Acute Toxicity of Fire Control Chemicals to Daphnia magna (Straus) and Selenastrum capricornutum (Printz)
Discussion
Acute Toxicity
Daphnia magna. According to acute toxicity rating scales developed by Passino and Smith (1987), the fire retardant and foam suppressant chemicals would be considered slightly to moderately toxic. Practically harmless chemicals produce an acute toxicity measure (EC50, or IC50) with values ranging from 100 to 1,000 mg/liter, moderately toxic chemicals produce an acute toxicity measure with values ranging from 10 to 100 mg/liter, and slightly toxic chemicals produce an acute toxicity measure with values ranging from 1-10 mg/liter. The 48-hr EC50 of daphnids exposed to the foam suppressants Phos-Chek WD-881 and Silv-Ex ranged from 4 mg/liter in hard water tests to 11 mg/liter in soft water tests, thus categorizing them as slightly to moderately toxic. Tests with the three non-foam retardants demonstrated 48-hr EC50s greater than 100 mg/liter, thus categorizing them as slightly toxic. However, in the field these formulations are used in highly concentrated solutions to fight fires. For example, Fire-Trol GTS-R is mixed at the rate of 1.66 pounds per gallon of water, which is equivalent to 200,000 mg/liter. Likewise, the field mixtures for the other four test chemicals are also formulated at concentrations substantially above their toxicity values presented here.
In general, foam suppressants were as much as 100 times more toxic to daphnids than non-foam retardants. Foam suppressants are typically composed of 30 to 40% surfactant, which is the apparent toxic constituent. These surfactants lower the surface tension of water thereby interfering with an aquatic organism's ability to obtain oxygen (Norecol, 1989). The actual toxic threshold of surfactants is dependent in part on its carbon-chain length (Swisher, 1987). The carbon-chain length of anionic surfactants contained in Phos-Chek WD-881 and Silv-Ex is unknown because they are proprietary formulas. D. magna has been reported as the most sensitive of the species previously tested to surfactant-containing chemicals, and other aquatic invertebrates and algae are comparably sensitive to surfactants (Lewis and Suprenant, 1983; Swisher, 1987). In the present study, the results from three of four tests with foam suppressants indicated daphnids were twice as sensitive to foam suppressants as algae.
Selenastrum capricornutum. Fire-Trol compounds were as much as 80 times more toxic to algae than to daphnids. Toxicity of the three fire retardant compounds may have been due to a nutrient limitation or other constituents present in the fire retardants.
Stimulation in response to the addition of low concentrations of ammonium and phosphorus would be expected. Additional nitrogen and phosphorus together stimulated growth more than either element added alone in tests conducted by Miller et al. (1978). Fire-Trol GTS-R powder has a nitrogen:phosphorus ratio of 14:1, Fire-Trol LCG-R concentrate has a ratio of 1:1.5, and Phos-Chek D75-F powder has a ratio of 3:1. When these compounds were added to the algal medium,the ratio of 11:1 in the medium is altered. A ratio greater than 11:1 indicates a potential for phosphorus limitations, whereas a ratio less than 11:1 indicates a potential for nitrogen limitation. This alteration in low test concentrations stimulated growth in tests with four of the five chemicals. The greatest stimulations occurred in tests with the three non-foam retardants containing ammonium compounds. Fire-Trol GTS-R stimulated twice as much growth in the two lowest concentrations as Fire-Trol LCG-R in similar concentrations, whereas the lowest Phos-Chek D75-F treatment produced the greatest stimulation of all five test chemicals. Phos-Chek WD-881 produced a growth stimulation in lower test concentrations even though the chemical does not apparently contain ammonium or phosphorus, based on available information. This stimulation suggests the presence of an important nutrient in the formulation. The nutrient imbalance created by the addition of fire control chemicals can stress aquatic plant species such as algae by limiting their growth and maturity, thus, inhibiting the cells.
Ammonia
The ammonia constituent has been determined to be the toxic portion of non-foam retardant chemicals (Inman, 1974). Macroinvertebrates are reportedly more tolerant to ammonia than fish species (USEPA, 1986). Toxicity of ammonia concentrations is apparently species specific for invertebrates and fish, and un-ionized ammonia is believed to be more toxic to aquatic organisms than total ammonia. Flowthrough tests determioned un-ionized ammonia to be acutely toxic to 19 freshwater macroinvertebrate species at concentrations ranging from0.53 to 22.8 mg/liter (USEPA, 1986). Studies conducted by Williams et al. (1986) reported 96-hr LC50s for un-ionized ammonia ranging from 0.71 to 2.95 mg/liter for 11 macroinvertebrate species, and Monda (1991) reported a 96-hr LC50 for un-ionized ammonia for Chironomus riparius as high as 9.4 mg/liter. USEPA (1985) reported 48-hr LC50s for daphnids of 2.4-2.8 mg/liter un-ionized ammonia in hard water (hardness 192-202 mg/liter as CaCO3) and 0.53-0.90 mg/liter un-ionized ammonia in soft water (hardness 42-48 mg/liter as CaCO3).
Un-ionized ammonia concentrations in the present study with Fire-Trol compounds were close to or higher than the above-reported values, thus indicating that un-ionized ammonia derived from the constituents probably caused the observed toxicity. The un-ionized ammonia concentration for Phos-Chek D75-F from this study and in the Johnson and Sanders (1977) study with amphipods were lower than the above reported values. This observation suggests that another constituent such as spoilage and corrosion inhibitors may have contributed to the toxicity of Phos-Chek D75-F to daphnids.
Few studies have been conducted to determine the acute toxicity of ammonia to aquatic plants, and no study with plants cited by USEPA (1985) provided un-ionized ammonia concentrations. Ammonia is toxic to plants at high concentrations and high pH suggesting that toxicity of ammonia is due to un-ionized ammonia rather than the ammonium ion. Previous tests conducted with algae measured inhibition as a reduction of oxygen evolution or CO2 photoassimilation rate (USEPA, 1985). Ammonia concentrations reported to inhibit 50% of algal growth ranged from 2.4 to 11 mg/liter and were obtained at high pH rather than low pH indicating that un-ionized ammonia concentrations were probably the cause of inhibition. In the present study, the pH of test concentrations above the 96-hr IC50 value was lower than 6.5, and there was very little un-ionized ammonia present in Fire-Trol GTS-R and Phos-Chek D-75-F test concentrations. Fire-Trol LCG-R contained no measurable un-ionized ammonia in ASTM algal assay medium. This absence of un-ionized ammonia suggests that some factor other than ammonia may have influenced the toxicity of non-foam retardants to algae.
USEPA (1986) has established an ammonia concentration criterion of 0.02 mg/liter as un-ionized ammonia as the concentration below which all aquatic life may be protected. Un-ionized ammonia concentrations of foam suppressants at the 48-hr EC50 for daphnids were below this criterion concentration, whereas the un-ionized ammonia concentrations of non-foam retardants exceeded the criterion concentration.
Nitrate and Nitrite
Few studies have been conducted to determine nitrate and nitrite toxicity to freshwater invertebrates. Nitrite is usually present only in trace amounts in most natural freshwater systems because it rapidly oxidizes to nitrate, whereas nitrate is considerably less toxic than nitrite (Rand and Petrocelli, 1985). A study testing the toxicity of oil shale leachate reported D. magna were sensitive to a concentration of the effluent which contained 66 mg/liter NO3--N (Woodward, et al., 1985). Camargo and Ward (1992) obtained 96-h LC50s of 97 mg/liter NO3--N and 113 mg/liter NO3--N when Hydropsyche occidentalis and Cheumatopsyche pettiti, respectively, were exposed to sodium nitrate in soft water (hardness 43 mg/liter as CaCO3). Nitrate-nitrogen concentrations present in the 48-hr EC50 concentration of the three non-foam fire retardants were 15 to 1,000 times less than those reported to be toxic to freshwater invertebrates and probably did not influence the toxicity of the fire retardants to daphnids.
Gutzmer and Tomasso (185) exposed crayfish (Procambarus darkii) to sodium nitrite in hard water (hardness 198 mg/liter as CACO3). They obtained a 96-hr LC50 of 8.4 mg/liter NO2--N. Nitrite-nitrogen concentrations in Phos-Chek D75-F were >30-times lower than the crayfish 96-hr LC50, whereas Fire-Trol GTS-R in both water qualities contained nitrite-nitrogen concentrations similar to the crayfish 96-hr LC50. Fire-Trol LCG-R in both water qualities contained nearly 4 times the crayfish 96-hr LC50. The toxicity of Fire-Trol GTS-R and Fire-Trol LCG-R in both water qualities may have been influenced by their nitrite concentration.
Trophic Interaction
Growth stimuli observed in algae tests with fire retardants could be beneficial to aquatic invertebrates feeding on them. For example, daphnids fed algae grown in medium containing 1 mg/liter hexavalent chromium experienced an increase in fecundity and growth as well as a decrease in the mean age at which reproduction began (Gorbi and Corradi, 1993). However, negative effects on fecundity, growth, and onset of reproduction were observed in daphnids fed algae grown in the presence of larger concentrations of chromium (10 mg/liter). Stimulatory effects on daphnids were attributed to larger quantities of algal biomass when algae was grown in low concentrations of chromium. Biomass as dry weight at this concentration was double that of the control treatment. Negative effects were attributed to algae being a poor food source as a result of severe intercellular alterations caused by high concentrations of chromium.
Sterner et al. (1993) also reported increased fecundity, rate of body mass accumulation, and survivorship of daphnids in response to moderately and severely nitrogen-limited algae, but not in response to severely phosphorus-limited algae. In their study, daphnids raised on a diet of severely phosphorus-limited algae were more sluggish and easily caught with a pipet. Mortality of these animals in a natural setting could be increased because sluggish daphnids would be easy prey for predators, and thus could lead to an altered food web. Daphnids were also observed with exuviae still attached at the posterior margin. Sterner et al. (1993) suggested that the diet of severely phosphorus-limited algae may have interfered with completion of the molt cycle in daphnids.
Daphnids were more tolerant to non-foam retardants than were algae. Daphnids consuming algae grown in concentrations of fire retardant chemicals that stimulate growth could respond favorably. However, other constituents of the fire retardant chemicals could produce toxic effects to daphnids. When algal growth is inhibited, daphnids could experience negative effects. The algae, being more sensitive to fire retardant chemicals, may be less abundant and a poor quality food source at chemical concentrations that are not detrimental to daphnids.
Interlaboratory Comparison
Other laboratories have conducted toxicity tests with daphnids and algae exposed to fire retardant chemicals (Table 7). Fire retardant chemicals used in previous testing with daphnids and algae were similar in their formulation to Phos-Chek D75-F and Phos-Chek WD-881.
Daphnia magna. Test methods and water quality (hard water) utilized by ABC Laboratories (1986a) in tests exposing daphnids to Phos-Chek D75-R and Phos-Chek WD-861 were similar to methods used in this study with Phos-Chek D75-F and Phos-Chek WD-881. Phos-Chek D75-R differs from Phos-Chek D75-F only in the coloring agent added. Phos-Chek D75-R contains a red coloring agent (pigment grade iron oxide), whereas Phos-Chek D75-F contains a fugitive (international orange) coloring agent (Monsanto Co., 1991). Phos-Chek WD-861 is similar to Phos-Chek WD-881 except for addition of a constituent to reduce the viscosity of Phos-Chek WD-881 in cold temperatures and a corrosion inhibitor (Monsanto Co., 1990).
Daphnids exposed to Phos-Chek D75-F (48-hr LC50 280 mg/liter) were greater than 3.5 times more sensitive than daphnids exposed to Phos-Chek D75-R (48-hr LC50 >1,000 mg/liter). Comparing results from both tests produces a high/low ratio of 3.6, which is within the range of typical variation, i.e., 4.0, for interlaboratory comparison of test results (Schimmel, 1981).
Daphnids exposed to Phos-Chek WD-881 (48-hr EC50 4 mg/liter) were nearly twice as sensitive as daphnids exposed to Phos-Chek WD-861 (48-hr EC50 7.8 mg/liter). Interlaboratory comparison produces a high/low ratio of 2.0, thus indicating the results of tests with Phos-Chek WD-881 are similar to those reported for Phos-Chek WD-861 by ABC Laboratories (1986b).
Selenastrum capricornutum. Tucker et al. (1987) exposed the algae S. capricornutum to Phos-Chek WD-861. Test methods exposing algae to Phos-Chek WD-861 in their tests were somewhat different from the present tests with Phos-Chek WD-881. Differences between the two studies include the following: for tests with Phos-Chek WD-881 algal assay medium was prepared and pH adjusted one day prior to testing, algae was exposed in 250-ml flasks which contained 125 ml algal assay medium, and each flask was inoculated with 2.0 x 104 cells/ml, whereas for tests with Phos-Chek WD-861 algal assay medium was prepared on the same day as test initiation, used 125-ml flasks which contained 50 ml algal assay medium, and inoculated each flask with 104 cells/ml. Inhibition of algal growth, i.e., biomass, was determined in a different manner. Biomass for tests with Phos-Chek WD-881 was determined by analyzing a sample of the solution for chlorophyll a content, whereas for tests with Phos-Chek WD-861 biomass was determined by cell counts and dry weight. Algae exposed to Phos-Chek WD-881 (96-hr IC50 24 mg/liter) were greater than three times more tolerant than the algae exposed to Phos-Chek WD-861 (96-hr EC50 7.6 mg/liter). A growth stimulus in lower treatments of tests with Phos-Chek WD-861 was not reported whereas a stimulus of 3 to 16% was observed in the chlorophyll aanalysis of tests with Phos-Chek WD-881. Interlaboratory comparison produces a high/low ratio of 3.2, thus suggesting results from both tests are similar in spite of the differences in study design and end points measured.
Relation to Environmental Considerations
Application of non-foam retardants in fire fighting 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 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 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). Non-foam retardants are prepared for field use by mixing 1.66 pounds of Fire-Trol GTS-R per gallon of water to produce 1.1 gallons of slurry, which is equivalent to 198,930 mg/liter. Field mixtures for other fire fighting chemicals are given in Table 8. Comparing the concentrations of field mixtures to the acute toxicity values for the three non-foam retardants for daphnids gives ratios ranging from 319 for Fire-Trol LCG-R tested in soft water to 1027 for Phos-Chek D75-F tested in hard water. Thus, an accidental drop of Fire-Trol LCG-R in an aquatic environment would require a dilution of 319- to 333-fold to dilute it to a concentration equivalent to the 48-hr EC50 -- a concentration which would cause a substantial amount of mortality. Similarly, application of Fire-Trol GTS-R would have to be diluted to at least 587- to 774-fold to reach a concentration equal to the 48-hr LC50; for Phos-Chek D75-F the dilution would have to be 514- to 1027-fold.
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" (maximum acceptable toxicant concentration, MATC) to the acute toxicity value, i.e., 48-hr EC50 (Cairns et al., 1978; 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 MATC. Applying a safety factor of 100 to the above toxicity information would require a 77,400-fold dilution of Fire-Trol GTS-R in soft water, 33,300 for Fire-Trol LCG-R, and 102,700 for Phos-Chek D75-F to approach a safe concentration, i.e., MATC.
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 daphnids gives ratios for Phos-Chek WD-881 of 909 in soft water and 2500 in hard water, and 1429 for Silv-Ex in either water type (Table 8). Applying a safety factor of 100 to these values would require a 90,900- to 250,000-fold dilution for Phos-Chek WD-881 and 142,900-dilution for Silv-Ex to approach a safe concentration.
Effects of surfactants on D. magna have been reported to be greater in the second and third generations than on the initial generation (Lesyuk et al., 1983). The degradation of Silv-Ex over a 20-day period was 42% for a 1% solution and 37% for a 0.5% solution (Norecol, 1989). The degradation of Phos-Chek WD-881 was 80% in 21 days but only 85% in 42 days in a shake flask test, which by its continuous mixing probably speeded up the degradation process (Norecol, 1989). Considering that D. magna are reproductively mature in 6 to 9 days (ASTM, 1984), the second and third generations could be exposed for a substantial time period after surfactants entered the water and before degradation was complete. Although these foam suppressants would degrade in the aquatic environment, the effects of residual amounts could be exerted on successive daphnid and algal populations, and consequently, through the food web.
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