WATER 51 



certain general and special facts in regard to the bacterial 

 content of different classes of waters must be considered. 



For present purposes water supplies may be divided into the 

 following groups : 



A. Upland surface waters. 



B. Deep water supplies deep wells and springs. 



C. Shallow wells and subsoil waters. 



D. River water. 



Each class of water must be separately considered. 



Upland surface waters. 



Supplies of this class are derived from rain water which has 

 been in contact with soil, usually uncultivated soil, but which 

 have not undergone any filtration through soil. Rain water itself 

 when pure contains but few bacteria and no excretal indicator 

 organisms. The bacterial content of upland waters will vary 

 with the bacterial nature of the soil they wash, the degree to 

 which they are liable to pollution from sources other than soil 

 and to some extent with the degree of storage to which they 

 have been subjected. 



Upland uncultivated soil may contain numerous bacilli but 

 is free from B. coli and other indicator organisms. The bacterial 

 content of water in contact with such soil may be considerable 

 but it will not contain B. coli. 



Moorland soil of this character is however usually used for 

 grazing to some extent and is the habitat of wild fowl and 

 other birds. The excreta of these birds and animals contain 

 numerous B. coli and other indicator organisms and thus serve as 

 a source of such organisms to the water. As a matter of prac- 

 tical experience therefore it is found that many upland surface 

 waters contain these indicator bacilli in considerable and some- 

 times in large numbers, and this even when the uplands which 

 serve as the gathering area are remote from human habitations 

 and free from all risk of contamination from human sources. 



As illustrations of results obtained with such waters in which 



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