TYPES OF WELLS 75 



against the entrance of contaminated waters and polluting solids. 

 The type of well does not affect the purity of the original source ; but 

 if the water supply is primarily pure, its maintenance in that con- 

 dition depends largely on construction that prevents contamination. 



Polluting matter finds entrance to wells in a variety of ways. 

 In dug wells it enters through the crevices in the stone, brick or 

 wood curbing, or possibly through the brick itself; in bored wells 

 it enters through the uncemented joints of the tiling or through 

 cracks between the staves of tubular wooden curbing; and in 

 drilled and driven wells, through leaky joints or holes eaten in the 

 iron casing by corrosive waters. By cementing the interior sur- 

 faces of stone or brick curbed wells, by replacing wood by cement 

 or other impervious curbs, and by substituting new pipes for 

 leaky iron casings the entrance of polluting matter through the 

 walls can be prevented. Little or nothing enters the small tubular 

 wells from the top and they may, therefore, be regarded as free 

 from danger of pollution from this point. The larger open wells 

 should be protected by a water-tight iron or cement cover standing 

 somewhat above the level of the surrounding ground and tightly 

 joined to the curb proper. The sloping of the earth away from the 

 well serves to turn rainwater or pump drippings away from it, so 

 that little will penetrate, even if the curb becomes cracked by frost. 



A particularly dangerous type of well the more so because 

 of the fancied security of the owners is the combination dug 

 and drilled type. Because of a slight saving of expense, drilled 

 wells are frequently sunk in old dug wells, the casing commonly 

 beginning at the bottom of the old well. Although the water en- 

 countered by the deep well may be perfectly pure, it is liable to be 

 contaminated, especially after rains, by the entrance of seepage 

 waters into the open well and thence into the drilled well. The 

 remedies are obvious; either the casing should be carried to the 

 surface of the outside ground, or at least above the highest li-\vl 

 ever reached by the water, or the open well should be converted 

 into a water-tight cistern by the application of a thick coat of 

 cement over both sides and bottom. 



