Acute Toxicity of Three Fire-Retardant and Two Foam-Suppressant Foam Formulations to the Early Life Stages of Rainbow Trout (Oncorynchus mykiss)
Introduction
Fire-fighting chemical formulations are used widely throughout North America and are an essential tool for the control and management of wildland fires. These chemical formulations were applied by aircraft in nearly 15,000 fire-retardant drop missions delivering about 64 million L in 1971 [1]. Approximately 91 million liters of ammonia-based fire-retardant mixtures were applied in the United States in 1992, (C. Johnson, personal communication), to lands often adjacent to aquatic habitats, thereby creating the possibility of incidental fish kills. Although most fire-retardant formulations are believed to have minimal toxicity to aquatic oganisms, fish kills have been associated with accidental stream contamination during fire-retardant drops [2]. Almost complete mortality of trout was reported in a section of the Little Firehole River due to a fire-retardant drop during the major 1988 fire in Yellowstone National Park [3]. There is a paucity of information about the toxicity to fish of fire-fighting chemicals currently in use.
Although studies have been conducted using constituent chemicals of fire-control chemicals [4-7], relatively few studies have been conducted to determine the acute toxicity of fire-fighting chemical formulations to freshwater fish [8-10]. In addition, most of these studies were conducted with formulations that are no longer in use. Toxicity testing has been conducted by the manufacturers or contract laboratories; however, several of these tests were not conducted in standardized waters. Moreover, experiments conducted by the manufacturers utilized only one life stage of fish and one type of water; thus, data are lacking on both the sensitivity of various life stages of fish and the influence of different water qualities on the toxicity of these chemical formulations [11-17].
Two general categories of chemicals are typically utilized in fire fighting. The first category is fire-suppressant foams, short-term wetting agents that enhance the ability of water to penetrate fuel sources, thus reducing the ability of the fuel to ignite. These formulations also act by slowing the evaporation of water, increasing water retention on fuel sources, and reducing air contact with the fuel by insulating the fuel source from the heat of the fire. However, fire-suppressant foams do not reduce combustion after water has evaporated from the fuel source. Application concentrations of fire-suppressant foams range from 0.1 to 1.0% concentrate in solution [18,19]. Fire-suppressant foams are generally composed of a mixture of surfactants, foam stabilizers, inhibiting agents, and solvents [18,19]. The use of fire-suppressant foams in fire fighting is becoming more prevalent because the amount of water required can be reduced by 60% [20].
The second category is fire-retardant formulations, which have the ability to form a long-term combustion barrier after the evaporation of the water. In general, the retardant chemicals are composed of ammonium salts with a thickening agent, such as ammonium sulfate or ammonium polyphosphate with an attapulgite clay thickener, or diammonium phosphate with a guar gum-derivative thickener. Formulation effectiveness depends on the amount of salt applied per unit surface area. Fire-retardant effectiveness is increased by utilizing highly concentrated solutions (144-199 g/L), which are very corrosive due to their high salt content. Fire-retardant formulations contain rust inhibitors such as sodium dichromate as well as trace amounts of colorants, such as ferric oxide, to mark drop sites.
The acute toxicity of five fire-control chemicals to the early life stages of rainbow trout (Oncorhynchus mykiss) was assessed using static 96-h toxicity tests. The chemicals tested were selected because they are the most common chemicals used to fight forest and rangeland fires and are currently applied to a wide variety of habitats. Studies were conducted with three fire retardants, Fire-Trol GTS-R, Fire-Trol LCG-R, Phos-Chek D75-F, and two fire-suppressant foams, Phos-Chek WD-881 and Silv-Ex.
Cold-water species such as rainbow trout are often associated with forested areas in the western United States where large amounts of fire-control chemicals are used to combat rangeland and forest fires. Inaccurate or deliberate drops of fire-fighting formulations near streams and rivers may damage those aquatic ecosystems. Rainbow trout were used in tests because they represent a top-level consumer in cold-water ecosystems.
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