summary 



and key 



criteria 



RECOMMENDATIONS given below are con- 

 sidered to be satisfactory for aquatic life. In 

 all instances where natural conditions fall outside 

 the recommended ranges, these conditions may 

 be marginal and should not be changed in such a 

 way as to make them more unfavorable. 



Freshwater organisms 



Dissolved Materials 



(1) Dissolved materials that are relatively in- 

 nocuous; i.e., their harmful effect is due to osmotic 

 effects at high concentrations, should not be in- 

 creased by more than one-third of the concentra- 

 tion that is characteristic of the natural condition 

 of the subject water. In no instance should the 

 concentration of total dissolved materials exceed 

 50 milliosmoles (the equivalent of 1500 mg/1 

 NaCl). 



(2) Dissolved materials that are harmful in 

 relatively low concentrations are discussed in the 

 section "Toxicity." 



pH, Alkalinity, Acidity 



(1) No highly dissociated materials should be 

 added in quantities sufficient to lower the pH be- 

 low 6.0 or to raise the pH above 9.0. 



(2) To protect the carbonate system and thus 

 the productivity of the water, acid should not be 

 added in sufficient quantity to lower the total al- 

 kalinity to less than 20 mg/1. 



(3) The addition of weakly dissociated acids 

 and alkalies should be regulated in terms of their 

 own toxicities as established by bioassay pro- 

 cedures. 



Temperature 



Warm Water Biota: To maintain a well- 

 rounded population of warm-water fishes, the fol- 

 lowing restrictions on temperature extremes and 

 temperature increases are recommended: 



( 1 ) During any month of the year heat should 

 not be added to a stream in excess of the amount 

 that will raise the temperature of the water (at the 

 expected minimum daily flow for that month) 

 more than 5 F. In lakes, the temperature of the 

 epilimnion in those areas where important orga- 

 nisms are most likely to be adversely affected 

 should not be raised more than 3 F above that 

 which existed before the addition of heat of artifi- 

 cial origin. The increase should be based on the 

 monthly average of the maximum daily tempera- 

 ture. Unless a special study shows that a discharge 



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