ways, even when the same toxicant is the active agent at low and high con- 

 centrations. A species of fish that is more resistant than another to 

 the lethal action of a toxicant can be more susceptible than the other to 

 sublethal injury by the same compound. 



The value of acute toxicity tests in the assessment and control of 

 pollution can be often reasonably questioned. For entirely different rea- 

 sons, chemical data may also be very misleading. Until analytical 

 methods are perfected, and the problems of interpretation of the chemical 

 information are solved, continued heavy reliance on toxicity tests of 

 short duration appears to be warranted. 



For some purposes, the acute toxicity test is clearly the best 

 possible test. Even when dilution of toxic effluents is sufficient to 

 prevent damage to aquatic life outside the mixing zone, fish may be 

 killed when they enter the mixing zones if the dilution is not very rapid. 

 To avoid fish kills in the immediate vicinity of wastewater outfalls, it 

 may be necessary to limit the toxicity of the effluent without regard to 

 the amount of further dilution. It is evident that the toxicity must be 

 sufficiently low so that the exposed fish will be overcome rapidly by the 

 effluent, unless they are known to be attracted by the effluent or likely 

 to remain in it for long periods for other reasons. The safe level of 

 acute toxicity varies with the area of the mixing zone as well as the 

 ability of fish to avoid high effluent concentrations. Acute toxicity 

 tests and limits are most appropriate for the regulation of some infre- 

 quent discharges of toxic wastes that are of sufficiently short duration, 

 that there is no need to protect organisms against chronic toxicity of 

 the wastes. The toxicity of intermittantly discharged wastewater can be 

 relatively high without causing serious damage, but the amounts or rates 

 of their dilution, and the duration of the discharges must be considered 

 prior to setting limits. 



Some regulatory authorities have favored much more stringent and uni- 

 form toxicity limits independent of the amounts of dilution of the ef- 

 fluents, the frequency, and the duration of the discharges. For example, 

 96-hour survival has been required of an average of at least 90 percent, 

 or even of no less than 80 or 90 percent at any given time (in every 

 test), of prescribed test animals (fish) in undiluted effluents of various 

 kinds. Specifically, application has been made to pulp and paper mill 

 effluents diluted to 65 percent of their full strength, without regard to 

 plant location. Various arguments have been advanced in support of such 

 uniform requirements unrelated to the assimilative capacities of waters 

 receiving the wastes. The enforcement of this type of regulation is 

 simplier than the limitation of toxic waste concentrations in the 

 receiving waters, and some people believe that this solution more equit- 

 able than the latter restrictions. However, when the dilution factor 

 related to wastewaters is great, the requirements in question can be 

 quite unnecessarily restrictive, necessitating costly waste treatment or 

 other expensive measures that result in no real benefits. On the other 

 hand, when the dilution is slight, the same requirements can be quite 

 inadequate for protection of aquatic life against chronic or sublethal 



toxicity of the wastes. 



4 



