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
  
Use of Carbon and Hydrogen Stable Isotopes to Investigate the
Production and Fate of Methane at a Toxic Waste Site, Bemidji, Minnesota
by
Kinga Revesz (U.S. Geological Survey, 431 National Center, Reston,
VA 22092), Tyler Coplen (U.S. Geological Survey, 431 National Center, Reston,
VA 22092), Mary J. Baedecker (U.S. Geological Survey, 431 National Center,
Reston, VA 22092), and Marc Hult (U.S. Geological Survey, 2280 Woodale Drive,
Mounds View, MN 55112)
Abstract
Stable carbon and hydrogen isotope ratios of dissolved
methane and aqueous carbon dioxide in the saturated zone of
a crude oil spill near Bemidji, Minn., support the concept of
methane production by acetate fermentation with concomitant
increase in bicarbonate concentration as opposed to carbon dioxide
reduction (and bicarbonate consumption). Oxidation of dissolved
methane along the lateral flow path seems to be minimal because
no measurable change in isotopic composition of methane occurs
with distance from the oil body. As methane from the ground
water diffuses upward through a 5- to 7-meter-thick unsaturated
zone, it is partially oxidized to carbon dioxide, increasing
the  13C
of the remaining methane, decreasing the 13C
of the carbon dioxide, and increasing the pressure of carbon
dioxide in the unsaturated zone.
No increase of concentrations of atmospheric methane
was detected at ground level directly above the oil >plume;
concentrations were identical to those of a background atmospheric
sample.
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