Abstract
Abstract Nonaqueous phase liquids (NAPL) such as petroleum products can pose significant long-term contamination risks to the groundwater when released in the unsaturated zone. Efficient remediation of contaminated sites should be guided by the interpretation of numerical simulations using some appropriate assumptions based on the gathering of field data. However, the prediction uncertainties of numerical models are accentuated by the way heterogeneity in natural geological formations influences the contaminant behaviour rending effective remediation strategies difficult. Hence the predictive capability of such models needs to be validated against quantitative laboratory experiments.Four two-dimensional multiphase flow experiments were conducted to investigate the migration and the distribution of light nonaqueous phase liquids (LNAPL) in the unsaturated zone under various hydrogeological conditions. Miniature resistivity probes were used to measure water saturation. Water and NAPL pressures were measured using hydrophilic and hydrophobic tensiometers. An innovative multispectral image analysis technique was developed to determine the dynamic NAPL, water and air saturation distribution in the three fluid-phase laboratory experiments. The method provided a non-destructive and non-intrusive tool for studying multiphase flow for which rapid changes in fluid saturation in the entire flow domain are difficult to measure using conventional techniques.
Modelling of multiphase flow requires that the constitutive relations between the pressure, the saturation and the relative permeability of the fluids be known in order to solve the governing flow equations. The applicability of analytical closed-form expressions representing the constitutive hydraulic relations and commonly used scaling procedures in modelling multiphase flow were investigated experimentally. Extending two-phase relations to three fluid-phase systems is often used in modelling multiphase flow and three fluid-phase behaviour is usually estimated from two-phase pressure-saturation relations established in the laboratory for static conditions. The limitations of these assumptions under dynamic and heterogeneous conditions are also addressed.
The effect of macro-heterogeneity on LNAPL migration was investigated by means of textural interfaces between layers of soil with contrasting textures. The simulation of LNAPL spill in the unsaturated zone under heterogeneous conditions showed that, depending on the hydraulic properties of the heterogeneities as well as the saturation history of the soil formation, the contaminant flow pattern and velocity could be largely affected and significant volumes ofNAPL could remain entrapped leading to long term contamination scenarios. Similarly, the mobilisation of entrapped LNAPL by simulated rainfall was infl uenced by the nature of the heterogeneities. It appeared that the constitutive three fluid-phase relations between fluid saturation, pressure and permeability are critical to the correct prediction of NAPL migration and distribution.
This study showed that the dynamic monitoring of the pressure and the saturation distribution of NAPL, water and air, under various hydrogeological conditions, provides essential quantitative data, which challenges the validity of numerical models.