Long-Term Geochemical Evolution of a Crude-Oil Plume at Bemidji, Minnesota
By Isabelle M. Cozzarelli, Mary Jo Baedecker, Robert P. Eganhouse,
Mary Ellen Tuccillo, Barbara A. Bekins, George R. Aiken, and Jeanne
B. Jaeschke
ABSTRACT
The long-term study of the development of the contaminant plume at
the Bemidji site has provided an excellent opportunity to
determine how natural attenuation of hydrocarbons is affected by
evolving redox conditions in the aquifer. During the 16 years
that data have been collected the shape and extent of the
contaminant plume have changed as redox reactions, most notably
iron reduction, have progressed over time. The downgradient
extent of the Fe2+ and BTEX plume did not change
between 1992 and 1995, indicating that, at the plume scale, the
supply of these compounds from the upgradient contaminated water
is balanced by the attenuation processes. However, depletion of
the unstable Fe (III) oxides near the subsurface crude-oil
source has caused the maximum dissolved iron concentration zone
within the plume to spread at a rate of approximately 3
m/year. The zone of maximum benzene, toluene, ethylbenzene, and
xylene (BTEX) concentrations has also spread within the anoxic
plume. Analysis of sediment and water, collected at closely
spaced vertical intervals, from cores in the contaminant plume
provide further insight into the evolution of redox zones at a
smaller scale. Contaminants that appeared not to be moving
downgradient from the oil based on observation well data, such
as ortho-xylene, are migrating in thin layers as the aquifer
evolves to methanogenic conditions.