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 



Fia. 22. Section showing relation of water table to surface irregularities. (From Water 

 Supply Paper 255, U. S. Geological Survey.) 



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 



