222 Microbes and You 



volume of water. The following table will be found helpful in 

 these calculations. 



In locating wells it is important to bear in mind that as much 

 distance as possible should be allowed between the sewage outlet 

 and the location of the well. While each installation is a separate 

 problem, a minimum distance of fifty feet is generally considered 

 sufficient to allow proper filtration of sewage through the soil to 

 eliminate pollution of water supplies. This may not be sufficient, 

 however, if the drainage slopes steeply toward the well, or if the 

 type of soil does not allow efficient filtration. If possible, the water 

 supply should be located on ground higher than the sewage outlet 

 with surface drainage directed away from the well. Unless the top 

 of the well is properly constructed, surface washings from barn- 

 yards, etc., may gain access to the water supply, particularly 

 during heavy rainstorms. The extra time and expense devoted to 

 the details of construction will more than pay for themselves over 

 a period of years. 



DRIVEN WELLS 



By drilling deep into the earth, often through a layer impervious 

 to water, a more abundant and usually safe water supply can be 

 expected. Just because water comes from a driven well, however, 

 is not reason enough to insure its potability without subjecting it 

 to bacteriological examination. In general the water from such 

 wells has come from a distance, and has undergone natural filtra- 

 tion as it seeped through the soil. But if, through improper con- 

 struction, surface water is allowed to gain entrance to the supply, 

 pollution problems might well arise. 



The depth of driven wells varies with the location, but in some 

 areas they average between fifty and one hundred feet. Many 



