WATEE SUPPLY AND SEWAGE DISPOSAL 163 



diseases are often more prevalent in the country than in the 

 city. City health officers have learned to expect an increase in 

 the number of cases of typhoid in the city at the time of year 

 when people return to the city from vacations in the country. 

 The water that finds its way into an ordinary well comes 

 from the rain water which falls upon the ground within a few 

 hundred feet of the well and percolates through the soil until 

 it reaches the well. Such water has usually been contami- 

 nated more or less at the surface of the earth with the various 

 bacteria which may be present upon the surface or in the 

 upper layers of soil. As the water percolates through the soil 

 many of these bacteria are removed by the filtering action of 



FIG. 82. Purity of farm wells 



Results of the examination of farm wells in Illinois. Deep wells are more 

 frequently pure. After data by Bartow 



the soil. If the bacteria are not too numerous, and if the soil 

 is sufficiently compact and the distance the water travels 

 through it is great enough, practically all of the bacteria may 

 be removed. On the other hand, if the well is located near a 

 compost heap, cesspool, pigpen, or other source of pollution, 

 it is almost certain that the water in the well will be contam- 

 inated. If the ground slopes toward the well, this will favor 

 contamination. It is very desirable, therefore, that a well be 

 located far from all possible sources of contamination, that the 

 ground slope away from it in all directions, and that it be 

 tightly curbed and covered. 



In many communities water is secured from deep wells, and 

 these are frequently several hundred feet deep. The water 

 which enters a well from the deeper layers of the earth's crust 



