In a crude-oil-contaminated sandy aquifer at the Bemidji site in northern Minnesota, biodegradation of petroleum hydrocarbons resulted in transient increases in magnetic susceptibility under high water table conditions. The magnetic susceptibility subsequently decreased again when the water table dropped. This data set was assembled to illustrate the cause of the magnetic susceptibility changes. These data show that alkalinity is high and dissolved oxygen is low in the area where the increased magnetic susceptibility is found. The decrease in the magnetic susceptibility can be attributed to the long term decline in sediment Fe(III) concentrations with time. The decrease is caused by iron reducing bacteria converting Fe(III) to Fe(II) as they degrade hydrocarbons. The decrease in sediment Fe(III) is associated with a decrease in dissolved Fe in the plume.