Progression of natural attenuation processes at a crude-oil spill site: I. Geochemical evolution of the plume
Isabelle M. Cozzarellia, , , Barbara A. Bekinsb, Mary Jo Baedeckera, George R. Aikenc, Robert P. Eganhousea and Mary Ellen Tuccillod
a U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, USA
b U.S. Geological Survey, MS 496, 345 Middlefield Road, Menlo Park, CA 94025, USA
c U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, USA
d Department of Environmental Sciences, Clark Hall, University of Virginia, Charlottesville, VA 22903, USA
Received 20 March 2000;
revised 10 January 2001;
accepted 8 February 2001.
Available online 5 December 2001.
A 16-year study of a hydrocarbon plume shows that the extent of contaminant migration and compound-specific behavior have changed as redox reactions, most notably iron reduction, have progressed over time. Concentration changes at a small scale, determined from analysis of pore-water samples drained from aquifer cores, are compared with concentration changes at the plume scale, determined from analysis of water samples from an observation well network. The small-scale data show clearly that the hydrocarbon plume is growing slowly as sediment iron oxides are depleted. Contaminants, such as ortho-xylene that appeared not to be moving downgradient from the oil on the basis of observation well data, are migrating in thin layers as the aquifer evolves to methanogenic conditions. However, the plume-scale observation well data show that the downgradient extent of the Fe2+ and BTEX plume did not change between 1992 and 1995. Instead, 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 concentrations of benzene, toluene, ethylbenzene and xylene (BTEX) has also spread within the anoxic plume. In monitoring the remediation of hydrocarbon-contaminated ground water by natural attenuation, subtle concentration changes in observation well data from the anoxic zone may be diagnostic of depletion of the intrinsic electron-accepting capacity of the aquifer. Recognition of these subtle patterns may allow early prediction of growth of the hydrocarbon plume.