Factors affecting diffusion of atmospheric fluorocarbons into unsaturated porous materials

Abstract

Atmospheric buildup of two fluorocarbon gases, CC1₃F (F-11) and CC1₂F₂ (F-12), during the past forty years has been documented by previous workers. The present study involved measurements of F-11 and F-12 concentrations in soil air within unconsolidated porous materials. Measurable concentrations of both gases were found at depths as great as forty-four meters at field sites near Lubbock, Texas. Agreement between measured concentrations and concentrations predicted using mathematical models based on molecular diffusion theory suggests that gaseous diffusion was the primary mechanism by which atmospheric fluorocarbons were transported within the subsurface. Analytical models were employed to derive effective diffusion coefficients of F-11 and F-12 under field conditions (0.005 cm²/sec. and 0.012 cm²/sec., respectively) and under controlled laboratory conditions (0.017 cm²/sec. for F-11 and 0.021 cm²/sec. for F-12). A finite difference model was used to estimate in situ tortuosity values (mean = 0.10) for unsaturated porous materials at one field site.