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.