On the use of magnetic properties to study soil hydrocarbon biodegradation

Since the beginning of the industrial era, petroleum hydrocarbons are becoming a global problem for the environment, especially when spilled. To remediate contaminated soils, bioremediation, involving microbial-mediated redox reactions can be used. Geophysical methods can be used to understand better the spatial extent and state of the degradation processes. Electrical methods have been successfully used to quantify the ionic conduction and estimate degradation areas based on precipitation/dissolution of metallic particles (e.g., pyrite, magnetite). However, electrical methods cannot give information on the type of metallic minerals forming in the soil, which are controlled by the redox conditions of the environment, directly linked to the degradation stage.

We propose a laboratory approach that includes quantitative analysis of different magnetic properties: thermomagnetic measurements, frequency dependent susceptibility (FDS) and isothermal remanent magnetization (IRM). We used soil samples from the Brandywine superfund site contaminated with dense non-aqueous phase liquids (DNAPLs) and remediated using amendment injections to enhance the hydrocarbon biodegradation. We collected soil samples over the entire aquifer depth (30 m), both in the bioremediated area and in an adjacent non-remediated area. We observe vertical variations of the magnetic signals between different areas of the aquifer that we can associated to water and lithological variations. Thermomagnetic analysis shows a difference between treated and untreated samples: magnetite is observed mostly in the untreated samples, when another magnetic mineral with a Curie temperature around 300°C is observed in the treated samples. This could correspond to a reduced form of magnetite such as pyrrotite or greigite that would indicate microbial reduction. IRM data support the interpretation of two distinct magnetic minerals. IRM parameters are in good agreement with low coercivity minerals (e.g. magnetite and pyrrotite). FDS interpretation suggests that size variations of the magnetic minerals are related to water and lithological variations in the aquifer. The integration of rock magnetism technics allows us to infer changes in magnetic particles which are related to the degradation stage.