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
by
Pierre D. Glynn (U.S. Geological Survey, Reston, Va.) and Eurybiades
Busenberg (U.S. Geological Survey, Reston, Va.)
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.
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