94 RUNNING WATER 



An artesian well is a boring which taps a subterranean stream 

 or sheet of water, confined under sufficient pressure to rise to the 

 surface, or even spout above it, like a fountain. Artesian wells 

 form most valuable sources of water supply, and it is important to 

 understand the conditions under which they may be successfully 

 bored. Many people have the impression that a boring anywhere, 

 if deep enough, will furnish artesian water, and much money has 

 been wasted by making this assumption. The only safe guide is 

 a careful examination of the geological structure of the region, 

 and failure may result even when everything seems favourable. 



The structural requisites for successful borings are as follows : 

 (i) There must be a porous water-bearing stratum, usually sand 

 or sandstone, enclosed between relatively impervious beds. The 

 impervious beds are necessary to enclose the porous beds and 

 prevent the water from escaping either upward or downward, 

 shutting it in as in a closed pipe, or underground siphon. (2) At 

 some point, which must be above the mouth of the well, the 

 porous stratum must reach the surface, so that it may receive its 

 supply of rain-water. If all the points where the porous bed 

 crops out be above the mouth of the proposed well, the conditions 

 are particularly favourable, because none of the water can escape, 

 except by very slow percolation through the relatively impervious 

 beds. If the water-bearing stratum communicates with the surface 

 at a level lower than the site of the well, success will depend upon 

 the ease with which the water can escape at the lower level. The 

 best conditions for a flowing well are, therefore, to be found in 

 a basin of folding, with the strata dipping toward the well from 

 all directions, and the porous bed cropping out around the edge 

 of the basin, at levels much above the mouth of the well. 



The outcropping edges of the porous strata may be very far, 

 even hundreds of miles, from the well ; the water will follow the 

 dip of the beds and rise to the surface wherever it is tapped, 

 provided that there is no easier path of escape. The friction of 

 the slow creeping through the water-bearing bed is so great that 

 a spouting well never throws the water up to the level of the 

 outcrop. 



