IN-SITU MEASUREMENT OF EFFECTIVE VAPOR-PHASE POROUS-MEDIA DIFFUSION-COEFFICIENTS

Knowledge of the effective vapor-phase porous medium diffusion coeffic ient is important for many applications, including risk-based volatili zation and vapor migration calculations and remediation performance es timates. A procedure for measuring the effective vapor-phase porous me dium diffusion coefficient in situ is presented here. The approach uti lizes transient changes in volume-averaged concentrations of an inert tracer gas, rather than changes in point concentration measurements, t o determine the effective vapor-phase porous medium diffusion coeffici ent D-v(eff). Measurements are made over intermediate time frames (min utes - hours), and the procedure is easily altered to probe scales ran ging from approximately 0.1-1 m. The data reduction leads to the deter mination of D-v(eff)/[theta(v)](1/3), where theta(v) denotes the air-f illed porosity. Given this reduced dependence on theta(v) (relative to approaches based on transient point concentration changes), in many s ettings it would not be necessary to make independent measurements of moisture content and total porosity to determine D-v(eff). For example , if theta(v) falls in the range 0.13 less than or equal to theta(v) l ess than or equal to 0.43 cm(3)-vapor/cm(3)-soil, then using an assume d value theta(v) = 0.28 would contribute to less than a 23% error in d etermining D-v(eff) from experimental data. The theory, a general prot ocol, and an example field protocol are presented along with sample fi eld data.