Electrical and hydraulic vertical variability in channel sediments and its effects on streamflow depletion due to groundwater extraction

SummaryThe stratification of channel sediments and their vertical hydraulic conductivity (Kv) are important hydrologic information in the analysis of stream–aquifer interactions. This paper describes the use of direct-push technology to generate electrical conductivity (EC) logs and collect continuous sediment cores beneath river channels. The techniques were applied to nine study sites along a 130 km reach of the Platte River in southeast Nebraska. EC logs for the channel surface down to as much as 24 m below the channel surface indicate that the channel sediments in the western part of the reach consist predominantly of sand and gravel; low-Kv silt–clay layers occur in the rest of the reach. These silt–clay layers are either interbedded with sand and gravel or occur as a major unit within the channel sediments. As a result, the values of Kv, determined from the sediment cores, can vary by four to five orders of magnitude in the same vertical profile of channel sediments at a number of sites. The river channel is not lined by a low-Kv layer at the surface. Instead, the Kv values for the top part of channel sediments were consistently greater than Kv values for sediments in deeper parts. They show a decreasing tendency with the depth. This paper also analyzes the effect of low-Kv layers within channel sediments on streamflow depletion induced by groundwater pumping. Simulation results suggest that the effectiveness of low-Kv layers on the calculation of streamflow depletion depends on their depth, thickness, vertical permeability, the length along the channel, and the width within the channel, as well as the extension into the aquifer on both sides of the river. A low-Kv layer present at the channel surface seems to be the most effective hydrologic feature in reducing the hydrologic connection of stream–aquifer.