116 CONSERVATION OF GROUND WATER 



reservoir, and most of the public supplies are obtained from 

 surface water. Five reservoirs on the Trinity River and its 

 tributaries have a combined storage capacity of about 2 

 million acre-feet, and five others under construction will 

 increase this storage by 1,600,000 acre-feet. Even this storage 

 does not represent full development of the Trinity River 

 potential. 



In many areas of concentrated draft water levels have de- 

 clined very little in recent years, and the present rate of pump- 

 ing appears to be approximately in balance with the rate of 

 inflow to the cone of depression. 



Rockjord, III., 49 pumps about 25 million gallons a day 

 from a sandstone aquifer for municipal and industrial use; 

 artesian pressure there declined about 24 feet from 1891 to 

 1938 but dropped no further in the next 10 years. More than 

 30 million gallons a day is reported to be discharged from 

 the aquifer into streams, and it is evident that a substantial 

 increase in pumpage would be practicable in the Rockford 

 area. 



lacksonville, Fla. 41 has until recently obtained its water 

 from a limestone aquifer by free flow, in order to be certain 

 that there should be no salt-water intrusion in its municipal 

 supplies from deep brine-bearing limestone. In the past 15 

 years artesian pressures at Jacksonville have declined only 

 3 feet, although current withdrawals are at a rate of about 

 50 million gallons a day. The total decline since wells were 

 first drilled is about 25 feet. 



48 Reference: Smith, H. F., and T. E. Larsen, "Ground-water Resources in 

 Winnebago County," 111. State Water Survey Rept. of Invest. 

 2, 1948, 33 pp. 



47 References: Cooper, H. H., Jr., Ground-water Investigations in Florida, 

 with Special Reference to Duval and Nassau Counties, Jour. 

 Am. Water Works Assoc, vol. 36, pp. 169-185, 1944. 

 Cooper, H. H., Jr., and M. A. Warren, The Perennial Yield 

 of Artesian Water in the Coastal Area of Georgia and 

 Northeastern Florida, Econ. Geol., vol. 40. pp. 263-282, 1945. 



