The exposed rocks and those underlying Karnes County dip toward the Gulf of Mexico at average rates ranging from 20 to more than 200 feet per mile. The oil fields are on structures associated with faulting; the effect of faulting on the occurrence of ground water has not been determined.
The principal water-bearing formations, from oldest to youngest, underlying the county are the Carrizo sand, Yegua formation, Jackson group, Catahoula tuff, Oakville sandstone, and Lagarto clay. They range in age from Eocene to Miocene and all are of sedimentary origin. About 40 million acre-feet of usable water (water containing less than 3,000 ppm) is stored more then 2,500 feet below land surface in the Carrizo sand; 30 million acre-feet is stored in the younger formations at depths less than 1,000 feet.
Ground-water withdrawals for municipal, industrial, and domestic use averaged about 1.7 million gpd in 1957. Irrigation and stock supplies were derived from both ground- and surface-water sources. In general, water levels from 1936 through 1957 were not affected appreciably by withdrawals.
Recharge to the ground-water reservoir from precipitation, although only a small percentage of total precipitation, exceeded withdrawals of ground water by wells in 1957.
Most of the usable ground water in Karnes County is of substandard quality; whereas water from the San Antonio River, although hard, is of excellent quality. Wells tapping the Carrizo may yield as much as 1,000 gpm in the northwestern part of the county; wells in the shallower formations may yield as much as 600 gpm in the most favorable areas, but in some places may yield only a few gallons per minute of water suitable only for stock.
