USING DIRECT-PUSH ELECTRICAL CONDUCTIVITY (EC) LOGGING TO IDENTIFY FLOODPLAIN TERRACE BOUNDARIES IN THE SUBSURFACE

A knowledge of variations in depths and thicknesses of alluvial sediments caused by river channel migration is important to studies ranging from aquifer management to stream channel stability. Surface features such as terraces and meander scars are commonly used to predict the positions of channel boundaries in the subsurface, however, because floodplains often host agricultural and human activities, original surface expressions of subsurface transitions are often obscured. Direct-push electrical conductivity (EC) logging has previously been demonstrated to provide rapid access to information on variations in pore-fluid chemistry, sediment texture, and soil moisture, and used to reveal small-scale vertical and lateral variations in alluvial sediments. We tested the ability of direct-push EC logging to locate the margins of suspected channel positions in floodplain deposits along the Neosho River where surficial indicators have been altered. Twenty-seven EC logs and four soil cores were collected along five transects across the 0.14 km² site. Two distinct sedimentary layers are indicated in most of the logs and cores. The upper layer consists mostly of interbedded silt and clay, and varies in thickness from 1.8 to 9.1 meters. The lower layer ranges from 0.3 to 3.1 meters in thickness, and consists mostly of coarser sand and gravel textures with intermittent clay lenses. When interpreted in cross sections, the logs indicate that both layers change dramatically from north to south. From the Neosho River to the bedrock wall, the composition of the upper layer increases in clay content near the Neosho River while becoming slightly siltier toward the valley wall. The lower layer contains more clay near the river, and grades laterally to a coarse sand and gravel texture toward the valley wall. Simultaneous variations in thickness and texture within the two layers across the site suggest that the Neosho River held three possible channel positions during migration to its present position, and different flow velocities. The results suggest that EC logging can be used to confirm the locations of sedimentary features in the subsurface when surficial features cannot. The method enables more accurate interpretations of the timing of paleochannel development and contaminant transport in flood plain aquifers.