WATER STANDARDS 713 



mation may be made if this happen. It is better to state 

 the result as " B. coli present in, or absent from, .... 

 c.c. of water " rather than to say that so many B. coli 

 are present, though as a matter of fact the latter state- 

 ment is probably approximately correct. 



If nothing is known about the water, the following 

 standards may be adopted : 



(a) Waters of first-rate quality. B. coli absent from 

 100 c.c. 



(6) Waters of good quality. B. coli present in 100 c.c., 

 but absent in 50 c.c. of the water. 



(c) Waters of medium quality. B. coli present in 50 c.c., 

 but absent in 25 c.c. 



(d) Waters of poor quality. B. coli present in 50 c.c. 

 and 25 c.c., but absent in 10 c.c. 



(e) Waters of suspicious quality. B. coli present in 

 50 c.c., 25 c.c., and 10 c.c., but absent in 1 c.c. 



(/) Waters unfit for drinking. B. coli present in 1 c.c. 

 or less. 



Waters which show no B. coli in 50 c.c. are of a high degree of 

 purity, and therefore the proved absence of this organism in this 

 amount, and still better in larger quantities, is of great value. 



B. coli should be absent from at least 50 c.c. of spring or deep 

 well water, possibly from greater amounts. 



In upland surface waters the presence of B. coli in 40, 10, or 

 even 2 or 1 c.c. means contamination, but not necessarily a 

 contamination which it is essential to prevent. It may arise from 

 contamination with the excreta of animals grazing on the gather- 

 ing areas, and is by no means necessarily from sewage or other 

 material containing specific organisms of infection. If B. coli are 

 present in numbers greater than, say, 500 per litre (or even in 

 that amount), such a water is suspicious, as it is rare to get so 

 many B. coli in a water from the kind of animal contamination 

 indicated, and further investigation is desirable. In filtered 

 samples the number of B. coli is, as a rule, considerably reduced. 



In surface wells B. coli in large numbers indicate surface or 



