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