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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Determination of Vapor-Phase Diffusion Coefficients for Unsaturated-Zone Sediments at a Gasoline-Spill Site in Galloway Township, New Jersey

by

Jeffrey M. Fischer (U.S. Geological Survey, West Trenton, N.J.), Ronald J. Baker (U.S. Geological Survey, West Trenton, N.J.), Matthew A. Lahvis (U.S. Geological Survey, West Trenton, N.J.), and Arthur L. Baehr (U.S. Geological Survey, West Trenton, N.J.)

Abstract

The rate of diffusion of gases through the unsaturated zone has been related to rates of microbial degradation of hydrocarbons at a gasoline-spill site in Galloway Township, New Jersey. The diffusion coefficient for the unsaturated porous medium is required to obtain rates of vapor movement from vapor-concentration measurements. Laboratory experiments were conducted on heterogeneous sediment cores to identify vapor-phase concentration profiles and vapor fluxes. A one-dimensional vapor-diffusion model based on the Stefan-Maxwell equations was used to calculate diffusion coefficients from these experiments. Diffusion coefficients obtained by using this method were smaller than those predicted by using the Millington-Quirk equation, and were particularly significant for clayey sediments. Use of the theoretically calculated diffusion coefficients to calculate vapor flux and biodegradation rates at the study site would have led to overestimates of these rates when compared to those determined by using the experimental method.

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