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
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 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.
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