Acute Toxicity of Three Fire-Retardant and Two Foam-Suppressant Foam Formulations to the Early Life Stages of Rainbow Trout (Oncorynchus mykiss)
Discussion
The fire-suppressant foams tested 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 GTS-R, FT LCG-R, and PC D75-F. Based on the hazard ranking presented by Passino and Smith [32] (0.01 - 0.1 mg/L, extremely toxic; 0.1 - 1.0 mg/L, highly toxic; 1 - 10 mg/L, moderately toxic; 10 - 100 mg/L, slightly toxic; 100 - 1,000 mg/L, practically harmless; and greater than 1,000 mg/L, relatively harmless), both PC WD-881 and Silv-Ex were considered moderately toxic formulations. Fire-Trol GTS-R and PC D75-F were ranked as practically harmless formulations, whereas FT LCG-R was ranked as relatively harmless. However, the fire-retardant formulations are used in highly concentrated solutions in the field (e.g., the mix ratio for FT GTS-R is 1.66 lb/gallon or 198,930 mg/L); hence the possibility of experiencing concentrations close to the 96-h LC50 in the environment are real and render the terms "practically" and "relatively harmless" in this scenario to be irrelevant.
Surfactants
Toxicity of the fire-suppressant foams PC WD-881 and Silv-Ex is probably due to the anionic surfactant portion of their formulation. Various authors have reported on the toxicity of surfactants, with results comparable to the 96-h LC50s determined in this study. Müller [33] reported a 24-h LC50 for 8-g rainbow trout of 8.5 mg/L for a commercial, non-ionic surfactant. 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 approx. 45-50 dynes/cm at the 24-h LT50 (the concentration at which 50% of the population survives exposure for the specified time period). In comparison, the 0.6% Silv-Ex field application mixture has a surface tension of 22.92 dyne/cm [19], about half the surface tension reduction that caused the mortality reported by Müller [33]. Holman and Macek [34] determined the 96-h LC50s for three different chain length linear alkylbenzene sulfonate (LAS) anionic surfactants with 2 - 3 month-old fathead minnow (Pimephales promelas) juveniles tested in soft water (40 mg/L as CaCO3). The 96-h LC50s ranged from 0.86 to 12.3 mg/L, with increasing alkyl chain length directly increasing toxicity. Although the exact anionic surfactants used in PC WD-881 and Silv-Ex were not known, the 96-h LC50 of the LAS anionic surfactant with a mean chain length of C11.7 (12.3 mg/L) [34] was extremely close to the 96-h LC50s determined in this study with nonegg life stages for Silv-Ex and PC WD-881 (10 - 22 mg/L; Table 3).
The effect of water quality on the toxicity of fire-suppressant foams, which are primarily composed of anionic surfactants, was somewhat contradictory in our study. No effect was observed in tests with PC WD-881, whereas the toxicity of Silv-Ex was significantly increased in hard water tests with four of the five life stages compared to soft water tests. Others have also reported conflicting results. Hokanson and Smith [35] and Holman and Macek [34] showed that the toxicity of the anionic surfactant LAS was increased in hard water, whereas McKim et al. [36] concluded that water hardness played a minor role in the toxicity of LAS.
Un-ionized ammonia, nitrate, nitrite
The toxicity of the three fire retardants seemed to depend on the amount of un-ionized ammonia available from the formulation. Little or no un-ionized ammonia was present in the two fire-suppressant foams. Ammonia estimates were compared to the un-ionized ammonia 96-h LC50 concentrations determined by Thurston and Russo [4], who used rainbow trout in their tests. Due to the prediction of extremely high concentrations of un-ionized ammonia at 96-h LC50 for tests conducted with rainbow trout eyed eggs and embryo-larvae, these life stages were not compared to the NH3 data of Thurston and Russo [4]. Estimated un-ionized ammonia concentrations at the 96-h LC50 in tests with FT LCG-R in both soft and hard water and the swim-up fry and 60- and 90-dph juveniles were very similar to those reported by Thurston and Russo [4]. This similarity of results for the three life stages suggests that the un-ionized ammonia derived from the ammonium polyphosphate component of FT LCG-R was the primary toxic component.
Likewise, the similarity of estimated NH3 concentrations at the 96-h LC50 for FT GTS-R with those reported by Thurston and Russo [4] strongly suggests that the un-ionized ammonia derived from the ammonium sulfate and diammonium phosphate components of FT GTS-R was the primary toxic agent. Although PC D75-F tests in soft water had the lowest un-ionized ammonia estimates of the three retardants tested (Table 4), NH3 estimates were within a factor of two to four of the values reported by Thurston and Russo [4] for similarly-sized fish. This similarity suggests that un-ionized ammonia probably contributed substantially to the toxicity of PC D75-F. Other components (corrosion inhibitors, colorants, etc.) may have also contributed some toxicity to the formulations.
Few studies have reported the toxicity of nitrate to fish because it is considered essentially non-toxic [37]. Westin [38] reported a 96-h LC50 of 1,362 mg NO3--N/L for rainbow trout, which is about 1,000-fold higher than that reported by Kincheloe et al. [39], who observed about 50% mortality in rainbow trout fry exposed for 30 d to nitrate-nitrogen concentrations of 1.1 to 2.3 mg/L. Kincheloe et al. [39] reported toxicity values that lie within the range reported here, but their tests were conducted for more than 30-d. This result may be comparable because Westin [38] reported that toxicity values declined 20% in a 7-d test from that in a 96-h test. Concentrations of nitrate-nitrogen present in test waters with the three fire retardants at the 96-h LC50 concentrations were below those reported by Westin [38] for rainbow trout and probably did not substantially influence the toxicity of the fire-retardant chemicals.
Nitrite-nitrogen probably influenced the toxicity of the fire-retardant chemicals to rainbow trout. Nitrite-nitrogen concentrations measured at the 96-h LC50 concentrations in tests with swim-up fry of rainbow trout and calculated for other life stages in tests with FT GTS-R and LCG-R were greater than those reported for rainbow trout by Russo and Thurston [40] (0.19-0.28 mg NO2--N/L), Russo et al. [41] (0.17-0.4 mg NO 2--N/L), and Russo et al. [42] (0.19-1.05 mg NO2--N/L). The measured concentration of nitrite-nitrogen in PC D75-F in the 96-h LC50 concentration for the swim-up fry life stage was similar to those reported in the literature and may have contributed to the toxicity of the fire retardant in this life stage.
Effect of life stage
The life stage at which rainbow trout were exposed to the various fire-fighting chemicals played a significant role in the toxicity of the formulation. Eyed eggs were always much more resilient than the later life stages, possibly due to their protective cell layers. Differences in membrane permeability and metabolic pathways may enable the embryo to withstand a chemical stressor because it is "isolated" somewhat from the aquatic environment. Other authors have reported that rainbow trout eggs were less tolerant of NH3 than was reported in our study. Solbé and Shurben [43] reported that rainbow trout eggs exposed within 24 h of fertilization to as little as 0.022 mg NH3/L were more sensitive than other life stages; however, eyed eggs 24 d post fertilization exposed to 0.22 mg NH3/L had no significant mortality. The older age of the eyed eggs used in our study probably accounts for the increased NH3 resilience compared to that reported by Solbé and Shurben [43] and may also indicate that eggs exposed at an earlier developmental stage to fire suppressants may be more sensitive as well. Solbé and Shurben [43] also used a flow-through exposure that continuously replenished ammonia concentrations, which were measured daily, whereas our tests were static. The continuous replenishment may have also contributed to the greater sensitivity reported by Solbé and Shurben [43].
The most sensitive life stage in our tests was the swim-up fry. This life stage is just beginning to actively search for food and change metabolic processing from endogenous to exogenous resources, which probably cause it to be more prone to the stresses placed on it by the test formulation. Also, because this life stage has just begun to utilize exogenous resources, it probably does not have a large store of endogenous resources or a 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 [44]. Gills of rainbow trout swim-up fry are presumably quite different than those of sac fry in which much of the gas exchange is accomplished cutaneously [45]. Cutaneous respiration may provide eggs and sac fry with the ability to cope with a stressor which affects gas exchange in swim-up fry.
Comparison to published and manufacturer data
Due to the paucity of toxicity information on fire-fighting chemicals, most of the comparisons made were with results reported by the manufacturers or by their contract laboratories. Available toxicity data for rainbow trout exposed to fire-fighting chemicals are summarized in Table 6.
Table 6. Summary of reported acute toxicities of fire-fighting chemical formulations to rainbow trout.
(g) | (mg/L) | °C | (mg/L) | ||
| Fire-Trol GTS-R | NAb | NA | NA | 1,000 | [16] |
| Fire-Trol GTS-R | 0.5 | NA | 15 | 899 | c |
| Fire-Trol LCG-R | NA | NA | NA | 790 | [17] |
| FIREFOAM 103B | 0.5 | NA | 15 | 41.1 | c |
| Phos-Chek D75-R | 0.44 | 40-45 | 12 | >1,000 | [11] |
| Phos-Chek WD-881 | 0.57 | 40-48 | 12 | 22 | [14] |
| Phos-Chek WD-861 | 0.83 | 40-45 | 12 | 18 | [13] |
| Silv-Ex | 0.37 | 53 | 13 | 25 | [15] |
| Silv-Ex | NA | NA | NA | 0.07-320 | [47] |
|
aLC50 = Median lethal concentration. bNA = Data not available. cC. Chang (Chemonics, personal communication). | |||||
Our results for tests with FT LCG-R are similar to those reported for rainbow trout tested by others with FT LCG-R [17] or with Fire-Trol 931 [8,10], an ammonium-based retardant similar to FT LCG-R except for the manufacturer of the polyphosphates. Johnson and Sanders [10] also reported that the sac fry and swim-up fry were more sensitive than fingerling salmonids in their study, as was generally found in our study. Our results with FT GTS-R are similar to results of tests with rainbow trout reported by others using the same compound [16].
In contrast, our results with PC D75-F suggest rainbow trout are more sensitive than others report (Table 6). Acute toxicity tests conducted by Monsanto's contract laboratory with PC D75-R, a liquid similar to PC D75-F except for the colorant, resulted in a 96-h LC50 of greater than 1,000 mg/L [12]. Their 96-h LC50 value was 4.3 times our 96-h LC50 obtained for 60-dph juvenile rainbow trout tested in soft water (234 mg/L) and 4.6 times the 96-h LC50 for 60-dph juveniles exposed to PC D75-F in hard water (218 mg/L). This amount of variation is greater than the typical range of four for interlaboratory comparison of acute toxicity values [46].
The results of our tests conducted with the two fire-foam suppressants, PC WD 881 and Silv-Ex, are similar to those of others who also tested rainbow trout [14,15,47]. Our results were also similar to those who tested PC WD 861, which is similar to PC WD-881 but contains a component to reduce viscosity in cold temperatures [13] (Table 6). However, acute toxicity tests by others with Silv-Ex produced 96-h LC50s ranging from 0.07 to 320 mg/L for several tests with rainbow trout [47]. The extreme points of the range were not explained, nor were descriptions of the test procedures given for all tests.
Relation to environmental conditions
Application of nonfoam retardants in fire-fighting situations is accomplished by use of a wide variety of aircraft equipped with different storage tanks, door sizes, and sequencing speeds, which results in a wide range of drop patterns [48]. 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 delivery of fire retardant in fixed-wing aircraft, the large number of combinations of aircraft types and tank configurations, retardant types, and environmental conditions precludes a straightforward 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 fire-fighting chemicals.
Retardant use ranges from 0.41 L/m2 (1 gallon/100 ft2 [gpc]) for fires in annual and perennial grasses or tundra to >2.44 L/m2 (>6 gpc) for fires in mixed chaparral or heavy slash [48]. For example, the fire retardant FT GTS-R is prepared for field use by mixing 1.66 lb/gallon of water to produce 1.1 gallons of slurry (referred to by fire fighters as a field mixture), which is equivalent to 198,930 mg/L. Field mixtures for other fire-fighting chemicals are given in Table 7. Comparing the concentrations of field mixtures to the acute toxicity values for the most sensitive life stage and FT GTS-R gives ratios of 548 in soft water and 961 in hard water. Thus, an accidental drop of FT GTS-R in an aquatic environment would require a dilution of 548- to 961-fold to reach a concentration equivalent to the 96-h 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 297- to 310-fold to reach a concentration equal to the 96-h LC50; for PC D75-F the dilution would have to be 515- to 660-fold.
Table 7. Formulation of field mixtures for five fire-fighting chemicals and the ratio of the concentration of the mixture to the acute toxicity value for rainbow trout swim-up fry.
LC50b | ||||
| (mg/L) | ||||
| Fire-Trol GTS-R | 1.66 lb/gal | 198,930 | Soft Hard | 548 961 |
| Fire-Trol LCG-R | 1 gal/4.5 gal | 270,400 | Soft Hard | 297 310 |
| Phos-Chek D75-F | 1.20 lb/gal | 143,800 | Soft Hard | 515 660 |
| Phos-Chek WD-881 | 1% (v/v) | 10,000 | Soft Hard | 769 909 |
| Silv-Ex | 1% (v/v) | 10,000 | Soft Hard | 500 769 |
| a Weight or volume of chemical concentrate combined with water to produce application mixture. bRatio: field mixture/median lethal concentration (LC50) (mg/L). | ||||
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., the ratio of the highest no-observed-effect concentration observed (NOEC; lower limit of the maximum acceptable toxicant concentration) to the acute toxicity value, i.e., the 96-h LC50 [49]. An application factor of 0.01 is typically used, which inversely gives 100 as a safety factor to use in estimating a possible NOEC. Applying a safety factor of 100 to the above toxicity information would require a 96,100-fold dilution of FT GTS-R in hard water, a 31,000-fold dilution for FT LCG-R, and 66,000-fold dilution for PC D75-F to approach a safe concentration (NOEC).
Foam suppressants are applied at about 1% foam, which is equivalent to 10,000 mg/L. Comparing the concentration of field mixtures to the acute toxicity values for the two foam suppressants for rainbow trout gives ratios of 769 in soft water and 909 in hard water for PC WD-881, and for Silv-Ex, 500 in soft water and 769 in hard water (Table 7). Applying a safety factor of 100 to these values would require a 76,900- to 90,900-fold dilution for PC WD-881 and a 50,000- to 76,900 dilution for Silv-Ex to approach a safe concentration.
Protection of important fishery resources from the adverse effects of fire-fighting 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 [50]. 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 resulted in almost complete mortality of trout [3]. Soon after the fire, dead trout were also 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 not the cause of the fish mortality [3].
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