56 WATER 



which is very difficult to locate and for which bacteriological 

 examinations are of the utmost value. Such cases are usually 

 met with in supplies from the limestone or chalk and numerous 

 examples might be quoted. 



The following is a good example of intermittent contamina- 

 tion of .a deep well in the chalk reported by Richards and 

 Brincker 1 . The well yielded from one to two million gallons per 

 day. The only source of contamination was from surface water 

 rather more than two miles away which was shown to gain access 

 to the deep well water. This was proved by adding a special 

 yellow bacillus to the surface water and tracing it into the well. 

 It was found that the test organism took in one experiment 78*5 

 hours and in the other 67*5 hours to pass from the s wallet hole 

 to the well. Its presence was associated with a marked increase 

 in the bacterial content of the water, together with the finding 

 of B. coli, in loc.c. or less of the water. The latter organism 

 was absent from the well when no pollution was taking place. 



Shallow wells and Subsoil water. 



Obviously bacteriological findings will vary greatly with this 

 class of waters and very widely varying results are in fact obtained. 

 Sources of bacterial contamination are mainly two, one due to 

 the entrance of bacteria through the mouth or round the sides 

 of the well and the other from bacterial contamination from sub- 

 soil water, due to imperfect filtration of bacteria through the soil. 

 In considering bacteriological analyses of surface wells it is always 

 most important to discriminate between these two sources of 

 pollution. 



Contamination through the open mouth (in draw wells) or 

 from imperfect fittings and covers round pumps is extremely 

 common and is usually very heavy. Indeed it is very excep- 

 tional with open draw wells for their water to show other than 

 gross contamination, and this whether or no the subsoil water 

 itself is contaminated. When such wells are properly covered 

 and their sides are effectively rendered to a depth of 1 2 ft. or so, 

 no harmful bacterial contamination is sometimes subsequently 

 present, showing that the subsoil water is not itself contaminated. 

 1 Proceedings Roy. Soc. Med- t Efidem. Sec., 1908, I, p. 191. 



