Evidence of Coupled Carbon and Iron Cycling at a Hydrocarbon-Contaminated Site from Time Lapse Magnetic Susceptibility

Abstract

Conventional characterization and monitoring of hydrocarbon (HC) pollution is often expensive and time-consuming. Magnetic susceptibility (MS) has been proposed as an inexpensive, long-term monitoring proxy of the degradation of HC. We acquired repeated down hole MS logging data in boreholes at a HC-contaminated field research site in Bemidji, MN, USA. The MS data were analyzed in conjunction with redox conditions and iron availability within the source zone to better assess whether MS can serve as a proxy for monitoring HC contamination in unconsolidated sediments. The MS response at the site diminished during the sampling period, which was found to coincide with depletion of solid phase iron in the source zone. Previous geochemical observations and modeling at the site suggest that the most likely cause of the decrease in MS is the transformation of magnetite to siderite, coupled with the exhaustion of ferrihydrite. Although the temporal MS response at this site gives valuable field-scale evidence for changing conditions of iron cycling and stability of iron minerals it does not provide a simple proxy for long-term monitoring of biodegradation of hydrocarbons in the smear zone.

Methanogens and Their Syntrophic Partners Dominate Zones of Enhanced Magnetic Susceptibility at a Petroleum Contaminated Site

“…How can we relate these results with the MS values that were measured at Bemidji? First, we have to mention that the high MS values that were measured in year 2011 around the water table in the free-phase petroleum decreased substantially (∼90%) in the following years when measured down the borehole (Lund et al, 2017). On the other hand, here we report that the elevated MS values measured around the water table in the retrieved cores did not substantially decrease over 5 years when measured in the laboratory (Figure 2).…”

“…Sediments were sampled from the cores, and subjected to MS measurements in the laboratory, augmenting our previous in situ measurements that were taken at the Bemidji site ( Atekwana et al, 2014;Lund et al, 2017). For core 1110, volume MS measurements ranged from a low of 29 × 10 −5 SI at 427.0 m to a peak of 287 × 10 −5 SI at 424.8 m at the water table (Figure 2B).…”

“…There is also a growing interest in relating environmental magnetic studies to microbial mediated iron cycling (Porsch et al, 2010;Mewafy et al, 2011;Byrne et al, 2015), and oil biodegradation after spills (Rijal et al, 2010(Rijal et al, , 2012Atekwana et al, 2014;Beaver et al, 2016;Lund et al, 2017). In particular, Rijal et al (2010Rijal et al ( , 2012 and Atekwana et al (2014) observed large MS anomalies within the water table fluctuation zone (WTFZ) associated with the presence of hydrocarbons relative to background measurements and related these changes to the precipitation of magnetite resulting from microbial degradation of the hydrocarbons by iron-reducing bacteria.…”

“…Nonetheless the relationship between MS and the activity of iron-reducing bacteria is not completely understood and the use of MS as a proxy for biodegradation is tenuous at best. This is underscored by the observation that the MS signature can be transient due to the creation and destruction of magnetic minerals by biotic and abiotic processes (e.g., Ameen et al, 2014;Lund et al, 2017). Some of these magnetic minerals may be used by microbes to transfer electrons, causing changes in mineral structure.…”

“…In the field, Rijal et al (2012) and Ameen et al (2014) documented temporal decreases in MS at a contaminated former military air force base in Hradčany, Czech Republic, and speculated this to be due to the dissolution of magnetite. Lund et al (2017) (Mewafy et al, 2011;Atekwana et al, 2014). It was hypothesized that the reduction of MS was due to the transformation of magnetite to siderite coupled with the exhaustion of ferrihydrite.…”