(1.4)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSilt (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e62.5\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e19.2\u003c/p\u003e \u003cp\u003e(2.9)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClay (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12.5\u003c/p\u003e \u003cp\u003e(0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.7\u003c/p\u003e \u003cp\u003e(1.4)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTextural Classification\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSilty loam\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSandy loam\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCEC (mEq/100 g) \u003csup\u003ea,c\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNM\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e28.1\u003c/p\u003e \u003cp\u003e(0.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.3\u003c/p\u003e \u003cp\u003e(1.1)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003ea\u003c/sup\u003e mEq/100 g\u0026thinsp;=\u0026thinsp;milliequivalents per 100 grams\u003c/p\u003e \u003cp\u003e \u003csup\u003eb\u003c/sup\u003e Available phosphate\u003c/p\u003e \u003cp\u003e \u003csup\u003ec\u003c/sup\u003e Cation exchange capacity (ammonium acetate distillation method)\u003c/p\u003e\n\u003ch3\u003eExperimental Design\u003c/h3\u003e\n\u003cp\u003eTo simulate environmental weathering of long-term retardants, we applied either LC95A-R, MVP-Fx, or control water (ASTM soft water) to substrate treatments and weathered them outside for 7 to 56 days. We used 4 replicates per treatment [4 replicates x 3 test waters (2 products and control) x 4 substrates x 4 weathering periods (7, 14, 28, and 56 days)] resulting in 192 total containers (i.e., experimental units). Each container (5-gallon [18.9-liter] polyethylene bucket) received 1 L of substrate, which amounted to a depth of approximately 3 cm. Product or control water was then applied to the surface of the substrate using a backpack sprayer to produce an even application of approximately 210 mL (200.1 mL [\u0026plusmn;\u0026thinsp;4.33 mL]) to achieve an application rate of 8 gallons/100 square feet (3.3 L/m\u003csup\u003e2\u003c/sup\u003e), which is the highest application rate the USFS uses (USFS 2011). Containers were covered nightly and during inclement weather to prevent debris from entering and further dilution of product by rain.\u003c/p\u003e \u003cp\u003eUpon the completion of a weathering period (7, 14, 28, and 56 days), sets of containers (48 buckets per weathering period) were transferred inside the U.S. Geological Survey, Columbia Environmental Research Center (CERC, Columbia, Missouri) where a 96-hr toxicity assay was conducted. Each container received 10 L of ASTM soft water resulting in nominal concentration of 5360 mg/L LC95A-R and 2300 mg/L of MVP-Fx. Containers were aerated and held in a water bath to maintain a temperature of 12\u0026deg;C\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C during the assay. After substrate settled for 1 hr, 10 juvenile rainbow trout (30\u0026ndash;60 days post swim-up) were added. For some treatments (i.e., duff), substrate was still suspended in the water column, hindering daily visual observations. If visible, fish that were dead, immobile with no visible opercular movement, were removed daily. All surviving fish at the conclusion of the 96-hr assay were enumerated and euthanized with an excess dose of buffered tricaine methanesulfonate (MS-222).\u003c/p\u003e\n\u003ch3\u003eWater Chemistry Measurements\u003c/h3\u003e\n\u003cp\u003eWe monitored water chemistry at the start of each assay, as fish were exposed to the highest chemical concentration during this period. Temperature, dissolved oxygen, specific conductance, pH, alkalinity, total hardness, and TAN were measured following established protocols (APHA 2023). Temperature and dissolved oxygen were measured using a YSI Pro20 dissolved oxygen meter (Xlyem Inc., Washington, D.C.). Specific conductance was measured using a portable Orion Star A325 multiparameter meter (Thermo Fisher Scientific, Waltham, Massachusetts). An HQD HQ440d benchtop meter (HACH Company; Loveland, Colorado) equipped with IntelliCAL electrodes was used to measure pH (PHC301), alkalinity (PHC301), and TAN (ISENH3181). Total hardness was measured using the EDTA (Ethylenediaminetetraacetic acid) titration method. Composite samples of all replicates per treatment were collected for pH, alkalinity, and total hardness. Temperature, dissolved oxygen, and specific conductance measurements were collected from one randomly chosen replicate. The TAN measurements were collected from a composite of replicate samples from the first weathering period treatment and each replicate per weathering period treatment thereafter. A duplicated sample was also collected for one random treatment each sampling event. Given that the relationship between TAN and un-ionized ammonia (NH\u003csub\u003e3\u003c/sub\u003e) is controlled by temperature and pH, proportions of un-ionized ammonia from the TAN concentrations were calculated using the following equation (Emerson et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1975\u003c/span\u003e; Guan et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2010\u003c/span\u003e):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{NH}_{3}=\\frac{{NH}_{3}+{NH}_{4}^{+}}{1+{10}^{\\left(pKa-pH\\right)}}\\:,$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere p\u003cem\u003eK\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e is a calculated (p\u003cem\u003eK\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e = 0.09018\u0026thinsp;+\u0026thinsp;2729.92/T; T\u0026thinsp;=\u0026thinsp;temperature in Kelvin) ammonia-TAN equilibrium constant value. When comparing our measured TAN concentrations in the overlying water to the acute water quality criteria set by the U.S. Environmental Protection Agency (U.S. EPA), we normalized those concentrations to a pH of 7 using the following equation (US EPA 2013):\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:A{V}_{t,7}=\\frac{A{V}_{t}}{(\\frac{0.0114}{1+{10}^{7.204-pH}}+\\frac{1.6181}{1+{10}^{pH-7.204}})},$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{AV}_{t}\\)\u003c/span\u003e\u003c/span\u003e is the TAN concentration. All instruments were calibrated prior to sample measurement and certified standards were measured at the beginning and end of each sampling event to verify the accuracy of those measurements. If standards measured outside of certified ranges, the instrument was recalibrated.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eTotal mortality of rainbow trout at the end of each assay was summarized. We tested for differences in mortality between products and among weathering periods and substrate types using generalized linear models (GLM) with a binomial distribution and a logit link. Mortality data were logit transformed to meet model assumptions of GLMs using the \u0026ldquo;glm\u0026rdquo; function in the \u0026ldquo;lme4\u0026rdquo; package (Bates et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). All fixed-effect variables were evaluated using Analysis of Deviance table (Type III tests) with the \u0026ldquo;Anova\u0026rdquo; function in the \u0026ldquo;car\u0026rdquo; package (Fox and Weisberg \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). For all significant relationships, Tukey\u0026rsquo;s multiple comparison test was performed to identify differences among treatments. Analyses were conducted in the R statistical programming language (v 4.3.3; R Core Team \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Trout mortality and water chemistry data are available in Puglis et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eMortality\u003c/h2\u003e \u003cp\u003eThe effects of product (F\u003csub\u003e2,144\u003c/sub\u003e = 10.1, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), substrate type (F\u003csub\u003e3,144\u003c/sub\u003e = 10.5, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), and weathering period (F\u003csub\u003e3,144\u003c/sub\u003e = 6.76, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) were significant factors in the model (\u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.85). There was a significant interaction among product and substrate type and weathering period (F\u003csub\u003e18,144\u003c/sub\u003e = 2.15, p\u0026thinsp;=\u0026thinsp;0.007), between product and substrate type (F\u003csub\u003e6,144\u003c/sub\u003e = 4.13, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), between substrate type and weathering period (F\u003csub\u003e9,144\u003c/sub\u003e = 3.26, p\u0026thinsp;=\u0026thinsp;0.001), and between product and weathering period (F\u003csub\u003e6,144\u003c/sub\u003e = 2.81, p\u0026thinsp;=\u0026thinsp;0.013). Mortality was highest in LC95A-R across all treatments compared to MVP-Fx or control, with near complete mortality on duff and gravel substrates. In general, mortality in MVP-Fx treatments was similar to controls, except in the gravel treatment, where mortality was greater in MVP-Fx. On average, duff and gravel substrates had 31% more mortality compared to the HOC and LOC soils. Also, there was a general trend of decreasing mortality as weathering period increased for each product, suggesting the toxicity of product decreased over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Average mortality ranged from 0\u0026ndash;40% across control treatments, with the highest mortality observed in duff substrate after 7 days of weathering.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eWater Chemistry\u003c/h2\u003e \u003cp\u003eWater temperature (10.3\u0026ndash;13.4\u0026deg;C) and dissolved oxygen (7.5\u0026ndash;9.9 mg/L) were similar across all treatments. Alkalinity was higher in the products than in the control group, while pH was lower in the products (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). We only assessed total hardness for control group water samples (38\u0026ndash;87 mg/L) as the products interfered with the water hardness method we used to measure hardness. Specific conductance was higher in the products (LC95A-R\u0026thinsp;=\u0026thinsp;464\u0026ndash;2125 \u0026micro;S/cm, MVP-Fx\u0026thinsp;=\u0026thinsp;398\u0026ndash;1859 \u0026micro;S/cm) compared to the control group (166\u0026ndash;197 \u0026micro;S/cm). There were differences in specific conductance across substrates, with 696\u0026ndash;2125 \u0026micro;S/cm in duff and gravel treatments and 398\u0026ndash;654 \u0026micro;S/cm in the soils between both products. The TAN concentrations were 38.6\u0026ndash;398 mg of N/L in LC95A-R and 30.3\u0026ndash;524 mg of N/L in MVP-Fx across all treatments. The average TAN concentration in the control group was 0.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.29 mg of N/L. There was a decrease of 20\u0026ndash;47% in TAN concentrations from day 7 to both 14 and 28 days of weathering and an increase of 56% after 56 days of weathering. The TAN concentrations were highest in duff and gravel substrates (0.08\u0026ndash;586 mg of N/L) across all treatments compared with the soils (0.20\u0026ndash;95.4 mg of N/L). The products corroded the probe membrane, which caused 50% of the ammonia standards to fall outside of the acceptable range.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn this study, the toxicity of long-term wildland fire retardants to rainbow trout was altered by other environmental variables such as the underlying substrate the product was applied to and the length of time product weathered on dry substrate before inundation with water. In previous work, we found that some environmental variables, such as water temperature, alter the toxicity and subsequent impacts of fire retardant to fish (Puglis et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). We also confirmed differences in toxicity between products as described in Puglis et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Lastly, the significant interaction observed between product and substrate highlights the need to consider underlying substrate when evaluating the toxic effects of weathered fire retardant in intermittent streams.\u003c/p\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eInfluence of Product\u003c/h2\u003e \u003cp\u003eToxicity differed between products, as LC95A-R was always more toxic than MVP-Fx, regardless of substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). This is consistent with previous studies in larval fish, which attributed the difference in toxicity to higher un-ionized ammonia concentrations in LC95A-R than MVP-Fx at the same application rate of retardant (Puglis et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Puglis and Iacchetta 2023). In aqueous solution, TAN is present as a ratio of un-ionized ammonia and ammonium (NH\u003csub\u003e3\u003c/sub\u003e:NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e). Temperature and pH affect this ratio such that increases in temperature and pH increase the concentration of un-ionized ammonia (Emerson et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1975\u003c/span\u003e); therefore, different environments can have varying ratios of un-ionized ammonia to ammonium. Un-ionized ammonia is relatively more toxic to fish than ammonium due to its neutral charge, which allows for greater uptake through membranes, including the gill epithelium (Evans et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). Exposure to elevated concentrations of un-ionized ammonia can cause convulsions, comas, and eventually lead to death in fish (Randall and Tsui \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Buhl and Hamilton (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) reported a 24-hr LC\u003csub\u003e50\u003c/sub\u003e of un-ionized ammonia as 0.125 mg of N/L for 40 days post swim-up rainbow trout larvae. Un-ionized ammonia concentrations in this study were below this threshold (except for one replicate of LC95A-R weathered on duff substrate after 7 days; NH\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.139 mg of N/L) and were lower than our previously reported values for similar concentrations of fire-retardant in water-only exposures (Puglis et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This suggests the ammonium, or some other unmeasured component of the products, contributed to the mortality observed across substrates in the current study. The acute water quality criteria for TAN, a concentration set by the U.S. EPA (US EPA 2013) to protect aquatic life, is 24.1 mg TAN/L at pH 7 and temperatures below 14\u0026deg;C (US EPA 2013). All TAN observed in product treatments within this study (n\u0026thinsp;=\u0026thinsp;160) were above the criterion, except for five replicates in the soil treatments of MVP-Fx and one replicate of LC95A-R in LOC at 14 days of weathering and all control treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eInfluence of Substrate on Product Toxicity\u003c/h2\u003e \u003cp\u003eFor both fire-retardants tested, we observed higher mortality in duff and gravel treatments compared to soil treatments for a given product (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Visual inspection of the data indicates that while the un-ionized ammonia concentrations appeared comparable across all substrates (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), specific conductance varied, with duff treatments showing relatively high values followed by gravel, HOC, and LOC (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). Specific conductance is an indirect measure of dissolved ions in a solution, including ammonium. Thus, the specific conductance of the overlying water in our treatments was influenced by substrate and is not independent of the TAN concentrations, but also includes other ions, which may contribute to toxicity. Our data suggest that, within a product, the composition of residual ions in the overlying water of duff and gravel are more toxic than in the overlying water of both soil treatments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOne mechanism that influences ion availability is CEC, a measure of a soil\u0026rsquo;s ability to adsorb and retain cations controlled by the negatively charged sites within a soil (Nommik and Vahtras \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Sumner and Miller \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1996\u003c/span\u003e). Ammonium is a positively charged ion and thus able to bind to negatively charged sites in soils, making it mostly unavailable for cation exchange reactions (Nieder et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Clay and organic matter content of soil are the primary constituents that influence the CEC to fix added ammonium (Nommik and Vahtras \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Kissel et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Generally, higher clay and/or organic matter content equates to greater CEC (Wright and Foss \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e1972\u003c/span\u003e; Martel et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1978\u003c/span\u003e). While not measured, the CEC was likely lowest in the gravel treatments and may have resulted in the product remaining suspended throughout the water column, increasing potential exposure to the trout. This may account for the high mortality observed in gravel treatments compared to soil treatments. We would have predicted that the TAN concentration and mortality would be lower in the HOC soil than the LOC soil treatments, given the CEC of the HOC soil was more than four times that of the LOC soil. Yet, we observed similar mortality and measured similar TAN concentrations between the soil treatments within a product. This suggests the concentration of TAN from the product saturated the negatively charged sites in both soils and exceeded the ability of the CEC to measurably affect the TAN concentration in the overlying water.\u003c/p\u003e \u003cp\u003eAlthough not measured here, duff presumably had a higher CEC compared to the soil and gravel treatments. The CEC of duff collected from a coniferous-deciduous forest in Oregon was more than 50 mEq/100 g (Warner \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1976\u003c/span\u003e), which is higher than our soil treatments (HOC\u0026thinsp;=\u0026thinsp;28.1 mEq/100 g, LOC\u0026thinsp;=\u0026thinsp;6.3 mEq/100 g; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Yet, the highest mortality was observed in duff treatments, and as previously stated, un-ionized ammonia concentrations did not vary substantially across substrates within a product. It is possible that the duff was not composed of enough decomposing material to elevate the substrate\u0026rsquo;s CEC, and thus there was a low binding affinity of ammonium. Additionally, trout may have experienced added stress from suspended duff material in the water column. The effect of suspended solids on Salmonids in controlled environments is associated with death, gill trauma, decreased osmoregulatory ability, and increased blood sugar levels (Redding et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Muck \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Therefore, potentially lower than expected CEC and additional stress from substrate suspension in the duff treatment may explain the observed mortality.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eInfluence of Environmental Weathering\u003c/h2\u003e \u003cp\u003eThere was a trend of decreasing mortality as weathering period increased (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Mortality observed in rainbow trout exposed to product weathered for 56 days was 15% lower than the observed mortality in those exposed to product weathered for 7 days. Physical and microbial processes such as ammonia volatilization, ammonia nitrification (Aislabie and Deslippe \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), or ammonium fixation could have influenced the bioavailability of TAN to fish. As a result, these products on the landscape may become less toxic over time but likely persist in the environment after 56 days.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOur results suggest that LC95A-R and MVP-Fx may persist in the environment after weathering for 56 days and that the underlying substrate may alter the toxicity of these products under static conditions. Although these data provide a better understanding of the environmental persistence of long-term fire retardants, aquatic organisms are likely to encounter effects from climate change, wildfires, and intrusion of fire retardant in an intermittent stream. The consequences of climate change could have direct and indirect impacts to organisms that utilize these highly productive, dynamic streams for spawning, refuge, and foraging (Mas-Martí et al. 2010; Vander Vorste et al. 2020). Ash inputs (Earl and Blinn 2003; Murphy et al. 2018), sedimentation (Rhoades et al. 2011), and increased temperatures (Dunham et al. 2007) can occur in these streams following wildfire. Additionally, organisms can experience additional stressors from exposure of long-term retardants, either through direct contact into the waterbody or via runoff from precipitation events after application. The combination of these effects may create a cumulative toxic effect to aquatic organisms. This study provides resource managers with key data characterizing potential toxicity to aquatic life in situations where fire retardant is applied to dry intermittent streambeds.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDISCLAIMER: Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAcknowledgement: This work was supported by the U.S. Forest Service (USFS) with additional funding through the U.S. Geological Survey (USGS) Contaminant Biology Program and USGS Toxic Substances Hydrology Program. We\u0026rsquo;d like to thank J. Orech, E. Scott, S. Tidwell, and T. Tidwell for assistance in data collection.\u003c/p\u003e\n\u003cp\u003eFunding: The work was supported using the funds provided by the USFS, U.S. Geological Survey (USGS) Contaminant Biology Program, and USGS Toxic Substances Hydrology Program.\u003c/p\u003e\n\u003cp\u003eConflict of Interest: The authors declare that there is no financial or non-financial interest to disclose.\u003c/p\u003e\n\u003cp\u003eAuthor Contributions: All authors commented and edited revisions to the manuscript. Holly Puglis conceptualized and designed the study. Material preparation, experimentation, investigation, and data collection were performed by Christina Mackey and Holly Puglis. Data analysis was performed by Christina Mackey and Michael Iacchetta. The first draft of the manuscript was written by Christina Mackey. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003eEthical Approval: The present study was reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) at the US Geological Survey \u0026ndash; Columbia Environmental Research Center under IACUC19-007.\u003c/p\u003e\n\u003cp\u003eData Availability Statement: The datasets generated during and/or analyzed during the current study are available in the ScienceBase repository, https://doi.org/10.5066/P9C94L2L.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAislabie J, Deslippe JR (2013) Soil microbes and their contribution to soil services. In: Dymond J (ed) Ecosystem services in New Zealand\u0026ndash;conditions and trends. Manaaki Whenua Press, Lincoln, New Zealand. pp 143\u0026ndash;161. https://oldwww.landcareresearch.co.nz/__data/assets/pdf_file/0018/77040/1_12_Aislabie.pdf\u003c/li\u003e\n \u003cli\u003eAmerican Public Health Association (APHA) (2023) Standard methods for the examination of water and wastewater. Lipps WC, Braun-Howland EB, Baxter TE (eds.), 24th edn. Washington DC\u003c/li\u003e\n \u003cli\u003eASTM International (2014) Standard guide for performing acute toxicity tests on test materials with fishes, macroinvertebrates, and amphibians. E729-96. https://doi.org/10.1520/E0729-96R14\u003c/li\u003e\n \u003cli\u003eBates D, M\u0026auml;chler M, Bolker B, Walker S (2015) Fitting Linear Mixed-Effects Models Using lme4. J Stat Softw 67(1):1\u0026ndash;48.\u0026nbsp;https://doi.org/10.18637/jss.v067.i01\u003c/li\u003e\n \u003cli\u003eBuhl KJ, Hamilton SJ (2000) Acute toxicity of fire-control chemicals, nitrogenous chemicals, and surfactants to rainbow trout. Trans Am Fish Soc 129(2):408\u0026ndash;418. https://doi.org/10.1577/1548-8659(2000)129%3C0408:ATOFCC%3E2.0.CO;2\u003c/li\u003e\n \u003cli\u003eDhungel S, Tarboton DG, Jin J, Hawkins CP (2016) Potential effects of climate change on ecologically relevant streamflow regimes. River Res Appl 32(9):1827\u0026ndash;1840. https://doi.org/10.1002/rra.3029\u003c/li\u003e\n \u003cli\u003eD\u0026ouml;ll P, Schmied HM (2012) How is the impact of climate change on river flow regimes related to the impact on mean annual runoff? A global-scale analysis. Environ Res Lett 7(1):14037. https://doi.org/10.1088/1748-9326/7/1/014037\u003c/li\u003e\n \u003cli\u003eDunham JB, Rosenberger AE, Luce CH, Rieman BE (2007) Influences of wildfire and channel reorganization on spatial and temporal variation in stream temperature and the distribution of fish and amphibians. Ecosystems 10(2):335\u0026ndash;346. https://doi.org/10.1007/s10021-007-9029-8\u003c/li\u003e\n \u003cli\u003eEarl SR, Blinn DW (2003) Effects of wildfire ash on water chemistry and biota in South‐Western USA streams. Freshw Biol 48(6):1015\u0026ndash;1030. https://doi.org/10.1046/j.1365-2427.2003.01066.x\u003c/li\u003e\n \u003cli\u003eEmerson K, Russo RC, Lund RE, Thurston R V (1975) Aqueous ammonia equilibrium calculations: effect of pH and temperature. J Fish Res Board Can 32(12):2379\u0026ndash;2383. https://doi.org/10.1139/f75-274\u003c/li\u003e\n \u003cli\u003eEvans DH, More KJ, Robbins SL (1989) Modes of ammonia transport across the gill epithelium of the marine teleost fish Opsanus beta. J Exp Biol 144(1):339\u0026ndash;356.\u0026nbsp;https://doi.org/10.1242/jeb.144.1.339\u003c/li\u003e\n \u003cli\u003eFox J, Weisberg S (2019) An R Companion to Applied Regression, Third edition. Sage, Thousand Oaks CA. https://socialsciences.mcmaster.ca/jfox/Books/Companion/\u003c/li\u003e\n \u003cli\u003eGimenez A, Pastor E, Z\u0026aacute;rate L, et al. (2004) Long-term forest fire retardants: a review of quality, effectiveness, application and environmental considerations. Int J Wildland Fire 13(1):1\u0026ndash;15. https://doi.org/10.1071/wf03001\u003c/li\u003e\n \u003cli\u003eGould JS, Khanna P, Hutchings P, et al. (2000) Assessment of the effectiveness and environmental risk of the use of retardants to assist in wildfire control in Victoria (Research Report No. 50). Department of Natural Resources and Environment, State of Victoria. https://www.ffm.vic.gov.au/__data/assets/pdf_file/0009/21060/Report-50-Assessment-of-the-Effectiveness-and-Environmental-Risk-of-the-Use-of-Retardants-to-Assist-in-Wi.pdf\u003c/li\u003e\n \u003cli\u003eGuan B, Hu W, Zhang T, et al. (2010) Acute and chronic un-ionized ammonia toxicity to \u0026lsquo;all-fish\u0026rsquo; growth hormone transgenic common carp (Cyprinus carpio L.). Chinese Sci Bull 55:4032\u0026ndash;4036. https://doi.org/10.1007/s11434-010-4165-5\u003c/li\u003e\n \u003cli\u003eJones MW, Smith A, Betts R, et al (2020) Climate change increases the risk of wildfires: January 2020. ScienceBrief Review 116:117. https://ueaeprints.uea.ac.uk/id/eprint/77982\u003c/li\u003e\n \u003cli\u003eKissel DE, Cabrera ML, Paramasivam S (2008) Ammonium, ammonia, and urea reactions in soils. In: Schepers S, Raun WR (eds) Nitrogen in agricultural systems, vol 49. Wiley, New York, pp 101\u0026ndash;155. https://doi.org/https://doi.org/10.2134/agronmonogr49.c4\u003c/li\u003e\n \u003cli\u003eLarned ST, Datry T, Arscott DB, Tockner K (2010) Emerging concepts in temporary‐river ecology. Freshw Biol 55(4):717\u0026ndash;738. https://doi.org/10.1111/j.1365-2427.2009.02322.x\u003c/li\u003e\n \u003cli\u003eLittle EE, Calfee RD (2002) Environmental Persistence and Toxicity of Fire-Retardant Chemicals, Fire-Trol\u0026reg; GTS-R and Phos-Chek\u0026reg; D75-R to Fathead Minnows. USDA Forest Service, Missoula, MT. https://www.fs.usda.gov/rm/fire/wfcs/documents/NWST-2475.pdf\u003c/li\u003e\n \u003cli\u003eMartel YA, De Kimpe CR, Laverdiere MR (1978) Cation‐exchange capacity of clay‐rich soils in relation to organic matter, mineral composition, and surface area. Soil Sci Soc Am J 42(5):764\u0026ndash;767. https://doi.org/10.2136/sssaj1978.03615995004200050023x\u003c/li\u003e\n \u003cli\u003eMas-Mart\u0026iacute; E, Garc\u0026iacute;a-Berthou E, Sabater S, et al. (2010) Comparing fish assemblages and trophic ecology of permanent and intermittent reaches in a Mediterranean stream. In: Stevenson RJ, Sabater S (eds) Global Change and River Ecosystems\u0026mdash;Implications for Structure, Function and Ecosystem Services, vol 215. Springer, Dordrecht pp 167\u0026ndash;180.\u0026nbsp;https://doi.org/10.1007/s10750-010-0292-x\u003c/li\u003e\n \u003cli\u003eMuck J (2010) Biological effects of sediment on bull trout and their habitat \u0026ndash; Guidance for evaluating effects. U.S. Fish and Wildlife Service, Washington Fish and Wildlife Office, Lacey, WA. https://pdf.wildearthguardians.org/site/DocServer/USFWS_BIOLOGICAL%20EFFECTS%20OF%\u003cbr\u003e20SEDIMENT%20ON%20BULL%20TROUT_2010.pdf\u003c/li\u003e\n \u003cli\u003eMurphy SF, McCleskey RB, Martin DA, Writer JH, Ebel BA (2018) Fire, flood, and drought: Extreme climate events alter flow paths and stream chemistry. J Geophys Res Biogeosci 123(8):2513-2526.\u0026nbsp;https://doi.org/10.1029/2017JG004349\u003c/li\u003e\n \u003cli\u003eNadeau T, Rains MC (2007) Hydrological connectivity between headwater streams and downstream waters: how science can inform policy. J Am Water Resour Assoc 43:118\u0026ndash;133. https://doi.org/10.1111/j.1752-1688.2007.00010.x\u003c/li\u003e\n \u003cli\u003eNathan MV, Stecker JA, Sun Y (2012) Soil testing in Missouri: A guide for conducting soil tests in Missouri EC923. Division of Plant Sciences, University of Missouri, Missouri.\u0026nbsp;https://hdl.handle.net/10355/50590. Accessed 21 March 2024\u003c/li\u003e\n \u003cli\u003eNieder R, Benbi DK, Scherer HW (2011) Fixation and defixation of ammonium in soils: a review. Biol Fertil Soils 47(1):1\u0026ndash;14. https://doi.org/10.1007/s00374-010-0506-4\u003c/li\u003e\n \u003cli\u003eNommik H, Vahtras K (1982) Retention and fixation of ammonium and ammonia in soils. In: Stevenson FJ (ed) Nitrogen in agricultural soils 22:123\u0026ndash;171. https://doi.org/10.2134/agronmonogr22.c4\u003c/li\u003e\n \u003cli\u003ePerimeter Solutions (2018) Phos-Chek\u003csup\u003e\u0026reg;\u003c/sup\u003e MVP-Fx Safety Data Sheet. https://www.perimeter-solutions.com/wp-content/uploads/2022/03/SDS-PHOS-CHEK-MVP-Fx-Version-1.4-OSHA-US-EN.pdf\u003c/li\u003e\n \u003cli\u003ePerimeter Solutions (2019) Phos-Chek\u003csup\u003e\u0026reg;\u003c/sup\u003e LC95A/Phos-Chek\u0026reg; LC95A-MV Concentrate Safety Data Sheet. https://www.perimeter-solutions.com/wp-content/uploads/2022/03/SDS-PHOS-CHEK-LC95A-PHOS-CHEK-LC95A-MV-Version-1.4-OSHA-US-WHMIS-CA-EN-1.pdf\u003c/li\u003e\n \u003cli\u003ePuglis HJ, Iacchetta M (2024) Toxicity of Wildland Fire Retardants to Rainbow Trout in Short Exposures. Environ Toxicol Chem.\u0026nbsp;https://doi.org/10.1002/etc.5791\u0026nbsp;\u003c/li\u003e\n \u003cli\u003ePuglis HJ, Iacchetta M, Mackey CM (2022) Toxicity of Wildland Fire‐Fighting Chemicals in Pulsed Exposures to Rainbow Trout and Fathead Minnows. Environ Toxicol Chem 41(7):1711\u0026ndash;1720.\u0026nbsp;https://doi.org/10.1002/etc.5347\u0026nbsp; \u0026nbsp;\u003c/li\u003e\n \u003cli\u003ePuglis HJ, Mackey CM, Iacchetta MG, Scott EL (2023) Water chemistry and biological data of Rainbow Trout following aquatic exposure to weathered wildland fire retardants after application to substrate. U.S. Geological Survey data release.\u0026nbsp;https://doi.org/10.5066/P9C94L2L\u003c/li\u003e\n \u003cli\u003eR Core Team (2022) R: A language and environment for statistical computing\u003c/li\u003e\n \u003cli\u003eRandall DJ, Tsui TKN (2002) Ammonia toxicity in fish. Mar Pollut Bull 45:17\u0026ndash;23. https://doi.org/10.1016/s0025-326x(02)00227-8\u003c/li\u003e\n \u003cli\u003eRedding JM, Schreck CB, Everest FH (1987) Physiological effects on coho salmon and steelhead of exposure to suspended solids. Trans Am Fish Soc, 116:737\u0026ndash;744. https://doi.org/10.1577/1548-8659(1987)116\u0026lt;737:PEOCSA\u0026gt;2.0.CO;2\u003c/li\u003e\n \u003cli\u003eRhoades CC, Entwistle D, Butler D (2011) The influence of wildfire extent and severity on streamwater chemistry, sediment and temperature following the Hayman Fire, Colorado. Int J Wildland Fire 20:430\u0026ndash;442. https://doi.org/10.1071/WF09086\u003c/li\u003e\n \u003cli\u003eRind D, Goldberg R, Hansen J, et al. (1990) Potential evapotranspiration and the likelihood of future drought. J Geophys Res Atmos 95(7):9983\u0026ndash;10004. https://doi.org/10.1029/JD095iD07p09983\u003c/li\u003e\n \u003cli\u003eSumner ME, Miller WP (1996) Cation exchange capacity and exchange coefficients. In: Sparks DL, Page AL, Helmke PA, et al (eds) Methods of Soil Analysis, pp 1201-1229. https://doi.org/10.2136/sssabookser5.3.c40\u003c/li\u003e\n \u003cli\u003eUSDA Forest Service (2020) Aerial firefighting use and effectiveness (AFUE) report.\u0026nbsp;https://www.fs.usda.gov/sites/default/files/2020-08/08242020_afue_final_report.pdf\u003c/li\u003e\n \u003cli\u003eUSDA Forest Service (2024) Qualified Products List.\u0026nbsp;https://www.fs.usda.gov/rm/fire/wfcs/products/. Accessed 19 November 2024\u003c/li\u003e\n \u003cli\u003eUS Environmental Protection Agency (US EPA) (2013) Aquatic life ambient water quality criteria for ammonia - freshwater (EPA-822-R-13-001). Washington, DC. https://www.epa.gov/sites/production/files/2015-08/documents/aquatic-life-ambient-water-quality-criteria-for-ammonia-freshwater-2013.pdf\u003c/li\u003e\n \u003cli\u003eUS Forest Service (USFS) (2011) Nationwide aerial application of fire retardant on national forest system lands: Final environmental impact statement. US Department of Agriculture https://www.fs.usda.gov/sites/default/files/media_wysiwyg/wfcs_final_feis_1.pdf\u003c/li\u003e\n \u003cli\u003eUS Forest Service (USFS) (2023) 2022 Aerial fire retardant use on national forest system lands. US Department of Agriculture. https://www.fs.usda.gov/sites/default/files/2023-05/2022-RetardantUse-NFSL.pdf. Accessed 10 Jul 2023\u003c/li\u003e\n \u003cli\u003eUS National Interagency Fire Center (2023) Interagency standards for fire and fire aviation operations (NFES 2724). Boise, ID. https://www.nifc.gov/sites/default/files/redbook-files/RedBookAll.pdf\u003c/li\u003e\n \u003cli\u003eVander Vorste R, Obedzinski M, Nossaman Pierce S, et al. (2020) Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams. Glob Chang Biol 26:3834\u0026ndash;3845. https://doi.org/10.1111/gcb.15116\u003c/li\u003e\n \u003cli\u003eWarner SA (1976) Cation exchange properties of forest litter as influenced by vegetation type and decomposition. Thesis, Oregon State University. https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj430691c\u003c/li\u003e\n \u003cli\u003eWright WR, Foss JE (1972) Contributions of clay and organic matter to the cation exchange capacity of Maryland soils. Soil Sci Soc Am J 36:115\u0026ndash;118. https://doi.org/10.2136/sssaj1972.03615995003600010027x\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"archives-of-environmental-contamination-and-toxicology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aect","sideBox":"Learn more about [Archives of Environmental Contamination and Toxicology](https://www.springer.com/journal/244)","snPcode":"244","submissionUrl":"https://submission.nature.com/new-submission/244/3","title":"Archives of Environmental Contamination and Toxicology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5829115/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5829115/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLong-term fire retardants are employed to combat and control wildfires by altering the way fuels burn, and they continue to decrease fire intensity after water in the retardant solution has evaporated. After application, fire retardants may persist on dry stream beds or in riparian habitats before precipitation events flush the retardant into intermittent streams. We exposed juvenile (30-60 days post swim-up) rainbow trout (\u003cem\u003eOncorhynchus mykiss\u003c/em\u003e) to fire retardants weathered for 7-56 days on different substrates (duff, gravel, high organic content soil, and low organic content soil) under static conditions for 96 hrs to evaluate the potential toxicity of two current-use long-term fire retardant (LC95A-R and MVP-Fx) products. Trout mortality was greater in LC95A-R treatments compared to MVP-Fx due to higher concentrations of LC95A-R in the applied product than MVP-Fx at the same application rate. Underlying substrate affected fire retardant toxicity, with 31% higher average mortality for products applied to duff and gravel compared to soil. Differences in mortality across substrates and products after weathering may be attributed to differences in the mix-ratio of applied product and interactions of product chemistries with underlying substrate. These interactions resulted in elevated ionic concentrations of the overlying water in duff and gravel treatments. Trout mortality decreased 15% for products weathered 56 days compared to 7 days. Our results suggest that long-term fire retardants may persist in the environment and that underlying substrate may alter the toxicity of these products upon entrance into an intermittent stream.\u003c/p\u003e","manuscriptTitle":"Environmental Persistence and Toxicity of Weathered Wildland Fire Retardants to Rainbow Trout","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-09 09:18:54","doi":"10.21203/rs.3.rs-5829115/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accept as is","date":"2025-04-10T15:59:58+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-07T19:46:50+00:00","index":"","fulltext":""},{"type":"submitted","content":"Archives of Environmental Contamination and Toxicology","date":"2025-03-27T19:13:02+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"archives-of-environmental-contamination-and-toxicology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aect","sideBox":"Learn more about [Archives of Environmental Contamination and Toxicology](https://www.springer.com/journal/244)","snPcode":"244","submissionUrl":"https://submission.nature.com/new-submission/244/3","title":"Archives of Environmental Contamination and Toxicology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"688973b7-8188-4b9b-9666-3db2390f2a1c","owner":[],"postedDate":"April 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-05-19T16:00:36+00:00","versionOfRecord":{"articleIdentity":"rs-5829115","link":"https://doi.org/10.1007/s00244-025-01131-y","journal":{"identity":"archives-of-environmental-contamination-and-toxicology","isVorOnly":false,"title":"Archives of Environmental Contamination and Toxicology"},"publishedOn":"2025-05-14 15:57:02","publishedOnDateReadable":"May 14th, 2025"},"versionCreatedAt":"2025-04-09 09:18:54","video":"","vorDoi":"10.1007/s00244-025-01131-y","vorDoiUrl":"https://doi.org/10.1007/s00244-025-01131-y","workflowStages":[]},"version":"v1","identity":"rs-5829115","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5829115","identity":"rs-5829115","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}