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26 JOSEPH B. HUNN AND ROSALIE A. SCHNICK individual water quality criteria established by EPA is provided in documents available from the National Technical Information Service (NTIS). See Appen- dix C. The EPA also issued a series of documents relat- ing to water quality criteria based on State regula- tions. These documents present the criteria for each State in alphabetical order. The documents are available through NTIS (Appendix D). In addition, these water quality standards are available for each State as a separate document or as part of a com- pilation in one document that can be purchased from NTIS. Factors that Modify Toxicity Laboratory and field studies have shown that many factors influence the toxicity of chemicals to fish. The origin of modifying factors may be either biotic or abiotic (Sprague 1985; Mayer and Ellersieck 1986). Biotic factors include species, life stage and size, nutritional state, general health, and parasit- ism. Abiotic factors include characteristics of the water (e.g., temperature, pH, hardness, alkalinity, osmolality, dissolved oxygen, salinity, dissolved organic carbon), possible binding to suspended or dissolved materials, and formulation of pesticide products. Water hardness has little effect on the toxicity of organic compounds. However, increased water hard- ness (as Ca and Mg) can reduce the availability of metals such as Al, Cd, Hg, and Pb (Hunn 1985; Mance 1987). Hardness, alkalinity, and pH all influ- ence the availability of metals, such as Cu (Sprague 1985). Hydrogen ion concentration (measured as pH) influences the toxicity of chemicals that ionize. For example, the toxicity of ammonia, cyanide, and hydrogen sulfide is influenced by the pH of the water. Un-ionized molecules usually are more lipid- soluble than ionized forms and thus penetrate mem- branes more readily (Hunn and Allen 1974; Spacie and Hamelink 1985). As noted by Mayer and Eller- sieck (1986) in a study of 410 chemicals, pH affected the toxicity of only about 20% of the organic chem- icals tested, but caused greater changes in 96-hour LC50 values than any of the other water chemistry factors examined. Results from the analysis of water samples tested for a suspected chemical should yield positive results if that substance is present. Analytical chemistry data generated should include the concentration found, limits of detection, quality assurance, and quality control information that will help determine whether the analysis was accurate and reliable. In comparing the results from the control or reference site with those from the kill site, there should be a definite difference in concentration of the chemical. If there is not, several possibilities exist: (1) the reference site was not a true control; (2) the chemical moved downstream (in running water); (3) the com- pound was removed by becoming bound to sediment; (4) the substance was biotransformed, degraded, or volatilized; or (5) a combination of these possibilities. Keup (1974) listed eight factors to consider when an investigator is attempting to interpret on-site evidence at a fish kill: (1) time of water travel (streams); (2) dilution; (3) lateral mixing; (4) season and temperature; (5) habitat characteristics; (6) de- layed reactions in fish and invertebrates; (7) syner- gism and antagonism; and (8) suspended materials. Time of travel and dilution of the chemical can be estimated after the fact by conducting a dye study if the hydrological conditions present during the in- vestigation are similar to those that existed at the time of the kill. For further information on how to conduct dye studies, see Slifer (1970). Toxicity data from acute tests are usually reported as LC50's in mg/L. An LC50 is the estimated con- centration of a substance in water that is lethal to 50% of the test organisms after exposure for a stated period of time (e.g., 24, 48, or 96 hours). Thus, the larger the LC50 value, the less toxic the chemical is to fish; and the smaller the value the more toxic the chemical. The relative acute toxicity of chemicals to fish (96-hour LC50) can be categorized as follows: Toxicity rating 96-hour LC50 Practically nontoxic 100-1,000 mg/L Slightly toxic 10-100 mg/L Moderately toxic 1-10 mg/L Highly toxic 0.1-1.0 mg/L Extremely toxic Less than 0.1 mg/L It is important to establish some measure of the relative toxicity at the site. A valid pH measurement may be sufficient to establish whether the hydrogen ion concentration was lethal (Table 4.3). In extreme- ly soft water, pH determinations should be made with a special electrode designed for use in waters of low ionic strength. Most substances are toxic to organisms if the concentration is high enough and