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Observed migration of a controlled DNAPL release by geophysical methods

; Dublin Vol. 33, Iss. 6,  (Nov 1995): 977.

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Observed Migration of a Controlled DNAPL Release by Geophysical Methods

by J. L. Brewster(a,b), A. P. Annan(a), J. P. Greenhouse(a), B. H. Kueper(c), G. R. Olhoeft(d), J. D. Redman(a), and K. A. Sander(d,e)


Ground-water contamination by dense, nonaqueous phase liquids (DNAPLs) is a common environmental problem throughout industrialized areas of North America (Mercer and Cohen, 1990; Mackay and Cherry, 1989). Chlorinated solvents make up one of the largest classes of DNAPL contaminants; other DNAPLs of concern include PCB oils, coal tars, and creosote. Typically, chlorinated solvents have a higher density than water, allow solubility in water, low viscosity, high volatility, and are mechanically immiscible in water. This combination of properties allows such DNAPLs to exist as a distinct liquid phase within the subsurface where they may give rise to evolving dissolved phase plumes for several decades or more, before they are depleted through natural dissolution.

A DNAPL released into the saturated zone of an aquifer will migrate downwards because of its high density, and spread laterally on horizontal capillary barriers. When DNAPL migrates through a porous medium, a portion of the mass will be retained as either a disconnected residual phase or a connected pool phase. The residual phase forms through bypassing and pore-scale snap-off mechanisms (Wilson et al,, 1990), trapping the DNAPL within individual pores so that it is no longer part of a connected phase. A DNAPL pool is formed when capillary pressure can no longer exceed the capillary resistance forces at the leading edge of a migrating body. In a pool, DNAPL saturations are higher than in a residual phase, and the DNAPL exists as a connected phase which is potentially mobile if hydraulic conditions were: to change. Recent field studies have shown that the capacity of sandy deposits to retain DNAPLs is on the order of...