WATER 57 



The following analyses from a well treated in this way between 

 the two analyses illustrates this point. 



Organisms "Excretal" B. coli 



Sample per c.c. c.c. Streptococci 

 37 C. 21 C. o'i i 10 40 i 10 40 

 With defective pump- 

 covering etc. ... 2350 over 5000 + + + + + + 

 After impervious covering 



and rendering ... 9 2020 _ _ _ _ 



The bacteriological results 1 obtained with surface wells are 

 variable, but if properly protected from surface contamination 

 and from the entrance of water which has not filtered through 

 at least 12 ft. of soil they will usually show no excretal indi- 

 cators in 50 c.c. and certainly not in 10 c.c. If " excretal " B. coli 

 are present in loc.c. or less of a surface well water it is evidence 

 of undesirable and possibly dangerous contamination. Even 

 when only present in 50 c.c. the well and its surroundings should 

 be subjected to very careful examination. The influence of rain- 

 fall upon the bacterial content of surface wells is considerable 

 and must be kept in view. 



The variable bacterial content of surface wells makes it 

 peculiarly dangerous to pass well waters as satisfactory from the 

 results of a single satisfactory bacteriological analysis. The 

 filtering action of the soil may be for the time satisfactory, but 

 at any moment it may break down and harmful bacteria be 

 washed through into the well water. 



For example a recent bacteriological analysis of a well water 

 supplying five houses gave the following results : Organisms 

 growing at 37 C. and 21 C. = 2 and 15 per c.c. respectively. 

 No B. coli or streptococci in 50 c.c. : larger amounts not examined. 

 This well was a shallow one sunk in porous ground and surrounded 

 up to a few yards by heavily manured gardens. It was on 

 inspection obviously liable to pollution, while a chemical analysis 

 disclosed a very high nitrate content (2*0 per 100,000) but low 

 ammonia figures. It is quite clear, as the very good bacterio- 

 logical analysis showed, that the soil was acting as an efficient 

 filter at the time the samples were collected. The bacteriological 

 data did not, however, disclose that the good quality of the 



1 For numerous analyses and data see Savage, Journ. of Hygiene, 1907, vn, p. 477. 



