A comparision of laboratory and field based determinations of molecular diffusion coefficients in a low permeability geologic medium.
Molecular diffusion is the dominant transport mechanism for contaminants in many saturated clay-rich aquitards. The effective coefficient of diffusion (Da) is traditionally determined by conducting laboratory tests on cm-scale core samples that may not be representative of the bulk geologic formation. Here we conducted the first long-term field based in situ diffusion experimentto compare the effect of experimental scale (5 x 10(-5) m3 in the diffusion cells and (5-20) x 10(-2) m3 in the in situ experiments) on De values for clay-rich aquitards. Using a conservative tracer (deuterium), our testing shows De values estimated from in situ testing ((2.5-3.5) x 10(-10) m2 s(-1)) are similar but lower than the average De values measured in the laboratory (4 x 10(-10) m2 s(-1)). The difference was attributed to greater porosity values in the laboratory samples resulting from core barrel extrusion and sample swelling. With representative core sampling and care, laboratory-based diffusion testing remains a viable method to assess solute transport mechanisms in clay aquitards.
Hendry
M Jim
MJ
Department of Geological Sciences and Department of Civil Engineering, University of Saskatchewan, Saskaroon, SK, Canada. jim.hendry@usask.ca
Barbour
S Lee
SL
Boldt-Leppin
Brigitte E J
BE
Reifferscheid
Laura J
LJ
Wassenaar
Leonard I
LI
D000538:Aluminum Silicates / Q000737:chemistry; D000077215:Clay; D004058:Diffusion; D004784:Environmental Monitoring / Q000379:methods*; D019015:Geologic Sediments / Q000737:chemistry*; D055593:Geological Phenomena; D008962:Models, Theoretical; D010539:Permeability; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000032:analysis
D000538:Aluminum Silicates; D014874:Water Pollutants, Chemical; D000077215:Clay