ide, etc.). The constant renewal test is superior for 

 monitoring effluents, water supplies, or streams on 

 a continuous or intermittent basis and is the only 

 suitable method for long-term tests. 



Several systems for adding the test material to 

 the water have been devised since this type of 

 bioassay has been in use. Lemke and Mount 

 (1963) describe a system using a controlled water 

 flow balanced against a chemical metering pump. 

 Henderson and Pickering (1963) describe a sim- 

 ple drip system and a controlled water flow; a 

 similar system is proposed by Jackson and Brungs 

 (1966). Both of these latter references describe 

 the use of fish and flowing systems as continuous 

 monitors. Mount and Warner (1965), Mount and 

 Brungs (1967), and Brungs and Mount (1967) 

 describe systems suitable for continuous short or 

 long-term tests. 



Most criteria for toxic substances must be based 

 on a bioassay made for each specific situation. 

 This is dictated by the lack of information and 

 the wide variation in situations, species, water 

 quality, and the nature of the substance being 

 added to the water. 



Most of the bioassay work on algae has meas- 

 ured the threshold concentrations that reduce phys- 

 iological processes by 50 percent rather than the 

 concentrations that cause 50-percent death in the 

 population tested. It is very difficult to determine 

 the death point of algae cells, but some workers 

 have used it as a criterion. Physiological measure- 

 ments have been based largely on 50-percent 

 reduction in photosynthesis and 50-percent reduc- 

 tion in number of divisions that have taken place 

 during a period of time. This is determined by 

 the number of cells present at the beginning and 

 end of the experiment. A bioassay method employ- 

 ing diatoms has been recognized by the American 

 Society for Testing Materials ( 1964) . 



Application Factors 



Short-term or acute toxicity tests provide in- 

 formation on the overall toxicity of a material 

 and thus precede meaningful long-term toxicity 

 studies. They may also be used to compare toxic- 

 ities of different materials. When water for dilu- 

 tion is taken from the receiving stream, these tests 

 may also indicate additional stresses due to mate- 

 rials already present in the receiving water. These 

 acute studies do not indicate concentrations of a 

 potential' toxicant that are harmless under condi- 

 tions of long-term exposure. It is desirable, there- 

 fore, to develop a factor that can be used with 96 

 or 48-hour TLn, values to indicate concentrations 

 of the waste or material in question that are safe 



in the receiving water. Such a factor has been 

 called an application factor. 



Ideally, an application factor should be deter- 

 mined for each waste or material. To do this, it 

 would be necessary to determine the concentration 

 of the waste or material in question that does not 

 adversely affect the productivity of the aquatic 

 biota on continuous exposure, in water of known 

 quality, and under environmental conditions (DO, 

 temperature, pH, etc.) at which it is most toxic. 

 This concentration is then divided by the 96-hour 

 TLn, value obtained under the same conditions 

 to give the application factor. 



safe concentration for continuous exposure 

 96-hour TL^ 



For example, if the 96-hour TL^ is 0.5 mg/1 and 

 the concentration of the waste found to be safe 

 is 0.01 mg/1, the application factor would be: 



safe concentration _0.01 _ 1 

 96-hour TU, ""050 "50 

 In this instance, the application factor is %o or 

 0.02. Then in a given situation involving this 

 waste, the safe concentration in the receiving 

 stream would be found by multiplying the 96-hour 

 TU by 0.02. 



To effectively determine the application factor 

 for a given waste, it is necessary to determine the 

 concentration of that waste which is safe under 

 a given set of conditions. For those materials 

 whose toxicity is not significantly influenced by 

 water quality and in streams free of other wastes 

 that influence the waste in question or that have 

 water qualities similar to those under which the 

 waste was tested, the above-mentioned concen- 

 tration would be the one that is safe in the re- 

 ceiving water. However, differences in water qual- 

 ity and lack of information on the toxicity of 

 waste materials already present make the direct 

 use of laboratory-determined safe levels unwise 

 at present, and a different approach is recom- 

 mended. 



In this approach, a 96-hour TL„, is determined 

 for the waste using water from the receiving stream 

 for dilution and, as test organisms, the most sensi- 

 tive species or life stage of an economically im- 

 portant local species or one whose relative sensi- 

 tivity is known. This procedure would take into 

 consideration the effects of local quality and the 

 stress or adverse effects of wastes already present 

 in the stream. The TL^, value thus found then is 

 multiplied by the application factor for that waste 

 to determine the safe concentration of that waste 

 in that stream or stream section. Such bioassays 

 should be repeated at least monthly and at each 

 change in process or rate of waste discharge. 



This procedure must be used because of the 



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