82 DOMESTIC WATER SUPPLIES FOR THE FARM 



the movement, though much faster than in uniform clay, is never- 

 theless not very rapid, rarely exceeding a few feet per day, and 

 pollution does not often extend much over 150 feet, 200 feet 

 usually being a safe distance. In open passages movement is 

 much more rapid and may amount to several hundred feet a day 

 in extreme cases. Under such conditions there is no purification 

 and relatively little dilution, and if the passage discharges into a 

 well dangerous contamination may result. In a thickly in- 

 habited region a well depending for its supply on passages of this 

 nature is never safe. 



A bed of sand is among the safer water beds. Being of an in- 

 coherent nature, the material rarely contains open passages, the 

 water circulating in general by a slow movement among the 

 grains. The rate, though sometimes amounting to 50 feet or 

 more a day, is usually under 5 feet and may be under I foot. A 

 well 200 feet from the nearest point of pollution is probably safe 

 in fine and medium sands, but in coarse sands and gravel a much 

 greater distance may be essential. 



The movement of water in sandstone is in part through the 

 body of the rock and in part through small open passages along 

 the joint or bedding planes. Owing to the greater density of the 

 rock resulting from the cementation of the grains the distance to 

 which pollution may extend through the pores of the rock is less 

 than in sand, 100 feet usually being a safe distance. Probably even 

 with the water moving along the joints and bedding planes 125 

 to 150 feet from the source of pollution is a safe distance for a well. 



In slate and shale the water follows in part the planes of 

 stratification or bedding and in part the more or less vertical joints 

 by which these rocks are usually cut. Unless certain of the 

 layers are sandy the movement along the bedding planes is gener- 

 ally slow and pollution is carried for only short distances. The 

 joints, however, are in many places fairly open and may conduct 

 the water within a short time to considerable distances, possibly 

 many hundred feet, like the granite joints described on p. 44. 

 However, unless the examination of the rock or the behavior of the 



