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Description
Leaks from underground storage tanks containing petroleum products represent a major environmental concern due to the potential contamination of soils and groundwater. Early detection of such leaks is essential to limit the spread of pollutants and reduce remediation costs. Conventional monitoring strategies rely primarily on groundwater sampling; however, these methods often detect contamination only after dissolved pollutants reach the water table. In deep groundwater contexts, this delay can result in large volumes of soil becoming contaminated before detection occurs. Soil-gas monitoring offers a promising alternative for earlier detection because volatile components of light non-aqueous phase liquids (LNAPLs) can migrate through the vadose zone in the vapor phase before the liquid phase reaches groundwater.
This study investigates the coupled processes of LNAPL infiltration, volatilization, and vapor transport in porous media, with the objective of identifying the key parameters controlling vapor detection and determining which gasoline components are the most suitable indicators for early leak monitoring. Controlled laboratory experiments were conducted in a decimetric two-dimensional tank filled with dry sand to reproduce the infiltration of gasoline and the subsequent generation and transport of vapors in the porous medium. Gasoline was injected at the top of the tank to simulate a leakage scenario, while soil gas was continuously extracted at the bottom of the system to reproduce the operation of a monitoring well.
The migration of the LNAPL phase within the porous medium was monitored using image analysis, which allowed tracking of the infiltration front and estimation of LNAPL saturation over time. In parallel, the extracted gas was analyzed continuously using a portable gas chromatograph, enabling real-time monitoring of vapor concentrations of major gasoline components, including butane, benzene, and toluene, for different gas pumping flow rates.
The experimental observations highlight the complex interactions between multiphase flow, vapor generation, and advective–diffusive transport in porous media. In particular, the high volatility of light hydrocarbons leads to rapid vapor generation and transient concentration peaks, whereas less volatile compounds exhibit slower but more persistent signals. The effect of pumping rate on vapor detection was also investigated, showing that airflow conditions strongly influence both the magnitude and timing of measured vapor concentrations.
To interpret these results and improve the understanding of the governing processes, the experiments were coupled with numerical modeling of two-phase flow and vapor transport in porous media. The model accounts for LNAPL infiltration, volatilization of multicomponent hydrocarbons, gas-phase transport, and mass transfer between liquid and vapor phases. Comparison between simulations and experimental data provides insight into the mechanisms controlling vapor generation and migration, including the influence of volatilization kinetics, gas flow rates, and porous media properties.
The combined experimental and numerical approach provides a better understanding of the processes controlling vapor detection in soil-gas monitoring systems. The results contribute to identifying suitable indicator compounds for early leak detection and to improving the design of soil-gas monitoring networks around underground storage tanks.
| Country | France |
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