Centrifuge and numerical modelling of dense non-aqueous phase liquid contaminants migration

Helena Coumoulos, Cambridge University
Geotechnical Engineering Group


Dense Non Aqueous Phase Liquids (DNAPLs) are common groundwater contaminants. They are of particular interest because they are denser and less viscous than water, whilst having very low solubility in water. Their properties render them particularly mobile in an aqueous environment. The understanding of the migration and entrapment of DNAPLs immediately after their spillage is necessary to determine an effective clean up operation of contaminated land.

This dissertation investigates DNAPL contaminant migration in saturated soil, focusing on the special problem of its migration in inclined layered soil profiles. It begins by describing the parameters affecting multiphase flow in porous media in general and then DNAPL contaminants in particular.

A series of 12 centrifuge tests was designed and carried out at the Schofield Centrifuge Centre using the 10m beam centrifuge. A DNAPL was injected in two-dimensional soil models with an inclined interface. The main varying parameters were the porous media and the interface inclinations. A unique pattern of pressure changes was registered by pore pressure transducers due to the DNAPL flow. This data were useful in indicating the level of DNAPL saturation, as well as the arrival of the DNAPL plume at a particular location. Digital images of the DNAPL migration were also recorded and analysed to determine the spreading along the interface and the penetration depth with time. The combination of these two types of data provided an insight to the mechanisms taking place during DNAPL migration.

A multiphase flow numerical code, TOUGH2/T2VOC, was used to model a well documented one-dimensional experiment from the literature. The effect of the porous material properties on the one-dimensional DNAPL flow was investigated. Simulations of two-dimensional DNAPL migration problems were also performed. They were primarily used in a parametric analysis examining the effect of the sand interface angle, the permeability contrast at the interface and the DNAPL injection type on DNAPL spreading and penetration at the soil layer interface.

It has been demonstrated that faster DNAPL releases result in bigger plumes and more spreading at the soil interfaces. For a given volume of released DNAPL, slower releases resulted in deeper DNAPL penetration in layered soil systems, whereas for the same injection rate, increased spreading was observed with increasing angles of interface. Increased spreading was also observed for increased permeability contrast at the interface between different materials.

Keywords: Dense Non Aqueous Phase Liquid Contaminants; DNAPL; centrifuge; saturated sand; layers; image processing; numerical modelling.