METHODS FOR DETERMINATION OF SEDIMENTARY STRUCTURES AND HYDRAULIC CONDUCTIVITIES OF STREAMBED

The architecture of channel sediments and their vertical hydraulic conductivity (Kv) are important hydrologic information in the analysis of stream-aquifer interactions. This paper describes a method of using direct-push technology to generate electrical conductivity (EC) logs and collect continuous sediment cores from river channels. EC logs are useful in differentiating low-Kv layers from high-Kv units, and sediment cores are used to estimate the vertical hydraulic conductivities. The techniques were applied to the river channel along a 130-km reach of the Platte River in southeast Nebraska. EC logs to a depth up to 24 m and sediment cores to a depth up to 15 m were collected from nine study sites. EC logs indicate that the channel sediments in the western part of the reach consist largely of sand and gravel and low-Kv silt-clay layers occur in the rest part of the reach. These silt-clay layers are either interbedded with sand and gravel or occur as a major unit in the channel sediments. As a result, the values of Kv can vary by four to five orders of magnitude in the same vertical profile of Kv. The Kv values for the top part of channel sediments were greater than Kv values for sediments in deeper parts. They show a decreasing tendency with the depth. The reverse correlation between EC and Kv was observed; it is nearly a log-log linear relation. MODFLOW was then used to simulate streamflow depletion caused by irrigation wells in nearby alluvial aquifers. Simulation results suggest that these low-Kv layers, depending on their depth in the channel, can largely reduce the impact of pumping on streamflow.