Delineating Hydraulic Conductivity with Direct Push Electrical Conductivity and High-Resolution Slug Testing

Direct push technology continues to make advances in efficiently measuring water related parameters in unconsolidated sediments. A direct push subsurface profiling technique used during field investigations measures the electrical conductivity (EC) of sediments and fluid surrounding the EC probe. The EC geophysical method is typically used for gross lithologic definition. When numerous direct push EC profiles are completed, a general impression of vertical and lateral variation of subsurface lithology can be inferred. Unfortunately, these EC profiles do not directly measure the hydraulic conductivity, even though the profiles may indicate the presence of fine-grained material such as silt and clay, which are known to affect the hydraulic conductivity. The direct push EC vertical profiles can be obtained quickly and efficiently over an extended area. These EC profiles can be examined for regions that display a subsurface EC response with a potentially interesting behavior of the hydraulic conductivity. At selected locations, 5cm (2 inch) PVC monitoring wells with appropriate screen lengths can be installed by direct push techniques. We have developed equipment and techniques for performing high-resolution slug tests efficiently in 5cm (2 inch) wells. Correlation of the EC response and the high-resolution slug test results can aid in developing a 3-D picture of the hydraulic conductivity distribution at a given site. In this paper, we present the results of such a correlation for a well located near the Geohydrologic Experiment and Monitoring Site (GEMS) at the University of Kansas in the Kansas River valley. During the installation of this well with direct push equipment, it was discovered that the EC log indicated a prominent but relatively thin silt-clay layer at depth, which is somewhat unusual for this area. We routinely perform high-resolution slug testing efficiently over intervals as small as 7.5cm (3 inches) to 15cm (6 inches); therefore, we decided to see if high-resolution slug testing could accurately delineate this layer of apparent reduced hydraulic conductivity. Results show good correlation between the EC profile and the high-resolution slug test profile. Coupling direct push EC profiling with high-resolution slug testing may provide an efficient way to establish a detailed representation of the hydraulic conductivity distribution in a given area.