Towards characterizing LNAPLs and DNAPLs co-transport under groundwater table fluctuation conditions

The transport behavior of pollutants under dynamic groundwater conditions has attracted significant attention recently. However, there is limited research on the simultaneous effects of groundwater table fluctuations on the transport of co-existing pollutants, especially combined dense and light non-aqueous phase liquids (DNAPLs and LNAPLs). In this study, column experiments investigated toluene and dichloromethane transport in a controlled water table system with varying fluctuation conditions. Results showed that both dichloromethane and toluene accumulated near the groundwater table under static water table conditions, but the concentration of dichloromethane declined more rapidly than toluene due to differences in their physicochemical properties, such as solubility, density, and hydrophobicity. Groundwater fluctuations facilitated pollutants transportation towards deeper layers, potentially resulting in unforeseen increases in pollutant volatilization and downward fluxes. The interactions between dichloromethane and toluene, including competitive adsorption, enhanced dissolution, and altered kinematic viscosities, resulted in the reduced transport potential of dichloromethane while enhancing that of toluene. Furthermore, compared to dichloromethane, the initial upward fluctuation of the water table had a more pronounced impact on toluene due to its lower solubility and volatility. The downward transport risk index assessment indicated that among various factors considered, groundwater fluctuation amplitude exerted the most significant influence on pollutant migration risk. These new findings will provide important insights into understanding and assessment of the potential transport risk associated with combined LNAPLs and DNAPLs in the natural environment.