PROBLEMS FROM DEVELOPMENT 135 



the pumpage may have reached 60,000 acre-feet. In addi- 

 tion some 17,000 acres is now irrigated by water from the 

 Nueces River, and farther east perhaps 5,000 acres is irri- 

 gated from flowing wells in Atascosa County, which is now 

 in about the same stage of development as Dimmit and 

 Zavala Counties were 10 years ago. 



The ground water comes from the Carrizo sand, which 

 crops out in a belt 1 to 5 miles wide. Recharge is chiefly 

 from precipitation on this outcrop, and records show that 

 most of the recharge occurs in wet years, with very little in 

 dry years. Southeast of the outcrop area the Carrizo is 

 covered by the relatively impermeable Bigford formation, 

 and the water is under artesian pressure. 



In 1940 artesian pressures were slightly higher than in 

 1930, reflecting a slight decrease in irrigated acreage during 

 the decade. Since 1941, however, artesian pressures in repre- 

 sentative wells have dropped 20 to 100 feet, of which 5 to 40 

 feet occurred in the single year 1948. In the recharge area 

 the water table is 5 to 8 feet lower than it was in 1941, having 

 dropped 1 or 2 feet in 1948. The precipitation in 1948 was 

 the least in 30 years, and the marked declines in water levels 

 in the recharge area that year reflect the small amount of 

 recharge and the increased pumpage for irrigation in a dry 

 year. 



The declines in artesian pressure are far greater than the 

 decline of the water table in the recharge area and indicate 

 that the rate of pumping from artesian wells is greater than 

 the rate at which water can move through the sand from the 

 recharge area. The water levels in the artesian wells will 

 continue to drop until the draft and replenishment balance. 

 The overlying Bigford formation contains salt water, which 

 can contaminate the developed ground-water reservoir, and 

 some contamination has already occurred at defective wells 

 that permit leakage of salt water into the aquifer. There 

 is likelihood of far greater contamination if part of the Car- 

 rizo sand in the artesian area is unwatered by pumping. 



The water table in the recharge area is of critical impor- 



