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Acute Toxicity of Fire-Control Chemicals, Nitrogenous Chemicals, and Surfactants to Rainbow Trout


Fire-control chemicals are an important tool used to protect forest resources from wildland fires. Fire managers in North America use a wide variety of fire-control chemicals, including long-term fire retardants and fire-suppressant foams. Long-term fire retardants slow the spread of the fire and reduce its intensity by changing the combustion properties of fuels. These chemicals are usually applied aerially and are typically composed of ammonium salts (which provide the fire-retardant action), thickeners, spoilage and corrosion inhibitors, and a pigment to mark the drop site (George 1971; George et al. 1991).

Fire-suppressant foams are used to extinguish fires and provide a short-term fire barrier. They are prepared by mixing foam concentrates with water and air, and the resulting foamed solutions are applied using aerial and ground delivery systems. Foams increase the effectiveness of water as a fire suppressant, and the bubbles within the foam insulate the fuel from heat and oxygen (National Wildfire Coordinating Group 1992; Schlobohm 1996). Foam concentrates are typically composed of anionic surfactants, foam stabilizers, and solvents (Norecol Environmental Consultants 1989).

Fire-control chemicals are often used in riparian habitats, and accidental inputs into streams have occurred. Because most fire-control chemicals are applied aerially, unintentional drops directly into streams have the highest potential for introducing significant amounts of these chemicals into streams and, in turn, adversely affecting aquatic biota. Although not well documented, most fish kills that have occurred during fire-control operations were associated with aerial applications of fire retardants (Norris and Webb 1989; Minshall and Brock 1991).

Although fire-control chemicals have been in use since the 1930s, little information on their toxicity to aquatic biota was available until recently. Moreover, these chemicals are continually being reformulated and earlier formulations that were tested for toxicity to salmonids (Blahm and Snyder 1973; Johnson and Sanders 1977) are no longer in use. Recent investigations have assessed the relative toxicity of three fire retardants and two foams that are currently in use to standard test species (Gaikowski et al. 1996a, 1996b; McDonald et al. 1996) and chinook salmon Oncorhynchus tshawytscha (Buhl and Hamilton 1998). These studies showed that fire-suppressant foams, Phos-Chek WD-881 and Silv-Ex, were considerably more toxic (48-h concentrations producing an effect to 50% of the test organisms [48-h EC50s] or 96-h concentrations lethal to 50% of test organisms [96-h LC50s] of 4- 22mg/L) than fire retardants (Fire-Trol GTS-R, Fire-Trol LCG-R, and Phos-Chek D75-F; 48-h EC50s 96-h LC50s of 135-1,797 mg/L) to Daphnia magna and postembryonic life stages of chinook salmon, fathead minnow Pimephales promelas, and rainbow trout O. mykiss. Also, these investigators attributed the toxicity of the fire retardants to un-ionized ammonia derived from ammonium salts and the toxicity of the foams to anionic surfactants in the formulations.

The purpose of this study was to determine the acute toxicity of three fire retardants and five foam concentrates currently in use to juvenile rainbow trout. In addition, the acute toxicity of ammonia, nitrate, nitrite, and two anionic surfactants were determined. Nitrate and nitrite were tested because the ammonia derived from the fire-retardant formulations can be oxidized in water to nitrite and then to nitrate by bacteria of the genera Nitrosomonas and Nitrobacter, respectively (Cole 1979). Rainbow trout are routinely used as a surrogate test species for Pacific salmon Oncorhynchus spp. (Sprague 1973; ASTM 1989), whose natal streams are located in areas where large amounts of fire-control chemicals are used (Norris et al. 1983; USDA Forest Service, Intermountain Fire Sciences Laboratory, unpublished reports).

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