Controlling Factors Affecting NAPL Residuals in Aquifers Containing Low-Permeability Lens Bodies

The presence of residual non-aqueous phase liquid (NAPL) in low-permeability lens aquifers of ten results in a “tailing” effect, ecological deterioration and poor sustainability, which is a primary factor contributing to remediation failures at NAPL-contaminated sites. This issue is largely due to the poorly understood mechanisms by which NAPL residuals interact with low-permeability lens aquifers. To elucidate these mechanisms, this study conducted a series of column experiments, varying the permeability contrast (Kmn), lens sizes (ϕ), and hydraulic gradients (I). Complementary techniques such as mercury intrusion porosimetry and particle size analysis were employed to characterize the aquifer and lens materials. The data obtained include the residual NAPL saturation (Sr), groundwater flow velocity (V), pore size distribution, particle size, and gradation under different experimental conditions. Sensitivity analyses using range and variance analyses identified the following order of effect on NAPL residuals in low-permeability lens aquifers: Kmn > ϕ > I. Correlation analyses further suggest that the governing mechanisms are predominantly mediated by changes in the average particle size, macroporosity (pores > 60 μm), mesoporosity (pores = 30~60 μm), and microporosity (pores = 2~30 μm), as well as abrupt changes in pore size at the interface between the lens and the aquifer, in addition to V. This study can provide a theoretical basis for green, low-carbon, and sustainable development, such as pollution remediation and ecological environment security.