112 DOMESTIC WATER SUPPLIES FOR THE FARM 



casings are left open at the top, even when care has been taken to 

 carry them to proper depths. Figure 51 shows a casing properly 

 protected, all openings being hermetically sealed by cement. 



A fourth and very common means of contamination of deep 

 wells is by leaks in the casing due to imperfect joints or to cor- 

 rosion. The process of corrosion may be very rapid, the pipe in 

 some wells with acid chalybeate waters lasting only a few years. 

 No one expects a pipe laid in the ground near the surface to last 

 many years, yet many seem to think that a well casing will last 

 indefinitely. Unfortunately, this is far from true. 



When the casing has been corroded, pollution from sources 

 near the surface is often admitted through the minute holes eaten 

 in the iron, spoiling the deep waters. Where the casing does not 

 entirely fill the hole, contamination may pass down outside of it, 

 while in uncased rock wells pollution may enter through any of 

 the numerous fissures that usually exist in the upper part of the 

 rocks. Even in such wells, however, the danger of contamina- 

 tion decreases rapidly with depth. 



The detection of leaks is somewhat difficult. In some wells, 

 however, water may be heard trickling in or may be seen by a 

 light ray projected down the well by a mirror when the pump is 

 withdrawn. The admixture of water from outside sources may 

 sometimes be detected by a change in the hardness of the well 

 water, by an earthy taste or taste of decayed vegetation, or by a 

 cloudiness due to silt brought in by superficial waters. 



The remedy is usually to pull the old casing and replace it by 

 a new one, the length of time the pipe is allowed to remain before 

 replacement being determined by an estimate of its life based on 

 the action of the water on the pump tube or other pipes. An alter- 

 native treatment sometimes employed when the leak is near the 

 surface is to set a packer designed for the purpose in the space 

 between the bottom of the pump tube and the casing and fill the 

 space above with cement. 



