Multiphase Transport of Petroleum Hydrocarbon Fractions in Groundwater Table Fluctuation Zone: Laboratory Experiments and TMVOC Numerical Simulation

ABSTRACT

Based on actual contaminated site conditions, this study combines two‐dimensional sand trough physical experiments with TMVOC numerical simulations to systematically investigate the multiphase transport patterns and differentiation mechanisms of mixed petroleum NAPL components within a zone of fluctuating water tables. The results indicate that during the wet season, high‐frequency hydraulic action primarily drives vertical convective transport of pollutants, while lateral diffusion is relatively weak. Significant differences in transport behavior were observed among the different components: Hexadecane exhibits weak mobility and strong adsorption, primarily accumulating at the leakage site; toluene, with its high volatility and low viscosity, diffuses extensively in both the free liquid and vapor phases, resulting in widespread contamination. During groundwater table fluctuations, contaminants first migrate vertically to form an elongated contamination plume, with the water table line serving as the primary zone of contaminant accumulation; subsequently, diffusion gradually drives lateral spread, continuously expanding the contaminated area. Groundwater table fluctuations induce dynamic changes in the saturation levels of pore water, NAPL, and the gas phase. During the early stage of transport, free‐phase contaminant concentrations were high, and three‐phase displacement was intense; in the later stage, adsorption and dissolution tended to dominate, and inter‐phase displacement effects weakened significantly. The water saturation in the saturated zone is generally high, with only a slight decrease at the lowest water table; the water saturation in the vadose zone is extremely low and relatively stable, undergoing only slight changes due to capillary action. The TMVOC simulation results show good consistency with experimental data, effectively reproducing the experimental process, compensating for sampling limitations, and accurately characterizing the dynamic evolution of a single‐component three‐phase system. Compared to traditional NAPL models, TMVOC can finely distinguish the transport differences among various components within a mixed system. This study refines the theory of multiphase separation in petroleum contamination within groundwater table fluctuation zones and provides a theoretical basis and technical support for site contamination investigations, risk assessments, and efficient remediation.