Sub-Surface Migration of an Oil Pollutant into Aquifers

Louise A. MacDonald

February 2000

Department of Environmental Sciences, University of Plymouth, Plymouth, U.K.

in collaboration with

The National Grid Company plc (NGC), U.K.

The risk to groundwater quality following a sub-surface spillage of immiscible pollutants such as oil, petroleum and other organic chemicals is an increasingly potent threat, through escalating industrial application of such pollutants.

This study significantly enhances the understanding of the flow of immiscible pollutants within soil, through field scale investigations to define the spatial variability and extent of a contaminated area and the development of a comprehensive framework for the analysis of oil pollutant migration. This study represents a first attempt by researchers to analyse oil pollutant migration on a wide range of scales, from pore- to field-level.

The research shows that quantity of pollutant is a critical factor in determining the extent of oil migration. Permeability and porosity of the sample material are also important secondary factors. High permeability assists the migration of oil pollutants. Soils with a high porosity allow the pollutant to migrate vertically under the influence of gravity, whereas soils with low porosity induce lateral oil migration, as the oil spreads from the point of injection. A full scale field study using contrasting soil types determines that oil migration is approximately symmetrical about the point of injection.

Experimental data is used to establish modelling capabilities for the characterisation of pollutant migration. Modelling is undertaken at two levels. The first consists of the development of simple Gaussian equations based upon observations of oil glomuses. The glomus approach, newly developed in this work, can be compared to a fractal model, with the glomuses observed in each of the different scales studied.

The second stage involves the use of Pore-Cor to determine the pore scale movement of pollutants. This suggests that during the early stages of pollutant migration, oil is often located in larger stagnant pores, enabling smaller pores to continue to carry water. Consequently there is little impact on permeability. Where greater concentration occurs, oil contaminates both larger and smaller pores, reducing permeability significantly. In addition, Pore-Cor also realistically reproduces the capillary fingering of oil.

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