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‐1l) 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‐1l 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 finite‐difference 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 coefficient 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‐1l 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.