Electromagnetic Properties Monitoring to Detect Different Biodegradation Kinetics in Hydrocarbon-Contaminated Soil

The electromagnetic properties (electrical permittivity and electrical conductivity) of three different soil mesocosms polluted with diesel oil were monitored using a time-domain reflectometry probe for 8 months. The main target of the research was to establish a relationship between the development of biological activity within the mesocosms and the temporal behaviour of electromagnetic properties. The trend of the electromagnetic properties exhibited different responses that could be related to the composition of the mesocosms and their variation with time during the runs. We considered three different mesocosms with similar soil conditions and the same diesel oil concentration: porosity of 45%, volumetric diesel oil content of 9%, and volumetric water content of 15%. The first one was subjected to a natural attenuation (NA), the second one was biostimulated without inoculation (BS), and the third one was biostimulated with inoculation (BS + IN). The biostimulated mesocosms showed a marked decrease in electrical permittivity and electrical conductivity, whereas the naturally attenuated mesocosm did not show these variations. Between the biostimulated mesocosms, the inoculated one showed the fastest variations in the electromagnetic properties. The microbial activity and the pollutant degradation were evidenced by the analyses performed at the end of the experiment. As demonstrated by the results for the fluorescein diacetate analysis, the microbial activity was a bit higher for the inoculated microcosm, which also showed faster variations of the electromagnetic properties. In the biostimulated mesocosms, the removal of diesel oil was 66% and 72%, respectively. With natural attenuation, there was a limited removal efficiency, in the order of 2%. Even if the electromagnetic properties evidenced different kinetics of bioremediation in BS and BS + IN, both were able to successfully degrade similar percentages of the contaminant after 8 months. The long monitoring revealed that a substantial decrease in the electromagnetic properties happened only in the first month after contamination. Additionally, an increasing trend of the permittivity was detected in the following months, before reaching a steady-state condition. This study revealed that biodegradation processes with acceptable overall removal efficiency can be monitored successfully by observing the variations in the electromagnetic properties.