582 
teria present in the ground, and the water which percolates through 
the soil, although it becomes contaminated in the upper layers, it is 
rid of bacteria on its passage farther downward. Deep ground water 
usually contains few bacteria, but it may become contaminated 
when it is tapped for a well. Of course if the layer of soil through 
which the water percolates on its way to the water table is saturated i 
^\dth filth some of the pollution may be carried down, particularly j 
if the layer of soil is not thick. 
PROTECTION FROM POLLUTION. 
The water supplies of farms consist of wells, springs, and cisterns. 
A recent inspection of the water supplies of some 300 dairies in 
Mar^^land and Yirginia showed that wells are used much oftener on 
these farms than either of the other two. The proportions are about 
5 wells to 3 springs to 1 cistern. 
Since, as has been stated, the sources of pollution are the entrance 
of sewage or other impurities through cracks and crevices or 
Fig. 64 .— Showing how a cesspool located on high ground may fail to pollute a well lower down. 
Harrington: Practical Hygiene; Phila. and N. Y., 1901, p. 324. 
through a porous soil that has become saturated or finally by the in- 
tentional or accidental introduction of impurities, it is necessar}^ to 
guard against each of these sources. 
WELLS. 
To guard against the pollution of wells the location is of impor- 
tance. Where it is possible the ground should slope away naturally 
on all sides, and the pump should be on top of a mound which should 
be well sodded or cemented all around. Sources of domestic or of 
other pollution should be separated from the well by an impervious 
layer below ground to avoid the danger of pollution from seepage. 
Figure 54 shows how a stratum of rock may protect a well from 
pollution, even where the drainage is toward the well. 
