Using direct-push EC logging to delineate heterogeneity in a clay-rich aquitard

Abstract

Heterogeneity exerts an important control on solute transport pathways in any subsurface environment, including aquitards where diffusion is the dominant transport mechanism. Direct-push (D-P) electrical conductivity (EC) logging has recently enabled high-resolution hydrostratigraphic characterization of predominantly coarse-grained sediments in low-salinity aquifers. In this paper, we apply D-P EC logging to characterize the spatial variability of physical and chemical properties in a clay-rich aquitard that has known vertical contrasts in pore water salinity. The D-P EC logging was conducted to a maximum depth of 17.5 m below ground (BG). Results obtained from 22 D-P EC logs across a 140- × 80-m field site were compared with pore water chemistry data from squeezed core samples (n= 21) and piezometers (n= 6) to reveal complex spatial variations in pore water salinity ranging from <5000 to ∼90,000 μS/cm. The EC distributions appear to be controlled by nonuniform salt fluxes from the unsaturated zone to the water table and subsequent downward diffusion. Detailed D-P EC logging also revealed the presence of a single discontinuous sand lens located between 9.4 and 15.9 m BG that may have important controls on solute transport pathways. The only limitation with using this method in very fine-grained sediments is that depths of tool penetration may be considerably less than those currently achievable in sand and silt deposits. © 2006 National Ground Water Association.