A field technique to measure the tortuosity and sorption‐affected porosity for gaseous diffusion of materials in the unsaturated zone with experimental results from near Barnwell, South Carolina

Abstract

A tracer experiment was conducted at the commercial low‐level nuclear waste disposal site near Barnwell, South Carolina, to test a new method for determining the tortuosity and sorption‐affected porosity for gaseous diffusion transport of materials in the Unsaturated zone. Two tracers, CBrClF2 and SF6, were released at constant rates of 105 and 3.3 ng/s, respectively, from permeation devices, which were placed in short screened sections in access holes. Soil gas was sampled from 15 piezometers located at various distances from the sources by sequentially pumping 60–160 mL of gas from the piezometers into a dual‐column gas chromatograph located at the test site. The CBrClF2 concentration data obtained from several of the piezometers were analyzed by use of type curves for a continuous point source in an areally extensive medium bounded above and below by planar no‐flow boundaries. The tortuosity of the geologic unit tested, an eolian sand, was determined to be about 0.4, and the sorption‐affected porosity to be 0.22. The tortuosity value is plausible, but the sorption‐affected porosity value is substantially less than that computed from the drained porosity, particularly if adjustments are made for retardation due to solution of the tracer in the liquid phase and sorption on the solid phase. The SF6 data could not be reliably analyzed.

Transport Models for Radioactive Carbon Dioxide at RWMC

“…The migration of a reactive gas moving through a variably saturated media has been investigated (Weeks, Earp and Thompson 1982;Kreamer, Weeks, and Thompson 1988;Gierke, Hutzler, and Crittenden 1990). Partitioning between the gas phase and the aqueous phase and between the aqueous phase and solid phases retards the diffusion of the reactive gas.…”

“…Many relations between porosity and gas-phase diffusion have been suggested and tested (Millington 1959;Currie 1960a;Currie 1960b;Weeks, Earp, and Thompson 1982;Sallam, Jury, and Letey 1984;Kreamer, Weeks, and Thompson 1988;Lerman 1988). Particle shape and particle size distribution affect gas diffusion and how diffusion changes with moisture content.…”

Using a Gas-Phase Tracer Test to Characterize the Impact of Landfill Gas Generation on Advective-Dispersive Transport of VOCs in the Vadose Zone

“…Travel times for diffusive transport of SF 6 were determined with Fick’s 2 nd Law, using the solution C/C 0 = erfc x/(2[D 0 t/T] 0.5 ), where C = SF 6 concentration detected, C 0 = initial SF 6 concentration, erfc = complementary error function, D 0 = gaseous diffusion coefficient (0.036 cm 2 /h, from Kreamer et al, 1988), T = tortuosity (assumed value = 2), and X = travel distance (15.2 m). This approach is based on the assumption that there was no vapor sorption or retardation of the SF 6 , which is to be expected based on its properties.…”

“…Gas tracer tests (GTT) have been used to characterize numerous properties for vadose-zone systems, such as water content (e.g., Nelson et al, 1999; Keller and Brusseau, 2003; Carlson et al, 2003; Han et al, 2006) and gas flow velocities and tortuosity (e.g., Kreamer et al, 1988;; Werner et al, 2004; Tick et al, 2007). Several GTT methods exist to characterize landfill gas generation, such as double tracer techniques (e.g., Scheutz et al, 2011), multiple tracer tests (e.g.…”

Field testing plan for unsaturated zone monitoring and field studies

“…This stage of the project depends on the availability of a portable gas chromatograph to conduct the tracer tests. Descriptions of tracer tests can be found in Kreamer et al (1988) where SF6 and BCF tracers were used. The results of this study will provide valuable information on subsurface gas transport processes and the various techniques to obtain data on parameters required for simulating diffusive transport in subsurface layers.…”