Estimating Diffusion Coefficients in Low-Permeability Porous Media Using a Macropore Column

Abstract

Diffusion coefficients in an aquitard material were measured by conducting miscible solute transport experiments through a specially constructed macropore column. Stainless steel HPLC columns were prepared in a manner that created an annular region of repacked aquitard material and a central core of medium-grained quartz sand. The column transport approach minimizes volatilization and sorption losses that can be problematic when measuring hydrophobic organic chemical diffusion with diffusion-cell methods or column-sectioning techniques. In the transport experiments, solutes (tritiated water, 1,2,4trichlorobenzene, and tetrachloroethene) were transported through the central core by convection and hydrodynamic dispersion and through the low-permeability annulus by radial diffusion. All transport parameters were independently measured except for the effective diffusion coefficient in the aquitard material, which was obtained by model fitting. Batch-determined retardation factors agreed very closely with moment-derived retardation factors determined from the column experiments, and no evidence of pore exclusion was found. A model with retarded diffusion was found to apply, and the effective tortuosity factor of the aquitard material was estimated at an average value of 5.1 (based on estimates that ranged from 3.7 to 7.7).

Effects of fundamental differences between batch and miscible displacement techniques on sorption distribution coefficient

“…A question of great interest is the compatibility of these two techniques. Good agreement between the results of the two techniques has been reported (Nkedi-Kizza and others 1987; Lee and others 1988;MacIntyre and others 1991;Spurlock and others 1995;Young and Ball 1998). In contrast, several investigators have observed that batch-determined distribution coef®cients are higher than their MD counterparts (Bilkert and Rao 1985;Bouchard and others 1988;MacIntyre and Stauffer 1988;Celorie and others 1989;Lion and others 1990;Piatt and others 1996;Maraqa and others 1998;Carmichael and others 1999;Thomsen and others 1999;Kim and others 2000).…”

A review of model applications for structured soils: a) Water flow and tracer transport

“…The model could be used to explain the transfer mechanisms that increased leaching by 20% in an intermittent (as compared to continuous) flow regime (Cote et al, 1999). In a cylindrical macropore system, a MIM with diffusion equation could be used to determine the effective diffusion coefficient of soil surrounding a sand filled macropore (Young and Ball, 1998 Ahuja et al (1995) used RZWQM to study transport of a surface-applied Br − tracer under rainfall in a soil column with a 3-mm artificial macropore. A good match of the rapid Br − breakthrough resulted when the macropore water was assumed to mix with only 0.5 mm of matrix solution, revealing restricted solute transfer through the compacted macropore walls (Ahuja et al, 1995).…”

Application of inverse methods to contaminant source identification from aquitard diffusion profiles at Dover AFB, Delaware

“…Effective diffusion coefficients are defined as molecular diffusivity divided by an effective tortuosity factor, as described elsewhere [Ball et al, 1997a;Young and Ball, 1998]. L is the thickness of the first layer (OSCL) and is a function of the coring location, x is distance from the interface between the aquifer and aquitard, and t is time.…”