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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

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Unsaturated Zone Diffusion of Carbon Dioxide and Oxygen in the Pinal Creek Basin, Arizona

by

Pierre D. Glynn (U.S. Geological Survey, Reston, Va. 22092) and Eurybiades Busenberg (U.S. Geological Survey, Reston, Va. 22092)

Abstract

Unsaturated-zone gases were sampled in November 1991 at two sites in the Pinal Creek basin, above the acidic to partially neutralized ground-water-contaminant plume. The gases were analyzed for carbon dioxide (CO2), oxygen (O2), nitrogen and argon. The isotopic composition of the CO2 gas also was measured. Results show a linear CO2 and O2 gradient. An oxygen flux towards the water table of approximately 1.1x10-2 (mol/m2)/d (moles per square meter per day) is estimated. This oxygen flux is probably caused by the reaction of dissolved oxygen with reduced manganese and iron in the ground water. A steady-state flux of approximately 9x10-3 (mol/m2)/d of carbon dioxide through the unsaturated zone can also be calculated. The CO2 flux is thought to emanate from the ground-water table. The reaction of acidic contaminated ground water with carbonate minerals produces ground waters with high equilibrium partial pressures of CO2 relative to the unsaturated zone atmosphere. The high carbon-13/carbon-12 (13C/12C) ratios of the unsaturated zone CO2 (as high as -10.75 per mil delta13C with respect to the Vienna Pee Dee Belemnite standard), and the increase in the 13C/12C ratio with increasing CO2 concentrations, suggests that degradation of organic matter and root respiration in the unsaturated zone are not contributing significantly to the CO2 flux. Field measurements show a difference of close to 4.4 per mil in delta13C between the 13C enriched CO2 sampled just above the water table and CO2 sampled 0.46 meters below the ground surface. A theoretical model constructed assuming (1) steady-state diffusion of CO2 from the water table, (2) a constant 13C/12C flux ratio determined by the isotopic composition of the CO2 exsolving from the water table and (3) a fixed 13C/12C ratio at the ground surface, predicts a difference of close to 4.4 per mil 13C fractionation between CO2 built up just above the water table and the CO2 coming out of the ground surface. This preliminary model does not, however, satisfactorily explain the field observations, and further research and field data are needed to fully understand our observations.

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