Use of atmospheric fluorocarbons F‐11 and F‐12 to determine the diffusion parameters of the unsaturated zone in the Southern High Plains of Texas

Abstract

Two stable, entirely man-made fluorocarbon gases, trichlorofluoromethane (F-11) and dichlorodifluoromethane (F-12), have been released into the atmosphere during the past 40 years as a consequence of their widespread commercial use. Production data for these compounds are well documented, and atmospheric buildup can be reconstructed on the basis of published release estimates. On-site measurements of F-I 1 and F-12 concentrations in soil air within unconsolidated sedimentary deposits were made using gas chromatography at four sites in the Southern High Plains of Texas. Measurable concentrations of both gases were found at depths as great as 43.9 m. An analytical model and a finitedifference model (both based on molecular-diffusion theory) were used to assess the on-site results. For the analytical model, effects of porosity, tortuosity, gas-liquid partitioning, and sorption were tested as part of a lumped diffusion coe•cient parameter. In the finite-difference model, effects of porosity, tortuosity, and gas-liquid partitioning were treated separately for individual layers, and sorption was treated as a single uniform parameter. Tortuosity values determined using the in situ measurement of F-11 and F-12 concentrations and the subsequent numerical modeling of results agree closely with values calculated using published theoretical and empirical relationships. These results thus confirm that the procedures for estimating tortuosity developed from theoretical considerations or by laboratory techniques are useful for predicting the transport of gases through even very thick and heterogeneous unsaturated zones. Such confirmation has not previously been available. the semiarid Southern High Plains of Texas (Figure 1) were investigated, and detectable concentrations of both gases were found at depths as great as 43.9 m. Soil gas samples were obtained from nests of air piezometers constructed by the U.S. Geological Survey for air permeability studies, as described by Weeks [ 1978]. All measurements of gas composition were made on site using a gas chromatograph.This study was designed to measure F-11 and F-12 concentrations in the interstitial gases of unsaturated soils, to derive effective diffusion coefficients for these gases, and to thereby determine the total retarding effect of tortuosity,

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 chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) to characterise groundwater movement and residence time in a lowland Chalk catchment

“…In deeper unsaturated zones of simple porous aquifers, there is a lag time for the diffusive transport of both CFCs and SF 6 through the unsaturated zone. The time lag is a function of the tracer diffusion coefficients, tracer solubility in water, and soil water content (Weeks et al, 1982;Cook and Solomon, 1995). In a deep unsaturated zone the apparent tracer age will be older than the true age of recharge.…”

Gas-phase Diffusive Tracer Test for the In-Situ Measurement of Tortuosity in the Vadose Zone

“…b from Weeks et al 1982 c from Moldrup et al 1997;and included in Moldrup et al 2000a (undisturbed soils) d from Moldrup et al 1997;and included in Moldrup et al 2000b (re-packed soils) e modified from Moldrup et al 1997;reported in Costanza-Robinson and Brusseau 2002 interval directly from the cylinder (Fig. 1).…”

“…Several methods exist for measuring or estimating tortuosities for unsaturated porous-media systems, including laboratory methods, empirical relationships, and field methods (e.g., Lai et al 1976;Weeks et al 1982;Reid et al 1987;Moldrup et al 1997Moldrup et al , 2000a. In-situ measurement of physical properties is generally considered to provide more representative results than laboratory-measured values for field-scale applications.…”