Ages of groundwater can be determined from the carbon 14 content of the carbonate dissolved in the water. The carbon 14 is derived from plant-produced CO2 in the soil of the recharge area and is usually diluted by carbon 14-free carbonate dissolved from carbonate minerals in the soil and in the aquifer. Techniques based on the ratios of the stable carbon isotopes and on the over-all carbonate chemistry of the water can be used to correct for this dilution and to allow the calculation of true water ages. Water samples from wells in the Eocene Carrizo Sand in Atascosa and adjacent counties, Texas, were dated by this method. The ages of the water samples ranged from 0 years at the outcrop to 27,000 years 35 miles downdip. Based on the carbon 14 ages, the water velocities were about 8 feet per year 10 miles from the outcrop and 5.3 feet per year at 31 miles. Flow rates calculated from available hydrologic data are in agreement with carbon 14 results.