194 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
pens, or other structures above springs is objectionable. 
Water from springs exposed to surface drainage should 
not be used for washing dairy utensils and vessels, but 
if no other source is available the spring should be pro- 
tected by impervious walls. Open springs are sometimes 
polluted by live stock or contaminated by dust, leaves, 
and other refuse blowing into them. Pollution of springs 
by subsurface drainage is not very common on farms, 
but may occur where the layer of earth above the water 
table is thin or readily permeable. The permeability will 
depend upon the material present. Fine sand is a good 
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Fia. 22.—Section ia ed Ks aoe rag ntn belie (From Water 
filter, but coarse sand and gravel permit the water to 
pass through rapidly, carrying some of the contaminat- 
ing material through with it. Clay and till (largely clay 
and sand) are good filters. When the layer of earth 
above the water table is thin or permeable, the water in 
the spring is likely to be warm. Cesspools, buildings, 
or anything else which may be a source of pollution 
should not be placed at a higher level than a spring be- 
cause of the danger of contamination by subsurface 
drainage (Fig. 23). When cracks or fissures extend 
from the surface into the water-bearing strata, as occurs 
in rock formation, or where tubular channels have been 
formed, the spring may be contaminated. This may 
