of laboratory studies in which median lethal concentrations of the toxi- 

 cants were related to the highest levels that were apparently harmless to 

 the test animals in experiments of long duration. The usefulness of 

 these widely ranging application factors in dealing with industrial wastes 

 that are complex mixtures of variable, and often incompletely known com- 

 position, is questionable. Application factors most appropriate to 

 different kinds of industrial wastewaters differ greatly. Prescribed ap- 

 plication factors also vary with the value of commercial or recreational 

 fisheries that are to be protected, some of which merit a high degree or 

 level of protection than others. Economic and social considerations can- 

 not be overlooked in deciding how much risk of impairment of fish produc- 

 tion by waste discharges is to be deemed acceptable (Warren, 1971, pp. 

 15-23, 375-386). 



After the test animal to be used, a suitable test temperature and ex- 

 posure period, and an appropriate application factor has been carefully 

 chosen, an announced enforcement of this regulation next must be under- 

 taken. To determine whether or not the concentration of an acutely toxic 

 effluent that has been judged permissible in a receiving body of water is 

 outside an allowable dispersion or mixing zone, the amount of dilution of 

 the effluent within the mixing zone must be known. Only a concentration 

 of the effluent (percent by volume) equal to the product of its concentra- 

 tion in the receiving water (at the boundary of the mixing zone) and the 

 reciprocal of the prescribed application factor needs to be tested for 

 toxicity. If, at any time, this concentration is found to kill more than 

 50 percent of the test animals in the specified exposure period, either 

 the toxicity of the effluent or the rate of its discharge may be regarded 

 as excessive. If 50 percent or more of the test animals survive in such 

 tests, the permissible concentration is not exceeded, and hence, the reg- 

 ulatory requirement is not being violated. 



The above procedure is applicable to all effluents that are toxic 

 enough to kill at least half of the test animals in the prescribed test 

 period when they are not diluted. If an effluent is of low toxicity, but 

 the product of its concentration (percent by volume) and the reciprocal 

 of the prescribed application factor is greater than 100 percent, one can- 

 not reasonably conclude from the result of the acute toxicity test that 

 aquatic organisms are not being endangered. The possibility of serious 

 chronic or sublethal toxicity of the effluent cannot be judged negligible 

 in the absence of better evidence. Tests for toxicity of such effluents 

 that have no readily measurable acute toxicity must be required to pro- 

 tect aquatic life adequately. 



Reliance on acute toxicity tests alone in regulating discharges of 

 any toxic wastes can occasionally lead to serious error. The regulatory 

 practices considered above are based on certain assumptions which cer- 

 tainly cannot be always valid. The chronic or sublethal toxicity of a 

 complex industrial waste at low concentrations can be quite independent 

 of its acute toxicity at much higher concentrations, since the causative 

 agents of these variations in toxicity can be entirely different com- 

 ponents of the wastes. Further, environmental variables, such as natural 

 water quality and temperature, can influence toxicity in very different 



