50 CONSERVATION OF GROUND WATER 



generally 50 to 150 feet thick, lies beneath the clay through- 

 out the Grand Prairie. 



Originally the water was under sufficient artesian pressure 

 to rise 20 to 35 feet above the top of the aquifer, but so much 

 water has been removed from storage by pumping that a 

 considerable part of the aquifer has been unwatered. The 

 aquifer had been partly drained under an area of 240 square 

 miles by 1929, and by 1944 the unwatered material was com- 

 puted to have an average thickness of 1 1 Y 2 feet in an area 

 of 610 square miles. The reduction in storage, amounting 

 to nearly 1,400,000 acre-feet by 1944, proves that pumping 

 for many years has exceeded the replenishment to the aqui- 

 fer under the Grand Prairie. 



The ubiquitous clay prevents recharge within the rice- 

 growing area, either from precipitation or from irrigation, 

 and the only replenishment is by movement of water from 

 beyond the borders of the Grand Prairie. The White River 

 along the northeastern border of the region is known to pro- 

 vide some recharge, and in a few places small sandy areas 

 may permit some recharge to the aquifer. The initial hy- 

 draulic gradient was to the southeast, suggesting that water 

 enters the aquifer northwest of the Grand Prairie. Since the 

 development of the broad cone of depression by pumping, 

 water has moved inward from all directions. 



As more and more of the principal aquifer has been 

 drained, an increasing number of wells have been drilled 

 to deeper aquifers. Little is yet known about the capabilities 

 of natural replenishment for these deeper wells, but there 

 is a good chance that here again the wells will pump from 

 long-accumulated storage, and the yield that can be peren- 

 nially sustained will be less than the rate at which the wells 

 can be pumped initially. 



To sum up, the ground- water reservoir of the Grand Prai- 

 rie is capped by impermeable materials ideal for rice- 

 growing but unsatisfactory for ground-water replenishment. 

 The reservoir has large storage capacity, and as the water is 

 removed by pumping, the utilization of that capacity be- 



