WHAT IS THE EFFECTIVE PORE THROAT DIAMETER IN A CLAY-RICH AQUITARD AND WHY IS IT IMPORTANT?

An understanding of the controls on the transport of colloids, including pathogenic microbes, dissolved organic carbon (DOC), and hydroxides, in clay-rich aquitards has become an increasingly important area of research because these aquitards are the key protective barrier to protect potable water supplies in aquifer. To date we know very little about the maximum size of colloids that can migrate via diffusion through these media. We determined the maximum size of conservative colloids (i.e., the effective pore throat diameter, EPTD) that can diffuse through a well characterized clay-till aquitard in southern Saskatchewan, Canada using laboratory experiments and field measurements. In the laboratory experiment, the EPTD of the clay till was determined by assessing the breakthrough of colloids in double reservoir diffusion cells using a variety of colloids with molecular weights from 920 Daltons (Da) up to 15450 Da (n=8) over a 230-550 day period. The colloids included natural groundwater DOC, standard humic and fulvic acids, and sodium poly(styrene sulphonate) standards. Identification of the breakthrough of the colloids was achieved using asymmetrical flow field-flow fractionation, which allowed for accurate separation, identification, and quantification of particles greater than 500 Da. In these experiments, only colloids with molecular weights < 2100 Da (approx. 0.001 µm) were observed to break through the till and reflected the EPTD of the medium tested. Retardation (including straining) of the colloids was observed and was shown to be a function of the colloid diameter. The laboratory determined EPTD was in agreement with that estimated from DOC samples collected from the aquitard. In addition to the EPTD, we were also determined the effective diffusion coefficients, effective porosities, and retardation factors for each of the colloids examined. This work shows that bacteria, hydroxides, and most DOC should not migrate through clay aquitards and that any bacteria (or their progeny) within these media were not introduced but present in the till at the time of deposition.