extreme difference in sensitivity among species and 

 among necessary fish food organisms. Henderson 

 (1957) has discussed various factors involved 

 in developing application factors. Results of studies 

 by Mount and Stephan (1967), in which con- 

 tinuous exposure was used, reveal that the appli- 

 cation factor necessary to reduce the concentration 

 low enough to permit spawning ranged from % 

 to /^oo- It is recognized that exposure will not be 

 constant in most cases and that higher concentra- 

 tions usually can be tolerated for short periods. 



At present, safe levels have been determined 

 for only a few wastes and hence only a few appli- 

 cation factors are known. Since the determination 

 of safe levels is an involved process, it will be nec- 

 essary to use indirect or stopgap procedures for 

 estimating tolerable concentrations of various 

 wastes in receiving waters. To meet this situation, 

 it is proposed to use three universal application 

 factors selected on the basis of present knowl- 

 edge, experience, and judgment. It is proposed 

 that these general application factors be applied 

 to TLn, values determined by those discharging 

 wastes in the manner described above to set toler- 

 able concentrations of their wastes in the receiv- 

 ing stream. 



It should be evident that when these general 

 application factors are used for all wastes the re- 

 sulting concentrations at times will be more strin- 

 gent than needed for some wastes and inadequate 

 for others. The derived concentrations wiU be 

 tolerable, however, for a considerable number of 

 wastes in the midrange of relative toxicity. 



Recommendation for the Use of Bioassays and Appli- 

 cation Factors To Denote Safe Concentrations of 

 Wastes in Receiving Streams : ( 1 ) For the deter- 

 mination of acute toxicities, flow-through bioassays are 

 the first choice. Methods for carrying out these flow- 

 through tests have been described by Surber and 

 Thatcher (1963), Lemke and Mount (1963), Hender- 

 son and Pickering (1963), Jackson and Brungs (1966), 

 Mount and Warner (1965), Mount and Brungs (1967), 

 and Brungs and Mount (1967). Flow-through bio- 

 assays should be used for unstable, volatile, or highly 

 toxic wastes and those having an oxygen demand. They 

 also must be used when several variables such as pH, 

 DO, CO- and other factors must be controlled. 



(2) When flow-through tests are not feasible, tests 

 of a different type or duration must be used. The kinds 

 of local conditions aff'ecting the procedure might be a 

 single application of pesticides or lack of materials and 

 equipment. 



(3) Acute static bioassays with fish for the deter- 

 mination of TLn, values should be carried out in ac- 

 cordance with Standard Methods for the Examination 

 of Water and Wastewater, 12th edition (1965). Such 

 tests should be used for the determination of TL,„ val- 

 ues only for persistent, nonvolatile, or highly soluble 

 materials of low toxicity which do not have an oxygen 

 demand because it is necessary to consider the amount 



added as the concentration to which the test organisms 

 are exposed. 



(4) When application factors are used with TLm 

 values to determine safe concentrations of a waste in 

 a receiving water, the bioassay studies to determine 

 TLn, values should be made with the most sensitive 

 local species and life stages of economical or ecological 

 importance and with dilution water taken from the 

 receiving stream above the waste outfall. Other species 

 whose relative sensitivity is known can be used in the 

 absence of knowledge concerning the most sensitive of 

 the important local species or life stages or due to 

 difficulty in providing them in sufficient numbers. 

 Alternatively, tests may be carried out using one species 

 of diatom, one species of an invertebrate, and two 

 species of fish, one of which should be a pan or game 

 fish. Further, these bioassays must be performed with 

 environmental conditions at levels at which the waste is 

 most toxic. Tests should be repeated with one of the 

 species at least monthly and when there are changes in 

 the character or volume of the waste. 



(5) Concentration of materials that are nonper- 

 sistent (that is, have a half life of less than 96 hours) 

 or have noncumulative effects after mixing with the 

 receiving waters should not exceed Ho of the 96-hour 

 TLm value at any time or place. The 24-hour average 

 of the concentration of these materials should not 

 exceed V20 of the TLm value after mixing. For other 

 toxicants the concentrations should not exceed %o and 

 Moo of the TLm value under the conditions described 

 above. Where specific application factors have been 

 determined, they will be used in all instances. 



When two or more toxic materials whose effects are 

 additive are present at the same time in the receiving 

 water, some reduction in the permissible concentrations 

 as derived from bioassays on individual substances or 

 wastes is necessary. The amount of reduction required 

 is a function of both the number of toxic materials 

 present and their concentrations in respect to the de- 

 rived permissible concentration. An appropriate means 

 of assuring that the combined amounts of the several 

 substances do not exceed a permissible concentration 

 for the mixture is through the use of following rela- 

 tionship: 



/C, Cb 



VlT+u 



c„ 



<1 



Where Ca, Cb, . . . Cn are the measured concentra- 

 tions of the several toxic materials in the water and 

 La, Lb, . . . Ln are the respective permissible concen- 

 tration limits derived for the materials on an individual 

 basis. Should the sum of the several fractions exceed 

 one, then a local restriction on the concentration of 

 one or more of the substances is necessary. 



Heavy Metals 



An extensive discussion of the physiological 

 mode of action of heavy metals is found in the 

 toxicity portion of the section on water quality re- 

 quirements for marine organisms. 



Zinc: While much information has been pub- 

 Ushed regarding zinc, a large amount of the data 

 cannot be used because of incomplete description 



59 



