WATER 



55 



B. coli Streptococci 



(per litre) 



1000 10,000 30 100 



absent in 50 c.c. absent in 50 c.c. 



30 100 

 absent in 50 c.c. 



30 100 

 100 1000 



A source of contamination sometimes met with as regards 

 deep wells is due to surface water gaining access either by the 

 side of the bore hole down to the deep supply or from the im- 

 pervious lining being faulty or not extending down far enough. 

 The following is an instructive illustration of this from the point of 

 view of bacteriological examinations. This water supply, which 

 supplies a considerable population, is derived from the mountain 

 limestone and is obtained from a well 3 1 feet deep with an 

 impervious lining and passing through a thin impervious layer 

 (lower Lias). Usually bacteriological analyses gave satisfactory 

 figures but after very heavy rain unsatisfactory bacteriological 

 results were obtained (e.g. Nos. 2, 3 and 4 below). Considerable 

 difficulty was experienced in tracing out the cause of the con- 

 tamination, but it was ultimately traced to surface water (from 

 the Fuller's earth formation) under certain conditions rising up 

 and passing down by the side of the bore hole to the deep supply. 

 Since steps have been taken to stop this source of contamination 

 the results have been very satisfactory (Nos. 5, 6 and 7) although 

 samples have been purposely taken after heavy rain. 



B. coli Streptococci 



(l er litre) 



*absent in 10 c.c. absent in 10 c.c. 

 absent in 50 c.c. 



IOO IOOO 

 IOO IOOO 



absent in 50 c.c. 



1000 10,000 

 absent in 50 c.c. 



[* By inadvertence larger quantities were not examined.] 



In addition to easily ascertained sources of surface contamina- 

 tion, deep water supplies are sometimes liable to contamination 



