102 CONSERVATION OF GROUND WATER 



stone yields water to wells are not shown on Plate I, because 

 of this inferior quality and because under present conditions 

 most wells do not yield as much as 50 gallons of water a 

 minute. 



We still do not have complete information about the hy- 

 drology of the Dakota-sandstone aquifer. Most of the out- 

 crops along the flanks of various mountain ranges seem un- 

 likely places for recharge. For instance, several perennial 

 streams rise in the Black Hills, but the channels are usually 

 dry where they pass the hogbacks of Dakota sandstone, partly 

 because of heavy seepage where those streams have crossed 

 outcrops of cavernous limestones such as those in Wind Cave 

 in South Dakota. Perhaps the geologic formation most de- 

 serving of fame for the artesian system is the overlying Cre- 

 taceous shale, so impermeable as to prevent the water in 

 any of the underlying beds of limestone, shale, or sandstone 

 from moving higher than the horizon of the Dakota sand- 

 stone. It is possible that the wells may have obtained water 

 and minerals in solution from a considerably greater thick- 

 ness of sediments than the relatively thin Dakota sand- 

 stone. 



Detailed studies of the artesian conditions in the vicinity 

 of Ellendale, N.D. were made in 1923 and 1938. In this area 

 the first well drilled to the Dakota sandstone, in 1886, was 

 1,087 feet deep and flowed 600 to 700 gallons a minute. 

 The original pressure head was variously reported as 270 

 to 400 feet above the surface, as compared to a present posi- 

 tion within a few feet of the land surface. Records indicate 

 that the pressure declined about 12 feet a year from 1902 

 to 1915, 4 feet a year from 1915 to 1923, and less than a foot 

 a year since 1923. In 1938 there were 815 wells in the Ellen- 

 dale area, and the average flow was less than 3 gallons a min- 

 ute. The discharge from the flowing wells had caused a drop 

 of several feet in the artesian pressure of wells as much as 

 100 miles to the west, and the western limit of the belt of 

 artesian flow moved eastward about 10 miles because of this 



