###### Abstract  
This study aims to monitor the biological processes ongoing in a hydrocarbon polluted soil. The experiments were carried out at a laboratory scale by measuring the evolution of its geophysical electromagnetic parameters. Time-domain reflectometry (TDR) probes were used to measure dielectric permittivity and electrical conductivity in columns of sandy soil artificially contaminated with diesel oil (V\({}_{\text{oil}}\)/V\({}_{\text{tot}}\) = 0.19). To provide aerobic conditions suitable for the growth of microorganisms, they were hydrated with Mineral Salt Medium for Bacteria. One mesocosm was aerated by injecting air from the bottom of the column, while the other had only natural aeration due to diffusion of air through the soil itself. The monitoring lasted 105 days. Geophysical measurements were supported by microbiological, gas chromatographic analyses, and scanning electron microscope (SEM) images. Air injection heavily influenced the TDR monitoring, probably due to the generation of air bubbles around the probe that interfered with the probe-soil coupling. Therefore, the measurement accuracy of geophysical properties was dramatically reduced in the aerated system, although biological analyses showed that aeration strongly supports microbial activity. In the non-aerated system, a slight (2%) linear decrease of dielectric permittivity was observed over time. Meanwhile, the electrical conductivity initially decreased, then increased from day 20 to day 45, then decreased again by about 30%. We compared these results with other researches in recent literature to explain the complex biological phenomena that can induce variations in electrical parameters in a contaminated soil matrix, from salt depletion to pore clogging. diesel oil; aerobic biological processes; time-domain reflectometry; dielectric permittivity; electrical conductivity +
Footnote †: journal: Applied Soft  
to degrade or accumulate pollutants. A previous laboratory-scale study showed that biostimulated indigenous microorganisms can successfully remove diesel oil from soil [3]. During the bioremediation process, biological activity (e.g., generation of by-products, co-metabolites, and biofilms) may induce variations in soil geophysical properties, such as electrical and acoustic impedance [4; 5]. Therefore, a time-lapse geophysical investigation could be a useful tool to monitor the biodegradation timeline, but the correlation between the spatial and temporal distribution of geophysical properties and the efficiency of pollutant removal is far from being completely understood [6]. Theoretical models have been proposed to describe the behavior of hydrocarbon fluids in soil and their effects on properties such as dielectric permittivity and electrical resistivity. [PERSON] and [PERSON] [7] developed a pore-scale fluid model for clay-free granular soils. In order to predict the geophysical evidence of a spill of hydrocarbons, [PERSON] and [PERSON] [8] proposed a model for the complex permittivity of a soil composed of sandy grains, clay, and silt, partially saturated with gas (air), water, and hydrocarbon based on the self-similar model [9; 10]. Thus, multiphase models were implemented such as the Complex Refractive Index Model (CRIM) to predict the dielectric permittivity of a matrix composed of soil, water, gas, and oil. They were validated by using an experimental set based on time-domain reflectometry (TDR) [11], similarly to the approach adopted in our study. These models work well when pollutants displace water contained in soil porosity, as hydrocarbons and water have very different electrical properties. They are less suitable when pollutants displace air because they have relatively similar dielectric permittivity. Hydrocarbon contaminants themselves are a weak source of geoelectrical signals. Otherwise, it is known that the biological activity associated with degradation generates larger electrical signals than the oil itself.