MOBILITY OF GEOGENIC TRACE ELEMENTS AND ORGANIC MATTER IN AN AQUIFER CONTAMINATED WITH CRUDE OIL

In 1979, a crude oil pipeline rupture spilled over 10,000 barrels of crude oil into a shallow, unconfined aquifer near Bemidji, MN, USA. Partial dissolution of the oil created a hydrocarbon plume in groundwater. Biodegradation of the hydrocarbons is coupled to microbial iron reduction and methanogenesis. Changes in aquifer chemistry due to hydrocarbon biodegradation (e.g., production of acidity, lower redox potential) triggered complex water-mineral-organic interactions that mobilized trace elements from native aquifer sediment into groundwater. Using sequential chemical extractions of sediments along the hydrocarbon plume transect, we show that native carbonates are dissolved due to acidity and serve as a source for boron, barium, calcium, magnesium, and strontium in groundwater. Barium, calcium, cobalt, iron, manganese, and lead are associated with oxides that are reductively dissolved due to iron and manganese reduction. Organic matter (OM) in native sediments at Bemidji contain a host of trace elements, but, statistically, only calcium was shown to be mobilized. The free oil product also serves as a potential source of nickel, iron, cobalt, chromium, and zinc in groundwater.

Despite their high concentration in reduced groundwater, dissolved trace element concentrations approach background concentrations near 100 m downgradient from the oil source due to sequestration by aquifer sediments in less reduced groundwater. Carbonate mineral precipitation was a significant sink for elements including lead, iron, and manganese, that have low carbonate solubility products. Oxyanions of arsenic, chromium, and boron were sequestered via the formation of organic complexes with heavy molecular weight OM in the plume sediments. Using excitation-emission spectroscopy, we further characterized OM in aquifer sediments and in the groundwater plume, and we completed a parallel factor analysis to better characterize OM in each phase.

Results from this study expand knowledge about the influence of oil spills on water quality by elucidating mechanisms triggered by hydrocarbon biodegradation that mobilize and sequester native trace elements. Furthermore, these results provide fundamental geochemical constraints that may advance remediation efforts of groundwater resources via a more informed understanding of geogenic contaminant behavior following oil spills.