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, 
