196 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



material may also enter through leaks or holes in the iron 

 casing of drilled wells. Cesspools and privies are the 

 most common sources of seepage. Material thrown out 

 upon the surface of the ground may also be carried down 

 through the soil by rain and seep into the well or reach 

 the underground water. The "safety distance" from 

 such sources of pollution will depend upon the character 

 of the soil and the quantity and concentration of the 

 polluting material, but a rule that may serve as a gen- 

 eral guide is the following: A well drains an inverted 

 cone of land whose top surface is four times as wide as 

 the depth of the well. Stone or brick curbs may be made 

 impervious by covering the interior surface with cement. 

 This will prevent contamination by seepage unless the 

 well is shallow, in which case the polluting material may 

 pass down and enter the well under the bottom of the 

 curb. 



One of the most common methods of pollution of 

 dug wells is the entrance of material through the top. 

 Dust and surface washings from pump drippings, waste 

 water, and rains are the principal sources of contami- 

 nation. Small animals, such as toads, mice, rats, moles, 

 and snakes, fall into open wells in seeking water in pe- 

 riods of drought. A water-tight iron or cement cover, 

 tightly joined to the curb, will protect the well from 

 these surface contaminations. 



Deep wells, driven or bored, are less likely to be pol- 

 luted than dug wells or shallow bored wells because the 

 underground water is protected from seepage by a much 

 thicker stratum of earth and the well is protected by an 

 iron casing. If cracks or leaks occur in the casing, pol- 

 luting material may enter. While deep wells are subject 

 to contamination, they are a much safer source of water 

 supply than shallow wells. 



