Biogeochemical Processes in a Contaminant Plume Downgradient from a Landfill,
Norman, Oklahoma
By Isabelle M. Cozzarelli, Joseph M. Suflita, Glenn A. Ulrich, Steve H. Harris,
Martha A. Scholl, Jamie L. Schlottmann, and Jeanne B. Jaeschke
ABSTRACT
Studies of an alluvial aquifer contaminated with landfill leachate,
at Norman, Oklahoma, indicate that the non-uniform distribution
of electron acceptors and biogeochemical reactions in anoxic ground
water result in steep chemical gradients and the formation of distinct
reaction zones. A combined geochemical and microbiological approach
was used to delineate different biogeochemical zones along a transect
parallel to regional ground-water flow downgradient from the landfill.
The important microbially mediated reactions in the anoxic plume
are iron reduction, sulfate reduction, and methanogenesis. Dissolved
H2 measurements in ground water
at several depth intervals at one location near the edge of the
landfill indicate that sulfate reduction is a dominant respiratory
process, but near- saturation levels of methane were detected in
some intervals. Cycling of sulfur was apparent in a thin interval
at the water table where the highest rates of sulfate reduction
(13.2 micromoles per day (µM/day)) and sulfate concentrations
well above background levels (up to 4.6 millimolar (mM)) were measured.
In this zone, sulfur isotope analyses indicate that the sulfate
is enriched in 34S (34S
of SO42-
was 67-69 per mil). Elevated concentrations of sulfate near the
water table appear to result from the oxidation of sulfides as oxygenated
recharge water mixes with anoxic plume water during recharge events.
Sulfate availability in this vertically thin zone near the water
table appears to control the high sulfate reduction rates measured
in the zone. Iron reduction occurred at the edges of the sulfate-depleted
plume, whereas methanogenesis was detected only in the center of
the plume. This study underscores the importance of examining the
availability of electron acceptors in contaminated environments
and using a combined geochemical and microbiological approach to
elucidate the spatial distribution of biogeochemical processes.