GEOCHEMICAL MECHANISMS OF TRACE ELEMENT MOBILIZATION AND ATTENUATION IN A CRUDE-OIL CONTAMINATED AQUIFER

Biodegradation of crude oil in a contaminated aquifer near Bemidji, MN has formed distinct geochemical zones downgradient of the spill site. The development of methanogenic, iron (Fe)-reducing, and interface zones have resulted in regions of distinct pH and/or redox state. As a result of these geochemical changes, geogenic trace elements were mobilized from sediment into groundwater, where they become human and environmental health hazards. However, the processes controlling trace element mobilization and attenuation are poorly characterized. This study seeks to identify specific geochemical mechanisms of trace element mobilization and attenuation by examining a comprehensive suite of elemental contaminants and their geochemical behaviors in different zones.

We used sequential extractions of sediment from geochemical zones along the plume path to elucidate the sediment phase origins of 30 trace elements and provide insight into attenuation mechanisms. Extractions targeted elements in surface exchangeable, carbonate, (hydr)oxide, and organic phases, which were then analyzed with inductively coupled plasma-mass spectrometry (ICP-MS) and -optical emission spectroscopy. Pairwise Wilcoxon tests of resulting data identified statistically significant differences between zones for each element. Principal component analyses examined elemental relationships for each phase.

Lead, manganese, and Fe, which were mobilized into groundwater by the reductive dissolution of (hydr)oxides, precipitated as carbonates in the Fe-reducing zone. Arsenic (As), boron, and chromium (Cr) were elevated in the organic fraction from Fe-reducing zone sediment, suggesting metalloorganic complexing with the oxyanions. Ultraviolet-visible spectroscopy, size exclusion-high-performance liquid chromatography, and ICP-MS showed the presence of organic complexes of As, Cr, and calcium in the interface zone. Further understanding the behavior of geogenic contaminants following an oil spill aids efforts to remediate secondary contaminants and protect groundwater resource quality.