HETEROGENEITY WITHIN A SURFICIAL, CLAY-RICH AQUITARD INFERRED FROM DIRECT-PUSH EC MEASUREMENTS AND POREWATER CHEMISTRY

Variations in the surface topography and hydraulic properties (K, η) of saturated porous media can exert important controls on solute transport pathways, particularly in clay-rich aquitards where diffusion is assumed to be the dominant transport mechanism. Delineating sub-surface heterogeneity, whether at the macroscopic (10⁰ m), field site (10¹ to 10² m) or regional (10³ to 10⁴ m) scale is notoriously difficult and therefore generally expensive. Recent advances in Direct-Push Electrical Conductivity (D-P EC) logging technology have provided a relatively low-cost, high-resolution method for characterizing changes in either lithology or porewater chemistry within coarse-grained sediments. Until now, this approach had not been tested in fine-grained deposits with large vertical contrasts in porewater salinity, such as the glacial till aquitards that cover much of North America.

We present 22 D-P EC profiles to depths of 10 – 17 m below ground surface (> 20,000 data points) from a well-studied, clay-rich aquitard site (140 m x 80 m) in southern Saskatchewan, Canada. The shape and magnitude of the measured profiles vary considerably across the site and with increasing depth below the water table. A comparison of bulk EC measurements with porewater EC and chemistry data obtained by mechanical squeezing of 21 core samples enabled the calculation of ground water EC distributions. Two detailed transects of inferred groundwater EC reveal complex spatial distributions of salts that are well correlated with topographical expression at the surface. Hydraulic conductivity of the aquitard was calculated from recovery rates in more than 30 piezometers across the site, and exhibits only minor horizontal variability. Three-dimensional diffusive transport modeling indicates that the heterogeneity observed at the site is controlled by non-uniform salt fluxes to the water table and subsequent downward diffusion, rather than changes in physical properties of the aquitard.