Influence of water table fluctuations and temperature on light nonaqueous phase liquid distribution in heterogeneous porous media

Abstract

The remediation of soils and aquifers contaminated by light nonaqueous phase liquid (LNAPL) relies on a precise understanding of the LNAPL distribution above the water table. This study investigates the impact of groundwater table fluctuations and temperature change on LNAPL redistribution in a heterogeneous porous medium through laboratory-scale experiments. Experiments were conducted in a two-dimensional tank simulating aquifer condition, using diesel fuel as the LNAPL. The reservoir, filled with coarse sand and fine sand low-permeability lenses, reproduced the subsurface heterogeneity. Following LNAPL infiltration from the top, controlled drainage and imbibition cycles simulated water table fluctuations. Experiments were conducted at 10 °C and 20 °C to characterize temperature effects. Fluid behavior was monitored using time domain reflectometry (TDR) probes and high-resolution image analysis. Time domain reflectometry measurements provided quantitative dielectric permittivity data, which were converted to saturation profiles. Simultaneously, an image processing approach using the Biodock platform based on AI and OpenCV was used to visualize the spatial distribution of LNAPL, water, and air. Applying the two methods allowed integrated methodology and a detailed understanding of the dynamics driving LNAPL migration. Results show that water table fluctuations significantly affect LNAPL redistribution, with each imbibition cycle leading to LNAPL entrapment in the capillary fringe due to wettability changes and capillary barriers. Higher temperature increased the mobility of LNAPL by reducing its viscosity, resulting in more efficient fluid displacement during drainage. This highlights the importance of studying the fate and transport of pollutants in the laboratory under temperature conditions relevant to aquifers. Low-permeability lenses further modulated LNAPL migration, emphasizing the critical role of subsurface heterogeneity. Overall, the comprehensive experimental design combining TDR and advanced image analysis provides insight into the mechanisms of LNAPL behavior under dynamic environmental conditions and hints at further improvements for predictive models and remediation strategies in contaminated subsurface environments.

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