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 (Voil/Vtot = 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.
Highlights
Industrial activities have often caused environmental damage to soil and water, with dramatic consequences due to pollutant diffusion along the food chain [1]
Geophysical monitoring was performed by time-domain reflectometry measurements, placing a probe into each column and monitoring the evolution of dielectric permittivity and electrical conductivity
The number of colony-forming units (CFU)/g of soil and the fluorescein produced by fluorescein diacetate (FDA) hydrolysis are shown in Figures 2 and 3, respectively
Summary
Industrial activities have often caused environmental damage to soil and water, with dramatic consequences due to pollutant diffusion along the food chain [1]. Remediation is one of the pillars to recover the polluted areas and give them back to local communities for safe use. The monitoring process should be efficient, simple, fast, and possibly low-cost. In the design of a remediation activity of a wide polluted area, geophysical surveys are often associated with hydrogeological surveys and drilling and coring campaigns, to assess the hydrology, geology, hydrogeological settings, and the contamination level. The main oil clean-up methodologies refer to bioremediation [2], which exploits the activity of microorganisms or plants.
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