ELECTRICAL AND MAGNETIC MEASUREMENTS AT THE BEMIDJI, MINNESOTA, CRUDE OIL CONTAMINATED SITE: IMPLICATIONS FOR THE BIOGEOBATTERY MODEL

Recent biogeophysical research suggests that microbial processes and resulting redox conditions at mature, hydrocarbon-contaminated sites undergoing biodegradation generate distinct electrical geophysical signatures. The National Crude Oil Spill Fate and Natural Attenuation Research Site at Bemidji, Minnesota, is a unique field laboratory for investigating the geophysical signatures of a mature oil-spill where natural attenuation is well documented at the field-scale. In August 1979, a high pressure crude oil pipeline ruptured, spilling 1,700,000 L of crude oil in an uninhabited area near Bemidji. Oil pooled in low lying areas (~2,000 m²) and sprayed over an area of 6,500 m². Field-scale geophysical measurements were focused to identify evidence for a geobattery mechanism in self potential (SP) signals, postulated to occur at the sharp redox front presented by the water table if/when long-range electron transport between reduced and oxidized zones occurs. SP measurements were recorded down a borehole drilled at a location where free product is thickest and natural attenuation is enhanced. Vertical SP profiles recorded on an electrode array immediately after the drilling of the borehole showed no evidence for a dipolar anomaly across the water table that would be expected in the presence of a geobattery. However, SP measurements recorded on a monthly basis for twelve months after installation of the array show the progressive development of a vertical dipole with the polarity consistent with a geobattery in which the anode is below the water table. Downhole magnetic susceptibility measurements show that the smear zone is characterized by anomalously high magnetic susceptibility, possibly due to magnetite precipitation caused by iron reduction and coupled to oxidation of aromatic hydrocarbon compounds. This zone of elevated magnetic susceptibility may provide the electron conductor needed to facilitate electron transport between anode and cathode and thereby serve as the current source for the geobattery. Water level measurements indicate that the strength of this geobattery could in part be controlled by the water table elevation relative to the zone of enhanced magnetic susceptibility.