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Observed migration of a controlled DNAPL release by geophysical methods

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; 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)

Introduction

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...