Acute Toxicity of Firefighting Chemical Formulations to Four Life Stages of Fathead Minnow
Discussion
Fire-suppressant foams were more toxic than the nonfoam fire retardants. Phos-Chek WD-881 and Silv-Ex were at least 10 times more toxic than the fire retardants FT LCG-R, FT GTS-R, and PC D75-F. Based on the hazard ranking presented by Passino and Smith (1987; 0.01 - 0.1 mg/liter, extremely toxic; 0.1 - 1.0 mg/liter, highly toxic; 1 - 10 mg/liter, moderately toxic; 10 - 100 mg/liter, slightly toxic; 100 - 1000 mg/liter, practically harmless; greater than 1000 mg/liter, relatively harmless), both PC WD-881 and Silv-Ex were considered moderately toxic formulations. Fire-Trol GTS-R and PC D75-F ranked as practically harmless formulations, whereas FT LCG-R ranked as relatively harmless. However, fire-retardant formulations are used in highly concentrated solutions in the field (i.e., mix ratio for FT GTS-R; 1.66 lb./gallon = 198,930 mg/liter), hence the possibility for experiencing concentrations close to the 96-hr LC50 in the environment become real and render the terms "practically" and "relatively harmless" in this scenario to be irrelevant.
Fire-Suppressant Foam Toxicity
Toxicity of the fire-suppressant foams PC WD-881 and Silv-Ex may be due to the surfactant portion of their formulation. Various authors have reported on the toxicity of surfactants, with results comparable to the 96-hr LC50s determined in this study. Müller (1980) reported a 24-hr LC50 of 8.5 mg/liter for a commercial, non-ionic surfactant using 8-g rainbow trout (Oncorhynchus mykiss) as the test organism. Müller determined that surfactant toxicity was related to the surface tension reduction caused by the surfactant. The greater the reduction in surface tension, the greater the toxicity of the surfactant. In Müller's study, surface tension was reduced to approximately 45 - 50 dynes/cm at the 24-hr LT50. In comparison, the 0.6% Silv-Ex field application mixture has a surface tension of 22.92 dynes/cm (Ansul, 1991), about half the surface tension reduction that caused mortality in Müller's study. Reduction in surface tension has also been found to have adverse effects on gill epithelia, ranging from epithelial swelling to complete destruction of the gill epithelia (Bock, 1967). Holman and Macek (1980) determined the 96-hr LC50s for three different chain length linear alkylbenzene sulfonate (LAS) surfactants with 2- to 3-month-old fathead minnow juveniles tested in soft water (hardness 40 mg/liter as CaCO3). The 96-hr LC50s ranged from 0.86 to 12.3 mg/liter, with increasing chain length directly increasing toxicity. Although the exact surfactants used in PC WD-881 and Silv-Ex were unknown because they are proprietary products, the 96-hr LC50 of the C11.7 chain length LAS surfactant (12.3 mg/liter) was extremely close to the 96-hr LC50s determined in the present study for PC WD-881 and Silv-Ex (13 - 32 mg/liter; Table 3).
The surfactants used in the fire-suppressant foams pose another threat to aquatic organisms besides their acute toxicity. Surfactants have been found to alter the permeability of biological membranes (Helenius and Simons, 1975). This change in permeability may be detrimental in situations in which multiple stressors are being placed upon an aquatic organism. LAS surfactants increased the uptake of cadmium across the perfused rainbow trout gill above that of gills exposed to cadmium without LAS (Pärt et al., 1985). Manner and Muehleman (1976) reported that LAS modified the uptake of tritiated uridine in fathead minnow eggs. Eggs exposed to LAS displayed a decreased uptake of uridine in comparison to that of control eggs.
LAS can modify the toxicity of various substances, as well as change their uptake. Solon and Nair (1970) reported an increase in the toxicity of various phosphorothionate pesticides, such as parathion, by as much as 49% when fathead minnows were exposed to the pesticide in the presence of a sublethal (1 mg/liter) LAS concentration. Thus, in aquatic ecosystems which are degraded by certain inorganic or organic pollutants, fire-suppressant foam toxicity may be altered or may alter the uptake and toxicity of the additional pollutants.
The effect of water quality on the toxicity of fire-suppressant foams, which is composed primarily of anionic surfactants, was minor. The toxicity of PC WD-881 was significantly greater only in hard water tests with 30-DPH and 60-DPH fish (Table 3). Hokanson and Smith (1971) reported that the toxicity of LAS to bluegill (Lepomis macrochirus) was increased in hard water. In a 30-day test with fathead minnow exposed to LAS, Holman and Macek (1980) demonstrated greater mortality in a hard water test (hardness, 200 mg/liter as CaCO3) than in a soft water test (hardness, 40 mg/liter as CaCO3). In contrast, McKim et al. (1975) concluded that water hardness played a minor role in the toxicity of LAS to fathead minnow.
Un-ionized Ammonia, Nitrate, Nitrite
The estimated un-ionized ammonia concentrations determined in this study were similar to those obtained by Thurston et al. (1983) who tested fathead minnows in flowthrough tests at 12- 22 C. Thirty-DPH juveniles were close in weight (0.03 - 0.04 g; Table 1) to the smallest fish tested by Thurston et al. (1983). Thirty-DPH juveniles tested with FT LCG-R in soft water had an estimated un-ionized ammonia concentration of 1.19 - 1.76 mg/liter at the 96-hr LC50; similar tests conducted in hard water had 0.86 - 3.22 mg/liter. In comparison, the 0.09-g fish tested by Thurston et al. (1983) had a 96-hr LC50 of 1.10 - 1.50 mg/liter un-ionized ammonia. Sixty-DPH juveniles (0.12 g; Table 1) tested with FT LCG-R in soft water had an estimated un-ionized concentration at the 96-hr LC50 of 1.22 - 1.28 mg/liter, whereas 60-DPH juveniles tested in hard water had an estimated concentration at the 96-hr LC50 of 0.95 - 2.77 mg/liter. Thurston et al. (1983) reported a 96-hr LC 50 value of 0.75 mg/liter for 0.13-g fish. The estimated un-ionized concentration ranges at the 96-hr LC50s of 30-DPH and 60-DPH juveniles were close to or greater than the 96-hr LC 50 values reported by Thurston et al. (1983), thus suggesting that un-ionized ammonia was the primary toxic agent of FT LCG-R. For fry (0.003 - 0.008 g), measured un-ionized ammonia at the 96-hr LC50 in the soft water test was 0.64 - 1.02 mg/liter and in the hard water test was 0.33 - 0.74 mg/liter. Although Thurston et al. (1983) did not observe differences in sensitivity of fathead minnows for sizes 0.09 to 2.3 g, the fry life stage in the present study was over 10-fold smaller. This earlier life stage could be more sensitive to un-ionized ammonia as are early life stages of fish to other contaminants (Rand and Petrocelli, 1985).
Likewise, the measured un-ionized ammonia for fry and estimates for 30-DPH and 60-DPH juveniles tested with FT GTS-R were close to or greater than the 96-hr LC50 for un-ionized ammonia determined with similar-sized fish by Thurston et al. (1983). Thus, it seems that un-ionized ammonia was the primary toxic agent of FT GTS-R.
Although PC D75-F had lower un-ionized ammonia estimates than FT LCG-R and FT GTS-R, the estimated un-ionized ammonia concentrations at the 96-hr LC50 of PC D75-F determined with fry, 30-DPH, and 60-DPH juveniles in soft and hard water are within a factor of four of the values reported by Thurston et al. (1983), except for that for 60-DPH juveniles tested in hard water. This similarity suggests that un-ionized ammonia was the principal toxic agent in the PC D75-F formulation.
There are few studies reporting the toxicity of nitrate to fish because it is considered essentially nontoxic (Russo, 1985). Studies with channel catfish (Ictalurus punctatus) reported 96-hr LC50s greater than 1300 mg/liter as NO 3--N (Colt and Tchobanoglous, 1976), whereas for bluegill 96-hr LC50s ranged from 415 mg/liter for KNO 3 to 1978 mg/liter for NaNO3 (Trama, 1954). The values in the present study were all less than 10 mg/liter and, therefore, nitrate probably did not contribute to the toxicity of the three fire-retardant compounds.
The toxicity of nitrite to fish is variable depending on the test organism. Russo and Thurston (1977) reported that the 96-hr LC50 to fathead minnow was 2.30 - 2.99 mg/liter as NO2--N, whereas for rainbow trout it was 0.19 - 0.28 mg/liter and for mottled sculpin (Cottus bairdi) it was >67 mg/liter. In contrast, Palachek and Tomasso (1984) found that nitrite toxicity to 0.3 - 0.8-g fathead minnow was 71 mg/liter as NO2--N and to 0.9 - 3.3-g fish was 46 mg/liter (reported graphically as 232 mg/liter NO2- and 150 mg/liter NO 2-, respectively). These values are 25-fold higher than those of Russo and Thurston (1977); however, tests by Russo and Thurston were conducted at 13°C, and those of Palachek and Tomasso were conducted at 23°C. Measured nitrite-nitrogen concentrations in the fry tests with FT GTS-R were higher than those reported by Russo and Thurston (1977) and those in tests with FT LCG-R were higher than those reported by Palachek and Tomasso (1984). Thus, nitrite probably contributed to the toxicity of these compounds, but not PC D75-F where nitrite concentrations were low.
It has been found that fish are able to tolerate greater concentrations of nitrite when chloride is also present. The 96-hr LC50 for rainbow trout increased 27-fold from 0.46 mg/liter in the presence of chloride concentrations ranging from 1 to 41 mg/liter (Russo and Thurston, 1977). Other anions have been found to inhibit nitrite toxicity to fish including bromide, sulfate, phosphate, and nitrate (Russo et al., 1981). Apparently, the fire-retardant compounds contain at least one constituent that ameliorates the toxicity of nitrite to fathead minnows for FT LCG-R and FT GTS-R because the nitrite concentrations at the 96-hr LC50 were above those reported to be toxic to fathead minnow.
Effect of Life Stage
The life stage exposed to the various firefighting chemicals played a significant role in determining the toxicity of the formulation. Egg life stages were less sensitive than the later life stages possibly due to the protective cell layers around the egg. Differences in membrane permeability and metabolic pathways may enable the embryo to withstand a chemical stressor because it is isolated in its own environment. Eggs of other species may be less tolerant of NH3 at various developmental stages. Solbé and Shurben (1989) reported that rainbow trout eggs exposed to NH3 within 24 hr of fertilization were sensitive to as little as 0.022 mg NH3/liter, whereas eyed eggs exposed 24 days postfertilization to 0.22 mg NH3/liter had no significant mortality. The comparably rapid development of fathead minnow eggs makes differentiation of variable toxicity difficult compared to what was reported by Solbé and Shurben (1989).
Fathead minnow fry was the most sensitive life stage tested. This life stage was just beginning to actively search for food; thus, a possible change in metabolic processing from endogenous to exogenous resources may cause this life stage to be more prone to the stresses from the test chemicals. Also, because this life stage had just begun to utilize exogenous resources, it probably did not have a large store of endogenous resources or physiological mechanism in place to deal with this additional stressor.
Another possibility is that the gills of this life stage have undergone various physiological changes in response to metabolic shifts. Ammonia in its un-ionized form passes freely across the gill epithelium (Randall and Wright, 1989). An additional characteristic that is noteworthy concerning life stage is the increased resilience exhibited by 60-DPH juveniles compared to fry and 30-DPH juveniles (Table 3), which may have been due to their relatively advanced maturational stage. Fathead minnow females have been reported to reach sexual maturity in as little as one year (Lord, 1927; Dobie et al., 1956). Based on age at first spawn, fathead minnow 60-DPH juveniles have completed approximately 16% of the growth required for maturity.
Comparison to Published/Manufacturer Data
Due to the paucity of published toxicity information on firefighting chemicals, most of the comparisons made were with results reported by manufacturers or their contract laboratories. Formulation toxicity data is given in Table 6. Comparisons made with data found in the literature were mainly with formulations no longer in use, although the main components of the compounds are still used in present-day formulations.
Table 6. Summary of Reported Acute Toxicities of Firefighting Chemical Formulations to Fish
| Species | Formulation | Mean weight (g) | Water hardness (mg/liter) | Temperature (°C) | 96-hrLC50 (mg/liter) | Referencea |
| Rainbow trout | FT GTS-R | NA | NA | NA | 1000 | 1 |
| FT GTS-R | 0.5 | 5 | 15 | 899 | 2 | |
| FT LCG-R | NA | NA | NA | 790 | 3 | |
| Firefoam 103 B | 0.5 | 5 | 15 | 41.1 | 2 | |
| PC D75-R | 0.44 | 40-45 | 12 | >1000 | 4 | |
| PC WD-881 | 0.57 | 40-48 | 12 | 22 | 5 | |
| PC WD-861 | 0.83 | 40-45 | 12 | 18 | 6 | |
| Silv-Ex | 0.37 | 53 | 13 | 25 | 7 | |
| Silv-Ex | NA | NA | NA | 0.07-320 | 8 | |
| Fathead minnow | PC D75-R | 0.15 | 40-45 | 22 | >1000 | 9 |
| Note. NA, data not available. a 1, Chemonics (1992a); 2, C. Chang (Chemonics, personal communication, 1993); 3, Chemonics (1992b); 4, ABC Laboratories (1986b); 5, ABC Laboratories (1988); 6, ABC Laboratories (1986c); 7, Springborn Bionomics (1986); 8, Norecol (1989); 9, ABC Laboratories (1986a). | ||||||
Data from Chemonics (1992a; C. Chang, personal communication, 1993) indicate that the 96-hr LC50 of Fire-Trol GTS-R to 0.5-g rainbow trout tested in soft water (hardness, 5 mg/liter as CaCO3) was 899 mg/liter in one test and greater than 1000 mg/liter in a second test. These values are 1.5 - 3.9 times the 96-hr LC50 determined for three life stages of fathead minnow tested in soft water (96-hr LC50, 233 - 605 mg/liter). This range is within the typical range of interlaboratory variation of four reported by Schimmel (1981).
Data from Chemonics (1992b) indicate that the 96-hr LC50 for FT LCG-R to rainbow trout was 790 mg/liter, which is within a factor of 1.5 of the 96-hr LC50 for fathead minnow fry. Johnson and Sanders (1977), as well as Blahm and Snyder (1973), conducted research with Fire-Trol 931, an ammonium-based retardant similar to FT LCG-R except for the manufacturer of the polyphosphates. Johnson and Sanders reported a 96-hr LC50 of 940 mg/liter for 0.8-g rainbow trout juveniles tested in soft water (hardness, 40 mg/liter as CaCO3), whereas Blahm and Snyder (1973) reported a 96-hr TLm of 1497 mg/liter at pH 7.0 with 129-mm rainbow trout juveniles. These values are similar to those found in the present study with three life stages of fathead minnow exposed in soft water (96-hr LC50, 1080 - 1797 mg/liter). Johnson and Sanders (1977) also reported that the sac-fry and swim-up fry were more sensitive than fingerlings, as was generally typified in the present study with fathead minnow.
The results of acute toxicity tests conducted for Monsanto with PC D75-R, which is similar to PC D75-F except for the colorant, determined a 96-hr LC50 of greater than 1000 mg/liter with fathead minnow and rainbow trout tested in soft water (hardness, 40-45 mg/liter CaCO3) at 22oC (ABC Laboratories, 1986a). This value is 1.6 to 2.4 times the 96-hr LC50s determined in the present study with fathead minnow tested in soft water (96-hr LC 50, 420-612 mg/liter).
Tests conducted for Monsanto with PC WD-881 in soft water (hardness, 40- 48 mg/liter as CaCO3) with 0.57-g juvenile rainbow trout resulted in a 96-hr LC 50, of 22 mg/liter (ABC Laboratories, 1988), which is slightly higher than the 96-hr LC50 for fathead minnow fry (the most sensitive life stage tested) tested in soft or hard water in the present study (96-hr LC50, 14 mg/liter). PC WD-861, a product similar to PC WD-881 but does not contain a component to reduce viscosity in cold temperatures, was also tested with rainbow trout under similar conditions and resulted in a 96-hr LC50 of 18 mg/liter (ABC Laboratories, 1986c), which was slightly higher than the 96-hr LC50 of PC WD-881 determined for fathead minnow fry in the present study.
Acute toxicity tests with rainbow trout exposed to Silv-Ex produced 96-hr LC50s ranging from 0.07 to 320 mg/liter (Norecol, 1989). The extreme points of the range were not explained, nor were descriptions of the test procedures given for all tests. Tests with 0.37-g rainbow trout exposed to Silv-Ex in soft water (hardness, 53 mg/liter as CaCO3) at 13°C reported a 96-hr LC50 of 25 mg/liter (Springborn Bionomics, 1986), which is similar to the 96-hr LC50 determined for three life stages of fathead minnow tested in soft and hard water in the present study (96-hr LC50, 19-22 mg/liter).
Relation to Environmental Considerations
Application of nonfoam retardants in firefighting situations is accomplished by a wide variety of aircraft equipped with different storage tanks configurations, door sizes, and sequencing speed, which results in a wide range of drop patterns (George, 1992). Drop patterns and the resulting ground patterns are also modified by drop height, retardant type, relative humidity, temperature, and wind speed and direction. Although there has been a substantial effort to improve performance of fire-retardant delivery in fixed-wing aircraft, the large number of combinations of aircraft types and tank configurations, retardant types, and environmental conditions precludes a straight forward comparison of laboratory toxicity data to potential effects in aquatic ecosystems.
Nevertheless, the wide variety of aircraft delivery systems have been modified to deliver similar amounts of firefighting chemicals. Retardant use ranges from 0.41 liter/m2 (1 gallon per 100 square feet; gpc) for fires in annual and perennial grasses or tundra to >2.44 liter/m2 (>6 gpc) for fires in mixed chaparral or heavy slash (George, 1992). For example, the nonfoam retardant FT GTS-R is prepared for field use by mixing 1.66 pounds per gallon of water to produce 1.1 gallons of slurry, which is equivalent to 198,930 mg/liter. Field mixtures for other firefighting chemicals are given in Table 7. Comparing the concentration of the FT GTS-R field mixture to the acute toxicity values for the most sensitive life stage gives ratios of 853 in soft water and 1474 in hard water. Thus, an accidental drop of FT GTS-R in an aquatic environment would require a dilution of 853- to 1474-fold to reach a concentration equivalent to the 96-hr LC50 -- a concentration which would cause a substantial amount of mortality. Similarly, application of FT LCG-R would have to be diluted to at least 250- to 521-fold to reach a concentration equal to the 96-hr LC50; for PC D75-F the dilution would have to be 342- to 856-fold.
Table 7. Formulation of Field Mixtures for Five Firefighting Chemical Formulations and the Ratio of the Concentration of the Field Mixture to the Acute Toxicity Value for Fathead Minnow Fry.
| Formulation | Field mixturea | Water type | Ratio to 96-hr LC50b | |
| mg/liter | ||||
| FT GTS R | 1.66 lb/gal | 198,930 | Soft | 853 |
| Hard | 1474 | |||
| FT LCG-R | 1 gal/4.5 gal | 270,400 | Soft | 250 |
| Hard | 521 | |||
| PC D75-F | 1.20 lb/gal | 143,800 | Soft | 342 |
| Hard | 856 | |||
| PC WD-881 | 1% | 10,000 | Soft | 714 |
| Hard | 714 | |||
| Silv-Ex | 1% | 10,000 | Soft | 455 |
| Hard | 500 | |||
| aWeight or volume of chemical concentrate combined with water to produce application mixture. bRatio: field mixture/96-hr LC50 (mg/liter). | ||||
A safety factor may be applied to toxicity data to estimate a safe concentration for aquatic organisms. The basis for a safety factor is the same rationale as an application factor, i.e., ratio of the highest observed "no effect concentration" (NOEC; lower limit of the maximum acceptable toxicant concentration) to the acute toxicity value, i.e., 96-hr LC50 (Rand and Petrocelli, 1985). An application factor of 0.01 is typically used, which inversely gives 100 as a safety factor to use in estimating a possible NOEC. Similar approaches to extrapolating from toxicity data to environmental concern concentrations using assessment factors of 100 (Okkerman et al., 1993; Schudoma, 1994) and 1000 (Okkerman et al., 1993) have been used in European countries in the hazard assessment process. Applying a safety factor of 100 to the above toxicity information would require a 147,400-fold dilution of FT GTS-R in hard water, 52,100 for FT LCG-R, and 85,600 for PC D75-F to approach a safe concentration, i.e., NOEC.
Foam suppressants are applied at about 1% foam, which is equivalent to 10,000 mg/liter. Comparing the concentration of field mixtures to the acute toxicity values for the two foam suppressants for fathead minnow gives ratios for PC WD-881 of 714 in soft and hard waters, and for Silv-Ex 455 in soft water and 500 in hard water (Table 7). Applying a safety factor of 100 to these values would require a 71,400-fold dilution for PC WD-881 and a 45,500 to 50,000 dilution for Silv-Ex to approach a safe concentration.
Protection of important fishery resources from the adverse effects of firefighting chemicals depends on adequate planning before a fire occurs, including identification of stream sections that need to be protected and development of application plans to minimize adverse effects on the stream (Norris and Webb, 1989). Small streams with little flow could be severely affected by an accidental fire-control chemical drop due to insufficient dilution. For example, during the 1988 fires in Yellowstone National Park, an accidental drop of ammonia phosphate retardant on the Little Firehole River during firefighting operations resulted in almost complete mortality of trout (Minshall and Brock, 1991). Soon after the fire dead trout were found in three creek drainages but the cause was unknown. Toxicity tests with larval cutthroat trout (Oncorhynchus clarki) exposed over a 3-week period to ash leachate from the Yellowstone fire showed no acute toxicity, thus suggesting ash leachate was probably not the cause of the fish mortality (Minshall and Brock, 1991).
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