Acute Toxicity of Firefighting Chemical Formulations to Four Life Stages of Fathead Minnow
Introduction
Firefighting chemical formulations are used widely throughout North America. These chemical formulations were applied by aircraft at up to 64 million liters per year in 1971 in the United States (Van Meter and Hardy, 1975). Approximately 91 million liters of ammonia-based fire-retardant mixtures were applied in 1992 in the United States (C. Johnson, U.S. Forest Service, personal communication, 1993). Applications often occur to lands adjacent to aquatic habitats, thereby creating the possibility of incidental fish kills.
Although most fire-retardant formulations are essentially fertilizer formulations and thought to have minimal toxicity, fish kills have been associated with accidental stream contamination due to inaccurate fire-retardant drops (Dodge, 1970). Almost complete mortality of trout was reported in a section of the Little Firehole River due to a fire retardant drop during the major fire in Yellowstone National Park in 1988 (Minshall and Brock, 1991). Toxicity information for fire-fighting chemicals currently in use is needed because of their relatively unknown toxic effects in aquatic ecosystems, the large volumes applied, and the variety of habitats where they are applied.
Although studies have been conducted using constituent chemicals of firefighting chemical formulations (Thurston et al., 1983; Singh et al., 1985; Sheehan and Lewis, 1986; Pramanik and Sarkar, 1987), relatively few studies have been conducted to determine the acute toxicity of fire-fighting chemicals to freshwater fish (Blahm and Snyder, 1973; Inman, 1974; Johnson and Sanders, 1977). In addition, most of these studies were conducted with formulations no longer in use, although the main constituents are present in currently used compounds. Toxicity testing has been conducted by the manufacturers or contract laboratories; however, several of these tests were not conducted in standardized water types. Moreover, experiments conducted by the manufacturers utilized only one life stage of fish and one water quality, so data are not available as to the toxicity to various life stages or how toxicity is affected by different water qualities (ABC Laboratories, 1986a,b,c, 1988; Springborn Bionomics, 1986; Chemonics, 1992a,b).
Two general categories of formulations are typically used in firefighting operations, short-term fire-suppressant foams and long-term retardants. Short-term fire-suppressant foams are wetting agents generally composed of a mixture of surfactants, foam stabilizers, inhibiting agents, and solvents (Monsanto, 1990; Ansul, 1991). The use of foam suppressants in firefighting is becoming more prevalent because the amount of water required can be reduced by over 60% (Schlobohm and Rochna, 1988). Foams enhance the ability of water to penetrate fuel sources, slow the evaporation of water, and create a barrier that reduces air contact with the fuel. Application concentrations of fire-suppressant foams range from 0.1 to 1.0% concentrate in solution (Monsanto, 1990; Ansul, 1991).
Long-term fire-retardant chemicals are composed of ammonium salts such as ammonium polyphosphate with a small amount of clay to maintain suspension, or ammonium sulfate, monoammonium phosphate, or diammonium phosphate, singly or in combinations, thickened with a guar gum-derivative. Fire-retardant formulations have the ability to form a combustion barrier composed of decomposition products of the formulation after the evaporation of the water carrier. The effectiveness of these formulations depends greatly on the amount of salt applied per unit surface area and is often increased by utilizing highly concentrated solutions (144 - 199 g/liter), which can sometimes be corrosive. Retardant formulations contain corrosion inhibitors such as sodium ferrocyanide as well as small amounts of colorants, such as ferric oxide, to mark drop sites.
This research was conducted to assess the acute toxicity of five firefighting chemicals currently in use to four life stages of fathead minnow (Pimephales promelas) in soft and hard water. Warmer waters are often associated with the open prairie rangeland and typically have different fish assemblages than mountain streams and rivers. Fathead minnows were chosen as a representative warmwater fish species for test purposes because rangeland fires are combated with many of the same formulations used in forest fires.
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