Carbon 14 ages and flow rates of water in Carrizo Sand, Atascosa County, Texas

Abstract

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.

Groundwater flow and the 4He distribution in the Great Artesian Basin of Australia

“…Velocity estimated from the distribution of a radioisotope, for example, can be compared to groundwater velocity calculated from the measured head gradient, hydraulic conductivity, and sediment porosity, giving an independent check on the flow rate [e.g., Pearson and White, 1967]. Alternatively, hydraulic conductivity can be calculated from the velocity inferred in an isotopic study, providing a way to determine conductivity on a scale larger than possible by laboratory measurement or well testing.…”

Qualilty, isotopes, and radiochemistry of water sampled from the Upper Moenkopi Village water-supply wells, Coconino County, Arizona

“…In the upper atmosphere, 14 C is rapidly oxidized to CO 2 which readily mixes into the lower atmosphere. Any material using or reacting with atmospheric CO 2 (plants and water) has a 14 C activity equal to atmospheric 14 C while it is in equilibrium with the atmospheric carbon reservoir (Pearson and White, 1967). Carbon-14 activity is reported as percent modern carbon (pmc) and, by convention, the modern pre-1950 (pre-nuclear weapons testing) activity of atmospheric 14 C is 100 pmc.…”

Methods for obtaining sorption data from uranium-series disequilibria

“…The 14 C age of Carrizo waters has been measured (Pearson and White, 1967). Based on extensive study, investigators believe that the recharge source is meteoric water and that the aquifer is well confined over the 100-km-long study area.…”