Migration of Colloids through Nonfractured Clay-Rich Aquitards

Abstract

This study examined characteristics and controls on diffusive transport of colloids through nonfractured, clay-rich glacial till. The range in molecular weight (M(w) of the colloids tested (five polymers and three natural dissolved organic carbons) was 910-15 450 Da. Hydrodynamic diameters increased from 1.45 to 6.05 nm, and aqueous diffusion coefficients decreased from 2.6 x 10(-10) to 6.3 x 10(11) m2 s(-1) with increasing M(w). All colloids were subjected to diffusion testing using undisturbed core samples placed in double reservoir diffusion cells. All colloids decreased in concentration with time in the spiked reservoirs. Concentrations in the receiving reservoirs increased for only the four smallest colloids. The lack of breakthroughs for larger colloids was attributed to straining. Transport modeling using data from colloids exhibiting breakthrough shows effective diffusion coefficients and tortuosity factors decrease from 1.5 x 10(-10) to 6.5 x 10(-11) m2 s(-1) and from 0.6 to 0.3, respectively, with increasing M(w). The effective porosities are slightly less than total porosity (0.31). Our data suggest diffusive transport through clay-rich aquitards is limited to colloids with mean diameters < 2-2.2 nm and that measuring the diameter of dissolved organic carbon from nonfractured clay-rich aquitards may be an effective method to estimate effective pore throat diameters of these media.