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

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

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 ¹³C 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 ¹³C 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.