342 WATER BACTERIOLOGY 



Numbers of Bacteria in Waters. The bacterial content of the 

 several waters varies greatly. Atmospheric waters after a long- 

 continued storm may be free from bacteria, whereas rain after a 

 long drought may contain many. There is also a variation in the 

 number, depending upon whether the rain is collected in the country 

 or city. Miquel obtained for the period 1883-1886 an average 

 of 4.3 bacteria per cubic centimeter in the country and 19 per 

 cubic centimeter in Paris. Snow contains rather higher numbers 

 than does rain. Janowski found in freshly fallen snow from 34 

 to 463 bacteria per cubic centimeter of snow-water. 



Surface waters are never free from bacteria, but the numbers 

 vary greatly from a few hundred, in the case of clear mountain 

 streams, to millions, in the case of the sewage polluted rivers. 



The number varies with the turbidity of the stream. The 

 Thames River carries 277 bacteria per cubic centimeter in April, 

 whereas the Illinois carries between 6000 and 8000 per cubic centi- 

 meter. The number also varies with the season of the year. In 

 May the Potomac River carries about 750, while in March it 

 carries 11,500 per cubic centimeter. The number is increased 

 when the drainage basin is manured with the various animal 

 manures, as it is also by the entrance of sewage into the streams. 



The bacterial content of lakes is usually lower than that of 

 streams, but shows wide variations. Lake Michigan near Chicago 

 gives count for from 68 to 2000 per cubic centimeter, while Lake 

 Lucerne's variation is from 8 to 51 per cubic centimeter. 



The same wide variation is shown in ground waters. Shallow 

 wells and springs often contain as many and just as dangerous 

 organisms as do surface waters. But deep wells and springs contain 

 few organisms, and it is not an uncommon experience to find some 

 which are sterile. 



The seasonal variation of bacteria in deep wells and springs 

 is zero, and where we have seasonal variation in these sources of 

 water it indicates surface contamination, and with shallow wells 

 and springs it is often enormous. 



Surface waters are subject to marked variations in bacterial 

 contents, especially during spring and fall, due to melting snow 

 and rains of these seasons. A heavy shower is likely to increase 

 contamination by introducing fresh material from the surface 

 of the ground. Prolonged moderate rains may have the opposite 

 effect and after the main impurities have been washed away may 

 dilute the stream with a better water than itself. The net effect, 

 therefore, depends upon the character of the stream as well as the 

 catchment basin. A stream highly polluted with sewage may 

 actually contain fewer bacteria after a heavy storm than before, 

 but a normal stream contains more, as emphasized by the following 

 data compiled by Prescott and Winslow: 



