discussion 



treating such supplies. This is the importance of 

 the factors of safety mentioned in the definition of 

 "desirable criteria". However, managers of all sup- 

 plies would welcome improved raw water quality. 



This Subcommittee believes that the criteria set 

 forth herein can be used in setting standards of 

 raw water quality only with a substantial amount 

 of understanding and discretion. To a considerable 

 extent this is related to the very great regional 

 variations in water quality entirely aside from 

 manmade pollution. In addition, human oc- 

 cupance and activity have inevitable effects on 

 water quality. These facts make it difficult and 

 sometimes impossible to develop uniform numeri- 

 cal criteria suitable for national application. 



The criteria selected by the Subcommittee are 

 listed in the table and discussed in the numbered 

 paragraphs cited in the table. The paragraphs also 

 include some rationale of the basis for the criteria. 

 The fact that a substance is not included in these 

 criteria does not imply that its presence is innocu- 

 ous. It would be quite impracticable to prepare a 

 compendium of all toxic, deleterious, or otherwise 

 unwelcome agents that may enter a surface water 

 supply. 



Sampling 



THE SUBCOMMITTEE recognizes that sur- 

 face waters are used for public water supply 

 without treatment other than disinfection. Such 

 waters at the point of withdrawal should meet 

 Drinking Water Standards (10) in all respects 

 other than bacterial quality. 



It should be emphasized that many raw water 

 sources which do not meet these permissible cri- 

 teria have been and are being used to provide satis- 

 factory public water supplies by suitable additions 

 to and elaboration of the treatment processes de- 

 fined above. In some instances, however, the water 

 delivered to the customer is of marginal quality. 

 Also the finished water is much more likely to be- 

 come unsatisfactory if treatment plant irregularities 

 occur. It is recognized that most of the surface 

 water treatment plants providing water for do- 

 mestic use in the United States are relatively small 

 (7) and without sophisticated technical controls. 

 Marginal quality characteristics, therefore, assume 

 considerable importance to the managers of plants 



Sampling should be of such frequency and of 

 such variety (time of day, season, temperature, 

 river stage or flow, location, depth) as to properly 

 describe the body of water designated for public 

 water supply. Sampling should also be conducted 

 in full cognizance of findings of the sanitary sur- 

 very. Judgment should be exercised as to the rela- 

 tive desirability of frequent sampling at one point, 

 such as the raw water intake, as compared to less 

 frequent sampling at numerous locations, such as 

 is required for stream profiles or cross sections. 



It is clearly not possible to apply these criteria 

 solely as maximum single sample values. The cri- 

 teria should not be exceeded over substantial por- 

 tions of time. If they are exceeded, efforts should 

 be made to determine the cause, and corrective 

 measures undertaken. 



Analytical methods 



The criteria are based upon those analytical 

 methods described in Standard Methods for the 

 Examination of Water and Wastewater (9) or 

 upon methods acceptable to water pollution con- 

 trol agencies. 



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