U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Dissolved Gas and Chlorofluorocarbon Content of Ground Waters in the Pinal Creek Basin, Arizona

Abstract

Dissolved nitrogen and dissolved argon concentrations in ground waters at the Pinal Creek toxic waste site were used to calculate recharge temperatures and excess air trapped during recharge. The average recharge temperature was about 12 ±2C -- 6 to 7 °C colder than the average ground water temperatures. Excess-air concentrations were up to 11 milliliters per liter, indicating very rapid recharge probably during flooding events in the winter and early spring. This hypothesis is supported by local precipitation and air temperature records as well as by the 10 °C temperature recorded in Pinal Creek during a large recharge event in February 1993. Nitrogen production by denitrification is not thought to be significant. Dissolved oxygen concentrations are high in all uncontaminated ground waters, a finding consistent with low organic carbon contents in the aquifer. High carbon dioxide partial pressures (pCO₂) were measured in the contaminated ground waters, particularly in the neutralized contaminated ground waters. Dissolution of carbonate minerals by acid-contaminated ground water is thought to be responsible for the high pCO₂ values. Chlorofluorocarbon-12 (CFC-12) and chlorofluorocarbon-11 (CFC-11) concentrations decrease with depth and distance downgradient in the metal- and acid-contaminated ground waters. CFC-12 concentrations are abnormally high and are thought to result from historically higher-than-normal atmospheric concentrations rather than from point-source ground-water contamination. Ground-water ages calculated from CFC-11 concentrations are reasonable -- 3 to 15 years for acid-contaminated ground waters and 20 to 30 years for neutralized contaminated ground waters. Deep uncontaminated ground waters have no significant CFC-11 or CFC-12 concentrations, and are believed to more than 50 years old. The CFC-11 recharge dates are consistent with available tritium, deuterium, oxygen-18, and dissolved-gas data.