INVESTIGATING IRON MINERAL TRANSFORMATIONS AT A HYDROCARBON CONTAMINATED SITE FROM TEMPORAL MEASUREMENTS OF MAGNETIC SUSCEPTIBILITY OF MINERAL PACKETS SUSPENDED IN WELLS

Long-term monitoring of biogeochemical transformations of hydrocarbon contaminants in the subsurface is important for assessing vulnerability of groundwater resources and habitats impacted by them. Geophysical measurements have the potential to improve understanding of the transformation of iron minerals at hydrocarbon-contaminated sites. At a long-term research site over an oil spill near Bemidji, MN, magnetite transformation to siderite has been postulated to explain long term decreases in magnetic susceptibility (MS) signals in boreholes. To further investigate this hypothesis, mineral packages (sands containing magnetite and hydrous ferrous oxhydroxide (HFO) minerals) were suspended in the smear zone and periodically retrieved over a 16-month period to assess change in MS over time. Time-lapse measurements on magnetite mineral packets show a progressive decrease in MS over the first eight months, with near constant values over the subsequent four months. This pattern is remarkably consistent for 15 different magnetite packets placed in both the free phase and dissolved parts of the plume. The HFO packets in the smear zone exhibit more variability, with some showing strong evidence for conversion of HFO to magnetite and subsequent decreases at later time. Additional mineral packets were installed in upgradient wells in the Fall of 2018. Preliminary results for measurements taken 4 months apart show that all mineral packets in this upgradient location exhibit only mineral variability in MS, lending confidence to the significance of the MS changes observed within the plume. These findings all support the argument that transformation of magnetite to siderite, coupled to exhaustion of ferrihydrite, is occurring at this hydrocarbon contaminated site and can be monitored with magnetic susceptibility.