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.