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.

Membrane Interface Probe Protocol for Contaminants in Low‐Permeability Zones

“…Elevated EC readings coincided with lower k units, and the EC data could be used to generally identify the upper and lower boundaries. These results are consistent with those obtained by other high-resolution characterization studies that included direct-push electrical conductivity logging (e.g., Schulmeister et al 2003;Wilson et al 2005;Harrington and Henry 2006;Köber et al 2009).…”

Challenges and a Strategy for Agricultural BMP Monitoring and Remediation of Nitrate Contamination in Unconsolidated Aquifers

“…Multilevel monitors for groundwater sampling were introduced to address the need for a detailed understanding of contaminant distributions in aquifers (Pickens et al ), and though for many years these were used almost exclusively as research tools, they are commonly used by professionals today. More recently, direct push technologies have been developed for unconsolidated sediments that deliver highly detailed depth‐specific datasets of both physical (Butler et al , ; Harrington and Hendry, ; Sellwood et al ; McCall et al ) and chemical (Pitkin et al ; Parker et al, ; Schulmeister et al ., ) conditions in single boreholes. The incorporation of boreholes and monitoring wells into fences for detailed two‐dimensional pictures of the subsurface and contaminant distributions has also been described (Einarson and Mackay ).…”

Chemical heterogeneity in diffusion‐dominated aquitards

“…This transposition is usually assumed valid because (1) the aquitard is laterally extensive and (2) the vertical hydraulic gradient through the aquitard is one to two orders of magnitude greater than the horizontal hydraulic gradient, so any solute transport by advection will primarily be in the vertical direction (upward or downward depending on the head gradient). Recent efforts to define the spatial distribution of pore water salinity at a well‐studied, surficial till aquitard site in southern Saskatchewan, Canada (hereafter called the King site) resulted in the discovery of lateral heterogeneity in pore water chemistry between ~3–18 m BG [ Harrington and Hendry , 2005]. Although Harrington and Hendry [2005] focus primarily on the direct‐push electrical conductivity (EC) logging method for characterizing heterogeneity in aquitards with known salinity contrasts, the paper also raises concerns about the validity of transposing hydrochemical data obtained from laterally distributed piezometers onto a 1‐D profile for interpretation.…”