Nondestr. Test. Evat., 1994, Vol. II, pp. 97-106 Reprints available directly from the publisher Photocopying permitted by license only © 1994 Gordon and Breach Science PublishersS.A. Printed in Malaysia THE IMAGING OF WETTING FRONT INSTABILITIES IN POROUS MEDIA USING NEUTRON RADIOSCOPY B. P. TULLIS, J. T. LINDSAY' and S. J. WRIGHT University of Michigan, Department of Civil and Environmental Engineering, 113 Engineering I-A, Ann Arbor, MI48/09-2/25, U.S.A., PH (313) 764-7/48; 'University of Michigan, 3051-B Phoenix Memorial Laboratory, 2301 Bonisteel Blvd., Ann Arbor, MI48/09-2/00, U.S.A., PH (313) 936-1583 (Received 10 May, 1992) The remediation of soil contaminated with organic substances such as gasoline products is a subject of growing concern. One method often employed is fiooding the soil with water or a water-surfactant mixture to wash the contaminant down to the water table. The organics are then pumped out of the subsurface with the ground water for treatment. This procedure has produced only limited results. It has been speculated that this lack of success is due in part to what is known as wetting front instability. Wetting front instability can occur during infiltration of a liquid into porous media, particularly when a fine soil layer is overlying a coarse layer. Instability of the wetting front implies that the interface between the advancing fluid and the portion of the porous media still at initial conditions does not advance as a fiat front, but rather it breaks up into finger-like flow paths. Through the usc of neutron radioscopy and three-dimensional, computer reconstruction, three dimensional infiltration experiments, with water and organic liquids infiltrating into layered porous media, were conducted in an attempt to evaluate the influences of initial moisture content, and type of infiltrating fluid on the formation of fingers. Experimental data was also evaluated for the purpose of calibrating dimensionless relations for predicting finger properties such as finger diameter and propagation velocity. KEY WORDS: neutron radioscopy, neutron radiography, flow in porous media, infiltration, instability, fingering INTRODUCTION The remediation of ground water contaminated with an organic liquid which is slightly miscible can be difficult. Interfacial tension causes a fraction of the organic phase to bind to the soil particles, both in the saturated and unsaturated regions. This creates a residual concentration of the contaminant in the subsurface. One approach to remediation of such an aquifer is to extract the water from the subsurface through capture wells and implement above-ground treatment processes to remove the organic contaminant. Residual concentrations of hydrocarbons, for instance, can remain bound to the soil particles and continue to contaminate the aquifer due to their eventual dissolution from the soil particles. To diminish the time required for the dissolution of the residual contaminants, artificial infiltration with surfactant 97